Process stability test injection molding

The purpose of the process stability test is to identify any gross mechanical design/construction flaws which will cause part variation over time. In addition, the process stability must be established with no short shots, flash, etc., prior to running designed experiments. The test is run at mid-range resin molding conditions for a period of time to allow thermal expansion equilibrium. In most instances, the mold builder may already have a process where the mold could run comfortably for several hours. If this is the case, skip to step 10.

During the process stability test the mold must run with one ejection stroke unless it was designed to cycle with multiple strokes. A robust ejection system will be able to eject the part and runner with one ejection stroke. Multiple ejection strokes per molding cycle increases the wear on the ejection system and increases the molding cycle.

All the steps during the procedure that involve intimate contact with the injection molding machine are to be done by a qualified injection molding machine operator.

Procedure:

1. Set melt temperature to resin manufacturer's recommended mid-range.
2. Set mold temperature to resin manufacturer's recommended mid-range.
3. Set hold pressure and time to zero, as well as pack time and pressure.
4. Set cooling time long enough so that parts eject consistently without distorting.
5. Adjust feed stroke and screw/ram position cutoff so that parts fill approximately 95% during injection.
6. Set adequate hold time and pressure, as well as pack time and pressure, so no sink marks are visible.
7. Record process settings of the machine, melt temperature and mold temperature.
8. Run the process long enough that you feel comfortable the mold and process has reached steady state, and there are no functional issues with the mold.
9. Evaluate visually for any gross part or mold variation, e.g., flash, short shots, scratches, blemishes, water (leakage) and voids.
10. Collect one full shot and weigh each part. Record individual part weight.
• Note: This will be used as the weight of a full part when determining 90% part weight during the following tests.
11. Explore the processing window of the mold. Vary parameters such as injection rate, hold pressure, hold time, cooling time in order to begin to understand the mold processing window. For each range of process settings collect at least one set of parts. These parts will be necessary to perform the gage R&R analysis. Recall for the gage R&R one is trying to minimize the variation within a sample but maximize the variation between samples.

Dry cycle injection mold

The purpose of the dry cycle mold is to evaluate the mold mechanical actions for any gross problems prior to injecting plastic into the mold. The machine operator at the mold builder or molder should know how to adjust the settings on the injection unit to cycle the mold without injecting plastic into the tool. If at anytime during the dry cycle procedure there is a possibility that damage is being done to the tool or if the mold is not operating as designed, pull the mold from the press and have it examined by an experienced mold builder.

This stage is often carried out prior to the customer eing invited to the mold builder for the debug. If there are any machine/mold operation issues it is preferable for the mold builder to remedy this before attendance, saving unnecessary travel. If there is a serious mold design flaw (as opposed to a mold assembly issue) it will then be necessary to attend to discuss next steps.

All the steps during the procedure that involve intimate contact with the injection molding machine are to be done by a qualified injection molding machine operator.

Procedure:

1. Put mold in press. Ensure that the cooling circuits are attached to the mold and that water is circulating through the mold. Also, if there is a hot runner manifold have the controller installed and temperature heaters turned on.
2. Verify water is flowing through the mold at resin manufacturer’s recommended mid-range; check with flow meter indicator or measure flow rate by having the returns empty into a bucket. Ensure heaters are working in the hot runner system by reading the thermocouple outputs.
3. Close the mold at a slow rate and pressure to verify that the mechanical, pneumatic or hydraulic actions of the mold have responded and are moving as designed. Listen for any suspect noises of possible binding of slides or galling.
4. Clamp up on the mold at a low tonnage. Be sure to have mold protection time, pressure and rate properly set so that no major damage can be done to the mold.
5. When the mold is in the press, open the mold at a slow rate to ensure that the mechanical, pneumatic or hydraulic slides are responding and moving as designed. Listen for any suspect noises of possible binding or galling. The slides should move with little resistance.
6. After mold is completely open, cycle the ejector plate manually at a low pressure and low rate, while visually inspecting to verify the ejector plate is performing as designed.
7. Cycle the mold without injecting plastic. Typically this is done by transferring from fill/pack stage to hold stage by position and setting the feed stroke and transfer position to the same position while having the hydraulic pressure set to 0 bar. Keep the clamp open/close rates and pressures to a minimum. Set the hold time to zero. Set cool time to a minimum machine value to decrease the cycle of the mold.
8. Evaluate the mold while it is cycling for any gross mechanical problems. Study the movements of the mold for possible binding or galling of slides, ejector pins and leader pins.
9. Increase the rate and pressure of mold open/close slowly until it replicates the speed and pressures expected during production or sampling. Also do the same with the ejection action.
10. Increase the tonnage slowly until it replicates the tonnage necessary to keep the mold closed during the injection stage of the molding process.
11. Continue cycling the mold until you determine that mechanically there are no fundamental flaws with the production or design of the mold.

Die ring extrusion blow molding

Die ring - The outside ring forming wall that determines the final orifice O.D. and shape.

Die pin extrusion blow molding

Die pin (lower mandrel) - The removable extension of the upper mandrel forming the inside wall of the final orifice.

Die mandrel extrusion blow molding

Die mandrel - The cylindrical section in the die head about which the plastics flow. The upper mandrel supports the lower mandrel.

Die land extrusion blow molding

Die land - A parallel section of the die ring just ahead of the opening.

Die head manifold extrusion blow molding

Die head manifold - A structure containing the flow passage(s) that carries the plastic to single or multiple die heads.

Die head extrusion blow molding

Die head - The structure which holds the die.

Die body extrusion blow modling

Die body - The main structure of the die head excluding the ring and pin.

Die and pin shaping extrusion blow molding

Die and pin shaping - Machining the pin and/or ring to produce a uniform or a non-uniform parison wall thickness or shape.

Die and pin set extrusion blow molding

Die and Pin Set - A matching ring and pin to form a parison of a given shape.

Desiccant

Desiccant - A substance capable of absorbing water vapor from air or other gaseous material, used to maintain low humidity in a storage or test vessel.

Polymer density

Density - Mass per unit volume of a substance, exposed in units such as grams per cubic centimeter, pounds per cubic foot or pounds per gallon.

Degree of polymerization

Degree of polymerization - The average number of monomer units per polymer molecule, a measure of molecular weight. In most plastics, the molecular weight must reach several thousand to attain worthwhile physical properties.

Polymer degradation

Degradation - A deleterious change in the chemical structure of a plastic.

Daylight opening

Daylight opening - Clearance between the two platens of the press in the open position.

Cycle time extrusion blow molding

Cycle - The complete sequence of operations in a blow molding process to produce a set of parts.
Cycle time - Time during which one molding cycle is completed.

Extrusion blow molding cuting off

Cut Off - A hot or cold device to cut the finished part or parison from the die head.

Polymer crystallinity

Crystallinity - A state of molecular orientation which denotes an orderly compact structure of the molecular chains forming the polymer.

Crystalline Polymer

Crystalline polymer - Plastic whose state of molecular chains are in a uniform, regular arrangement.

Creep polymer

Creep - The dimensional change with time of a material under load, following the initial instantaneous elastic deformation.

Crazing

Crazing - Fine cracks which may extend in a network on or under the surface or through a layer of a plastic material.

Cooling channels mold

Cooling channels - Channels or passageways located within the body of the mold through which cooling medium can be circulated to control temperature on the mold surface.

Core (Core Pin)

Core (Core Pin) - Mold component which forms internal surface of the molded part.

Control Chart and Control Limits

Control Chart - A graphical method for evaluating whether a process is or is not in a state of statistical control (stable). The decisions are made through a comparison of the values of some statistical measurement calculated from the data with control limits.
Control Limits - Limits on a control chart that serve as a basis for judging whether or not a process is in a state of statistical control (stable)

Continuous Parison

Continuous Parison - A parison extruded through the die head continuously.

Plastic part conditioning

Conditioning - Subjecting a material to standard environmental and/or stress history prior testing.

Compression Ratio

Compression Ratio - In a helical extruder screw, the ratio of volume available in the first flight at the hopper versus that available in the last flight at the end of the screw.

Colorant

Colorant - Any substance that imparts color to another material or mixture. Colorants can be either dyes or pigments

Color Concentrate

Color Concentrate - A measured amount of dye or pigment incorporated into a pre-determined amount of plastic. This pigment is then mixed into larger quantities of plastic material to be used in molding.

Coefficient of Expansion

Coefficient of expansion - The fractional change in length of a material for a unit change in temperature

Closure

Closure - A device used to seal off the opening of a container, so as to prevent loss of contents.

Clearance

Clearance - Controlled distance by which one part of an object is kept separated from another part.

Clarifier

Clarifier - An additive that increases transparency of a plastic material.

Clamp stroke

Clamp stroke - The maximum travel distance the opening and closing mechanism can traverse the platens.

Clamp Opening Force

Clamp opening force - The maximum force in tons (2,000 pounds) possible to exert upon the molds at the initiation of opening.

Clamping Force

Clamping force - The maximum force in tons (2,000pounds) exerted upon the molds when closed and held against blowing pressure.

Clamp Speed injection molding

Clamp Speed - Speed at which the mold platens will open and close; controlled by operator settings.

Child Resistant Closure (CRC)

Child Resistant Closure (CRC) - A closure that requires dissimilar motions which may make removal by a child difficult. Subject to government regulations.

Polymer chemical resistance

Chemical Resistance - Ability of a material to retain utility and appearance following contact with chemical agents.

Charge injection molding

Charge - The amount of material volume or weight required for the injection unit to plasticize for each molding cycle.

Centerline

Centerline - A line drawn down on part drawing that represents the geometrical center of the part in that axis.

Cavitation and cavity

Cavitation - The number of molding stations within a mold.
Cavity - Mold component(s) which form the exterior or external surfaces of a part.

Capacity for polymer processing

Capacity - The rated amount of volume (# parts) produced by a mold/molding machine per year.

Bushing extrusion blow molding

Bushing - The outer ring of any type of circular tube or pipe die which forms the outer surface of the tube or pipe.

Burned extrsuion blow molding

Burned - Showing evidence of excessive heating during processing or use of a plastic, as evidenced by blistering, discoloration, distortion or destruction of the surface.

Breaker Plate

Breaker Plate - A metal plate installed across the flow of the melt between the end of the screw and the die with openings through it such as holes or slots, frequently used to support a screen pack.

Blueing Off

Blueing Off - A mold making term for the process of checking the accuracy of mating of two surfaces by applying a thin coating of Prussian Blue on one surface, pressing the coated surface against the other surface, and observing the areas of intimate contact where the blue color has been transferred.

Bottom Blow Pin Stand

Bottom Blow Pin Stand - A device used to support blow pin(s) - stationary of moveable.

Blow Time extrsuion blow molding

Blow Time - The length of time the blow medium is contained in the mold.

Blowing Pressure extrusion blow molding

Blowing Pressure - The pressure in psi of the medium blowing the plastic to the shape of the mold.

Blow Pins - Blow mandrels

Blow Pins - (Blow mandrels) devices used to inject blowing medium into a mold, from the neck dimensions of a blow molded container , locate inserts to form threads, stretch the parison for pre blow of large parts.

SAFETY PRECAUTIONS FOR BLOW MOLDING

Safety is everyone's responsibility in the workplace. Safety is most often related to good maintenance and good housekeeping. Safety needs to be an attitude that is always present in your daily activities. Employees should not be hesitant to voice safety concerns in the workplace. Management is just as committed to safety as the operators on the floor; the primary difference is that the operators are usually the closest to unsafe conditions; keep management advised of unsafe conditions.

The following list includes items which should be maintained to assure a safe working environment:

1. Floor and machine should be kept free of oil.
2. Floor and machine should be kept free of pellets.
3. Never reach over or under machine guards.
4. Never climb between the tie bars when hydraulic pumps are running.
5. Never disconnect or by-pass safety switches on guards.
6. In order to prevent mechanical hazards such as limbs being drawn into or trapped in the machinery, operators should not wear personal effects such as bracelets, watches, rings or chains during work shifts engaged in operating the blow molding machine.
7. Process only materials that are specified for use with the blow molding machine by the machine manufacturer.
8. Always wear adequate noise protection.
9. Use caution and wear protective gloves when making adjustments on hot die head components and manifolds.
10. Catwalks or platforms with railing should be present if hoppers such as drying hoppers stand tall enough whereby access requires climbing onto machine.
11. Know location of portable fire extinguishers; there should be an extinguisher no farther than 75 feet.
12. All electrical outlets should be marked as to the line voltage.
13. Never reach into the throat of an operating granulator. Unplug granulators before working on.
14. Always wear suitable foot and eye protection; safety glasses should be worn and steel toed shoes are recommended; soft soled shoes should not be worn.
15. Doe not operate any equipment unless suitable training has been completed.
16. All employees should be advised of any chemicals in the facility which are considered hazardous; read further about "Right To Know" laws for each particular state.
17. First aid kits should be available.
18. Advise operators that blow molding resin pressure can reach 4,000 psi and that hydraulic line pressure can reach 2500 psi. Clamp tonnage developed equals 2000 lbs of force for each ton; operators be advised.
19. Be conscious of sharp square corners on mold components and cavity parting line edges.
20. Razor knives also require extreme caution as their use results in many cuts.
21. NEVER use steel tools on the mold cores, cavities or parting line... Use brass, copper or aluminum. Brass can scratch highly polished steel, so use caution.
22. Do not stick fingers or rods into the barrel/screw feed throat area.
23. Examine air hoses and electrical cords to verify condition is proper; do not use cords with damaged insulation. Be especially observant when working near nozzle heater bands as these wires are easily
24. damaged.
25. Use only swivel type safety eyebolts; screw eyebolts far enough in such that thread engagement is 1.5 times the diameter.
26. Never stand directly below a mold suspended in air.
27. Avoid back injuries; lift properly with the back upright and straight; know your limitations and do not exceed them; use proper tools and get help when needed.

Design of experiments, DOX, DOE in extrusion blow molding

A statistical design of experiments, or DOX, is the preferred method to determine an optimum blow molding process and processing window. Initial mold/process evaluations should be completed on a small prototype mold when possible. The mold is normally shipped from the tool builder to the molder's development laboratory. The molder's development laboratory should have the time and resources available to evaluate the molding process for that mold and to perform any initial Design of Experiments.


The objective of a design of experiments is to not only identify an optimum process and processing window, but also to identify the main effects from process variables, interactions between them, and possible curvature effects. Initial experiments are run to determine which combination of process control parameters yields the lowest dimensional variability. Once this is determined, the tool steel can be adjusted to meet the molded container dimensional specifications. It is important NOT to choose process control parameters solely based on the part specifications. If your tool builder constructs the mold using a "Steel Safe" approach, you will be able to easily cut and ‘adjust’ the steel for the most stable and efficient process.

It is recommended that the Design of Experiments be performed on the blow molding machine on which the mold will run on during production or sampling. Only a skilled molding company can accurately translate the process control parameters from one blow molding machine to another. Even if the machines are similar, there are always differences that will affect the attributes of the molded part. The amount of wear on the extruder, the mold clamp system design, hydraulic valves, screw, barrel and timers can differ and can have large effects on process control parameter adjustments. In addition, the molder’s development laboratory often utilizes a different (mold) cooling system than the system used in a manufacturing area. Differences in development laboratory (individual mold temperature control systems) and manufacturing plant (central) coolant system performance can be significant. Careful attention must be given to the actual cooling (heat transfer) variables in the mold.

In Mold Labeling IML setup procedure

The IML system must be adjusted to accommodate the mold cavities, label size, and label position on the container. The adjustments must be done in relation to the molds, with the proper placer arm assembly, vacuum cup heads, and label magazines installed.


Procedure for Shuttle Process:

A. Carriage and Placer Arm Assembly Setup

1. Adjust the carriage and placer arm assembly. Disconnect the actuator links from the front and back sides of the placer arm assembly at the drag links.
2. Check that the angle between actuator arms and the top of the carriage frame is 75 degrees.
3. On the cylinder side only, reconnect the actuator links to the drag link. Check to assure the actuator arm is still in contact with the latch link. Adjust the link as required so that the placer arms are flat against the placer arm bracket and the actuator arm is against the latch link. Adjust the opposite (non cylinder) side, similarly. It is important to ‘balance’ the force on both the front and back side of the actuator links, taking care not to put a
‘jacking force’ on the components.

B. Centering the Placer Arm Assemblies Horizontally Between the Molds

1. Assure adequate daylight opening between mold halves exist on both side A and side B. Back-off the deployment roller (move towards the mold).
2. Move the placer arm assembly between the mold halves to the full in position and set the carriage lock.
3. Loosen the (4) screws that bolt the slide base to the upper frame.
4. Measure the distance from the inside face of the placer arm to the front face of the mold. Repeat the procedure on back face of the mold. If the measurement is not the same turn the adjustment screw located on the upper frame and below the slide base. The placer arm assembly is centered when this distance is the same on both the front and back halves of the mold.
5. Retighten the (4) screws on the slide base.

C. Centering the Vacuum Heads Vertically in the Cavity

1. Back off the deployment roller on the lift-up cam.
2. Move the inserter carriage between the mold halves to the full in position and set the carriage lock.
3. Slightly loosen the (4) screws on the back of the lower frame.
4. Turn the adjustment jackscrew: clockwise to raise, counterclockwise to lower.
5. Check to assure the vacuum cup head is approximately centered vertically in the cavity.
6. Retighten (4) screws.
7. Release the carriage lock and move the carriage out and in by hand to insure that no part of the placer arm interferes with the mold cavities or platen tie rod as it is lifted up into position. Readjust as required, then reset carriage lock to continue.


D. Setting the Tow Bar Length

1. With the placer arm ‘in molds’ and carriage lock engaged, loosen tow bar jam nuts (R.H. and L.H. nuts).
2. While manually extending placer arms, turn the tow bar until vacuum cups are located in the mold cavities as required, usually centered in the label panel area.
3. Tighten the jam nuts.

E. Adjusting the Deployment Roller

1. Assure the carriage lock is engaged.
2. Loosen the lock screw on front of the deployment roller housing.
3. Turn the adjustment screw: clockwise to extend vacuum cups deeper (moves roller away from molds); counterclockwise to retract arms (moves roller towards molds).
4. Check vacuum cup centering, readjust as required.
5. Retighten the lock screw. Placer arm extension should allow for only slight flattening of the vacuum cups against the walls of the mold. Over extension and excessive force may cause excessive wear and premature failure of the placer arm assembly components.

F. Adjusting the Label Magazines

1. Unlock and open the door of the magazine guard.
2. Pull back the pusher plate while holding the remaining stack upright.
3. Load new labels in until desired amount is reached.
4. While releasing pressure on the pusher plate shuffle stack down into and even stack.
5. Close and lock the magazine guard door. Label stacks should be ‘riffled’ several times to eliminate ‘edge welding’ due to die cutting and to allow some air to become entrapped between labels.
6. Set the label stops to allow the labels to be picked easily without double picking or moving the next label out of position. The overlap of the stops onto the label may be different from side to side or top to bottom. The overlap should be about 1/16” initially.
7. Adjust the magazine position (amount of contact force) between the label stack and the vacuum cup heads by moving the magazines in and out. Loosen the lock-down collars and release the lock-down clamp that holds the magazine in place.
8. With the blow molding machine and IML system in ‘Manual’ mode, press the ‘Step/Home’ button to move the first pair of placer arms to the first label pick position. Note how the vacuum cups contact the end label on the stack in the magazine.
9. Adjust the label magazine in or out as required to achieve proper contact. Secure lock-down collars after proper magazine position is achieved.

G. Pick Cylinder Speed

1. Ensure that adequate plant air pressure is available.. Adjust the regulator located on the pneumatic control assembly to 80 psi.
2. Adjust extension speed of the placer arms on valve connected to rod end of cylinder, turn stem: clockwise to slow; counterclockwise to speed up.
3. To adjust retraction speed of the placer arms turn valve connected to other end of cylinder do likewise.

H. Pick and Mold Vacuum

1. The IML system vacuum pumps require 80 psi for optimum performance. Adjust the pick vacuum with the regulator located on the pneumatic control assembly. Settings lower than 60 psi may result in missed picks.
2. Adjust mold vacuum with the regulator located on the pneumatic control assembly. Mold vacuum requirements will vary depending on the number of vacuum ports in the mold, label materials, container shape, etc.

I. Adjusting Label Position on Container

1. The adjustments are side to side, height, and skew (rotation). Adjust the label position by loosening the appropriate clamps and moving the adjustments in direct relation to the container. Retighten clamps to prevent changes in settings due to vibration.

J. Setting Up and Modifying Programs

1. Refer to the IML operating instructions manual for specific program modifications. The number of picks and the position of label picking are normally modified whenever the number of cavities or cavity spacing is changed. No other program modifications are normally required.

Manifold Balance extrusion blow molding

The purpose of the Manifold Balance step is to evaluate the thermal and melt flow balance of the head tooling manifold distribution system and to establish uniform melt flow to each die on multi-head blow molding systems. The melt flow control (normally a choker valve) must adjust freely at normal operating temperature. The valve design must be capable of adjusting material flow to each die head so that the melt flow is stable over a range of extrusion rates.

Both the parison weight and length differences are an indicator of the degree of melt balance control and the quality of the manifold system design. A typical, well-balanced manifold will be balanced to within 5%. It is critical to have the flows balanced to each die or the part-to-part variation may be large and process capability may not be achievable.

Initial manifold control valve balance adjustments should be made during the Parison Centering step to optimize the manifold system flow balance. The Manifold Balance step may have to be repeated at higher extrusion rates, i.e. production rate, as melt flow pressure gradients change with melt flow rate and can affect the manifold melt flow balance.

Proceed to the Verify Mold Operation step if the blow-molding machine is a single die head system.

All the steps during the procedure that involve intimate contact with the bow-molding machine are to be done by a qualified blow molding machine operator.

Shuttle Process Procedure:

Task Description:
1. Verify that the zone temperature controller displays indicate a relatively uniform temperature distribution for the manifold system and head tooling.

2. With the extruder operating at a low rate, cut the parison close to the die face (using the cut device where applicable), extrude a parison and note a reference length for the parison. Typically the reference parison length will be the required parison length for the mold. Shorter parison reference lengths should be used for larger containers to avoid parison sag effects. Use a stopwatch and record a reference parison extrusion time from when the parison was cut until the end of the parison reaches the reference length position. Cut the parison again and record the parison weight for that die head.

3. Adjust the flow control valve(s) as required to achieve relatively uniform melt flow rates to each die head.

4. Repeat steps 2 through 5 until a satisfactory flow balance is achieved.

5. Calculate the percent imbalance.

Parison centering extrusion blow molding

The purpose of the Parison Centering step is to establish a preliminary die position adjustment to achieve stable parison extrusion and uniform, circumferential parison wall thickness distribution.

Die bolts should be free of any degraded material and turn freely over the adjustment range at normal operating temperature for the die bushing. Appropriate repairs should be made for damaged threads on either the die bolt(s) or the die bushing.

Shuttle Process Procedure:

Task Description

1. Switch on the main power to the zone temperature controllers. Set the extruder barrel and head tooling temperature controllers to the material manufacturer’s recommended mid-range melt processing temperature. Verify that there is no error or open thermocouple circuit message on the temperature controller display. If an error or open circuit message is displayed, check all connections, thermocouples, heater bands and replace or repair as required. Allow 90 minutes for the extruder barrel and head tooling heating zones to reach the recommended material processing temperature.

2. Set the mold temperature system controller to the material manufacturer’s recommended mid-range mold surface temperature.

3. Switch on the melt pressure transducer and melt pressure monitor display for the melt distribution manifold and die head(s). Verify that there is no error or open circuit message on the melt pressure monitor display. If an error or open circuit message is displayed, check all connections and replace or repair the melt pressure transducer and/or melt pressure monitor unit as required.

4. Switch on the main power to the blow molding machine control panel. Set the process control mode selector switch to manual. Ensure that the mold(s) are in the open position and/or there is no obstruction in the path of the extruded parison(s) during the centering procedure.

5. Start the extruder at low RPM and monitor the ammeter and/or melt pressure monitor for any significant increase in current (for electric extruder drives) or significant increase in melt pressure, respectively, during start-up. Stop the extruder immediately if a high current or high melt pressure is indicated. Check for degraded material blockage and clean melt flow paths as needed. Increase temperature zone settings as required, allow for thermal soak and restart the extruder.

6. Note the melt pressure during extrusion at low RPM for future reference.

7. Verify that melt is emerging from the die head(s) and allow the material to purge until new material emerges from the die head(s). Note the relative melt strength of the parison(s) and adjust the melt processing temperature as required.

8. For blow-molding machines with multiple die heads, cut each parison close to the die (at the same cut position) during extrusion, measure the rate (length of parison per unit time) and note the results for each die head. Go to the Manifold Balance step 4.2.3 (page 23) if there are significant differences between parison extrusion rates from each die head and ‘balance’ the rate of melt flow to each die head.

9. Observe a parison as it extrudes from a die head and note how the parison drops away from the die. If the parison curls during extrusion (greater melt flow along one edge of the die), then the die bushing must be adjusted and centered.

10. Adjust the die bolts so that the parison drops straight down with minimal curl and melt flow appears stable and uniform. Note that cutting the parison close to the die face and observing the initial parison extrudate allows for the best parison centering evaluation.

11. Repeat the same procedure for each die head as required.

Mold certification extrusion blow molding

The purpose of Mold certification step is to verify cavity and mold base dimensions and materials of mold construction. Cavity and neck insert dimensions, mold cooling channel diameter dimensions, cooling line connector diameter dimensions, mold backing plate mounting hole pattern, mounting bolt hole diameter dimension and mounting bolt hole thread type should be verified with the mold and blow molding machine platen prints. Dimensions for the mold mounting bolt holes and the bolt hole pattern in the mold backing plates are verified to help avoid machine scheduling delays during mold installation, initial start-up and process optimization procedures.

The Mold Certification procedure should be performed at the mold builder to expedite any required mold modifications and to help avoid delays during mold installation, process start-up and process optimization procedures. Mold certification documentation must be provided by the mold builder for review before the molds are shipped to the production blow molding plant.

Use of the specified materials in mold construction help to ensure the required level of mold mechanical integrity necessary to meet production run requirements. The specified materials of mold construction also provide the required level of heat transfer characteristics in the neck insert, cavity and pinch-off areas during production.

Mold cooling channel and cooling line connector dimensions should be verified so that heat transfer in the mold is optimized and so that cooling control may be maintained during production. Cavity and neck insert dimensions should be verified to establish the mold dimension ‘base line’ before process optimization procedures are initiated. The ‘base line’ dimension data may be used later in the process optimization procedures and for process centering purposes

SHOT SIZE, SHOT INVENTORY, RESIDENCE TIME

SHOT SIZE, SHOT INVENTORY, RESIDENCE TIME
Sizing your mold to the proper molding equipment may seem obvious but many people don’t do it. If you want to get the best out of your tryout, the clamp tonnage must be enough to keep the mold closed and the shot capacity of the machine sized properly to the amount of plastic it will inject. Each of these three items including a density calculation are outputs of your Setup sheet.

SHOT SIZE:
Shot Size is the volume (usually expressed in grams or ounces) of the amount of plastic injected into the mold. The general Rule of Thumb is to use 50% of the shot capacity + 20%. Machines were originally quoted in Ounces or Grams. However, you had to understand this was in Ounces of General Purpose Styrene. So you had to go back and calculate the density to see if your material could be used in the machine. Today many machines are sized in maximum theoretical injection volume (cubic inches or cubic centimeters). This makes the conversions easier because you simply multiply the density of your plastic (grams per cc) time the volume of the machine of the machine and you get the machine’s capacity (in this case) in grams. Using less than 20% of the machine’s capacity increases the time the material will remain in the barrel (residence time). While some materials are very heat resistant, others will quickly degrade and substantially loose both their physical and chemical properties. Using more than 70% of the machine’s shot capacity overcomes the problem of overheating the material. However, we now encounter the problem of not completely melting all the material we intend to inject.

SHOT INVENTORY
Shot Inventory is the number of shots residing in the barrel. It is used to calculate residence time. Here is how it works: The shot begins in the material hopper. As the screw turns, it is fed into the screw flights. As the molding cycle continues, the screw continues to turn pushing our shot either forward into the barrel, where it is finally pushed into a hot runner system or directly into the mold where it is cooled. Our calculation for shot inventory is how many shots are in the screw flights, barrel, and hot runner system.

RESIDENCE TIME
Residence Time is the amount of time (expressed in minutes or seconds) it takes a theoretical pellet of plastic from the time it enters the barrel to the time it enters the mold. It is probably the most important variable when considering a properly sized machine.

When the material drops from the hopper onto the screw flights, it begins to see heat. The screw then turns mashing the material up against the heated barrel walls, crushing the pellets and pushing it forward. This causes the screw to be pushed backward to where it finally stops. Some of this material remains in the screw flights, merrily cooking. The screw moves forward, injecting the shot. The screw turns again and our shot goes beyond the check ring into the barrel cavity. Again it cooks. As the molding cycle repeats itself, our sample shot is finally injected into the mold. Here it may be directly injected into a conventional sprue and runner system OR it may move into an extension nozzle or hot runner system. If it is residing in either of these heated systems, again it is cooking. Residence time is the time a particular pellet (for our calculations it will be a shot) takes to get from the raw material hopper to be cooled in the mold. Its calculation would be the number of shots residing in a heated system (the shot inventory) divided by the cycle time. Obviously, the two determining factors for this calculation will be the cycle time and shot volume.

SAMPLING:
Random sampling plans tell you about how consistent (precise) the process is. Sequential samples minimize the process component and give an excellent picture of the variation on the mold.

Injection molding tryout and qualification

PROCESS
This is the marriage of mold, material, and machine into an on-going process to continuously manufacture a part of consistent dimensions and physical characteristics.

TRYOUT
This is the initial test of a mold to check and repair any mechanical or dimensional defects. Once the mold is proven mechanically, the next tryout’s purpose is to optimize the molding cycle and obtain parts of sufficient quality and quantity for submission for testing and approval. A tryout is never a qualification run.

QUALIFICATION
A qualification run is a sustained run to show the mold and process can consistently produce parts of acceptable quality. This is generally understood to be a sustained run with little or no adjustments after the mold is brought on-cycle for a period of hours. Since this must be done for a sustained time, it can only be done after the parts produced from the tryout have been approved.

STARTUP
This is taking the conditions identical to those approved during the qualification run, translating them to a production machine and beginning a production run.

Injection molding tryout and qualification

PROCESS
This is the marriage of mold, material, and machine into an on-going process to continuously manufacture a part of consistent dimensions and physical characteristics.

TRYOUT
This is the initial test of a mold to check and repair any mechanical or dimensional defects. Once the mold is proven mechanically, the next tryout’s purpose is to optimize the molding cycle and obtain parts of sufficient quality and quantity for submission for testing and approval. A tryout is never a qualification run.

QUALIFICATION
A qualification run is a sustained run to show the mold and process can consistently produce parts of acceptable quality. This is generally understood to be a sustained run with little or no adjustments after the mold is brought on-cycle for a period of hours. Since this must be done for a sustained time, it can only be done after the parts produced from the tryout have been approved.

STARTUP
This is taking the conditions identical to those approved during the qualification run, translating them to a production machine and beginning a production run.

Injection molding tryout and qualification

PROCESS
This is the marriage of mold, material, and machine into an on-going process to continuously manufacture a part of consistent dimensions and physical characteristics.

TRYOUT
This is the initial test of a mold to check and repair any mechanical or dimensional defects. Once the mold is proven mechanically, the next tryout’s purpose is to optimize the molding cycle and obtain parts of sufficient quality and quantity for submission for testing and approval. A tryout is never a qualification run.

QUALIFICATION
A qualification run is a sustained run to show the mold and process can consistently produce parts of acceptable quality. This is generally understood to be a sustained run with little or no adjustments after the mold is brought on-cycle for a period of hours. Since this must be done for a sustained time, it can only be done after the parts produced from the tryout have been approved.

STARTUP
This is taking the conditions identical to those approved during the qualification run, translating them to a production machine and beginning a production run.

Polymer and plastic material

In injection molding there are two major classifications: Thermosets – compounds that go through a chemical reaction when exposed to heat and crosslink into the molded part. The second classification is Thermoplastics – compounds that are only melted, injected and cooled into their new shape of the molded product. Thermoplastics come in two (sometime three) sub classifications: Amorphous – those polymers with no discrete molecular geometry, Crystalline – those with fixed polymer geometry, and Semi-Crystalline – polymers possessing both crystalline and amorphous properties.

Mold polymer processing

A mold is literally the hole into which the plastic is injected to form the part. It must be vented enough to let the air out that is displaced by the plastic, stable enough to not distort under the pressures and heats of processing, have an adequate fill system of sprues and runners, and it must fill evenly.

Molding machine in polymer processing

The molding machine is nothing more than a tool. By analogy what a molding machine is to a molder would be what a table saw is to a cabinetmaker: The saw blade must be sharp with all it’s teeth in alignment and not broken or bent, the motor must have enough power so that the blade can cut, The table must be square and level, and all the adjustments for speed, tilt, and cutting angles must be accurate. While the table saw does not make the cabinet, the cabinetmaker relies on its consistent precision so that his materials are not wasted and his craftsmanship is efficient.

A molding machine MUST deliver a consistent volume of material, at a consistent rate, at a repeatable temperature, and be able to maintain enough pressure to mold the parts. It must also be able to recover a consistent shot of material and open and close with repeatable speed and positions. Without a machine that can deliver the variables in a controlled manner any attempt at molding that will produce useable parts is senseless.

How to determine if your molding machine is fit to produce high quality consistent parts is shown in the section titled “Machine Consistency.”

Select head tooling extrusion blow molding

The purpose of the Select Head Tooling step is to provide a guide for determining the correct mandrel and die bushing size for the blow molding machine head tooling assembly. Mandrel and die dimensions are estimated based on container dimensional data, container symmetry, blow-up ratio, targeted container weight, neck finish requirements and the type of material (degree of parison swell) that will be used to produce the container.

An initial blow-up ratio must be calculated using the container design dimensions and the required parison diameter.
The required parison diameter will depend on the relative size of the container, the container design (handle or no handle) and the container neck finish requirements. Initial blow-up ratios may be calculated using the following equation.

Blow up ratio = Bd / Nd

where: Bd = Bottle diameter, in
Nd = Minimum neck diameter, in

The blow-up ratio is compared with the maximum recommended blow-up ratio of the selected material.
Figure 2 shows a typical blow molded container with dimension and design nomenclature for reference.

Blow up ratios of 2 or 3 to 1 are considered normal when molding commodity resins such as polyethylene. A blow-up ratio as high as 4:1 is a practical upper limit. The blow up ratio for large containers with a small neck, is generally extended to 7:1 so that the parison fits within the neck and so that there is no mold parting line mark on the neck finish. Blow up ratios for a containers with a handles are generally in the 3 or 5 to 1 range as the die diameter must be larger to allow the handle to be blown.

In order to properly estimate and ‘size’ mandrel and die geometry for the blow mold(s), and to effectively control the process, a thorough understanding of parison swell and draw down phenomena is required. Parison swell is a combination of diameter swell and weight swell. It is a difficult blow molding property to estimate and to control. The parison diameter swell is a complex function of the weight swell, the rate of swell, and the melt strength.

Parison swell behavior varies significantly depending on material type, material processing conditions, machine processing parameters, basic die design (diverging vs. converging), container geometry (required parison diameter), container weight (shuttle process) and type of blow molding process. Some of the wheel type blow molding processes clamp (pinch off) and hold the parison at both ends during the blowing sequence in the process. The parison swell effects are normally more readily controlled on the wheel process compared with the shuttle process.

Parison swell data for a given material is often not available for mandrel and die calculations. The alternative is to proceed in a stepwise approximation towards the desired mandrel and die dimensions, and through trial and error, towards the targeted container weight with the aid of an interchangeable set of dies.

Internal die design dimensions including approach angles and land lengths vary significantly with blow molding machine capabilities and machinery manufacturers experience. Calculations for these dimensions are beyond the scope of this document and will not be discussed here.

However, as a rule of thumb, when blow molding commodity materials (PE, PP), a die land length of at least 8 times the annulus gap (die gap) is typical.

Bimodal polyethylene

A polyethylene with a relatively high molecular weight fraction and a relatively low molecular weight fraction is called bimodal polyethylene. The comonomer contents of the two fractions are often different.

Extrusion blow molding

Extrusion of a parison (hollow melt tube) which is forced to a mould caving by internal pressure and cooled down to form a hollow article. Used for the production of bottles, wide-mouth containers, petrol tanks etc.
Blow molding press or clamp unit - That portion of a blow molding machine that contains and supports the platens and molds in any manner required to produce the blown part.

Barrier layer and properties

A layer of material designed to limit migration or infiltration of undesirable elements through the plastic or limit the loss of desirable elements through the plastic.
Barrier Properties - Material; permeability characteristics which limit the migration or permeability of elements.

Barrel (Extruder)

Hollow tube in which plastic is gradually heated and melted. Inside the barrel is a helical screw which compresses and moves the plastic from the feed throat (hopper end) to the manifold and die head(s). Outside the barrel are band heaters.
Barrel Lining - Centrifugal cast boron alloy or nitrided integral surface of the barrel bore.
Barrel Liner - A removable boron alloy or nitrided sleeve pressed or shrunk into the barrel bore.
Barrel Vent - An opening through the barrel wall , intermediate in the extrusion process, to permit the removal of air and/or volatile matter from the material being processed.
Barrel Flange - For attaching a melt pipe, die head, accumulator head or nozzle.

Gate freeze test injection molding

The purpose of the gate freeze test is to identify the hold conditions necessary to freeze the gate. The extent and duration of hold pressure has a large effect on the dimensional stability and outer appearance of the molded part. If the hold time is too short, the gate will not have had enough time to freeze off and sink marks could appear on the part. This is especially true of larger parts and when higher hold pressures are employed. After the mold gates "freeze", hold pressure has no effect and should be terminated at that point. It is typically better to be a little high on the hold pressure timer setting. This will cause a slight wear increase on the hydraulics of the injection press but the molder will be able to ensure higher dimensional accuracy.

Hold pressure is set at a pressure which allows no plastic melt to enter or leave the gate as the part solidifies. A high hold pressure setting could pack the part excessively, beyond dimensional tolerances. A low hold pressure setting will allow the melt to exit through the gate causing sink marks and voids in the part.

The gate freeze test is designed to achieve minimal dimensional variation by ensuring no plastic leaves the cavity before the gate is frozen. Hold times in the sharply rising part of the weight curve introduce additional variation. The gate freeze can be accurately determined via flow analysis programs which utilize the cooling circuitry layout, mold geometry, tool steel and hot/cold runner configuration. A predominant amount of the flow analysis programs assume isothermal conditions in the mold. This would produce best case results and typically the gate will never freeze off in this short of time. Properly interpreted, the flow analysis results serve as a good starting point.

Notes: 1) When using a valve gated system, no hold time is required. Once the valve is closed, no material will enter or leave the cavity. The valve is held open long enough to properly pack the part, and then closed. At this point, no pressure needs to be applied. 2) Having poor shot size control on your molding machine and an imbalanced mold will lead to less than desired results (lack of precision).

All the steps during the procedure that involve intimate contact with the injection molding machine are to be done by a qualified injection molding machine operator.

Procedure:

1. Set melt temperature to resin manufacturer's recommended mid-range.
2. Set mold temperature to resin manufacturer's recommended mid-range.
3. Set cooling time long enough so that parts eject without being distorted.
4. Set fill rate from results of mold viscosity test and if desired, profile the injection stroke to have velocity controlled pack. At this moment, record the dosing stroke and change over position. For the remainder of the validation process this will remain constant.
5. Set hold time based on the machine operators experience, take their estimate and multiply by 1.5. Use this as the starting point for hold time. If after running the test you do not identify a point at which the gate freezes off, increase the hold time incrementally. Caution: On some molds, high hold pressure and hold times can cause ejection issues. Pay heed to the advice of the mold builder and do not increase the hold pressure and time to the point the parts are difficult to eject off the cores. If you see this issue on your pilot mold, you may want to modify the mold or part design to make your part ejection more robust.
6. Set hold pressure so that there are no visual sink marks.
7. Collect and weigh 3 consecutive shots to 0.01 grams or better.
8. Subtract one second from hold time.
9. Add one second of time to cooling in order to maintain a consistent molding cycle.
10. Graph the shot weight versus the hold time.
11. Repeat steps 7 - 9 until the part weight begins to decrease.
12. Repeat the test with a high mold temperature and high melt temperature to document the worst case scenario for hold time.

A region on the graph where small changes in hold time result in large changes in part weight. These large changes in part weight may result in part quality variation with regards to dimensions or mechanical performance. The region indicating that the part weight is more stable and that the gate has frozen off, i.e., no polymer melt enters or leaves the cavity.

When the gate never froze. This is typical for hot tips directly gated onto the part. The thicker the part, the more likely this will occur. When such a curve is graphed, identify the region on the curve where a change in slope is evident.


It is possible the part will become over-packed and either flash or stick in the core, causing ejection problems, before obtaining a level curve.

Band heaters in polymer processing

Electrical heaters surrounding the extruder barrel, die head(s) and manifold.

Ball check valve for injection molding

An anti-back flow valve which prevents the plastic melt from flowing back over the screw during injection.

Baffle - polymer processing

Baffle - A plug or other device inserted in a flow channel to divert cooling medium to a desired path.

Back pressure injection molding

Back Pressure - Pressure exerted by the injection unit's cylinder against the screw as it recovers or plasticates. Used to increase the melt homogeneity. Increases melt temperature.

Back Draft

Back Draft - A slight undercut or tapered area in a mold tending to prevent removal of the molded part.

Antioxidants prevent degradation processes

Additive that minimizes or prevents the effects of oxygen attack on the plastic (e.g. yellowing or degradation). Such additives may render the plastic brittle or to lose mechanical properties. A group of substances being able to inhibit radical reactions in the polymer and thus prevent degradation processes. Different types are available: sterically hindered phenols and phosphites as base for polyolefins, sulphur based heat stabilisers and C-radical scavengers for special applications.

Annealing

Annealing - A process of holding a material at a temperature near, but below, its melting point. The objective being to permit stress relaxation without distortion of shape.

Polymer amorphous phase

Amorphous Phase - Devoid of crystallinity; no definite order. The plastic is normally processed at the amorphous phase temperature.

Plastics additives

Additives - Used to enhance processing, performance, appearance, and/or economics of the basic plastic formulation. Additives are added to the polymer to protect it from degradation and to give the material desired properties. Normally the additives are mixed with the powder before the extruder.

Acrylic - Polymethylmethacralate

Acrylic - Polymethylmethacralate

Adjustable Die Head

Adjustable Die Head - A die head whose orifice opening is adjustable.

Accumulator Head

Accumulator Head - A melt accumulator(s) vertically connected to the discharge end of an extruder and programmed for weight and thickness distribution of the formed parison by means of controlling the movement up and down of the inner die mandrel or outside die ring.

Accumulator Adapter

Accumulator Adapter - The structure containing the flow passage connecting an accumulator or reciprocating screw extruder to a die or die head manifold.

ABS copolymer

ABS (Acrylonitrile Butadiene Styrene) - Blends of copolymers or styrene-acrylonitrile copolymer with butadiene-acrylonitrile rubber.

Crystalline Streaks PET processing

Crystalline resin extends upward from the gate for about 20~30mm in streaks or bands. Typically seen if the machine has been stopped for just a few minutes. May be accompanied by other swirls or clouds of crystallized material in the body of the preform.
It is normal to see this fault on the first one or two shots after a start-up from cold because decomposed material will have come from the hot runner. Do not recycle preforms since they will have a lowered I.V

Injection velocity for PET processing

Keeping the injection velocity low will reduce the shear that occurs in the material. Shear is a major factor affecting overheating of the material and I.V. reduction, therefore reducing velocity will protect the PET resin from excessive damage.
When working with machines (hot preform method), the injection velocity will also make a significant difference to the material distribution in the finished container. Filling slower means that the preform will be hotter when the mold opens and its temperature balance will also have changed. Typically, the shoulder area will become relatively hotter than the base area giving more stretch at the top of the preform.
Excessive injection velocity can also disturb the alignment of the injection core, especially if the design is long and thin.
Reducing the injection velocity will also have the effect of making the holding time shorter since the V/P time will increase.

For machines fitted with electronic injection control, reduce the velocity percentage value on the injection screen of the electronic injection control system. Minimum usable value is typically around 15~17% but beware of making a short shot at very low settings.
For machines without electronic injection control, reduce the setting of the valve found on the operator side of the injection unit. Beware of making a short shot at very low settings.

In most cases, the five steps of injection control can be set to the same value. Different values may be advantageous in cases of complicated preform design or technically difficult bottles.
Reducing the injection velocity too far may cause other preform defects such as specks of crystal in the gate area.

Raising the injection velocity will reduce the time taken to fill the cavity and it is therefore possible to achieve faster cooling of the preform. However, it will also increase shear in the material. Shear is a major factor affecting overheating of the material, A.A. Generation and I.V. reduction, therefore increasing velocity will damage the PET resin.
When working with machines (hot preform method), the injection velocity will also make a significant difference to the material distribution in the finished container. Filling faster means that the preform will be colder when the mold opens and its temperature balance will also have changed. Typically, the shoulder area will become relatively cooler than the base area giving less stretch at the top of the preform.

For machines fitted with electronic injection control, increase the velocity percentage value on the injection screen of the electronic injection control system. Maximum allowable setting is 99%.
For machines without electronic injection control, increase the setting of the valve found on the operator side of the injection unit.

In most cases, the five steps of injection control can be set to the same value. Different values may be advantageous in cases of complicated preform design or technically difficult bottles.
Increasing the injection velocity too far may cause other preform defects such as lowered I.V., increased A.A., silver streaks and internal sink marks.

Resin drying

A fault with the dryer will lead to hydrolysis of the material in the barrel of the machine. This will cause lowered I.V. of the material. Lowered I.V. is the major cause of bad quality in preforms and bottles. More than 60% of all PET processing related faults can be traced back to the dryer.
Maintaining the dryer in optimum condition will allow the molding machine to perform at maximum efficiency and quality.

Airflow - The most important parameter, there should be nothing causing a restriction in the process and regeneration air flow.
Airflow - Most common problem is a blocked process filter, but also check blowers are operating correctly and the delivery hoses have not been squashed.

Temperature - Process Temperature should be in the region of 145~170ºC Depending on the resin supplier and the drying time. Also check the Regeneration Temperature which should be around 200~230ºC depending on the maker.
Temperature - Confirm process air temperature is correct on the display panel. The regeneration air temperature is not so easy to check since most regeneration controllers are hidden away in the electrical panel.

Time - Calculate (or measure) the time the material is in the hopper, this should be at least 3½-4 hours.
Time - If you know the capacity of the hopper in liters, you can calculate the PET quantity using a figure of 0.84kg/L as the bulk density. Compare this figure (kilograms of PET) against the current consumption of PET by the machine in kg/hr. If you are not sure, the safest method is to switch off the hopper loader and check the time necessary for the material to be consumed.

Dewpoint - Correct dewpoint may vary according to the manufacturer of the dryer, consult the maker's manual before assuming an error exists.
Dewpoint - Use a commercially available dewpoint meter to sample the air coming from the desiccant chamber. Typical vales may be anything between -20ºC and -50ºC. Check the manual for correct specification. In machines with more than one desiccant chamber, be sure to check all of them.

Check the correct function of all heaters on a regular basis using a clip-on ammeter. If one or more has broken, the others will be taking extra load.
Always check the calibration of instrumentation and thermocouples before assuming a problem exists

The dewpooint temperature

The dewpoint (or more correctly, the dewpooint temperature) is the temperature at which air can no longer support the existing quantity of water in vapour form.
Air always has a certain amount of moisture in the form of gas which is therefore normally invisible to the human eye. As the temperature of the air is lowered, the ability of the air to support this moisture is also lowered. At some temperature, a point will be reached where the excess moisture will change to liquid and will either become visible as a mist, or will be deposited on nearby surfaces as condensation. The temperature where this phenomena occurs is known as the dewpoint temperature.
Since the dewpoint temperature is not a real measure of the current air temperature, it is normal that the actual air temperature and the dewpoint temperature will be at different values. For example, in a PET resin dryer, the actual air temperature is typically 160ºC, while the dewpoint temperature is around -20ºC.
The dewpoint measurement is taken on the process air line just after the desiccant chamber(s). Many dryer makers fit an air sampling point with an outlet coming to the front panel of the dryer. When taking an air sample, allow adequate time for a stable result to be obtained and make sure to check all desiccant chambers. Each dewpoint meter may have different operating methods so please refer to the instruction manual. Although dewpoint is important for correct drying, the process airflow is by far the most critical. A dryer with good airflow and poor dewpoint is still partially effective, whereas a dryer with excellent dewpoint but poor airflow is useless.

Calculating the consumption of PET

A simple calculation that tells you the consumption of PET by an injection molding machine.

The calculation is:

kg / hr = W x C x S
W = Weight of the container in kg. (e.g. 45 grams = 0.045 kg)C = Number of injection cavities S = Number of injection cycles per hour (Shots)(3,600 / Cycle Time (secs) for one set of preforms)

Example:

machine with a cycle time of 14 seconds making a 35 gram bottle
W = 0.035C = 8S = (3,600 / 14) = 257.1
kg / hr = .035 x 8 x 257.1 = 72 kg / hr

Antistatic agents

An additive to dissipate static charges eliminating charge build-up and dust collection; an example of a widely used antistatic agent is glycerine-monostearate (GMS). Different types of antistatic agents are used in polymerisation reactors to prevent the formed polymer powder from adhering to the reactor wall.

Acetaldehyde A.A.

Acetaldehyde is a colorless gas at room temperature, it has a strong fruity smell. It occurs naturally in many fruits and other foodstuffs and is used in the food industry as a flavor enhancer for certain products. It is also generated during the production and injection processing of PET (Polyethylene Terephthalate) material. Generally, most acetaldehyde problems originate in the barrel during the injection process. All PET resins have some residual acetaldehyde after being manufactured, the quantity will vary according to the grade. Acetaldehyde is only generated while the PET resin is in its melt condition, therefore it can only be controlled by adjustments in the barrel (90%) and hot runner (10%) of the machine. The generation of acetaldehyde is not linked to moisture content of the material, although in the process of being dried, acetaldehyde can also be driven off. Therefore, correct drying is also important for acetaldehyde control.