Showing posts with label polymer engineering. Show all posts
Showing posts with label polymer engineering. Show all posts
Polycarbonate Resins processing
Polycarbonate Resins - Polymers derived from the direct reaction between aromatic and aliphatic dihydroxy compounds with phosgene or by the ester exchange reaction with appropriate phosgene- derived precursors.
Parison ejection rate
Parison ejection rate - The rate of plastic flow through the die body in cubic inches per second, or pounds per second.
Plastic mold
Mold - The cavity or matrix of cavities into which plastic melt is placed and takes form.
Melt Flow Index/Rate
Melt Flow Index - The amount, in grams, of a thermoplastic resin which can be forced through a 0.0825 inch orifice when subject to 2,160 grams force for ten minutes at 230°C, per ATSM D1238
Melt decompression
Melt decompression - Reducing pressure of melt during molding cycle, usually by reverse movement of screw.
Lead In
Lead In - Initial clearance through an initial angle or chamfer that helps align two close mating parts.
Knockout injection molding
Knockout - Any part or mechanism of a mold used to eject the molded article.
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)
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)
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.
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.
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.
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.
Subscribe to:
Posts (Atom)