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.