Method for adjusting the length of the preform in the extrusion blow molding machine

How to Adjust Parison Length in Extrusion Blow Molding Machines

Understanding Parison Length Fundamentals

Role of Parison Length in Blow Molding

The parison, or hollow tube of molten polymer, determines the final shape and wall thickness of the blow-molded part. Its length directly impacts material distribution, cycle time, and part quality. An improperly set parison length can lead to issues like uneven walls, flash, or incomplete cavity filling. Adjusting this parameter requires balancing extrusion speed, die gap, and blow timing to match the mold’s dimensions.

Factors Influencing Parison Length

Several variables affect parison length, including screw speed, melt temperature, and die head design. Higher screw speeds increase material output, potentially lengthening the parison if other settings remain unchanged. Conversely, cooler melt temperatures reduce flowability, shortening the parison unless compensated by slower extrusion or wider die gaps. Understanding these interactions is critical for precise adjustments.

Manual Adjustment Techniques

1. Modifying Screw Rotation Speed

The screw’s RPM controls the rate of melt delivery to the die head. To lengthen the parison, gradually increase the screw speed while monitoring the parison’s appearance. If the tube becomes too thin or wavy, reduce speed to stabilize flow. Use a tachometer or machine display to track changes and avoid sudden shifts that could disrupt consistency.

2. Altering Die Gap Settings

The gap between the die lips dictates the parison’s initial diameter and length. Widening the gap increases material flow, producing a longer parison for larger molds. Narrowing the gap reduces output, shortening the parison for smaller parts. Adjust the gap in small increments (0.05–0.2 mm) and observe the parison’s behavior during extrusion to prevent sagging or uneven walls.

3. Adjusting Extrusion Head Height

On vertical machines, the extrusion head’s vertical position influences parison length. Lowering the head reduces the distance the parison travels before reaching the mold, effectively shortening it. Raising the head increases this distance, lengthening the parison. Ensure the head is aligned with the mold’s center to avoid tilting, which can cause uneven stretching during blowing.

Programmable Control Methods

1. Using Parison Length Controllers

Modern machines often feature programmable controllers that automate parison length adjustments. Input the desired length based on mold dimensions, and the system adjusts screw speed, die gap, or extrusion time accordingly. Calibrate the controller using sample parts to ensure accuracy, and refine settings iteratively to account for material variations or environmental factors like ambient temperature.

2. Synchronizing with Blow Timing

The parison must reach the mold at the optimal moment for successful blowing. Program the machine to delay or accelerate extrusion based on the mold’s closing time. For example, if the parison is too short, delay the blow signal to allow more material to extrude. Conversely, advance the signal if the parison is too long and risks sagging before the mold closes.

3. Implementing Multi-Stage Extrusion

Some applications require varying parison lengths along its length. Use multi-stage extrusion to adjust die gap or screw speed dynamically during production. For instance, widen the gap mid-extrusion to create a thicker section for handles, then narrow it for the body. This technique demands precise timing and coordination between the controller and die head actuators.

Troubleshooting Parison Length Issues

1. Parison Sags or Droops

Excessive length or low melt viscosity can cause the parison to sag before reaching the mold. Increase the die gap slightly to reduce flow rate, or lower the melt temperature to stiffen the material. If using a vertical machine, raise the extrusion head to shorten the hanging distance. Check for uneven cooling around the die head, which may exacerbate sagging.

2. Inconsistent Parison Length

Fluctuations in screw speed or melt temperature often lead to uneven parison lengths. Verify that the machine’s temperature controllers are calibrated and that the screw drive operates smoothly. Clean the die lips to remove residue that could disrupt flow. For programmable systems, recheck the input parameters and ensure no conflicting signals are sent during operation.

3. Short Parison Leading to Incomplete Parts

If the parison is too short to fill the mold cavity, increase screw speed or widen the die gap to boost output. Alternatively, lower the extrusion head to extend the parison’s reach. For horizontal machines, adjust the mold’s closing speed to give the parison more time to stretch. Monitor wall thickness to ensure uniformity after making these changes.

By mastering these adjustment techniques, operators can optimize parison length for diverse mold sizes and material types, enhancing productivity and part quality in extrusion blow molding processes.