Adjustment techniques for the bending and skewing of the preforms in the extrusion blow molding machine

Tips for Adjusting Parison Deflection in Extrusion Blow Molding Machines

In extrusion blow molding, a well - formed parison is crucial for producing high - quality hollow plastic products. However, parison deflection, where the parison bends or  deviates from a straight path during the extrusion process, is a common issue that can lead to product defects. Here are some effective adjustment techniques to tackle this problem.

Analyzing the Root Causes of Parison Deflection

Uneven Material Flow

One of the primary reasons for parison deflection is uneven material flow within the extruder. When the plastic melt does not flow uniformly through the die head, it can cause one side of the parison to be thicker or thinner than the other. This imbalance in the parison's cross - section can lead to it bending as it exits the die.

To identify this issue, closely observe the parison as it is being extruded. Look for any visible differences in thickness along its circumference. Additionally, check the temperature and pressure settings in the extruder. Inconsistent temperatures in different heating zones can affect the melt flow rate, while incorrect pressure can disrupt the even distribution of the material.

Die Head Misalignment

A misaligned die head can also result in parison deflection. If the die head is not properly centered or is tilted, the parison will not emerge in a straight line. This misalignment can be caused by improper installation, vibrations during operation, or wear and tear of the die head components.

Inspect the die head for any signs of misalignment. Check if it is level and centered with respect to the extruder and the overall machine frame. Use precision measuring tools to ensure accurate alignment. Also, look for any loose bolts or damaged parts that may be contributing to the misalignment.

External Forces and Interference

External factors such as air currents, vibrations from nearby equipment, or obstacles in the parison's path can cause it to deflect. Air currents, for example, can exert a force on the soft, molten parison, pushing it to one side. Vibrations can disrupt the smooth extrusion process, leading to irregularities in the parison's shape and direction.

Evaluate the workshop environment around the extrusion blow molding machine. Check for any drafts or air movement near the parison exit point. Ensure that the machine is placed on a stable foundation to minimize vibrations. Remove any obstacles or equipment that may be interfering with the parison's path.

Adjustment Techniques Based on the Causes

Optimizing Material Flow

To correct uneven material flow, start by adjusting the temperature settings in the extruder. Gradually increase or decrease the temperature in different heating zones to achieve a more uniform melt flow. For example, if one side of the parison is thinner, slightly increase the temperature in the corresponding heating zone to increase the melt flow rate in that area.

Also, check and adjust the pressure settings in the extruder. Ensure that the pressure is consistent throughout the extrusion process. If necessary, clean the extruder screw and barrel to remove any build - up or blockages that may be affecting the material flow.

Realigning the Die Head

If die head misalignment is the cause of parison deflection, realign the die head carefully. Loosen the bolts that hold the die head in place and use a leveling tool to ensure it is horizontal. Then, use a centering device or measuring tape to center the die head with respect to the extruder. Tighten the bolts securely once the die head is properly aligned.

Regularly check the die head alignment during machine operation, especially after maintenance or when changing production runs. Small vibrations or movements can cause the die head to shift over time, so periodic realignment is necessary to maintain consistent parison quality.

Minimizing External Interference

To reduce the impact of external forces on the parison, take steps to control the workshop environment. Install air curtains or barriers around the parison exit point to prevent air currents from affecting it. If vibrations from nearby equipment are a problem, isolate the extrusion blow molding machine by placing it on vibration - dampening pads or mounts.

Clear the area around the parison's path of any obstacles. Ensure that there is enough space for the parison to hang freely without coming into contact with any objects. This will help maintain a straight and stable parison during the extrusion process.

Continuous Monitoring and Fine - Tuning

Real - Time Parison Observation

Once the initial adjustments have been made, continuously monitor the parison during the extrusion process. Use visual inspection or, if available, a parison monitoring system to track its shape and direction. Look for any signs of deflection or irregularities and make note of them.

Real - time observation allows for quick identification of any new issues that may arise during production. It also helps in determining if the adjustments made have been effective in reducing parison deflection.

Fine - Tuning the Adjustments

Based on the real - time observations, fine - tune the adjustments as needed. If the parison still shows slight deflection, make small incremental changes to the temperature, pressure, or die head alignment settings. Keep detailed records of the adjustments made and their effects on the parison quality.

Fine - tuning is an iterative process that requires patience and attention to detail. By making small, controlled changes and observing the results, you can gradually achieve a straight and well - formed parison, leading to high - quality extrusion blow - molded products.