Why do blow mold products have uneven wall thickness？

We all know that during the process of plastic products, it's inevitable that some of the items produced may not meet the required standards. One of the common issue is uneven wall thickness in the final product. Here we'll explain some reason that causes of it in blow-molded products and offer some solutions to address this problem.

1. Mold design problem

Reason:

• Improper mold design, such as asymmetric mold cavity distribution or uneven flow channel design.
• Uneven mold gap leads to inconsistent melt outflow.

Solution:

• Optimize mold design to ensure symmetric cavity and uniform flow channel distribution.
• Check whether the mold gap is uniform, and adjust or reprocess the mold if necessary.

2. Raw material problem

Reason:

• Inconsistent raw material properties, such as large fluctuations in melt flow rate (MFR).
• Uneven distribution of additives leads to uneven material melting.

Solution:

• Select raw materials with stable quality to ensure batch consistency.
• Improve raw material mixing uniformity, use a mixer or improve the formulation process if necessary.

3. Processing parameter problem

Reason:

• Unstable extrusion pressure or temperature leads to uneven melt flow.
• Inconsistent stretching or blowing speed leads to changes in wall thickness.

Solution:

• Stabilize processing temperature and pressure to ensure parameter consistency in each area.
• Adjust blowing pressure and stretching speed to find the best process parameters.

4. Equipment problems

Reason:

• Extruder screw or die head wear, resulting in uneven melt flow.
• Blow molding machine fixture or stretch rod unstable movement.

Solution:

• Regularly check and maintain equipment, and replace worn parts when necessary.
• Calibrate fixtures or stretch rods to ensure consistency of movement.

5. Environmental factors

Reason:

• The workshop temperature is too high or too low, affecting the material cooling speed.
• Uneven air circulation affects the molding conditions of the product during blow molding.

Solution:

• Control the workshop ambient temperature and humidity and keep them constant.
• Adjust the cooling system to ensure uniform cooling.

6. Problems with the blow molding process itself

Reason:

• Irrational design of the blow ratio leads to local stress concentration.
• The wall thickness of the initial parison (embryo) is inherently uneven.

Solution:

• Re-optimize the blow ratio to make it more reasonable.
• Ensure the uniformity of the parison and use a wall thickness control system (such as MOOG controller) when necessary.