9 Common Defects and Detailed Solutions in PVC Compound Production for Pipe Fittings

1. Defect: Brittleness / Cracking

Causes:

• Incomplete Fusion/Gelation.
• Insufficient amount of Impact Modifier or Plasticizer.
• Processing temperature is too low (insufficient energy for fusion).
• Using PVC resin with a very low K-value.

Solutions:

• Enhance the use of Processing Aid (ACR) and Impact Modifier (CPE/ABS).
• Increase the barrel temperature profile and/or holding pressure in injection molding.
• Control the high-speed mixing temperature to reach the optimal Fusion Temperature range of 110°C – 125°C to ensure complete homogenization.
• Use K-67 to K-71 resin for pressure fittings.

2. Defect: Poor Surface Finish (Rough, Pitting, Matte)

Causes:

• Poor dispersion of raw materials (powder/additives) during mixing.
• Residual moisture in the compound or filler.
• Imbalance between Internal Lube and External Lube.
• Overheating during extrusion/molding causing additive volatilization or slight degradation.

Solutions:

• Increase the time and speed of high-speed mixing to achieve uniform dispersion.
• Dry the compound/raw materials before processing.
• Balance the Lube system: Slightly increase External Lube (PE Wax, Paraffin) for gloss, but avoid excess to prevent plate-out.
• Lower the temperature of the metering zone and die head if signs of burning/volatilization are observed.

3. Defect: Poor Fusion / Delamination

Causes:

• Low degree of fusion (incomplete gelation).
• Extrusion/injection speed is too fast, giving insufficient heating time.
• Screw design does not generate enough shear stress.

Solutions:

• Increase ACR in the formulation to promote the fusion process.
• Adjust the screw speed (slightly decrease) to increase the residence time and degree of fusion.
• Slightly increase the processing temperature or increase the Back Pressure in injection molding to boost shear friction.
• Check the heat stabilizer system and supplement if necessary to maintain material stability throughout the fusion process.

4. Defect: Low Impact Strength, Low Elongation

Causes:

• Insufficient amount of Impact Modifier such as CPE, ACR.
• Poor homogeneity of the compound.
• Unstable quality of the input PVC resin.

Solutions:

• Add an appropriate amount of CPE or ACR to the formula to increase toughness.
• Control the mixing process to ensure uniform dispersion of the Modifier in the compound.
• Use resin from reliable suppliers with the appropriate K-value (typically K-67).

5. Defect: Yellowing / Discoloration

Causes:

• Thermal Degradation due to excessively high temperatures in certain zones.
• Insufficient heat stabilizer to protect PVC during processing.
• High friction due to lack of Internal Lube or excessive screw speed.
• Excessive material residence time in the machine.

Solutions:

• Increase the amount of heat stabilizer (Ca-Zn) or combine with Tin Stabilizer (if necessary).
• Reduce localized temperature (especially in the die head and nozzle area).
• Increase Internal Lube to reduce frictional heat generation.
• Shorten the material residence time by optimizing the cycle or speed.

6. Defect: Bubbles / Voids

Causes:

• High moisture content in the compound or filler.
• Decomposition due to heat, generating gas.
• Excessive plasticization, causing some additives to volatilize easily.

Solutions:

• Mandatory drying of raw materials before extrusion/injection molding.
• Control the heat profile and reduce the temperature if degradation is detected.
• Check and replace easily volatile additives with those that have a higher volatilization temperature.

7. Defect: Failure to Meet Standard Pressure (PN10, PN16, …)

Causes:

• Use of unsuitable K-value resin for pressure fittings.
• Low tensile and impact strength due to insufficient Modifier.
• Inhomogeneous product due to poor fusion.

Solutions:

• Select K-67/68 resin for pressure fittings.
• Optimize the amount of ACR & CPE to improve mechanical strength.
• Check the Fusion Level and optimize screw speed and temperature to achieve high homogeneity.

8. Defect: Color Inconsistency

Causes:

• Masterbatch (Color Concentrate) poorly dispersed.
• Unstable temperature during extrusion/injection molding.
• Masterbatch quality is incompatible with the base compound.

Solutions:

• Use high-quality and compatible PVC-based Masterbatch.
• Increase mixing time and speed to ensure color dispersion.
• Maintain stable temperature during processing to prevent color changes due to heat.

9. Defect: Sink Marks / Excessive Shrinkage

Causes:

• Mold temperature is too high or cooling time is too short.
• Holding Pressure is insufficient to compensate for material shrinkage.
• Product wall thickness is too thick or non-uniform.

Solutions:

• Reduce the mold temperature for faster and more effective cooling.
• Increase the Holding Pressure and Holding Time (Packing Time) to compact the material and compensate for shrinkage.
• Increase the Cooling Time.
• Optimize the gate location and size for easy material filling and compression.