Top 10 Common Injection Molding Defects Caused by Worn Screws, Valves, or Barrels

Injection molding is a highly efficient process for producing precision parts, but equipment wear can introduce defects that compromise quality and efficiency. While many troubleshooting guides focus on processing parameters, issues often originate from worn components like the screw, check valve, or barrel. These parts are critical in material flow, pressure consistency, and melt homogeneity. When they degrade, they can lead to persistent molding defects that cannot be corrected through machine settings alone.

Here are 10 common injection molding defects that arise due to worn screws, valves, or barrels – along with possible solutions to address them:

1.) Short Shots

Description: The mold does not fill completely, leaving sections unformed.

Causes:

• Worn check valve causing inconsistent shot size.
• Screw wear leading to poor material plasticization.
• Barrel wear causing inconsistent feeding and or slippage over the check ring OD.

Possible Solutions:

• Inspect and replace the check valve if it is leaking or not sealing properly.
• Evaluate screw condition; if excessive wear is present, refurbish or replace.
• Assess barrel wear and replace if the ID is out of spec.

2.) Warping

Description: The part bends or distorts after cooling.

Causes:

• Uneven material temperature due to a worn screw.
• Poor melt homogeneity from screw and barrel wear.
• Inconsistent injection pressure caused by leaking check valve.

Possible Solutions:

• Ensure the screw is not worn to the point of poor melting efficiency.
• Replace barrels with excessive wear that cause inconsistent material temperatures.
• Inspect and replace the check valve if it is allowing backflow.

3.) Sink Marks

Description: Depressions form on the part surface due to uneven cooling and material shrinkage.

Causes:

• Worn screw leading to inconsistent material density.
• Poor melt mixing from excessive screw flight wear.
• Degraded packing pressure from a leaking check valve.

Possible Solutions:

• Replace worn screws to restore proper plasticization.
• Inspect the check valve and replace if necessary.
• Ensure the barrel is not excessively worn, as this affects melt uniformity.

4.) Flash

Description: Excess material forms at the part edges or seams.

Causes:

• Excess injection pressure compensating for poor material flow due to worn screw and barrel.
• Check valve failure causing overpacking.

Possible Solutions:

• Replace worn screws and barrels to maintain proper melt flow and pressure.
• Inspect and repair or replace the check valve if it is not sealing properly.

5.) Flow Lines/Splay Defects

Description: Streaks or lines appear on the part surface, indicating inconsistent flow.

Causes:

• Inadequate material mixing from screw wear.
• Temperature variations from a worn barrel or screw.
• Check valve not sealing properly, causing erratic flow.

Possible Solutions:

• Inspect and replace worn screws to improve mixing and melting.
• Replace the barrel or screw if it is worn enough to cause inconsistent heating.
• Ensure the check valve is functioning properly to prevent variations in shot consistency.

6.) Bubbles or Voids

Description: Air pockets form within the molded part.

Causes:

• Worn screw failing to properly vent gases from the melt.
• Inconsistent back pressure due to check valve wear.
• Excessive material degradation in a worn barrel.

Possible Solutions:

• Replace worn screws to improve venting efficiency.
• Inspect and replace the check valve to ensure proper melt compression.
• Replace barrels with excessive wear to prevent degradation and gas entrapment.

7.) Burn Marks

Description: Dark or blackened areas appear on the part due to overheating.

Causes:

• Increased shear heat from excessive screw wear.
• Material degradation from prolonged residence time in a worn barrel.
• Check valve malfunction causing inconsistent flow and overheating.

Possible Solutions:

• Replace worn screws that generate excessive friction and shear heat.
• Inspect the barrel for wear and replace if material degradation is evident.
• Ensure the check valve is functioning properly to maintain consistent flow.

8.) Weld Lines

Description: Visible lines form where two melt fronts meet.

Causes:

• Poor melt quality due to worn screw flights.
• Inconsistent temperature distribution from a worn barrel.
• Check valve allowing uncontrolled material movement.

Possible Solutions:

• Replace worn screws to restore optimal melt flow and quality.
• Inspect the barrel for wear and replace if it affects temperature consistency.
• Ensure the check valve is not allowing excessive material leakage.

9.) Delamination

Description: Layers of material separate, leading to a flaky or peeling appearance.

Causes:

• Poor material bonding due to inconsistent melting from a worn screw.
• Contamination due to degraded barrel surface.
• Check valve allowing inconsistent backflow, leading to improper melting.

Possible Solutions:

• Replace the screw to improve material melting and bonding.
• Inspect and replace barrels with excessive wear that may introduce contamination.
• Ensure the check valve is sealing properly to prevent material inconsistency.

10.) Burned Sprue or Gate

Description: The material around the sprue or gate appears charred.

Causes:

• Excessive residence time due to poor screw feeding.
• Barrel wear causing hot spots and localized overheating.
• Check valve failure leading to irregular flow.

Possible Solutions:

• Replace the screw if material feeding is inconsistent.
• Inspect and replace the barrel if worn areas are causing overheating.
• Ensure the check valve is properly sealing to maintain consistent injection cycles.

Many injection molding defects stem from mechanical wear rather than processing parameters. Worn screws, barrels, and check valves can introduce inconsistencies that lead to short shots, warping, sink marks, and other defects. Regular inspection and maintenance of these components are essential to maintaining efficiency, reducing scrap, and ensuring high-quality molded parts. By focusing on mechanical wear as a root cause, manufacturers can avoid costly adjustments to processing parameters that may not address the real problem.

How can we help you?

The doors have been open at Santa Fe Machine Works, Inc. since 1923. For the past 45 years, we have been dedicated solely to manufacturing and supplying new & rebuilt injection & extrusion screws, barrels & valves to the plastics industry. We also offer custom-tailored screw, barrel, and valve combinations, allowing our customer’s production goals to be met in the best, fastest, and most cost-effective ways possible. Our employees have over 200 years of combined experience in the plastics industry and are eager to support your specific extrusion or injection needs. Have a question, need assistance, or looking for a quote? Contact us today!