Solutions For Injection Molding

Solving Common Injection Molding Issues

Injection molding process problems encountered can arise from variety of circumstances. Having a plastics contract manufacturer with the ability to provide complete solutions for your manufacturing needs provides the quality and results you are looking for. The following examples give some quick insight into common problems and tips to assist in finding the best practical solutions for injection molding process issues.

7 Common Molding Process Defects and Solutions

Solutions for injection molding process problems depend on the defect. Common defects during the molding process include flow lines, burn marks, warping, air pockets, sink marks, weld lines, and jetting.

1. Flow Lines: Discolored, wavy patterns on a component; happens when there are variations in the cooling speed of the material as it flows through the mold. Solutions need to keep mold flow rates consistent, yet quick enough to fill the entire mold before it cools. Options include:

  • Increasing the injection speed, pressure, and material temperature.
  • Rounding the corners of the mold.
  • Relocating mold gates.
  • Increasing the nozzle diameter.


2. Burn Marks: Black or rust-colored discolorations at an edge or surface. Usually caused by trapped air or overheated resin. To prevent burn marks:

  • Reduce melting temperature.
  • Reduce the injection speed.
  • Enlarge gas vents and gates.
  • Shorten the mold cycle time.


3. Warping: Deformations that are due to uneven shrinkage; occurs during the cooling process. Warping can be prevented by:


  • Slowing down the cooling process to prevent uneven stress on the material.
  • Reducing the temperature of the material or of the mold.
  • Switching to a resin or thermoplastic that has less shrinkage during the cooling process.
  • Redesigning the mold to achieve uniform wall thickness and part symmetry.


4. Air Pockets: Also called vacuum voids, these are trapped air bubbles that appear in the finished product, which can weaken the component. Often due to uneven pressure on the mold or too-fast cooling. Solutions include:

  • Raising the injection pressure to force out trapped air.
  • Choosing a lower viscosity material.
  • Placing gates close to the thickest parts of the mold to prevent premature cooling.


5. Sink Marks: Small recesses or depressions in flat surfaces; occurs when the inner part of the component shrinks and pulls material inward. Caused by cooling that is too slow, most frequently in the thicker parts of a component. Sink marks can be eliminated by:

  • Increasing holding pressure and time to allow complete surface cooling.
  • Increasing cooling time.
  • Designing molds with thinner walls to speed up cooling near the surface.


6. Weld Lines: Appear on the surface of a part, most often at the point where the material has diverged into separate directions to fill the mold or where two different resins or flows meet. Caused by inconsistent temperatures that prevent materials from bonding evenly. Remedies include:

  • Increasing the temperature of the materials.
  • Raising the injection speed and pressure to limit early cooling.
  • Redesigning the mold to eliminate partitions.
  • Switching to materials with lower melting temperatures or viscosity.


7. Jetting: A deformation that occurs when the initial “jet” of molded material begins to solidify before the cavity is filled. Appears as a squiggly line in the finished part and can cause part weakness. Usually, the result of too much injection pressure, which fills the mold too quickly for even cooling to occur. Jetting can be prevented by:

  • Reducing injection pressure.
  • Increasing material and mold temperatures to prevent early cooling/solidifying.
  • Changing the mold design so that the material is directed across the mold, as opposed to lengthwise.


Contact Baytech Plastics For Help Designing Solutions For Injection Molding Problems

Baytech Plastics has decades of experience troubleshooting and developing solutions for injection molding process problems. If you are having trouble with your project, contact our team at (705) 526-7801 or online to discuss your challenges and discover how we can help you.