7 Ways to Fix Injection Tool Process

December 31, 2021

1. Watermark of  the injection tool

Injection tools are small dents that form in plastic products. This usually occurs when the interior of the component solidifies and contracts rapidly, and the outer material inward by the shot.


Sinking occurs because the molten plastic material cools too slowly in the mold. As a result, the outer material inject before it has had a chance to cool completely, causing material degradation.

This usually occurs in the thickest parts of the injection tool due to the uneven cooling of the plastic material.

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How to fix the watermark of the injection tool:

  • Increase holding pressure and increase time to allow material near the part surface to cool
  • Increase cooling time to limit shrinkage
  • Design your mold with thinner components to allow for faster cooling near the surface.

2. Welding line

Welds may appear on the surface of the injection tool part, where the molten material has converged after separating into two or more directions in the mold. A hair-like weld seam results from weak material bonding, which reduces the strength of the part.

This leads to a weakening of the plastic component in the weld

Welding occurs when molten plastic converges from different channels at a certain point in the mold and weakly bonds with each other. This leads to a weakening of the plastic component in the weld.

How to fix it:

  • Increase material temperature to prevent partial solidification
  • Increase injection speed and pressure to limit cooling before the material fills the mold
  • Redesign the injection tool to remove the baffles
  • Switch to a material with a lower melting point or viscosity to allow faster flow and prevent premature quenching.

Type of deformation in a molded part that can occur

The injection typically appears as a squiggly line on the surface of the finished part, usually leading from the original nozzle. This visible flow pattern can lead to partial weakening.

3. High pressure of the injection tool

This may be due to the high injection pressure, which results in the material injecting through the nozzle into the injection tool instead of gradually filling the mold.

This stream of molten plastic begins to cool before the rest of the cavity fill, resulting in the formation of ripples on the surface of the finished product.

How to reduce injection pressure

  • To prevent material from rapidly injecting into the mold cavity
  • Raise the material and mold temperature to keep the initial jet material from solidifying prematurely

4. Common Defects Relating to the Use or Care of the Material

Defects in injection tools can often stem from the material itself or the way the manufacturer stores and treats the material before manufacturing. These defects can range from minor cosmetic problems to compromise the durability of the finished part. Serious safety concerns may also result, depending on the intended application of the affected product.

5. Discoloration occurs when a part has different colors

Discoloration, also known as “staining,” occurs when a part has a different color than it was intended. Often the discoloration is limited to a localized area or a few unusual streaks of color on the injection molded part. This defect usually affects the surface of the part without reducing the quality of the product.


One possible cause of discoloration is pellets of plastic material leftover from a previous run in the machine. It is also possible that some residual material is stuck in the nozzle or mold due to low pressure.

Other reasons may be the colorants not being mixed well with the ingredients.

How to fix:

  • Ensure that workers clean hoppers, nozzles, and molds properly between production runs to remove residual pellets or base material.
  • Consider using a bleach compound to remove excess color from the machine
  • Make sure you or your supplier are using a colorant with the proper heat stability

6. Surface delamination of the injection tool

If you see thin layers on the surface of a part that easily separate or peel off from the underlying material, you are seeing a casting defect called delamination. Delamination is a defect characterized by a flaky surface layer, similar to what you would normally see on flake mica. This is generally considered a relatively serious defect because of the reduced strength of the material.

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Reason for Surface delamination of the injection tool

Usually, surface delamination occurs because the raw material is with particles of foreign matter such as sheet metal. The product’s surface begins to peel when the materials cannot bond together properly. And very dangerous if the finished product is a critical ingredient in terms of safety.

Another reason for surface flaking because the material has not been properly dried before use, and there is moisture on the surface.

How to fix Surface delamination of the injection tool:

  • Raise the injection tool temperature or pre-dry the appropriate material if excess moisture is an issue
  • Ensure workers are properly storing and handling plastic pellets or base materials to prevent contamination
  • Consider redesigning the mold with a focus on the nozzle to limit your reliance on release agents

7. Injection Molding Defects Due to Poor Mold Design

Short shot

If the molten plastic material does not fill the mold cavity during the running process, the final product is defective and incomplete after cooling and solidifying. This is a crucial defect that greatly affects the functionality of the product and increases the cost of production.

Block lines or ports in the machine are one of the main causes of short shots. This can especially happen when the nozzles are narrow.  If the material is too viscous or the mold is not at a high enough temperature, the injection tool will not fill before the material solidifies.

Material can also affect the flow of the molten plastic

Air trapped in the material can also affect the flow of the molten plastic. Another reason could be insufficient pressure during material injection.

How to fix:

  • Redesign mold with wider runners or gates for better flow
  • Increase injection speed or pressure or choose thinner base material to improve the flow
  • Increase the mold temperature to prevent the material from cooling down too quickly

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