Common injection molding defects and how to solve them?


When it comes to injection molding, there are many common problems. Quality problems in injection molded products can range from minor surface defects to more serious problems that affect the safety, performance and functionality of the product. They can be caused by problems related to the molding process, material use, tool design, or a combination of the three. In this blog, we’ll go through the various challenges users face when injection molding and provide advice on how to achieve the best results.

There are 11 common injection molding defects as below:

1. Weld lines

2. Burn marks

3. Warping

4. Air pockets

5. Sink marks

6. Flash

7. Discoloration

8. Delamination

9. Short shot

10. Black spots

11. Water splash

Weld lines

Description: Weld mark refers to the surface defect caused by the fusion of two material flows. The fusion surface of a molten resin that often flows in a mold cavity. It mainly affects the appearance, coating and electroplating.

Causes: If there are holes in the parts, inserts or multi-gate injection modes or uneven wall thickness of the parts, there may be a solution mark.

Improvement method: increase the fluidity of plastic melt.

Change the position of the gate.

Additional exhaust groove

In terms of technology:

Increase injection molding pressure and retaining pressure.

Increase the melt temperature.

Reduce the amount of release agent used.

Burn marks

Description: Burn marks usually appear as black or rusty discoloration on the edges or surfaces of molded plastic parts. Burn marks generally do not affect the integrity of the part unless the plastic has been burned to the point of degradation.

Cause: The common cause of burn marks in injection molding parts is the lack of exhaust and the injection speed is too fast.


· Reduce the injection temperature

· Reduce injection speed to limit the risk of trapped air in the mold

· Expand the exhaust port and gate, or shorten the gate to allow trapped air to overflow the mold

· Reduce injection pressure

· Reduce the injection speed of the last stage


Description: Warping is the deformation that can occur in an injection molded product when different parts of a component contract unevenly. Uneven shrinkage can put excessive pressure on different areas of the molded part. This excessive stress can cause the finished component to bend or twist as it cools.

Cause: One of the main causes of warped injection molding plastics and similar materials is that the cooling time is too short, the material is too hot, there is a lack of cooling around the tool, the water temperature is incorrect (parts bend inward towards the hot side of the tool), and there is uneven shrinkage between parts areas.


· Ensure that the cooling process is gradual and long enough to prevent uneven stress on the material

· Reduce the temperature of the material or mold

· Try to switch to materials that shrink less during cooling (e.g. particle filled thermoplastics shrink much less than semi-crystalline materials or unfilled grades)

· Redesign the die with uniform wall thickness and part symmetry to ensure greater stability during cooling

Air pockets

Description: A void is a phenomenon in which air is trapped in the cavity, causing bubbles to occur in the product. Voids are usually located where the melt is last filled.

Reason: Lack of pressure holding (which is used to pack parts during pressure holding). Filling is too fast and does not allow the edge setting of the part. The die may also be misaligned (when the two halves are not centered correctly, and the parts have different wall thicknesses). The information provided is generally understood, corrected: lack of pressure holding (non-pressure holding) pressure (pressure holding is used for pressure holding, even during the part of the pressure holding). Filling too fast won't cause this. In other words, when the part shrinks, the resin separates from itself because there is not enough resin in the cavity. Voids may occur in any area or part not limited by thickness but by resin flow and thermal conductivity but are more likely to occur in thicker areas such as ribs or bosses. The other root cause of the void is the failure of the molten pool to melt.

Common ways to prevent voids include:

· Increase injection pressure to release trapped air pockets

· Select material grades with low viscosity to limit the risk of bubble formation

· Place the gate near the thickest part of the mold to prevent premature cooling where gaps are most likely to occur in the material

Sink marks

Description: Shrinkage mark is the phenomenon of concave surface in the wall thickness of the molded part. Usually found in stiffeners, countersunk holes, or internal gratings.

Cause: Shrinkage occurs because the hold time/pressure is too low and the cooling time is too short. For hot runner without gate, this may also be caused by gate temperature set too high. Too much material or too thick walls.


· Increase the holding pressure and time to allow the material near the surface of the part to cool

· Increase cooldown time to limit shrinkage

· Design molds with thinner component walls for faster cooling near the surface


Description: Flash refers to the appearance of excess plastic on the parting surface of the die or on the ejector bar, etc., resulting in irregular corners of the workpiece in these places.

Causes: Insufficient closing force

In the process of injection molding, if the closing force is too small to offset the injection pressure, it is easy to produce flying edge/rough edge phenomenon on the parting surface:

Mold defect

Forming conditions are not reasonable.

The exhaust system is poorly designed.

Improvement method:

· Design the mold reasonably to ensure that the mold can be closed without gaps

· Set an appropriate clamping force

· Increase injection time and reduce injection speed


Description: Discoloration or "color streaking" occurs when the color of the molded part is different from the intended color. Usually, discoloration is limited to a local area or a few abnormal color streaks on the molded part. This defect usually affects the appearance of the part without reducing its strength.

Cause: Common causes of discoloration are leftover particles in the hopper or residual resin in the nozzle or mold from a previous production run. Poor thermal stability of colorants or improper mixing of master materials are other potential causes.

Take the following precautions to limit the risk of discoloration of injection molded products:

· Ensure that workers properly clean the hoppers, nozzles and molds between production runs to eliminate any residual particles or substrates

· Consider using a cleaner to remove excess color from the machine

· Ensure that you or your supplier uses colorants with adequate thermal stability

· Ensure that the masterbatch is evenly mixed to achieve consistent color output


Description: Layer peeling means that the surface of the product can be peeled off layer by layer.

Causes of delamination in injection molding

There are several reasons for layering.

A) Mix with other incompatible polymers

B) Excessive release agent used in molding

C) the melt temperature in the cavity is too low

D) too much water

E) the gate and runner have sharp corners

F) resin temperature is inconsistent

If layering is found to affect molded parts, consider the following corrective actions to prevent recurrence:

· Avoid mixing incompatible impurities or contaminated feedstock into raw materials

· All runner and gate with sharp Angle should be chamfered to achieve smooth transition

· Increase cylinder and mold temperature

· Properly dry the material before forming

· Avoid using too much release agent

Short shots

Description: Short shots refer to the phenomenon that the mold cavity cannot be completely filled.

Causes: Any factor that prevents the flow of the polymer melt or causes the polymer to be injected in insufficient quantities may cause gel deficiency. These factors include:

A) too low melt temperature, mold temperature or injection pressure and velocity

B) uneven plasticization of raw materials

C) bad exhaust

D) Insufficient liquidity of raw materials

E) the parts are too thin or the gate size is too small

F) premature hardening of polymer melt due to improper structural design or delay in injection molding.


· Increase the fluidity of the melt

· Fill thick wall before filling thin wall to avoid retention and premature hardening of polymer melt

· Increase the number of gates and reduce the flow ratio

· Increase runner size and reduce flow resistance

· The position of the exhaust port is set properly to avoid the phenomenon of poor exhaust

· Increase the number and size of exhaust ports

· Increase injection pressure

· Increase injection molding speed and shear heat

· Increase injection molding volume

· Increase cylinder temperature and mold temperature

Black spots

Description: Black spots or stripes are brownish black spots or stripes, or brown stripes, on the surface of the piece.

Two reasons are expected:

A. Material degradation: the decomposition of plastic over heat will lead to black spots or streaks. If the plastic in the closed cylinder, screw surface stay too long, will lead to carbonization degradation, so in the injection process of black spots or stripes.

B. Material pollution: the presence of dirty recycled materials, foreign bodies, other colors of materials or easily degraded low molecular materials in plastics may cause the above phenomenon. Dust in the air also tends to cause black spots on the surface of the parts.

Improvement method:

· Non-polluting raw materials are used

· Place the material in a relatively closed storage bin

· Increase the thermal stability of the material

· Clean ejector rod and slider

· Improved exhaust system

· Clean and polish any dead corners in the runner to ensure that no accumulation occurs

· Clean the surface of the mold before injection

Water splash

Description: Splash refers to a surface defect in which water, air or carbonized matter is distributed in a emitting manner on the surface of the workpiece along the direction of flow.

Two reasons are expected:

A) The content of water in raw materials is too high

B) There is air in the raw material

Improvement method:

· Dry raw materials according to data provided by raw material supplier before injection molding

· Increase the size of main channel, diverter and gate

· Check for adequate exhaust space

· When switching materials, clean the old materials completely from the cylinder

· Increase back pressure

· Improved exhaust system

· Reduce melt temperature, injection pressure and injection speed


Injection molding defects can be harmful to the final product, but fortunately, they can be fixed relatively easily. With the right procedures and careful personnel, most defects can be detected before they have a chance to cause any serious damage. By taking the appropriate steps and paying close attention to the process, Alpine mold have a strict quality control process to ensure that your injection molding project proceeds as you expect and provides you with reliable and high quality products.