Injection molding is a manufacturing process for the production of parts by injecting molten material into a mold.

It can be done primarily with a host of materials including:-

Metals, glasses, elastomers, confection and most thermoplastic and thermosetting polymers.

Raw materials are melted and injected into plastic mold or pieces of metal and the size of the mold will take until raw material becomes rigid.


Making an injection molding prototype is both an art and science.

High level of technical expertise and attention is required to focus on expansion because it prevents small companies from spending large sums of money for large-scale production of novel parts.

Preventing this kind of situation is about highly capable design.

 This article discusses some molding defects that may occur in one part during the injection molding and the ways to fix them and avoid them.

We will discuss the design deficiencies:

  • Flow Lines
  • Sink Marks
  • Vacuum Voids
  • Surface Pollution
  • Weld Lines
  • Short shots
  • Warping
  • Burn Marks
  • Jetting
  • Flash

Most of the mistakes are due to the necessary experience or nescient personnel without the proper equipment at their disposal.

On the contrary, simplicity in personnel with creative solutions and the right experience and the right combination of hardware and software.

Finding the right team of people with relevant expertise is the most important part of the process.

Flow Lines

Flow Lines

Flow lines are streaks, patterns, or lines – usually off-to-paint in color – which appear on the prototype part as a result of the physical path, and as a cooling profile of molten plastic, the injection flows in the mold tooling cavity.

The injection mold plastic starts its journey through a gateway through part tooling called what is called “gate”, then it flows through the cavity and cools (finally hardened in a solid).


Flow line fault is due to the different speed at which the molten plastic flows because it changes the direction through the shape and bends inside the mold tool.

They are also when plastic flows in sections with a different thickness of the wall, or when the speed of injection is very low so that the plastic accumulates at different speeds.


1. Increase the injection speed and pressure to the optimum level, which will ensure that the cavities are filled properly (whereas molten plastic time is not allowed to start cold at the wrong location).

The temperature of the molten plastic or mold itself can also be elevated to ensure that the plastic is not sufficiently cold to cause defects.

2.The round corners and the position where the wall thickness changes to avoid sudden changes in direction and flow rate.

3.Locate the gate in one place in the device cavity with thin walls.

Sink Marks

sink marks

Sink traces are small craters or depressants that grow in dense areas of injection, which prototype is molded when there is a contraction in the internal parts of the finished product.

The effect is somewhat similar to the sinkhole in topography but is caused by shrinkage rather than erosion.


 The signs of the sync often occur when the cooling time or cooling mechanism is completely cold for the plastics and is cured while still in the mold.

They may be due to insufficient pressure in the cavity or excessive temperature at the gate.

The rest are becoming similar, the thick parts of the edged portion take more time to cool down than the thin ones, and hence the chances of the sink are more likely to be located.


1.Mold temperature should be reduced, increased pressure, and should be holding for long periods to allow for more adequate cooling and treatment.

2.Reducing the thickness of the thick wall squares will ensure faster cooling and will help reduce the chances of sink scarcity.

Vacuum Voids


Vacuum voids are pockets trapped in or around the surface of an injection molded prototype.


Vacuum voids are often due to uneven frozen between the surface and the internal sections of the prototype.

 This can increase when the holding pressure is insufficient to condense molten plastic in the mold (and thus throws out the air which will otherwise be trapped).

Voids can also be developed from a part which is inserted with a hamstring with two parts which are not properly aligned.


1.Locate the gate on the thickest part of the molding.

2.Switch on a less sticky plastic. This will ensure that the gas is trapped because the air is able to escape faster.

3.Increase pressure along with holding time over time.

Surface Delamination

Surface Delamination

Surface refining is a condition where thin surface layers appear on the surface due to contaminated material.

These layers look like coatings and usually can be peeled (i.e. “delimited”).


Foreign material that finds its way in molten plastic apart from the finished product because the contaminated and the plastic can not be tied.

The fact that they can not bind, not only affects the prototype’s appearance, but also on its strength.

Works as a trapped localized defect in a contaminated plastic.

 An over-dependence on mold release agents can also cause delamination.


1.Dry the plastic properly before molding.

2.Increase the mold temperature.

3.Smooth the corners in corners and smooth design to avoid sudden changes in melt flow.

4.More attention to the ejection system in mold design to reduce or eliminate reliance on mold release agents.

Weld lines

weld Lines

Weld lines are actually more like a plane than a line, which is visible in one part where molten plastic meets each other because they flow through two separate parts of the mold.


Weld lines are due to the insufficient connection of two or more flow fronts when partially freezing of the plastic.


  1. Increase the mold or melt plastic temperature.
  1. Increase injection speed.
  1. Adjust the design for the flow pattern to be the single source flux.
  1. Switch on with a low sticky plastic or low melting temperature

Short Shot

short shot

As the word implies, short shots can be described as a condition where a molding shot is reduced.

This means that for some reason the molten plastic does not completely capture the mold cavity or cavities, resulting in a part where there is no plastic.

The finished product decreases because it is incomplete.


Short shots can be due to many things. Due to incorrect calibration of shot or plasticizing capability, the plastic material can be insufficient to fill the cavities.

If the plastic is very sticky, then it can be solid before fully capturing all the cavities and this can result in a smaller shot.

Insufficient degassing or gas ventilation techniques can result in small shots as the air is trapped and there is no way to escape;

Plastic material can not occupy the place which is already capturing air or gas.


  1. Select less sticky plastic with high flux. This plastic will fill the most difficult-to-reach cavities.
  1. Increase the temperature of the flow or increase so that the flow can increase.
  1. Design the mold by designing the gas so that the gas cannot be trapped inside the mold and worked properly.
  1. Raise the material feed in the molding machine or switch to a machine that has high content feed in the event that has reached the maximum content feed.


molding warpage

Warping (or warpage) distortion occurs when there is uneven shrinkage in different parts of the molded component.

The result was a twisted, uneven, or folded shape where one was not intended.


Warping is usually caused by non-uniform cooling of the mold material.

Different cooling rates in different parts of the mold cause the plastic to cool differently and thus generate internal stress.

This tension, when released, leads to war.


  1. Make sure that the time of cooling is long enough and it is very slow to avoid the evolution of residual stress.
  1. Design the mold with the thickness of the equal wall and so that the plastic flows in one direction.
  1. The select plastic material that has fewer chances of shrinking and degrading. Semi-crystalline material is generally more prone to war.

Burn Marks

Burn Marks

There are burn marks discoloration, usually the colors of the rust, which appear on the surface of the injection molded prototype.


Due to the dehydration of plastic material due to excessive heating or injection speed, the burn marks are either very fast.

Burn marks can also be cause by the hot air being trap, which digs the surface of the molding portion.


  1. Reduce injection speed.
  1. Optimizing gas ventilation and degassing.
  1. Reduce the mold and melting temperature.



Jetting refers to a situation where the molten plastic sticks to the surface of the mold due to the injection speed.

Being fluid, the molten plastic is accumulate in a condition that shows the wavy layers of the jet stream on the surface of the injection molding portion.


Jetting mostly occurs when the molten temperature is very low and the melt plastic viscosity becomes very high, which increases the resistance of its flow through the mold.

When the plastic comes in contact with the walls of the mold, it becomes cool quickly and the viscosity increases.

The material flowing from behind that sticky plastic gives the sticky plastic forward, making the scraps mark on the surface of the finished product.


  1. Boost the temperature and melt the mold
  1. Increase the size of the gate so that the injection speed is slowed.
  1. Optimize the gate design to ensure adequate contact between molten plastic and mold.



Flash is a molding defect that occurs when some molten plastic is avoided by the mold cavity.

Typical routes for escape are through parting line or ejector pin locations.

This extrusion remains attached to the cold and finished product.


Flash may occur when the mold is not clamped together with sufficient force (enough force to face the anti-forces produced by the molten plastic flowing through the mold), allowing the plastic to leak.

The use of molds crossing their life will get worse and contribute to the possibility of flash.

Additionally, excessive injection pressure can make the plastic out of the path of least resistance.


  1. Increase the clamp pressure to ensure that parts of the mold are closed during the shots.
  1. Ensure that the mold is properly maintained and cleaned (or has reached the end of its useful lifespan).
  1. Adopt optimal molding conditions like injection speed, injection pressure, mold temperature and proper gas ventilation.

A large number of the above defects can be prevented in the design process by including the proper tooling design in the repetition process.

Using moldflow software such as SolidWorks Plastics will help you identify the ideal gate location, air pockets, flow or weld lines, and vacuum weds.

Most importantly, it will help you to design solutions to these problems ahead of time so that when it comes to production, you do not have to worry about the flaws you spend on the money.

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