Plastic Injection Molding Machine Problems and Preventions

Jan 29, 2021

Injection Molding Problems and Their Prevention

Injection plastic molding is a complex technique of creating plastic using a plastic injection molding machine.  They incorporate the technique of filling injections with molten plastic or other malleable materials and inserting them in the molds to form the desired shapes. A plastic injection molding machine needs a lot of high pressure to perform this function.

Since this is a highly complex procedure done by a plastic injection molding machine; it has a lot of room for error. The types of errors in production can vary from surface defects to safety hazards in the design of the output. 

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This can also cause major defects in the functionality of the product.  The occurrence of errors can be due to poor quality of raw materials, missing specifications during the procedure, and a defective design. 

Sometimes it can be a mix of all these defects too. Large amounts of defects can ruin the design as well as the reputation of the manufacturing company.

plastic injection molding machine

Molding Defects Caused by Process Problems

The cost of correcting molding errors can vary.   Very highly defective errors can be costly for the production as well as the consumer.  In order to prevent such high-end errors, it is advised to adjust molding processes, redesign the mold or replace the tools before producing the final output. 

It is better to face a loss at an initial stage rather than facing a big loss after the mass production of the plastic molds. But the best part is that it is avoidable to prevent the defects from happening altogether.  You can take easy steps of adjusting the pressure, heat-intensity, and flow rate to prevent defects from happening.

Here are some common defects that can occur during the injection molding process and some of its preventions.

Flow lines

They act as a passageway or a gate for the molten material to enter.  It is possible for a product to have these curvy designs imprinted on its own design which is not desirable by any consumer. It can be unsightly if they leave a mark on the product mold.

To prevent this from occurring, you need to make sure that the cooling speed of the molten material stays uniform.  Another remedy for this problem can be the even flow of material through the flow lines. When the speed of the molten material increases it becomes impossible for the plastic residue to stick to the pathway which is the best way to prevent clogging and unnecessary imprinting in this area. 


Burn Marks

This is a very common type of problem in plastic injection molding. Sometimes the product might have burn marks or discoloration on some of its surfaces.  Although they do not affect the functionality of the product itself, it can lead to some very unpleasant looking surfaces that might not be acceptable to the consumer.

Burn marks only occur when air is trapped in some parts of the machine.  Air cavities help the residue to accumulate in certain places. When the residue area heats up constantly it starts to burn and as the molten material passes by it leaves a burning imprint on the product.

To avoid this you need to lower the melting and molding temperatures in order to prevent overheating. Another remedy is to reduce the speed of injecting as well as shortening the molding time span.


Warping is an error in which the product deforms.  Some areas of the product might shrink while the others remain the normal size causing warping to occur. This can also cause due to high pressure that is uneven on different areas of the mold. One of its major causes is due to sudden cooling.  Sometimes this can also happen due to an unusual mold design. It is important to inform the design team to make a mold that can facilitate the plastic injecting in every nook and corner of the mold.

To prevent this from occurring, the manufacturers must make sure to create a cooling process that is gradual in nature. Another way to prevent this is to lower the pressures and temperature. A consistent and continuous heat and pressure flow will ensure an un-warped product.

Vacuum Voids and Air Pockets

This is the most common type of error that can be seen in injection-molded plastic.  Sometimes the design may feature air pockets in the form of air bubbles that can trap on the surface of the design.  It is a minor defect but an undesirable one.  Too many air bubbles in the design can affect the functionality of the design by weakening it. It can also cause safety hazards at the user’s end.

The major cause of the formation of air bubbles on the surface of the injection-molded plastic is uneven and insufficient pressure.  In order to prevent such occurrence, the manufacturers need to increase the injection pressure and choose a material that has a low viscosity.

Sink Marks

Sink marks feature the presence of breaks or hollows on the flat surface of a consistent design mold in the parts of the molded product where the material flow was insufficient.  Insufficient provision of material in the desired area during the process can cause depression in the design.  The reason behind the sink marks is the very slow cooling of the melted material.

In order to prevent this, the machine needs to hold the pressure for the exactly specified time and not a second less.  The cooling time must increase to prevent sink marks on the surface of the mold. What more you can do is that the design the mold with thin walls to compensate the cooling process.


Welding Lines

Welding lines appear when the molten material congregates after piercing in more than two directions of the mold.  Due to the weak binding of the molten material, welding lines can appear on its surface. They decrease the strength and functionality of the product significantly.

The reason may be the uneven merger of the polymers at an unspecified temperature. This can result in an even solidifying or an uneven liquefying of the polymers. 

The cooling time must increase to prevent sink marks on the surface of the mold. What more you can do is that design the mold with thin walls to compensate for the cooling process.


Jetting is the formation of zigzag lines in the design mold. It appears as a deformation in the design surface. The occurrence of jetting takes place when the molten material enters the mold suddenly or with too much pressure.  This can weaken the design of the mold substantially.

Moreover, too much pressure while injecting the molten material can cause jetting. Usually, the injection plastic molding machines have a very small entrance. When the polymers enter the small entrance with too much pressure it can harden the material undesirably.  This can cause leave an imprint on the finished design.

To prevent jetting to happen, you need to reduce the injecting pressure. Also, increasing the material and mold temperature can prevent it from happening.  Make sure that the polymer flow is nice and easy that increases gradually.

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Plastic Injection Molding Machine -- Take Away 

Making plastic using injection plastic molding machines is becoming a worldwide phenomenon. Due to its easy production, many manufacturers prefer using injection molding techniques for mass production of different parts and products. 

Sometimes defects can occur during the production process.  In order to produce defect-free products, the manufacturers need to make sure that the equipment, pressure, raw material, and other such factors are up-to-date.

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