Plastic injection mold

The 5 Most Common Plastic Injection Molding Defects & Their Causes

Plastic injection molding is used in multiple industrial, commercial, and consumer markets when applications require durable, high-strength plastic components. 

Although plastic injection molding provides many advantages, defects are not rare; molding at too high or too low a temperature, ejecting the plastic part too soon from the mold cavity, low clamping pressure, and poor mold design are just some of the reasons defects occur. And defects are not just a visual concern—they can dramatically impact the performance of the finished part. 

In this article, we’ll cover the five most common plastic injection molding defects and their causes.

1. Warping

Warping is relatively easy to identify. You can measure the plastic part against other parts or lay the defective plastic part on a given surface. If the part is uneven against the surface or appears buckled or bent, it’s a sure sign of warping.


Warping can be caused by molding at too high a temperature or improper cooling, where a specific area or region of the plastic component shrinks. The shrinkage stresses the remaining plastic, and the part buckles or warps. Another cause is ejecting the plastic part too soon from the mold. The mold’s design can also cause warping if the wall thickness isn’t uniform across the finished part.

2. Sink Marks

Sink marks are more of an aesthetic issue where a portion of the plastic part dips or recesses. These marks are often small and don’t impact the strength of the part or its performance. To identify them, you must tilt the part in the light to identify any circular depressions. A good rule of thumb is to tilt the part upwards against a light source and then rotate it until you find the sink mark.


Exceeding a thermoplastic’s molding temperature and low injection pressure are some of the reasons sink marks appear. Other causes include trying to cool the plastic part too soon and ejecting the plastic component too quickly from the mold. An obstruction inhibiting the molten plastic from properly feeding through the sprue opening can also cause sink marks.

3. Excessive Flash

Flashing may occur naturally throughout the plastic injection molding process. However, excessive flashing adversely affects how the plastic part mates with the surfaces of other parts, impacting assembly times. A thin layer of flash can often be removed with hot air, but thicker amounts of plastic often render the part unusable. In this case, there’s usually no recourse but to scrap the part, recycle it, or reuse the plastic in future production runs, if possible.


The most common cause of flashing is obstructions within the mold cavity, dirt, and debris near the gate or sprue. Beyond obstructions, there are several other causes of excessive flash, including:

  • Overfilling or excessing packing of the mold with molten plastic
  • Higher than normal barrel and nozzle temperature
  • Low clamping pressure
  • Poor airflow ventilation

In these situations, excess material builds up until it’s eventually released and causes flashing.

4. Voids

Voids are essentially air pockets that appear as bubbles within plastic molded parts. They appear inside the walls of a plastic part and occur when the exterior or surface of the part cools faster than its interior. When this happens, the interior of the plastic part is essentially stretched until a void appears. These voids are easily identified regardless of the color of the plastic used in molding.


Voids are commonly referred to as vacuum voids. They can be caused by moisture within the material or moisture build-up within the hopper, screw, barrel, and mold cavity. Low injection pressure and an injection speed that’s too high or too low can also cause voids. Poorly designed, improperly sized, or poorly-positioned gates and runners where channels with different materials meet can also cause voids.

5. Short Shots

A short shot refers to a plastic part that wasn’t properly molded. The piece appears incomplete as certain portions of the plastic part don’t have enough material. Sometimes there’s no material, and the part doesn’t respect its original design.


Short shots can occur due to low injection pressure, low mold temperature, and when not enough material fills the mold. Another cause includes issues with channels and gates in the mold that prevent the right amount or flow of material. These issues include channels and gates that are too narrow, don’t have enough clearance, or have dirt and debris, and other obstructions.

Preventing Defects

There are some proactive measures you can take to mitigate the incidence of defects in your plastic components. The first involves selecting the right material. You need a material that fits your application and can withstand the temperatures and operating conditions for which your product is designed.

Another important aspect is ensuring you work with a plastic injection molding partner. They should be able to take you through the injection mold design process and advise you on the right material for your product. A reliable plastic injection molding manufacturer should also provide you with a list of precautions and quality steps they take to prevent defects.

Turn to a Professional Plastic Injection Molding Manufacturer

Get mold design advice and plastic injection molding assistance from Silver Basis Engineering. With over 20 years of manufacturing experience, we’re a reliable resource and partner for all plastic injection molding projects. Our commitment to excellence is defined by our 20 National Fine Model Awards and 511 patents alongside a committed and dedicated workforce exceeding 9,000 employees.

Visit our blog for more plastic injection molding resources, or contact us today to learn more about our experience, capabilities, and dedication to producing high-quality, defect-free plastic molded components.