Top 10 Injection Molding Defects (And How To Prevent Them)

Hello people; today, I’m going to share a detailed and thoroughly researched guide on the most common injection molding defects and their prevention. So without further ado, let’s cut to the chase.

What is an Injection Molding Defect?

Injection molding defects are quite common in any shop floor extensively manufacturing using the injection molding Process.

The injection method is quite popular among plastic manufacturers because of its versatility in producing parts in bulk. However, sometimes small and sometimes big errors can always happen, but they’re always there.

Injection defects are not easy to avoid. Several reasons can cause defects like process issues, excess or lack of material, human error, tooling issues, designs, or all of them combined.

However, every problem comes with a solution:) So, just identifying the problem is a battle half won.

Here in this post, we will discuss the 10 most common injection molding defects and their remedies.

Injection Molding Defects and their Preventions 

injection molding defects and preventions

  • Flow Lines 
  • Warping 
  • Sink Marks 
  • Vacuum voids/air pockets
  • Discoloration
  • Jetting 
  • Flashing 
  • Short Shots 
  • Delamination 
  • Burn marks 

#1 Flow Lines:

Flow lines as an injection molding defect are the most common to appear in mode components. Flow lines usually appear as discolored “bands” or even “wavy” discoloration on the component’s narrower sections.

They may also appear on the entry points of mold and gates. Flow marks wouldn’t make or break the product’s wholeness, but it can be dis-appealing to see.

The most common cause of flow lines is inadequate variations in the material’s cooling speed as it spreads inside the mold. Any variations in wall thickness can cause the materials to cool at unusual rates.

Molten plastic will cool rapidly during the injection molding process, but flow marks are bound to happen when the injection speed is slow.

Due to slow speed, the plastic becomes solid and gluey while still filling the mold, causing the flow “wave” or “band” to appear.


There are some legit countermeasures to prevent flow lines.

  • Increasing the injection speed, material pressure, and material temperature ensures the material fills the mold properly.
  • Relocating mold gates to create more distance between them and the coolant so that the material wouldn’t cool earlier in the injection cycle.
  • Round the corners of the mold. When the mold corners are circular in shape, the flow rate will remain consistent, helping prevent the flow lines.
  • Increasing the nozzle diameter is also a quick solution to the problem, as it will increase the flow speed and prevent early cooling.

#2 Warping:


Uneven shrinking and bending of molded products during the molding process are called warpingWarping occurs when uneven stress is kept but shrinkage on different parts of the molded component.

The undue stress results in bending on the component, which is quite noticeable with flat surface products.

One of the main reasons behind warping is premature cooling. Excessive temperature and poor thermal conductivity of the material also play a key role in causing warpage.

Sometimes uneven wall thickness in the mold can also provide for creating warping.

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  • Lowering the temperature of either the material or mold can do the trick.
  • A consistent and long enough cooling process will help in avoiding uneven stresses on the material.
  • Switching to a material with good thermal conductivity during the cooling process.

 #3 Sink Marks:

sink marks

Sink marks are one of the major injection molding defects. It usually appears on the thicker parts of the molded component. It looks like small indentions or depressions and is quite stubborn to prevent.

The main reasons behind sink marks are insufficient cooling time before the molded component’s ejection, low pressure in molding cavities, and too high molding temperature at the gates.


  • Increasing the cooling time and keeping the part in the mold for a longer time.
  • A slight decrease in the mold temperature can also be useful.
  • Designing a mold with thinner component walls should drastically reduce the chances of sink marks.

#4 Vacuum voids/air pockets:

vacuum voids

It is exactly what it sounds like. Air stuck in the mold cavity can be noticed in the shape of a “bubble” in the finished component.

Inadequate cooling, material shrinkage, and incomplete injection pressure are some of the main reasons causing air pockets.

Air pockets are not easily visible, making them more of a threat to the product’s wholeness.


  • Following a design practice of keeping the wall thickness below 6 millimeters can be very helpful.
  • In other situations, relocating the gate to the thickest part of the mold can prevent premature cooling, which is the main cause of uneven shrinkage.
  • Increasing the air pressure can also be useful as it will push air out of the material.
  • Choosing a material with a low viscidity eliminates the risk of bubbles forming.

#5 Discoloration:


Discoloration means there is a valuation in the molded components’ color than the intended color.

Discoloration is one of the common injection molding defects that doesn’t affect the molded part’s strength or shape but is limited to few streaks of unusual color found anywhere on the molded part’s surface.

Common causes of discoloration are residual resin in the nozzle, vestige pallets in the hopper, or a mold from earlier production.

Once in a while, reasons like unsuitable mixing of the masterbatch below par thermal conductivity of the coloring agent can also pop up to cause discoloration.


  • Verifying that the machine, hopper, nozzle, and mold are cleaned properly during the production runs eliminates any leftover resins or materials.
  •  Using a color agent with excellent thermal conductivity will make a significant difference.
  • Utilizing a purging compound to remove extra material from the machine can be a wise choice.

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#6 Jetting:


The term jetting means a kind of distortion in the molded part, which occurs because an initial “jet” of a molten material injected into a mold cavity begins to become solid before the cavity is filled.

Jetting can be a reason for a component’s weakness because it appears mostly in the components’ middle as a visible flow pattern.

Excessive injection pressure is the prime cause of jetting. Often, injected molten material, which runs through a small gate rather than gradually filling the mold, surges inside very quickly. Then, when the material starts cooling and becomes hard, the remaining material pushes it, leaving marks on the finished part’s surface.


Increasing mold and material temperature ensure that the initial jet of material doesn’t become solid earlier than needed.

Rapid spurging of materials can be prevented by reducing the injection pressure.

Tweaking the mold and injection gate designs to direct the material across the mold rather than splurging end to end.

#7 Flashing:


When the molten plastic escapes the mold cavity during the molding process through parting lines through cavities halves or ejector pins is called flashing.

The main cause of this injection molding defect is poor mold condition or maintenance. The flash mainly occurs when the mold cavity halves are not held together perfectly, leaving additional space for the molten material to come out.


  • Frequent repairs and maintenance to keep the mold in good shape.
  • Improving the material flow can also help significantly in preventing flashing. To do that, tweak mold temperature, injection pressure, and ventilation.
  • Some amount of flash is acceptable and wouldn’t hurt the integrity of the molded component. Manual laborers can be hired to cut out the flash using knives after the component is ejected.

#8 Short Shots:

short shots

Short shots occur when the material is cooling rapidly and doesn’t fill the mold’s cavities. That results in the production of incomplete components.

Short shots are classified as one of the most serious injection molding defects and can severely hurt the molded component’s quality and appearance.

Poor mold design, incorrect settings in the injection molding machine, and bad choice of plastic resin are some of the main causes of short shots.


  • If the material seems too thick, changing it to something smooth will improve the flow rate and reduce short shots’ chances.
  • Increasing the mold temperature will allow the material to flow in the hard and tiny cavities.
  • If venting is the main concern, adding an extra vent will also allow trapped gases to leave the mold, allowing the resin to spread within the mold properly.

Interesting Read – Top 10 Injection Molding Materials (And Why They are the Best)

#9 Delamination:


If you notice a thin layer on the surface of the molded part, which is easily separating or peeling off the material, you’re witnessing a material-related injection molding defect called delamination.

This molding defect can severely impact the component’s strength and thus should be taken seriously.

The main cause of delamination is when the resin pallet comes in contact with some foreign material. The phenomenon is quite noticeable when the two materials cannot bond to each other.

Delamination might be inevitable when two materials contrasting in nature are combined.

The material is poured into the hopper, and excess moisture caused by improper dying, release agent coating the mold for smooth component detachment is the prime cause for delamination.


  • Properly strong handling of resing pallets to avoid any contamination.
  • If the excess temperature seems to be the cause, increasing the mold temperature should help.
  • Avoiding release agents completely or meager usage can be beneficial. However, to do that, some design changes in the mold are a must.

#10 Burn Marks – 

burn marks

Burn marks are a patch of discoloration that can be seen on the surface of the material. The discoloration is mostly dark, black, red, and rusty.

Burs marks occur because of high temperature that degrades the molten material or too high injection speed, which does the same damage.

The high injection speed results in the brisk air, which doesn’t always edge properly. That trapped air often overheats the plastic within the injection molding cycle.


  • Lowering the melt and mold temperature to ensure overheating doesn’t happen.
  • Adding an extra vent will also help the trapped air to escape.
  • Cut short the cycle time so any trapped air and material wouldn’t overheat.

Thus, were the top 10 injection molding defect and troubleshooting.

Why is Prevention Must?

Prevention or Injection molding troubleshooting is significant as most of the defects mentioned above can substantially hamper your production quality. Manufacturers tend to neglect these defects, assuming them as minor flaws, but these so-called minor flaws can have a dampening effect on your overall production cycle.

Let’s take an example of a flow line. As I mentioned earlier, it doesn’t physically hurt the product’s integrity, but it looks unprofessional from an appearance standpoint. The short shot is another defect that can significantly hurt the final product’s appearance and cause havoc on the shop floor.

Identifying the problem is the first step towards finding a solution that can help you in a big way with a smoother production cycle and happy customers.

Engaging Read – Types of Gates for Injection Molding – A Detailed Guide


What is the biggest disadvantage of the injection molding process?

The biggest disadvantage of the injection molding process is that it’s costly. The setting-up costs are very high because the machinery and mold are costly, and even the initial installation would need a significant amount of money, labor, and time. However, if you’re going to produce in bulk, injection molding is the best option, and it will reap fantastic profits in the long run.

What are the types of injection mold?

The most common types of plastic injection molds are as follows:

  • Family mold
  • Unscrewing molds
  • Hot runner molds
  • Cold runner molds
  • Insulated runner molds
  • Single cavity molds
  • Two/three-plate molds

How to improve injection molding productivity?

Making smart and efficient use of the material is the key to improving your injection molding process and improving your finished components’ quality. Another useful thing that helped me personally when working in a plastic workshop is re-evaluating the largest shot size your equipment will allow.

What are the most common defects in the Injection molding sister process called Casting?

In die casting, the most commonly seen defects are misruns and cold shuts. Low melt temperature, cold dies, lack of venting, dirty metal, gas porosity, shrinkage porosity, flow marks, hot tears, and excess lubricant trigger these defects.

What is cycle time in injection molding?

The total time needed to complete all the injection molding cycles is called cycle time. The phenomena consist of four stages  – 1. Fill time, 2. Pack time, 3. Cooling time 4. Mold open time.

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The Takeaway

Those molding defects and troubleshooting methods are legit and can be utilized extensively at your shop floor to improve your production, productivity, and, most importantly, profits.

That’s it on injection molding defects and prevention. You can add your valuable thoughts in the comment box.

Have a lovely day.

4 thoughts on “Top 10 Injection Molding Defects (And How To Prevent Them)”

  1. I was not a blog reader, but your unique style has made the true reader. Your blogs are really informative and stylish. Your style is reader friendly and provides the knowledge.

  2. Injection molding defects are something no one gets. Good that someone is actually knowledgeable on the topic and is sharing it on the internet.


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