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Why Do Injection Molded Products Crack?

Discover the main causes of cracks in injection molded products and learn effective strategies to prevent and fix these issues, ensuring product quality and durability.

Injection molding is an important process in manufacturing plastic products. They are one of the most used processes today. But, you might have seen that their products get cracked several times. This further creates problems for the manufacturer, which leads to compromise of structural integrity and functions of the plastic parts.

Knowing why these cracks have taken place should be necessary before you reach the answer. This will involve understanding the main cause of the crack, and the ways you can use it to fix the problem.

Precision machining in the production of injection molded products

This article describes in depth why the parts get cracked. And what should be done to fix them?

What Are The Main Causes Behind Cracks?

Cracks in injection molding products are very common due to internal and external forces on them. The following are the detailed reasons for the cracks.

Stress and Deformation

One of the most common causes of cracks is stress and deformation. It usually happens when something affects it either during or after the process. 

Internal Stress

Internal stress or residual stress occurs inside the parts. This is due to three main reasons.  

  • Overfilling: Sometimes, the mold cavity gets overfilled with molten plastic parts. Due to less space inside the mold, even after the whole cooling process, it doesn’t get relaxed. This is the main reason behind internal or residual stress.

This occurs especially in the thicker areas because they are difficult to bear. Further, this stress causes cracks around sharp corners or the gate. 

  • Demolding: During the injection molding process, when the molded part gets removed from the mold, internal stress can be caused. 

This is because when the ejection force is too high, or the part’s corners are too sharp, it can break down.

  • Metal Inserts: Plastic and metal parts are very different from each other based on their melting point, temperature, thermal properties, etc. 

When the metal parts are inserted into the plastic, there is a chance that the metal will create stress inside the plastic and cause cracks in the part.  

External Stress: 

Even if the whole injection molding process has been completed successfully, there is a chance external stress can cause damage to the parts.

  • Environmental Factors: When the parts are exposed to harsh climates, such as during extreme sunlight or rain, or if harsh chemicals enter them, the parts can be cracked within seconds or minutes.

The main reason behind this is that plastic is a very sensitive material and cannot bear degradation. 

Detailed view of the entire injection molding process in action

Deformation Stress

The plastic can easily shrink when it doesn’t get enough cooling during the process. This whole thing is called deformation. This can easily cause stress inside the material, and if the stress level is too high, it can easily put cracks in the completed product.

This is further responsible for creating shrinkage and cracks in the material.

  • Part Formation: How is the part made? What materials are used inside it? And what is their thickness and thinness? All these things together contribute to part formation.

Remember that the thicker section takes a long time to cool down, whereas thinner sections can be easily cooled down.

Injection molded plastic elbows showcasing resistance to external stress

These were some of the main reasons why injection molded products get cracked. It can be due to both internal and external stress. But what can we do if such a situation ever occurs to us? Following are a few factors that should be followed to avoid part cracks.

Strategies To Prevent Cracks

Locate the CrackToo much moisture on pellets can crack the plastic. Make sure to dry the pellets.
Select Right MaterialTemperature, time, and pressure should always be accurateDon’t exceed the limit in any of those.
ParametersTemperature, time, and pressure should always be accurate. Don’t exceed the limit on any of those.
MoistureToo much moisture on pellets can crack the plastic. Make sure to dry the pellets

Final Summary

Cracks on injection-molded products occur for various reasons, including external or internal. It is very important to check the main cause of the problem and use a correct solution to fix the problem.

Talk to a professional for better guidance if you ever face any problem during or after the process, specifically if the cracks take place.

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