Injection molded parts play a critical role in modern industrial production. However, even minor details can lead to unnecessary defects, posing challenges in injection molding processes. One common issue is the occurrence of cracking defects. So, what are the primary causes of cracking defects in injection molded parts?
- High Proportion of Recycled Material in Raw Materials: Using too much recycled material can increase shrinkage directionality or lead to uneven filler distribution in the raw material.
- Contaminated or Poorly Dried Raw Plastic Material: Contaminants, volatiles, or insufficient drying in raw plastic can contribute to cracking.
- Improper Ejection Design: Insufficient ejection angle, thin walls, sharp corners, or notches in the product can lead to stress concentration.
- Insufficient Hold Pressure and Injection Time: Short holding and injection times, or improper cooling times, can also be contributing factors.
- Uneven or Low Mold Temperature: Low or uneven mold temperature, combined with slow injection speed and low pressure, may result in cracks.
- Improper Molding Conditions: Excessive stress or poor ejection can cause structural weakness.
- Improper Use of Release Agent: Incorrect use of release agents or uneven cooling after demolding can lead to defects.
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Cracking Root Causes, Ranked by Frequency
When cracked parts come off the line, the diagnosis usually lands on one of five causes — in roughly this order of likelihood:
| Rank | Cause | Mechanism | First fix |
|---|---|---|---|
| 1 | Residual stress | Frozen-in orientation from high injection speed or low melt temp | Raise melt temp; slow fill; anneal if needed |
| 2 | Sharp internal corners | Stress concentrates at radii below ~0.5 mm | Add radius; redesign rib roots |
| 3 | Over-packing | Holding pressure too high or too long locks in stress | Profile holding pressure down |
| 4 | Moisture in resin | Hydrolysis embrittles PEI, PC, PA families | Verify dryer output dew point |
| 5 | Ejection damage | Unbalanced ejector layout drags the part | Rebalance pins; add draft |
The ordering matters because plant time is finite: check the process window before blaming the steel, and check the steel before blaming the resin. A short cross-section under a microscope settles most arguments in minutes. The full defect catalog sits in our injection molding defects guide, and the fracture-specific deep dive in solving fracture and cracking. Reference: injection moulding and polymer degradation.
Field Case: The Crate That Cracked in Transit
Not every crack is born at the machine. A recurring pattern: parts molded and packed warm, sealed in a container, then subjected to cold nights — the differential contraction adds to residual stress and hairlines open before unboxing. The countermeasures are unglamorous: cool parts below 40°C before packing, avoid tight stacking that loads flanges, and anneal stress-sensitive parts (PC, PA) when the geometry has thick-thin transitions. When a crack claim arrives, the first questions are about storage and transport temperatures, not only the molding record — the microscope shows whether the failure grew from a molding stress riser or from post-molding loading. Both are preventable; only one happens at the machine.