Author: Shenye Plastic TechnologyDate：2021-10-12Pageviews：1126
In the precision manufacturing of drawn parts—such as deep shells, enclosures, or containers—cracking is one of the most frustrating defects. It doesn't just ruin the aesthetic; it compromises the structural integrity of the entire component, leading to wasted raw materials and reduced production efficiency.
If your production line is facing cracking issues, a systematic approach is required to identify the culprit. Here is how to diagnose and solve the problem effectively.
Cracking often begins with the state of the raw material before it even enters the mold.
Moisture Content: Materials like PC, PA (Nylon), or PBT are hygroscopic. If not dried properly, the water molecules cause "hydrolysis" at high temperatures, weakening the molecular chains and making the part brittle.
Solution: Ensure strict adherence to drying temperatures and times using a desiccant dryer.
Regrind Ratio: Excessive use of recycled material reduces the average molecular weight, significantly lowering the material's toughness.
Solution: Keep regrind ratios below 20% for critical structural parts.
The geometry of a drawn part naturally creates areas of high stress. If the mold design is too "sharp," cracking is inevitable.
Radius (R-Angle) Optimization: Sharp corners are the most common sites for stress concentration.
Solution: Increase the radius at all corners and transitions. A larger R-angle allows for a smoother flow and better stress distribution.
Draft Angles: If the draft angle is too small, the part will experience excessive friction and tension during ejection.
Solution: Increase the draft angle (ideally to 1.5° or more) to ensure the part slides off the core without mechanical strain.
Venting and Polishing: Poor venting can trap air, creating "gas burns" or weak spots.
Solution: Improve mold venting and ensure the core surface is highly polished to reduce demolding resistance.
Sometimes the mold is perfect, but the "how" of injection is causing the failure.
Injection Pressure and Speed: High injection pressure traps residual stress inside the part. When the part cools or is exposed to the environment, this stress "relaxes" by forming cracks.
Solution: Use a multi-stage injection profile—slow down the speed as the melt reaches the end of the cavity.
Mold Temperature: If the mold is too cold, the plastic freezes too quickly, creating massive internal tension.
Solution: Increase the mold temperature to allow the molecular chains more time to relax and orient themselves properly.
For extremely deep or complex drawn parts, "annealing" may be necessary.
Annealing Treatment: Placing parts in a temperature-controlled oven for several hours allows the internal stresses to dissipate without deforming the part.
Example: For PC shells, annealing at 100°C–120°C can significantly reduce post-molding cracking.
At Sunye Plastic, we use Moldflow Analysis during the design phase to predict high-stress regions before the steel is even cut. By simulating the flow and cooling of drawn parts, we can proactively adjust wall thicknesses and gate locations to ensure uniform pressure distribution.
Solving cracking is about balancing material properties, mechanical design, and thermal dynamics. When these three elements are aligned, you achieve a stable, low-scrap production cycle.
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