What Is Cracking in Injection Molding?
When a part is physically fractured or broken, you’re experiencing cracking. Cracks usually occur due to post-molding conditions, but when troubleshooting cracking, you want to know exactly when the cracking is happening as it will help you efficiently address potential issues and ultimately solve the problem.
In addition to post-molding conditions, cracking can be caused by a number of process settings, including second-stage pressure, melt temperature, residence time in barrel, ejection, part removal, operator error, mold breakaway and gate seal.
|High second-stage pressure||Undercuts||Overheating||Moisture content|
|Melt temperature||Sharp corners||Lack of process control||Contamination|
|Residence time in barrel||Lack of draft||Regrind|
|Ejection||Lifter or slide corners|
|Mold breakaway speed|
Table 24.1 Cracking Troubleshooting Chart, found in Injection Molding Advanced Troubleshooting Guide: The 4M Approach (p. 224)
How to Eliminate Cracking in Injection Molding
While cracks often occur post-mold, identifying the underlying issue is extremely important in being able to prevent cracking from happening. Check the part design—are there areas with sharp corners that act as stress risers and initiate cracks? If so, adding more radius to those areas can alleviate the stress and thus the cracks.
During the molding process, you could be facing high second-stage pressure, causing the material to stick which can lead to cracks when the mold opens or during ejection. A high melt temperature or improper residence time in the barrel could degrade the part and make it susceptible for cracking during ejection.
Since cracking usually occurs during ejection, it’s important to determine if the ejection itself is causing the issue or if it’s another issue that leaves the part vulnerable to ejection. Stop the mold process prior to ejection and look for cracks. If you see them, you know you have another issue. If there are no cracks, eject the part and check again. If you see cracks this time, you know there’s an ejection issue.
Nanoplas mold-release sprays are invaluable when it comes to ejecting parts and can be a tremendous help if you’re experiencing cracking.
If the part shows no cracks after ejection, it could be a part-removal issue or even an operator error. Something as simple as an operator handling the part could lead to cracking, which generally means there’s a design flaw that needs to be addressed. Simple handling of a part shouldn’t lead to cracking.
Cracking is a troublesome issue, but it can give you a lot of necessary information as to what needs to be improved to eradicate the problem.
Want to learn more? Check out these other great articles that solve common injection mold issues:
- Fix Ejector Pin Marks in Your Injection Mold
- Eliminate Parts Sticking On Ejectors
- Injection Molding Buildup: Causes and Solutions
- How to Eliminate Plastic Delamination
Source: Injection Molding Advanced Troubleshooting Guide: The 4M Approach by Randy Kerkstra and Steve Brammer.