English
How to Fix a Crack Defect in Injection Molding?
Created on 12.17
Cracks in injection molding are one of the most common and frustrating defects that plastics manufacturers face every day. They destroy appearance, weaken parts, and drive up scrap rates. The good news? Almost every crack in injection molding can be eliminated once you understand the real causes and apply the right fixes.
This practical guide focuses on solving crack defects in injection molding—why they happen and exactly how to stop them for good.
Top 6 Causes of Injection Molding Cracks (and How to Eliminate Each One)
To really fix cracks, you need to figure out what's causing them. Here's what usually leads to cracks, based on what people in the business know:
1. Too Much Stress Stuck Inside
One of the main reasons for cracks is stress that gets trapped inside the plastic as it cools down unevenly or too quickly in the mold. More pressure from the machine means more of this stress. Also, if the melted plastic is way hotter than the mold, or if you don't let it cool long enough, that makes it worse.
For example, if you cool things down too fast, the plastic bits don't have time to settle, so they pull against each other inside, which can split the surface.
Solutions:
- Improve the Mold: Use entrances that let the plastic flow easily without losing pressure, like direct entrances that can handle high pressure. Or switch to several smaller entrances. If you use entrances on the side, make them easy to remove after molding so they don't cause stress.
- Adjust Settings: Lower the pressure to reduce stress. Turn up the heat on the machine and mold to get the plastic and mold closer in temperature. Cool things down slowly to let the plastic settle.
2. Pushing Too Hard from the Outside
Sometimes, the way you pop the plastic out of the mold can make cracks. If the pins that push the plastic out aren't in the right spot, or if there aren't enough of them, or if the plastic is hard to get out, that can create pressure points. Also, if the mold isn't angled right, it can scrape the plastic as it comes out, making lines that turn into cracks.
If the system for pushing parts out is badly planned, especially for bendy shapes with extra bits, it just makes the problem bigger.
Solutions:
- Fine-Tune the Ejector: Check the ejector system. Place pins where there's a lot of resistance, like near bumps or ribs. If you need more pins, add them, but make them smaller to spread out the force.
- Angle It Right: Make sure the mold has enough of an angle so the plastic comes out without damage, which stops scratches that can turn into cracks.
3. Different Materials Expanding at Different Rates
If you use metal pieces inside the plastic, problems can happen because the metal and plastic grow and shrink differently when the temperature changes. This is really obvious at the start when the metal is colder. When the hot plastic goes around the cold metal, it cools unevenly, which causes stress where they meet.
Mixing the wrong plastics, like polyethylene and polypropylene, makes this worse because they don't play well together.
Solutions:
- Warm It Up: Heat the metal pieces before molding, especially when starting, to even out the temperatures and cut down on stress.
- Pick the Right Metals and Plastics: Use metal that grows and shrinks at about the same rate as the plastic. Go for plastics that are strong. If you use weaker plastics, make the plastic thicker around the metal to give it more support.
4. Picking the Wrong Stuff or Using Dirty Materials
Using the wrong plastic or stuff that's not clean can cause cracks. Plastics that are thin and loose don't crack as much, but if you mix plastics that shouldn't be mixed (like the polyethylene and polypropylene mentioned earlier), you're asking for trouble. Using too much mold release agent is like adding dirt, giving cracks a place to start.
If your raw materials aren't pure or if you're using recycled plastic that's not consistent, that also ups the chances of cracks.
Solutions:
- Right Plastic for the Job: Choose plastics that don't crack easily. Don't mix plastics that don't go together.
- Keep It Clean: Use just enough mold release agent to avoid causing cracks. Make sure your materials are clean.
5. Bad Design
If the shape of your part has sharp corners or notches, those spots will have more stress, which can lead to cracks. If you don't round off those edges, forces can build up as the plastic cools and when people use the product, leading to cracks.
This is super important for parts that are thin or have to handle weight.
Solutions:
Round Things Off: Round off corners as much as possible. A good rule is to make the rounded edge about the same thickness as the wall of the part. This spreads out the stress and prevents cracks.
6. Mold Problems Showing Up on the Parts
If the mold has cracks or sharp bits, the plastic part will get those same marks. This often happens near the cooling pipes or spots that get worn down, causing the same cracks on many parts.
Solutions:
Keep an Eye on the Mold: Check the mold for cracks often. If you see problems showing up on the parts, smooth out the mold with tools.
Stopping Cracks Before They Start
The best way to deal with cracks is to stop them from happening. Keep up with injection mold maintenance, test materials to see if they work well together, and use software to see where stress might build up. Train your workers to use the right settings and keep an eye on the process.
By doing these things, you'll have fewer cracks and better products.
Conclusion
Cracks in injection molding can be fixed if you know what to do. From controlling stress to picking the right materials and designs, these tips can help you make things perfectly. If the cracks keep coming back, look closely at your setup and try these fixes. If you need more help, talk to a plastics expert or get some testing tools.
Remember, keeping an eye on product quality will not only solve the problems you have now but also make your factory better in the long run.
Leading provider of industrial machinery and comprehensive manufacturing solutions since 1955.
Quick Links
Quick Links
Our Services
Our Services
Contact Us
Contact Us
Unit 04-05 16/F The Broadway
No.54-62 Lockhart Rd Wanchai .HK
Av. Sunset #415, Punta Hermosa, Lima Peru
+86 18767169649
+51 994 484 114
© 2025 Sountec Machinery. All rights reserved.
English
WhatsApp