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Cracks are one of the injection molding defects that should be handled quickly. A small crack can affect strength, assembly, sealing, appearance, or long-term use. Sometimes the part looks fine after molding, but cracks during screw assembly, snap-fit installation, storage, or customer use.
I usually do not suggest changing process settings right away. A crack defect in injection molding can come from material, mold design, process conditions, ejection, or assembly stress. The faster way is to check when the crack appears, where it starts, and what condition triggers it.
What Are Cracks in Injection Molded Parts
A crack in injection molding is a visible or hidden fracture in the molded plastic part. It may appear as a fine surface line, edge break, white stress mark, deep split, or complete part failure.
Cracking is different from normal flow marks or surface lines. If the plastic has already fractured, the part has lost local strength. Crazing and stress whitening may appear before full cracking, especially in brittle or highly stressed parts.
Common crack locations include:
- gate area
- weld line area
- screw bosses
- snap-fits
- thin walls near load points
- ejector pin marks
- insert or press-fit areas
- sharp internal corners
Among common injection molding defects and solutions, cracking needs deeper review because it often involves more than one cause.
First Check When and Where the Crack Appears
Before making changes, identify the crack timing. This step can save a lot of trial-and-error work.
| Crack timing | Likely direction to check |
| Before ejection | High internal stress, material degradation, weak weld line, poor cooling, overpacking |
| During ejection | Poor draft, undercut, part sticking, ejector layout, rough core surface |
| During assembly | Screw torque, snap-fit strain, press-fit load, weld line weakness, tight tolerance |
| After storage or use | Residual stress, chemical exposure, environmental stress cracking, low-temperature brittleness |
The crack location also gives useful clues. Cracks near the gate often point to molded-in stress or poor gate design. Cracks near weld lines may come from weak flow-front bonding or poor venting. Cracks around bosses and snap-fits usually relate to part design or assembly force.
I find this timing check especially useful during trial molding. If the crack is already visible before ejection, the cause is usually different from a part that only cracks after screw assembly.
Main Causes of Cracks in Injection Molding
Cracking is usually caused by stress, weakness, or material damage. In real production, several factors may appear together.
| Cause | What usually happens |
| Molded-in residual stress | The part is packed or cooled under stress, then cracks during ejection, assembly, or use |
| Poor gate location | High local stress or strong flow orientation appears near the gate |
| Weak weld lines | Two flow fronts meet poorly and create a weak area |
| Sharp corners | Stress concentrates at rib roots, boss bases, snap-fits, or internal corners |
| Uneven wall thickness | Uneven cooling and shrinkage create internal stress |
| Ejection stress | The part sticks to the mold or ejector force is not balanced |
| Material degradation | Moisture, overheating, long residence time, or contamination reduces toughness |
| Assembly stress | Screws, clips, inserts, or mating parts apply too much load |
One point is worth stressing: cracks are not always caused by poor molding operation. Many cracks are already built into the part design before the mold is made.
How to Fix Cracks Caused by Processing Conditions
If the crack comes from process-related internal stress, process adjustment can help. The goal is to reduce stress inside the molded part while keeping dimensions and appearance acceptable.
Start with these checks:
- Reduce excessive injection pressure if the part is being filled too aggressively
- Review holding pressure and holding time to avoid overpacking
- Check melt temperature to avoid poor flow or material degradation
- Check mold temperature and cooling balance
- Extend cooling time if the part is too weak during ejection
- Confirm drying conditions for moisture-sensitive materials
- Check regrind ratio and material contamination
- Compare part weight stability during production
Do not only reduce pressure blindly. If packing becomes too low, you may solve the crack but create sink marks, voids, or dimensional problems. A stable process needs balance.
Moisture control is also important. Materials such as PA, PC, PBT, PET, and some blends can degrade if they are not dried properly. Poor drying may cause brittleness, silver streaks, bubbles, or reduced strength. Drying should follow the resin supplier’s datasheet.
How Mold and Part Design Affect Cracking
Part design and mold design have a direct effect on cracking. For plastic injection molding services, this is where DFM review becomes valuable.
Gate location and weld lines
Gate position controls how plastic flows into the cavity. A poor gate location can create:
- high local stress
- weak weld lines
- uneven shrinkage
- visible flow marks
- weak areas near bosses or snap-fits
Weld lines should be kept away from high-stress areas when possible. If a weld line must remain in the part, venting, melt temperature, and filling conditions should be checked carefully.
Sharp corners and wall thickness
Sharp corners are common crack starters. Internal corners, rib roots, boss bases, and snap-fit roots should have proper radii. The correct radius depends on material, wall thickness, part size, and function.
Uneven wall thickness is another common issue. Thick sections cool slower than thin sections, which creates uneven shrinkage and internal stress.
A better design usually uses:
- balanced wall thickness
- gradual thickness transitions
- ribs instead of thick solid sections
- proper radius at stress areas
- enough draft for ejection
Ejection and draft
If a part sticks to the mold, the ejector system may force the part out unevenly. This can create cracks or stress whitening.
A plastic mould manufacturer should review:
- draft angle
- ejector pin size and position
- undercuts
- lifter and slider movement
- core surface finish
- rib depth and texture
- part deformation during ejection
If cracking happens during ejection, changing injection pressure may not solve the real problem. The mold structure needs to be checked.
Why Parts Crack During Assembly and How to Prevent It
Some molded parts pass inspection after molding but crack during assembly. I often see this around screw bosses, snap-fits, press-fit areas, inserts, and weld lines.
Screw boss cracking
Screw bosses may crack when the boss wall is too thin, too thick, poorly supported, or matched with the wrong screw. High torque can split the boss. A weld line through the boss makes the risk higher.
Possible fixes include:
- adjust boss wall thickness
- add support ribs
- change screw type or screw size
- control assembly torque
- move weld lines away from the boss
- use a tougher material grade if needed
Snap-fit cracking
Snap-fits crack when the strain is too high for the material. A snap arm that is too short or too thick may not flex enough during assembly.
Check the snap-fit design together with:
- material strain limit
- snap arm length
- root radius
- wall thickness
- gate location
- flow direction
- weld line position
Press-fit and insert area cracking
Press-fit parts and insert areas need enough material support. If the mating part is oversized or tolerance is too tight, the plastic may split during assembly.
For insert molding, metal and plastic shrink and expand differently. Insert design, material selection, wall thickness, and molding conditions should be reviewed early.
A part should be tested under real assembly conditions before mass production. Visual inspection alone is not enough.
Troubleshooting Checklist and Mold Repair Review
When cracks keep returning, use a checklist instead of adjusting the machine randomly.
| Check item | What to look for |
| Crack location | Gate, weld line, boss, snap-fit, insert, ejector mark, corner |
| Crack timing | Before ejection, during ejection, during assembly, after storage |
| Material | Grade, drying, moisture, regrind, contamination, color masterbatch |
| Process | Injection speed, pressure, holding pressure, melt temperature, mold temperature, cooling time |
| Mold | Gate wear, venting, cooling, ejector pins, sliders, lifters, undercuts |
| Design | Wall thickness, radius, boss design, snap-fit strain, assembly load |
| Assembly | Screw torque, press-fit force, ultrasonic welding, clip installation |
Mold repair may be needed if the issue comes from worn or damaged tooling. Common mold-related checks include gate wear, poor venting, damaged cavity surfaces, ejector pin wear, slider or lifter issues, and cooling blockage.
If the crack comes from poor part design or a bad gate position, mold modification may be needed. If the crack comes from wrong material selection, process adjustment will only give temporary improvement.
This is where experienced plastic injection molding companies and plastic mold suppliers can help. A good supplier should not only mold parts. They should help identify whether the cracking issue belongs to material, process, mold, or product design.
FAQs About Cracks in Injection Molding
Can a crack in plastic be repaired?
A crack in plastic can sometimes be repaired for temporary use, depending on the material and part function. Possible methods include adhesive bonding, plastic welding, solvent bonding, or mechanical reinforcement. For production parts, repair is usually not the best answer. The root cause should be fixed so new parts do not crack again.
Conclusion
Cracking means the plastic part has stress, weakness, or material damage somewhere. To fix a crack defect in injection molding, do not rely on one machine adjustment. Check the timing, location, material, process, mold design, and assembly conditions together.
If you need plastic injection molding services for parts with bosses, snap-fits, inserts, tight tolerances, or strict assembly requirements, HingTung can support DFM review, mold design, trial molding, and production defect analysis. As a plastic mold manufacturer, we help customers find cracking risks before mass production, not after large batches of parts have already failed.