Views: 0 Author: Site Editor Publish Time: 2026-04-10 Origin: Site
Let's be honest: injection molding is a fantastic manufacturing process — when it works. When it doesn't, you're left staring at a pile of defective parts, wondering where it all went wrong.
The good news? Most injection molding defects are predictable, preventable, and fixable.
Whether you're molding thermoplastics, Liquid Silicone Rubber (LSR), or solid silicone, the same core issues tend to pop up. Here's your practical guide to identifying the most common defects — and exactly how to solve them.
Before we dive in, here's a secret: 80% of injection molding defects can be fixed by adjusting just four parameters:
Mold temperature
Melt temperature
Injection speed
Pack/hold pressure
If you can't solve a defect by tweaking these four, then start looking at your mold or material.
What it looks like: The part isn't fully formed. Edges are rounded, not sharp. Material stopped flowing before reaching the end of the cavity.
Common causes:
Low injection pressure or speed
Low melt or mold temperature
Poor mold venting
Undersized gate or runner
How to fix it:
Increase injection pressure and speed
Raise barrel and mold temperatures
Add venting or enlarge gates
Check that enough material is being fed
What it looks like: Thin excess material leaking out along the parting line or around ejector pins.
Common causes:
Insufficient clamping force
Worn or damaged mold mating surfaces
Excessive injection pressure
Melt temperature too high (material becomes too fluid)
How to fix it:
Increase clamp force
Repair mold surfaces (refit the tool)
Reduce injection pressure or melt temperature
For LSR (extremely fluid): clamp gap must be ≤0.002mm
What it looks like: Shallow depressions on the surface (sink marks) or internal bubbles (voids), typically in thick sections.
Common causes:
Insufficient pack/hold pressure or time
Uneven cooling
Excessively thick wall sections
How to fix it:
Increase hold pressure and duration
Reduce melt and mold temperatures
Design uniform wall thickness
Position gates at thickest sections
What it looks like: A visible line where two melt fronts meet — often around holes, inserts, or multi-gate parts.
Common causes:
Low melt or mold temperature
Poor venting at the meeting point
Multiple gates or inserts splitting the flow
How to fix it:
Increase melt and mold temperatures
Add venting at the weld line location
Adjust gate position to improve flow-front temperature
For LSR: higher injection speed improves weld strength
What it looks like: Silvery-white streaks on the surface (silver streaks) or yellow/brown burn marks (gas burns).
Common causes:
Wet resin (not properly dried)
Melt temperature too high (decomposition)
Trapped air due to poor venting
Excessive injection speed
How to fix it:
Pre-dry hygroscopic materials (nylon, ABS, PC, etc.)
Lower barrel temperature and screw speed
Add venting or reduce injection speed
Clean nozzle and hot runner of degraded material
What it looks like: A snake-like, squiggly flow mark originating from the gate.
Common causes:
Gate is too small or poorly positioned
Melt shoots freely into an open cavity instead of hitting a wall
How to fix it:
Use a fan or tab gate
Direct the gate at a cavity wall or obstacle
Reduce injection speed and use multi-stage injection
What it looks like: Alternating light and dark wavy bands on the surface.
Common causes:
Melt front temperature dropping unevenly
Fluctuating injection speed
Low mold temperature
How to fix it:
Raise mold and melt temperatures
Use fast, steady injection speed
Enlarge gates or switch to a hot runner system
What it looks like: White stress marks or actual punctures/cracks at ejector pin locations.
Common causes:
Insufficient ejection area
Inadequate draft angle
Incomplete cooling (part too soft)
Excessive ejection force
How to fix it:
Add more or larger ejector pins
Increase draft angle (typically 1°–3°)
Extend cooling time; lower mold temperature
Use mold release spray (especially for LSR and soft rubbers)
What it looks like: The part is bent, twisted, or curved — it doesn't sit flat or match the intended shape.
Common causes:
Non-uniform cooling
Anisotropic shrinkage (different shrinkage in different directions)
Uneven packing
Premature ejection
How to fix it:
Optimize cooling channel layout for uniform temperature
Adjust hold pressure and duration
Add ribs or adjust wall thickness
Lower melt and mold temperatures
Extend cooling time before ejection
What it looks like: Parts are consistently too large, too small, or specific features are out of spec.
Common causes:
Unstable packing/shrinkage control
Mold manufacturing error
Process parameter fluctuations
Change in raw material lot
How to fix it:
Stabilize hold pressure and switchover point
Verify cavity dimensions
Use a mold temperature controller
For crystalline plastics (POM, PA), strictly control cooling rate
Defect | Cause | Solution |
|---|---|---|
Uncured / Sticky surface | Wrong A/B mix ratio; mold too cold | Check metering pumps; raise mold temp (150–200°C) |
Bubbles / Internal voids | Too fast injection traps air; poor venting | Reduce injection speed; add vacuum venting |
Orange peel surface | Mold too hot; cure too fast | Lower mold temperature; extend cure time |
Sticking to mold | Insufficient draft; no release agent | Increase draft angle; use LSR-specific mold release |
Defect | Cause | Solution |
|---|---|---|
Short fill | Underweight charge | Use preforms or automated weighing |
Bubbles | Air trapped in compound; cure too fast | Bump (vent) the press; lower cure temperature |
Non-uniform thickness | Charge placed off-center | Use locating fixtures or switch to injection |
Tearing | Forced demolding | Slightly under-cure; use mold release |
When you encounter a defect, follow this sequence — it works every time:
Mold first → Check venting, gates, ejectors, and mating surfaces.
Process next → Adjust temperature (melt/mold), pressure (injection/hold), speed, and time.
Material check → Is it dry? Consistent lot? Contaminated?
Machine condition → Clamp force? Barrel temp accuracy? Non-return valve?
Environment → Humidity (for hygroscopic materials), room temperature swings.
Defect | Primary Parameter Adjustment | Secondary |
|---|---|---|
Short shot | ↑ Injection pressure/speed | ↑ Melt/mold temp |
Flash | ↑ Clamp force / ↓ Injection pressure | ↓ Melt temp |
Sink marks | ↑ Hold pressure & time | ↓ Melt temp / ↑ Cooling time |
Weld lines | ↑ Melt/mold temp / ↑ Speed | Add venting |
Silver streaks | ↓ Melt temp / Dry resin | Add venting |
Warpage | Optimize cooling / ↓ Hold pressure | ↓ Melt temp |
Oversized parts | ↓ Hold pressure / ↑ Cooling time | Modify mold (remove steel) |
Undersized parts | ↑ Hold pressure / ↑ Melt temp | Modify mold (add steel) |
Defects in injection molding are frustrating — but almost never mysterious. Most follow predictable patterns with straightforward fixes.
Start with the four parameters: mold temperature, melt temperature, injection speed, and hold pressure. If you've tuned those and still see issues, then — and only then — dig into your mold design or material handling.
And if you're working with LSR or solid silicone, remember: they have their own quirks. LSR hates poor venting and loves precise A/B mixing. Solid silicone hates off-center charges and loves generous draft angles.
Master these basics, and you'll turn scrap bins into shipping boxes.
