Injection Mold Troubleshooting: Solving 12 Common Problems

author
4th May 2026

Introduction

Look, we’ve all stood by the press watching a bin fill with scrap and felt that exact same frustration. Whether you’re fighting a short shot that won’t fill or a burn mark that just won’t quit, troubleshooting is where the real work happens.

You can’t just cross your fingers and hope the machine fixes itself; you have to figure out exactly why the physics of the mold are fighting you today.

But here is the secret—most defects aren’t “mysteries.” They are the result of physics being ignored.

To save your uptime, you need to stop guessing and start diagnosing.

Here is how to tackle the most common headaches on the production floor.

1. The Frustration of Short Shots

A short shot is exactly what it sounds like: a cavity that just won’t fill. If you’re looking at a partial part, you’re essentially losing the race against the “freeze.”

What’s happening?

When you’re staring at an incomplete part, you’re basically losing a race against the material’s freeze point.

Most of the time, the melt is behaving like cold molasses, or your injection pressure is hitting a wall of resistance—it just can’t climb.

But don’t overlook the air—if it’s trapped in a dead-end pocket with no way out, there’s simply no room for the plastic to enter, period.

The Fix

Before you start blindly cranking up the pressure, take a hard look at your heats.

If the resin is fluid enough, go hunt for gunked-up or blocked vents.

If you’re still coming up short after that, you’re likely looking at a design flaw—that gate might just be way too tight for the resin you’re trying to push through it.

2. Dark Scorch Marks (The Diesel Effect)

Those ugly black streaks at the end of the fill path are almost always “burn marks.”

In the shop, we call this the diesel effect because you’re essentially creating a miniature combustion engine inside the tool.

The Culprit

Air is getting trapped, compressed, and heated to the point where it literally chars the plastic.

This usually happens because the injection speed is too aggressive or the venting is nonexistent.

The Fix

Slow down the injection speed during the final stage of the fill to let the air escape.

If that doesn’t work, you need more vents (or deeper ones) exactly where that burn is appearing.

3. Sink Marks: The Nemesis of Thick Parts

Sink marks are those irritating dips that always seem to haunt the thickest areas of your part, like ribs or bosses.

It really boils down to a lack of “feed.”

If you don’t shove enough extra plastic into the cavity during the packing stage, the molten core will actually tug the surface inward as it cools and shrinks.

You’re essentially watching the part cave in on itself because it didn’t have enough material to fill the volumetric void left by the cooling process.

The Fix

You need more “pack and hold.”

Increase your holding pressure or extend the hold time.

Also, take a look at your cooling—if the core is staying too hot, the sink will never go away.

Long term? Redesign the part with more uniform wall thicknesses.

4. Warpage: The “Potato Chip” Effect

There is nothing worse than a part that looks perfect in the mold but curls like a potato chip once it hits the bin.

The Root Cause

This is almost always a result of “differential shrinkage.”

If one side of the part cools faster than the other, the internal stresses will pull it out of shape.

It can also happen if your packing pressure is so high that you’re locking stress into the molecular structure.

The Fix

Balance your cooling.

Check the inlet/outlet temps on both halves of the mold.

You want the part to cool as uniformly as possible.

If the part is still twisting, try backing off the pack pressure or adjusting the mold temperature to relieve that internal tension.

5. Flash: Plastic Looking for an Exit

Flash is that paper-thin “wing” of plastic leaking out at the parting line.

Flash is more than just a messy waste of resin—it’s a tool-killer.

When you see plastic blowing past the parting line, you’re usually looking at a power struggle between injection pressure and clamping force.

Most of the time, either the press doesn’t have the muscle to keep the halves shut, or your shut-off surfaces have finally been beaten out of alignment after a few thousand too many cycles.

Whatever the cause, it’s a red flag that you’re either overpowering the machine or your tool needs a serious regrind.