Injection Molding Defects: Causes and Fixes for Sink Marks, Warpage, Flash and More

Most injection molding defects have a short list of causes. The skill is working out whether the fix belongs in the process, the mold or the part design, because the cost of each is very different.

Quick answer

Start with the lowest-cost fix and work outward: adjust the process (temperature, pressure, time, speed), then correct the material handling (drying, regrind, color), then modify the tool (gates, venting, cooling), and only then change the part design (walls, ribs, radii). The table below maps common defects to their likely causes and fixes. These are general guidance, so the right answer for your part depends on the resin, tool and machine.

Key takeaways

• One defect can have several causes. Change one variable at a time.
• Moisture, venting and gate position explain a large share of problems.
• Defects that repeat after process tuning usually need a tool or design change.
• Prevent them in DFM: see the plastic part DFM checklist.

Defect guide

Defect	What it looks like	Common causes	Typical fixes
Sink marks	Depressions over thick sections, ribs or bosses	Thick wall, rib too thick, low packing, hot spot	Increase packing pressure and time, lower melt temperature, redesign ribs and bosses, move the gate
Warpage	Part bends or twists after ejection	Uneven wall, uneven cooling, high stress, resin shrinkage	Balance cooling, uniform walls, adjust packing, change resin or fill, add ribs
Flash	Thin excess plastic at parting lines	Clamp force too low, worn or poorly fitted tool, excessive pressure	Reduce injection pressure, check clamp, repair or re-fit the tool
Short shot	Cavity is not completely filled	Low shot size, low temperature, restricted gate or vent, thin wall	Raise melt temperature or speed, enlarge the gate, improve venting
Weld or knit lines	Visible line where flow fronts meet	Cold flow fronts, gate location, geometry	Raise melt and mold temperature, move the gate, change geometry
Burn marks	Dark or brown marks at the end of fill	Trapped air, poor venting, high injection speed	Improve venting, reduce speed, clean vents
Flow marks, jetting	Waves or snake-like streaks near the gate	High speed through a small gate, cold melt	Slow the first stage, enlarge the gate, raise mold temperature
Splay, silver streaks	Silver streaks along the flow	Moisture in resin, degradation, trapped air	Dry resin per datasheet, reduce residence time
Voids	Bubbles inside thick sections	Insufficient packing, shrinkage in thick areas	Increase packing, reduce wall thickness
Drag marks, ejection damage	Scratches on vertical walls	Insufficient draft, rough surface, uneven ejection	Add draft, polish, balance ejector pins
Delamination	Surface layers peel	Contamination, incompatible regrind or resin	Clean the line, remove contaminants, control regrind
Discoloration, black specks	Dark specks or color shifts	Degraded resin, dirty barrel, hot spots	Purge, lower temperature, shorten residence time

Where does the fix belong?

Fix type	Examples	Relative cost and time
Process	Temperature, pressure, time, speed	Lowest, quickest
Material handling	Drying, regrind, color	Low
Tool	Gate size or position, venting, cooling, polishing, repair	Medium, needs tool time
Part design	Walls, ribs, radii, draft, texture	Highest, may need new steel

A flash defect on a new tool can be a fit problem in the mold, not a pressure setting. A sink mark over a thick boss is more likely a design problem than a packing problem. Learn more about the cost and delay of late changes in mold lead time.

A simple troubleshooting sequence

1. Record the symptom with photos and where it occurs relative to the gate.
2. Check material: grade, drying, regrind percentage, color masterbatch.
3. Check the process window: melt and mold temperatures, injection speed, pack pressure and time, cooling time.
4. Look at the tool: gates, vents, parting line fit, cooling circuits, polish.
5. Review the design: wall thickness, ribs, radii and draft. Use the DFM checklist.
6. Change one variable at a time and record the effect.

Prevent defects before steel is cut

• Keep walls uniform and ribs thinner than the walls they join.
• Place gates at thick sections and away from cosmetic faces.
• Plan venting at the end of fill.
• Choose resin and fillers with shrinkage and warpage in mind: see plastic material selection.
• Decide runner and gate type with the part: read hot runner vs cold runner.
• Use mold-flow analysis for complex, thin or cosmetic parts. It is included in HC-Mold's mold project.

Cosmetic zones and acceptance

Not every surface needs the same standard. Agree zones on the drawing or a marked 3D model:

Zone	Example	Typical approach
A (visible, customer-facing)	Front face, logo area	Strictest: no sink, weld lines placed away, tight flash limit
B (visible in some use)	Side walls, handles	Moderate: minor marks acceptable
C (hidden)	Inside faces, under covers	Function first: cosmetic marks acceptable

When to stop tuning and change the tool

If a defect needs process settings at the edge of the resin's window, or returns whenever production runs at normal speed, it is a tool or design issue. Typical signs are flash that grows as the tool warms, a weld line that cannot move without moving the gate, or sink at a rib that is too thick. Record the evidence and agree the modification in writing. HC-Mold's mold project includes T0 trial and modification.

Quality control and acceptance

Agree acceptance criteria before the trial: what size of flash, what depth of sink and what weld-line visibility are acceptable on which faces. HC-Mold uses SPC monitoring and full batch inspection in production. See the inspection page, and the mold cost guide for what trials and modifications include: how mold cost is built up.

What to send for a defect review

Photos of the defect, the STEP file, resin grade and color, shot weight if known, process settings, and where in the tool the defect occurs. Our engineers reply within 12 hours. See the injection molding page.

FAQ

What causes sink marks?

Thick sections, ribs or bosses that are too thick, and low packing pressure. Fixes include more packing, a lower melt temperature and redesigning the thick feature.

Why do molded parts warp?

Uneven wall thickness, uneven cooling and residual stress. Balance cooling, make walls uniform and consider a different resin or fill.

How do I stop flash?

Check clamp force and injection pressure first, then the fit of the tool at the parting line. Worn or poorly fitted tools need repair.

Is a defect a process problem or a mold problem?

If it improves with process changes, it is a process issue. If it persists or returns, check the tool and then the design.

Can mold-flow analysis prevent defects?

It predicts filling, weld lines, warpage and gate effects before steel is cut. It is included in HC-Mold's mold project.

Have a molding defect to solve?

Send a photo, your STEP file and the resin. Engineers review the cause, whether it is a process, tool or design issue, and suggest fixes. We reply within 12 hours.

Request an engineering review