A container of parts arrives. The surface has dimples where it should be flat, one corner is lifted, and there is a faint line across the boss where two flow fronts met. The molder says the tool is fine. Your engineer says the tool is not. Nobody has data, and the email thread gets longer every day.
Here is the part nobody says out loud at the start of that argument: most defects are decided long before the molding machine runs. They are set by wall thickness, gate location, cooling layout, and draft — the things fixed in the design and the tool. The press can hide them for a while. It cannot remove them.
Below are the eight defects that account for most of these conversations, what actually causes each one, and where to fix it — in the part, in the tool, or in the process. If you are still preparing data for a supplier, the same principles apply when you prepare a CAD file for quoting.
Key takeaways
- Nearly every defect traces to one of three root causes: uneven shrinkage, restricted flow, or trapped gas.
- Sink marks, warpage, and weld lines are usually design-level issues — fix them in the part and the tool, not by adjusting the press.
- Short shots, flash, burn marks, and jetting are usually process and tool-condition issues — but a design that leaves no room for them makes the process window tiny.
- Silver streaks are almost always moisture. Dry the resin to the supplier’s specification before you look anywhere else.
- Fix the cause, not the symptom. Raising pack pressure to hide a sink mark usually moves the problem to warpage.
The eight defects at a glance
| Defect | Primary root cause | First place to look |
|---|---|---|
| Sink marks | Uneven shrinkage from thick sections | Part design — wall and rib thickness |
| Warpage | Uneven cooling and directional shrinkage | Cooling layout and gate position |
| Short shots | Flow restriction or insufficient pressure | Wall thickness, gate size, venting |
| Flash | Parting line opened by pressure | Clamp tonnage, mold condition |
| Weld lines | Flow fronts meeting cold or misaligned | Gate location and count |
| Burn marks | Trapped gas igniting under compression | Venting and injection speed |
| Jetting / flow marks | Melt entering the cavity as a free stream | Gate size and injection profile |
| Silver streaks | Moisture or volatile gas in the melt | Drying, then shear and contamination |
1. Sink marks
What it looks like: a shallow depression on a surface, almost always sitting directly above a rib, boss, or any locally thick section.
Root cause: the thick region cools and shrinks after the surrounding thin wall has already frozen. The surface gets pulled inward by the volume loss underneath it.
How to fix it: in the part, thin the feature. Keep rib and boss walls at roughly 50–60% of the nominal wall so they cool at the same rate as the surrounding surface, and core out thick bosses from the back. In the process, a slightly longer hold time can help — but only after the geometry is right. Using pack pressure to force a thick section flat just transfers stress into the part and tends to reappear as warpage. The design rules behind this are covered in detail in our guide to wall thickness guidelines.
2. Warpage
What it looks like: the part is not flat, or twists out of tolerance after it cools. It may pass inspection hot and fail after 24 hours.
Root cause: uneven cooling between the two sides of the part, and shrinkage that differs by direction — especially in fiber-filled materials, where glass fibers align with the flow and shrink less along their length than across it.
How to fix it: balance the cooling circuit so both sides of the tool remove heat at similar rates. Move the gate if the flow path forces a long, narrow fill. For fiber-filled resins, gate position effectively decides fiber orientation, so it decides warp — accept that and design the gate around it rather than fighting it in the press. Material selection sets how much this matters in the first place; the shrinkage and fiber behavior of each family is covered in choosing the right plastic material.
3. Short shots
What it looks like: the cavity is not completely filled. Edges, thin ribs, or the far end of the flow path are missing material.
Root cause: the melt froze before it reached the end. That comes from a wall that is too thin, a gate that is too small, low melt or mold temperature, insufficient injection pressure or speed, or gas trapped in the last place to fill.
How to fix it: check venting first — a venting problem looks exactly like a pressure problem. Then look at wall thickness and gate size. If the part has a section below roughly 0.8 mm in an engineering resin, the process window will always be narrow, and chasing pressures upward leads straight to flash.
4. Flash
What it looks like: thin fins of material along the parting line, at ejector pin locations, or around inserts.
Root cause: the cavity pressure exceeds what the clamp can hold shut, or the parting surfaces no longer seal. Excessive melt temperature, over-packing, a worn parting line, or a mold that was simply undersized for the part do it.
How to fix it: verify clamp tonnage against the projected area and cavity pressure, then inspect the parting surfaces. Flash that appears only after months of production is usually tool wear, not a process change — and it is the most common early symptom of a mold that needs maintenance. If you are specifying a new tool right now, hardness and heat treatment decide how long that parting line holds — see how to choose the right mold steel grade.
5. Weld lines
What it looks like: a visible line, sometimes a faint color difference, where two flow fronts met and joined.
Root cause: the flows met after cooling below the temperature where they can knit properly. The joint is mechanically weaker than the surrounding material — often the weakest point in the part.
How to fix it: reduce the number of gates if possible, relocate gates so the meeting point sits in a low-stress area rather than at a corner or on a loaded rib, raise melt temperature, or add a flow leader to move the meeting point out of a critical region. If a weld line must land in a load-bearing area, say so at the design stage so it can be evaluated — not after the tool is cut.
6. Burn marks
What it looks like: dark brown or black discoloration, typically at the end of the flow path, on a rib, or at a blind corner.
Root cause: trapped air is compressed by the incoming melt until it ignites — the diesel effect. The gas is not being evacuated, and injection speed is high enough to trap and compress it.
How to fix it: vent the location where the gas ends up, not the location that is convenient to machine. Add vents at the last-fill point and at weld-line positions, open existing vents, and reduce injection speed through the section that traps air. Burn marks appearing on a mature tool often mean a vent has been blocked by mold release spray or flash.
7. Jetting and flow marks
What it looks like: a snake-like or wavy pattern leading away from the gate, sometimes with a dull or rippled surface.
Root cause: the melt enters the cavity as a free stream instead of spreading from the gate, then freezes where it lands before the rest of the flow reaches it. Small gates combined with high injection speed cause most cases.
How to fix it: enlarge the gate or change its type so the melt spreads immediately, aim the gate at a wall or pin rather than into open space, and profile the injection speed so the first part of the shot is slower. Gate design is one of the decisions that belongs to the part designer — if you leave gate location entirely to the mold maker, expect to discover its consequences in the first sample.
8. Silver streaks
What it looks like: silvery or mottled streaks on the surface, often fanning from the gate.
Root cause: moisture in the resin, or volatiles released by excessive shear or contamination. Hygroscopic materials — polycarbonate, nylon, PET, ABS in humid conditions — absorb water from the air, and that water turns to steam inside the barrel.
How to fix it: dry the material to the supplier’s specification and verify it, rather than assuming the dryer is doing its job. Typical settings for polycarbonate sit around 120 °C for three to four hours, and nylon typically needs 80–100 °C for about four hours — but always confirm against your specific grade’s data sheet. If the streaks persist on properly dried material, check for shear and contamination next.
Why most of these come back to the design stage
Look back at the fixes: they are dominated by wall thickness, rib geometry, gate location, cooling layout, and venting. Four of those are frozen the moment the tool is cut, and one — cooling — is the hardest thing to change later.
That is why a structured DFM review before tooling catches more defects than any amount of press-side troubleshooting. Draft, wall transitions, undercuts, gate position, and vent locations are all visible in the CAD data before steel is cut. Running a mold flow analysis on the design moves the conversation from “why is it warping” to “here is where we expect the weld line to land, and here is why we accepted it.”
The checklist before you blame the molder
- Is the material dried and verified to the data sheet specification?
- Is the venting clear at the last-fill location?
- Are the machine settings documented against a known-good setup sheet?
- Is the defect on the first shots as well as the current ones — or did it appear after a maintenance event?
- Do you have the first article inspection data from when the tool was approved?
If the answers are yes and the defect is still there, it is a tool or design issue. If the defect was there on the first shots and got approved anyway, it is neither a process problem nor a molder problem — it is a specification problem, and it is worth fixing before the next mold is built.
Frequently asked questions
Can a defect be fixed without modifying the mold?
Sometimes. Warpage, short shots, flash, and burn marks can often be improved through process changes or venting. Sink marks from thick geometry and weld lines landing in a load-bearing area usually cannot.
Is warpage in glass-filled parts avoidable?
It can be predicted and managed, but rarely eliminated entirely. Gate position determines fiber orientation, which determines warp. Design the gate and the tolerance around that reality.
How do I tell a moisture problem from a shear problem?
Moisture streaks usually appear even at low injection speeds and often follow drying failures; shear-induced splay tends to intensify with higher speed and higher back pressure. Dry the resin first — that removes the most common cause.
Fix the cause, not the last symptom
If you are chasing a defect right now, send us the part data, the material grade, and a photo of the defect with the gate location marked. We will tell you whether it is a design issue, a tool issue, or a process issue — and what to change first. You can send it to our engineering team here.
Author: Jack Qiao works with the engineering team at Kysen Mold, a Shanghai-based injection mold manufacturer serving overseas OEMs.
Website: www.kysenmold.com
