Injection Molding Defect Acceptance: What to Eliminate and What to Set a Limit On

At first article review you point at a mark on the show face. The supplier says: “That is normal for molding.”

Sometimes that is accurate. Sometimes it is only true because a design decision months earlier locked it in. That distinction decides whether you can demand a fix or must accept a limit. Categorize defects during tool qualification under the broader injection molding defects frame.

Categorize by Root Cause, Not Severity

Buyers often sort defects by severity: minor cosmetic, major cosmetic, functional. That helps sort an incoming box. It does not tell you what to do with the mold.

The useful question is origin: Is this defect the result of a decision, or the result of geometry?

A sink mark over a heavy boss is a decision. Someone designed the boss that thick. A weld line downstream of a hole is geometry. Plastic must flow around the hole and rejoin. Process tuning will not erase flow fronts rejoining. Both marks can look equally bad. Only the sink mark has an owner who can eliminate it.

Capable molders track defects with a target per item: close it or control it.

The Close vs. Control Split

Use this breakdown when reviewing T1 samples.

Defect	Target	Justification
Sink marks	Close it	Driven by local thickness. The design can be cored out.
Gloss or shine marks at wall steps	Close it	Caused by abrupt thickness changes. Blending the transition removes the mark.
Gas traps, short shots	Close it	Predictable in simulation. Venting and tool geometry correct them.
Gate blush	Close it	Gate type, size, and position are configurable choices.
Flow marks	Close it	Driven by gate location and injection profile.
Ejector whitening, pin marks	Close it	A result of tool layout and ejection balance, not physics.
Drag marks on shut-off faces	Close it	Indicates insufficient shut-off draft angle.
Flash	Close it	A failure of tool fit, clamp tonnage, or process control.
Parting lines and witness lines	Set a limit	Every mold has them. The step height and location are negotiated.
Weld lines	Set a limit	Unavoidable around holes and multi-gate fills. Location and severity are controlled.
Warpage	Set a limit	Never zero. The question is how much is acceptable and whether assembly corrects it.
Fiber float on glass-filled resin	Set a limit	Inherent to the material. A hot mold reduces it but does not eliminate it.
Gloss level consistency	Set a limit	Must be matched across mating parts, not measured against an absolute ideal.

This list is not absolute. A weld line in a highly loaded structural corner is not a cosmetic negotiation. It is a structural failure and must be moved. The split still changes the negotiation. “This must be closed” versus “we need a written limit” shows you understand the mechanics.

Defects Locked in at Design Freeze

A third group can neither be closed nor tuned once the tool is cut. Those need part revisions or expensive steel work.

Sunburst under a hot drop: If a hot runner drop feeds directly onto an appearance surface, the thermal halo is permanent. Heat location drives it, not process settings. Secondary weld lines: When a flow front stalls, restarts, and leaves a raised ridge, it stands proud. Texture does not hide it. Paint follows it. Stress marks and pressure lines: By the time these appear, the fill pattern that made them is cut into the steel. Ghosting after texturing: A shadow that appears only after the tool is grained, often mirroring a feature on the back face. Late by definition.

For these, your bargaining power ends at design freeze. That is why moldflow analysis matters. The design review where an analyst flags a gas trap is the last meeting where fixing it costs nothing.

The Limits of Texture and Paint

A common claim is that mold texture will cover cosmetic defects.

Coarse grains hide flow marks and mild surface variation. Fine grains, bead-blast, and low-gloss finishes hide almost nothing. They scatter light evenly, so any localized defect stands out.

Texturing can also make some defects worse. Gate blush can wipe out grain locally, leaving a shiny patch in matte texture. Anything with height (a secondary weld line or parting-line mismatch) stays raised after graining.

If a supplier says a mark will disappear under the grain, ask which grain and whether they have masked that exact defect before. For appearance-critical parts, request smooth, pre-texture samples first. Flow marks and weld lines that texture might partly obscure are fully visible on a polished surface.

Documenting Acceptance Criteria

Settle this on paper before tooling, not as a fight at T1.

Define close/control targets early. Fill the defect list with realistic targets in the RFQ. Map unavoidable marks. Agree in writing on locations of parting lines, weld lines, gates, and ejector pins. Require a marked-up 3D model for approval before cutting steel. Establish a surface hierarchy. Define Class A, secondary, and non-appearance assembly faces. Without that hierarchy, every mark on every surface becomes its own negotiation.

When you separate what physics dictates from what design caused, you spend escalation capital on defects that actually threaten the product.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, diagnose production problems remotely, or certify suppliers. Whether a specific mark can be closed, tuned, or must be accepted depends on that part, tool, resin, and process. Confirm with your supplier against physical samples.