Injection Molding Defects: A Buyer's Guide to Causes and Fixes

When the first samples arrive and something looks wrong—a dimple on a smooth face, a faint line across a panel, an edge that won’t quite fill—the buyer’s first question usually isn’t “what is this defect called.” It’s “whose problem is this, and what do I do now?” Is it the design? The tool? The way it’s being run? The material? Without a way to think about that, it’s easy to either accept a bad part or blame the supplier for something the design baked in. If you’re holding a part with a mark you can’t name, the Defect Diagnostic Wizard narrows it to the likely patterns in three questions.

This guide is a buyer-facing tour of the common injection molding defects: what they look like, where their causes come from, and what you can actually do about each one. You won’t be adjusting machine settings yourself—that’s the molder’s job. But understanding defects well enough to have an informed conversation, evaluate whether a supplier has them under control, and recognize the issues your own design can prevent is squarely a buyer’s job. For the design side of prevention, this pairs with the plastic part design for manufacturing guide.

Why Buyers Should Understand Defects

You don’t run the press, so why learn this? Because defects sit exactly at the intersection of the three things a buyer controls or chooses: the design you hand over, the supplier you select, and the requirements you set. Many defects are designed in before a tool is ever cut. Others reveal whether a supplier has a disciplined, repeatable process or is fighting the part every shot. Reading a defect correctly tells you which lever to pull—revise the design, push the supplier, or adjust an unrealistic expectation.

It also changes the quality of your conversations. “The part has sink near the boss—can we look at the wall thickness there?” gets a different response than “the parts look bad, fix them.” The first shows you understand that sink near a thick boss is often a design-and-process interaction, not simple carelessness.

The Common Defects at a Glance

Most of the defects buyers encounter fall into a familiar set. Each has its own guide; this table is the map.

Defect	What it looks like	Most often points toward
Sink marks	Dimples or depressions over thick areas, ribs, or bosses	Wall thickness / cooling / packing
Warpage & shrinkage	The part bows, twists, or finishes out of dimension	Uneven cooling, geometry, material
Flash	Thin excess plastic along edges or the parting line	Tool fit / clamp / process
Weld (knit) lines	A faint line where two flow fronts met	Gate location, flow, temperature
Short shot	The cavity doesn’t fully fill; missing material	Fill / venting / gate / material flow
Splay / silver streaks	Silvery streaks radiating from the gate	Moisture / material / shear
Burn marks	Scorched discoloration, usually at fill ends	Trapped air / venting / speed

The Five Places a Cause Comes From

Experienced molders troubleshoot defects by working through the same five areas, in roughly this order. It’s a useful mental model for a buyer too, because it shows that “fixing a defect” is rarely one thing:

• Machine — Is the press the right size and in good order? Material running out, an undersized machine, or inconsistent equipment behavior can all show up as defects.
• Process — The settings: injection pressure and speed, hold pressure and time, cooling time, back pressure. A huge share of defects are process-tuned out by a capable molder.
• Temperature — Melt temperature (the barrel and nozzle) and mold temperature. Too hot, too cold, or uneven, and the part suffers in characteristic ways.
• Mold (tooling) — Gate size and location, venting, cooling layout, polish, and wear. Some defects are built into the tool and can’t be fully processed away.
• Material — The resin itself: whether it’s dried correctly, contaminated, the right grade, or too much regrind. Material problems often masquerade as process problems.

The reason this matters to a buyer: when a supplier says they “can’t get rid of” a defect, the honest follow-up is which of these five is the limit? If it’s process, a better molder may solve it. If it’s mold or design, it may need a tooling change or a part revision—and that’s a decision you’re part of.

What a Buyer Can Actually Do

Three levers, all upstream of the press:

1. Prevent what design can prevent. A large share of sink, warpage, weld-line, and short-shot problems trace back to geometry—uneven walls, thick sections, awkward gate-driven flow. Running a design review before you tool catches these while they’re cheap. The DFM guide and its companions on wall thickness and ribs, gate design, and the parting line cover the specifics.

2. Choose a supplier who controls their process. Defects are where process discipline shows. A molder who works to a documented, data-driven process—sometimes called scientific or decoupled molding—and who validates the tool through structured sampling is far less likely to ship inconsistent parts. The supplier capability checklist covers what to look for, and tooling and production covers the sampling and qualification stages where defects should surface and get resolved.

3. Set requirements that make defects visible early. Define your cosmetic surfaces and critical dimensions up front, in the RFQ, so the supplier knows where a defect is unacceptable and you both inspect against the same standard. A “minor” sink mark is a non-issue on a hidden boss and a reject on an A-surface—the spec is what decides.

When Defects Show Up—and What It Tells You

The best time to find defects is at first samples (often called T1), before the tool is hardened into production. Sampling exists precisely to surface these issues while changes are still practical. A defect that appears at T1 and is understood and resolved is the system working. A defect that appears intermittently in production—present on some shots, gone on others—is a different and more worrying signal: it usually points to a process that isn’t fully in control or a tool that’s marginal, and it’s worth taking seriously before it becomes a stream of mixed-quality parts.

This is an independent buyer resource, not a process-engineering manual or a substitute for your supplier’s expertise. Defect diagnosis depends on the specific part, tool, resin, and equipment, so use this to inform the conversation and your decisions—not to direct the molder’s process.

Buyer FAQs

What are the most common injection molding defects?

The ones buyers encounter most are sink marks (depressions over thick areas), warpage and excess shrinkage (the part distorts or finishes out of dimension), flash (thin excess plastic along the parting line), weld or knit lines (a line where two flow fronts met), short shots (incomplete fill), splay or silver streaks (usually moisture- or material-related), and burn marks (scorching from trapped air). Each has its own causes and its own guide in this section.

Are molding defects the supplier’s fault or the design’s?

It depends on the defect, and that’s exactly why understanding them matters. Many defects—sink over thick sections, weld lines from flow, warpage from uneven walls—are influenced or caused by the part design and can’t be fully processed away. Others reflect how well the supplier controls their process. Reading a defect correctly tells you whether to revise the design, work with the supplier, or adjust an expectation, rather than guessing.

Can injection molding defects be completely eliminated?

Some can be designed or processed out entirely; others can only be minimized or placed where they don’t matter. A weld line, for example, will form somewhere two flow fronts meet—the realistic goal is to move it off a critical face, not to make it vanish. Defining which surfaces and dimensions are critical lets you and your supplier focus effort where it counts instead of chasing perfection everywhere.

How do I prevent defects before tooling?

The highest-leverage step is a design-for-manufacturing review before you commit to a tool: uniform wall thickness, sensible gate location, adequate draft, and realistic tolerances prevent a large share of common defects. Pair that with choosing a supplier who runs a documented, controlled process and with a clear RFQ that defines cosmetic surfaces and critical dimensions, so problems surface at sampling rather than in production.

What is the difference between a defect at first samples and one in production?

A defect at first samples (T1) is the qualification process doing its job—it’s the expected time to find and resolve issues while tooling changes are still practical. A defect that appears intermittently in production, present on some parts and not others, is more concerning: it usually signals a process that isn’t fully in control or a marginal tool, and it warrants attention before it produces a stream of inconsistent parts.