Injection Molding Defects & Troubleshooting

When a part comes back with a dimple, a line, or an edge that won't fill, the buyer's real question is whose problem it is and what to do next, design, tool, process, or material. These injection molding troubleshooting guides explain the common defects in those terms.

You won't be adjusting the press yourself. But understanding defects well enough to have an informed conversation, evaluate whether a supplier controls their process, and catch the issues your own design can prevent is squarely a buyer's job.

Where a defect's cause usually comes from
  • Machine, press size and condition
  • Process, pressure, speed, hold, cooling time
  • Temperature, melt and mold temperature
  • Mold, gate, venting, cooling, polish, wear
  • Material, drying, grade, contamination, regrind
  • And upstream of all of it: the part design and the requirements you set

Defect Guides

Anyone trying to read a defect and decide what to do Injection Molding Defects (Overview) The buyer-facing map: what the common defects look like, the five places their causes come from (machine, process, temperature, mold, material), and which lever a buyer can actually pull. Read the guide → Cosmetic parts with ribs, bosses, or thick areas Sink Marks Dimples over thick sections, ribs, and bosses, why they form during cooling, and the design and process choices that prevent them. Read the guide → Large, flat, or dimensionally tight parts Warpage & Shrinkage Why parts bow, twist, or finish out of tolerance, uneven cooling, wall thickness, fiber-filled materials, and how it ties to your tolerances. Read the guide → Buyers seeing trimming or edge defects Flash Thin excess plastic along the parting line: what it signals about tool fit and process, and when recurring flash is a tooling-condition question. Read the guide → Parts with holes, multiple gates, or load at the line Weld Lines (Knit Lines) The line where two flow fronts meet, when it is cosmetic versus structural, and how gate location moves it off the faces that matter. Read the guide → Parts with thin walls or long flow paths Short Shot Why a cavity fails to fill, venting, gate sizing, thin walls and long flow, material, and whether the fix is process, tooling, or design. Read the guide → Hygroscopic engineering resins; supplier vetting Splay & Silver Streaks Silvery streaks from the gate, usually moisture or material handling, and why splay is a window into a supplier’s process discipline. Read the guide → Parts with deep ribs, corners, or end-of-fill traps Burn Marks Scorching at the last areas to fill, usually trapped air and venting, not just heat, and why burns and short shots travel together. Read the guide → Parts gated into open volumes Jetting The snake-like squiggle from the gate, melt shooting across an open cavity, and why the durable fix is usually gate design, not process tuning. Read the guide → Thick sections, bosses, load-bearing parts Voids & Bubbles Holes hiding inside the part, vacuum voids in thick sections versus gas bubbles from moisture or trapped air, and why they matter structurally. Read the guide → Glossy cosmetic surfaces showing patterns Flow Lines Wavy rings, halos, and streaks recording how the cavity filled, what drives them, and which fixes belong to process, gate, or design. Read the guide → Buyers reviewing pale marks at T1 Stress Whitening, Buyer Review White marks at ejector pads, gates, and clips: cosmetic or structural? Questions, evidence, and approval gates. Read the guide → Loaded features and assembly cracking Cracking, Buyer Review Cracks at bosses, holes, and clips are structural findings, the patterns to verify and the strength evidence to demand before approval. Read the guide → Trials with manual ejection or release spray Sticking & Demolding, Buyer Review A part that needs wrestling out of the tool is a program risk, what to verify, ask, and require before production. Read the guide → Cosmetic programs with gloss disputes Gloss Problems, Buyer Review Uneven sheen, dull zones, and lot-to-lot mismatch, how to verify causes and turn gloss into an inspectable standard. Read the guide → Color-critical and matched-assembly parts Discoloration, Buyer Review Yellowing, streaks, and color drift: separating degradation signals from mixing and standards issues before approval. Read the guide → Clear, light-colored, or cosmetic parts Black Specks, Buyer Review Dark particles are a housekeeping and equipment signal, the purging, regrind, and maintenance questions that reveal supplier discipline. Read the guide → Toughness-critical engineering-resin parts Brittle Parts, Buyer Review When a tough resin snaps like a cracker, the material in the part may not match the datasheet, drying, heat, and regrind patterns to verify. Read the guide → T1 reviews with visible witness marks Ejector & Drag Marks, Buyer Review Separating normal tooling witness from trouble signals at T1, classification, cosmetic-face conflicts, and approval gates. Read the guide → Cosmetic parts or deep-rib geometry with end-of-fill marks Gas Marks & Trapped Air, Buyer Review Silvery patches and surface blistering from air the mold couldn't exhale, venting questions, the diesel-effect overlap, and approval gates before sign-off. Read the guide → Anyone holding a defective part without a vocabulary for it Defect Diagnostic Wizard (Tool) Have a mark you can't name? Three questions, location, appearance, behavior, narrow it to the most likely patterns, each linked to its review guide and evidence list. Read the guide → Replying to a supplier's corrective-action claim Defect Evidence Generator (Tool) Select the defects on your parts and get a printable list of the records, photos, and documents to request before accepting "we fixed it." Read the guide → Structural parts, or any part shedding flakes near a gate Delamination, Buyer Review Layers that peel or fish-scale off the surface, usually a contamination, moisture, or release-agent signal, and a structural hold rather than a cosmetic waiver. Read the guide → Cosmetic or handled parts approaching tooling or T1 Gate Vestige & Blush, Buyer Review The mark where plastic entered the part: when a vestige is a safety finding, why blush is usually geometry wearing a process costume, and the acceptance criteria to write before tooling. Read the guide → Any program shipping production parts Defect Log Template A copyable log that turns scattered complaints into trends, supplier accountability, and dispute evidence. Read the guide →

Prevent Defects Upstream

Buyer Explainers (Animated) Animated review workspaces for T1 samples, defect evidence, and approval decisions, the interactive companions to these guides. Read the guide → Plastic Part Design for Manufacturing A large share of defects are designed in. Run a DFM review before tooling to prevent them. Read the guide → Supplier Capability Checklist Defects are where process discipline shows. Evaluate whether a molder controls their process. Read the guide →
Before you tool

Many defects are designed in before a mold is cut. A design-for-manufacturing review and a clear RFQ that defines cosmetic surfaces and critical dimensions catch them while they're still cheap to fix.

Review the DFM guide
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