2026-10-17 · JCH Design & Manufacturing

Injection molding defects: what causes them, and whether the fix is the tool or the process

Flash, short shots, sink marks, warping, knit lines and splay: what usually causes each one, how to tell a tool problem from a process problem, and what to put in writing so defects are caught before they ship.

Filed under Molds and tooling

Short answer: Most injection molding defects come from one of four places: the part design, the mold, the material or the process settings. Flash and short shots are the most common, followed by sink marks, warping, knit lines, splay, burns and voids. A buyer does not need to fix them, but does need to know which ones mean a steel change on the tool, which mean a process change, and which mean the material was not dried. That decides who pays, how long the fix takes, and whether the next lot will have the same problem.

Every new mold makes some bad parts. The first trial shots exist to find them. What separates a good supplier from a bad one is not whether defects appear, but whether the cause is found and fixed in the tool or the process, instead of being sorted out of each lot by hand. This note covers the common injection molding defects, what usually causes each one, how to tell a tool problem from a process problem, and what to write into your quality requirements so a defect is caught before it ships. It builds on how an injection mold is built and the first article inspection that approves the tool.

The four sources of a defect

Before arguing about a defect, place it in one of four buckets:

Shelves of red, pink and black molded plastic parts stacked by color in a factory building — Molded parts racked by color at the works. On solid-color parts like these, color streaks and marbling are easy to see.

Part design. Walls that change thickness abruptly, ribs and bosses that are too thick for the wall they sit on, sharp corners, features too tall to fill. These defects come back on every tool built to the same drawing.
The mold. Gate position, venting, cooling lines, parting-line fit and wear. Fixes mean machining the steel, so they cost money and take days or weeks.
The material. Wrong grade, wet resin, contaminated regrind, poorly mixed colorant. Fixes are cheap if caught at incoming inspection and expensive if caught after the lot is molded.
The process. Melt and mold temperatures, injection speed, pack and hold pressure, cooling time, shot size. Fixes are fast, which is exactly why they get overused.

Wikipedia's summary of molding defects puts it the same way: defects can come from flaws in the mold, but more often they come from the molding process itself. That is a good default, with one warning: a process change can hide a tool or design problem for a while without fixing it.

Common injection molding defects and their usual causes

Flash. Thin fins of plastic along the parting line, around ejector pins or at vents. M. Holland's Bill Fierens calls it "one of the most common reasons injection molded parts get scrapped". Typical causes are too much material or pressure, too little clamp force, or a parting line that no longer seals. He also notes that "mold parting lines tend to wear over time and are a common maintenance item." Flash that appears on an old tool is often a maintenance issue, not a setting.

Short shots. The cavity does not fill, so the part is missing a corner, a rib tip or a clip. Causes include poor venting, too little material or pressure, a cold mold, or a design feature such as a very tall rib that cannot fill. Fierens names short shots and flash as the top reasons molded parts are scrapped.

Sink marks. Shallow dents on the surface, usually opposite a thick rib or boss. Fierens says they can come from too little plastic, "but more commonly they are the result of a part design issue." His design rule for projections: redesign so "the projection thickness does not exceed 50% of the wall it is attached to." Packing more plastic in can hide a sink, but he warns this can lead to flash, sticking parts and higher residual stress.

Warping. The part does not hold the shape of the cavity: a flat lid that rocks, a housing that bows. Uneven cooling, uneven shrinkage and molded-in stress are the usual causes. Fierens finds warping is often a material issue: the shape of a filler such as glass fiber can stop the part shrinking evenly, so the fix may be a filler change rather than a process tweak.

Knit lines (weld lines). A visible line where two flow fronts meet, often behind a hole or a core pin. It is a weak spot as well as a cosmetic one. Fierens says "the best option to solve knit lines is to relocate the mold's gate", which is a tool change. Once the gate is cut, process changes can only make the line less visible.

Splay (silver streaks). Silver streaks along the flow direction. The common cause is moisture in a resin that absorbs water and was not dried properly; overheated material causes a more localized version.

Burns and blisters. Dark or discolored marks at the end of fill, where trapped air is compressed and overheats. Usually a venting problem in the tool, sometimes made worse by injecting too fast.

Voids and bubbles. Empty spaces inside thick sections, often found only when a part is cut open or X-rayed. Usually a packing problem in a thick wall, which also makes them a design question.

Flow marks, jetting and color streaks. Wavy lines, snake-like patterns from the gate, or uneven color. These point to injection speed, gate design and colorant mixing.

Why fixing one defect causes another

Plastics Technology columnist Robert Gattshall describes the most common trap in molding: flash and short shots are "in most cases" direct opposites, and "many of the actions we take to eliminate shorts can result in flash and vice versa." He calls the result "the chase": a technician raises hold pressure to cure a short shot, the parts look good, and a few hours later the same press is making flash.

A hand holding a clear plastic molded frame with screw bosses and clips over a concrete factory floor — A clear molded frame. On clear parts, flow lines, bubbles and knit lines are all visible, so cosmetic limits need to be agreed in writing.

His advice is blunt: "process changes should be the last thing we do to correct an issue. If the machine was running good parts and suddenly is producing shorts, something changed in that process, and it must be identified to resolve the issue." For a buyer, that is the question to ask when a supplier reports a defect: what changed?

How to tell a tool problem from a process problem

A defect that appears in the same place on every shot, from the first trial onward, usually lives in the design or the steel. A defect that comes and goes, or appears after weeks of good parts, usually lives in the process or the material. Three things help you tell them apart:

Look at every cavity. On a multi-cavity tool, a defect in one cavity only points to that cavity's steel, gate or cooling.
Compare against the documented process. Andy Routsis, writing in Plastics Technology, describes a good troubleshooter as one who "knows the history of a properly documented process, makes each adjustment based on knowledge, and verifies and documents the result of each change." He lists outputs to record when good parts are made, including melt temperature, coolant temperature and flow, fill time, gate-seal time, cycle time, part weight and dewpoint. A supplier that has those records can compare today's run with the approved one. A supplier that does not is guessing.
Check the material first. Routsis gives an example where a part with both flash and splay may point to moisture in the material, not to the clamp or the pressure settings. Drying records and the resin lot number are quick to check.

What to write into your quality requirements

Defects are cheaper to prevent in writing than to argue about on the dock:

Name the defects and classify them. List flash, short shots, sink, warping, knit lines, splay, burns and contamination, and say which are critical, major or minor for your part. This feeds the pre-shipment inspection and its AQL.
Mark the cosmetic surfaces. Show on the drawing which faces are visible to the end user and where a gate mark, ejector mark or knit line is acceptable.
Set limits on the approved sample. A golden sample with notes on what is acceptable (flash height, sink depth, flatness) is the reference for every later lot.
Ask for the cause, not just the fix. When a defect is found, ask whether the correction was made in the steel, the process or the material, and whether the process record was updated.
Agree who pays for steel changes. A design-driven defect is often the buyer's cost; a tool-build or maintenance defect is often the toolmaker's. Settle that before the mold is quoted, not after T1.

How JCH handles defects on a new tool

On an injection molding project, Jack reviews the part for the tool before steel is cut: draft, wall thickness, gates and undercuts. After the first trial shots, critical dimensions are measured on parts from each cavity and corrections are written down. Production lots are then checked against the approved sample, and every order is inspected at incoming material, first article, in-process and pre-shipment, as set out in step 9 of our process. If you have a part with a defect that keeps coming back, send us the drawing and photos.

Straight answers

What are the most common injection molding defects?

Flash and short shots are the most common, and the top reasons molded parts are scrapped. Other common defects are sink marks, warping, knit (weld) lines, splay or silver streaks, burns, blisters, voids, flow marks, jetting and color streaks.

What causes sink marks in injection molding?

Sink marks are shallow dents, usually opposite a thick rib or boss. They can come from too little packing, but more often from part design: a projection that is too thick for the wall it is attached to. One common guideline is to keep a projection at or below 50% of the wall thickness.

Is a molding defect caused by the mold or the process?

A defect in the same place on every shot from the first trial onward usually comes from the part design or the mold steel. A defect that comes and goes, or appears after a period of good parts, usually comes from the process or the material. Checking each cavity separately and comparing the run against the documented process helps separate the two.

Why does fixing a short shot cause flash?

Flash and short shots are near opposites. Raising pressure, shot size or hold pressure to fill a short part can push plastic past the parting line once the original cause, such as debris in a valve, clears. The fix is to find what changed before adjusting the process.

What causes silver streaks (splay) on molded parts?

Splay is most often caused by moisture in a resin that absorbs water and was not dried properly before molding. Overheated or degraded material causes a more localized form of splay.