Injection molding defects rarely come from one bad decision — they’re usually the visible symptom of a mismatch between part design, mold design, material, and process settings. Whether you’re reviewing a first-article sample from a supplier or designing a part for moldability, knowing what causes each defect makes it much easier to ask the right questions and get to a fix quickly. This guide walks through the most common injection molding defects, why they happen, and how they’re typically resolved.
Short Shots (Incomplete Filling)
A short shot is a part that doesn’t fully fill the mold cavity before the melt solidifies.
Common causes:
- Injection time too short, or the switch to holding pressure happens too early
- Injection speed too slow
- Nozzle blockage, leakage, or a burnt heater band
- Runners or gates that are too small or too long
- Poor mold venting, trapping air ahead of the melt front
- Unbalanced filling across multiple gates
Typical fixes: increase injection time or delay the pressure switch-over, raise injection speed, clean or realign the nozzle, enlarge runners/gates or shorten the runner length, and add or resize venting grooves at the last-fill location.
Flash
Flash is excess material that escapes along the parting line, ejector pins, or slides.
Common causes:
- Insufficient clamping force relative to the part’s projected area
- Injection pressure or speed too high, especially at the end of fill
- Unbalanced gates causing localized overpacking
- Low-viscosity material (some nylons are prone to this)
- Worn ejector pin holes or slides
Typical fixes: increase clamping force or move to a higher-tonnage machine, reduce injection pressure/speed near the end of fill, rebalance the gating layout, and inspect/repair worn mold components.
Sink Marks
Sink marks are shallow depressions that form over thick sections, ribs, or bosses as the part cools unevenly.
Common causes:
- Thick sections, ribs, or posts relative to the nominal wall
- Mold or melt temperature too low
- Insufficient shot volume or holding time
- Non-uniform wall thickness
Typical fixes: redesign for uniform wall thickness where possible, increase mold/melt temperature, extend holding pressure and time to compensate for shrinkage, and enlarge the gating system feeding thick areas.
Warpage
Warpage is dimensional distortion caused by uneven shrinkage across the part.
Common causes:
- Asymmetric part geometry or wall-thickness variation
- Unbalanced filling from multiple gates
- Uneven cooling across the cavity
- Unbalanced ejection, which can distort the part on the way out
Typical fixes: rebalance gate locations, improve cooling-channel layout (for box-shaped parts, cooling channels or high-conductivity inserts at the corners help), fix the ejection system, and — where the geometry allows — hold the part in a shaping fixture immediately after demolding.
Weld Lines
Weld lines appear where two melt fronts meet and re-fuse, and can be both a cosmetic and a strength issue.
Common causes:
- Injection speed or pressure too low
- Poor venting at the meeting point
- Insufficient material drying or melting
- Thin walls or large wall-thickness transitions
Typical fixes: increase injection speed/pressure, use sequential (valve-gated) injection to control where fronts meet, relocate the gate to increase the angle at which flows meet, and make sure hygroscopic materials are properly dried before molding.
Silver Streaks
Silver streaks are thin, silvery surface marks that trace the direction of melt flow.
Common causes:
- Insufficient drying of moisture-sensitive resin
- Overheated barrel or feed-throat temperature
- Foreign material contamination
- Blocked mold venting
Typical fixes: dry the material to spec before molding, lower feed-throat temperature, reduce screw suck-back, and increase back pressure slightly to improve melt homogeneity.
Bubbles and Voids
Trapped gas or vacuum voids show up as bubbles inside or just under the part surface.
Common causes:
- Long barrel residence time degrading the melt
- Poor material flowability
- Moisture or heat-sensitive additives in the resin
- Insufficient venting or ejector-pin clearance
- Large wall-thickness variation causing uneven solidification
Typical fixes: reduce residence time, dry the material properly, add or resize vent pins, and revisit wall-thickness transitions in the part design.
Burn Marks
Burn marks are localized dark or scorched spots, usually near the end of fill.
Common causes: excessive injection speed at the end of filling, and poor mold venting that traps and compresses air until it overheats.
Typical fixes: reduce injection speed near the end of fill, and improve venting at the affected location.
Water Wave Patterns (Ripples / Tiger Stripes)
These are ripple-like bands on the surface, often near the gate.
Common causes: a cold slug of material at the nozzle tip entering the cavity directly, or the first melt into the cavity cooling too quickly before hotter melt pushes through behind it.
Typical fixes: add a cold slug well at the end of the sprue, and adjust melt/mold temperature and injection speed to keep the flow front hot enough to avoid premature skinning.
Color and Gloss Variation
Inconsistent color or gloss on the same visible surface is usually a cooling-uniformity problem.
Common causes: cavity temperature too low or uneven cooling across the surface.
Typical fixes: raise and stabilize cavity temperature, and extend cooling time to even out the surface finish.
Where Precision Injection Molding Differs
Standard defect-prevention practices still apply, but precision injection molding adds tighter controls: mold cavities and runners are typically made from high-hardness alloy steel heat-treated to roughly 52 HRC for wear resistance, barrel temperature is held to about ±1°C via PID control, and dimensional tolerances on the mold itself are generally held to less than one-third of the finished part’s tolerance. Precision machines also use multi-stage injection control — separate speed, pressure, and position settings for different phases of the fill — to approach ideal, consistent melt flow and minimize the defects above before they start.
This guide covers general injection molding process knowledge and does not describe specific tolerances or outcomes guaranteed for any particular part. If you’re evaluating a manufacturing partner for an injection-molded component, our injection molding services page covers what we handle in-house, or you can send us your drawings for a specific quote.