7 Injection Molding Defects and How We Prevent Them at NEWBASE
Every injection molding manufacturer claims "high quality" — but true quality means consistent, dimensionally perfect parts, even under the extreme conditions of injection molding (100+ MPa pressure, rapid cooling, mass production).
At Anhui NEWBASE New Energy Technology Co., Ltd., we maintain a first-pass yield (FPY) above 99.2% — not by luck, but by engineering out defects before they reach your shipment.
In this article, we’ll break down the 7 most common injection molding defects, their causes, and our specific prevention steps. Perfect for engineers designing plastic parts or procurement managers evaluating suppliers, this guide helps you ask the right questions and identify reliable partners.
1. Sink Marks
What They Look Like
Small depressions or dimples on the surface of a molded part — typically appearing on the side opposite a rib, boss, or thick section. They're one of the most common cosmetic defects in injection molding, and they're instantly noticeable on smooth, painted, or high-gloss surfaces.
How We Prevent It at NEWBASE
During DFM review:
We flag any rib-to-wall thickness ratio exceeding 60%. Our standard guideline: if the nominal wall is 2.5mm, ribs should be ≤1.5mm thick. We also check for abrupt thickness changes and recommend gradual transitions (coring out thick sections where possible).
During mold design:
We use mold flow simulation (Moldflow) to predict sink mark locations before cutting steel. If simulation shows sink risk above our threshold, we adjust gate position, add gas-assist channels, or redesign the cooling circuit to equalize cooling rates.
During production:
We optimize packing pressure profiles — not just a single holding pressure, but a staged pressure curve that compensates for volumetric shrinkage as the part cools. Our process engineers document the exact packing parameters for every part number, so results are repeatable across shifts.
2. Warping and Deformation
What It Looks Like
The part bows, twists, or won't sit flat on a surface. Edges curl up. Mating features don't align. For parts that need to snap together or mount flush against another surface, even 0.5mm of warp can mean a failed assembly.
How We Prevent It at NEWBASE
During DFM review:
We analyze the part geometry for wall thickness uniformity. If we see sections varying from 1.5mm to 4.0mm in the same part, we recommend coring, ribs, or geometry changes to equalize thickness. We also flag potential issues with glass-filled materials early — differential shrinkage (along-flow vs. cross-flow) is a known warp driver in GF-reinforced resins.
During mold design:
Cooling channel layout is where warping battles are won or lost. We design balanced cooling circuits — ensuring both core and cavity sides cool at similar rates. For critical flatness requirements, we use conformal cooling channels (3D-printed inserts) that follow the part geometry instead of straight-line drilled channels.
During production:
We control mold temperature on both halves independently (using separate temperature control units). Cooling time is set based on the thickest section, not the average. And we validate flatness on the first 30 parts of every production run using a granite surface plate and feeler gauges (or CMM for tight-tolerance parts).
3. Flash
What It Looks Like
A thin film or "fin" of excess plastic extending beyond the intended part boundary — usually along the parting line, around ejector pins, or at vent locations. Flash is both a cosmetic defect and a functional one: it can interfere with assembly, create sharp edges, and indicate underlying mold problems.
How We Prevent It at NEWBASE
During mold design:
We specify parting line fit tolerance at ≤0.02mm and design proper venting channels (typically 0.02–0.03mm deep for most resins, shallower for low-viscosity materials like PA). All slide and lifter fits are precision-ground.
During production:
We calculate clamping force requirements for every mold based on projected area × cavity pressure — and add a 10–15% safety margin. We never run a mold on an undersized machine. Our operators perform parting line checks at the start of every shift and after every mold maintenance event.
Maintenance protocol:
Every mold undergoes a scheduled maintenance cycle based on shot count (typically every 10,000–30,000 shots, depending on material). Parting line surfaces are inspected, cleaned, and re-polished as needed. We track mold condition in our maintenance management system — so issues are caught before they produce defective parts.
4. Short Shots (Incomplete Fill)
What They Look Like
The part comes out of the mold missing material — features are incomplete, edges are rounded where they should be sharp, or entire sections are simply absent. It's an obvious defect, but the root causes aren't always obvious.
How We Prevent It at NEWBASE
During DFM review:
We check the flow-length-to-wall-thickness ratio for every part. For standard resins (ABS, PC), we flag ratios exceeding 150:1 as high risk. If the geometry requires long flow paths, we recommend multiple gates or a hot runner system.
During mold design:
We run filling simulation to verify that the cavity fills completely at reasonable pressure levels. Gate size, runner layout, and vent placement are all optimized based on simulation results — not guesswork. Vents are placed at the last-to-fill locations identified by simulation.
During production:
We set injection speed profiles (not just a single speed) that accelerate through thin sections and slow down at the end of fill to prevent burn marks (see Defect #6). If a new mold shows short-shot tendencies during T1 trials, we adjust venting or gate dimensions before approving the mold for production.
5. Weld Lines (Knit Lines)
What They Look Like
A visible line on the part surface where two flow fronts met and re-joined during filling. Weld lines can appear as faint hairlines (cosmetic issue only) or as structural weak points — depending on the angle at which the flow fronts met and the temperature at which they merged.
How We Prevent It at NEWBASE
During DFM review:
We can't always eliminate weld lines — they're inherent to parts with holes or multiple gates. But we can control where they appear. We work with clients to identify cosmetic surfaces and structural load paths, then position gates so that weld lines form in non-critical areas (hidden surfaces, areas covered by labels, or regions not subject to mechanical stress).
During mold design:
For critical applications, we use overflow tabs (small sacrificial areas beyond the weld line location that allow the cold flow front tip to be pushed past the weld zone). These are trimmed off after molding.
During production:
We increase melt temperature (within material spec) and injection speed to ensure flow fronts are still hot when they meet — improving molecular bonding at the weld. For glass-filled materials where weld lines are structurally critical, we provide weld-line tensile strength data from test bars to help clients validate their designs.
6. Burn Marks
What They Look Like
Dark brown or black discoloration — usually at the end of fill, in corners, or at vent locations. In severe cases, the plastic is actually degraded and brittle at the burn location. Sometimes you'll also smell a burning odor during production.
How We Prevent It at NEWBASE
During mold design:
We place vents at every last-to-fill location identified by mold flow simulation. Standard vent depth for most materials: 0.02–0.03mm (shallow enough to prevent flash, deep enough to let air escape). For deep ribs or blind pockets, we use vented ejector pins or sintered metal vent inserts that allow air through but block plastic.
During production:
We use staged injection speed profiles: fast fill for the first 90% of the cavity (to prevent premature solidification), then a controlled slowdown for the final 10% (to give air time to escape). This is programmed into the machine for every part and documented in the process sheet.
Maintenance protocol:
Vents are inspected and cleaned at every scheduled maintenance. Over time, mold vents can become clogged with off-gassing residue (especially with flame-retardant materials). Our maintenance schedule accounts for material-specific fouling rates — flame-retardant compounds get more frequent vent cleaning than standard ABS.
7. Surface Splay (Silver Streaks)
What They Look Like
Streak-like marks on the part surface, typically radiating from the gate in the flow direction. They have a silvery, splashed appearance — sometimes described as looking like water marks or brush strokes. They ruin cosmetic surfaces and indicate a process issue that may also compromise mechanical properties.
How We Prevent It at NEWBASE
During production:
We monitor barrel temperature zone by zone and set residence time limits based on material thermal stability. Screw speed and back pressure are optimized to minimize shear heating while maintaining melt homogeneity. For sensitive materials (PC, PMMA), we purge the barrel between color changes or material switches to prevent contamination-related splay.
Contact US
Mobile Phone :+86 15250936161
Email : melissa@newbasen.com