Introduction
You approve a mold trial sample, sign off on production, and receive your first shipment — only to find warped parts, surface sink marks, or flash on the parting line.
Injection molding defects are costly. They delay shipments, increase scrap rates, and erode trust between buyer and manufacturer. But most common defects are predictable and preventable — if the mold is designed correctly and the process is controlled properly.
We see these defects in our factory — and we prevent most of them at the DFM stage, before steel is cut. Here are the seven most common defects, what causes them, and how we prevent them in our mold designs.
1. Warping (Deformation)
What It Looks Like
The molded part bends, twists, or deviates from the intended shape after ejection and cooling. Warping is most visible on flat surfaces and long, thin-walled parts.
Root Causes
- Uneven cooling: If one side of the mold cools faster than the other, the plastic shrinks at different rates, creating internal stress that warps the part.
- Non-uniform wall thickness: Thick sections cool and shrink more slowly than thin sections, pulling the part out of shape.
- Insufficient cooling time: Ejecting the part before it reaches a stable temperature allows post-mold shrinkage to deform the part.
- Improper gate location: If the gate is positioned where melt flow orientation creates anisotropic shrinkage, warping is likely.
- Material selection: Semi-crystalline materials (PP, PA66, POM) shrink more than amorphous materials (ABS, PC, PMMA) and are more prone to warping.
Prevention
- Mold design: Design uniform wall thickness throughout the part. Add ribs for stiffness rather than increasing wall thickness. Place cooling channels to achieve even mold surface temperature on both cavity and core sides.
- Process: Increase cooling time, reduce mold temperature differential between cavity and core, and optimize holding pressure and time.
- Material: For parts with inherent warping risk, consider glass-fiber-filled grades (e.g., PP+GF30), which reduce shrinkage and increase stiffness. Confirm material recommendation during DFM review.
Business Impact
Warped parts cannot be assembled. Even 0.5 mm of warp on a housing may prevent PCB fitment or snap-fit engagement. The cost of a full production batch with warp defects includes part cost, assembly labor, shipping, and — worst case — customer returns.
2. Sink Marks
What It Looks Like
A shallow depression or dimple on the surface of the part, typically above thick sections, ribs, or bosses.
Root Causes
- Thick sections: Plastic in thick areas cools more slowly and shrinks more. The outer skin solidifies first, and as the inner material cools and shrinks, it pulls the surface inward.
- Insufficient holding pressure: If packing pressure is too low or holding time too short, there is not enough material to compensate for shrinkage.
- High melt temperature: Hotter melt shrinks more as it cools.
- Small gate: A gate that freezes off too quickly prevents packing material from entering the cavity.
Prevention
- Part design: Keep wall thickness uniform. Rib thickness should not exceed 50–60% of the adjacent wall thickness. Boss OD should be approximately 2× the screw diameter, with a wall thickness of 50–60% of the main wall.
- Mold design: Position the gate near the thickest section so packing pressure reaches the shrinkage zone. Increase gate size if freezing occurs too early.
- Process: Increase holding pressure and holding time. Reduce melt temperature slightly. Increase cooling time.
Business Impact
Sink marks on cosmetic surfaces are a visual defect that customers notice immediately. On functional surfaces, they may affect fit. The mold design fix (reducing rib/boss thickness) should happen at DFM stage — after mold steel is cut, sink marks can only be reduced by process adjustment, not eliminated.
3. Flash
What It Looks Like
A thin layer of plastic that escapes from the mold cavity and solidifies along the parting line, around ejector pins, or at insert boundaries.
Root Causes
- Insufficient clamping force: If injection pressure exceeds the clamping force, the mold halves separate slightly, allowing plastic to escape.
- Worn or damaged mold: Ejector pin clearance, parting line wear, or insert gaps allow plastic to flow into tiny openings.
- Excessive injection speed or pressure: High fill speed creates peak pressure that can momentarily open the mold.
- Low viscosity material: High-MFI (melt flow index) materials flow more easily into small gaps.
Prevention
- Equipment: Use a machine with adequate clamping force. As a rule of thumb, 3–5 tons of clamp force per square inch of projected part area.
- Mold maintenance: Regularly inspect parting line fit, ejector pin clearance, and insert seating. A well-maintained mold is the best defense against flash.
- Process: Reduce injection speed and peak pressure. Use a profiled injection speed (slower at fill completion).
- Mold design: Design robust parting line support. For large projected areas, add support pillars to prevent platen deflection.
Business Impact
Flash requires manual trimming, adding labor cost. It also signals that either the mold needs maintenance or the process is out of control — both warrant investigation before the next production batch.
4. Short Shots (Incomplete Filling)
What It Looks Like
The mold cavity is not completely filled. The part is missing material at the flow front, typically at thin sections, far from the gate, or at the end of long flow paths.
Root Causes
- Insufficient injection pressure or speed: Melt solidifies before reaching the end of the cavity.
- Flow restriction: Thin walls, narrow flow paths, or poorly designed runner systems restrict melt flow.
- Material viscosity too high: Higher-viscosity grades (e.g., PC, high-MW PP) flow more slowly.
- Air trap: Trapped air compresses and prevents complete filling.
- Low melt or mold temperature: Cold material or mold surface causes premature solidification.
Prevention
- DFM review: Use mold flow analysis to verify that the selected gate location and wall thickness allow complete filling at reasonable pressure.
- Part design: Increase wall thickness at thin flow-restricted areas. Add flow leaders if needed.
- Process: Increase melt temperature, injection speed, and injection pressure. Increase mold temperature to slow cooling and extend flow distance.
- Venting: Add vents at the end of fill to allow air to escape. Insufficient venting is one of the most common and most overlooked causes of short shots.
Business Impact
Short shots are 100% scrap. If discovered during production, the entire batch from that period is suspect. Good DFM catches flow issues before the mold is built.
5. Weld Lines (Knit Lines)
What It Looks Like
A visible line or groove on the part surface where two melt flow fronts meet. Weld lines are common around holes, inserts, and areas with multiple gates.
Root Causes
- Flow front collision: When melt splits around a core pin or flows from multiple gates, the two fronts meet and cool before fully bonding.
- Low melt temperature at the meeting point: The flow front cools as it travels, so the meeting point may be below optimal bonding temperature.
- Air or contaminants at the weld point: Trapped air, mold release residue, or material degradation products weaken the weld.
Prevention
- Mold design: Position the gate so the weld line forms in a non-structural, non-cosmetic area. Use a single gate when possible. Add venting at the weld line.
- Process: Increase melt temperature, mold temperature, and injection speed to keep the flow front hotter. Increase holding pressure to improve bonding at the weld.
- Material: Higher-MFI grades flow more easily and produce stronger weld lines. Avoid over-drying, which can degrade material and weaken weld strength.
Business Impact
Weld lines are both cosmetic and structural defects. On a cosmetic surface, they are a visual reject. In a structural area, a weak weld line can cause field failure. Mold flow analysis can predict weld line location before mold manufacturing begins.
6. Burn Marks (Diesel Effect)
What It Looks Like
Brown or black discoloration, typically at the end of fill or in trapped-air zones. In severe cases, the plastic is actually charred.
Root Causes
- Trapped air compression: Air trapped in the cavity is compressed and heated by the incoming melt (adiabatic compression), reaching temperatures high enough to burn the plastic.
- Insufficient venting: No vent or clogged vents prevent air from escaping.
- Excessive injection speed: Melt rushes into the cavity faster than air can escape through existing vents.
Prevention
- Mold design: Add vents at the end of fill, at weld lines, and in deep ribs or bosses. Vent depth is material-dependent (e.g., 0.02–0.03 mm for PP, 0.03–0.05 mm for ABS).
- Process: Reduce injection speed, especially at the end of fill. Use a profiled injection speed curve.
- Mold maintenance: Clean vents regularly. Blocked vents from mold release buildup or material residue are a common cause of burn marks that appear mid-production.
Business Impact
Burn marks are a reject-level cosmetic defect. On functional parts, charred material indicates polymer degradation that may compromise mechanical properties.
7. Jetting
What It Looks Like
A snake-like or worm-like mark on the part surface, typically starting from the gate and extending into the cavity. The melt shoots across the cavity as a jet rather than filling progressively.
Root Causes
- Small gate opening: Melt shoots through a small gate at high velocity.
- Gate positioned over an open cavity: If the gate is not positioned so that melt hits a wall or pin immediately, it can jet into open space.
- Excessive injection speed: Melt enters the cavity too fast to form a smooth flow front.
Prevention
- Mold design: Position the gate so that melt impinges on a cavity wall or core pin immediately after entering. Use a fan gate or tab gate to slow and spread the melt. Increase gate size.
- Process: Reduce injection speed during the initial fill phase. Increase melt and mold temperature to improve flow.
Business Impact
Jetting creates surface streaks that are visible even after painting or texturing. It is a mold design issue that should be caught during DFM review — a gate relocation or gate type change is the permanent fix.
Where Defects Are Prevented in Our Factory Process
In our factory, defect prevention is not a single checkpoint — it is built into four production stages. When we skip or rush any stage, defects appear downstream.
Stage 1: DFM Review — Before Steel Is Cut
Every mold project starts with a documented DFM review. Our engineers check wall thickness uniformity, draft angles, undercut feasibility, gate placement, and cooling layout against the part design. This is where warping risk, sink mark potential, and weld line placement are predicted and designed out. We do not cut steel until the DFM report is approved.
Stage 2: Mold Flow Analysis — Simulating Fill Before Machining
For complex parts with thin walls, multiple gates, or tight cosmetic requirements, we run mold flow simulation before CNC machining begins. The simulation predicts fill pattern, pressure distribution, cooling uniformity, and potential air traps. Adjustments to gate size, gate location, or wall thickness are made in the 3D model — not on a completed mold.
Stage 3: Mold Trial — Verifying the Tool on Our Injection Machines
Every mold we build is trialed on our own injection molding machines. During mold trial, we run progressive short shots to confirm fill pattern, measure trial samples on the CMM against every dimension on the 2D drawing, and evaluate surface finish, weld line visibility, and ejection. Process parameters — injection speed, melt temperature, holding pressure, cooling time — are documented and locked as the production baseline.
Stage 4: In-Process QC — Monitoring During Production
Once mass production begins, quality inspectors pull samples at regular intervals — typically every 1–2 hours — and check critical dimensions, appearance, and weight against the approved trial sample. Injection pressure, temperature, and cycle time are monitored against the documented baseline. Drift in any parameter triggers investigation before defective parts are produced.
These four stages run inside one factory: our injection molding workshop and mold manufacturing facility share the same quality system, the same CMM equipment, and the same engineering team. There is no handoff between a mold maker and a molder where defect responsibility disappears.
Evaluating a Factory’s Quality Capability
When sourcing injection molded parts from China, ask potential manufacturers these questions:
- “Do you perform DFM review before mold manufacturing? Can you share an example DFM report?”
- “Do you have in-house CMM and optical measurement equipment, or do you outsource inspection?”
- “How do you document and control process parameters between production batches?”
- “What is your procedure when a defect is detected during production?”
A manufacturer that answers these questions clearly and confidently has a quality system that prevents defects. One that dismisses them or says “don’t worry, we’ll fix it during production” is relying on luck.
Free Design Review — Catch Defect Risks Before Tooling
Worried about potential defects in your part design? Send your 3D drawings to Hua Zi Industrial for a free DFM (Design for Manufacturing) review.
Our engineers will analyze your part for warping risk, sink mark potential, weld line placement, venting requirements, and gate location optimization — identifying and resolving defect risks before mold steel is cut.
Upload your drawing for a free defect risk analysis →
- ✅ Wall thickness and warping risk assessment
- ✅ Gate location and weld line prediction
- ✅ Venting and cooling layout recommendations
- ✅ No obligation — prevent defects at the design stage, not during production
Frequently Asked Questions
What is the most common injection molding defect?
Warping is the most frequent and costly injection molding defect, especially on flat, thin-walled parts. It is caused by uneven cooling, non-uniform wall thickness, or improper gate location. Warping is best prevented during DFM review by designing uniform wall thickness and optimized cooling channels — once the mold is built, warping can only be reduced by process adjustment, not eliminated.
Can sink marks be fixed without modifying the mold?
Minor sink marks can sometimes be reduced by increasing holding pressure, extending cooling time, or lowering melt temperature. However, significant sink marks caused by thick sections (ribs, bosses) relative to the adjacent wall require mold modification or, ideally, should have been prevented during DFM review when rib/boss thickness could be adjusted. The rule of thumb: rib thickness should not exceed 50–60% of the adjacent wall thickness.
How do I know if my mold maker will prevent defects?
Ask these three questions: (1) “Do you perform DFM review before manufacturing and can I see an example report?” (2) “Do you use mold flow analysis for complex parts?” (3) “What is your inspection procedure during mold trial?” A professional mold maker answers these questions with specific processes, not general reassurances. At Hua Zi, every injection mold manufacturing project includes DFM review, CMM inspection, and documented trial reports as standard.
Are some plastic materials more prone to defects than others?
Yes. Semi-crystalline materials (PP, PA66, POM) have higher shrinkage rates and are more prone to warping and sink marks. High-viscosity materials (PC, high-MW grades) are more likely to cause short shots if the mold design does not account for flow distance. Amorphous materials (ABS, PMMA) are generally more forgiving. Your plastic injection molding manufacturer should confirm material-process compatibility during DFM review.
How much does it cost to fix a mold after defects appear in production?
Mold modification to fix a defect found during production typically costs $500–$3,000+ and adds 5–15 working days, depending on the change. If the defect requires replacing a cavity insert or re-machining a major component, costs can exceed $5,000. This is why DFM review before mold manufacturing is the most cost-effective quality investment you can make — it prevents these costs entirely.
Read the full system guide: Industry Injection Molding Applications Hub — Quality control, defect prevention, and industry-specific manufacturing.
Related Services
Hua Zi Industrial performs injection molding and mold manufacturing in a single facility with in-house quality control:
- Custom Plastic Injection Molding Services — 45 injection molding machines (90T–800T), in-house DFM, material selection, and ISO 9001:2015 certified quality system. Defect prevention starts at DFM review.
- Injection Mold Manufacturing Services — In-house mold design, 20 CNC machining centers, CMM inspection, and process-controlled mold trials. Most injection molding defects are prevented at the mold design stage.
Get a Free Quote Within 24 Hours
Sourcing injection molded parts from China and want to work with a manufacturer that prioritizes quality?
Send your 3D drawings and product requirements to Hua Zi Industrial. Our engineers will review your part design for manufacturability, identify potential defect risks, and deliver a complete quotation within 24 hours.
Upload your drawing and request a free engineering review →
- ✅ DFM review identifies defect risks before mold steel is cut
- ✅ Response within 24 hours (working days)
- ✅ Upload STEP, IGES, DWG, or PDF files
- ✅ ISO 9001:2015 certified quality management system