Common Injection Molding Defects: Causes, Fixes & Prevention Tips

Injection molding is a reliable way to make plastic parts at scale—but common injection molding defects like flash, warping, or short shots can ruin quality, waste materials, and delay production. Even small defects (e.g., a tiny sink mark) can make parts unfit for use, especially in industries like medical or automotive where precision matters. The good news? Most defects are predictable and fixable—if you know their root causes. This guide breaks down 16 common injection molding defects, explains why they happen, shares practical solutions (for both design and process), and uses real-world cases to help you avoid costly mistakes.

First: Why Injection Molding Defects Happen (Key Categories)

Most injection molding defects fall into two categories—mold design-related (caused by poor mold geometry or maintenance) and process-related (caused by wrong machine settings or material handling). Knowing which category a defect belongs to saves time troubleshooting:

• Mold design defects: Require changes to the mold (e.g., adjusting parting lines, adding exhaust channels) — more costly upfront but long-lasting.
• Process defects: Fixed by tweaking machine parameters (e.g., lowering injection speed, increasing mold temperature) — faster and cheaper to resolve.

Key Statistic: A 2023 study by the Plastics Industry Association found that 60% of injection molding defects are process-related (easily fixable) — only 40% need mold adjustments.

Part 1: Mold Design-Related Defects (Causes & Fixes)

Mold design defects are often hidden until production starts. They stem from poor initial design (e.g., uneven wall thickness) or lack of maintenance (e.g., worn mold edges). Below are the 5 most common ones:

1. Short Shots (Incomplete Filling)

A short shot happens when molten plastic doesn’t fill the entire mold cavity—leaving missing parts (e.g., a handle without a grip) or thin walls. It’s one of the most costly defects, as short-shot parts are usually scrapped.

Root Causes:

• Thin walls (<0.8mm) or narrow channels that slow plastic flow.
• Long flow paths (plastic cools before reaching the end of the cavity).
• Poor gate placement (gate too far from hard-to-reach areas).

Real-World Case:

A startup making plastic toy cars had 25% short shots on the car’s wheels (thin 0.6mm spokes). The mold’s gate was placed on the car’s body—plastic cooled before filling the wheel spokes.

Fixes (Design & Mold):

• Wall Thickness: Keep walls ≥0.8mm (or follow resin-specific minimums—e.g., 0.5mm for low-viscosity resins like PP).
• Gate Placement: Move gates closer to thin or distant features (e.g., the startup moved the gate to the car’s axle, near the wheels).
• Flow Simulation: Use software like MoldFlow to test flow paths—identify cold spots before building the mold.

2. Flash (Excess Plastic Burrs)

Flash is thin, flaky plastic that seeps into mold gaps—usually along parting lines (where the mold halves meet) or ejector pins. It’s mostly a cosmetic defect but may require post-processing (trimming) that adds cost.

Root Causes:

• Mold halves don’t align (worn or poorly machined mold edges).
• Sharp corners near the parting line (create pressure points that push plastic into gaps).
• Too high injection pressure (forces plastic into tiny mold gaps).

Real-World Case:

A kitchenware brand had flash on 30% of their plastic spatulas—along the handle’s parting line. The mold’s parting line had sharp corners that trapped pressure, pushing plastic into gaps.

Fixes (Design & Mold):

• Parting Line Design: Move parting lines to hidden areas (e.g., the spatula’s handle bottom instead of the top).
• Rounded Corners: Replace sharp corners near the parting line with 0.5–1mm fillets (reduces pressure).
• Mold Maintenance: Resurface worn mold edges to ensure tight alignment.

3. Improper Parting Line Placement

The parting line is where the mold’s core and cavity meet. If placed poorly, it creates visible seams or flash on critical areas (e.g., a phone case’s front face).

Root Causes:

• Parting line crosses cosmetic or functional features (e.g., a logo, snap fit).
• No DFM (Design for Manufacturability) review—mold maker didn’t check feature placement.

Real-World Case:

A electronics company’s phone case mold had a parting line through the camera cutout. The seam was visible, and flash got stuck in the cutout—ruining 15% of cases.

Fixes (Design & Mold):

• Plan Early: Place parting lines on hidden areas (e.g., the phone case’s bottom edge) before finalizing design.
• Conceal with Features: Align parting lines with ribs or grooves (hides the seam).
• DFM Review: Work with mold makers to check parting line placement during design—avoid last-minute changes.

4. Bubbles & Voids (Air Traps)

Bubbles (visible on the surface) or voids (hidden inside) weaken parts and ruin appearance. They form when air gets trapped in the mold or plastic shrinks unevenly.

Root Causes:

• Uneven wall thickness (thick areas cool slowly, trapping air).
• No exhaust channels in deep cavities (air can’t escape).
• Moisture in the resin (evaporates during heating, creating bubbles).

Real-World Case:

A medical device maker had voids in 20% of their plastic syringes—inside the thick barrel base. The mold had no exhaust channels, and the barrel’s 5mm base was too thick (trapped air).

Fixes (Design & Mold):

• Uniform Thickness: Keep thickness variation ≤±10% (e.g., a 2mm wall should not jump to 4mm).
• Exhaust Channels: Add 0.1–0.2mm wide exhaust channels in deep cavities or near the end of flow paths.
• Hollow Thick Areas: Use cores (internal holes) in thick parts (e.g., the syringe’s 5mm base became 2mm with a 1mm core—no more voids).

5. Gate Vestige (Visible Gate Marks)

Gate vestige is leftover plastic at the gate location (where plastic enters the mold). It’s a cosmetic issue but problematic for parts with tight fits (e.g., a gear that needs to spin freely).

Root Causes:

• Gate too large (leaves a big bump).
• Gate placed on a visible or functional surface (e.g., a lid’s top face).

Real-World Case:

A container brand’s lid had a 1mm gate vestige on the top—consumers complained about the bump. The mold used an edge gate on the lid’s top.

Fixes (Design & Mold):

• Gate Type: Use tunnel gates or latent gates (auto-remove vestige—no post-processing).
• Hidden Gates: Place gates on non-visible areas (e.g., the lid’s inner rim instead of the top).
• Gate Size: Keep gates small (1–2mm for small parts)—reduces vestige size.

Part 2: Process-Related Defects (Causes & Fixes)

Process defects happen during production—usually from wrong machine settings or material handling. They’re easier to fix than mold defects, as you don’t need to modify the mold. Below are the 11 most common ones:

1. Flow Lines (Wavy Surface Stripes)

Flow lines are wavy or streaky lines on the part’s surface—usually near the gate or corners. They’re cosmetic but make parts look unprofessional.

Root Causes:

• Low mold temperature (plastic cools too fast, creating uneven flow).
• Sudden geometry changes (e.g., a 1mm wall jumping to 3mm—disrupts flow).

Real-World Case:

A toy maker had flow lines on their plastic dolls’ faces—near the gate. The mold temperature was set to 40°C (too low for ABS resin).

Fixes (Process & Design):

• Mold Temperature: Increase to resin-specific levels (e.g., 60–80°C for ABS).
• Smooth Transitions: Replace sudden thickness changes with gradients (e.g., a 1mm to 3mm wall over 5mm length).

2. Burn Marks (Dark Discoloration)

Burn marks are yellow, brown, or black spots—usually at the end of flow paths. They form when trapped air heats up (from compression) and burns the plastic.

Root Causes:

• Poor exhaust (air can’t escape, so it compresses and heats up).
• Too high injection speed (forces air into small spaces).

Real-World Case:

A automotive supplier had burn marks on 15% of their plastic sensor housings—at the end of the flow path. The mold had no exhaust channels, and injection speed was set too high (100mm/s).

Fixes (Process & Design):

• Exhaust Channels: Add exhaust near the end of flow paths.
• Lower Speed: Reduce injection speed (e.g., the supplier dropped to 60mm/s—no more burns).

3. Sink Marks (Surface Depressions)

Sink marks are small pits or dents—usually in thick areas (e.g., a boss on a bracket). They form when the outer layer cools fast, but the inner layer shrinks and pulls the surface inward.

Root Causes:

• Thick areas (>4mm) with no cores.
• Insufficient holding pressure (can’t compensate for shrinkage).

Real-World Case:

A furniture brand had sink marks on their plastic chair legs—around the thick 6mm boss. The mold had no cores, and holding pressure was too low.

Fixes (Process & Design):

• Core Thick Areas: Add cores to reduce thickness (e.g., the 6mm boss became 3mm with a 2mm core).
• Holding Pressure: Increase to 80–90% of injection pressure (keeps plastic packed while cooling).

4. Warping (Part Deformation)

Warping is when parts bend or twist after cooling—common in flat parts (e.g., a plastic tray) or parts with uneven thickness. It makes parts unfit for assembly (e.g., a warped lid won’t close).

Root Causes:

• Uneven cooling (thick areas cool slower than thin ones, creating stress).
• Wrong material (semi-crystalline resins like PP shrink more than amorphous ones like ABS).

Real-World Case:

A packaging company’s plastic trays warped by 2mm—they were flat with 1mm walls (no ribs). The mold’s cooling channels were uneven, so one side cooled faster.

Fixes (Process & Design):

• Cooling Channels: Ensure even spacing (20–30mm apart) in the mold.
• Add Ribs: Add 1–2mm thick ribs to flat parts (reduces stress—tray warping dropped to 0.5mm).
• Material Choice: Use low-shrinkage resins (e.g., ABS instead of PP) for flat parts.

5. Weld Lines (Weak Seams)

Weld lines form when two molten plastic flows meet but don’t fuse—creating a visible seam and weak spot. They’re common in parts with holes or multiple gates.

Root Causes:

• Low mold temperature (flows cool before fusing).
• Obstacles in flow (e.g., a hole that splits the flow).

Real-World Case:

A tool maker had weld lines on their plastic pliers—near the hole for the pivot pin. The flow split around the hole and didn’t fuse, making the pliers weak.

Fixes (Process & Design):

• Mold Temperature: Increase to improve fusion (e.g., 70°C for ABS instead of 50°C).
• Gate Placement: Move gates to reduce flow splits (e.g., the tool maker added a second gate on the other side of the hole—no more weld lines).

Common Injection Molding Defects: Impact & Cost Comparison

Use this table to quickly assess how defects affect your production—from scrap rates to cost increases:

How to Prevent Injection Molding Defects (5 Pro Tips)

Prevention is cheaper than fixing defects. Follow these tips to avoid common issues:

1. Do a DFM Review Early: Work with mold makers and engineers to check your design for manufacturability—catch issues like thin walls or poor gate placement before building the mold.
2. Test with Prototypes: Use low-cost prototypes (e.g., 3D printed molds for small batches) to validate your design—fix defects before scaling to mass production.
3. Train Operators: Ensure machine operators know resin-specific settings (e.g., temperature, pressure)—wrong settings cause 60% of process defects.
4. Maintain Molds: Clean and inspect molds weekly—worn edges or blocked exhaust channels cause flash and bubbles.
5. Use Quality Resins: Avoid contaminated or degraded resins—they cause discoloration and delamination. Store hygroscopic resins (e.g., PA, PC) in dry containers.

Yigu Technology’s Perspective on Common Injection Molding Defects

At Yigu Technology, we tackle injection molding defects by combining design optimization and process control. For mold-related issues, we use DFM reviews to fix wall thickness and parting line problems early. For process defects, we train teams on resin-specific settings and use real-time monitoring to adjust pressure or temperature. We also recommend prototype testing—3D printed molds let clients validate designs without costly steel mold changes. Our goal is simple: help clients reduce scrap rates to under 5% and keep production on track. For us, defects aren’t just problems—they’re opportunities to improve efficiency.

FAQ About Common Injection Molding Defects

1. Can I fix flash without modifying the mold?

Yes—if flash is process-related. Try lowering injection pressure (reduces force pushing plastic into gaps) or increasing clamp force (tightens mold halves). If flash persists (mold wear), you’ll need to resurface the mold edges.

2. Why do my parts have both sink marks and short shots?

This usually happens from conflicting settings: low holding pressure causes sink marks, but increasing pressure too much may lead to short shots (if plastic can’t flow). Fix it by balancing holding pressure (start at 80% of injection pressure) and using uniform wall thickness (avoids thick areas that need extra pressure).

3. Are some resins more prone to defects than others?

Yes—semi-crystalline resins (PP, PA, POM) shrink more, so they’re prone to warping and sink marks. Amorphous resins (ABS, PC, PMMA) have less shrinkage but are prone to flow lines (if mold temperature is too low). Choose resins based on your part’s needs—e.g., ABS for flat parts (low warping) or PP for flexible parts (accepts higher shrinkage).