Common Injection Molding Defects: What Causes Them and How Do You Fix Them?

Introduction

Injection molding is a powerful process for making plastic parts at scale. But even with the best planning, things can go wrong. Parts can come out incomplete, warped, or marked with unsightly lines. These common injection molding defects cost time and money. They lead to scrap, rework, and delays. The good news is that most defects are predictable. They have known causes and proven fixes. This guide will walk you through 16 of the most frequent defects. We will explain why they happen and show you exactly how to fix them, whether the problem is in the mold design or the machine settings. Real-world examples will help you see these solutions in action and avoid costly mistakes on your own projects.

Before we dive into the specific defects, it helps to understand where they come from. Most problems fall into one of two categories.

• Mold design-related defects: These are built into the tool itself. They come from poor geometry, bad gate placement, or lack of maintenance. Fixing them often means changing the mold, which costs more money and takes more time.
• Process-related defects: These happen during production. They come from wrong machine settings, like temperature or pressure, or from issues with the material. These are usually faster and cheaper to fix. You just adjust the machine.

Here is a useful statistic: A 2023 industry study found that 60% of injection molding defects are process-related. That means most problems can be solved without expensive mold modifications.

Part 1: Mold Design-Related Defects

These defects are baked into the tool. They often do not show up until you start running parts. Catching them early, during the design review, is the best way to avoid them.

Short Shots: Why Is My Part Incomplete?

A short shot happens when the molten plastic does not fill the entire mold cavity. You end up with a part that is missing chunks, has thin walls, or has incomplete features. These parts are almost always scrap.

What causes it?

• The part has walls that are too thin, usually under 0.8mm for many materials. The plastic cannot push through before it cools.
• The flow path is too long. The plastic cools down before it reaches the far end of the cavity.
• The gate, where plastic enters, is in the wrong place. It might be too far from a thin or hard-to-fill area.

Real-World Case: Toy Car Wheels

A startup was making toy cars. They had a 25% reject rate on the wheels. The wheels had thin spokes, only 0.6mm thick. The mold’s gate was on the car’s body. By the time the plastic reached the wheel spokes, it had cooled too much to fill them completely.

The Fix: They moved the gate closer to the wheel area. The new gate position let hot plastic reach the thin spokes quickly. The short shots disappeared.

How to prevent it:

• Design for uniform thickness. Keep walls above the minimum recommended for your material. For most resins, aim for at least 0.8mm to 1.0mm.
• Place gates strategically. Put them near the thickest sections or areas that are hard to fill.
• Use mold flow analysis software. This lets you simulate how the plastic will fill the cavity before you cut steel.

Flash: Why Is There Excess Plastic on My Part?

Flash is a thin layer of excess plastic that squeezes out where the two halves of the mold meet, or around ejector pins. It looks like a feathery edge. It is often just a cosmetic issue, but removing it adds labor cost.

What causes it?

• The mold halves do not close tightly enough. This can happen if the mold is worn, damaged, or if there is debris on the parting line.
• The part design has sharp corners right at the parting line. These create pressure points that force plastic out.
• The injection pressure is simply too high, pushing plastic into any tiny gap.

Real-World Case: Kitchen Spatulas

A kitchenware brand found flash on 30% of their plastic spatulas. The flash ran along the handle’s parting line. The mold’s parting line had a sharp corner. This corner acted like a nozzle, forcing plastic into the gap.

The Fix: They machined a small radius onto the sharp corner at the parting line. This eliminated the pressure point. They also reduced the injection pressure slightly. Flash dropped to under 2%.

How to prevent it:

• Design the parting line carefully. Place it on a hidden edge, not on a wide, flat surface.
• Add radii to corners. Avoid sharp transitions at the parting line. A small fillet of 0.5mm to 1.0mm can make a big difference.
• Maintain the mold. Keep the parting line surfaces clean and free from damage.

Bubbles and Voids: Why Are There Holes in My Part?

Bubbles are air pockets you can see on the surface. Voids are empty spaces trapped inside the plastic. Both weaken the part and ruin its appearance.

What causes it?

• The wall thickness changes too suddenly. Thick areas cool slowly, and the shrinking plastic can pull apart, creating a void.
• Air gets trapped in the mold because there is no way for it to escape. This is common in deep ribs or blind holes.
• The plastic resin itself is wet. Moisture turns to steam in the hot barrel and creates bubbles.

Real-World Case: Medical Syringes

A medical device maker had voids inside the thick base of their plastic syringes. The base was 5mm thick. The mold had no small channels to let air escape from that deep cavity. As the plastic shrank, a void formed inside.

The Fix: They redesigned the base. Instead of a solid 5mm thickness, they added a hollow core. The wall became a uniform 2mm thick. They also added tiny exhaust channels, just 0.1mm deep, to let trapped air escape. The voids were gone.

How to prevent it:

• Keep wall thickness uniform. Avoid sudden jumps from thin to thick. If you must have a thick section, consider coring it out to make it hollow.
• Add exhaust channels. These tiny grooves, about 0.1mm to 0.2mm deep, let air escape without letting plastic out.
• Dry your resin. Many plastics, like Nylon (PA) and Polycarbonate (PC), absorb moisture from the air. They must be dried before use.

Part 2: Process-Related Defects

These defects happen during the production run. They are usually caused by incorrect machine settings. The good news is that you can often fix them by tweaking the parameters on the machine, without changing the mold.

Flow Lines: Why Are There Wavy Stripes on My Part?

Flow lines are wavy patterns or streaks on the part’s surface, usually near the gate. They are a cosmetic problem but can make a product look cheap.

What causes it?

• The molten plastic is too cool when it enters the mold. It starts to harden immediately, and the flow front becomes wavy.
• The plastic has to flow through sudden changes in the part’s geometry. A thick section flowing into a thin section disrupts the smooth flow.

Real-World Case: Dolls’ Faces

A toy maker saw flow lines on the faces of their plastic dolls. The lines appeared right near the gate. The mold temperature was set to only 40°C. For the ABS resin they were using, this was too cold.

The Fix: They increased the mold temperature to 70°C. This kept the plastic hot and fluid as it filled the cavity. The flow lines disappeared.

How to prevent it:

• Increase mold temperature. Check the resin data sheet for the recommended mold temperature range. Running at the higher end often helps.
• Increase melt temperature. Make sure the barrel is hot enough to keep the plastic fully fluid.
• Smooth out geometry changes. If the wall thickness must change, make the transition gradual, not sudden.

Burn Marks: Why Are There Black or Brown Spots on My Part?

Burn marks look like small black, brown, or yellow spots, usually at the very end of the flow path. They look like the plastic has been scorched.

What causes it?

• Air gets trapped in a corner of the mold. As the high-pressure plastic rushes in, it compresses this air. The air gets incredibly hot and literally burns the plastic.
• The injection speed is too fast, trapping and compressing air before it can escape.

Real-World Case: Automotive Sensor Housings

An automotive supplier found burn marks on 15% of their small sensor housings. The marks were at the far end of the part. The mold had no exhaust channels in that area. They were also injecting the plastic very fast, at 100mm per second.

The Fix: They added small exhaust channels to the mold at the problem area. They also reduced the injection speed to 60mm per second. This gave the trapped air time to escape. The burn marks were eliminated.

How to prevent it:

• Add exhaust at the end of flow. This is the primary fix. Air needs a place to go.
• Reduce injection speed. A slower fill gives air more time to escape through existing exhaust channels.
• Check for material degradation. If the barrel temperature is too high, the plastic itself can start to burn.

Sink Marks: Why Are There Dents on My Part?

Sink marks are small depressions or dimples, usually found on thick sections like ribs or bosses. The surface looks like it has been pulled inward.

What causes it?

• The part has a local thick area. The outer skin of plastic cools and hardens. But the inner core is still hot. As it cools, it shrinks and pulls the outer skin inward, creating a dent.
• There is not enough holding pressure after the mold is filled. This pressure packs more plastic into the cavity to compensate for the shrinkage.

Real-World Case: Chair Legs

A furniture maker had sink marks on their plastic chair legs. The marks were right on top of the bosses where the leg attached to the seat. These bosses were 6mm thick. The holding pressure was set too low.

The Fix: They could not change the boss thickness easily, so they focused on the process. They increased the holding pressure to 85% of the injection pressure. This packed more material into the boss area, compensating for the shrinkage and eliminating the sink mark.

How to prevent it:

• Core out thick sections. This is the best design fix. Make the wall thickness uniform.
• Increase holding pressure and time. The machine needs to push more plastic in as the part cools and shrinks.
• Increase cooling time. Give the part more time to solidify completely in the mold.

Warping: Why Is My Part Bending or Twisting?

Warping means the part is distorted. It is not flat or straight. It might have a twist or a bend. This often makes the part impossible to assemble.

What causes it?

• The part cools unevenly. One area cools and shrinks faster than another. This creates internal stresses that pull the part out of shape.
• The part design is not rigid enough. Long, flat, thin walls are very prone to warping.
• The material itself has high shrinkage. Semi-crystalline materials like PP and Nylon shrink more than amorphous materials like ABS.

Real-World Case: Plastic Trays

A packaging company made flat plastic trays. After ejection, the trays were warped by 2mm. They were not flat enough to stack properly. The mold’s cooling channels were spaced unevenly, so one side of the tray cooled faster than the other.

The Fix: They redesigned the cooling system in the mold to be more uniform, with channels spaced evenly. They also added small ribs, only 1mm tall, along the length of the tray. The ribs added stiffness. The warping dropped to under 0.5mm.

How to prevent it:

• Design for uniform cooling. Work with your mold maker to ensure cooling channels are evenly distributed.
• Add ribs for stiffness. Ribs add strength and resistance to bending without adding a lot of thickness.
• Consider material choice. For very flat parts, an amorphous resin like ABS or PC might be a better choice than a semi-crystalline one like PP.

Weld Lines: Why Are There Weak Lines on My Part?

Weld lines (also called knit lines) occur where two flow fronts of plastic meet and join together. They often look like a faint line or a slight groove. They can also be a weak point in the part.

What causes it?

• The plastic flow is split by a feature in the part, like a hole or a core. The two streams flow around the obstacle and then meet on the other side. If they are too cool when they meet, they do not bond together perfectly.

Real-World Case: Plastic Pliers

A tool maker had a problem with plastic pliers breaking at the pivot point. The pliers had a hole for the pivot pin. The plastic flow split to go around the hole. The two streams met on the other side, but they did not fuse well. This created a weak weld line right at the stress point.

The Fix: They increased the mold temperature. This kept the plastic hotter, so the two flow fronts fused better when they met. They also increased the injection speed slightly to get the plastic to the meeting point faster.

How to prevent it:

• Increase melt and mold temperature. Hotter plastic fuses better.
• Increase injection speed and pressure. This forces the flow fronts together with more force.
• Move or add gates. Sometimes changing where the plastic enters can change the flow pattern and move the weld line to a less critical area.

Common Defects at a Glance

Here is a quick reference table to compare the impact of these common defects.

How to Prevent Defects: 5 Pro Tips

Fixing defects after they happen is costly. Prevention is always better. Follow these five tips to keep your production running smoothly.

1. Do a Design for Manufacturing (DFM) Review Early. Before you cut the mold, have an expert review your part design. They can spot potential problems like walls that are too thin, sharp corners, or bad gate placement. Fixing these in the CAD model costs nothing. Fixing them in the steel costs thousands.
2. Test with Prototype Tooling. If you are unsure about the design, make a prototype mold from aluminum. Run a small batch of parts. Test them. See if any defects appear. This is much cheaper than fixing a production steel mold.
3. Train Your Operators. Most process defects come from wrong machine settings. Make sure your operators know the correct temperature, pressure, and speed ranges for the specific material you are running.
4. Maintain Your Mold Regularly. Clean the mold. Check the parting line for damage. Make sure exhaust channels are not clogged. A well-maintained mold produces consistent, high-quality parts.
5. Use High-Quality, Dry Material. Store hygroscopic resins (like Nylon and PC) in sealed containers with desiccant. Dry them according to the supplier’s recommendations before use. Contaminated or wet resin is a recipe for defects.

Conclusion

Injection molding defects are frustrating, but they are not a mystery. Each one has a specific cause, and that cause points to a specific solution. Some defects, like short shots and flash, often require a look back at the mold design. Others, like flow lines and burn marks, can be fixed by adjusting the machine. By understanding the root cause, you can stop wasting time on random guesses. You can go straight to the fix. Whether you are designing a new part or troubleshooting a production run, keep this guide handy. It will help you identify problems quickly and get your project back on track.

Frequently Asked Questions

1. Can I fix flash without changing the mold?
   Yes, sometimes. If the flash is minor, try reducing the injection pressure or increasing the clamp force. This can close the mold tighter. If the flash persists, it usually means the mold surfaces are worn and need to be repaired.
2. What is the difference between a sink mark and a void?
   A sink mark is a depression on the surface of the part. A void is an empty space inside the plastic. Both are caused by shrinkage in thick sections. Sink marks happen when the outer skin is strong enough to resist being pulled in completely. Voids happen when the skin is strong, but the inner core pulls apart.
3. Why do my parts have both sink marks and short shots?
   This is a tricky situation. It often means your settings are out of balance. You might have low holding pressure, which causes sink marks. But if you increase the pressure too much, you might be forcing plastic into areas that are already full, which can cause a different problem. Focus first on making the wall thickness uniform in your design. Then, adjust the holding pressure carefully, usually starting at 80% of the injection pressure.
4. Are some plastics more prone to defects than others?
   Yes. Semi-crystalline plastics, like Polypropylene (PP) and Nylon (PA), shrink more than amorphous plastics, like ABS and Polycarbonate (PC). This higher shrinkage makes them more prone to warping and sink marks. Amorphous plastics are more sensitive to mold temperature and can show flow lines more easily if the temperature is too low.
5. How do I know if a defect is a design problem or a process problem?
   A good rule of thumb is this: If the defect happens in the same place on every single part, it is likely a design or mold problem (like a short shot at the end of a thin wall). If the defect happens randomly, or its severity changes from part to part, it is likely a process problem (like flash that comes and goes with pressure changes).

Discuss Your Injection Molding Project with Yigu Rapid Prototyping

At Yigu Technology, we have seen and solved almost every injection molding defect there is. Our team of engineers starts every project with a thorough DFM review. We look for potential problems in your design before they become expensive mistakes in the mold. We guide you on wall thickness, gate placement, and material selection to prevent warping, sink marks, and short shots from the start. When issues do arise during production, we have the process expertise to adjust machine settings and get quality parts running fast. If you are planning a new project or struggling with defects on an existing one, let’s talk. We can help you get your production on track.