Common Injection Moulding Defects and How to Avoid Them

Have you ever gazed upon a freshly crafted plastic product and said, “Hold on, that’s not exactly how it’s supposed to look?” Perhaps the finish has some odd streaks; there is an indentation where one shouldn’t be, or the component doesn’t conform. If you’re in plastic manufacturing, this is your reality. Though miniature, such little things—the so-called ‘flaws’—can lead to a sizable expenditure spanning time, money, and an abundance of patience.

So, what causes these imperfections? Why do injection molding defects happen even with the most high-end machines? Let’s find out!

What Are Injection Moulding Defects?

In short, injection molding defects are discrepancies in the final product due to a plastic part’s process, material, or tool. Some are obvious, like even unwanted bubbles or fractures. Other times, they show up later when the part doesn’t hold up.

Here’s the thing: These aren’t just annoying—they can mess with a product’s look, feel, and function. So, understanding what causes them (and how to avoid them) is half the battle won.

Also Read: Precision Manufacturing Tips & Benefits from Chem-Trend Expe

1) Sink Marks

Have you ever noticed a drop or depression on the surface of a plastic part? That’s a sink mark. They usually occur in wider sections when the plastic inside chills slower than the surface.

Why it happens:

• Uneven cooling
• Low injection pressure
• Poor mould design

How to fix it:

• Increase cooling time
• Raise injection pressure slightly
• Modify part design to avoid thick areas

Tip: Uniform wall thickness goes a long way in preventing sink marks.

2) Flow Lines

These are streaks or wavy patterns, usually a different colour than the rest of the part. 

They peek like the plastic was pulled across the surface.

Why it happens:

• Plastic cools too quickly
• Slow injection speed
• Low mould temperature

How to fix it:

• Increase mould temperature
• Adjust injection speed
• Keep the flow path as smooth as possible

You want your plastic to flow like honey, not like a slow-moving river full of bumps.

3) Short Shots

Imagine pouring pancake batter into a mould but stopping halfway—only part of the pancake forms. That’s what a short shot looks like. The plastic doesn’t fill the mould.

Why it happens:

• Not enough injection pressure
• Blocked vents
• Poor flow of the material

How to fix it:

• Increase injection pressure
• Check for venting issues
• Use a material with better flow properties

Short shots are one of the most common plastic injection moulding defects—and one of the easiest to avoid with the proper setup.

4) Warping

Have you ever had a plastic part come out of the mould all twisted or bent? That’s warping. It makes the part unusable and frustrating to deal with.

Why it happens:

• Uneven cooling across the part
• Poor mould design
• The moulding material defects are usually related to shrinkage

How to fix it:

• Use cooling channels to cool evenly
• Choose materials with low shrinkage rates
• Adjust mould temperature settings

We get it—warped parts look like they’ve been through a bad day. But it’s avoidable with better thermal control.

5) Flash

This one’s hard to miss. Flash looks like a thin layer of plastic that leaks out of the mold’s parting line. It’s usually sharp and completely unnecessary.

Why it happens:

• Worn-out mould
• Excessive injection pressure
• Improper clamping force

How to fix it:

• Maintain and inspect moulds regularly
• Reduce injection pressure if needed
• Make sure the clamping unit is doing its job

Flash may sound cool, but it’s anything but stylish in injection moulding faults.

6) Air Bubbles (Gas Traps)

Tiny air sacs tangled inside the part can strain it. These bubbles are usually caused by poor venting or incorrect injection speed.

Why it happens:

• Air can’t escape the mould
• Fast injection traps gas
• The molding material defects are sometimes related to moisture

How to fix it:

• Improve mold venting
• Dry the material properly
• Adjust injection speed and pressure

Drying the material and checking vents regularly helps clear these hidden defects in plastic molding.

7) Discoloration

Have you ever seen a plastic part that is supposed to be bright white but ends up yellow or burnt-looking? That discoloration is often related to plastic injection molding defects during material preparation.

Why it happens:

• Contaminated material
• Excessive heat
• Improper color mixing

How to fix it:

• Clean hoppers and barrels regularly
• Avoid overheating
• Use compatible colorants

Also, keep the release agent clean and apply it correctly—too much can cause colour issues.

8) Jetting

Jetting looks like a squiggly line on the surface of the part. It happens when the plastic hits the mold wall too quickly before it smooths out.

Why it happens:

• Very fast injection
• Improper gate design
• Cold mold surface

How to fix it:

• Slow down injection speed
• Improve gate positioning
• Raise mould temperature slightly

Want smoother parts? Think of the plastic as paint—you want it to flow, not splatter.

Also Read: How to Solve Silver Streaks and Splay Issues in Injection

So, How Can We Avoid These Defects Altogether?

Now that we’ve looked at some of the most common injection molding defects let’s talk about prevention. Because yes, most of these issues can be avoided!

Here’s your checklist:

• Use high-quality mold designs
• Choose the right molding material—because often, the molding material defects are to blame
• Maintain machines and tools regularly
• Keep injection parameters optimized
• Apply release agents carefully and correctly
• Dry the materials properly before processing
• Avoid shortcuts (trust us, they’ll cost you more later!)

Every little tweak makes a difference. From cooling times to clamping force—being mindful of each step in the process helps you reduce injection molding faults and make top-quality parts every time.

Final Thoughts

There is no guesswork to be done in plastic manufacturing. Knowing these plastic injection molding defects and how to resolve them can safeguard you from expensive rework, material loss, and angry customers.

Your parts will improve as the processes improve. And when in doubt, always go back to the basics—because that’s where most of the fixes begin. Let’s make better, cleaner, defect-free plastic products—together.

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