Injection Moulding Quality Control: Detecting and Preventing Defects

If you’ve ever used a plastic spoon, a phone case, or even the dashboard of your car, chances are, it came from an injection moulding machine. But, now, imagine a company producing 10,000 medical syringe plungers a day. If even 100 of them come out defective, this can be dangerous. Which is why quality checks exist! Especially for industries like automotive, medical, and consumer electronics, accuracy is everything. 

this article, we will be talking about detecting and preventing defects, and how you can assure accuracy through different tested methods. So, let’s jump into injection moulding defects and remedies​, and common injection moulding defects​ that you should avoid!

Types Of Injection Moulding Defects​ (And What They Mean)

Let’s dive into the most common problems that happen during moulding and how you can spot them:

 

Flow Lines

If we talk about the looks of it, they often look like wavy patterns or lines that you can see on the surface of the part. These lines or patterns can happen as the plastic cools too quickly as it flows into the mould. This well is what causes those streaks, or defects. 

How to prevent them:

• Increase injection speed
• Raise the mould temperature
• Improve gate design (the point where plastic enters the mould)

Burn Marks

Burn marks appear as black or brown discoloration on the plastic part, usually around the edges or near thin sections. And no, these marks aren’t just cosmetic! They indicate that something went wrong inside the mould during the injection process. This usually occurs in high-speed or high-pressure scenarios where the air has nowhere to go.

How to prevent them:

• Add vents to let air escape
• Lower injection speed
• Adjust the clamping pressure

Sink marks

Sink marks show up as small dents or dimples, usually on thicker areas of the part. Although you might think they are harmless, no! These marks are a clear sign that there is a problem with how the part cooled after moulding.

How to prevent it:

• Use uniform wall thickness
• Optimize the cooling system
• Balance the mould temperature

Warping

Warping is when a part bends, twists, or becomes misshapen instead of coming out flat and straight. No doubt, it’s one of the most frustrating injection moulding defects because even if everything looks right on paper, internal stresses can cause the part to shift while it cools. The major reason why this happens is because of uneven cooling across different areas of the part. Basically, one part cools faster than the other!

How to prevent it:

• Use uniform wall thickness
• Optimize the cooling system
• Balance the mould temperature

Short Shots

In the simplest way possible, a short shot happens when a plastic part doesn’t fill completely. So, you might see a corner missing, an edge that didn’t form, or a gap in a detailed section. Basically, the mould didn’t get enough material to create the full part. The reason? Low injection pressure, a temperature that’s too low, or a blockage in the mould.

How to prevent them:

• Increase injection pressure or temperature
• Enlarge the gate
• Use a lower-viscosity material

Flash

No matter how fancy this one sounds, this is what happens when extra plastic seeps out at the seam between two mould halves, forming a thin, unwanted flap. So, it might look like a thin ridge or overgrowth along the edge of the part. So, if the mould was not clamped tightly enough, or the pressure was too high, which then forced the plastic through. 

How to prevent them:

• Ensure proper mould fit
• Calibrate injection pressure
• Maintain the mould regularly

How Do You Detect These Types Of Injection Moulding Defects​?

 

• Visual Inspection

This is the old school method where trained workers would check the moulded parts by eye, usually under bright lights. This way, they would catch anything that looks off, like marks, warping, or incomplete sections. The good thing about it is that this way is cheap and pretty flexible. Although people get tired, and their focus can shift, and might as well skip some defects.

 

• Automated Vision Systems

This is like giving your production line a set of robot eyes. In this method, the cameras are paired with smart software, which then scans each part as it comes out. This instantly flags any defects like wrong shape, color differences, flash, dents, you name it. The good thing is that it is super fast, accurate, and can run nonstop. However, the upfront cost is pretty high, and it needs maintenance, so it is kind of an investment!

 

• In Machine Sensors

There is no doubt that modern injection moulding machines are getting smarter. In fact, many of them now come with built-in sensors that keep an eye on parameters like pressure, temperature, and flow, while the part is being made. So, if something’s off, the machine either stops or flags the part as potentially defective. This way, the problems are caught in real-time, even before there is a defect. However, these machines are new and expensive!

 

• Quality Sampling

Now, this one is also a pretty old-school method in which, instead of checking every single part, this method pulls out a few pieces from each batch and gives them a closer look. Basically, randomly picking up samples, which are then tested for size, strength, flexibility, and overall quality. With this, you save hours and hours of checking each piece. The cost you pay? Because it is random picking, it does not guarantee that every part is defect-free!

 

How to Prevent Defects

 

• Design It Right from the Start

If you think most defects aren’t just bad luck. No! They’re born way back in the design stage. If the mould or part design isn’t thought through properly, problems are bound to show up later. For example, when the wall thickness changes across the part, the plastic doesn’t flow evenly, which can disrupt cooling and lead to warping or sink marks. Sharp corners? Yeah, they might look cool in theory, but they cause unnecessary stress and break the flow.

 

• Keep the Machine in Shape

One thing that is often ignored is how injection moulding machines are workhorses, but even the best ones need regular care. Basically, you need to calibrate them regularly so temperatures, pressures, and timings are always on point. Also, you have to swap out worn parts before they give up mid-production. And yes, lubricating the moving bits is the most important thing to do! After all, a glitchy machine can throw off the entire process, and even a tiny such inconsistency can mean a pile of injection moulding defects!

 

• Monitor Process Parameters

Injection moulding is a lot like baking. If the temperature is too high, things burn, and if it’s too low, nothing cooks properly. Which is exactly why one should keep their eyes on process parameters like the temperature, pressure, and cycle time. It is because, let’s say, if the temperature isn’t right, then the plastic might burn! So, once you figure out the “perfect recipe,” stick to it and repeat it for consistency in quality. 

 

• Use Quality Control Tools

Quality isn’t just about catching mistakes, but it’s also about setting up systems that stop them from happening in the first place. That’s where QC checklists come in. Before any batch goes out, there are assigned manufacturers who run through a detailed checklist like: how it looks, if the part fits, if it bends like it should, and if the material’s consistent. This way, people save themselves from “Oh no, why didn’t we catch that earlier?” moments.

 

• Trace Every Part

Let’s say a customer calls you up complaining about a defective part. If you can trace that part back to the exact day, machine, operator, and setting, then you’ve just saved a ton of money and stress. This can happen through barcodes or batch numbers to track every part! These tiny tags can help you save time, and energy without needing to recall an entire shipment!

Final thoughts

Quality control in injection moulding means knowing what can go wrong, spotting it fast, and having the systems in place to stop it. The goal isn’t just to produce parts in bulk, but to maintain consistency, efficiency, and accuracy!