How To Fix Injection Molding Defects

Even though society as we know it right now would be radically different without injection molding, it’s a process few people would stop to give any thought. But in truth, injection molding is a fascinating process that involves involvement from highly skilled professionals working in a variety of different fields.

Injection molding combines art and design with science, technology and precision engineering. And while the basic idea of injecting molten plastic into a shaped cavity for mass production seems simple enough, the process is reasonably delicate, and requires special care attention along the way to ensure the desired results. Troubleshooting can easily occur if a pivotal part of the process isn’t being done right, and this will usually show up on the molded parts as defects.

The good news is that defects in your molded parts don’t necessarily indicate any significant issues with your molds. It’s completely possible for a good mold to produce bad parts when parts of the molding process aren’t being performed correctly.

Flow Lines

Flow lines show up as darker streaks in the molded part. As the name suggests, the patterns of these lines look similar to the pattern you’d expect the molten plastic to flow in, with darker lines occurring close to where the mixture enters the mold.

Assuming your mold is in good shape, the best advice for preventing flow lines is to increase the rate at which plastic flows into the mold a bit. Increasing the injection speed can prevent the mixture from solidifying at different times during the process, which should help the get rid of the lines.

However, if using the optimal injection speed doesn’t change the problem, it might be best to have your mold inspected. Flow lines are more like to occur in areas where the wall thickness isn’t equal throughout. A flow gate located in the wrong area of a mold can also cause these kinds of problems.

Weld Lines

Weld lines look a bit like seams in a garment. They’re often visible in areas around a shaped area of your molded part. And these lines are, in fact, somewhat like the injection molding version of clothing seams. They’re caused when molten plastic flowing into the mold from two different directions of flow come together.

Once again, it’s worthwhile trying to increase the injection speed to prevent this problems from occurring. You can also look into using a different kind of plastic.

If all else fails, you might be able to have your mold adjusted to change the flow pattern of the molten plastic in the mold.

Surface Delamination

Like most plastic molding defects, surface delamination is very noticeable problem. In this case, your molded prototype parts will have layers on the surface instead of appearing smooth. These layers are bad news for the part, as they easily peel away.

One of the main causes of surface delamination is the presence of a foreign material, or contaminant, that cannot mix with your plastic. This contaminant then causes the plastic to separate in the way you see with this molding defect.

A good call for preventing surface delamination is to use only as much mold release agents as is absolutely necessary. However, if your molds are poorly designed, or have faulty ejection mechanisms, your best choice is to have the mold inspected and adjusted by a qualified mold professional.

Short Shot

Short shots might cause a bit more panic for some molders. In the case of a short shot, your molten plastic will actually fail to fill up the entire mold cavity during the injection process. This will cause your molded part to be incomplete when it’s ejected, as there will be missing areas in your part where it didn’t flow into the mold.

Your first choice in fixing this problem would be to reconsider the plastic you’re using. If you’re using the wrong material, it can actually set before filling the mold cavity, meaning never flow into the parts of the mold that would usually be filled last. Increasing the temperature can also help to prevent your plastic from solidifying prematurely.

Another cause for this problem can be that your mold isn’t allowing gas to effectively escape from the mold cavity as the molten plastic flows in. This trapped gas will be condensed in the mold cavity, creating too much pressure to allow the molten plastic to flow into that area.

These are just some of the problems you might experience during molding. When trying to successfully create your first molded prototype, you’ll soon see that injection molding isn’t as simple as getting your equipment set up and starting the process. A lot of factors will contribute to creating a molded part that lives up to your expectations.

Your team of professionals will arguably play the biggest role in whether or not your part turns out as you planned. A team of qualified professionals should be able to help you with choosing the right plastics and injection speeds to mold quality parts.

But having the best team behind you is essential in all stages of part production. Including the very first stages of designing and building your mold. If your mold isn’t functioning properly, nothing else you can do will compensate for this malfunction. At Quality Mold Shop, we can help you fix and adjust dysfunctional molds, as well to design molds specifically to avoid common part defects.

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