Podcast – How Can You Identify Shrinkage Defects in Injection Molded Parts and Improve Them?

Close-up of an injection molded part with visible shrinkage defects
How Can You Identify Shrinkage Defects in Injection Molded Parts and Improve Them?
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All right, welcome back, everyone. Today we're going to be tackling something that you might not think about every day.
Definitely not an everyday topic.
Yeah, it's a little bit niche. Right. We're going to be looking at shrinkage defects in injection molded parts.
Yeah.
You know, you sent over some really cool research on this, and I have to say, I'm kind of fascinated by it.
Yeah. It's one of those things that you probably never even think about.
Right.
But it affects pretty much every product you use.
Yeah, exactly. I mean, think about, like, your phone case.
Yeah.
Or like, parts in your car. Right. Getting those smooth, perfectly shaped plastic components, it's not as easy as it seems.
There's a lot of science that goes into preventing those imperfections.
Yeah. So before we get into how you do that. Right. Can you just, like, set the stage for us a little bit? What exactly are we talking about when we say shrinkage defects?
Okay, so imagine you've got this perfectly designed mold for a plastic part.
Right.
And on paper, everything looks great, but when the actual part comes out of the mold, it's not quite right. Maybe it's a little bit smaller than it should be.
Okay.
Or there's a weird dent on the surface. Or the whole thing is, like, warped out of shape.
Okay. Yeah.
So those are shrinkage defects.
So it's like we're detectives, right? We've got this crime scene, these shrinkage defects, and now we need to find out who are the usual suspects.
Exactly. Like, and there are three main culprits, really.
Okay.
We usually look at the material itself, the design of the mold, and then the actual process parameters used in injection molding.
Gotcha. So each one of those could kind of throw a wrench in the works.
Yeah. They can each play a big role in how the plastic behaves as it cools and solidifies.
Okay. So let's start with the material then.
Okay.
How can, like, choosing the wrong type of plastic mess things up?
So different plastics shrink at different rates when they cool down. Some are naturally more prone to shrinkage than others. So, for example, polyethylene and polypropylene tend to have higher shrinkage rates.
Okay.
Which can be tricky.
Yeah. So if you're making something that you really need to be, like, dimensionally accurate.
Right.
You know, like, it has to fit into something.
Yeah.
You don't want to use those.
You might be better off going with something like polycarbonate or ABS in those cases because they shrink less.
Gotcha. So it's all about picking the right plastic for the job. Right. But what if you are, like, stuck using something that has a high shrinkage rate? Is there anything you can do to. To work around that?
Absolutely. So you can actually modify the material itself to reduce shrinkage. You can add fillers, like glass fibers. Think about it. Like reinforcing concrete.
Right. Like rebar.
Yeah, rebar. Exactly. So it adds strength and stability, but in this case, it also helps restrict how much the plastic wants to shrink.
That's cool. Okay, so we've tackled material selection. What's next?
Mold design.
Okay.
And this is where things get really interesting, because the mold itself can either help prevent shrinkage defects, or it can make them a lot worse. Think about baking a cake in a wonky pan. You know you're not gonna get a perfect cake, right?
Right. It's gonna be all messed up.
So even with the right plastic, a bad mold design can really throw everything off.
Okay. So even seemingly small details in the mold could have a big impact.
Oh, yeah, definitely. So, for instance, the position and number of gates in the mold, which are those channels where the molten plastic flows in.
Okay.
They can have a huge impact on how evenly the plastic fills the cavity.
Okay.
And if it's not even, you get those sink marks on the surface.
Right, right. So it's, like, strategically directing the plastic so it gets everywhere it's supposed to go.
Yeah.
What else?
Wall thickness is another big one. If the walls of the part are uneven, you get different cooling rates, and that can cause warping and dimensional changes.
Okay.
And then you've got temperature control within the mold itself. You don't want hot or cold spots because that can lead to uneven shrinkage and distortions.
Wow. It sounds like every little thing has to be, like, perfectly planned out to avoid those problems.
Yeah, it does. And it gets even more complex when you consider that different types of plastics behave differently at a molecular level. So, for example, you've got amorphous polymers like ABS and polycarbonate. They have a more random molecular structure, so they tend to cool and solidify in a more predictable way. And that makes them less prone to shrinkage surprises.
So because it's random, it's actually more consistent.
Exactly. But then you have crystalline polymers like polypropylene and polyethylene. They have a more ordered molecular structure, so they actually shrink more as they cool because those molecules align and pack together really tightly.
So with those crystalline polymers, you've got, like, a whole other level of shrinkage you gotta worry about.
Exactly.
That's fascinating how something so tiny like that can make Such a big difference.
It really is. And it gets even more interesting because environmental conditions like temperature and humidity can also affect how the material behaves. All of these factors need to be considered when you're designing a mold to try and prevent those shrinkage defects.
So it really is like this giant puzzle, huh? You got the material, the mold, the environment, and it all has to work together.
You got it. And each one of those plays a critical role.
Okay. So we've talked about material selection and mold design. What's the last piece?
The last piece of the puzzle is process parameters, which are the settings and adjustments that are made during the actual injection molding process. Gotcha. It's kind of like cooking.
Okay.
You can have the best ingredients, a perfect recipe, but if you mess up the oven temperature or the cooking time.
Yeah. You're not going to get a good result.
You're not going to get a good result at all.
Right. So I have a feeling this is where all the, like, fine tuning comes in.
It does.
So where do we even begin with all these process parameters? There's got to be, like, a ton of them.
There are a lot, but there are a few key one that we focus on. Injection pressure, holding time, injection temperature, and cooling time.
Okay. So it sounds like we got a lot to unpack there.
We do.
Let's. Let's take a deeper dive into each one of those and figure out how they all fit in.
Sounds good. Let's do it.
All right, so we're back and ready to dig into those process parameters.
Okay.
You were saying that they're kind of like the knobs that you tweak, you know, to get the injection molding process just right.
Yeah. You can fine tune the outcome by adjusting those parameters. Just like you'd adjust the oven temperature and the cooking time to bake something.
Okay. So let's start with injection pressure and holding time.
Right.
How do those affect shrinkage?
So imagine you're filling up a water balloon.
Okay.
If you don't have enough pressure, it's not going to fill up all the way.
Right. You'll get all those, like, wrinkles and.
Exactly. And the same thing happens in injection molding.
Okay.
If the injection pressure isn't high enough, the plastic might not completely fill the mold.
And then you end up with those sink marks and things like that.
Exactly.
Gotcha. Okay, so enough pressure. Make sure that the plastic gets everywhere it needs to go. Right.
What about holding time?
So holding time is about maintaining that pressure.
Okay.
Once the mold is full. So think about holding your hand on that water balloon.
Okay.
Even after it's full, to make sure.
It doesn't, like, squirt out.
Exactly. So in injection molding, the holding time keeps the pressure on the plastic while it's cooling and solidifying.
So it holds its shape.
Right.
Okay. So it's like a two step process. You blast it with pressure.
Yeah.
And then you hold it there until it's set.
Exactly.
What happens if you don't hold it long enough?
Then the plastic can shrink as it cools.
Then you get the wrong size part.
Yeah.
It's amazing how much precision goes into all of this.
Yeah. It's definitely not just melting plastic and shoving it in a mold.
Not at all. Okay, so let's move on to injection temperature.
Okay.
That's the Goldilocks parameter.
Right.
Not too hot, not too cold.
Exactly. The molten plastic needs to be at the right temperature for that specific material.
Okay, can you give us an example of, like, how that would actually play out? Yeah. So let's say we're working with ABS plastic, which is used in all sorts of things, like LEGO bricks to car dashboards.
Oh, wow.
Now, if the injection temperature is too high, the ABS can actually start to degrade.
Okay.
And you might end up with discoloration or the part might become weaker.
So you could end up with a LEGO brick that, like, crumbles too easily.
Exactly. But on flip side, if the temperature is too low, the plastic might not flow properly into the mold.
Oh, okay.
And then you might get an incomplete or, like a deformed part.
So it's about finding that sweet spot where it flows, but it doesn't get too hot.
Exactly. And then finally, we have cooling time, which is probably one of the most important parameters when it comes to shrinkage.
Okay, so how does cooling time affect shrinkage?
So if you rush the cooling process and you eject the part from the mold too early, the plastic might still be soft.
Oh, okay.
And it could continue to shrink outside of the mold.
So even if everything else went perfectly, you could still mess it up at the end.
Exactly. And this is where things get a little tricky, because longer cooling times mean slower production.
Right. So it's that balance of quality versus efficiency.
Exactly. And that's where the skill of the engineer comes in. They need to balance all those things to produce high quality parts, but without sacrificing speed and efficiency.
Right, right. It's like they're conducting an orchestra, making sure that everything's working together.
That's a great analogy. It shows just how much expertise goes into making these Products.
You know, I never really thought about all the stuff that goes into just a simple plastic bottle cap.
Right.
Or, you know, it's easy to overlook a LEGO brick, but there's so much there.
Yeah.
It's amazing.
And that's the beauty of these deep dives. Right. We get to peel back the layers and see the hidden wonders, and it's.
A whole new world. You know, I feel like I'm seeing everything differently now.
V2.
So where do we go from here? So I feel like we've covered a lot of ground already, you know, about shrinkage, but I'm sure there's more. Right. What else did you find interesting in the research that you sent over?
Well, one thing that stood out to me was how important it is to think about what the part's actually going to be used for.
Okay.
When you're trying to prevent shrinkage defects.
When you say what it's used for, what do you mean?
Like, its application.
Okay.
So, you know, does it need to be super strong and rigid, or does it need to be flexible? Will it be exposed to high temperatures or chemicals? All of those things affect the choices you make about the material, the mold design, even the process parameters we were just talking about.
So you're saying it's not just about knowing, like, the science of shrinkage, but also about understanding how the part's actually going to be used in the real world.
Exactly. So, for example, we were talking about water bottles earlier, right?
Yeah.
You'd probably want to use a flexible material like polypropylene for a water bottle.
Right.
But if you were designing, like a hard hat, you'd need something much stronger and more rigid, like polycarbonate.
Right. And those two materials would require totally different approaches to prevent shrinkage.
Exactly. So polypropylene has a much higher shrinkage rate than polycarbonate.
Okay.
So you need to adjust the mold design and the process parameters accordingly. So for the water bottle, you might need to make the mold slightly larger to account for the shrinkage, to compensate for the shrinkage.
Yeah.
And you might need to use a lower injection pressure and a longer holding time.
Okay. And for the hard hat, you'd be, like, you know, focusing on different things altogether, just making sure that it, you know, sets properly and keeps its shape.
Right. So it's not one size fits all Right.
It's all about the application.
Yeah. You need to understand the specific demands of the application if you want to prevent shrinkage defects.
It's like a tailor making a suit. You're not going to use the same, you know, pattern and techniques for every single person. You're going to have to tailor it to. To their, you know, measurements and all that.
Exactly. And just like a tailor, engineers who work in injection molding need to have this really deep understanding of how to prevent these shrinkage defects.
Right. And from what we've talked about today, it sounds really complicated.
It is. But it's also really rewarding.
Yeah.
When you see a perfectly molded plastic part.
Yeah.
It's amazing to think about all. All the skill and precision that went into making that.
I'm, like, looking around my house right now at all the plastic things, and I'm thinking, I know. Wow. I had no idea.
That's a whole new appreciation.
That's the beauty of these deep dives, right?
Yeah.
We get to learn about all this stuff that we never even think about.
Exactly.
I feel like I'm walking away with this whole new perspective on the world.
Yeah, me too. And hopefully next time you use a plastic product, you'll think about all the science and engineering that went into making it.
I definitely will. Thanks again for, you know, taking us on this journey.
Oh, of course. It was my pleasure.
It's been really, really interesting.
And to all our listeners out there, thanks for joining us for another deep dive.
We'll see you next