Welcome back, everybody, to another deep dive. This time, we're taking a look at injection molding.
Oh, injection molding.
But specifically back pressure. Ah, back pressure, which I've been reading this article you sent me called how does Back Pressure affect Injection Molding?
Right.
And good stuff. I think this is one of those things in injection molding that people maybe don't consider enough 100% when they're trying.
To troubleshoot even one of the hidden gems.
Yeah. And I mean, there are so many factors.
Oh, there are.
In injection molding. But this one really stood out to me as we were going through this article, and.
Absolutely.
We'll see if we agree on that by the end of this deep dive.
Okay, sounds good.
But so what is back pressure? Like, how would you define that for someone?
So back pressure is the resistance that the screw encounters as it's going backwards to compact the plastic.
Okay.
During the plasticizing stage.
So it's not the pressure of injecting. No, it's before that. The pressure of, like, getting.
Yeah, it's like making sausage before you push it out of the tube.
Okay.
So you're compacting all the pellets together.
So it's like the packing pressure almost.
Yeah, you could say that.
Okay. And so this article talks about how that affects mixing.
Absolutely.
So. So how does. How does back pressure affect how well your. Your plastic gets mixed up before it goes into the mold?
So when that screw is moving backwards and it's compacting the pellets, that's when the shearing happens. And the shearing is what mixes it.
So it's almost like kneading dough.
Exactly. Like, think about that.
Yeah.
You're compacting and shearing at the same time. That's what creates a homogeneous mixture.
So, you know, they always say, like, don't over mix your batter. You don't want to over knead your bread.
Right. Yeah.
Is that a factor in injection molding or.
No, not so much. You want it to be homogenous.
Okay.
So you want to mix it. Well.
Got it.
Especially if you have colors or additives.
Yeah.
You want to make sure that it's right.
Because I was thinking, you know, one of the things I've noticed in injection molding sometimes is you'll have, like, a part that maybe has like.
Like a swirl.
Little swirl of color or is not quite uniform.
Yeah. Or black specks where the master batch hasn't been mixed right in properly.
Okay.
So this helps with that.
Yeah. So this is one of those things where it can actually prevent some of those Visual defects.
Absolutely.
Okay, So I noticed that the article also talked about density.
Yes.
How back pressure affects density and how that affects void formation.
Right.
So can you talk about that a little?
So when you have low back pressure, you can end up with voids in your parts.
Oh.
And that is because the air is not being pushed out during the plasticizing stage, so it's trapped in there. And when it goes into the mold, it creates those little pockets.
So just for. For people who maybe aren't as familiar, what are voids?
Voids are basically air pockets in the molded parts.
Okay. And those are bad because you create weak points.
Okay.
And they can also affect the appearance.
Gotcha. So, like, if you have a part that needs to be right. Structurally sound, you don't want.
You don't want it to break.
Right.
Where there's a void.
So back pressure is kind of ensuring.
That it's compacting the material.
Yeah.
To eliminate those voids.
So it's like squeezing all the air out before you bake it.
Exactly.
Okay. So I was going to ask, like, is more always better. But I feel like we're getting to that point. We are in the article where it's like, you don't want too much back pressure.
Exactly. Either because it can cause other problems.
Okay.
Like, it can increase the injection pressure, which puts stress on the machine and the mold.
Okay.
It can also increase the cycle time because it takes longer to pack the material.
Ah, okay. So it's kind of like if you pack something too. Paint. It's going to take longer to get it out.
Exactly.
Okay. So then the last thing I wanted to touch on in this section was how back pressure affects flow characteristics.
And this is where I was like, wait, what?
Yeah. A lot of people are surprised by.
This because, you know, you would think that more pressure equals less flow.
Right.
But that's not always the case.
It's counterintuitive.
Yeah.
But it's true.
So how does. How does back pressure sometimes actually improve flow?
So in some cases, increasing the back pressure can actually reduce the viscosity of the plastic.
Okay.
So it'll flow better.
So remind me again what viscosity is?
Viscosity is the resistance of a fluid to flow.
Okay.
So like, honey has eye viscosity.
Right.
Water has a low viscosity.
So you're saying in some cases, increasing this pressure.
Right.
Can make the plastic act more like water than like, honey.
Exactly.
Okay.
And that's because of those shear forces.
Right.
It helps untangle the molecules.
So it's not just about compacting it's about.
It's about the shear. It's about the movement.
Interesting.
And the friction.
But what are the benefits of having that?
Better flow, More flowable. So you can fill complex molds.
You could get better dimensional accuracy.
Okay.
Less warping.
So this is another one of those where it's like finding that sweet spot of back pressure.
It's all about balance.
Because if you have too much, you're going to have those negative consequences.
Yes.
But if you don't have enough, then you might have problems with flow and filling.
Right. Okay. And voids.
And voids.
Yeah.
So it's a lot to consider.
It is.
When you're setting up your injection molding process.
It is. But it's worth it.
Yeah. And this is a great lead into our next section where we talk about actually how to optimize back pressure.
Absolutely.
Because I imagine there's no magic number.
There isn't.
It depends on the type of plastic.
Yeah.
The mold design, your machine, the part itself.
Absolutely.
So when we come back, we'll dive into all of that.
Sounds good.
And how to actually figure out what is the right back pressure for your specific situation.
Let's do it.
Stay with us. So we've been talking about all the benefits of back pressure, like.
Right. How it helps with mixing and density and flow. But the article also mentioned something about how it affects the appearance of the part.
Absolutely.
And I thought that was interesting.
It is.
Because, you know, it can make a big difference.
It's not just about strength and filling the mold.
Yeah.
It's also about.
It's about the aesthetics.
Right.
Yeah.
So how does back pressure affect the way a part looks?
So one of the things that back pressure can help with is preventing sink marks.
Okay.
And those are those little depressions on the surface of a part, you know, where you have, like, a thick section and a thick, thin section.
Yeah.
The thick section will shrink more.
Oh, okay.
And it'll create a little dip.
So it's like differential shrinkage almost.
Yeah.
Okay.
And back pressure helps to push that pack that material in tighter.
Okay.
So it reduces that shrinkage.
So it's kind of evening out the shrinkage.
Exactly.
Gotcha.
And it can also help with weld lines.
Yeah.
Okay, now, what are weld lines?
Weld lines are those faint lines you see where two flow fronts meet. So, like, if the plastic has to flow around a corner and then it meets up again, sometimes you'll see a little line, and that's the weld line.
So back pressure helps with that because.
It helps to fuse those two flow fronts together.
Okay.
So you don't see that line.
So it's almost like it's creating a stronger bond between those.
It's like welding. Yeah, but with plastic.
So it's not just about the internal structure. It's also about the surface finish.
Right.
Really?
Yeah.
That's fascinating.
It is.
So, you know, we keep coming back to this idea that more is not always better.
That's right.
So what are some of the challenges that people face with back pressure? When they're trying to optimize back pressure.
One of the biggest challenges is finding that sweet spot between too much and too little.
Right, right. Because we've talked about the downsides of having too much.
Yeah.
What about too little? What kind of problems can that cause?
So if you don't have enough back pressure, you can have problems with short shots, which is where the plastic doesn't fill the mold completely. You can have flash, which is where the plastic squeezes out of the mold.
Right, right.
And you can also have problems with dimensional accuracy.
So it's almost like it's not packed in there tight enough to hold its shape.
Exactly.
Okay. So it's really a balancing act.
It is.
And I imagine it also depends on.
Oh, yeah.
A lot of factors.
The material.
The material. The mold. The machine.
The machine.
Everything.
The temperature.
The temperature. Yeah.
The humidity.
So there's no magic formula.
No, there isn't.
That you can just plug in and say, this is the Right back pressure for everything.
Nope.
Okay. So. So then what advice would you give to somebody who's.
Yeah.
Maybe new to injection molding or just trying to figure out.
Yeah.
How to get this. Right.
So I would say start with the material supplier's recommendations. They usually have some guidelines. And then experiment. You know, try different settings.
So it's a lot of trial and error.
It is.
And observation and taking notes and.
Yeah. Keep good records of what you're doing and what the results are so that.
You can kind of hone in on that sweet spot.
Exactly.
That's really helpful advice.
I hope so.
So this has been a really great overview, I think, of back pressure.
We've covered a lot.
We have. And it's really highlighted for me how important this one parameter is.
It really is.
And how it affects so many things.
It's like a dominant effect. One thing affects another.
But. But before we wrap up this deep dive, I want to. I want to go back to something you said earlier about how back pressure can actually change the flow characteristics of the plastic.
Yes.
Like, it can make It.
It can change the way it behaves in the mold.
And I thought that was so fascinating because it's not just about making it more or less viscous.
Right.
It's about, like, fundamentally changing the way it flows.
Yeah. It's about melt elasticity.
Okay. So when we come back, we're going to. We're going to dive into that a little bit more.
Sounds good.
Because I think that's where it gets really interesting.
It does.
So stay with us. All right, so we're back, and we're talking about melt elasticity.
Melt elasticity.
I gotta admit, this is not a term I was familiar with before reading this article.
It's not something that comes up every day.
No, it's not, but it sounds really important.
It is.
So can you explain what melt elasticity is?
So melt elasticity is basically the ability of the molten plastic to stretch and recover.
Okay.
So think about, like, a rubber band. You stretch it and it bounces back. That's elasticity.
Okay.
And molten plastic has that property, too.
Interesting.
To a certain extent.
Yeah. So how does that relate to back pressure?
So back pressure can actually increase the melt elasticity of the plastic.
Okay.
Which means it can flow better into those tight corners and intricate details.
So it's almost like it's making the plastic more flexible. You could say that, but not in the sense of like.
Not in the sense of, like, bending.
It's more like a rubber band.
Yeah. It's like it can stretch and then go back to its original shape.
Okay. So it's almost like it's giving the plastic more give.
Yeah.
So that it can.
So it can fill those complex geometries without breaking or tearing.
Okay. And that's a good thing because we want our parts to be strong and have good dimensional accuracy.
Exactly.
So melt elasticity helps with all of that.
It does.
But I'm guessing there's a limit.
There is.
To how much you can't just crank.
Up the back pressure.
Right, right. Because we talked about how too much back pressure can be bad. Yeah. So. So what happens if you go too far with melt elasticity?
If you push it too far, you can actually degrade the plastic. So instead of making it flow better, you're actually making it weaker.
So it's kind of like stretching that rubber band too far.
Exactly.
And it snaps.
Yeah. You break the molecular chains.
So. So it's all about finding that sweet spot.
It is.
Of back pressure.
The Goldilocks zone.
The Goldilocks zone. Yeah. Not too much. Not too little.
Just right.
Just right. And that's going to depend on all the factors. All the factors we've been talking about.
The material, the mold, the machine.
So it's really important to understand how these things all interact with each other.
It's a complex system.
It is. But I think we've done a pretty good job.
I think so.
Of breaking it down.
We've covered a lot.
We have. And I hope our listeners have a better appreciation now. I hope so too, for this often overlooked parameter.
It's a hidden gem.
It is. And it just goes to show that sometimes the smallest things.
Yeah.
Can make the biggest difference.
Absolutely.
So to all of our listeners out there, keep experimenting, keep learning and keep pushing the boundaries in your own field.
That's great advice.
And who knows? Maybe you'll find the back pressure equivalent in your own world.
The hidden gem. The hidden gem that unlocks new possibilities.
Exactly. So until next time, stay curious.
Still