All right, so you're dealing with sink marks in your injection molding, huh? Frustrating, right? Well, today we're going deep on this to arm you with the knowledge to get those surfaces smooth.
Yeah. And strong too. We're diving into a whole bunch of research.
I've stack of it, expert advice, all that good stuff. Because those little depressions, those sink marks.
Not only do they look bad, they.
Can actually compromise the strength of your part.
Exactly.
Which is a no go big time. So let's jump right in. One of the things that really stood out to me was the whole idea of unified, uniform wall thickness.
Oh, absolutely.
Like, one source described it as, like, smoothing out a bumpy road.
Oh, I like that.
Yeah, it was a good one. Advocating for those gradual transitions.
Gradual is key.
Rather than those abrupt changes in thickness.
Makes a huge difference. It's all about that. Even cooling. You know, when you have uniform wall thickness, the plastic cools down at the same rate consistently throughout the entire part. And that minimizes the internal stresses which cause sink marks, which are often the root cause of those sink marks.
Okay.
You ever notice how, like, unevenly cooked parts of a cake can, like, sink in the middle?
Yeah.
Similar idea here.
Okay. So you want that nice, even cooling.
Yeah.
But what about those situations where you just can't have a completely uniform thickness because of the part's design?
That's where things get really interesting.
Okay.
And this is where ribs come in.
Now, I'm thinking ribs for strength, like structural support.
You're already thinking in the right direction, but they can also be your secret weapon against sink marks.
So ribs aren't just about strength.
Exactly. Ribs are like these carefully placed reinforcements.
Okay.
They're helping distribute that stress and heat more evenly throughout the part. It's like adding support beams to a structure.
Okay, good.
You're essentially guiding the flow of that molten plate elastic.
Right.
Helping it cool down in a more controlled way.
So I'm sensing a theme here. Controlled cooling.
Control is key.
Right. So it's not just slapping any old rib onto the design.
No, definitely not.
Placement.
Placement's important.
Size matters.
Size is critical.
Okay.
Yeah. The research we looked at actually suggested aiming for rims.
Okay.
That are about 0.6 to 0.8 times the thickness of the wall. Of the wall.
Not too thick, not too thick, not.
Too thin, not too thick. You want to find that Goldilocks zone.
You know, the sweet spot.
The sweet spot for optimal performance.
Okay. So we've talked about the design aspect with uniform wall thickness.
Yes.
Rib placement.
Strategic rib placement.
Now let's get into the nitty gritty of the actual injection molding process.
Let's do it.
One of the sources had a great analogy.
Okay.
Comparing the process parameters.
Right.
To tuning a musical instrument.
I love analogies.
Yeah, this one was good. Each adjustment, whether it's holding pressure.
Right.
Injection speed or temperature, it all matters as a ripple effect on the final product.
It does. It's like a chain reaction.
Okay, so let's break it down. What is holding pressure?
Holding pressure is the pressure that's applied to the molten plastic.
Okay.
After it fills that mold cavity, it's almost like you're giving the part a firm hug to make sure it keeps.
Its shit as it cools.
And it cools and shrinks.
Right. So there's gotta be a sweet spot there.
There is. There is a sweet spot.
Too much. Yeah, not enough.
If you have too much pressure, what happens? You can actually lead to over packing.
Okay.
Which can cause other issues like flash or even part deformation.
Okay. So you don't wanna overdo it.
No, you don't wanna overdo it.
And then not enough holding pressure.
If you don't have enough holding pressure.
Or not enough holding time or you're.
Not holding it for long enough, that can allow the plastic to shrink too much as it cools, as it crawls.
And then you're getting those sink marks.
Exactly. It can pull away from those mold walls.
Right.
Create those dreaded sink marks.
It's like a dance between all those different variables.
It is. It's a delicate dance between all the variables.
So injection speed.
Yeah, Injection speed, temperature.
We got to talk about those too.
We do. So a faster injection speed can actually help you fill that mold quickly, Reduce the cooling time, which can beneficial. Right. Preventing those sink marks. But you've also got to consider, okay, the material's viscosity and the mold design. If that material is too viscous, too thick. Too thick. Or the mold has intricate details, you might need to slow down that injection speed to avoid things like short shots or other defects. So there's always a balance.
It's a balancing act.
Constant balancing act.
You really have to know your material and your mold. You got to know your material inside and out.
You got to know your mold.
Yeah, for sure.
Speaking of molds.
Okay.
We should probably talk about the unsung hero.
Okay.
Of injection molding.
Who is that?
The cooling system.
Oh, the cooling system.
Yes.
Yeah. Okay.
One of the sources actually said that a well planned cooling system is like having a reliable friend.
Ooh, I like that.
Always there to support You.
That's good.
I thought that was a great way to put it.
Yeah. Yeah. So it's key.
It is.
To preventing those sink marks.
Absolutely.
But it's not as simple as just blasting it with cold water.
No, not at all.
Right.
It's about achieving that uniform cooling.
Okay.
Across the entire mold surface.
Right.
To minimize.
Oh. Shrinkage differences.
Those dreaded shrinkage differences. And this often means designing custom cooling channels.
Oh, wow.
That are targeting specific areas and making sure that cooling process is consistent.
So it's about strategically directing that flow of coolant.
Exactly.
Or even heat dissipation.
That's a great way to put it.
That sounds pretty complex.
It can be. Yeah.
In itself.
But remember, we're aiming for precision.
Right.
You're essentially guiding that heat away.
Right. Okay.
From that molten plastic in a very carefully orchestrated way.
Right.
To control that solidification process and prevent those localized shrinkage areas which cause the sink marks that lead to those sink marks.
Right.
Yeah.
It's all about understanding those thermal dynamics.
It is, Right. It all comes down to heat transfer.
And on that note.
Okay.
We should probably talk about material selection.
Oh, yeah. Material selection, which is huge. It's critical.
Right?
Yeah. Each material.
I mean, this is where I get really interested.
Yeah.
I love the material science part of it.
I find it fascinating. Each material has its own personality, you know, when it comes to shrinkage and cooling. Some materials, like polystyrene.
Okay.
Are known for their low shrinkage rates.
So those are your friends.
Those are your friends.
If you're worried about sink marks.
If sink marks are a major concern, polystyrene is a good choice. Others, like polypropylene, tend to shrink more.
Okay.
So you really have to factor that into your design.
So it's not just about strength and flexibility.
Yeah.
It's about how it behaves.
It's about understanding how that material is going to behave during cooling. During that cooling process.
Okay. So I think we've laid some good groundwork.
We have. We've covered a lot of groundwork.
The basics of wall thickness, rib design, process parameters.
You talked about holding pressure.
Yeah.
Injection speed, cooling systems, cooling systems, material selection. Material selection is crucial.
But I want to go deeper.
Let's go deeper.
I mean, we're doing a deep dive.
We are. We are doing a deep dive.
What are some of the more advanced techniques?
Oh.
Get ready to combat these pesky sink.
Marks, because we're about to enter the world of gas assisted and microcellular injection molding.
Hey, how you're talking.
Get excited.
Let's do It.
Yeah. All right, so we're back.
Back for more.
Ready to dive deeper.
Deeper into those sink marks.
Yes, exactly. And how to get rid of them.
As promised, we're stepping into the world of some advanced techniques.
Okay.
Specifically gas assisted and microcellular injection molding.
All right, so these are really gas assisted. Yeah.
From what I've read, it involves introducing a gas.
Yes.
Usually nitrogen.
Nitrogen is the most common one into the mold, Right alongside the molten plastic.
Okay, so why would you want to add gas to the mix?
That's a great question.
It seems like it would complicate things.
It might seem that way on the surface. Yeah. But it's actually quite ingenious because that gas serves a couple of key purposes.
Okay.
First, it helps to push that molten plastic into those hard to reach areas of the mold.
Okay.
So you're making sure even those most intricate details.
Right. Are completely filled so you don't have incomplete parts.
It minimizes the chances of those incomplete parts or those short shots.
Right. Okay.
And then secondly, and this is where things get really cool. The gas actually creates a hollow core.
A hollow core Inside the part.
Inside the part.
So the part isn't solid plastic all the way through.
Exactly.
I'm trying to picture this. Yeah, it's like a chocolate Easter Bunny with nothing inside.
There you go. That's a perfect analogy.
Okay.
And that hollow core is the secret weapon.
Okay.
Because by reducing the volume of that solid plastic, you're essentially speeding up that cooling process.
Oxtric cooling, faster cooling, fewer sink marks.
And more even cooling, which means fewer sink marks.
Okay. So not only are we getting potentially fewer sink marks.
Potentially fewer sink marks.
We're also getting lighter parts.
Yes, exactly.
Which could be a huge advantage, depending on what you're making.
Absolutely. Especially if you're making something that needs to be lightweight.
Right.
Here's another bonus.
Okay.
That gas pressure inside the part can actually enhance its structural strength.
Oh, interesting.
So you can achieve rigidity with thinner walls.
So it's not just about avoiding the sink marks.
It's about optimizing the whole part.
It's optimizing the whole part.
It really is.
Okay, so gas assisted is pretty cool.
It is very cool.
What about microcellular.
Microcellular injection molding?
Yeah. They call it foaming sometimes. Right?
Yeah, Sometimes called foaming.
That makes me think of a cappuccino.
It's a much more appealing image than sink marks, I'll say that.
Right. But we're talking about.
But we're talking about introducing a chemical blowing agent.
Okay.
To that plastic resin. And as the plastic is injected into the mold, this blowing agent decomposes and creates tiny gas bubbles.
So we're creating a bubble bath, a miniature bubble bath inside the plastic.
Inside the plastic.
What effect does that have?
So instead of having that solid mass of plastic.
Right.
You end up with a part that has this cellular structure.
Okay.
Almost like a sponge.
Okay.
And just like with that hollow core in gas assisted molding.
Okay.
This cellular structure reduces the amount of solid plastic.
So less solid plastic.
Less solid plastic.
Less shrinkage.
Less shrinkage.
Fewer sink marks.
Fewer sink marks.
I'm seeing a theme.
It's a recurring theme.
So we're manipulating the internal structure of the plastic.
You got it.
To control how it behaves, to control.
Its behavior during the cooling process.
Okay. So microcellular.
Microcellular.
It's pretty amazing. It is when you think about it.
It's like we're fine tuning those material properties at a microscopic level.
Yeah.
To achieve those macroscopic results that we want.
So we're getting pretty advanced here.
We are.
We've covered the basics.
Yeah.
We've covered some of these advanced techniques.
Techniques.
I like to do a recap.
Always good to recap.
Make sure I'm following everything.
Make sure everything's sinking in.
Yeah.
Pun intended.
So we started with the fundamentals.
We did. Yeah.
Wall thickness.
Uniform wall thickness.
Rib placement. Strategic rib placement.
We talked about holding pressure.
Holding pressure.
Injection speed.
Injection speed.
Cooling systems.
Cooling is critical material selection.
Yeah. And all of those play a role in preventing sink marks.
We got into gas assisted. Gas assisted and microcellular.
Microcellular. Yep.
It's been a lot.
It's been a lot of information, but I'm feeling good. Good.
About my understanding.
I'm glad to hear that.
So what's next?
Well, now that we've laid all this groundwork, I thought it'd be fun to explore.
Okay.
A really intriguing concept.
Right.
Turning shrinkage from a foe to a friend.
Wait, what?
I know, right?
We can use shrinkage to our advantage.
To our advantage?
I mean, it's the thing we've been trying to prevent this whole time.
It is. Okay.
I'm intrigued.
We'll save that for the grand finale. Stay tuned. Okay, so we're back, and I'm ready to have my mind blown. How do we use shrinkage to our advantage?
Well, it's a whole new way of thinking about it.
Right.
Instead of dreading shrinkage, we can actually see it as a tool.
Okay.
A way to achieve specific things in our design.
So how do we go from dreading it to, like, using it? It really starts with understanding how different areas of a part are going to shrink based on their geometry, the wall thickness, the material you're using. So let's say you're designing, like, a snap fit enclosure for, like, an electronic device. You could actually strategically design those wall thicknesses and rib structures so that certain areas shrink more than others during cooling. And that creates those precise undercuts and protrusions.
Oh, wow.
That you need for a secure snap fit.
So it's like you're predicting the shrinkage.
Exactly.
And manipulating it.
Predicting and manipulating to create the feature. To create functional features.
That's wild.
Yeah. You're working with the material's natural tendencies.
Right.
Instead of fighting against them.
Okay, so snap fits, but what about. What are some other examples?
Well, think about adding texture to a surface.
Okay.
You can vary the wall thickness in certain areas.
Okay.
And that will control how that surface shrinks and create patterns.
So you could have a grippy texture on a handle.
Exactly. You could have a drippy texture on.
A handle or like, a decorative element on a panel.
A decorative element on a panel. You can get really creative with it.
It's like you're sculpting with shrinkage.
Yes, exactly. Shrinkage becomes your artistic tool.
That's amazing. It's a whole new way of thinking about injection molding.
It is. The more you understand about it, the more creative you can be with your designs.
It's like this blend of art and science.
It is a beautiful blend of science, engineering, and artistry.
So as we wrap up this deep dive.
Yeah.
What's the key message you want to leave our listeners with?
I want them to walk away feeling empowered.
Okay.
Knowing that they have all these tools and techniques at their disposal.
Right.
To tackle those sink marks and maybe even use them. And maybe even use them to their advantage.
Exactly. It's not always a bad thing.
It's not always the enemy.
Right.
Sometimes what we see as a problem can be an opportunity.
Yeah. To innovate.
To innovate.
That's great.
Yeah.
So I think we did it.
I think we did too.
We went from sink marks, being the.
Enemy, the dreaded sink mark, to being.
Like maybe our friend.
Maybe our friend. Potentially.
Potentially. We learned about wall thickness, rib design, holding pressure, injection speed, cooling systems, material selection, glass assisted molding, microcellular molding, all sorts of things. I mean, it was a deep dive.
It was deep dive.
We went deep.
And you know what? We're just scratching the surface.
I know. There's so much more to learn.
There's always more to learn.
So keep experimenting. Keep pushing the boundaries.
Don't be afraid to experiment.
And who knows? Yeah, you might just stumble upon that next big breakthrough.
You might just revolutionize the injection molding world.
Well, that's it for this deep dive into sigmarx.
Thanks for joining us.
We'll see you next