Design for Manufacturing Podcast Transcript

Kelly

Hello everyone. My name is Kelly Riley and I’ll be your host for today’s podcast. In this week’s episode, we will be discussing design for manufacturing with Jon Cottrell, who is the lead program manager for PTI’s commercial engineering group. When designing a plastic part, there’s so many items that are important and things to consider from uniform wall thickness to gate location to applying the proper draft to your part model…

So this podcast is intended to shed light on the key areas of design.

Welcome to the podcast, John, and tell us a little bit about yourself and what’s your background in the industry?

Jon

Well, thank you. I’m glad to be here. As mentioned, I’m the lead program manager here at PTI. I’ve actually been in the plastics industry for the better part of 25 years, starting right out of high school.

Throughout my career, I’ve launched products in many different industries, from automotive to consumer products, furniture, robotics, defense and more recently here at PTI, with the high focus on medical devices and monitoring devices. 

Kelly

Fantastic! And in particular industry that you really like to work within or do you find find little things here and there that are appealing?

Jon

Each industry has their own pros and cons. 

Kelly

Pros and cons. Okay, So… all right, well, we’ll dive right in. So why should a designer consider the manufacturing process when they’re in the design phase of the product? Why is it important?

Jon

Really understanding the processes that you’re going to use to manufacturer will allow a designer to apply principles for how to design a part, knowing how it’s going to be made lends itself to how you should design it.

All too often we receive designs that look great on paper. However, design will not work for injection molding. They’d actually be better suited for like a machining application. As I said, I can’t tell you how many times I received a part, only to realize that the designer had no idea or took no consideration of that it was going to be an injection molded part.

And by not understanding the requirements of an injection molding design. That’s a lot of cost and time to deliver the part.

Kelly

Yeah. Which is going to set your project back right from the get go.

Jon

And not only my project but their project.

Kelly

 Their project, and ultimately the timeline they’re trying to deliver on.  Okay, so what are the main design aspects that a designer should be considering when they’re developing that part to be injection molded?

Jon

As as we know, we could go on and on about this, but it will kind of focus on some of the main items to consider.

Those include materials you use, which some people call resin. I’ll refer to it as material. Wall thickness, draft angles, parting lines where we want to gate the part, how you want to inject it, the bossing and ribbing on the backside side. And if it’s not a stand alone component, how’s that part going to be joined together?

Kelly

Okay. So when you’re looking at your resin or your material, what should you consider when you’re when you’re considering your your materials that you’re going to use?

Jon

The first question to ask is what environment is the part going to live in? Is it going to encounter chemicals or extreme temperatures? Will it be a cosmetic part or something that people touch on?

Do we need a highly engineered material for performance? And I always recommend people lean on the resin manufacturers for material guidance.  Molders such as PTI, we have our preferred suppliers that can help recommend the grade. However, fully understanding who’s going to use, what’s it being used for, where is it being used and how are they going to use…goes a long way.

It’s kind of like developing a story the way we learned to do in grade school. The whole who, what, where, when, why and how.

Kelly

Why and how. Okay. Right, Right. Because if it’s going to be under the hood of a car, no one’s going to see it. Well, you’ve got maybe. And I imagine cost is even a consideration, too, with materials like why would you spend money on a highly engineered resin that no one’s going to see?

So there’s probably a lot of different things that go into that. Okay.

Jon

Definitely. But most designers really don’t care about cost.

Kelly

Oh, oh, that might be another episode. Okay, so what about things like the wall thickness? I think you talked a little bit about that, but so how would the resin play into that? And is there too thin of a wall or too thick of a wall?

Like, where would you go with that?

Jon

Well, wall thickness can be a tricky response. However, a good rule of thumb is to keep a wall stock between two and three millimeters and also keeping it consistent. Wall stock will help the post injection shrink to be more consistent and predictable.  And one thing I do want to point out is that this is coming from a perspective of a non thin wall molding.  

Thin wall molding is a whole other animal that PTI doesn’t get into. So that’s why we do recommend a 2 to 3 millimeter wall stock. But thin wall molding, those are the containers and bottles that you see in packaging.

Kelly

Okay. Yeah, more like packaging materials. Okay.

Jon

I want to bring up an example from early in my career in regards to wall thickness.

And it’s something I’ll never forget and I always go back to.  One of the first companies I worked at we were making is very cool handles for large furniture manufacturer and they look like wood and they’re about three quarters of an inch thick. And the wood or the material was probably even had a wood filler in it, because I remember it always smelled like a bonfire when we ran the part.

But the intent was to replace the wood handles that they historically had used on their furniture. However, that large mass caused a whole lot of processing issues and the parts actually had to be dropped into a chilled bath of water to process through.

Kelly

Every part had to go in

Jon

Every part. So I added a ton of time, a ton of cost

Kelly

That must have been a cost driver.

Jon

It definitely was. And if the parts didn’t go in that cold chill bath, they fell on the floor for whatever reason, they would actually shrivel up and look like a coiled snake on the floor.

Kelly

Oh my God.

Jon

So anytime somebody comes to me with a thick wall stack, I always go back to looking at that part and be like, All right, this is why we want a consistent, relatively thin wall stock.

Kelly

Yeah, and consistent. Okay.  Very good.  Yeah, that that had been an interesting process.

Jon

It definitely was, so…that’s we always want to keep that wall stock to a good manageable level. And then one last point I wanted to make on wall stock, material selection does play into that, as you alluded to earlier, because a part gets too thin, the material may have a harder time filling, which would cause another level of processing issues.

Kelly

Processing problems. Okay. Okay. So talking about processing and then like material flow and everything, how I’m sure again, the material matters there. Then what about when you have to decide where to put a gate in on the part design.

Jon

Gating a part. If you don’t know what it is, is where the plastic enters the mold and flows into the part geometry.

This dictates where the flow fronts and knit lines are going to be on the finished part. Also, getting can affect the visual and tactile surfaces. For instance, getting on the inside of a part could leave a vestige that a rubber glove would tear on. And we don’t really want that to happen. So a designer would need to understand what options are available for gating.

Kelly

So that the A-side is the visible part.

Jon

That that is correct.

Kelly

Yeah. Yeah. So right. If you had something that a doctor was going to handle or a medical part and it wasn’t gated properly, you’d be causing a lot of problems in the medical setting, in an operating room or something like that. You wouldn’t want to tear a glove. Okay.

Jon

And that’s one thing. Like during the tool design development process, we ask these questions.

A lot of times we have to make part changes to allow for a specific gating scenario, which if this was vetted out during the design phase, it would save time and we wouldn’t have to be doing any post design for these changes to allow for the propagating based on the use of the model.

Kelly

So there’s different types of gates that are involved, I’m sure depending on the type of material that you’re using…So in different situations. So what kind of tell us what kind of gates there are and where they’re used at?

Jon

Well, the most common type of gate is an edge gate. An edge gate is something that a lot of us are familiar with. If you remember back to being a kid and building a model, the model parts came on like a a runner system or a frame that you had to break the gates off on.

And now some of us now are doing that for our own kids. Well, those tend to leave a large amount of vestiges, if not trimmed or broken off properly. That’s the analogy about the gloves and the the doctor’s gloves. But there’s also what we call a sub gate. These are sheared off during the part ejection.  A little more fancy little less vestige.

Then there’s cashew gates, which are typically wrap around to the backside of the part or the B side, as we call it. Th A side is our show part, the B side of the backside. Okay. And then if we want to get even more fancy, there’s several types of direct gates or some people can call them veiled gates.

But at this point, a designer really should start consulting with a tooling expert during that part development to really understand the limitations of each. Quick internet search can provide pictures and a general overview of what these look like. We also need to be wary of material selection because that does play in the gates selection or gates style, because not all materials work with all Gates styles and dispersal.

Kelly

Right.  So you could find some references out there, but really you need to lean on your potential tool builder to help you vet those things out to make sure that you’re going down the right path.

Jon

And just like I said, just a little bit of homework can go a long way so we can get those things designed in or at least identify locations and type before we even get to that tool design process.

Kelly

Okay. Okay. So then the part ejection… So what types of variables do we have to consider about…we’ve got our part molded, now we got to get it out of the mold.

Jon

Well, as you put it, we’ve got to get out of the mold, it’s a very critical part of the production process of making sure the ejection system and points of push are not going to interfere with any of the functional areas should be noted.

We love to see areas where on drawings, ejections okay here, not okay here…specifically if you have areas that are highly functional on the B side of the part and also a draft is very important for the parts to come out of the mold.

So what is draft?  You’ll hear us talk about draft or draft angles throughout this discussion or you know, in general design work.

So draft is if a wall is perpendicular to the surface of the tool, it can’t be a straight a straight wall is going to cause a lot of friction and potentially the part will sticking out at all or we will get what’s called pin push because we have to pull that straight wall or push that straight wall out of the tool.

And yeah, we always joke sometimes if a parts not designed right, I’ll give you a great look plastic part, but it comes with a big piece of steel.

But all joking aside, the draft angle allows the feature to release from the tool as it starts to move out of the steel.

Kelly

Okay. Okay. When the tool or the part is cooling, like…does that…how…that plays into draft too?

Like so you have to consider how a part is going to shrink?

Jon

Shrink does play into draft a little bit because…features that are inside of the tool potentially may need to have a little more draft and stuff on the outside.  And something I’m asked all the time is “how much draft do you need?”

We…our initial response is going to be one and a half degrees, which doesn’t seem like a lot, but it adds up really fast.

We do have parts that have half degree or quarter degree draft, something that we really don’t like to do…It can be done, but it’s just not ideal.  And it potentially could cause some problems. Through the years, I’ve seen, like I said, I’ve seen parts with little draft and lots of ribs sticking because of it.

It’s pretty cool sight when you see a part stick in a tool, but it causes a LOT of problems that the tool shop, the process engineers, the product engineer we all have to take care of.

Kelly

Right. That you would be vetting all this out in your prototype phase. And when you’re you’re developing all this. So you kind of you want to get ahead of it.

You don’t even want that to happen in the prototype phase.

Jon

Correct. Or just sort of the design phase. And one thing that should be noted, if a part has any texture or surface finish on it, that there are specifications for how much draft those surfaces need. There’s all sorts of SPI finishes and then in different industries they have different textures, such in the automotive industry, there’s a lot of animal prints or highly engineered finishes and they provide specifications on how much drafting angle is required for those.

Kelly

Okay. So that’s kind of like the B side of the background…What about improving the esthetics of a part the A side of the part?

Jon

Well, most designers, they’re trying to make a good looking part anyways and typically they achieve it. But executing from a full design standpoint, its sometimes missed.

When I say full design standpoint, they make it look good on the two and what your eyes see, but how does the whole system work together?

Many of the items we already discussed play into that, such as the draft on the visual surface, because if you don’t have enough draft, you’re going to get what we call scuffing where that texture, the peaks and valleys of the texture, are going to scrape kind of like sandpaper, or if you scrape it across the part one time.

But another area that needs attention are the features on the back side or the B side. And we kind of alluded to that. Those do play a major role in what the A-side looks like.

Kelly

Okay.  A supporting role.

Jon

You got it. Back side supporting the A.  Alright, these features could include the structural, the connection points of the project or products such as ribs and bosses.  As with some of the items mentioned before there’s documented design principles to follow, such as a ribs should only be 50% of the thickness of the walls connected to…I guess, to explain what that means, you know, this morning, as I was getting prepared for our discussion, I was looking at the speakers on my desk and just like anybody else in the classical world, when we look at plastic parts we critique them.

My first impression was, hey, they look pretty good. But then I notice there’s some read through all the way around the part. And that was the ribs on the inside that gave it structure. They were either designed improperly or processed improperly, but I could see them.

Kelly

 You can see on the A side where those ribs are faintly, like just a little…

Jon

You got it.

Kelly

Yeah. But enough that, hey, it’s there and it’s not supposed to be.

Jon

Possibly most people want to see it, but people that are working in a place where we definitely pick up on that stuff.

Kelly

Right, right. And then, I mean, if it’s a high quality, if it’s in a consumer good world and it’s a very expensive speaker, you don’t want to see anything like that.

Jon

That is correct.

Kelly

Or even a medical device where all those things are critical.

Jon

And then another thing to consider with the esthetics of the part is actually having enough support on the back side. We don’t want the part NOT to have structural integrity or if it needs…or if it has a mating part that those mating features are…provide a nice clean assembly.

Something I learned a long time ago while working with Japanese OEMs on some automotive projects was about a tight fit or finish with making parts.

Once we were…got our parts dimensionally correct, the focus then turned to the gap and flushness of the system and making sure the parts not only function properly but had great mating seam was imperative. And really, to this day, understanding that gap and flush requirement and how it translates into the quality level of part, it sticks with me and I even kept the tools utilized for checking those.

Kelly

Okay.

Jon

But again, that goes back to the…what’s on the back side of the part and having enough structural integrity that those parts fit together properly.

Kelly

Fit the way they’re supposed to. Okay. And so, then that leads me into the next question about tolerances. So, I imagine that that’s where tolerances become critical, too, if you’re mating a part.

But so, what does a designer need to consider with part tolerance? And again, the material selection?

Jon

So earlier I mentioned I get asked a lot about draft. The number one question I do get asked is what tolerance can you hold?  Fortunately, there’s a pretty easy answer on that as there’s a LOT of documentation out there…White papers, books, industry guides that have been published over the years that contain tables and recommendations to been relating to materials and their expected tolerances based on part size.

My goal when asked this question is to help the designer to…or guide the designer to look at those guides and to even provide samples of those guides to them so they have something to look at and base their tolerances on.

Kelly

Right. Right. Use the reference materials that are out there. Work with your team.  Okay.

Jon

But one thing I do want you to know is…we always break the rules. Those are guides…we break them.

Kelly

They’re suggestions.

Jon

Yep. It’s…we break them both in a good way and bad way. Because the thing is, we never see a part that is a perfect part relative to how those design guides were developed. They were designed off of basic shapes and part geometry.

We don’t make basic shapes and part geometry… we make complex shapes.

Kelly

So you have a little wiggle room and decision making. Okay. Okay. Back to, again…like the point you made about joining parts together…So how…there’s probably a lot of different ways that plastic parts are joined together. That’s got to be critical in the design phase, too.

Jon

It is. It’s not my specialty, but I do know enough to be dangerous. The first comment I would like to make is we recommend you stay away from adhesives such as glue or epoxies. They can be messy and consistent, if not vetted out correctly, have an impact on the plastics…but…

Kelly

It could degrade the material. Okay.

Jon

Yep.  However there’s…I’ve seen or the last few years of better and better application processes, so it’s slowly changing my perspective on it, but it’s something that PTI we’re really not comfortable with.

We believe that there’s better ways to do this.

There’s a, you know, the number one way I would look at is using snap fits or press fit joints. There’s a…I would like to use a litany of them. There’s a lot of different designs out there that are proven for snap fit joints. We have a docket of our own designs that we’ve used over the years that we really like.

We know that work well. We’ve also seen a lot of our customers using thread forming screws. Those are used a lot for non-sterile small occasions, i.e. projects that we have to assemble one time and that’s it. If something needs to be serviceable and they want to use screws like cap end screws, though a lot of times we’ll see threaded inserts or heat staked inserts that screw can go in and out of multiple times with no degradation to the joints.

Additionally, we do a lot of joining with Ultrasonics. It’s a process that can give you a very nice seal, goes well with products that have geometries that don’t allow for snap fits or force screws or bolts or nuts. And it’s a very repetitive process, very easy to develop and to have a good seal.

Kelly

Repeatable…quality, all that kind of thing.

Okay, so there’s probably everchanging rules, ever changing advancements…So what kind of maybe technical advances in injection molding does a designer have to be aware of, or would that be, you know, your tool builders role too and helping you sort through those and stay on top of it?

Jon

Like we said earlier, you know, we’re kind of looking at it from a starting point of, like a centralized general design principles.

With any industry, there’s continual advances. Plastic’s no different.

As we look at new technologies, you know, where, I guess, as part of this discussion and my recommendation with a designer would be let’s start with the basic principles. And then as that design develops, if we as a team want to look at some newer technologies or newer ways to manufacture, then we’re looking at system design, and that’s equipment.  That’s an injection molding, that’s auxiliary capital, you know, something bigger than just the product design.

Jon

So I guess in summary, my overall suggestion with designing for manufacturing is to start with the general principles that we’ve discussed today. We have some other guidelines that we can provide customers, engineers, designers as to get a good understanding of what these principles really look like, more graphical, more technically detailed than what our conversation is today.

Kelly

Right. But in like engage early, get your team together, probably use the references that are out there. But early engagement with your team is probably the best advice that you could offer?

Jon

Yep. And as I mentioned, we know we have a documentation and design guidelines that we can supply to customers and those can be found on our website.

Kelly

Very good. Well, thank you for joining us today, John. I hope, you know, that everyone was able to take away a little something.  You gave some good advice, some food for thought and yeah, we’ll hope that we can continue these kinds of discussions and help people out early on in their design process. So thanks for joining us today.

Jon

That sounds good. Thanks for having me.

Kelly

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