1

2

It is immensely satisfying when you pull off a complicated recipe and the same goes for designing parts in semi-crystalline polymers.

3

With the right application knowledge, the right process, and the right material, it can be surprisingly effective.

4

But approaching the design and worrying about how to make it as an afterthough will inevitably lead to trouble. Worst still, making simple errors (that could be so easily corrected early on) can lead to costly mistakes, and or dissatisfaction with the results.

5

6

7

8

There are a lot of potential material candidates out there, and at DuPont Performance Polymers, we choose to focus on semi-crystalline ones, as you can see in the ‘Pyramid of Plastic’. Needless to say, there are sometimes multiple good materials for a design, but not always. Knowing which material to choose and when is an important part of a design project. Far from being a constraint, early identification of potential material candidates is critical to the success of many projects.

9

Due to the limited time available, focus here is mainly on injection moulding (and even then, it is impossible to cover the breadth of design and engineering knowledge relevant to all semi-crystalline polymers).

10

I realize that there are many people with significant experience in the world of Plastics, and I cannot make everyone happy with the specific technicity of this presentation, so I will focus on some ‘Joker’ cards for semi-crystalline material part design. Think of them as like ‘get out of jail free’ cards. If you’re armed with these pointers, you’ll likely be well on your way to robust cost effective designs…

11

Here is the thing though, you would already have considered how you would go about cooking your meal if you were inviting people over, but the context of the meal is sometimes critically important. The same goes for the end-use of your design. You simply cannot neglect the operating environment of your design. Think Hot/Cold, think biology, think

12

This is the only graph, I promise. I just want to reinforce the fact that materials semi-crystalline polymers behave very differently over a temperature range. And don’t forget, the long tails are not the area you want to design in. Whilst the yellow box is a gross simplification, the area shown can at least be considered almost ‘linear’ in terms of material behavior, and that is where ideally you want to operate. Also, the data presented here is relevant for a 4mm thick ISO bar in tension… unless you are designing 4mm thick ISO bars, you will need to apply knock-down factors that represent processing, environment and safety….

13

14

Whilst your loaf of bread might be preferred with the right size and distribution of bubbles, a part in semi-crystalline polymer with a similar structure is very undesirable.

15

Here is where semi-crystalline materials may diverge a little from the cooking analogy. As the crystalline structure of the material tries to organize itself on a micro level, the part can undergo some changes on the macro level. Remember to position the gates in the thickest part of your design, and aim to have the thickness profile of your part decrease as you move from the gate. I grew up in the eighties, hence the Tetris™ style analogy.

16

We say standard applications as there is nothing stopping you from designing a part thicker or thinner than this recommendation. However, processing is almost certainly going to become complicated. Difficulty to fill a thinner part can be expected, and voids/warpage can be expected at the thicker end.

17

So you need a simple part, and you’d like it flat… The design is simple, and not so simple… Differing thicknesses could well be a problem. It is not always avoidable but when you can, it’s best to try and keep things constant…

18

The reason comes back to the Tetris concept a few slides ago. The material is going to try and reorganize itself, and in the thick sections of such a part, there is more room for reorganization, and thus there will be more shrinkage in this area…

19

You can try and correct your tooling for this, however it may be less expensive and easier to avoid the change in thicknesses…

20

And do something like this…. Now granted, perhaps you cannot for some functional reason, this is not an absolute rule… It is merely a heads-up that this is the type behavior you can expect… And what if we throw glass fibers into the mix…?

21

Design shouldn’t be stressful… But with sharp corners it can be… So the next thing you should do is put radii on any corner that is likely to see some deformation… And then put radii on the corners you don’t think will see some deformation… The go back and add radii to the corners you missed… then increase their size… (to at least 0.5mm).

22

So now your part has a constant thickness, or close to it… and you have added some nice radii…

23

Well, in a perfect world, that would be all. However, due to the same behavior explained via Tetris, there could be an unpleasant surprise waiting for you…

24

So you have to go back and change things around a little.

25

These are recommendations, all designs cannot always follow the rules. But if you have them in the back of your mind, you might be able to avoid getting into trouble.

26

And, well, sometimes you might need to perform a little magic trick, and make things disappear in front of your very eyes….!

27

If you can’t find a way to weld your part to another part, or snap it to another part, you might need to uses some screws… but before you go choosing those nice conic head screws that sit flush with the surface… What’s at play here is your processing. The material already has to weld itself together around the pins in the mould. If you then try and push it apart at that point, something’s got to give (any guesses?).

28

So by adding a screw boss, or at the very least reinforcing the weld-line, and choosing flat head screws, you can avoid problems before they happen….

29

30

31

32

33