casting vs injection molding for TPE"S and design process


I’m havin trouble on how to begin a project I’m trying to develop. Basically I need to make a semi soft joint that wooden sticks can be inserted into and let the joint be pliable but stiff enough to hold the sticks up. The end product is for a childrens learning toy, so hazardous chemicals and off gassing are an important issue. I was looking into silicones but I do not need their extreme physical and chemical properties and their costs are high. I have been researching thermoplastic elastomers from kraton, sabic, gls etc. This seems to be a more correct material for my application. I’m unsure of the hardness I need, I like the of the roller balls in some dell mouses and the hardness of some calculater buttons. Sorry for the vagueness. How do other designers determine hardnesses? Without a hardness scientific test. Do material suppliers send out material samples?

Also since we are a small startup we are looking into alternatives to injection molding since our beginning runs will be low and dies may be cost prohibitive at this point. Can TPE’s be casted with a silicone rtv mold? Or any other alternatives?

How do design firms go from sketching ideation to touch and feel prototypes?

Any comments, suggestions and critisicms are greatly appreciated

The measurement of TPE is based on the Durometer of the material and measured in the Shore manner Shore durometer - Wikipedia

While they give a rough table of Shore, and mathematical calculations, the only way to start picking one is the get samples in different Shore levels and feel them. Most TPE suppliers have a “key chain” of different shore levels you can test for how they feel.

Even than you have to test what you think is correct and modify from there. Different shapes create different feeling in softness.

Hope this helps

Thanks Tim that does help. I have spoken with a few TPE Reps today so hopefully I can get some samples. Would I be expected to pay for the keychain?

Thanks again

Any comments are greatly appreciated


TPE is a thermoplastic, it needs to be heated to the melting point in order to flow. Although I have read references to casting thermoplastics in literature, I have yet to find an actual example of someone doing it. You would need pressure as well to get the molten plastic to flow nicely into a mold. The RTV mold is not suited to handle this.

Cast two part urethanes are the best material to use with silicone molds, you can experiment with a range of durometers from soft to rigid. You can do prototyping and test the material, is is quite strong.

Skateboard wheels are all done with cold cast urethanes for production, some other products requiring lots of resilience are also cast. Thermoplastic do not have the bounce that cast plastics have. You can go to a skate shop and get an idea of the hardnesses as they are usually marked on the product. 75A bouncy soft up to the high 90’s rock hard.

Process to get to prototype: you have your 3D file machined into ABS plastic or aluminum, a RTV mold is made from the first machined part, a urethane (2 part resin) is cast in the RTV and extracted. Multiple parts can be made in a variety of hardnesses.

If your volumes were low and the geometry of the part was suited to it, cold casting in machined aluminum molds might be lower start up cost. Injection molding is the only real way to get economical larger scale production.

You might want to look into compression molding

The tools are much cheaper and you can still do silicone. You can also do EPDM and natural rubber.

Thank you both so much for the excellent help. I will do some more research and post my results here. Again thank you very much

It sounds like PVC mixed with a plasticizer might be an option if you’re on a budget. It’s used in everything (not just pipe), has many options for additives, is chemically resistant, and it’s dirt cheap.

Injection molding machines are great, but mostly only for long term production. Dip Casting(or Dip Coating) might be the option if you are just looking to prototype at this stage. A CNC aluminum cast and an oven would be all you need. The mold would also be reusable, allowing for testing of different softness/desired mechanical properties. A disadvantage is that you would be required to cut out the plastic prototype from the mold, but that all depends on the design of the product, i.e. whether it needs to be hollow, how big it is, etc.

You’re best bet would be to call the company up and describe what you need. Also, a plastics encyclopedia would be a good idea( I got mine, Modern Plastics Encyclopedia, for free from my University).

Some excellent resin information can be found at the following website: (Search Engineering Material by Property Value)
and my personal favorite!..
The Macrogalleria

Also, for your application, you might be more interested in flexural modulus than hardness(Rockwell or Brinell usually).