Plastic selection for vacuum forming

Several materials are commonly used; I'd think of ABS (acrylonitrile-butadiene-styrene) alloy first, but it may or may not suit your needs. You can search for "Vacuum forming" or thermoforming (slightly more generic term), but one starting point might be http://support.knowlton.ohio-state.edu/files/FormechVacuumGuide.pdf. I've made some parts from a vacuum-formed material that softened just enough when in the sun on a car dashboard that they lost their shape (never actually melted, but sort of drooped); I think that was Kydex (acrylic-PVC alloy). It may come in grades with higher heat deflection temperatures - and you'll need the better rating, too, for situations like a dark car sitting in the sun after being shut down, engine still hot and radiator radiating nicely.

Hi,

I can't make any specific recommendations, but I can give you some questions you need to answer in order to find a solution ... IF there is a solution.

First, I assume the detail gained by thermoforming is good enough for your application. Generally the detail 'against the forming tool' is pretty good whereas the level of detail on the opposite side of the material is much worse. General shapes and profiles are good, but if you are after strong details (lines, shallow engraved surfaces, textures, sharp corners) a thermoformed part is not nearly as detailed as other processes. A lot depends on the thickness of the material being formed, how well your vac-form tool is designed (vac holes in critical positions), and how deeply you are drawing the material.

Secondly, you need to know the final thickness of the material, and in this, you need to realize the deeper the draw, the thinner the material will become at the bottom of those draws. (consider if you are working with 1.0mm thick material, and you are pulling that material into a 25mm diameter hole, 25mm deep. The sides of this example cylinder are ~2000 sq.mm. and the bottom is ~500 sq.mm, BUT you only have ~500 sq.mm to 'stretch' into that space. It won't do so evenly, but be thicker at the 'top' and thinner at the 'bottom', but if you just (for the moment) assume it is even, the final thickness of the material in that 'hole' will only be .2 mm thick). In reality the way the material stretches across a form is very complicated, but you need to know what is the final thickness you want in order to know what beginning thickness you need.

Next, you need to know what is the operating temperature of your finished part. This is the place where you may discover that thermoforming just won't work for your application. It was already mentioned before when it was mentioned about the material becoming 'soft'. When you learn what is the operating temperature, you need to find a material with a "creep temperature" that is higher than that. For general automotive, I don't think you will find an appropriate material. Generally, automotive materials must be rated at 90C, and I'm not sure if there are any thermoforming materials rated that high.

Next, and this relates to the temperature issue, sheet stock has a "grain" and the "shrink factor" differs between the 'roll direction' and across the sheet. Generally, this won't have much effect until you are at the creep temperature, but at that point, the part will begin to 'relax' differently in different directions.

Next, you need to know how you plan to finish the part. If you plan to 'plate' (actually, vacuum-metallized) the part, or paint it, you need to know the nature of the material you are forming in order to do any finishing process. Some materials, such as PE (polyethylene) are very difficult (if not impossible) to finish.

Finally, once you have all the details gathered, I suggest you talk to a "sheet converter", and you can find them on-line, or via other source books. Plastics companies (GE, Borg-Warner, Bayer, etc.) don't sell sheet goods, but only pellets. They sell pellets to the converters who sell sheet or rolled goods.

I know I raised maybe more questions than answering questions, but these are things you need to know before you can really find the correct answers.

I know you already have your form tooling, but if this is truly for an automotive application, you would be much better to make a fiberglass part than to use thermoforming as your manufacturing method.

Good luck, and best regards.

DCad is correct in stating that you need to determine the useful life of the product and the specifications that must be adhered to. With that said, both ABS and PC/ABS alloy are used in automobile and truck grills (International uses a PC/ABS platable grade). It would be wise for you to contact a material supplier, such as SABIC (formerly GE Plastics) for the proper grade for your specific application.

http://www.geplastics.com/sfs/SFS/en/IndustryApplications/IndustryDetail/transportationoutdoorvehicles.html

Thank you very much for the information

The parts are indeed for a grill, and headlight surround, and the company who will eventually be doing this is taking the Ford approach, "any color you want as long as it is black".

My first suggestion to them was fiberglass or other 'applied material' form of molding, they do not want to environmental problems that go with storage of resins and hardeners.

They also don't have the ability to go with injection molding of the parts. So vacuum forming it is. No great detail is required on the outside of the parts, it is the shape they are after. I will do some testing to find the radiator temperatures for the vehicles under various typical usage scenarios.

Again thank you very much for your comment, they have been most helpful.

You should touch base with the engineers from FRIMO, or BROWN, or whoever the thermoforming equipment came from. They should be very familiar with materials that will suit your needs. They will also be able to work with you to be sure that you have the correct set up on the equipment for the selected materials.