Vacuum Forming ABS - minimum form radius 1

[KENAT]

I'm looking at a vacuum formed design. We worked with a couple of potential vendors and came up with a design that was well suited to manufacture, they especially liked the large radii we'd used. Then our VP questioned the Industrial Design of it so we had it industrial designed by a consultant and their design concepts have much smaller radii as well as what appear to be small steps or the like.

I was wondering if any one could point to a design guide that would help or maybe answer the specific question of minimum suggested bend/form radii on ABS around 1/8" (3mm) sheet material?

I found this

http://support.knowlton.ohio-state.edu/files/FormechVacuumGuide.pdf

but didn't see an answer.

thread710-137720 was about the only thing I found on here.

I'd go back to our potential vendors but I feel we've pushed the relationship far enough without committing, and I'm still waiting for purchasing to say which vendor they prefer. - So I'll take advantage of you guys instead;-).

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[MikeHalloran]

You can probably do 1/4*T _inside_ radius, and still stretch a little material over the edge. You can almost do a sharp corner if you don't need to pull any material over it. Think of it as a drawing operation. ... which it is.

What matters most is where the material in the finished part comes from. E.g., you can vacuum form a part in the shape of a Bundt pan, but you get severe wall thinning as you decrease the inside radius of the torus. As the central cone approaches the size of a finger or thumb, the material either thins to transparency or fails in tension.

It may help to do a thought/ pencil experiment. Imagine that the raw material is covered with a continuous array of small circles inscribed in squares. (Diemakers actually do this for trying out sheet metal stamping dies, and images of such sheets or formed parts sometimes find their way into advertising material for the metal trades, so you may have seen it) Try to estimate where each circle will end up in the finished part, and how big and how oval it will be, remembering that the number of circles doesn't change even when the surface area does.



Mike Halloran
Pembroke Pines, FL, USA

 

[HDS]

look through the thermoforming quartiles and you will learn more about the process. There was an article in the thermoforming 101 series about radi.

http://www.thermoformingdivision.com/thermoformingquarterly/index.htm

There are also design guides from extruders and resin suppliers like Spartech and Sabic/GE.

Be carefull of thinning with sharp radi.

Any radi on the opposite side of the sheet from the mold will have a radi about the sheet thickness at that point. Did you decide on a male or female tool?

If you are using textured sheet be careful about how stretching will change the texture.

Pressure forming can give better radi and crisp lines.

Did the ID know you were thermoforming the part? Good ones will design for the process. The rest won't.

 

[KENAT]

Thanks guys.

Your point on where the material is coming from is well taken Mike.

HDS -

http://www.thermoformingdivision.com/thermoforming101/articles/pdfs/2007/thermo101_corners.pdf

talked about corners, thanks.

The thinning was my concern, we've already considerably decreased the 'sheet thickness' to aid manufacture and too much thinning could really be an issue.

I think they said female tool.

We want to stick with vacuum forming as we have a very aggressive cost target.

The ID was told a lot of things, however whether he paid attention/fully comprehended the implications of some of them is another matter.

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[HDS]

You can specify a minimum part thickness in the drawing notes. Also point out the areas that are critical and they can be focused on during the tool design. I highly recommend you visit the vendors plant to discuss the details of the part and see the process in action.

I think there is another 101 article about thickness.

Pressure forming may not ad too much cost if all that is added is a pressure box. It depends on how it is done, what you need and the equipment the vendor has. The small increase in tool cost could be offset by reduced scrap rates.

 

[KENAT]

Thanks HDS, as I mentioned in the OP I've been in touch with the likely vendors and we actually did visit their sites. They very much liked our original design. However, now that it's been industrial designed some of the radii have been reduced by as much as an order of magnitude so I'm a little concerned.

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