Best production method for simple piece of plastic 4

[raphjutras]

Hi everybody

I'd like to have some tips from a professionnal who could tell me which production method would be appropriate for the type of plastic part I want to do... Could anybody help me to decide what would be the most appropriate production method and please consider that I know absolutly nothing about plastics and that is why I'm here...In fact, I have a project in mind and I would like to concretise it, I know I want a plastic piece, but I don't know what would be the appropriate production method, something not to expensive so please consider I can't afford a 20K$ mold...

I would like to produce ex: max 50 piece/year (for now)
The piece of plastic i'd like to do would be a cover that will go over a wood plate. The plastic part would be the "visual" part of the product so if there is any engineering method that produce a clean finish, it would be perfect!
Imagine like a puzzle box cover, but in plastic, that's what looks the most to what I want. (PS: I've included an image 3D of what it look like)

To give an idea: the piece would be 18" x 14" x 2" width, thickness: about 1/16"

I've heard about thermoforming, do you think that it would be appropriate?

Thank-you in advance for any answer!

Raphael

 

[KENAT]

Well, you'd need some radii on the corners and draft etc. but it sounds like thermoforming may be a good place to start so long as the tolerances don't need to be too tight. Getting a consistent thickness makes it a bit trickier - if you really need that - but a good shop should be able to do it. You may have issues with the large flat surface 'sagging' so may want to incorporate some stiffening feature or allow it to be thicker or variations there on.

Many paddling pools and similar are made by this process as I understand it.

I was able to get aluminum vacuum form tooling for a slightly larger, much deeper and more complex part for much less than $20k a year or so ago. My memory is a bit vague but it may have been around 7k or even 5k.

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[KENAT]

Oh, another thing, with vacuum forming you can even make temporary molds out of wood/mdf etc for prototyping, though shop around for price. I had one place charge almost as much for the wood tooling as for the aluminum tooling on the above mentioned part.

These molds might be good for 5-10 part or similar, sometimes you can even develop an epoxy mold or similar from it which will last longer. However, while I had places claim they could do this, when actually asked to do it they declined.

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[raphjutras]

Very interesting KENAT, thank-you very much for sharing your personnal experience!
I will look for a good shop that does thermoforming around here if it seems to be the appropriate method

 

[MikeHalloran]

For 50 pcs/ yr, it might be possible to do it by solvent bonding acrylic rectangles. You'd probably need to go to 1/8" thick so it wouldn't be too fragile. People who make trophy cases can do this sort of stuff. It's fairly skill-intensive, so the price isn't going to come down much with quantity. The part will naturally have sharp corners. The joints can be made transparent with some care.

For vacuum forming or thermoforming, you'd need to go to 1" radius on the long edges, and probably 2" radius on the short edges. Smaller radii may be possible with thinner material. You'd probably want to incorporate some shallow ribs to stiffen up the large flat surface. I will second KENAT's caution about vacuum formers; all of the few who have crossed my path were congenital liars; you don't find out what they can actually do until you've already paid them for a mold or two. It's _really_ hard to design/make a thermoformed/vacuum formed part, especially a transparent one, that doesn't look like a cheap bubble pack part.

Injection molding is just flat not possible within your budget.

It might be possible to CNC machine or injection mold two dark or black parts that assemble around a cut sheet of acrylic or polycarbonate like a picture frame. Hell, if you can stand an opaque frame like that, you could use wood.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Like Mike says.

For the sharp edges, cut and glue from flat sheet.

Required rigidity will determine sheet thickness.

If you can tolerate radii on the corners, thermoforming is the best method. Vacuum forming will still be more expensive. The cheapest way is a Kraftwood mould that is the male part, heat the sheet and drape it over the mould and press down with hands protected by leather gloves.

If you want sharper edges, cut squares from each corner so the 4 edges fold down easy, then use a strip heater to heat a straight narrow line that will bend or fold tight. Then solvent weld the 4 vertical joins.



Regards
Pat
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[KENAT]

Doh, I can't believe this wasn't the first thing I suggested, you should take a look at:

http://www.envplastics.com/

. They essentially do plastic forming like 'sheet metal' - or vaguely similar.

I haven't used them but saw them at a trade show and for the kind of volumes I often deal with (similar to yours) they seem appropriate . Unfortunately our marketing folk have fallen in love with sexy free form curves etc. on our covers which rule this process out for most stuff, but it may work well for you.

Not sure if you need clear parts, I'm not sure if they work with any transparent materials, but worth a look.

On the thermoforming, Mikes suggested radii will definitely help, and the guy running the tool may love you for it! However, a good shop that knows how to control the heating of the part and that can do a slight pre stretch/blow or similar may be able to get smaller radii. Going to pressure forming might help too but will increase cost as you'd now need 2 parts to the mold, not one - though should still be below 20k from what I remember of prices I got last year.

Or the suggestions Mike & Pat say about bonding from flat sheet parts or similar may well work. Our sister division makes some covers in this way - usually out of transparent 'perspex'/'plexiglass' or similar. However, they tend to be much thicker walled than you're talking about - doesn't necessarily mean they can't do thinner though.

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[btrueblood]

Excellent suggestions here from reputable people.

The only somewhat disreputable suggestion I can give is a fiberglass lay-up, i.e. single layer of random glass in an open mold, and hand-laid resin. Cheap in low quantities, and might give you required rigidity without ribs, if thickness of your sheet is a driving concern...

 

[raphjutras]

Wow! I'll be able to send some bids sooner than I thought,
I wasn't expecting so much information and different ideas!
And that envision custom plastic company is very interesting!!

Thank-you very... very much!

Raphaël

 

[btrueblood]

raph...

If you click the little pink star under Mike, Pat, and Kenat's posts, you can say thank you in a way that gives them a real pat on the back.

 

[cowski]

Injection molding was mentioned as out of your budget, but you should check out some prototype shops. They will do small runs and may be cheaper than you expect.

http://www.xcentricmold.com/

http://www.quickparts.com/

http://www.vistatek.com/

http://www.phoenixproto.com/

http://www.protomold.com/

 

[ornerynorsk]

Think clear plastic packaging. There are companies that will make clear vinyl or acetate slip covers and boxes very economically. It all depends on your requirements.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[KENAT]

If you need any 'strength' to this thing DO NOT think typical light gauge clear plastic packaging.

Thermo-forming, or at least vacuum forming, seems to be split into 2 fields, heavy gauge and light gauge.

We had an existing light gauge vendor that made lightweight packaging etc. try to make a more durable/structural cover for one of our products and it was a night mare.

It seems the 2 sectors are significantly different, this is based on talking to folks at trade shows and trying to get bids on subsequent jobs.

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[btrueblood]

Agree with Kenat. Find somebody who can show you parts they make, and are of the same thickness as the parts you want.

 

[patprimmer]

I think we still have zero info on the application and service environment, so we really cannot start to guess on what thickness or reinforcement the part needs.

Also, if strength is more important than clarity, then FRP as suggested by Btrue is a valid method.

Blister pack type process whie cheap and fast, still requires reasonably expensive toolong to produce an accurate exact size and shape. It does not give good large flat areas.

Regards
Pat
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[berkshire]

If you do not have to see through the part, it would be cheaper and quicker to form this in sheet metal, weld and clean up the corners at the production rate you are asking for.

B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[KENAT]

No necessarily Berkshire, that company I gave the link to claim to be cost competitive with sheet metal from what I recall, and because of the typically thicker section and different material properties can do some things a bit different from typical sheet metal shops.

However, as Pat points out we don't have a lot of information on the OP's requirements/situation so who knows.

Heck, maybe some kind of cardboard or something with some kind of overlay or similar would work.

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[raphjutras]

Well, I know i'll need something colored because it'll hide the wood part under... the surface is the "clean" visual part, so I cannot go with something transluscent, and i'll need something that can get scratched because sharph edges will get in contact of the surface but in the same time I don't want it to scratch with a little touch.. And it's a surface that will get screwed on top so if it's too rigid, it'll crack easily, something I don't want!