Prototyping in plastic - hub caps?

[andrewm]

I have been asked to help out on a very different project than any I have tried so far. A fiend has a design for a hub cap and wants me to prototype it for him. Has anyone done anything like this on the Shopbot. I can easily model the hub cap in 3D Studio Max, and I figure I can get a toolpath from this with MillWizard. The question is what would be the best way to make this. Cut it in plastic, or in some other material and make a mold then cast it in some kind of resin. I am not too familiar with plastics but I am willing to try anything. Any ideas or help anyone could provide would be very much appreciated.

This prototype something he needs to help sell the idea, it doesn't have to actually work on a car.

[paco]

If the design does not included 3D designing, I would look to do this in 2D... if it only the "top" that has 3D relief than I would do this second part (the 3D relief) on a secondary file in a gig... Woods (MDF, hard) and HDU scraps could be a good designing/prototyping substrat... maybe UHMW too which machine quite good... as for a mold to cast; I very don't know... depend on the "finishing" material...

[billp]

Andrew,
You might consider using a foam for this. They cut easily, and you can fine tune the shape with sandpaper manually afterwards. For making the masters of molds you'd either want a harder density foam like the HDU's ( Precision Board, Signfoam, etc.), OR a lower density foam with a surface coating to stiffen the shell. There's a lot of info about this process already on the Forum in the 3D carving links I believe... ( try Styrospray, and epoxies in the search feature..).
Another big advantage of the foams is that in smaller sizes the prices aren't too bad so you can make some smaller maquettes, or demos to show a potential customer...

[Brady Watson]

Andrew,
There are a number of ways that you can accomplish this, the 1st of which would be using foam as Bill recommends.

Another possibility is to cast foam, Rayite100 or a plastic resin, such as Polytek EasyFlo 60 a little larger than your part to machine. The R100 can be cast and machined within hours with a plaster-like finish. I would wait as long as possible after casting because if the center is wet...it gets messy (but will still machine just fine). EasyFlo 60 machines beautifully and can be sanded and worked with saws and hand tools if necessary. Depending on mass, it cures in less than an hour and is ready to machine right away. I like machining the liquid plastic because it will give you a nice tight finish that only needs buffing for a perfect finish. From that point you can either machine more of them, or use your perfect master to make a silicone mold and make the rest. You can also mix EasyFlo 50% resin to 50% metal powder (AL, Brass, Brinze etc) to create a cold-cast metal finish.

All of the things I am recommending fall under the 'best bang for the buck' category. You could go out and spend a lot more on resin tooling board, but I don't think it is necessary. EasyFlo 60 is $60 for 1.5 gallons of resin. Rayite100, a bit hard to find, is about $110 for a 50# bag. I had a really nice sample of a 3D part cut out in Rayite...but the dog ate it (for real!) I might have a pic on it somewhere on the board.

Let me know if you have any questions about the stuff mentioned.

-Brady

[stevem]

Andrew,

Probably the easiest and cleanest (except for MDF dust) method is to vacuum form your hub cap in an MDF female mould. If you plan your mould well, you will be able to produce several parts off the same mould and then use the mould as a holding fixture for finish machining the hub caps on the Bot.

[Brady Watson]

Steve,
Do you have any links for a recommended vacuum forming machine? I left out vacuum forming because it gets expensive to do it right...aside from using an upside down grill from Walmart etc.

-Brady

[stevem]

Brady,

No. I use a contractor to do vacuum forming.

Most companies that do vacuum forming have a variety of different vacuum boxes that will allow you to simply place your mould on the box and form parts. The process is relatively inexpensive compared to machining from solid blocks of material.

[andrewm]

http://www.studiocreations.com/howto/index.html

This site had a lot of info on vacuum forming and instructions on making a vacuum forming system.

Never tried this, but I remembered this site and had it in my favorites list. Might be worth trying for this project.

[Brady Watson]

Good point, Steve...

Andrew, the problem with many of the designs is that they require you use your kitchen oven...not the best idea going. Have you ever smelled hot styrene?

Some of the smaller units *might* work for you in a hobby capacity...but probably not very well for anything larger than what you are doing.

-Brady

[btk]

Brady,

In your post above you mention Rayite100 and EasyFlo 60 are good for making mold masters.....are one of these the small carved head profile that you showed at the jamboreee. If so....
(a) Does it make a lot of dust?
(b) What type of bit do you use and at what speed?


I have been using Butterboard and have found that the rubber still shows the grain in the finished part.

Thanks,
Brian

Sorry for not taking down during your greate presentation at the jamboree.

[Brady Watson]

Yes Brian...the Shopbot-Head piece was done in Rayite100. If you rush things and don't let it cure 100% (made for rapid prototyping and can be cut in 3-4hrs with fans blowing on it)...it will put off dust and sling uncured mud all over the place. This initially turned me off about the product...HOWEVER, after letting the stuff cure a 2nd time for a full 72hrs, it was a dream! It throws off nice soap shavings very similar to Corian, and NO dust. It needs no coolant, and can be cut fairly aggresively with carbide or HSS bits. I used carbide for the cutting and had no issues. What you saw at the Jamboree was right off of the machine with NO sanding or post finishing; however it will buff up to a nice finish if you go over it with a scotch pad.

If you seal it in shellac (perfect sealer for any gypsum based product) you can make a mold right off of it.

The EasyFlo60 is another option as well. It too machines nicely, throwing off a soap shaving. A bit more pricey ounce for ounce than the Rayite, but still cheap at about $60 for about 2 gallons of resin. You have to be careful when mass casting it because it (and the Rayite for that matter) throw off quite a bit of heat. While the Rayite is virtually unaffected by this heat, the EasyFlo, when cast in large volumes, will distort the final shape and show some shrinkage as well. If you are simply going to cast an 'ingot' to be machined, then this is pretty much a moot point.

I would stick to the Rayite as a machining media for the time being since it has a long working time and is very forgiving. Pick up a concrete vibrator and hold it against your mold after pouring in the Rayite. This will minimize any bubbles left suspended in your Rayite and in turn, eliminate porosity in your finish after machining. The stuff is pretty forgiving if there is porosity, and that is easily filled with some light spackle and your finger.

This was the 1st thing that I machined in Rayite...

4818
before this one...

4819
ate the entire thing and I scrambled for the MSDS sheets...Yes it is non-toxic!

-Brady

[btk]

Brady,
I will give Rayite a Shot. Where do you purchase?
You list Plastermasters on your wiki page.

We use Polytek Urethane to make molds...is there any reaction (or does the shellac prevent and interaction).

My Dog has not had any gypsum, she prefers swallowing Golf Ball sized stones from the yard.


4820

[Brady Watson]

Brian,
All plaster has small pockets on the surface that are hidden from the naked eye. If left unsealed, the urethane will key into it and either lock itself to the master or give you a stippled surface. Polytek recommends shellac as the perfect sealer for plaster when I attended their course.

If I were making a urethane mold of a center cap, I would mill out the rayite, buff with a scotchpad if necessary, apply 3 coats of shellac, lightly buffing between coats and follow it up with a spritz of PolEase 2600.

After I came home and found the receipt...You can buy Rayite100 from Chembar in Ohio. I believe that I paid $110 for a 40# bag (haven't even put a dent in it yet...much cheaper than any tooling board by FAR!) plus shipping. This was the only place I could find (after calling USG and ALL of their reps for a week stright) that would sell my company a single bag. Everyone else wants you to buy a full pallet...BIG $$$

-Brady

[t_fiddler]

Brady
Don't overlook cheap quartz work light bulbs for
heating the plastic to vacuum mold
Thant all we used when I worked for Coka-Cola to make 2 liter bottles from 1" diameter slugs at 300 per minute

Just a thought
Tom

[Brady Watson]

Tom,
Very interesting...and I have been researching the thermoforming idea for a while now. I bought some plans, but don't like some things about the design. Things are pretty busy right now, but I'd like to pick your brain about this in the near future if that's OK w/you.

Thanks!
-Brady

[t_fiddler]

Anytime Brady
Thanks
Tom