Plaster mold, what material to use to copy old part?

I would need to copy an existing plastic part from an old robotic arm (it's the base) it's a 6" x 6" square piece of plastic, and about 3
inches high.
The part is quite simple and I have succeeded in making a plaster mold out of it.
Think of it as a fancy giant ashtray, with a nice, complex shape and 4 "legs" to make it stand
Now... I know nothing about plastic injection.... is there any plastic resin I could pour into the plaster mold to get a similar part and with the same resistance as the original plastic part?
Anyone familiar with metal molding, with a plaster mold?. Any FAQs on the net about this?
I read much talk about molding in the RC newsgroups, but very few specific info.
Thanks Willy
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Hi Willy, You can get liquid polyurethane that you can pour into your cast. It's a two pot mix that hardens into a good, reasonably tough plastic in a few minutes. The stuff we use is called 'FormCast' or something like that. It should be available at good hobby or art shops. Regards Olaf

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Thanks so much Olaf!. It looks like Por-A-Kast is what I need... http://www.synair.com/products/pak /
"POR-A-KAST® is a liquid urethane casting material. This two-part room temperature curing system, is measured at equal volumes (1 to 1); therefore, no special equipment is required for weighing or curing. Por-A-Kast quickly cures at room temperature to a rigid Shore D-73 solid. It can be tooled, turned, drilled, ground, sanded, pigmented, stained, and filled, yet has the high strength typical of more flexible materials. This means that Por-A-Kast can be trusted for thin delicate parts as well as parts requiring a machine finish. "
Is this like the one you're using?
Regards Willy
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Hi,
I am looking for a professional research grade mobile robot development kit.
The robot must have facilities for adding external specialised hardware (sensors etc) and must provide wireless communication to a PC base station.
Effectively, the robot needs to relay video and other sensor information to a MATLAB or C based development suite, which will respond by transmitting movement / actuator commands.
Does anyone know of any suitable kits?
Many Thanks
Scott Page
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Hi, I regularly cast using "CastnCraft" polyester resin available from craft/hobby stores. Two part system,uses a catalyst in proportion to the amount your are mixing up. Cures in about 2 or 3 hours. It is drillable,tappable, and fully machinable. I also have a foundry here if you want to make it out of aluminum. I work pretty cheap on simple patterns.The parts would be more durable and not weigh much more than the resin. If you are going with the resin(should be fine) You can also buy latex rubber mold builder to make your molds. Just paint it over your pattern and let it dry and repeat until its thick enough to support itself. It will be simple to pop out your parts and you can make as many as you like. The craft stores also sell this(made by the same folks as the resin.) Watch the fumes!!!They will make you high and sick without good ventilation.
Mark
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I'd use an epoxy resin (2-part), and glass-fibre mat. The epoxy itself is not very strong, but combined with glassfibre bulletproof. Is the mold only of the outside, or from the inside and outside??
-- Bye, Ray
http://home.hccnet.nl/raymond.van.elst / Mobile robot, walker and I-Cybie
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http://www.polytek.com /
read the tips and techniques which will apply to all vendors
I use a couple of vendors, polytek has always been helpful and I like their product ... won;t say it's better than anyones else, but it always seems to work as advertised which is good if you are not a pro at this :)
look over the different resins, and figure out which meets your need
polyurethane or epoxy
I suspect you want a poly
plaster isn't the "best" mold material, since it tends to have imperfections and voids on the surface which will be replicated in the final. You might need to coat the mold to fill in the voids you may not even see.
Is this a solid base or is it hollow or multiple pieces ?
if solid, look closely at the plastic specs to determine cure time and if the object is too thick for the plastic ...
all in all good preparation is the answer when working with anything that has a short working time .. some of these have a 2-3 minute work time ... and while it might seem silly, a dry run through how you're going to stir, pour and let set might help make the first pour as good as you'll get
good luck
        - Ed got talc ? Drysuit talc bags http://www.underwaterusa.com
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excellent suggestions there.
Silicone compounds are the preferred materials for making molds. They are durable, and holds their shape well. A vacuum pump can also be of great aid when casting. Mix up you goo, then outgas it in a chamber. A guy in our club made a great chamber out of a big piece of drainage pipe, and 2 slabs of kevlar. Be careful of putting molds into a chamber though. If there are any air bubbles trapped in cured silicone, they can distort the mold, or worse.
Dentist have these great mixing bowls that have a vacuum attachment. I built a motor with a drill chuck that would fit in my chamber. I was working with a 1 hour pot life, so there was no chance to mix and de-gas separately.
good luck
mike
(Willy Kreim) wrote:

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