Making plastic parts

Hi, i've been searching around for the cheapest way to make plastic parts from molds. I've looked at alumilite/alumalite. The stuff is expensive. What is the cheapest way to make plastic parts from a mold?
They don't have to be exceptionally strong or tough or anything, just cheap.
Is epoxy any cheaper or some other kind of resin perhaps? Have you rocket, railroad or scale model guys found anything economical.
Thank you to whoever replies.

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I don't know how cheap epoxy out of a tube is, but I do know I used to use it and it worked for me.
I also made my molds from silicon bathroom caulk and that worked, too.

It depends on how accurate you need the parts to be, the cheaper materials are more prone to shrinkage and the mold durability. I have used Dow Corning RTV for molds and melted styrene crystals in the oven in them for small parts where the shrinkage factor is not noticeable. Works fine for things like mirrors and other small parts. Sam
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zalzon wrote:
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i've been searching around for the cheapest way to make plastic parts from molds. I've looked at alumilite/alumalite. The stuff is expensive. What is the cheapest way to make plastic parts from a mold?
They don't have to be exceptionally strong or tough or anything, just cheap.
Is epoxy any cheaper or some other kind of resin perhaps? Have you rocket, railroad or scale model guys found anything economical. <<<<<<<
Well, you need to define a little better what you want to do. Roughly how many parts do you figure to make. And does the turn-aroudn time and quality matter much?
The reason I mention this is that while you could get hold of some resin that is cheaper than Alumilite (or similar isocyanate types), they tend to take longer to cure. For example , polyester resins can often take all day to cure, though they can be accelerated by using a little more hardener than normal, and adding some heat, and there are some that cure faster than 24 hours, but I don't think many (or any) under one hour.
Now if you were making maybe 6 to 12 parts, and not intending ot make any more parts, then the cure time might not matter much since you could mix, pour, and de-mold one time a day. But if you wanted to make more parts and/or wanted to make them with a lot faster turn-around time, you’d want to use a resin that cured a lot faster.
The regular Alumilite is very fast. It starts to gel in 2-3 minutes, depending on temperature. The parts are usually stiff enough to de-mold in 6-10 minutes, also depending on temperature, plus how stiff they need to be and the shape in order to get them out of the mold without warpage.
Now, you could use “5 minute” epoxy, which would cure in about, well, 5 minutes. But it takes a lot longer to get stiff, and generally not as stiff as Alumilite. As well, epoxies and polyester resins often tend to have a bit of a gooey film on top of them, depending on the epoxy or resin it requires something like alcohol or even acetone to clean that film off or actually requires sandpaper to remove it. Alumilite does not require that.
Hey, around 1990, I saw Alumilite in a store and figured it was not worth it. I made a scale model that involved a lot of vacu-formed parts, some of which I wished I’d cast. In 1991, I finally took the plunge and bought some Alumilite. Boy what a mistkae. Yes, a mistake I had not bought it EARLIER! It worked out great. I use it to this day.
About the only drawback is that you have to mix it up just right. I like to use just a hair more of the clear part “A” than the part “B”. I usually make small mixes, where I count out the drops from the bottles. So I like to use 1 drop of “A” for every 10 drops of “B”. And then mix them together thoroughly. In my early mixes, sometimes I didn’t mix the cup completely and when the parts cured, then either there were some external areas of uncured resin. Or the part seemed to cure OK skin-wise, but there would be a small pocket of uncured resin inside the part, and over time the uncured resin would tend to ooze out. Don’t let that scare you off though, by reading this you should be able to avoid those little bugs I ran into.
The other half of the equation is a good mold material to pour the resin into. I like using RTV silicone for molds. They are self-lubricating, so no mold release is required. The tip there is to avoid old stale stock, since RTV has a shelf life of about 6-12 months. One time I bought some in a store, it was apparently about a year old. What I have often done is buy Dow Corning 3110 RTV at the same time I bought Alumilite by mail order from Alumilite Corp, that assured of getting it fresh.
Alumilite also has a shelf life of around a year.
I wish they’d sell Alumilite in a smaller “starter” version so more people would try it, say 8 ounces for $10 rather than the 28 ounces for $26-28 or whatever it is right now (I need to get some more when I start casting parts next spring)
Now, I have tried an isocyanate resin by another brand. I didn’t like it since it took longer to cure, and did not pop free from styrene plastic which I like to use as a 4th side plate (top) over open-face one piece molds.
There is one tricky thing about casting any noses, whether Alumilite, polyester resin, epoxy, or whatever. Shrinkage. They all shrink to some extent. The polyester resins shrink a lot. Alumilite shrinks some. The molds also shrink. The Dow Corning does not shrink much, but it does shrink a little bit (the faster it cures, the more it shrinks, so for size-critical parts I use as little catalyst as practical to still cure, and put it in a not-warm place to cure (I don’t mean cool since too cool and it won’t cure right). So, for a nose cone to fit just right, the master part would need to be made oversized so that the cast part would come out to the desired diameter. I myself have not cast noses that were that critical. What I would do would be to make a test mold of just the shoulder and outer first 1/2” of the planned real nose cone (using a scrap cone). After curing, cast a test alumilte part. And after that part cured, use calipers to find out the shrinkage from the original part. That would tell me how much larger to make the custom original nose, for the resulting RTV mold and cast part.
Now, another alternative to cast parts would be vac-forming. I have an article on a simple and cheap homemade one that borrows the use of a shop-vac for air and household oven for heat, so the only parts that have to be made are the vacuum box and the frame to hold the plastic.
http://members.aol.com/GCGassaway/vacuform.htm
It depends on what sort of parts you want to make as to whether vac-forming might be suitable. And again how many, if you only ever wanted to make 6-12 parts, sort of hard to justify it. But if you wanted to make 6-12 parts now, and be able to make others later, it could be worth it. I finally got into it when I was making a Delta 3920 scale model, needing 9 identical light noses and 9 identical light nozzles for the strap-ons. It does take some experience to get the knack of it, and apply experience to different nose shapes and ratios. For example it’s easier to form and remove the mold from an Apollo-type conical capsule shape than an egg capsule half. And the taller the nose to diameter ratio, the harder it is to reliably form.
- George Gassaway

Wow! Great post George. Thanks!
-- Drake "Doc" Damerau www.rocketmaterials.org NEPRA President NAR Section 614 NAR 79986 L3 www.nepra.com Remove "My Shorts" to reply
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Thanks so very much for your post, George.
I've been curious about Alumilite since I picked up a brochure a fair while ago. Your informative, insightful info is greatly appreciated.
Dwayne

I wrote, about Alumilite mix ratios:
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About the only drawback is that you have to mix it up just right. I like to use just a hair more of the clear part “A” than the part “B”. I usually make small mixes, where I count out the drops from the bottles. So I like to use 1 drop of “A” for every 10 drops of “B”. <<<<
Arrgh.
Well, obviously one drop of "A" per 10 drops of "B" is wrong, since it’s supposed to be about a 50-50 mix.
That should have been:
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So I like to use 11 drops of "A" for every 10 drops of "B". <<<<
The "1 drop" came from adding an extra drop to "A", for 11, when there’s 10 drops of "B".
- George Gassaway

Polyester casting resin is cheaper than urethane, though it does not make as finely detailed parts.
Also, you can make molds from Plaster of Paris, which is cheap compared to RTV mold materials.
However, with plaster molds they are rigid and fragile, so you absolutely cannot undercut mold angles. In fact, you must allow a clearance angle as you would in metal die casting.
zalzon wrote:
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zalzon wrote:
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TAP plastics has a "quick cast" resin (2 part white polyurethane) - seems pretty similar to what PML uses for the Cirrus Dart nosecone.
(They also have a rubbery urethane "mold maker" resin.)
-dave w

http://www.info-central.org/index.cgi?construction
Click on 'mold making'. Haven't tried it myself yet but it's on the big list.
Ted Novak TRA#5512
zalzon wrote:
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Try the mold making materials and resin from Micro-Mark. These are the most forgiving for starters. After you get the hang of it try some of the resins and mold materials such as Quantum Silicones especially QSIL 40 RTV.
Keith
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For what it's worth, I make all my molds and castings at "Moldin' Oldies" using 2-part silicone RTV and 2-part polyurethane resin from a company called Smooth-On. (www.smoothon.com). The 2-part materials all mix 1:1, so it's extremely easy to measure them out. There is no vacuum de-gassing required. There is no bubbling. Shrinkage is virtually non-existant. Both the molds and castings reproduce even tiny details. The RTV material becomes fully cured and ready for use in about 6 hours. The castings resin flows like water and starts to set in 3 minutes, is fully cured in 10 minutes and rigid in less than 30 minutes. They have a starter package that gives you everything you need, including a detailed instruction book for around $25-$30. If you ever want to try making your own molds or castings, this is the set I'd recommend. Mike

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Get a Micromark catalog,they have a section with supplies for mold-making and casting in resin and low-melt point metals. How-to materials,too,IIRC.
www.micromark.com

Try Bondo (auto body filler) cheap, strong, light and easy to work with.
zalzon wrote:
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<< zalzon wrote: <<i've been searching around for the cheapest way to make plastic parts from molds. I've looked at alumilite/alumalite. The stuff is expensive. What is the cheapest way to make plastic parts from a mold?>>
It depends on several factors. What is the size of the part? How strong does it have to be? How complex is the shape? How many do you want to make from the original mold?
I've been making molded plastic nosecones and booster hardware. These have moderately simple shapes but do include some "undercuts". Some of the nosecones are fairly large -- 38mm diameter by several inches long. I use a two-part silicone molding compound called "Oomoo 30" which I get from Smooth-On. It's easier to use than most silicone mold compounds, as it only needs a 1-to-1 mix ratio (by volume). It's cheaper than most too, though still pretty steep IMHO. You can get the "trial size" for $22, which 1 pint each of part A and part B.
For plastic, I use "Smooth Cast 300" from the same company. It's also easy to mix, sets quickly (about 10-15 minutes), fairly low viscosity, reasonably durable, and accepts paints and glues.
Smooth-On sells a starter kit that includes the trial size silicone mix, plastic mix, sealer and mold release spray all for about $25, if I remember correctly, which is an excellent price. You're limited to one purchase of the starter kit at that price.
They also have a lot of useful info on their site about making molds and doing castings.
http://www.smooth-on.com /
If you're just making a few small, simple parts for your own use, you _might_ be able to do it with RTV silicone from an auto parts store and epoxy. The trick here is to prevent the silicone from sticking to the original part, and also to make sure there are no voids in the mold (easier said than done).