Experiments in Polymer casting

[pstockley]

I am intrigued by the possibility of casting polymer components for metal working machines at home. I have quite a bit of experience in using epoxy for laminating but not much in casting. So I decided to do some research into the area. I looked at the big guys such as http://www.itwpolymercastings.com/

In theory it is relatively simple in that you cast a mixture of epoxy resin, metal fillers and Quartz aggregate into a mould at room temperature. Metal reinforcement will be required in certain areas including inserts for screws etc. The tricky bit is selecting the right resin, fillers and aggregate. The normal resin used for laminating isn't any good because it sets too quickly and causes exothermic runaway when large volumes set i.e. the depth of the resin is more than a few millimeters. Just try leaving a pot of epoxy to set and you will see what I mean.

I have been in contact with a local producer of epoxy (http://www.crosslinktech.com/ ) and they think they have a suitable product that is used to make moulds. This is a blend of epoxy and aluminum power. This has a much longer cure time so the heat buildup is less. However, even this you can only cure to a depth of about 4" at a time. The trick is to pour a little in, let it get to the point it is just about to gel and then pour more in. Once it is fully poured, an elevated post cure is recommended to get the full mechanical strength of the epoxy.

To make the mould, it should be possible to use a split wood structure which is sealed and a suitable release agent applied. For one-offs it wouldn't even mater if the mould was destroyed. For bigger runs, a GRP mould or steel mould could be made.

At the moment, I am trying to locate a suitable source for quartz aggregate. If I can find one, I am going to do some small scale experiments. There are still lots of unanswered questions:

1) What is the best type of aggregate

2) How to compact the mixture to avoid air bubbles

3) What is the best design/material for structural re-enforcements

[vacpress]

what sort of metal working machine components do you see being cast of this material? bases?

[High Seas]

Is there any reason you havent considered using silicone/urethane RTVs for mould making?
There are some great products along this line that have high Shore-A hardness with good tear strength, and give very precise results. If you consider this approach Smooth-On has a 'site and have a sample pack on offer to try out.

They even have a set of pourable plastics that might meet your design requirements.
I've only used their stuff and don't work for them - but its good.
Jim

[pstockley]

I can see beds, columns and maybe headstocks being possible.

[ozzie34231]

A while back I read a few posts, (on some group), regarding casting acme lead screw nuts from "Moglice". Moglice is very expensive and a lot of folks had some doubts, fearing the machining of the screws was not good enough to optain the desired near zero backlash.
Much later I read that a person, (I think Ballendo), suggested using regular epoxy with the slowest catalyst, spiked with graphite powder.
I've just completed a first test trying that, with what I think are good enough results to go farther, at least for my purposes.

The nut fit very snugly, but was easily run from end to end of the screw with a drll on the screw and the nut in my hand. ...20" screw.

I used lard as a release agent and sealed the mold around the screw with a hot glue gun.

I'm putting together an "IH" mill from parts I've aquired and I think before I spend the $600 to $800 for ball screws and nuts I'm going to persue trying to get near zero backlash from the stock screws, (they look beautiful and the ends are 17mm). I just happen to have a bunch of angular contact bearings with 17mm bore.

What is the purpose of the quartz?

If Ballendo is listening and I have the referance correct, perhaps he can add his experience.

Jerry

[WhiteTiger]

Granite chips can usually be had from masonry suppliers, sometimes even lawn and garden outfits. Not pure quartz, but then the purity of the fill/wear media is pretty irrelevant when your strength limit is defined by the resin(s)

Best solution I know for escaping the bubble problem is to arrange to vibrate the mold while applying vacuum at the sprues and vents.

EDIT> forgot to mention, if you go with the granite, make sure they don't sell you *decayed* granite, which is sometimes fashionable as lawn decoration.


Tiger

[High Seas]

Just reread your post and you do know you can slow the exotherm on castings - right?
Just a few ideas:
1. Use a purpose-made slow hardner check with your supplier and have them promote it for you
2. Increase the amount of fillers (pro'ly stay at or below 50%)
3. Cool the resin and hardner then pour
4. Pour and then cool the whole mould and resin in an ice bath/fridge

WEST system has some outstanding fillers - sounds like you might like to use the 404 High Strength - works good for high strength apps - mount winches and similar bits on deck

One major caution - and its good you'll do some sample parts first - shrinkage can bite you in the butt like a tight pair of jeans!
Jim

[vacpress]

along the same lines, i used to be a researcher at a material science magazine. while there we ran an article about a low-temperature IJ material with properties very similar to nylon...

I took a bag home and did some experiments with using it as linear bearings on stainless rod.... it seemed rather promising... it can be melted in boiling water.. around 180F.

so.. this material i am speaking of.. it is similar to this: http://www.micsaund.com/2006/03/13/p...-diy-projects/

but it may not be the same, and i know it can be gotten cheaper than $25.00/500g. that seems about twice as much as i saw it on ebay.(auctions all gone now)...

Molding this material could be done into really inexpensive molds, possibly with a hot glue gun... in fact, it would be easy to build a simple IJ machine around this material... it would be similar to the gingery machine.. This stuff is neat. I am glad ive got 300g~ of it around...

also, on the snap on stuff.... i have used a few types of it. in general i consider it expensive... for general RTV molds, there are far more economical solutions... I had sorta bad experiences with most snap-on products... i would recomend looking for a local manufacturer... maybe this is easier for me, being in chicago..

g'luck.

[WhiteTiger]

Alumilite, Hapco and Reynolds taken as a group cover a pretty good range of both resins and rtv's. Dow has a lot of interesting products, but you almost have to find out about them from distributor websites.


Tiger

[CarveJunky]

Pstockley,

I have been doing some research on this for quite a while, it turns out there is a division of ITW polymer concrete locally that I am trying to get into to tour the plant. From what I have been able to find, the material is 75% to 90% aggregate and the remander resin. Most of the molds are open molds and they are placed on a vibrator when pouring and left till the material sets. Here is an informative site: http://www.itwpolymercastings.com/ . OOPS... I see you found that site. I also have a pdf somewhere that goes into a lot of the vibration dampening and reasons for using PC. I will try to find it for you. I think I saw on Globalspec that they also sell the resin and aggregate, not real sure about that tho as I saw it in passing. Here is the PDF I was talking about. The file is too large to upload, e-mail me and I will send it to you....

HTH

Carvejunky

[vacpress]

i have thought about cement as a machine component also... thought about how easy it might be to cast it into hot-wire CNC cut foam molds...

of course for the machine i am building.. i would much prefer to just use the aluminum plate and steel tube i am planning on..

casting parts IS intriguing though... especially since i am well equiped for such things..

[pstockley]

The thing that puts me off concrete is the water content. It generally isn't that stable and takes months to fully cure. Plus it has a nasty habit of shrinking and cracking.

To answer the question about why use quartz aggregate. There are 3 reasons I can think off, firstly it damps vibrations really well, increases compressive strength and is a lot cheaper than epoxy.