plastic for finboxes?

Lets say I wanted to mill some plastic blocks into some prototypes for a fin system I was thinking of, what kind of plastic would I want to find? Something that would stick well to resin, and still be cheap and easy to find.

What if I wanted to mill a mold and rig up a Gingery-style prototype-sized home-injection-molding machine? What kind of plastic then?


Resin reinforced polycarbonate?

I suspect lots of research is done on material that molds well, is semi strong, and cheap.

For a true strongbox, maybe straight multi layered combo of carbonkev and epoxy resins, but not feasible for mass production dats cheap.

Cool, some innovations from within our own !

My partner and his brother made their first injection molding machine. The 300-500 Degrees F is not hard to get. But, the 20,000 psi injection pressure and 20 to 200 tons clamp pressure can be a real bitch to achieve and that’s a small to mid size machine.

Many of the most popular boxes out their are a combination of ABS and PC.

Aloha Schwuz:

If you have a 3D design for your idea there are companies that will machine prototypes for you from a design. They are not cheap enough for actual real world prototypes (by this I mean putting them in boards) but they could be depending on the complexity of the system.

The prototypes I have done this way have been machined from acrylic but they can do almost anything. They are extremely fast, I will typically get a prototype in two days using this service.

Another option is to have the design run on an FDM machine but this also requires a 3D design of the idea. These tend to not be as well finished as the machined ones but are excellent for getting a visual on a design.

Now if your intent is to use these in a board in order to test them that can get a little expensive, speaking from experience.

For the final product you will probably want some fiber in the plastic!


You know tom, I actually had you in mind when I posted this. I figured if anyone could shed some light it would be you. Thanks for the quick reply. That’s some serious pressures you’re talking about… That clamping force, you mean on the outside of the mold, holding the halves together? I’m betting I could get adequate pressure with a small cylinder and a big lever, but I’d hate to have a catastrophic phailure with hot plastic spewing all over. Yeah, the injection molding might be a bit over the top, but it’d be good to keep in mind if I want to make a few more. For now though, I think I could keep it within reason if I machined them up myself. Not to mention I’ve been wanting one of those little benchtop mills for a while now. It’s not a very complex shape, and having the ability to adjust on the fly would be advantageous over having someone else machine a CAD design to spec. So, if I wanted to source a block of something that fits the bill, what kind of plastic should I look for? I know delrin is kind of the go-to for milling prototype stuff, and that crap is way too slippery to get a good grip with resin. My main concerns are compressive strength and resin adhesion.

Polycarbonate is harder to cut. But, the result will be stiffer. ABS will be easier to cut. But, lacks the tensile strength. Either one will get you a chemical etch with polyester or epoxy. ABS more PC less. Personally, I still prefer Isoplast. But, I don’t think you’ll find that in block from.

Ok, in looking up plastics I’m finding polycarbonate as in Lexan, the clear stuff… Is that what you’re talking about? Or something like a glass-filled polycarbonate product?

ABS is definitely way cheaper, that’s for sure. Rigidity isn’t so much an issue for what I’m trying to do, but compression strength is.

Brother J

what about this stuff?

Need some good grit router/drill press bits

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Lets say I wanted to mill some plastic blocks into some prototypes for a fin system I was thinking of, what kind of plastic would I want to find? Something that would stick well to resin, and still be cheap and easy to find.

What if I wanted to mill a mold and rig up a Gingery-style prototype-sized home-injection-molding machine? What kind of plastic then?


Okay, back to the original question. Something that would stick to resin G-10, PC and ABS will all fit the bill.

Still cheap and easy to find ABS first, PC second, G-10 last.

Want to be able to injection mold G-10 is out. It is compression molded not injection molded.

Now I don’t live in Texas and I rarely cut proto-types from block plastics. So, on-line suppliers are my only recommendation there. But, anything you mill from a block is going to have different physical characteristics from the injection molded version. So, I think I’ve given you the direction. Now, it’s time to see what you can find.

Awesome Tom,

Thats helped me out as well!

You’re a wealth of knowledge mate, thanks for sharing.

I couldn’t find a definitive answer for epoxy sticking to ABS, now I know!


Excellent tom, I appreciate the input! I think I’ll try the ABS first, although that G10 seems like it would be damn near bulletproof… Maybe too bulletproof, I wonder how well it would mill.

G-10 mills great, is very precise.

Bonding is the main problem, and $$$$$ of course.

Wears out bits like you wouldn’t believe.

Is heavy for it’s volume.

ABS is the way if production is a consideration.

Plain old PVC machines very well for finboxes, and bonds well to epoxy or poly resins. Comes in lots of shapes. Machined plastics suffer from lots of cracks due to internal stresses, unlike molded parts. Because of this you normally need about 15% heavier wall thickness over a molded part (my rule of thumb). Glass-reinforced epoxy (like G10) does absorb water over time, and will play hell on your cutters. ABS is too flexible even with lots of glass reinforcement and also soft. Polycarbonate is very brittle and has lots of the cracking issues that I mentioned. If your design has thin sections or other intricate features, make your prototypes out of aluminum. It glasses fine if you rough the surfaces.

For milling work avoid glass-loaded plastics and epoxies.

I’d use s high density nylon, delrin, or polycarbonate. In fact I did. They all machine easily and are all comparable in strength. To get more strength out of a plastic will require a lot more $$$. PVC, polypropylene and the like are cheaper and much less stiff/strong,

To bond with the resin simply rough the surface and clean with an appropriate solvent.

For casting work you can add glass reinforcement for strength. The glass will add substantial strength but reduce precision workability. Some high density plastic with glass impregnated is the standard for commercial finboxes. I think Tom uses polyurethane (high density), Bahne uses glass/polycarb.