Aluminum molds for thermoforming

[fer_mayrl]

Hello,
Im looking into making vacuum forming molds for production, im told they are made from aluminum. and that they have strategically placed holes that are 1/64" diameter, is it fesible to drill this small with a homebuilt cnc router? (im thinking the drills might break very fast) (any ideas of feeds and speeds for drilling with a drill this small in aluminum?) another thing, what finishing steps are needed to get a surface that would be flat and leave the plastic without marks and lines?
Regards
Fernando

[igor]

First off, what material and what thickness are you molding. The material used will dictate the vacuum hole size allowable. In most situations 1/16" is acceptable, but depends on the plastic, the contours and type of mold. If the mold is used for high volume it should be aluminum and it should be cooled with internal cooling or temperature control. If the mold is too cold then no matter the mold surface finish the part finish will have chill lines. Only testing will dictate mold temp. Part finish is only as good as the mold.

[drcrash]

Hmmm, I've been wondering whether alloys like ZA-27 would be suitable for thermoforming molds. (Or maybe some other easy-to-make ZA formulation, that's even softer and easier to machine.)

I'm looking for something that's castable with minimal equipment, and easily machined. As I understand it, at least some ZA's are easy to make because you don't need a furnace that's actually hot enough to melt aluminum---the aluminum dissolves in the zinc, rather than actually melting---so you can do it with cheap and easy homebrew equipment.

(Like this: http://www.gizmology.net/stovetop.htm)

If that's no good, would pewter work okay?

I have almost no experience with metal casting and machining, so this may be a hopeless noob question. (Please don't hit me! :-) ) I don't know if it makes any sense to try to cast this stuff on the cheap, with probably a fairly rough finish, and then try to smooth it.

[slipstick]

I have been working in aluminum thermal form molds for about 12 years and find that with the vacuum holes are best put in with a high speed servo drill or a right angle air tool with a small chuck in it. I vac drill holes down to .008 and use WD-40 as a lube. 99% of the jobs are in 6061 T6. Most of the finishes we deal with requite some polishing. Especially on cavity style molds. Finish out with 600 grit and some water. Finalize with light Scotchbright pad.

[One of Many]

When I was in this trade there were several methods I used.

Basic vacuum holes can be put in with a Dremel tool using one of their keyless chucks. WD-40 was a must for drilling aluminum, both cast and plate. It just takes a bit of peck drilling to draw out the chips so the drill won't sieze. .015-.03 should be fine. Some holes, you do not want to show, go small. Another factor is how rapid the material needs to be pulled before it chills off and you lose detail. Larger holes help, but could also pop and become a vacuum loss. A certain amount of finesse comes with experience.

You can always back drill the tooling with a larger bit so the small hole from the mold side need not be a teeny tiny deep hole. On built up plate tooling, you can drill larger holes hidden by add on details(blocks, bosses, logo's etc.) and port the tooling with grooves on the back side of the add on's. For large flat surfaces, it helps to either sandblast or rough orbital sand the die to obtain some capillary action(between the die and the plastic during forming) rather than a perfectly smooth surface that can trap air pockets that never get evacuated. For the most part, only inside corners were drilled, minimal on flats or where any fine details may be needed. Strategic is nothing more than sucking the air out of places you want pulled tight against the tool. The finished parts fit form and function will dictate much of what you can and cannot get away with. I.E. a clear lens verses a rough textured shroud.

Not all of these holes are for vacuum only. For instance, deep draws with very little side draft on the die can be a B*tch to get off the die sometimes. Conveniently placed holes on the top of the tool can be used for air eject of the part. A puff of air lets the part float off.....most of the time, but not all.

Plastic sheet has a shrink factor that needs to be included into the die dimensions to get finished part dimensions, unless it isn't critical. 2 Points being here. First, that if the part cools too much on the die, it's not going to be any easier to get off said die. Second, take into account that if accuracy is essential, don't expect a net die to produce a net part. Shrink factors are available as a guidline, but precision is relative to one batch, one test. Expect slight variances between manufactures and batches.

For High production, aluminum is best. We never used water cooling. Once the tool was up to temp, it prevented a lot of the chill off, odd/inconsistant material shrink rates and really helps the material flow against the tool, going on and coming off. All they used were fans to reduce some of the heat buildup in the tool. The productivity may not be the same, but part quality to many of the 25-30 year forming veterans in the shop turned these methods into an art.

For low production there were high temp epoxy's. I would add a filler to the inside of this type of die. The epoxies were not cheap and only the outer skin was a concern. The filler could be resin beads, MDF, chunks of aluminum, old epoxy dies etc. whatever was not nailed down.

Then there was the cheap prototype tooling made of Sugar pine, MDF, lead, bondo, and even chunks of Teflon we had for other uses around the shop. Most of this was good for a couple pulls as long as it held up. Heck, 15-18 years ago, some of the tools I made for prototypes are still producing parts!

Now I hear some firms are forming parts on Fiberglass resins, dense foams and even rapid prototype Stereo Laser lithography resins. Imagine the possibilities......

Sorry, I got carried away with the long winded post! Most of which may not even matter to the home shop.

DC

[fer_mayrl]

Thanks to all for all your inputs.
just a little info about how i tackled the first attempt prior to all your suggestions.
We CNCd a foam pattern and cast it in alumminum in a lost foam process.
of course the surface finish was very bad, lots of pores. well we hand polished with a hand drill, and a sand paper pad, and drilled the holes 1mm dia by hand with a dremmel.
the mold came out good, but the surface finish not too good
i guess it is just more elbow grease,
the drilling was a lot of work, and in some parts of the finished part it shows. I guess smaller drills. I was hoping they could be done at the cnc but didint try, mainly because I didnt have a way to re-reference the part to be able to locate the holes.
Ill be more careful next time and leave referencing marks on the next part, which will not be cast, but rather machined out of a billet of aluminum.
Thanks again to all for your inputs
Regards
Fernando

[One of Many]

Devcon aluminum filler epoxy works great to patch up castings for thermal forming. Then there is aluminum sanding dust and a bit of super glue for a quick fix too. Drilling out the hole or casting flaw a bit bigger and beat in a soft aluminum rivet then redrill with something smaller or finish blend the plug for a more permanent rework.

The lost foam process may not be the greatest, but cast a bit oversize and finish machined would be reasonably efficient compared to mountains of chips hogging bar stock. I sure hope to gain that capability....some day!

DC

[fer_mayrl]

we did thought about machining the casting, but since we didnt compensate for aluminum shrinkage in the casting, it came out too small to be able to machine it. I think it would be a little hard to get it right so you could machine it, but then again, what do I know :-)
I think next time Id rather machine it from a solid block, even if it takes a day to machine.

[One of Many]

We used some very talented independent Pattern Makers for many of the castings. I think aluminum shrinkage was 1/4" per foot if I recall correctly.

Including the cast shrinkage and plastic shrink factors in a pattern does add some challenges. I couldn't tell you how many times I had to split a pattern or casting to compensate for the customers drawing changes. Sometimes it was easier/cheaper to use the first casting as a pattern, just to get double shrinkage without modifying the pattern or original casting.

I had seen a demonstration of those parafin wax vats that women use for hand and foot moisturizing. I noticed you can build up wax on a dipped finger. Hmmmm.....A few dips in that vat could build up layers to make patterns grow.

DC

[fer_mayrl]

Now that you mention it, My pattern was cast two times, once in the lost foam, it didnt came out too well because of thin walls. then they used the casted part as a pattern and cast again. Didnt thought of it till you mentioned it, the part actually came out 4%smaller than the first foam pattern.
Regards
Fernando

[drcrash]

For low production there were high temp epoxy's. I would add a filler to the inside of this type of die. The epoxies were not cheap and only the outer skin was a concern. The filler could be resin beads, MDF, chunks of aluminum, old epoxy dies etc. whatever was not nailed down.

I was just reading in a rapid prototyping book about small-run "CAFE" molds---that is, molds made of Composite Aluminum Filled Epoxy, good for a dozens to a few hundred (depending on shape, etc.).

The molds they were talking about were made with aluminum-filled epoxy for the impression coat, and aluminum-filled epoxy mixed with aluminum shot for the rest. (And with copper tubing simply bent to shape and cast in place around the plug, for heating or cooling the mold.)

The aluminum shot combined with the aluminum (powder) filling raises the thermal conductivity of the epoxy/aggregate a lot, because the small shot mostly fills in the interstices between the bigger shot, and the aluminum powder filling mostly fills in the spaces between the small shot. This reduces the amount of epoxy that heat must be conducted through, and provides mostly-aluminum paths for conduction between the molding surface and the copper tubes or the outside.

The thermal conductivity of this stuff is significantly lower than for solid aluminum, so that per-part cycle times are higher, but the time and cost to make the mold is much lower.

I'm wondering whether it would work better to use metal "fibers" (chopped wire) along with shot and powder, to provide longer paths of solid aluminum through the epoxy.

Anybody know where to get a lot of JB Weld cheap? :-)

[One of Many]

This is who we purchased most of our compounds from.

Freeman Supply

The only time it would be of benefit to use water cooling is in very large tooling, thick material forming that transfers a lot of heat and in-line forming where cycle time is critical. The complications of water lines and cooling towers might not be worth the maintenance hassles in most job shop environments. Even less for home shops.

We had one customer that would carve out wax figures, hotel and hardware store logo's, then pot them in silicon to make a mold. Then use the silicon mold to make multiple epoxy dies to form 10-200 pulls of .020 PETG on, for custom chocolate candy trays. That little company did around $500k in candy production annually. Not a bad gig!

DC