Aluminum molds for thermoforming

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Thread: Aluminum molds for thermoforming

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    Default Aluminum molds for thermoforming

    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

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    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.



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    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.



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    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.



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    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



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    Default great info guys

    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



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    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



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    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.



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    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



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    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



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    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? :-)



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    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



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    Quote Originally Posted by One of Many View Post
    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.
    I sorta guessed as much. I'm toying with some related ideas, though, that might likewise not be worth the trouble.

    I'm wondering if it would be worth embedding some thin aluminum rod in an aluminum-filled epoxy mold, rather than tubing, just to provide long paths of solid aluminum for heat to travel through, or maybe putting strips of windowscreen in the layup.

    Two applications of that are for a small rotomolder, and for a vacuum forming mold (& a plug-assist plug) for rigid sheet foams like Depron (an extruded polystyrene). Vacuum forming that stuff seems to be pretty tricky, and to depend on very even heating in a narrow range of temperatures. Molds and plugs that even out the heat well might be useful.

    (Then again, maybe aluminum-filled epoxy is already plenty conductive enough that if you preheat the mold properly, the problem is solved.)



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    Fernando, I think you'll need to answer Igor's questions about what material and what thickness and what draw depth are you molding before you can get a decent answer that relates to your case. There's been some fantastic and very knowledgeable advice given above but you must admit that vac forming 5 million .020 PETG chocolate trays on a high speed vac former requires a totally mould to vac forming 200 signs (or face masks or whatever) using 5mm acrylic or polycarbonate. No one wants to know what the actual product is as that is your business.



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    Quote Originally Posted by drcrash View Post
    I sorta guessed as much. I'm toying with some related ideas, though, that might likewise not be worth the trouble.

    I'm wondering if it would be worth embedding some thin aluminum rod in an aluminum-filled epoxy mold, rather than tubing, just to provide long paths of solid aluminum for heat to travel through, or maybe putting strips of windowscreen in the layup.

    Two applications of that are for a small rotomolder, and for a vacuum forming mold (& a plug-assist plug) for rigid sheet foams like Depron (an extruded polystyrene). Vacuum forming that stuff seems to be pretty tricky, and to depend on very even heating in a narrow range of temperatures. Molds and plugs that even out the heat well might be useful.

    (Then again, maybe aluminum-filled epoxy is already plenty conductive enough that if you preheat the mold properly, the problem is solved.)
    I cannot comment on rotomolding. I've seen it, but that is all.

    For what it is worth, styrene has the lowest forming temp and the most rapid chill off as I recall. I would expect the heat time verses the form time to transfer the least amount of heat into the die. Unless automated to the point the short heat cycle time does not allow the tool any time to disipate heat with a simple fan array. Forming a couple scrap blanks PE was what we used to pre-heat the tool, because it held heat the longest and at a higher temp.

    Having a screen tray between the heat source and the plastic sheet will allow shadow screening of either top and/or bottom ovens. This is simply a way to add in screen mesh of different densities to vignette heat off areas for better control about the sheet. You might not want the whole sheet to be all at the same temp.

    You are in control of how much material thickness you hope to manipulate to your advantage. Be that with plug assist, snap back, bubble pre-stretch, placing the die off-center of the blank or pedestal mounted to retain thickness at the trim edge are tricks often not easily learned without going through tons of plastic over years of trial and error.

    The best path to take is with your specific equipment and ways that you learn to make it work for you.

    DC



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    Sorry, with all this info, i just forgot to answer that,
    Im molding 12 and 15 mills PET for some food product trays. And I hope a lot of trays will be made, since its my brother who is starting his food business.



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    In that case, a rough basis for vacuum hole diameters would be around 1 material thickness up to around .040 and a .06 hole size limit there after. Much more than that risks pulling the material into the holes or they could show up as dimples in the part.

    As with all of the cycle events that make up your sequence to produce a good part. Having reliable cycle timers that either turn on a bulb indicator that informs the operator the heat cycle has completed or tiggers a chain reaction to say heat for xx seconds, open door, slide out material, activate upper and lower plattens, turn on and off vacuum, fans or compressed air for ejection. The fancy features need automation for consistancy.

    You might look into getting PET in rolls and do some looking around for an in line former that either includes or can append a trimming die punch. This is how coffee cup lids, plastic cups and plates are made efficiently, but those are primarily styrene. I cannot imagine anyone trying high production food service items on manual forming machines.

    I'd at least get a part price quote from those already in the business to see how much an initial investment would be if YOU produced the tooling to fit their equipment. If the parts costs run in the 5-35 cents by the 1000. You will have a hard time competing with that as a lone single product former. I'd question if quantities at 10,000/month would make it a worth while side line business.

    DC



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    Thanks for your insight,
    We dont own a thermoforming machine, we are having the trays made, we just provide the tooling.
    Thanks Again
    Fernando



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    Okay, I'am going to tell ya how I do it at my work place. We use cnc's to cut pine or mdf patterns with shrinkage included. We sample parts then send pattern out to be cast.We used to Drill 1/64 holes with high speed drills using wax, 1/16 holes if apperance does'nt matter. But know we use a vac. button,looks like a .500 inch button with small hole in it, we just place them in strategic ares and eliminate all the hours of drilling the small holes. Now some people are going to ? this saying that there is a formula for how many hole per inch with draw of vac ect... We build around 200 vac. molds a year and they are working great with no forming problems We use cooling line to cool or heat mold, do not use copper,get some high pressure water line(300psi).
    p.s. if apperance is a factor then do not use buttons drill the 1/64 hole



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    Unless your vac form is going to be high volume or very complicated why not do as we do at school and use MDF as a pattern?

    It CNC routes well and gives a reasonably long lasting pattern. In general we don't need to put draw down holes in the pattern unless very deep or a complicated shape.

    Give it a try MDF is cheap at least it will serv as a test for your Drawing.



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