A question on vacforming...

[KOTR]

I build me a small vacforming-box for a couple of parts I will have to do for my XP-38-conversation, and it works in general as like intended, but yet I havent got a usable part out of it. So here are my questions to the master scratchbuilders: how much space has to be between my master and the inner edge of my frame, where I put my plastic sheet into. Or in other words, how much bigger than my master does the plastic sheet has to be to successfully form into some parts? And how influence do the masters measurements have on the thickness of the plastic sheet that can be used?

 

Thanks in advance

[maverick728470]

KOTR,

For vac parts and this depends on the thickness as to the ID kits that i have built.It's .40 and sometimes .030.The gap as to pulling or covering a vac parts is about 1/16th of an inch or slightly more.One as to the ID kits was the plastic that the guy who owned ID molds before MDC bought them up.Wherever he got his sheets was something that i never found out.For combat as of lately.The thickness is almost the same as the ID sheets that were used.

I would use the rule of thumb here as to doing what u have in mind and keep it at a close talerance as possible.U might have to do some testing as to making what u'er trying to achieve.And sometimes the castings can be just alittle bigger than u might not want.But that's the trial and error deal here.

Even the combat and the ID kits had some over sizing here and there.But i think it was due to the way the forming was done.Cold weather can really mess with as well nad it happened to john who owned combat before he past away four years ago.And one just might talk to Jeff Robert's who owns the molds.He can give u some intel that could help u out.

Cat not think of anything else for hwta u'er trying and can do as to what u find out from jeff.Larry

[LSP_Mike]

KOTR, I f I understand you , you want to correct frame size for the part being formed. If you have a tall part(Spinner) on the platen you need to compensate for the draw -down of the hot plastic. A 1/32 spinner for a P-38 is 3/4 inch or so(.750) so determine desired thickness of finished part and use thicker plastic sheet if required. The critical part is where the blades come thru the spinner, so adjust accordingly. For this example, you will notice "webs" forming off of your part. This happens with many circular patterns. To avoid this put small blocks where the webs form, this will suck up the extra plastic, and keep it from gathering. For say, gear covers, you can use thinner plastic, but you are trying to match frame size so allow 3/8 inch or so on each side. If you make a machine and match the frame and platen, you can adjust the frame size by sliding one side in or out, and match it to your platen by putting a pre-drilled block onto your existing platen. Cut grooves in the bottom of your spacer block to allow lots of air flow, and tape over any holes not needed. If you notice "sags" on your part(in the plastic) this is because your mold is cold, heat it gently with a hair dryer, to more closely match the temperatures of the plastic and mold.

HTH, Mike in calif

[KOTR]

Hm I perhaps expressed myself a little unclear. By frame, I meant a wooden frame, where I hang my plastic sheet into. How much larger has this to be than the master? Ah well, I'll have another shoth later, I also suspect, my sheet is not warm enough and perhaps the vacbox pulls some air where its not supposed to do so...

[maverick728470]

KOTR,

OK,Now i know what u'er needing.What size would the frame piece as to making a new parts.Figure on at least another two inches as to the frame itself.U do want enough for the sheet to cover the wooden mold.If one doesn't add to the sheet,then u'll poak a hole through the mold or master it'self.Hope this helps.Larry

[KOTR]

Yeah, why explain a problem so that everyone understands it, when you can do it the hard way?

 

Thanks for your efforts, I'll have a shot at it tomorrow

[LSP_Matt]

I find at times that the height of the master can be an issue, but also the shape.

 

If you are vacuforming down onto a sharp edge or triangular shape the plastic will be thin at the top and thicker at the sides. If the master is high then sometimes the plastic sheet can come slightly out of the frame that carries it down onto the master...this can reduce the effectiveness of your vacuum and hence clarity in the detail picked up from the master.

 

I read UMI's posts the other day on Roberts models and found it interesting that vacs are done on male masters but also female molds. I would imagine the female mold would allow the plastic that is vacumformed into it to be a more consistent thickness.

 

Any pics KOTR...it sounds interesting.

 

Cheers Matty

[KOTR]

Hey ho, it works!

 

Made the carrier frame to small, and the plastic was a little to cool. Holding it directly over the slits of the toaster for a few seconds made the difference

 

Well, cellphone cams are by far not the best choice for making pictures, but to give you a little idea of what I'd done, this is my little box. It measures 10 cm x 15 cm x 7.5 cm (4" x 6 " x 3 " for the metrically challenged ), made from 2 mm plywood over a pine tree frame and features 213 handdrilled holes on top and obviously the opening for the vacuum cleaner hose. Sufficient for small jobs.

 

Next to it the master for the radiator air intakes, made from balsa and dipped in resin, and the result. Well, perhaps to be binned, too, because I placed it offcenter on the master. But I'm still learning...

[LSP_Mike]

Exactly! So, if you want you can tape over holes not needed to draw down your parts. Your part was placed in the centre of your platen, so you can cover some holes to concentrate the vacuum around your part,and control the draw-down. another finer control would be to put a ball valve to your box. pay attention to the "sag" of your hot plastic, you want a nice symetrical sag, and your oven may heat unevenly. Remember to gently heat your mold before pulling a part.

[KOTR]

Ah, heating the master was one thing I hadn't thought of! Thanks for the hint. Well, the holes were covered with some tape and cardboard, I just removed it for the picture