Vac forming (again)

[Agent Orange]

Hi,

It seems to me that 99% of home vacforming machines tend to be the type where the plastic is sucked down over the original.

Industrial places seem to have ones that allow the original to be pushed into plastic.

Why would you need to push it into the plastic rather than suck it down?

Are there any benifits to doing it that way and could that be done at home?

Cheers.

Mike

[naoto]

If I'm not mistaken, the reason for a positive "buck" (pattern from which the vacuum-formed part is made) is the ease with which you can create the "buck." The disadvantage would be "soft" detail (e.g. panel lines). A female mold on the other hand would have the advantage of sharper surface detail -- but the mold would be harder to make.

Another factor would be the location of the holes through which you're drawing the air out from between the master and the plastic. With a positive mold, the holes would generally be located on the surface on which you'd place the pattern (with a spacer), so any dimples formed because of these vent holes would end up being on the area outside of the main pattern (i.e. it's not on the part). In a female mold, you'd probably want to locate strategically some of the vent holes so that the plastic will get drawn completely into the mold -- possibly resulting in dimples on the actual part.

In some cases if the part is simple and small enough, you might not have to use vacuum forming. The old "heat-n-mash" technique worked well enough for me when making bubble canopies for my model aeroplanes (okay... so I mainly build sport "peanut scale" FF-rubber, so these parts aren't too complicated and either paint on or use tape for the canopy frames).

[TER-OR]

If you look at the high quality vacuforms, they are all concave molds. The plastic is pulled down into a mold. This allows the exterior to form around and into details. I believe the mold itself is a permeable material in some cases, in others, there are pinholes in the mold allowing air to pull.

You can get finely engraved panel lines with a concave mold. You cannot with a convex, you can at best get a sharp trough.

Good vac models:
http://www.donsmodelworks.com/

Some of the best are Dynavector:
http://modelingmadness.com/reviews/kore ... verjav.htm

[naoto]

It's amusing to note that Dynavector is a manufacturer of phonograph cartridges that just happens to make models on the side. IIRC the model division was in the UK
http://www.dynavector.co.jp

[Andrew Gorman]

I ran across a good David Merriman article on scratchbuilding a Lost in Space Jupiter II that goes into some detail on making a female mold out of portland cement- the vacuum reaches the mold cavity by way of channels made by sticking greased wires into the master. I'm really intrigued by the idea.
Andrew

[Dr. Yo]

Andrew, would you have a link to that article? I've had moderate success
using plain old plaster-of-paris for female molds, and the greased wires
sounds like a good way to go.

[Andrew Gorman]

There is no link- it's a hard copy from a 1990's Finescale Modeler. BUT I'd be happy to snail mail a copy. Just let me know your address! The article doesn't go into a lo of detail on the process, but there's enough there to get me thinking.
Andrew

[Dr. Yo]

I've sent a PM-Thanks, Andrew!

[Andrew Gorman]

PM received, and a copy will be winging it's way to you tomorrow! The article is in the May, 1990 issue of Scale Modeler. My copy came from a box of old magazines at the local hobby shop.
Andrew

[jack wendt]

in the '90's mattel released a female vacuform machine which was about the same size as their male machine from the '60's

[Robobuild]

in the '90's mattel released a female vacuform machine which was about the same size as their male machine from the '60's

Yes, I have that machine, but it is kinda useless, as I cant make moulds for it. It also used a light bulb to heat up very thin plastic, which took allot of time

-Robobuild

[Agent Orange]

Thanks for the replies - you've really cleared up some gaps for me!

Cheers,

Mike