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about to design a mold on the computer - advice needed

 
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spacephrawg
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Joined: 02 Jul 2010
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PostPosted: Sun Jul 25, 2010 8:51 pm    Post subject: about to design a mold on the computer - advice needed Reply with quote

on the general discussion forum and elsewhere on the net, I've been picking people's brains about how to do the following and every new bit of information i get, begs more.

So in this case, I want to make a mold for this thing:
http://www.the-blueprints.com/blueprints-depot-restricted/cars/uaz/uaz_3151-19691.jpg
Its the Uaz 3151, one of the hard top variants of the Soviet answer to the Land Rover and Jeep.

Anyhow, this thread isn't going to be redundant in the context of the others i've posted: i was wondering if there was such a thing as too many vacuum holes. I want the plastic to pick up every single negative right angle, like where the hump around the middle of the vehicle's sides meets the vertical of the upper half of the body, and/or any details that may reside on the front, or the corners of windows and things.

For background, the mold would be CNC'd out of aluminum and would be made of a few pieces so it could be disassembled inside the plastic shell, so as to accommodate for undercuts.

By vacuum holes I mean more like tunnels. The idea is based on seeing images of these holes being used in a female vacuum form mold, as well as having them described to me by someone elsewhere on the net, which he used on his RC car body mold with great effect.

I am hoping to get this all right the first time so that my development and tinkering costs are kept relatively low.

If anyone has done this, how small does one of these tunnels have to be for it not to show up on 1mm or .75mm thick ABS plastic? Is there such a thing as too many vacuum holes or is more the merrier when it comes to detail?

Thank you and sorry about all of they hypothetical questions; I like to know what I am doing before i waste energy on something that doesn't work at all.

Your help is much appreciated!
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jdougn
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PostPosted: Tue Jul 27, 2010 11:31 am    Post subject: Reply with quote

Consider making a rough mock up of the mold from wood or other easy to work, & cheap, materials. Use that to trouble shoot and determine the best design, location and frequency of vac holes.

This is just my opinion, but I prefer fewer vacuum holes. Maybe a small 1/16" or smaller hole every few inches. (This is on 3/16" ABS and the vacuum holes are located off the finished product.) The plastic doesn't slam down as fast and seems to web less with fewer holes.
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spacephrawg
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Joined: 02 Jul 2010
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PostPosted: Wed Jul 28, 2010 11:56 am    Post subject: Reply with quote

jdougn wrote:
Consider making a rough mock up of the mold from wood or other easy to work, & cheap, materials. Use that to trouble shoot and determine the best design, location and frequency of vac holes.

This is just my opinion, but I prefer fewer vacuum holes. Maybe a small 1/16" or smaller hole every few inches. (This is on 3/16" ABS and the vacuum holes are located off the finished product.) The plastic doesn't slam down as fast and seems to web less with fewer holes.


I will make the rough mockup as you suggested.

the 3/16ths stuff is the stuff you are forming, yes?

I would've thought with more holes you'd have less webbing and more detail capture. not so?
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jdougn
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PostPosted: Fri Jul 30, 2010 7:36 am    Post subject: Reply with quote

spacephrawg wrote:
I will make the rough mockup as you suggested.

the 3/16ths stuff is the stuff you are forming, yes?

I would've thought with more holes you'd have less webbing and more detail capture. not so?


I use 3/16" ABS for most of what I am forming at this time. ABS cools slowly so if the sheet is heated properly it will capture all possible detail before it cools without a large quantity of vent holes. Webbing is a difficult issue to control since there seem to be so many variables on what causes the webbing. You have to learn what works best for each mold. FYI, oil based modeling clay will quickly plug a small hole if it seems to be causing a problem. For larger holes a few layers of wide tape works but tape will show through the formed plastic.
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spacephrawg
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PostPosted: Sat Aug 21, 2010 7:05 pm    Post subject: Reply with quote

Thank you for your input. Since I first posted this, I have found out some more info:

Lexan would be the best material for vacuum forming - I want to sell these things and the market wants Lexan shells.

I've been poking around on here and have read there are cheaper mold materials than aluminum that are good for production runs. Mysterious urethane things and odd plaster compounds too. I want my mold to have a lot of detail as I said and I'd like the mold to be able to reproduce the detail and last for many many pulls. What are some good materials for that?

One thing I've read of on here is a rather complicated process wherein someone makes a mold out of something that holds detail but wouldn't hold up for many pulls, pulls just one copy in HIPS or something, and then fills that copy with high-temp polyurethane or something and the result is the new vacuum forming tool, right? Assuming I've gotten it right this far, how many pulls is a HIPS mold good for? It sounds like a rather roundabout process for reproducing a mold and it sounds like there would be some loss of detail from the original tool, amirite?

I would still prefer to make the original design in a CAD program rather than by hand.

Can anyone advise further? thanks again!
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