vacuum forming mold/tooling techniques

Don't know enough about the molds but as far as the vacuum, are you sure you have enough for the draw? Wouldn't you want to use something approaching 25 to 29 inches? Max

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It is only .030 sheets...and I have heard of people doing it with shop vacs that only pull 5-8 inches. I am using two shop vac motors (10A each) directly mounted to the underside of the platen which is only

I noticed that no one replied to your post, so I'll give it a go. Not sure I follow your positive negative description but if the form you are molding the sheet over has rectangular shapes protruding from a flat surface, then you are right, the material will probably web the space between. If the rectangles are recessed then there would be no webbing. The basic difference in forming into a cavity or over a core is the material displacement. In a cavity, only the material area described by the perimeter is available to form the walls and floor of the cavity. In your case that would be an area 2x2. That area must cover a larger area because of the depth, .75x2x4x2x2. The material will thin out as it moves to the bottom, and may get too thin for the application. On the other hand, a core shape can draw material from a greater area unless there is another core close by. This is probably not very clear, it would be much easier to show in a diagram. You might do a search on the subject to get some better explanations.

What plastic are you using?

Thanks Billy. Now that I think about it my reference to positive vs. negative is relative to the application. I understand what you are saying I think: If you were going to vacuum form a coffee cup you would NOT likely start with a large square mold with a big hole in it...because your material would not only have to draw down into the hole to make the cup, it would have to draw up the side of the mold. So to reproduce 1 coffee cup you would have a core (or an upside down coffee cup on your platen). But to produce an array of coffee cups from the same sheet, you might have 10 wells representing 10 cups...with maybe 2" between each cup and the next. Otherwise you would web between the cores. I am basically making a plastic tray, similar to an ice cube tray but not nearly as deep (nor as thick, only .030 HIPS). If there are 12 wells in the tray (i think they will be about .625" deep now, spaced .25" apart) then I assume I should make a mold consisting of 12 cavities rather than 12 cores. I was planning on drilling .125" ports in the bottom corners of each cavity so that the suction from the platen could draw into the wells. I'm still not sure what I am going to even make the mold out of. I was going to have emachineshop.com do it out of aluminum but it would be $180...and I would like a cheaper alternative as i am quite poor.

Thanks again. By the way... your website has some cool stuff on it. That stereolithography based epoxy mold is something. Might I ask how you keep the epoxy from sticking to the SL? Bonehead question but it seems like it would be a mess.

.030 high impact polystyrene sheet, white. I am looking for a clear sheet for this same mold but it seems a lot more expensive. What is a clamshell made out of? PETG?

It sounds like you are on the right track with the cavities. And yes you need some air vents in the cavities, but smaller is better, usually less than 0.062". If you have a table saw, you could easily make the mold from wood, with the cavities tapered for easily realease.

Thanks for looking at my site. A mold release agent is used to prevent adhesion, and in addition all patterns need draft to aid in removal of both the pattern and the injection molded part. And yes, it is quite a mess if not done properly!

Putting some vent holes around the base of each of your compartments might help if using a positive mold. Make sure the sides of the cavities are not at right angles to the base. Slope them like an icecube tray is sloped. If possible set up your mold so that you are removing the smallest amount of air possible. The more time it takes to suck the air out the more likely it is that your plastic will start to cool before you want it to. A plastic like HDPE might be better with the setup you describe. It seems to stay formable longer.