I was responding to a post by davidvillajr http://www.jalopyjournal.com/forum/showthread.php?t=148698 He wanted to vacuum form some faces for the dash in his 1951 Ford. I thought I would do a Tech about vacuum forming. Years ago I owned a company that formed large plastic faces for signs and this is a quick look at how it is done. Basically, you have a way to heat the plastic, lay it on a form that is on a vacuum table, and then suck the air out of the chamber and the vacuum sucks it down over the mold. Just as a large press stamps out sheet metal. this is done the same way, except that the vacuum does the forming over the mold. I will try to explain this in a series of posts here and I hope it won't be too confusing. Most large sign companies or plastic companies have vacuum forming equipment. They take a sheet of plastic in a frame and raise it up to a uniform heating source to get the plastic to a molding temperature. They then lower the plastic over a mold and suck the air out and cause the plastic to conform to the mold. I hope the following posts and graphics will make it clearer.
Fig 1 This is the basis for the vacuum forming process. A frame of angle iron that can be clamped or screwed together with the plastic to be formed sandwiched in between. The bottom is a flat table with a hole in the middle that is the source for the vacuum Next is a box form that would be sealed to the table with duct tape or silicone seal so as to be air tight. Now if you turned on the vacuum at this point it wouldn't work because the top of the form is open. The sandwich of the frame and the plastic will seal the top of the form and provide a vacuum. The unheated plastic would not form a vacuum, but when it is heated, it will become soft and pliable and will seal on the top edge of the form. Whatever the plastic is that you will be using will have a temperature that you use that will make the plastic pliable without bubbling or sagging too much. This will require a certain amount of experimenting. You may get lucky on the first pull, but not likely. I used to heat at 400 for twenty minutes on a 6' by 10 ' piece of Uvex, but that will vary according to thickness and size. Now we need a form or mold to keep the plastic from sucking into the hole.
Fig 2 To seal the top of the vacuum box, we will put a flat top forming a air tight chamber. Now if we would apply the full vacuum it would probably sag down or collapse, so we build a brace to put in the center for support. The center support needs holes in it so the air can get down the vacuum hole. Now we place the part to be formed on the top flat surface and we will drill holes around the object so that the air can be sucked through the holes into the vacuum chamber.
Fig. 3 This is the basic vacuum set up. Vacuum source is at the bottom, can be a large vacuum tank, or as simple as a vacuum cleaner. As with everything, the more power the better. The vacuum chamber must be air tight, down around it's base on the table and where the plastic meets the top edge. The filler bracing with the hole in it, braces the top of the vacuum chamber and yet allows the vacuum to go through the holes. Without the bracing the vacuum could collapse the upper plate.
Fig. 4. This is the part that you are trying to form. In the example it is a Letter N. Notice that it has holes around the perimeter to allow the air to be sucked down into the chamber. This is the object that you will be most concerned with in doing your projects. In the object N the edges should have 15 degrees return outward. This angle should prevent the vacuum to suck under the object so that you can get the mold out. If you are forming over an existing object, like your dashboard, you must build support underneath and add a taper away from the object, so it will pop off the mold. If you concern yourself with just the object that you are trying to form, you can take it to someone that does vacuum forming and they can mold it for you, without you having to build the vacuum apparatus yourself. In the thread where someone was trying to make a plastic cover for a dash panel, you would need to support underneath, and allow some of the support to protrude out from the object. See Fig. 5
Figure 5 shows the dash piece that we were talking about and how to allow the lip and support to hold it correctly. I think that I will stop here and see what I haven't made clear and can clarify.
With some of the new high density foams, you could literally make complete new dashboards with styling for different gauges. Carve what yoiu like and vacuum away. The one tip to remember is the the outsides edges should have a 15 degree slant to facilitate removal from the mold.
saw the mythbuster guys doing this all the time, never thought to apply it to grandpa's car..... gonna keep watching dv
Your tech post was very informative and really shows practical use. I hope others can gain some knowledge from these posts. Thanks for you post.
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