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Frequently asked questions about pumps for vacuum forming
Answers to FAQ's
Diaphragm pumps - Usually smaller pumps with low flow, they are quiet, but only pull 10-20 IN.HG. Not a good choice for vacuum forming.
Oiless Piston Pumps - These can make good vacuum pumps and can also be used as a compressor. They offer decent flow and a maximum vacuum of 27-28.5 IN.HG. with low maintenance, and good service life. Oil filled piston pumps used for air compressors shouldn't be adapted for vacuum use because oil can suck past the rings and blow out the exhaust.
Dry Rotory Vane Pumps - These use no lubrication at all, just carbon vanes that wear down slowly. These offer a surprisingly long life, are easy to rebuild and virtually maintenance free. They usually cost less than an oilf filled pumpl and are what most commercial machines will use. However, they usually only pull 26 - 27 inches of vacuum. Sometimes they come with a drip feed lubrication that can increase the vacuum levels to around 28.5 IN. HG.
Oil filled Rotory Vane Pumps - These have an oil reservoir and the vanes
are always lubricated. This lets them pull over 29 inches. They may
be one or two stages allowing them to reach deeper vacuum levels. These
pumps cost more and last even longer than dry pumps. Often they will
blow a fine oil mist out the exhaust that must be piped away or trapped
and returned with a filter.
As a general rule most people could use 25 IN. HG. or even less for vacuum forming. If using thin easy to form plastics, some people even get by with the 4 to 6 inches provided by a shop vac, but more is always better for crisp definition and neccesary for thicker plastics. There is rarely ever a problem with too much vacuum, and its easy to turn it down. Its just prudent to buy as much forming power as you can afford, just in case you need it. Keep in mind that things like altitude above sea level and even barometric pressure can reduce the forming power you have available!
Well that depends on everything!.. How much money does a person need?.. More is usually better. I make specific recommendations in my Hobby-Vac and Proto-Form construction plans, but I often get this question from people building their own designs.
I'll try to keep this simple.. When dealing with vacuum forming machines using the industry standard "pump and Tank" systems, make the tank large enough so that there isn't too much vacuum loss at the end of the forming cycle (large parts are worse). Then make the pump large enough to recharge the tank quickly to a usable level before the next part is ready to form. When evacuating a tank, the pressure will drop very quickly at first, then get real slow at the very end. If your pump is too small, it will never seem to get that last few inches of vacuum that the pump is capable of.
I compared many commercial machines and talked to way too many pump salesmen, but there were no clear patterns. Apparently there is a wide range of acceptability depending on your needs. Here are some guidelines I use:
- Use one gallon of tank capacity for every 25 to 45 square inches of platen area.
- Use 1 CFM of pump flow for every 2-4 gallon of tank size.