vacuum forming equipment

Vacuum Forming Equipment

The equipment required for vacuum forming consists of a source of vacuum, an accumulator tank, piping, valves and fittings, and the vacuum pot or chamber in which the actual forming is done.

The vacuum chamber is generally made of steel Plate, all seams being welded and airtight. The chamber should have a flange around the edge for attaching clamping rings and should be as small as the part will permit to minimize the amount of air which must be removed during forming. The completed part should be at least 2" from the inside of the vacuum pot to prevent touching and to prevent uneven heat transfer from the hot sheet to the cool tank wall.

If only a few pieces are to be made the chamber can be built of heavy Plywood or Masonite die stock on a frame of wood twobyfours. The joints are sealed by a caulking compound applied to all edges before assembling. Any gun or knifing compound can be used and will maintain the seal even though there is a certain amount of movement of the joints as the pressure within the pot is reduced. Such movement often cracks a glued joint and causes leakage. For additional sealing, masking tape can be applied to the outside of all seams and Spraylat or similar material may be brushed on the outside.

Clamping rings can be made of Plywood, rigid phenolic laminate or Masonite die stock, and with a center hole cut to the desired shape. Interchangeable clamping rings will permit production of a variety of parts on a single vacuum chamber. Metal rings should not be used unless they are temperaturecontrolled, because they chill the Chemcast® GP too rapidly. Plywood may be used in some cases if markoff in the flange area is not objectionable. Hardwood plywoods give better results than softwood plywoods.

When detachable rings are used, a tubular gasket or "O" ring can be aplied between the metal flange and the removable ring. The greater the pressure on such a gasket, the tighter the seal.

In practice, the hot Chemcast® GP sheet is clamped between the detachable ring and a holddown ring, by any of the various means suggested. This second ring may be knurled or scored to prevent the sheet from slipping. Often the edge of the detachable ring is beaded to make an airtight seal. Such a seal is important not only for preserving the vacuum, but also to avoid drawing in cool air, which may chill the Chemcast® GP unequally. A small leak can make the difference between the production of good and bad parts.

A vacuum pump, driven by an electric motor which will handle 25 to 50 cubic feet of air per minute at 27" of mercury, is satisfactory for forming all but the largest parts. Steam, water or air venturi or ejector valves can also be used. Whatever source is used, it should maintain at least 22" of mercury at the rated volume to make the system most useful.

The vacuum system should include an accumulator tank to prevent fluctuations in air pressure during forming. Galvanized steel domestic hot water tanks, or any tank able to withstand external pressures of 15 psi, are recommended in capacities of 30 to 100 gallons. The larger sizes give extra capacity at little extra cost and permit greater flexibility in the system. The vacuum pump should be capable of pumping a volume of air per minute (at a minimum of 22U mercury) approximately twice the volume of the system, including the accumulator tank and the vacuum pot. The accumulator tank should be fitted with a drain.

Standard steel piping and valves of 1u diameter or larger are normally used. At least one section of the piping between accumulator and forming chamber should be made of flexible metal reinforced hose so that the chamber can be moved or the connection changed easily. Short, large diameter pipes and valves reduce losses and make a more efficient system.

A pipe from the vacuum chamber is connected with a standard pipe tee. One side of the tee is connected through a valve to the vacuum line; the other, through to a second valve, is open to the atmosphere. The pressure, and hence the rate and depth of draw, can be accurately controlled by adjusting the two valves.

This control may be manual or automatic (Figure 27). In one mechanical system the Chemcast® GP, as it is being formed, touches a micro switch which activates a solenoid. This solenoid operates an airline controller, which, in turn, controls the modulating valve on the vacuum line. In another system, the Chemcast® GP, as it is being drawn, interrupts a beam of light focused on a photoelectric cell. The change in current in the cell operates a solenoid valve, through relays, to open and close the vent line on the vacuum pot. A highlift or needlevalve is adjusted on the vacuum line so that the volume of air being drawn from the vacuum pot is balanced by the volume of vented air. In this way the correct rate of draw and depth is maintained.

Forming Acrylic Sheet