vacuum drawing

Vacuum Drawing or Blowing into a Form

This method is also based on airpressure differentials. The heated Chemcast® GP sheet is clamped directly to the edges of a female form, and the sheet is either drawn down by vacuum or forced down by air pressure into the form.

When the shape closely resembles a "free form" shape, parts formed by this method will have fairly good optical properties. Every part of the sheet comes in contact with the form at approximately the same time, and the pressure can be controlled so that mark~off is held to a minimum.

When the shape is not a "free form" shape, one area of the sheet comes in contact with the form before the other areas are fully drawn, and the pressure at the areas in contact with the form will be great enough to cause surface defects.

If good optical properties are most important, the form can be greased as in "Grease Forming". It is very difficult to attach a felt grease blanket to the inside of a female mold and none is ordinarily required. Special polybutene greases (Sources of Supply) which do not change their viscosity radically with changes in temperature should be used. The mold should be warmed with electric elements, infrared lamps, steam, or oil to approximately 170°F. for Chemcast® GP and then coated with a film of grease approximately 1/16' thick. The grease film must be reasonably uniform. It should be smoothed and fresh grease added as needed. The surface of the grease should be heated to as near the forming temperature as possible with infrared lamps just before the heated sheet is clamped in Place for forming.

Forms used for vacuum drawing or blowing into a form should be well made of sturdy materials and adequately reinforced. The mold should have a uniform thickness in the forming area to ensure equal deflection under forming pressure or vacuum and constant heat transfer rates. When the mold is greased, positive air pressure is preferred as vacuum tends to draw entrapped air from the pores of the mold, causing bubbles in the grease layer and distortions in the formed Chemcast® GP part. When the mold is not greased, either positive pressure or vacuum may be used. Pressure or vacuum molds should have outlets at the points of deepest draw and should provide a tight seal between flange and Chemcast® GP to avoid air leaks.

Vacuum drawing or blowing into a form is used often for forming parts which differ quite radically from free form shapes, but in which mark-off is not objectionable. In fact, the method can be used to reproduce in Chemcast® GP the mirror image of any pattern or device in the female mold. Very fine detail can be picked up in this way, depending on the amount of pressure used. Close approximations of geometric shapes can also be produced. Corners, of course, will tend to be round, the radius of curvature depending on the pressure used. Sharp corners will tend to thin out due to the greater depth of draw in the corners. Heavy clamping pressure is required when high positive air pressure differentials are used.

To calculate the clamping pressure required to prevent leaks, multiply the pressure required to form the part by the projected area of the part at the clamping ring.

Example:

50 psi required to form

Area: 50" X 60" = 3000 jn 2

Clamping pressure:

50 X 3000 = 150,000 Pounds

Forming Acrylic Sheet