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Vacuum-snap back is an excellent and often used process for forming deep draw products with uniform wall thickness. Vacuum is used to pre-stretch the hot plastic before the mold makes contact with the sheet. Vacuum snap-back, while more complex than plug assist, can produce deeper drawn products with better wall uniformity and less mark-off. A vacuum pre-stretch box is required. The pre-stretch box is sealed against the hot sheet and vacuum is applied. The plastic is drawn into the box as a hemisphere with the height of the hemisphere usually controlled by a photocell. Other methods can be used to control the hemisphere height, but a photocell works well.

The steps of vacuum snap-back are:

• After the plastic sheet is heated and the sheet cart returns to the forming station, the bottom platen moves up sealing the vacuum pre-stretch box against the hot sheet. Vacuum is then applied. When the stretching plastic crosses the photocell beam, vacuum is turn off.

• The mold is moved into the formed hemisphere. When the mold is sealed against the hot plastic, vacuum is applied to the mold and vacuum is released from the pre-stretch box causing the plastic to snap to the contours of the mold.

• The pre-stretch box is then lowered and cooling air is blown against the hot plastic. After the plastic cools, the mold vacuum is released, air eject is applied through the mold and then the mold is removed from the formed plastic part.

•  Placing the mold on the bottom platen and the pre-stretch box on the top platen will also work for vacuum snap-back.

Advantages

• well controlled part thickness
• though longer cycle times