Friday, April 23, 2010
Stretch Forming A plastic sheet forming technique in which the heated thermoplastic sheet is stretched over a mold and subsequently cooled. It is quick, efficient, and has a high degree of repeatability.
Advanced pre-stretch forming or mechanical assisted forming techniques require thermoforming equipment with both top and bottom platens. Automatic machine sequence control is also usually required.
The only real advantage of this process is that only a male form is needed. The disadvantages are many, and include requiring the male form to beconstructed strong enough to resist thelarge forces exerted by the mechanicalequipment that is needed to stretchmany lineal inches of plastic in onedirection. That force may reach manytons.Additionally, minor dirt particles on,or minor deviations of the molds exteri-or surface, will show up as opticaldefects (mark-off) on the concave innersurface of the part. Such defects arevery difficult to remove by later effortsusing abrasives and polishing products.Other problems are excessive thin-ning of the plastic at the deepest por-tion of the formed part and the intro-duction of severe internal stresses thatusually result in early failure when thepart is exposed to sunlight