Thermoplastic Sheet Forming Method
Cold bending. Simple circular or cylindrical Vacuum forming or blow molding in female mold shapes. The thicker the sheet, the larger the bend diameter must be to prevent high stress concentrations. Ex: consumer packages.
Hot line bending. Two-dimensional shapes with arc, angle or channel cross section. Gradual to sharp bends, depending on heating procedure. Ex: safety guards.
Stretch forming (hot draping, yoke forming). Two-dimensional or slightly compound shapes, especially large shapes. Subject to surface distortion. Not suitable for reproduction of fine details. Yoke forming provides more uniform thinning out and is suitable for longer runs than manual stretch forming. Ex: aircraft canopies.
Plug and ring forming. Shallow to deep drawn shapes with gradual to sharp angles and curves. Not suitable for highly accurate contours or fine mold details. Considerable surface distortion, especially at inside comers一unsuitable where optical properties are critical. Ex: trays.
Slip forming. Same as yoke forming or plug and ring forming except that much more uniform thickness and less optical or pattern distortion can be obtained throughout the part, especially in deep drawn shapes. Fluted edges obtained by increasing clearance. Ex: carrying cases.
Free blowing or free vacuum forming. Somewhat modified surface tension shapes (tending toward flshbowl shapes) determined by shape or clamping ring and depth of draw. Internal flanges. Tolerances in. Good optical properties. Vacuum method easier to control and preferred where 15 psi is sufficient pressure. Ex: aircraft domes.
Ridge forming. Simple or complex (with vacuum and/or air pressure) shapes with minimum surface distortion and low mold cost. Generous radii necessary to avoid tearing at ridges during forming. Combines possibilities of plug and ring forming, free blowing or free vacuum forming, vacuum forming and vacuum snapback forming. Ex: equipment housings.
Usually limited to relatively shallow draws, and relatively large angles and radii—more so the thicker the sheet. Blow molding offers higher pressures and is therefore more suitable for smaller angles and radii, especially in thicker sheet; also reduces forming temperature required for thick sheet. Both produce some surface distortion一less so the more the shape of the mold resembles a surface tension shape. Ex: display signs.
Blow-dieing. Small, thin shapes having greatest perimeter larger than neck — especially closed spherical or modified spherical shapes. Not suitable for close tolerances. Ex: hollow toys.
Vacuum forming over male plug (drape forming). Shallow to deep drawn shapes with relatively sharp angles and curves can be made with good accuracy and reproduction of fine mold detail, and without excessive thinning out. More accurate than plug and ring forming but not as suitable for thick sheet. Subject to surface distortion, especially at bottom of draw. Ex: display signs. Vacuum snap back forming. Especially suitable for modified surface tension shapes with minimum mark-off and closer tolerances (% in.) than obtainable in free blowing or free vacuum forming. Simple snap back method not suitable for reverse curves or sudden contour changes, but more complex shapes and finer reproduction of mold detail are provided by additional vacuum or air pressure. Integral flanges. Combines possibilities of free vacuum forming and plug and ring forming. Ex: aircraft domes.
Matched die molding (embossing). Especially suitable for shallow draws, long runs and embossed surfaces. Relatively uniform thickness, good mechanical properties. Subject to surface distortion on both sides. Ex: watch crystals.
Die pressing. Small, simple shapes that can be approximated in precut blank and have somewhat varying thickness, or relatively great thickness compared to over-all size. Ex: brush backs.