Processes

Thermoforming tool design guidelines

As with all processes, there are processing limitations. The following are guidelines to assist you with designingyour product for optimum strength, appearance and performance.

•  Minimum draft angle should be 2° to 5° on male molds ormale portions of female molds and 1/2° to 1° on female molds. For textured mold surfaces, the draft angle should beincreased 1o per 0.2 thousands of an inch or 5 µm of texture.
• To minimize nipple height, the diameter of any vacuum hole should not exceed the local sheet thickness. For very thin sheets, alternate means of air evacuation, such as slot vents or porous plugs, should be considered to avoid nipple formation. If the rate of air evacuation is too low, the sheet will not fully form against the mold. This indicates that there are an insufficient number of vacuum holes.
• Undercuts should be avoided. If undercuts are necessary, they should be discontinuous around the periphery and should be shallow. If deep or continuous undercuts are required, breakaway portions of the mold will be needed to affect part removal without scuffing.
• Molds must be oversized to allow for polymer shrinkage. On male molds and male portions of female molds, 0.3% to0.5% shrinkage allowance is recommended. On female molds, 0.5% to 0.8% shrinkage allowance is recommended. The polymer grade, coefficient of thermal expansion, part geometry, mold temperature, initial sheet temperature, initial sheet thickness, and forming cycle all affect polymer shrinkage.• Radii on ribs and fillets should not be less than the localsheet thickness. The radii should be as much as four times the local wall thickness in areas where high loading is encountered or good stiffness is required.
• The draw ratio is given as the surface area of the formed part divided by the surface area of the sheet used to form the part. The average thickness reduction is the reciprocal of the a real draw ratio. Often, the depth-to-width ratio, viz,H:D, is used for axisymmetric parts but it is not accurate for rectangular parts since it ignores the effects of the length dimension. • In vacuum or drape forming, the depth of draw is usually limited to the narrowest width of the part, viz, H:D <1.Greater draw ratios are achieved with prestretching techniques such as plug assist forming.
• The best part-to-part dimensional tolerance is achieved by forming against a heated mold. However, cooling cycletimes increase with increased mold temperature.