Calculations

Draw Ratio

Draw ratio is the ratio of part depth to part width, . The part is formed from a plastic sheet that is stretched over a male mold, or stretched into a female mold. As the sheet is stretched, thin and thick spots will develop. Maintaining a low draw ratio is important to maintaining a somewhat uniform wall thickness, and avoid excessively thin areas. In general, draw ratio is more critical for female molds than male. A draw ratio of 2:1 is recommended for female molds. If plug assist thermoforming is used, the draw ratio can be greatly increased, up to 7:1.

Area Ratio

Area ratio is the ratio of the sheet area before thermoforming to the area of the part after forming, . The area ratio will give some indication to the amount of wall thinning the sheet will see. This calculation is also commonly used to determine the thickness of the sheet that is needed to achieve the desired wall thickness.

Example

Shown above is a design for a thermoformed cup. We will determine the draw ratio, the area ratio, and then the necessary thickness of the sheet to achieve the desired cup thickness.

Using the area ratio, the sheet thickness needed for a 0.060” wall cup is 0.060” / 0.17 = 0.352” thick. This is a very thick sheet for thermoforming; this part design should not be used. Either the wall thickness of the cup needs to be reduced, or the cup can be injection molded.

The draw ratio of 1.675 : 1 is good.