DIMENSIONAL TOLERANCES

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GUIDELINES:

For molded in dimensions of parts from a machined aluminum mold use +/-.015" for the first inch adding an additional +/-.001" for each subsequent inch. For molded in dimensions of parts from a cast aluminum mold use +/-.030" for the first 16" adding an additional +/-.001" for each subsequent inch. Trimmed dimensions, regardless of the mold but using CNC trimming equipment, should have a general tolerance of +/-.015". For hole diameters use +/-.005".

OVERVIEW:

Thermoformed part tolerances are determined by the coefficient of thermal expansion of the resin, extrusion conditions, type and temperature of the mold, consistency of the forming process, method of trimming and quality of trimming fixtures.

SPECIFICS:

Most thermoforming resins have a coefficient of thermal expansion in the range of .000060" to .000120" per ºF per inch [6.0 - 12.0 x 10^-5 in./ºF/in.]. This will be a more significant factor when the part is large and the "in use" temperature of the part varies. On parts over 48" it is good practice to add a note to the drawing specifying a temperature at which the dimensions should be measured. We have found 40-50 ºF temperature variances in our plant summer to winter.

Extrusion quality will affect the part many ways. The control of the extruder from run to run is very important. Changes in extrusion speed, direction, temperature, and gauge will modify the amount of stress that the sheet has when it is delivered to the thermoformer. Differences in that stress will change the rate at which the part molds thus changing the dimensions.

Molds must be temperature controlled with internal cooling channels to allow for consistent mold temperature. Aluminum is the material of choice because its very high coefficient of thermal conductivity allows for very consistent cooling cycle times through the entire production run of parts. Wood, epoxy, or plaster do not allow for this. Because of the amount of shrink that takes place at the aluminum foundry when a mold is cast the typical tolerances (from the print to the mold) begin at +/-.030" for the first 16", adding an additional +/- .001" for each subsequent inch.

The forming process must be very consistent from run to run to insure dimensional consistency. The use of digital controls on the forming machine allows for a high degree of accuracy in the cycle times. A change in the amount of time the part spends in the mold has a direct effect on the amount of part shrinkage that occurs. The part must be held in the mold until the set temperature of the resin has been reached. A quality thermoformer will have fully digital controls on their forming equipment along with the capability to continually monitor the sheet temperature during the heating cycle. CNC trimming allows for tighter tolerances and consistent parts. A typical CNC trimming machine controls the movements to +/-.0002"; the trimming machine itself is designed for +/-.005". For cost effectiveness we use holding fixtures that allow a tolerance of +/-.015". Hole sizes and hole patterns can be held to a tolerance of +/-.005". To try and hold tighter tolerances would require fixturing approaching that of the original mold cost.