Tolerances

Design engineers have a need to know what dimensional tolerances are possible with a given processing technique. It is always difficult to be definitive about what tolerance can be guaranteed as the final dimensions on a Thermo Pressure Formed part since they are dependent upon many factors. In general, small parts can be held to closer tolerances than large parts.

It is well-known that parts formed with a plastic material with a low mold shrinkage factor will provide more uniform dimensions than the same size and shape of part made with a high mold shrinkage material.

The final dimensions on a formed part can never be better than the dimensions on the die. There is no substitute for a good quality, precision forming die. This is a compromise situation as top quality dies are more costly and many purchasers are considering Thermo Pressure Forming because of its low tooling cost.

The repeatability of the forming process itself also contributes to dimensional consistency or the lack thereof. Good manufacturing practices, top quality processing machines and a formal quality control program can spell the difference between success and failure. The single most important decision to be made in this regard is to choose a first class supplier with proven experience in the type of thermoforming being considered.

The size and shape of a formed part also contributes to the tolerances that can be maintained by Thermo Pressure Forming.

Referring to Figure 18, outside dimensions A, B, C and D are free to pull away from the die as the material cools and shrinks. These dimensions will never be held as closely as dimensions E and F which are formed over male projections on the die that restrict the material's shrinkage and stabilize the dimensions.

Dimension G is subjected to a minimal amount of stretching and this dimension can normally be held reasonably constant, except for the increased thickness that will be present at the corners of square parts.

Dimensions H, I, and to a lesser extent J, could be precision dimensions since their routing can be numerically controlled and is performed after the part has shrunk and stabilized.

If dimensions G or K were critical, the bottom of the flange could be machined to a closely controlled size.

As pointed out previously, the final dimensions on a formed part can never be better than the dimensional accuracy of the forming die. Similarly, the final trim dimensions can never be better than the dimensional accuracy of the trimming fixturing. These "tooling-associated" inaccuracies, however, are just one source of overall final piece part inaccuracies. There are many other contributors including: sheet-to-sheet and run-to-run variations, female versus male tooling, piece part geometry, specifics of the vendors' manufacturing processes, operator skill and training, tooling maintenance procedures, etc. Some of these are virtually impossible to control economically. Others are controllable by a competent thermoforming vendor. Any customer for thermoformed piece parts should be careful to select a vendor with proven capabilities to meet the designer's tolerance requirements.

The Thermo Pressure Forming process is too new to have settled down and established industry-wide allowable tolerance standards. Until such time as tolerance standards are evolved however, the designer can assume the following tolerances can be met by a competent thermoforming vendor using reasonably priced tooling. It is important to note however, that tighter tolerances are possible if required, at some one-time additional tooling cost and/or some continuing additional piece part cost.

All other things being equal, Thermo Pressure Formed parts are always more precise than similar parts formed by other lower pressure thermoforming processes.