Nominal Walls

The cardinal rule for the design of all molded or formed plastic parts is to maintain a uniform wall thickness. This is especially true with high pressure processes such as injection molding. While designing parts for the injection molding process, it is not always possible to maintain a uniform wall thickness while providing all of the functional features that are possible with this closed mold process. Unintentional increases in wall thickness frequently occur at the junction of two walls or between one wall and a hollow boss, stiffening ribs or other details. These thicker sections take longer to cool and they shrink more than the surrounding thinner sections. This lack of uniform mold shrinkage produces a high level of molded-in stress that often results in post mold warpage and reduced impact strength. Unsightly sink marks on appearance surfaces also result from these increases in the nominal wall thickness. This non-uniform shrinkage condition represents one of injection molding's primary limitations. This is one of the reasons why injection molding loses some projects to the structural foam process which minimizes this condition.

On the other hand, Thermo Pressure Forming starts with a sheet of thermoplastic material which has a uniform wall thickness. This sheet of material may be somewhat thinned out by the stretching that takes place during the forming process. However, it is visually impossible to produce sections which are thicker than the original sheet thickness. As a result, Thermo Pressure Formed parts do not contain sink marks.

During the injection molding process the plastic is heated to the point that it melts and becomes a thick viscous liquid capable of flowing to fill the cavity.

The Thermo Pressure Forming process operates at a much lower temperature that softens but does not melt the material. These lower forming temperatures result in smaller differences in thermal expansion as the plastic material cools from the processing temperature down to room temperature. The differences in expansion and contraction that result from the variation in the wall thickness of Thermo Pressure Formed parts are much less than for injection molding.

Closed mold processes are capable of intentional or inadvertent abrupt changes in wall thickness. The nature of the Thermo Pressure Forming process which stretches the sheet produces only smoothly blended changes in wall thickness.

The injection molding process requires matched metal molds or cores and cavities which is one of the reasons that the tooling is so costly. The presence of the core pin inside of a three-dimensional part prevents the molded part from shrinking as much as it would if the core pin was not present. This condition produces high levels of molded-in stress that can result in post mold warpage.

Thermo Pressure Formed parts are normally formed in female dies. There are no cores inside of the part to interfere with the natural shrinking of the part as it cools.

The lower processing temperatures, the gradual changes in wall thickness and the lack of shrinkage-restricting core pins allow Thermo Pressure Formed parts to be produced with relatively low levels of molded-in stress. These reductions in molded-in stress allow the production of tough parts which are dimensionally stable with less tendency toward post mold warpage.

The higher forming pressures associated with Thermo Pressure Forming make it an ideal process for thick walled parts. The nominal wall thickness of Thermo Pressure Formed parts is in the range of .062 to .500 inches. Thicker plastic sheets have been formed into large parts demanding high strength. Thinner parts find their primary use in special cases and packaging applications. However, an ideal sheet thickness for a Thermo Pressure Formed part would be in the .125 to .250 inch thickness range, depending upon the proportions of the part and the load-bearing requirements of the application.

Thermo Pressure Formed parts have varying wall thicknesses which are determined by the amount of stretching of the sheet required to form each detail on the part. Considering this fact, the nominal wall thickness cannot be specified in the normal manner. It is standard practice in the thermoforming industry to specify the original starting sheet (or gauge) thickness and the minimum allowable thickness that will be acceptable at any location on the finished part. A less desirable but common practice is to specify the average wall thickness of the finished part.

The various thermoforming processes were originally created to convert flat sheets into three-dimensional parts. Thermo Pressure Forming is recognized as an ideal process for producing deep draw, three-dimensional parts. When a flat, basically two-dimensional sheet is formed into a larger, generally more complex three-dimensional shape, there are four factors to which the designer must pay careful attention. These four factors are:

1. the depth of draw ratio
2. the stretch ratio
3. corner radii
4. draft angles