Stiffening Details

One of thermoforming's primary advantages over injection, structural foam and reaction injection molding is its ability to produce large parts with relatively thin walls. This advantage is made possible because thermoforming starts with a sheet of material about the same size as the finished part. Closed mold processes on the other hand, require the plastic to flow from a centrally located nozzle out to the periphery of the part. As the size of the part becomes progressively larger the wall thicknesses must be increased in order to provide for the flow of the plastic.

This weight-saving advantage of thermoforming becomes obvious as parts increase in size. However, the same situation exists with small-to-modest sized parts. The common thermoformed vending machine cup is always lighter in weight than its injection molded competitor. The two well plates shown in Figure 12 are a classic example.

These two similar parts which are used in the same diagnostic procedure are both clinically acceptable. The Thermo Pressure Formed part is only .012-.018 inches thick and weighs 22.09 pounds per 1,000 parts. The thicker walls required for filling the cavity of the injection mold are an average of .051 inches, which produce a part weighing 179.74 pounds per 1,000 parts. The thinner walls which were attainable with Thermo Pressure Forming resulted in a significant savings in the cost of the plastic material used. These thinner walls also reduced the energy and cycle time required to heat, form and cool the thinner parts.

Thermo Pressure Forming is capable of producing parts which are either thick or thin. However, the cost advantages to be realized with the thinnest acceptable wall thickness are worth consideration. One way of achieving greater rigidity while keeping the nominal wall thickness to a minimum is the use of shapes that provide a stiffening or reinforcing effect.

Solid ribs of the type commonly used on injection and compression molded parts are not possible with thermoforming. Hollow ribs of the type shown in Figures 13 can however be used to add stiffness in heavily loaded areas.

It is generally accepted that the outside width of a hollow, female thermoformed stiffening rib should be at least 1-3/4 times the depth of the rib. Wider ribs may be required for relatively thick walled parts. Traditionally the edges of these ribs were always well radiused in order to minimize thinning out of the material as it stretched into the shape of the rib.

Thermo Pressure Forming's higher forming pressures allow the forming of sharp rib details whose width in some cases can be as little as one to two times the thickness of the wall if the depth of the rib is not greater than the sheet thickness.

The shape and location of stiffening ribs can be designed in many different ways to satisfy the needs of the part. The different types of ribs in Figure 14 are examples of just a few.

Figure 14 A shows a method of stiffening the bottom center portion of the part, B1 refers to the use of the edge flange to stiffen the side walls. B2 shows an additional step back at the flange to provide even more strength in this area. C illustrates a method of adding strength to the junction of the bottom of the part and the vertical side walls. D describes a way to provide double walls in the corners of the vertical walls.

This type of ribbing can be used to add crush strength to a part. It is also obvious that all four types of stiffening ribs could be added to the same part to supply a great deal of reinforcement to a thin wall product.

Another approach to adding stiffness to a thin walled part is to add a slight dome or radius on large flat surfaces. A rise of as little as .015 inches per inch is enough to begin to provide a reinforcing effect. The slight dome also does a good job of hiding the warpage which can sometimes be a problem with large, flat surfaces (Figure 15)

These large radii do add somewhat to the cost of the die. However that is a one-time charge which can normally be justified if strength and good appearance are important.

Yet another type of stiffening that is common with thermoformed parts is the flange at the trim line at the open end of the part (Figures 14, 16 and 17).

These flanges provide a great deal of reinforcement to a thin walled part. The presence of these relatively thick flanges often allows the use of thinner original sheet stock. This results in cost savings in both material and cycle time.