Corner Radii

Whenever a flat plastic part turns a corner to make a three-dimensional part, the designer must decide on the best way to make the transition from one plane to another. The best way to make this transition on any plastic part is with a properly proportioned corner radius.

The radii which are so common on plastic parts have two primary functions. First, they simplify the manufacturing process. Secondly, they strengthen the corner by eliminating stress concentration and by distributing the corner stress over a broader area.

It is a well-known fact that there will be a gradual increase in stress in the corner of a plastic part whenever the size of the inside corner radius is less than 75% of the thickness of the wall (Figure 7). There will be a very rapid increase in stress when the inside corner radius is less than 25% of the thickness of the wall to which the radius is attached.

Considering these facts, the inside corner radius on a plastic part should not be less than 25% of the wall thickness as shown in Figure 8. If maximum strength is required, the size of the radius should be at least 75% of the thickness of the wall to which it is attached.

Historically, thermoformed parts have been characterized by having relatively large corner radii (Figure 9).

An inside corner radius of four times the thickness of the wall has traditionally been considered to be good. An inside radius equal to the wall thickness was normally possible, but only with a significant reduction in wall thickness and an increase in forming difficulty (Figure 5). An ideal corner radius was always "as large as possible."

These large corner radii came about due to the understandable fact that large radii were thinned out less and were easier to produce than small corner radii while using the older, more traditional methods of thermoforming.

As marketplace preferences changed, designers began to ask for smaller outside radii. Thermo Pressure Forming, with its higher forming pressures, was able to give designers what they were asking for. Thermo Pressure Forming allows the forming of outside corner radii which are as small as .015 inches with most materials and most shapes. Outside corner radii of as little as .005 inches have been formed under special circumstances.

These sharper corners and other similarly clearly defined details opened up totally new markets for Thermo Pressure Forming. The designers' objectives were met and the end-users got what they wanted. It must be remembered however, that these small corner radii, just like the sharp corners on injection molded parts, violate the strength considerations illustrated in Figure 7.

The designer must also remember that all plastic materials cannot be handled in the same way. Notch-sensitive materials such as nylon or polycarbonate are very susceptible to a loss of strength in sharp corners. Other materials such as PVC and ABS are more tolerant of small corner radii.

The shape of a part leading up to a corner also has an effect on the amount of thinning out of the sheet and the size of the corner radius that can be formed. A corner with an angle of greater than 90 degrees is ideal. Angles of less than 90 degrees start to become troublesome. A material such as acrylic which retains a lot of its strength at forming temperatures (hot melt strength) will be more difficult to form into small corner radii than a material like polycarbonate which is not as strong at forming temperatures.

The proper proportions for the corner radius on a plastic part will add significantly to the part's strength. The strength of radiused corners is determined primarily by the size of the inside radius. Designers therefore tend to specify the inside corner radius on a plastic part. In the case of thermoformed parts, only that corner on the side of the part that comes into contact with the die can be controlled accurately. The designer must bear this fact in mind while specifying the corner radii on pressure formed parts.