Part Two: Designing Products for Rotational Molding

Nominal Wall Thickness & Wall Thickness Limits

Nominal Wall Thickness & Wall Thickness Limits

The nominal wall thickness is the average wall thickness that the designer assumes to be prevalent around the part. This figure will typically be used for performance calculations, such as the determination of stress in different sections.

A practical rotomolding design should recognize that there will be an inevitable variation in wall thickness around the part; this is a known feature of the process and is described in more detail in the next section. Therefore it may also be necessary to specify a minimum wall thickness, below which no section of the part will fall. This will provide the molder with a guide to the quality tests and limits that must be applied during manufacture.

The type of plastic material used, plus the nominal wall thickness, will determine the strength and load bearing capability of the finished part. The wall thickness required to sustain a given load can be determined by hand or by computer-assisted calculation methods. However, selection of material property data should be made carefully and preferably after consultation with the material supplier; it is known that as-molded physical properties may be significantly different to properties shown on Technical Data Sheets. The selection of appropriate Factors of Safety can mitigate some of these issues.

In the past, there have been instances (eg the rupture of large liquid storage tanks) where product failure occurred because a low wall thickness coincided with an area of high loading in the part. This created a local stress that was in excess of the material capabilities; a design that assumed even wall thickness throughout may not have predicted this occurrence.

Many rotomolded products, especially those of small / medium size, may not require particularly high structural strength because they are not subject to high loading. In such cases, a nominal wall thickness may be decided by comparison to similar parts, experience, or even overall product weight considerations.

Polyethylene nominal wall thicknesses for small / medium sized parts are often in the range of 0.125 inch to 0.250 inches, which represents a good compromise between strength, cycle time, ease of processing and cost. Larger parts have been made with much greater wall thicknesses, up to two-plus inches.

The known tendency for wall thickness variation in rotomolded parts means that nominal wall thicknesses below approx. 0.075 inches run the risk that areas of the part may be of inadequate, or even zero, thickness. This depends on the practical circumstances prevailing, over which the molder will have some control. When envisaging thin-walled parts, the designer would be well advised to specify a minimum wall thickness (see above).

Some speciality materials (especially amorphous polymers like polycarbonate) experience a weakness in physical performance, due to stress concentration, if sections are too thick. It is advisable, when using such materials, to check whether there are maximum recommended wall thickness requirements.

Rotomolding provides the design engineer with the unique capability to increase or decrease the wall thickness of a part, after the mold has been built and sampled. Once the outside shape has been defined, the part will be free-formed and its internal size and shape will be dependent upon the outside size and shape, minus the wall thickness. The final wall thickness of the part can therefore be modified after actual in-use testing of the part.

For products such as tanks and containers, there may be a design specification for the total internal volume required to be provided. The designer should bear in mind that increasing the wall thickness of the part will reduce the resultant volume of the internal cavity of the product. This may well be an insignificant factor in large tanks and containers, but for small items it could be significant. It would be advisable to do a check calculation to ascertain whether there are practical limitations to how much the wall thickness can be increased.

Wall thickness of rotomolded parts can be altered by changing the shot weight of material inserted into the mold at the start of the molding process. An increase in shot weight will create a proportional increase in nominal wall thickness. This means that, for many non-structural items, a nominal wall thickness will be assumed in the design, in the knowledge that it may be increased or decreased somewhat, once the mold is available for product trials.

It should be noted that increasing the wall thickness of the part will affect the production cycle time. A larger shot weight in the mold will lengthen the time needed for initial melting and sintering of the material. It may also lengthen the time required for cooling. Consideration should also be given to the possibility of thermal degradation of the material, if prolonged oven cycles are used; this is especially true when using non-polyethylene materials, which can be more heat sensitive.

The nominal wall thickness that is ultimately selected will have a direct effect on the cost of the finished part. In addition to the added cost of the material, the cycle time and the energy consumption in molding may all be significantly affected.