Part Two: Designing Products for Rotational Molding

Development of Product Designs and Tooling using Digital Methods

Development of Product Designs and Tooling using Digital Methods

Although many simple rotomolded parts are still designed by traditional means, the use of digitally based methods has become established for the development of rotomolded product designs.

3D computer-aided design (CAD) software is readily available and is commonly used to create, modify and optimize the design of a product. The use of CAD has the following advantages:

• Once a design has been created in CAD, review and modification can be conducted more easily and more rapidly. A variety different design options can be tested. Design changes can be made in an efficient manner and the use of color renderings helps non-technical personnel to fully understand the design.
• Multiple views and sections through the product can be generated, enabling checks to be made on form, fit and function.
• CAD software will enable associated calculations to be made quickly and easily. For example, physical properties of the finished part, such as shot weight, can be accurately assessed; this allows for more accurate product costing at an early stage of the design process. The volume of the resultant mold cavity can be calculated, to ensure that there will be sufficient space for the powdered material required by the shot weight.
• Design details can be rapidly disseminated to interested parties, by electronic means.

Completed CAD data files can be exported for subsequent operations. This may include computer-aided engineering / structural analysis by finite element analysis (FEA), as well as computer-aided manufacturing (CAM).

FEA can be an expensive technique to carry out properly and requires a degree of skill and experience to produce a realistic output. Many rotomolded part designs produced with CAD will not go on to be analysed in depth; use of FEA will often be limited to larger parts that are known to be subjected to significant loadings or movements. In order to increase calculation efficiency, a detailed CAD file will often have some detailed features removed before it is migrated to FEA, particularly if these features have no bearing on the structural performance of the part.

FEA techniques can be applied to rotomolded product designs, but caution is advised in an over-reliance on the results of such calculations. Most rotomolded parts are made using semi-crystalline polymers, like polyethylene, which do not fully comply with the standard FEA assumptions that are made for materials such as metals. Typically, polymer physical properties such as Flexural Modulus, Yield Strength and Poisson’s Ratio will be called for in a standard FEA computation. This will enable stress and strain levels to be calculated across the product, to ensure that there are no areas of excessive stress concentrations or deflection. A typical FEA output will enable a rendering of the product to be produced with stresses and deflections mapped across its surface, often as a multi-colored display for easy identification of areas that are in excess of limits.

It should be noted that polymer properties will be significantly affected by operating temperature; for example, the stiffness of polyethylene reduces by approx. 50% as the ambient temperature increases from 70⁰F to 100⁰F. These effects may not be adequately reflected on standard Technical Data Sheets available from polymer manufacturers. In addition, it may also be necessary to consider long-term properties of polymer, such as its resistance to creep.

Generally, a Factor of Safety (FoS) will be incorporated into design calculations, to account for variables in both the material performance and the manufacturing process. The choice of a suitable FoS should be carefully considered and discussed, taking account of previous experience and the extent of uncertainties in the design parameters used.

In the context of tool manufacture, CAD data can be used to direct the computer numerical control (CNC) devices that manufacture patterns and plugs (for cast tooling) or to directly machine tooling from metal billets. Care needs to be used in the migration of data from product designers to tool makers. One extremely important area for clear communication is how allowance will be made for material shrinkage, especially in the case of polyethylene. In most cases, the product designer will produce a “net finished shape” and the toolmaker will be responsible for making allowances for shrinkage and for other manufacturing issues.

CAD files can be used as input to control 3D printers, for several purposes. Scale models can be produced, for visualization purposes, to investigate detailed aspects of usage / installation and to carry out limited physical testing. In principle, 3D printers could also be used to manufacture models for casting and even tooling components. The production of product prototypes by 3D printing is less common for rotomoulding than for other molding processes, mainly because rotomolded parts are usually of relatively large size.