Part Two: Designing Products for Rotational Molding

Minimum Wall Separation & Bridging Defects

Minimum Wall Separation & Bridging Defects

As described previously, the first stage of the rotational molding process involves the powder being able to move and flow, in an unimpeded manner, across the inside surface of the tool cavity. A simple hollow shape is unlikely to provide any difficulties in this respect. However, in a more complex shaped cavity, there may be restrictions to powder flow in some sections.

Powder flow issues can become particularly acute for molds that have parallel walls close to each other; this is the situation in double-wall parts. Parallel walls which are too close may cause the powder to bridge over the space between them and the cavity will be prevented from filling completely (Fig 16). After melting and cooling, a solid bridge of material will be formed and, in most cases, a defect in the continuity of the wall will be apparent from the outside of the part. One part of the wall may have a reduced wall section or may even be completely missing.

In order to reduce the possibility of bridging, a separation between parallel walls (outside part wall surface to outside part wall surface) of five times the nominal wall thickness of the part is recommended (Fig 17). This will allow the molding of good quality parts with little or no difficulty.

Wall separations of as little as three times the nominal wall thickness have been molded successfully, with only occasional bridging defects. However, parallel walls which are this close to each other require considerable care and attention and perfect parts may not be produced in a consistent manner.

A poor quality plastic powder may also promote bridging problems, especially powders with poor initial Dry Flow (Fig 18). The rotomolding industry has an agreed test protocol by which powder quality of batches of material can be determined. It should be noted that the flow characteristics of the powder inside a heated mold will deteriorate as the powder heats up, so an initial (ie pre-molding) dry flow figure is of significant importance.

Another powder quality parameter that can be easily measured is the Bulk Density of the powder. Bulk Density is likely to be roughly one third of the molded density of the material. This means that a given mass of powder will take up approximately three times as much volume within the mold, at the start of the process, as the material does when fully molded. This raises the possibility, with some sizes and shapes of part, that the mold cavity volume will be insufficient to contain the required charge of powder. In a relatively small mold cavity volume, the powder flow around the cavity may be retarded to the extent that bridging problems and excessive wall thickness variation may occur.

Mold cavities that are otherwise non-problematic, but have constraints to flow in some sections, may also be prone to bridging problems. If such issues are unresolvable at the design stage, there are some expedients that the molder can introduce into the process that may help to avoid problems. These mainly involve making changes in mold mounting, rotation and processing parameters.