Secondary Operations and Their Effect on Design

How important is it for a designer to understand secondary operations associated with rotational molding?
The answer is very important. I’ll describe the interrelationships between design and a number of secondary operations in this editorial. Let’s first list a few of the many types of post molding operations that are often included during the production of rotationally molded parts. A short list is provided below. I’ll only discuss the first three.

• Deflashing

• Shrink fixtures

• Trimming and machining

• Spin welding

• Welding

• Decorating-decals

• Painting

Deflashing

Virtually all rotationally molded parts undergo some sort of post molding operation requiring excess flash to be removed. This operation is usually achieved manually, resulting in some residual scarring of the part’s surface. Flash is always located along parting lines, which tend to wear as time passes and tools age. The deterioration of parting lines results in increased flash thickness, which can often become a major aesthetic detraction. Workers remove light flash with a scrapper or sharp knife, which leaves a thin line on the surface of the part. However, if flash tends to thicken, more aggressive methods of flash removal must be employed, including grinders and sanders which scar the surface with very noticeable unsightly irregular marks. Designers should therefore try to design parts with minimal parting lines and locate them in areas which are less noticeable.

Shrink Fixtures

Sometimes tolerances, flatness, or warpage must be controlled to within very tight specifications requiring shrink fixtures to be used after parts are molded. The construction and design of shrink fixtures are directly related to the design intent and critical dimensions for each mating part. Designers should discuss their expectations of flatness, warpage limits, and critical hole to hole tolerances with a molder before the design is released for tooling or production. This early collaboration will enable the molder to begin planning the additional production fixtures required to attain the desired design intent.

Trimming and Machining

Virtually all rotationally molded parts undergo some sort of post molding trimming or machining operation. The quality and consistency of these operations are highly dependent on the methods employed. Either operation can be accomplished manually by an operator or robotically with a multi-axis CNC router/robot. The former method will yield very inconsistent features which are highly dependent on the operator, tool and fixture, if it is used. This method is often done for short run production or parts that don’t require high quality. CNC machined parts require programming, high quality fixtures to secure parts during the machining operations, and an investment in the equipment. Robotically trimmed parts are much more consistent, accurate, and cost effective for higher production volumes. A designer should understand how the parts will be machined and which method will be used to achieve his or her design specifications. In addition, designers should understand basic machining limitations including cutter size and accessibility into deep wells or cutter radii within internal corners. Designers must avoid designing parts which must be trimmed but cannot be easily machined because of accessible areas, which prevent the router from access those regions.

I hope you enjoyed reading this short editorial on secondary operations and their effect on design. If you have any questions or comments, please feel free to contact me directly at paloian@idsys.com. I will be more than happy to reply.

from an article at RotoWorld®
https://rotoworldmag.com/secondary-operations-and-their-%e2%80%a8effect-on-design/