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Complexity versus ‘manufacturability’. No compromise so long as you follow the rules

05 January 2010

Part thickness and rib design - both easily applied with common CAD functionality, but two issues that dramatically affect design integrity. In the first of a series of articles to be published in future editions of DPA, Marc Freebrey focuses on design fundamentals and the impact they have on manufacturing and production processes

The rapid development of modern 3D CAD systems have facilitated the evolution of product design and resulted in a switch to more organic forms of increasingly complex geometry. Just think about the change in design from the conventional box shaped vacuum cleaner to the modern Dyson appliance.

Design engineers need to try and maintain a consistent wall thickness throughout the entire model. Any significant change in part thickness can incur major moulding issues, such as internal voids, surface sink marks, unpredictable shrink rates and ultimately, longer cycle times.
If a wall thickness change is necessary, it should be a smooth transition to ensure ease of material flow, preventing critical stress points that may cause part failure during product testing and result in an updated part design or additional tooling costs. Detecting manufacturability issues at the design stage will prevent costly rework and save valuable time during later production stages.

Rib design
When creating rib patterns, it is important to remember that ribs are only there to help strengthen a part and should not be compromised for aesthetical reasons. Design engineers typically follow standard guidelines for rib design. If possible, a combination of thin and thick ribs should be avoided. Some of the most common design guidelines include:
*Rib thickness should be 60% - 80% of the nominal wall thickness.
*Maximum rib height should not exceed 3X the nominal wall thickness.
*Minimum spacing between ribs should be 2X the nominal wall thickness.
*Fillet radii applied to ribs should be no greater than 50% of the rib thickness.
*Extra thick ribs should be cored out.
*Cross-ribbed patterns are preferred (if the design allows) as they offer greater loading permutations and ensure uniform stress distribution

Fillet radii should be applied at the base of a rib pattern to allow better stress distribution. If no fillets are applied, high stress concentration peaks occur and this will often lead to cracking and part failure. Please note, however, that if the fillet radii applied at the rib base are too large, excess material accumulation can occur, leading to voids or sink marks during the moulding cycle. This same principle is also true where a rib pattern meets the edge of the component.

Mould tool design
As far as gating is concerned, it is often preferred to gate on the thickest section of the component to reduce the possibility of sinking due to insufficient material packing. Close attention must also be paid to cooling and ejector pin location; moulds must be designed with adequate cooling in order to take advantage of the faster cooling rates of reinforced compounds. Poor cooling rates lead to rising mould temperatures and longer cycle times. Therefore, if the mould design allows, cooling and heating channels should be located directly in the mould inserts and cores.

Fortunately, CAD systems are beginning to introduce analysis tools to calculate and display the thickness of a CAD model and help identify potential problem zones. Typically, two methods are used. The first is based on the largest sphere that can be placed within the model without intersecting any other face. The second is the more traditional shooting ray method which shoots a ray through the model along the surface normal until it hits a second face.

So, next time you shell out a model or drop in some strengthening ribs, don't forget to consider the impact these decisions have on the production cycle - and don't forget to add draft to the features, but that's a subject for another edition of DPA.

Marc Freebrey is with Vero Software

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