Plastic Product Design Principles (Part 1)

This is the first in a two part series that focuses on plastic product design fundamentals and the impact they have on the manufacturing and production processes. Part 1 will look at part thickness and rib / boss design - both easily constructed using common CAD functionality, but two issues that can dramatically effect design integrity.

Consistent Wall Thickness
Design engineers need to try and maintain a consistent wall thickness throughout the entire model. Any major change in part thickness can cause major moulding issues such 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 which 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 reworking 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 increase part rigidity 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 are listed below:

• Rib thickness should be 60% - 80% of the nominal wall thickness.
• Maximum rib height should not exceed 3X the nominal wall thickness. To increase product rigidity, it is better to increase the number of ribs rather than the rib height.
• 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.

Bosses
Bosses are a fundamental component in plastic part design as they offer strengthening properties and provide alignment during assembly. Similar to rib design, it is important to consider the wall thickness when designing bosses. The design guidelines listed below will help avoid surface imperfections such as internal voids, surface sink marks and unpredictable shrink rates.

• The boss thickness should be 60% of the nominal wall thickness. If the part thickness is greater than 4mm, the boss thickness can be reduced to 40% of the nominal wall thickness.
• The boss height should not exceed 2.5 x the diameter of the hole in the boss.
• Corner bosses integral to side walls will result in excess material accumulation.

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

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

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