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Rethink design for manufacturability rules with FDM
For years, design engineers have been bound by design-for-manufacturability rules for injection molded plastic parts. These rules, although usually resulting in accurate and repeatable end use parts, have restrained design possibilities.
Stratasys’ Fused Deposition Modeling (FDM) technology has changed that. It allows for production without tooling which opens up doors for complex geometries, organic shapes, hollow interiors, negative draft and more. When designing for FDM, the following design-for-manufacturability rules no longer apply in most applications:
Add shrink factors
When you are designing a mold, you have to take shrink rates into account to achieve an accurate part for your application. With FDM, shrink rates are automatically factored in to the part when the CAD file is analyzed and processed in Insight (software that communicates with FDM systems). Often Paradigm 3D engineers will adjust the default values to fit specific geometries depending on the part’s intended use.
Design your part with consistent wall thicknesses to avoid warp
Thinner areas of an injection molded part will cool faster than thicker areas so stresses build up between them and can cause walls to warp. Since FDM adds very small amounts of molten material in a heated environment rather than all molten material at once, warp is a very uncommon problem
Part XY dimensions must be less than 20 in. by 20 in., maximum part volume cannot exceed 36 cubic inches, [insert size limitation here], etc.
Every injection molding system has size limitations. The biggest FDM build platform in the Fortus 900mc can make single parts as large as 36”x24”x36”, but even larger part designs can be split, built in sections and bonded together with the same material to achieve consistent tolerances and strength.
Avoid undercuts and overhangs
Undercuts for features such as o-ring grooves and overhangs can’t easily be achieved through injection molding. If the application requires these features, an injection molded part would need secondary operations, increasing costs and production time. FDM, being an additive layering process, allows for overhangs and undercuts to be built into the design with support materials. Supports create a base to build the actual part material on, then can be removed manually or dissolved in a water-based solution after the build process
Ribs, bosses, and gussets should be 50 to 80 percent of the base wall thickness to avoid sink marks
Sink marks are localized shrinkage in areas of an injection molded plastic part that not only create an uneven surface finish, but also hinder accuracy. Sink can be caused by a number of factors, including design, molding conditions or ventilation. It rarely appears on parts produced using FDM technology because of the ability to add support structure for varying wall thicknesses. In fact, it is unnecessary to reduce wall thickness of a boss, rib or gusset in FDM parts at all.
Provide sufficient draft and fillets
Incorporating draft and reinforcing fillets into a part design aids mold ejection and prevents the part surface from getting damaged. When designing for FDM, you don’t have to sacrifice design to fit the mold—you have complete design freedom. However, fillets can be applied to increase overall strength if the part application involves high-stress concentrations.
To fully leverage the benefits of 3D printing, you must embrace new ways of thinking about design. Challenge yourself to break the design for manufacturability rules and find ways to maximize strength, combine multiple components in one design, lightweight structures, etc.—then decide how to build it.
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