3D Printing SLS Design Guide

Selective Laser Sintering Design Guidelines

SLS Design Guide

Selective Laser Sintering Design Guidelines

This SLS Design Guide is to guide those interested in the layered Additive Manufacturing process known as Selective Laser Sintering (SLS) which allows for the direct digital manufacture of complex parts that may be cost-prohibitive and/or impossible to produce through traditional manufacturing processes. eg: internal features, undercuts, and negative drafts.

PRODUCTION PARTS

Meeting all design requirements including physical properties, dimensional tolerances, appearance and cost is what differentiates a prototype from a production part. Typically, prototype parts involve compromises in one or more areas and almost always involve compromises in physical properties.

SLS produces parts from impact-resistant engineering plastic, great for low- to mid-volume end-use parts, enclosures, snap-fit parts, automotive moldings and thin-walled ducting.While the SLS process has been around for years, recent advancements in materials and process control have resulted in parts that are suitable for many production applications.

BENEFITS & LIMITATIONS OF SLS PRODUCTION

SLS Design Guide

Figure 1

PRO – Complex shapes can be produced without tooling in a very short period of time.

CON – As production volume increases, other production methods are usually more cost effective.

Since SLS is an additive manufacturing process that involves no tooling, the benefits increase as the geometric complexity increases. This is quite the opposite to traditional manufacturing processes.

Maximum benefit is achieved if the manufacturing process is considered at the design stage. For example, the duct assembly in Figure 1 was originally manufactured in 15 parts reduced to 1, had 10 assembly checks reduced to 1 resulting in a reduced risk of human error, and lighter-weight part.

SLS production parts material options include various nylon materials with optional fillers such as glass, carbon and aluminum can be used to enhance the physical properties, careful consideration must be given to the material properties.

SLS PRODUCTION DESIGN CONSIDERATIONS

  • Undercuts, negative draft and interior features are not an issue
  • Should have a minimum wall thickness of 0.040 inches (1.0 mm).
  • Holes in designs will be smaller than specified due to hoop shrinkage.
  • Wall thickness of 0.120 inches (3.0 mm) or less will minimize hoop shrinkage.
  • SLS adds a natural radius of 0.015 inch (0.4 mm), it is not necessary to add a break edge radius, unless additional stress relief is required.
  • Radii of less than 0.015 inch (0.4 mm) will be constructed with the natural 0.015 inch (0.4 mm) radii.
  • A 0.015 inch (0.4 mm) radius fillet is recommended to be designed on all interior corners for stress relief.

MATERIAL CONSIDERATIONS

SLS Design Guide

Figure 2

SLS production materials are typically based on nylon powder, with optional fillers such as glass, carbon or aluminum. Evolv3D provides a wide variety of SLS materials and the information below is helpful in understanding some of the things to look for when designing SLS parts.

COLORS

SLS materials are available in white, grey and black, although not all materials are available in each color.

BOSSES

Bosses are used for attaching fasteners or accepting threaded inserts. The boss diameter should be 2.0 and 3.0 times the diameter of the insert to provide sufficient strength and to minimize hoop shrink. The height of the insert should not exceed the height of the boss, as hoop shrinkage may occur below the level of the boss. Ribs and gussets can be added to the boss for increased strength.

FEATHERED EDGES

Feathered or knife-edges should taper to no less than 0.030 inches (0.8 mm).

DRAFT

Not an issue for SLS parts.

INSERTS

Threaded inserts with adhesives are recommended.

INTERIOR FEATURES

Interior features, such as stiffeners, baffles, ribs and struts can be designed and constructed as one integral part.

MINIMUM FEATURE SIZE

SLS Design Guide

Figure 3

The minimum suggested feature size is 0.030 inches (0.8 mm).

RADIUS

The SLS process adds a natural radius of 0.015 inches (0.4 mm) to all sharp corner radii.

RIBS, GUSSETS, FILLETS & BULKHEADS

No special design requirements

SURFACE FINISH/TEXTURE

Average surface finish is 125-250 RMS finish, depending on the SLS material selected. Surfaces can be sanded smoother. SLS materials can also accept most coatings, textures, printing or other special finishes.

THICK WALLS

SLS plastic materials shrink as they solidify. Thick walls and large blocks of material will cause excess heat and shrinkage, resulting in geometric deformations. The process is much more tolerant of thick walls than injection molding, but these should still be avoided where unnecessary.

WALL THICKNESS

Wall thickness should be between 0.040 and 0.120 inches (1.0 to 3.0 mm).

DIMENSIONAL ACCURACY

Typical tolerances are ± .015 inches (0.4 mm) or ± .003 inch/inch (0.1 mm/mm), whichever is greater. Tighter tolerances may be offered on a case-by-case basis.

STEEL THREAD INSERTS (STI)

SELECTION

Evolv3D recommends the use of miniature stainless steel non-locking inserts (Heli-Coil, Kato, Keensert) and an epoxy adhesive for SLS production parts. The recommended adhesive is 3M Scotchweld Gray #2216.

INSTALLATION

Follow the specified installation instructions for drilling and tapping the part. Prior to threading the insert, apply a thin coat of epoxy to the threaded part and install the insert per the manufacturers instructions.

CONCLUSION

SLS Design Guide

Figure 4

SLS production parts are ideal for addressing a number of applications:

  • The geometric complexity of the part makes it difficult to produce through traditional manufacturing processes.
  • The anticipated production volume does not justify the time and expense of tooling.
  • Time is critical and initial production parts must be available before traditional tooling can be completed.
  • Weight must be reduced by eliminating fasteners and mounting components.
  • Physical space is limited and conformal designs (such as fuel tanks and cooling ducts) can be used to maximize the available space.

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