This article is a SLS 3D printing design guide. It includes technical design specifications, materials, limitations and an introduction into the post-processing options available.
SLS is an ideal solution for producing functional products with complex geometries. The technology has very few design constraints when compared to other 3D printing technologies and is also suitable for of batch manufacturing.
This SLS 3D printing design guide will discuss the advantages of using SLS to 3D print parts, present the most common SLS materials and establish a clear set of design rules for designers to follow when printing with SLS.
Selective laser sintering (SLS) is a powder-based fusion technology that uses a laser beam to locally sinter polymer powder to build parts layer by layer.
A bin of the powder material is heated to an elevated temperature. A recoating blade deposits a very thin layer of the powdered material (typically 0.1 mm) onto a build platform. After deposition of a layer, a laser beam starts to scan the surface. The laser beam selectively sinters the powder and solidifies a cross-section of the part. When the entire cross-section is scanned, the building platform moves down one layer thickness in height. Unsintered powder remains in place to support subsequent layers eliminating the need for support structures. The recoating blade deposits a new layer of powder on top of the scanned layer and the laser beam starts to sinter the successive cross-section of the part onto the previously solidified cross-sections. This process is repeated until all parts are fully manufactured.
The result is a container filled with powder and consolidated products. Since multiple products can be produced simultaneously the process can be used for batch manufacturing. The placement and orientation of parts is optimized to maximize part occupancy in the powder box during each print.
When the printing process is complete and the powder container and product have cooled down, the powder container is unpacked. The solid products are parted from the unsintered powder and cleaned with compressed air and a blasting medium. The remaining (unsintered) powder is collected and reused. The parts are then ready to use or are post processed to improve mechanical properties or appearance.
Like all manufacturing techniques there are several design recommendations that exist to improve the quality, surface finish and functionality of SLS parts. One of the most advantageous characteristics of designing and printing parts using SLS is that there is no need for support structures. The unsintered powder surrounding the part removes the need for support allowing highly complex and intricate designs to be printed.
General guidelines for designing SLS parts are:
Wall thickness – The minimum wall thickness to ensure a successful 3D print varies between 0.7 mm (for PA12) up to 2.0 mm (for carbon filled polyamide).
Hole size – All holes should be larger than 1.5 mm diameter.
Escape holes – To save weight (and sometimes costs) SLS parts are printed hollow. To remove unsintered powder after production escape holes must be included. Escape holes must be a minimum of 3.5 mm diameter.
Feature size (pins, protruding features etc.) – A minimum size of 0.8 mm is recommended.
Embossed and engraved details – To ensure small details are visible the following rules apply:
Text – To ensure readability of text the following rules apply:
Tolerances – Typical tolerances for SLS parts are ± 0.3 mm or ± 0.05 mm/mm, whichever is greater.
Joints – LS materials can be bonded with a variety of adhesives. Lap joints, with a 0.010 inch (0.3 mm) bond line clearance, are the preferred joint method. The recommended joint overlap is 3-5 times the wall thickness. Joint performance can be adversely affected by temperature, bonding and mixing techniques, joint geometry and other factors. Stratasys Direct Manufacturing strongly recommends that a vigorous prototyping program be used to validate any LS production designs that include joints.
We continuously evaluates potential LS adhesives. Good results at room temperature have been obtained with the following adhesives:
Axles – Nylon as a natural bearing material will provide a smooth low friction mechanism for low load, low velocity applications. For running axles a bearing surface clearance of 0.3 mm is recommended. It is important to remember that powder needs to be removed after the printing process to ensure a smooth running shaft. Include escape holes (minimum 3.5 mm diameter) wherever possible. A 2 mm between the running shaft axle and clearance shaft hole is recommended to also allow for powder removal.
Integrated hinges – Integrated hinges can work very well with SLS nylon when designed properly. A trapezoid shaped pocket that accepts a semi-spherical ball allows for low friction and good stability. 0.2 mm of clearance between the sphere and the pocket is recommended with 0.3 mm clearance between all other gaps.
Tanks – SLS nylon offers good chemical resistance and is often implemented in custom tank design. For extra watertightness or when aggressive fluids such as fuel or solvents are to be used the tank can be coated or lined. A wall thickness of greater than 1 mm is recommended. Excess powder must be able to be removed from inside the tank.
Threads – The rough surface produced by SLS printing results in increased friction and can cause some issues when connecting threaded SLS parts together. It is possible to drill and tap SLS nylon. An ideal solution is only using SLS nylon for one of the threaded connections (either the hole or the bolt, not both).
Living hinges – SLS is one of the only 3D printing methods that can produce functional living hinges. For SLS hinges, anneal the hinge by heating it up (dipping in boiling water will often suffice) and then flexing the hinge back and forth. It is recommended living hinges are 0.3 – 0.8 mm thick and a minimum of 5 mm in length.
Due to the high temperatures experienced by SLS components during the printing stage some shrinkage and warping can occur. SLS parts are typically cooled slowly to limit the impact of warping and shrinkage.
SLS offers a number of different material with the majority being polyamide based. Polyamides are synthetic thermoplastic polymers, more commonly known as nylons. The table below summaries the most common materials that are printed via SLS.
PA12 – Mechanical properties comparable to injection molded polyamide, good dimensional stability, good wear resistance and high chemical resistance
PEBA (TPA) – Rubber-like, strong yet flexible material.
Alumide (Aluminium filled polyamide) – High stiffness and good post-processing abilities.
Carbon filled polyamide – High stiffness and strength.
Glass filled polyamide – High stiffness and wear resistance.
PA11 – High impact resistance and elongation at break, environmentally friendly.
PEEK – Excellent mechanical properties, high temperature resistance, potential for biocompatibility and sterilizability.
The size a part is able to be printed at is limited by the size of the nylon container used in the SLS machines. Currently the average build volume is around 300 mm x 300 mm x 300 mm with the bigger machines offering a build volume of 700 mm x 380 mm x 580 mm.
Since every SLS printed part consists of hundreds (or even thousands!) of layers, small variations between products can occur (dimensions, surface quality). In addition, due to the uniqueness of the products most post processing steps are done manually. This also means that minor variations can occur (e.g. small colour or coating variations).
While SLS produces a consistent surface finish the surface appearance is a satin-like matte finish that is slightly grainy to the touch. If a shiny and smooth finish is desired post processing is recommended.
For detailed information of each of the post processing finishes offered for SLS parts refer to this article. The most common SLS post processing methods are:
The addition of coatings can also improve the functionality of SLS parts resulting in:
|Wall thickness||0.7 mm – 2.0 mm depending on material|
|Hole size||Greater than 1.5 mm diameter.|
|Escape holes||A minimum of 3.5 mm diameter|
|Text||Minimum font height of 2 mm|
|Feature size||A minimum size of 0.8 mm|
|Embossed and engraved details||Minimum depth of engraving 1 mm & Minimum height of embossing 1 mm|
|Tolerances||± 0.3 mm or ± 0.05 mm/mm, whichever is greater.|