ALM technology – manufacturing processes

1.1        Stereolithography (SLA)

Also known as VAT photopolymerization, the part is built one layer at a time from CAD data that guides an Ultra Violet (UV) laser beam directed by a computer guided mirror onto the surface of the UV sensitive liquid epoxy resin. The UV light precisely solidifies the resin it touches. Every pass of the laser fuses another 0.1mm to the preceding layer by constructing the “skin” and then filling in the “core” material. Each layer is applied by submersion of the build platform into the resin.

slaTypical Vat Photopolymerization Process

The paddle sweeps across the surface of the resin with each step downwards, to break the surface tension of the liquid and control layer thickness. The part gradually develops below the surface of the liquid and is kept off the build platform by a support structure. This is made in the same way, prior to building the first layer of the part.

The finished parts are left to drain and then removed from the tank along with any uncured epoxy resin residue. The parts are separated from the build platform and the support structure is detached. An alcohol-based chemical (isopropinol alcohol) is used to clean off the uncured resin liquid and the parts are then fully cured under intensive UV light for about a minute. The build strata are just visible in the finished part and can be removed with abrasive blasting, polishing or painting.

Some considerations:

  • Minimum wall thickness 0.5-0.7mm Horizontal (X&Y) and 0.8mm Vertical (Z – direction of build)
  • Left untreated, SLA resin is not UV stable
  • Reduced strength compared to SLS and moulded parts
  • Limited temperature range (up to 50 °C)
  • Tolerances depend on geometry but typically ±0.15mm up to 100mm, ±0.15% over 100mm

As mentioned, parts made with SLA are only partially cured by the laser to speed up the process. After the part is taken out of the tank it is further cured under UV light. The curing however can continue for another 18 months. If a part is subjected to uneven UV lighting during its use, it can warp because one side cures faster than the other. Therefore always paint SLA parts black or use in a dark or evenly lit environment.

Materials:

Several materials are available for SLA and are generally Acrylic and Epoxy photopolymer resins. Most used are Acurra 55 which is white and rigid and 7870 which is a clear plastic similar to ABS.

Stronger materials are ‘Bluestone’ and ‘Ceramax’ which is a high temperature resistant ceramic filled material.

As materials are evolving and becoming more readily available all the time, please refer to the latest selection suitable for application at the time of design.

Cost:

Similar to FDM & DMLS, parts can be roughly costed to a typical model:

Part Cost = Material + Machine Time + (Finishing Costs) + Preparation Fee

The time it takes to build a part depends upon:

  • Mass of the part
  • Height of the part
  • Amount of support structure

The preparation time depends on:

  • Quality of the supplied CAD file
  • Amount of support structure to put on and remove afterwards

 

1.2        Selective Laser Sintering (SLS)

Also known as polymer powder bed fusion, a CO2 laser fuses fine nylon powder in 0.1mm layers, directed by a computer guided mirror. The build platform progresses downward in layer thickness steps. The delivery chambers alternately rise and provide the roller with a fresh change of powder to spread accurately over the surface of the face build area. Non-sintered powder forms a “cake”, which encapsulates and supports the model as the build progresses.

sls.png

Typical Polymer Powder Bed Fusion Process

The whole process takes place in an inert nitrogen environment to stop the nylon oxidizing when heated by the laser beam. The temperature inside the building chamber is maintained at 170˚C, just below the melting points of the polymer powder, so that as soon as the laser makes contact with the surface particles they are instantly fused by the 12˚C rise in temperature.

Once it is complete, the build platform is raised, pushing the mixture of non-sintered and sintered parts into a clear acrylic container. The block of powder, once cooled, is disposed of in a clean-up booth where the parts are removed. The non-sintered “cake” encapsulates the parts and has to be carefully brushed away so that individual parts can be removed and blasted with a fine abrasive powder.

Some considerations:

  • Minimum wall thickness is typically 0.8mm (X, Y & Z directions)
  • The SLS process is based on a powder so parts do not have a completely smooth surface without further finishing
  • Parts are porous (but can be sealed post build)
  • Best results are obtained when parts are designed along the same guidelines as for injection moulding or casting, i.e. hollow with even sections and wall thicknesses
  • Tolerances depend on geometry but typically ±2mm up to 100mm, ±0.2% over 100mm

Materials:

Materials used for SLS manufacture are mainly Nylons, with or without fillers.

PA12 Polyamide material: used in the SLS system offers the most durable prototype parts from any RP system. PA12 is heat resistant up to 150°C and is the obvious choice for functional prototypes.

PA12-GF (Glass Filled): offers a stiffer and slightly higher heat deflection temperature, when a more rigid structure is required.

PA12-AL (Aluminium Filled): is made up of 50% fine aluminium powder suspended in PA12 Polyamide. This gives a very stiff, durable, high heat deflection component.

Available finishes:

SLS parts can be painted in any required colour.

SLS parts can also be vacuum metalized for chrome like finish as well as normal plating processes.

Cost:

The price of SLS parts is calculated by the percentage of volume they take up in the bed. This means that the amount of material used doesn’t influence the price. If you can pack a lot of parts close together a cost saving can be made. Since the parts are surrounded by the powder in the bed, they are fully supported – there is no support structure to take into account. This makes it possible to stack parts close together to reduce cost.

1.3         Direct Metal Laser Sintering (DMLS)

Also known as metal powder bed fusion, DMLS is based on the same principle as SLS, but here a more powerful laser and a metal powder are used to create the part. The end product is typically a fully dense material if the correct post processing steps have been followed. The built part can then be welded, polished, machined, plated and many other common post production operations.

dmls.jpg

 Image Taken During Fusing in DMLS System

Some considerations:

  • Minimum wall thickness 0.4-0.7mm Horizontal (X&Y) and 0.3mm Vertical (Z – direction of build)
  • Typical tolerance ±0.05mm
  • Maximum build size 250 x 250 x 190mm
  • Typical lead-times 1 – 2 weeks
  • Typical surface finish 4.5Ra before any machining or polishing

DMLS parts are bonded to the platform with a small support structure. Because of the high temperature the part can have very high internal stresses which can warp the part if not connected. Stresses can be relieved post process by heat treatment.

Materials:

A variety of metals are available for DMLS from maraging steel to stainless steel and nickel-bronze to titanium.To ensure the quality of the part, a separate bar of material can be built up next to the part. Once the part is finished this bar can be used to do a pull test to ensure all the layers of material have been built up as intended and provide the required strength.

Cost:

Height of build dictates price – more components per build reduces unit cost.

For simple, uncomplicated parts, machining is often a cheaper option.

Please also see the cost influences describes under section 4.1 which also apply for DMLS parts.

 

1.3.1        DMLS  Design/Manufacture Process

The DMLS design process is an iterative one, to ensure that a part will meet its functional requirement and also be optimised for the DMLS process the part manufacturer should be consulted from the earliest point possible. This will allow for problem areas of the part to be identified and possible design solutions explored before the part is sent out for manufacture. The part manufacturer should be able to give advice relating to the build orientation of the part and support material requirements. Including these factors into the design will not only help to optimise the cost of the part but also reduce the likelihood of parts failing during the manufacturing process.

Design optimisation through collaboration with manufacturer
Initial Design
Manufacture
Heat Treatment
Post Build Surface Finish (e.g. shot peen, tumble)
Inspect
Machine critical to quality features
Apply Surface Coatings

 DMLS Manufacturing Process

1.4        Fused Deposition Modelling (FDM)

FDM works on an additive principle by laying down material in layers; a plastic filament or metal wire is unwound from a coil. An extruder head heats the material to melting temperature and deposits it on the working platform in layers to produce a part.

fdm.png

Typical Material Extrusion Process

The parts that are built this way also need a support structure for overhangs. This is taken off post process by either washing it off with a solvent or heating it until it melts off.

[Typical layer heights, 0.1 – 0.5mm]

Some considerations:

  • Minimum wall thickness 1mm Horizontal (X&Y) and 0.8mm Vertical (Z – direction of build)
  • Typical tolerance 0.05mm

Materials:

Typical materials available for FDM manufacture include Polyoxymethylene (POM), Acrylonitrile Butadiene Styrene (ABS), Poylcarbonate (PC), ULTEM resin, Nylon and Polyphenylsulfone PPSF.

Costs:

Please see the cost influences describes under section 4.1 which also apply for FDM parts.

 

1.5        Material Jetting (Inkjet Material Systems)

The inkjet systems work in a very similar way to regular inkjet printers. Instead of ink, a resin is deposited in small droplets onto a platform by means of a piezoelectric nozzle. Once a layer is deposited, the platform moves down and the next layer is created.This technology is also used in combination with a powder bed. The droplets are then used to fuse together the particles in the powder bed to build up parts at a faster rate.