Introduction

For a startup manufacturer getting that first product right is crucial for your reputation and future sales. First impressions really do count when you are building up your company and brand equity.

Time is another critical factor as you strive to get a product to market and begin to generate revenue. Yet it is a mistake to skip a thorough prototyping and testing programme because if you discover an error during manufacturer, the cost of putting it right is far higher in money, time and reputation. It is far better to make any adjustments in design, material, size, shape, colour, manufacturability and strength following testing and analysis. To balance this tightrope between testing
time and cost, it pays to understand all of the rapid prototyping technologies that are available to you.

There are numerous ways prototypes can be made. Some prototyping processes use traditional manufacturing methods. Other technologies have emerged and have been improved upon over a relatively short period of time. As prototyping processes continue to evolve, the product designer is constantly trying to determine what process or technology is best for their unique application.

This technical brief explores the advantages and shortcomings of the major prototyping processes available to designers. It also describes the processes and discusses material properties of parts produced by each. In addition, a helpful decision tree highlights key questions designers must consider. Ultimately, the goal of this paper is to help you select the best prototyping process for your product development process.

3D Printing – SL Stereolithography

SL is an additive fabrication process that builds parts in a pool of UV-curable photopolymer resin using a computer-controlled laser. The laser is used to trace out and cure a cross-section of the part design on the surface of the liquid resin. The solidified layer is then lowered just below the surface of the liquid resin and the process is repeated. Each newly cured layer adheres to the layer below it. This process continues until the part is completed. SL was the first ‘rapid prototyping’ technology.

Pros

For concept models or patterns to be used as masters for other prototyping methods, SL can produce parts with complex geometries and excellent surface finishes
when compared to other additive processes. If you need micro resolution features then there are materials, such as MicroFineTM Green and MicroFineTM Grey,
that can achieve features down to 0.07mm (in X-Y direction) using SL. The cost is very competitive and the technology is available from several sources.

Cons

Prototype parts are not as strong as those made from engineering-grade resins, so the parts made using SL are normally unsuitable for functional testing. Also,
since the resin is UV-curable, exposure to sunlight continues to cure the resin and parts can become brittle over time.

3D Printing – Selective Laser Sintering (SLS)

The SLS process uses a laser to build parts by sintering (fusing) powdered material layer by layer from the bottom up. SLS parts can be accurate and more durable than SL parts, but the finish is relatively poor with a grainy or sandy feel. Though SLS parts are fairly strong, there is reduced strength between the fused particles, so the parts will tend to be weaker than machined or moulded parts made from the same resin. Nylon-based materials are currently the primary resins available.

Pros

SLS parts tend to be more accurate and durable than SL parts. The process can make durable parts with complex geometries.

Cons

The parts have a grainy or sandy texture and are typically not suitable for functional testing due to their reduced mechanical properties.

3D Printing – Multi Jet Fusion (MJF)

Multi Jet Fusion uses an inkjet array to selectively apply fusing and detailing agents across a bed of nylon powder. This is then fused by heating elements into a solid layer. After each layer, powder is distributed on top of the bed and the process repeats until the part is complete.

Pros

MJF offers engineering grade materials with great overall properties. Additionally, MJF offers an enhanced surface finish, fine features, consistent mechanical properties, and fast build times.

Cons

While the surface quality is good, it is rougher than that produced by photopolymer-based technologies (Stereolithography). The raw parts are grey, which can be dyed but this is an extra process.

Direct Metal Laser Sintering (DMLS)

Direct metal laser sintering is an additive manufacturing technology that produces metal prototypes and production-quality parts. DMLS uses a laser system
that draws onto a surface of atomised metal powder. Where it draws, it welds the powder into a solid. After each layer, a blade adds a fresh layer of powder
and repeats the process. DMLS can use most alloys, allowing prototypes to be a full-strength, functional part made out of the same material as production
components. It also has the potential, if designed with manufacturability in mind, to transition into metal injection moulding for when you need to increase
production.

Pros

DMLS produce strong (typically, 97 percent dense) prototypes from a variety of metals that can be used for functional testing. Since the components are built layer by layer, it is possible to design internal features and passages that could not be cast or otherwise machined.

Cons

If producing more than a few DMLS parts, costs can rise. Due to the powdered metal origin of the direct metal process, the surface finish of these parts are slightly rough.

3D Printing – Fused Deposition Modeling (FDM)

The FDM process builds parts from the bottom up through the use of a computer-controlled print head. The feedstock for the process is a filament of extruded resin, which the machine selectively re-melts and deposits on the prior layer for each cross section of the desired part. FDM primarily produces parts in ABS or PC, so they tend to be stronger than parts from other additive processes. However, the parts are sometimes porous and have a pronounced stair-stepping or rippling texture on the outside finish, especially at layer junctions. It may also be difficult to achieve tight tolerances with the process.

Pros

FDM parts are relatively strong and can be good for some functional testing. The process can make parts with complex geometries.

Cons

The parts have a poor surface finish, with a pronounced rippled effect. It is also a slower additive process than SL or SLS for build time.

3D Printing – PJET PolyJet

PJET uses inkjet heads to jet a UV-curable material in very thin layers at high resolution. The materials are jetted in ultra-thin layers onto a build tray, layer by layer, until the part is completed. Each photopolymer layer is cured by UV light immediately after it is jetted. The gel-like support material, which is specially designed to support complicated geometries, is easily removed by hand and water jetting.

Pros

This process yields a good surface finish; one of the best of the additive processes. It is a good additive choice for complex parts with undercuts. The process can make parts with complex geometries.

Cons

PJET parts have poor strength (comparable to SL). While PJET can make parts with complex geometries, it gives no insight into the eventual manufacturability of the design.

3D Printing – Three Dimensional Printing

In 3DP an inkjet print head moves across a bed of powder, selectively depositing a liquid binding material, and the process is repeated until the complete part has been
formed. After completion, the unbound powder is removed leaving the finished object.

Pros

3DP offers one of the fastest build times of any additive process, and is also among the least expensive options for prototype quantities. Coloured models can communicate more information and have aesthetic appeal. This plaster material is non-toxic, inexpensive and readily available. The process can make parts with complex geometries.

Cons

Parts are rough and weak, and there are very few material options. While 3DP can make parts with complex geometries, it gives no insight into the eventual manufacturability of the design.

IM – Injection Moulding

Rapid injection moulding works by injecting thermoplastic resins into a mould, just as in production injection moulding. What makes the process ‘rapid’ is the technology used to produce the mould, which is often made from aluminium instead of the traditional steel used in production moulds. Moulded parts are strong and can have excellent finishes. It is also the industry standard production process for plastic parts, so there are inherent advantages to prototyping in the same process if the situation allows. Almost any engineering-grade resin can be used, so the designer is not constrained by the material limitations of the prototyping process. Additionally, metal injection moulding (MIM) and liquid silicone rubber (LSR) moulding offer rapidly moulded parts in metals like stainless steel and LSR, respectfully. There is an initial tooling cost associated with rapid injection moulding that does not occur with any of the additive processes or with CNC machining. So, in most cases, it makes sense to do one or two rounds of rapid prototypes (subtractive or additive) to check fit and function before moving to injection moulding.

Pros

Moulded parts are made from an array of engineering grade materials, have excellent surface finish and are an excellent predictor of manufacturability during the
production phase.

Cons

Front-end costs can be higher due to tooling costs.

CNC Machining – Computer Numerically Controlled Machining

A solid block of plastic or metal is clamped into a CNC mill and cut into a finished part through a subtractive process. This method produces superior strength and surface finish to any additive process. It also has the complete, homogenous properties of the plastic because it is made from solid blocks of extruded or compression moulded thermoplastic resin, as opposed to most additive processes, which use plastic-like materials and are built in layers. The range of material choices allows parts to be made with the desired material properties, such as: tensile strength, impact resistance, heat deflection temperatures, chemical resistance and biocompatibility. Good tolerances yield parts suitable for fit and functional testing. Prototypes can be delivered in days just like additive processes. Because the process involves removing material instead of adding it, milling undercuts can sometimes be difficult. Machining also tends to be somewhat more expensive than the additive processes.

Pros

Machined parts have a good surface finish and they are very strong because they use real engineering-grade thermoplastics and metals.

Cons

There are some geometry limitations associated with CNC machining, and it is much more expensive to do this in-house than the additive processes due to the cost of the programmers and machinists needed to create CNC toolpaths and fixturing for the parts.

VC – Vacuum Casting

The process starts with making a mould by encapsulating a master model in two-part liquid silicone rubber. A vacuum is then applied to remove any trapped air, then it is cured in an oven. Master models can come from a number of 3D printing technologies including stereolithography. Vacuum casting allows the production of small batches of high quality mouldings in a range of polyurethane resins that replicate the performance of engineering plastics without the high costs of hard tooling associated with injection moulding.

Pros

Vacuum casting can be used for small batches of high quality prototypes or low volume end use parts. A wide range of resins are available and master models can come from a range of sources, the most common being stereolithography models. Good dimensional stability.

Cons

Rapid wear of the mould, only used for a few dozen parts.

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