Which materials to use with which prototyping technologies

Which materials to use with which prototyping technologies?

A key question when designing any production part is what material to use, then the follow-on question is what material to choose for prototyping that part? Prototypes should represent production items as closely as possible, which implies that the materials are, ideally, the same. However, the choice of manufacturing technologies for the production and prototype parts can complicate matters. Manufacturing technologies and materials for production parts are interlinked and if the prototyping technology is different, then the material is likely to be different too.

Most of the enquiries we receive at Prototype Projects are from designers who have already selected a prototyping technology and material. But sometimes designers want to discuss alternative options and, from time to time, we talk to relatively inexperienced designers who are somewhat overwhelmed by the choices available. Today there are so many different prototyping technologies – from CNC machining to SLA, SLS, FDM, DLP, vacuum casting and more – that it is more important than ever to understand what the optimum technology and material are for prototyping.

Material choices for CNC machined prototype parts

CNC (computer numerical controlled) machining is incredibly versatile and can be used with almost any metal or plastic. If customers wish to use our Express service, then the choice of material is limited to what we hold in stock, otherwise we can procure a vast range of commonly available metals and plastics. Typically, customers ask us to CNC machine prototype parts in aluminium, acetal, clear acrylic, mild steel, carbon steel, stainless steel and brass. Machining of exotic alloys such as Inconel, Nimonic and Hastelloy alloys requires specialist tooling, so we entrust that to subcontractors, managing the work on behalf of our customers.

A major advantage of CNC machining is that the finished part is produced from an isotropic solid material, which can be vital for functional parts. Depending on the geometry, CNC machining is excellent for prototyping parts that will be injection moulded from plastics, die cast in zinc alloy or mass-produced in machining centres. For some customers, such as special-purpose machine builders, the parts we CNC machine are the final parts, as there is no pre-production prototyping phase.

Material choices for 3D printed functional prototype parts

3D printing is often used for prototyping parts that will be injection moulded once they are in production. Some materials are available in various forms of supply for both 3D printing and injection moulding. For example, ABS filaments can be used for FDM (fused deposition modelling) 3D printing and granular ABS for injection moulding. Although this is nominally the same material, care must be taken because the mechanical properties for the prototype part and moulded production part will not be identical. Variations in material grade have a small effect but the main difference stems from the way the material is processed. In injection moulding, the material is plasticised and flows into the mould as a liquid, whereas 3D printing with FDM is more akin to a continuous welding process, with molten material being fused to material that has already solidified. Furthermore, the FDM process usually leaves small voids where the molten material does not flow perfectly into the surface beneath. As a result of these two points, the moulded material will be more isotropic, though not perfectly so due to the flow inside the mould influencing the molecular alignment. In contrast, FDM prototype parts tend to be stronger in the X and Y axes than the Z axis (in terms of build orientation). Care must therefore be taken when selecting the build orientation to ensure the best mechanical properties are where they are needed most – though sometimes compromises must be made. Post-processing can seal porous surfaces if watertightness is important, but this still leaves internal voids and anisotropy.

Similarly, SLS (selective laser sintering), which builds the part in layers by fusing plastic powder, also tends to produce parts that are anisotropic and have a degree of porosity.

In SLA (stereolithography), layers of photopolymer liquid resin are cured in such a way that they bond to the partially-cured layer beneath, resulting in a material that is effectively isotropic. Moreover, the use of a liquid avoids the formation of voids. The same is true for both conventional SLA technology and the newer, faster DLP process.

3D printing materials with similar properties

While this isotropy and lack of voids sounds ideal for functional parts, bear in mind that the cured resins do not have the exact same mechanical properties as plastics that will be used for injection moulding plastic parts, though they are close. For example, we process three materials in our SLA machines: Accura Xtreme is similar to ABS, Accura ClearVue and similar to PC, and Accura 25 is similar to PP. Likewise, a range of materials is offered for 3D printing using the DLP printer. For both types of machine, the prototype part’s mechanical properties will usually be close enough to those of the production part, but designers need to be aware of the differences when prototyping functional parts.

If you need parts for patient contact or drug delivery devices, we have an SLA machine installed permanently in our medical room, and our DLP machine can be moved into the medical room when necessary. The SLA machine builds parts from Accura ClearVue resin, which has been tested to pass USP Class VI bio-compatibility uses, while the DLP machine builds parts in a choice of biocompatible resins, or another that can be sterilised in an autoclave.

Materials for visual models

For parts that are to be used in visual models only, the differences in material properties do not matter, which gives the broadest choice of prototyping technologies. Of course, designers should not overlook CNC machining for non-functional parts. If the geometry lends itself to CNC machining, this can be the better process for producing accurate, non-functional prototype parts in plastics and metals.

Multiple and hybrid 3D printing materials

Polyjet is a 3D printing process that build parts, layer by layer, by spraying small droplets of photopolymer material and curing these with ultraviolet light. Because Polyjet printers have multiple spray heads, they are capable of printing two materials concurrently. Mixing materials can effectively create ‘intermediate’ grades. In addition, the machine can build prototype parts that mimic overmoulded production parts.

Materials and processes

To help designers select the optimum material and process for their application, we have produced a table that shows which materials are compatible with different processing technologies.

Visit the Prototyping Materials page to see this detailed table.

Can prototyping technologies be mixed?

Yes, different prototyping technologies can be combined – within reasons. Prototype Projects is fortunate to have invested heavily in its facilities and equipment, so we have, in-house, a wide variety of prototyping technologies: CNC machining, SLS, SLA, DLP, Polyjet, FDM, vacuum casting and laser cutting. If customers require other prototyping processes – ranging from photochemical etching and sheet metal work, through to 3D printing of metals, investment casting and coil springs – we can utilise our trusted network of subcontractors and manage the project on behalf of the customer.

With all these technologies available under one roof, we often combine them as necessary. For example, we can CNC machine components in metal, then use these for insert moulding in polyurethane, with the mould produced by casting silicone rubber around a 3D printed master. Another popular option is to use the vacuum casting process to overmould with polyurethane, thereby adding elastomeric grips or sealing features to parts produced by 3D printing or CNC machining.

As well as the prototyping technologies mentioned above, we also offer finishing and assembly operations, whether parts are produced by CNC machining or 3D printing, and in plastic or metal.

Talk to us

Whatever your need for prototype parts, talk to us on 01763 249760 if you would like to explore your options for prototyping materials and technologies.