What do we mean by additive manufacturing?

You may have seen our recent article about additive prototyping. In this article, we will explain what we mean by additive manufacturing.

Terminology

When 3D printing technologies became capable of making production-quality end-use parts, the term additive manufacturing became popular. The inference is that additively manufactured parts possess the mechanical properties and resistance to environmental conditions that are often lacking in additively prototyped parts.

Within the mainstream media there tends to be no differentiation between additive prototyping and additive manufacturing. Instead, journalists resort to the catch-all term 3D printing because non-technical audiences can visualise ‘printing’ parts in three dimensions.

Additive versus subtractive

When we talk about additive technologies it means parts are built by adding material. For example, in SLS, a high-energy point of laser light fuses a fine powder, with the laser tracing a path to build a complete layer (or ‘slice’) of the part. Similarly, in SLA, a point of laser light traces a path, though in this case, the light cures a photopolymer resin, transforming it from liquid to solid. All such 3D printing processes build parts one layer at a time.

Contrast this with subtractive (or reductive) technologies such as CNC milling and turning. These manufacturing processes start with a solid blank, then remove material to leave the required geometry.

Subtractive production technologies are often criticised for being wasteful because material is removed and discarded – indeed, in some cases the volume of material removed from the blank far exceeds the volume of the finished part. However, while the cost of the material should always be borne in mind, it is often less significant than other costs such as for the time involved in file preparation, processing and secondary finishing operations. Furthermore, subtractive production processes sometimes necessitate the production of bespoke fixtures or tooling before the part can be made.

What is manufacturing?

Earlier we mentioned that ‘manufacturing’ often implies something about the properties of the finished part that make it suitable for end-use applications. Manufacturing can also refer to mass production but, if only small quantities are required, the term is equally applicable to low-volume production. Sometimes only one part is required, perhaps for a special-purpose machine, research instrument or test rig. This part, just as with parts produced in volumes, also needs to be manufactured.

If you have already read our article about additive prototyping, you might feel that there is some overlap here. That would be a fair observation, and it is true that some one-offs or low-volume parts could be said to be either additively prototyped or additively manufactured. Nevertheless, the important point is that the part is fit for purpose; semantics is secondary!

Which parts benefit from additive manufacturing?

There are so many potential applications for additive manufacturing that it is not possible to list them all. In truth, the scope is limited only by the designer’s imagination, so get in touch with our additive manufacturing experts if you want to discuss whether your part is suitable for additive manufacturing. Meanwhile, here are a few examples:

• jigs, fixtures, nests, gripper jaws and guides for assembly or test systems;
• production aids such as trays for transporting delicate parts or for organising kits of parts;
• sensor clamps;
• enclosures for electronics or electromechanical/electro-pneumatic systems;
• replacements for obsolete parts that are no longer available;
• components for sports equipment, including parts for motorsport applications;
• prosthetics or other bespoke parts that need to fit an individual’s anatomy perfectly;
• mass-customisation of consumer products;
• fluidics or microfluidic systems; and
• seals and gaskets.

Typically, additive manufacturing is beneficial if the predicted production volumes do not justify the investment in production tooling such as injection mould tools. Alternatively, additive manufacturing can be a cost-effective and time-efficient stopgap if the timescales for getting a product to market are insufficient for toolmaking.

Additive manufacturing is often the best approach if a part has complex geometry, internal voids or other features that are difficult or expensive to CNC machine.

Additive manufacturing technologies

Of the five 3D printing technologies that we have in-house, some are more suitable than others for additive manufacturing. Most importantly, the customer needs to understand what material properties are required of the finished part, and what the operating conditions will be. Only then can the material be selected, enabling the material compatibility to be cross-checked with the 3D printing technologies.

We have a wealth of detailed information on our website about 3D printing technologies but the following summary addresses each one’s suitability for additive manufacturing.

SLS (Selective Laser Sintering)

With SLS, a laser beam fuses a thin layer on the top of a bed of fine powder. The bed then descends, and fresh powder is spread across the top so the next layer can be fused. The material we use is PA2200 Nylon, so the resultant parts have good accuracy, surface finish, strength and durability. SLS is popular for additive manufacturing provided the parts are sufficiently strong and durable for the application. Parts perform well when there is sliding contact, so SLS parts can be good for mechanisms.

SLA (stereolithography)

This is a high-quality 3D printing technology that builds parts by using a spot of laser light to trace a path and cure a photopolymer resin. We offer a choice of resins for SLA, so parts can benefit from a range of material properties. The surface finish is good but, if required, hand polishing and lacquering or painting can improve the finish further.

PµSL (Projection Micro Stereolithography)

For small parts with fine detail, tight tolerances and exceptionally smooth surfaces, PµSL is unbeatable. The technology is excellent for manufacturing small parts with complex geometries and internal voids. Customers often find that we can use PµSL to additively manufacture parts that could not be made any other way, including micro injection moulding and CNC machining.

PolyJet

In common with SLA, DLP and PµSL, PolyJet uses photopolymer resins. However, that is where the similarity ends. Unlike those other technologies, PolyJet applies the resin by means of printheads rather like in an inkjet printer. Moreover, multiple materials can be printed, so parts can have different material properties in different areas, or materials can be combined to create intermediate grades. PolyJet materials can simulate relatively stiff and strong materials through to soft elastomers. PolyJet can be a very versatile and cost-effective alternative to CNC machining, other 3D printing technologies or injection moulding.

DLP (Digital Light Projection)

DLP technology is similar to SLA but quicker because each layer is cured in a single, short exposure. This is achieved by using a digital mask to expose selected areas on the surface of a liquid photopolymer resin. We offer a wider range of materials for DLP than for SLA, which can make DLP more suitable than SLA for additive manufacturing.

Materials for additive manufacturing

What material properties do you need for your additively manufactured parts? With a choice of materials for most of our 3D printing technologies, we can offer everything from stiff, strong materials through to soft elastomers and clear grades.

The exception is SLS, as we run just one material on our SLS 3D printers, namely PA2200 Nylon (base PA12). This material is characterised by high strength, stiffness, abrasion resistance and long-term dimensional stability. It is also biocompatible and can therefore be used in certain medical applications (EN ISO 10993-1 and USP Class VI).

PµSL builds small parts with high resolution and tight tolerances. We currently offer three materials for PµSL, all of which are relatively stiff and strong for the reproduction of fine details. Two materials are biocompatible and the third withstands elevated temperatures of up to 114 °C.

We use a variety of photopolymer resins in our SLA, PolyJet and DLP 3D printers. If customers tell us what material properties they need, together with other relevant information about the part’s requirements, we will advise on the optimum combination of 3D printing technology and compatible material. We can cater for any requirement, ranging from materials with properties similar to polycarbonate, polypropylene, ABS, acrylic and elastomers (Shore A 30 to 95).

Specialist material grades for PolyJet and DLP come with properties including high clarity, biocompatibility, USP Class VI approval, heat resistance (up to 300 °C), flame retardancy, chemical compatibility and long-term environmental compatibility.

Some materials are better suited to end-use applications than others, but it often depends on the operating conditions and required durability. Our additive manufacturing experts can discuss materials and applications with customers to ensure the optimum grades are selected and parts are fit for purpose. You can access our library of 3D Printing material datasheets here.

Finishing options for additively manufactured parts

Additively manufactured parts can usually be finished in many different ways. The available choices relate partly to the 3D printing technology but a key factor is always the material, as some accept particular finishes better than others. Finishes, whether functional or aesthetic, also need to be appropriate for the intended operating conditions.

SLA, DLP, PolyJet and PµSL all build parts from UV-curing photopolymers. The resultant parts can degrade in sunlight or other sources of UV unless an appropriate finish is applied. This is usually possible, though it can be problematic applying secondary finishes to elastomers.

PµSL parts are almost always used without any secondary finishing, just cleaning, as they tend to be functional parts with fine tolerances and an inherently smooth surface.

Other popular finishing options for additively manufactured end-use parts include blackout/RFI/EMC coatings on internal surfaces, light bead blasting of external surfaces or, if a high-quality aesthetic is required, parts can be sanded and lacquered, or primed and painted. Other finishes range from a rubberised ‘soft feel’ coating through to vacuum metallisation. SLS parts can be dyed any RAL or Pantone colour.

Additive manufacturing of overmoulded parts

Occasionally parts need soft features. This might be for ergonomic reasons, perhaps to improve grip or simply the way a part feels in the hand. Alternatively, soft features can be functional, such as to protect other components at points of contact, or to provide integral seals or gaskets. Such features can be achieved in several ways, so it is always best to talk to our additive manufacturing experts to decide on the best option.

For example, PolyJet can 3D print soft areas directly as part of the build process. Or it could be that a minor redesign enables replaceable inserts to be 3D printed in an elastomeric material. Another option is to use vacuum casting to overmould a part that has been produced by 3D printing, CNC machining, vacuum casting or almost any other manufacturing technology. Although vacuum casting is not an additive manufacturing technology, it is so versatile that it is often used in a complementary way.

Assembly options

We are often asked to manufacture a single part or a short run of identical parts. But sometimes customers request that we incorporate these parts within assemblies. We can undertake any level of assembly operation, from installing threaded inserts, to building sub-assemblies or finished assemblies from numerous parts. As well as assembling parts we have additively manufactured or that we have CNC machined, we can also incorporate parts supplied by the customer or standard off-the-shelf parts that we have sourced.

What are the limitations of additive manufacturing?

Additive manufacturing is versatile, quick and cost-effective, especially if the alternative requires dedicated tooling. Nevertheless, there are times when other manufacturing technologies are better. We have already mentioned vacuum casting and CNC machining, but there are many other production technologies that we can provide in-house or outsource on behalf of customers. These include laser cutting, sheet metalwork, investment casting, and metallic 3D printing.

Furthermore, remember that manufacturing technologies can be combined to very good effect. Earlier we mentioned overmoulding 3D printed parts by means of vacuum casting, but we can also CNC machine additively manufactured parts if particular features need tight tolerances, or we can CNC machine inserts for fixing within additively manufactured parts.

Talk to us

If you need additive manufacturing or would like to explore whether it is beneficial for your parts, talk to our experts by calling 01763 249760.