A Guide to Additive Manufacturing for Plastics Engineers
By Phil Kilburn, medical markets manager at 3T RPD.
There are a number of terms used to describe the process of building a solid object, layer by layer, but the principles are the same.
The object to be created is first modelled in a 3D data format (known as a CAD design), then the data is sliced into thin layers and transferred to a machine which then gradually builds the object, one layer at a time, slice on top of slice, using a range of materials from nylon through to steel and titanium.
Known most widely in the consumer press as 3D printing, amongst the industry it is more frequently known as additive manufacturing or rapid prototyping. Such printers range in sophistication from a home version that can be bought for US$1,000 to a full industrial 3D printer which can cost anything up to US$1 mn.
Additive manufacturing was originally conceived as a prototyping technology which produced models to demonstrate fit, form and function. But things have developed to such an extent that in 2013 the range of sectors and uses that parts produced by additive manufacturing is wide and continuing to expand.
For example, end-use parts manufactured using the technology are on cars racing in Formula 1, are in space on experimental vehicles and are implanted into humans. In addition to these bespoke utilisations, the technology is being used to build production parts for engines, manufacturing lines and consumer products, all of which would be either impossible or prohibitively expensive to produce by traditional, subtractive technologies.
Types of additive manufacturing technologies
There are a range of types of additive manufacturing systems, including stereo lithography (SLA), fused deposition modelling (FDM), 3D printing, and plastic laser sintering (also known as selective laser sintering—SLS). The systems build parts in a range of materials including epoxy-based materials, ABS, wax, polystyrene, ceramic and nylon. All the systems conform to the principles of building layer by layer, but vary in how the materials are applied (for example, as a fine powder, liquid polymer or molten plastic) and how they are cured (for example, by melting with a laser or activating UV resin with a laser).
The material most commonly used in non-implantable medical applications is nylon 12.
Parts made with nylon 12 have good long-term stability, offer resistance to most chemicals and are easily sterilised. High levels of complexity are achievable in the finished part and the material delivers good impact strength and durability. Tensile and flexural strength combine to make tough plastic parts, with the flex associated with many production thermoplastics. Nylon 12 is non-hygroscopic, thereby negating the requirement to seal the surface on components being used with liquids.
The process by which a part is created is that a fine layer of nylon powder (between 0.1 mm and 0.15 mm thick) is laid on to a base platform. One or more powerful lasers melt the fine powder in accordance with the outline described by the sliced data which creates solid nylon where the laser has acted leaving the remaining nylon as powder. A further layer of nylon powder is then applied and the laser melting process is repeated.
As each layer is completed, the platform upon which the process is taking place drops by the depth of one layer. At the end of the build cycle, the platform has a cake of nylon sitting on top of it—of which some is still fine powder sitting around a series of solid parts. The platform is removed, cooled, and the parts removed and air-blasted to clean off the loose powder. A limiting factor to the current technology is the boundary box of the build chamber of the machine—700 mm x 380 mm x 580 mm, but this is an area that continues to develop.
At the point the part is removed from the powder cake and cleaned, it can be used as a finished piece or have a number of post-process finishes applied to it. These include colouring, plating in metal, surface finish refinement and having inserts applied. However, the finished piece can have hinges and threads designed in to the part from the outset, meaning that designers have a wide range of options available to them.
Tips in designing for additive manufacturing
When designing parts for additive manufacturing, in addition to designing in specific features that are required, there are a number of pointers to help produce accurate and appropriate designs.
For example, a minimum wall thickness of 1 mm is advised and a minimum layer thickness of two layers. Many file types can be accepted, but whether the source is a CT scan or Google Sketchup, the data will be translated into an STL file—a standard triangulation language file—which represents all points on the part via triangles. Scaling of a part is crucial and the team at 3T RPD works with all customer data to ensure that accuracy is maintained as the heat generated by the build process will result in parts shrinking during cooling.
Some examples of additive manufacturing in the medical market
Over the last ten years the use of additive manufactured models for the treatment of craniomaxillofacial trauma and reconstruction has become the gold standard.
The use of additive manufacturing models has resulted in a reduction in the time the patient spends in theatre, a shorter recovery and an improved clinical outcome for the patient.
These models are used in a variety of ways:
Implants: The patient is scanned using a computerised tomography (CT) scanner. This generates thin slices though the body. The slices from the scans are virtually reconstructed using computer software to recreate the scanned area as a 3D model. This model is then built with an additive manufacturing machine which re-assembles the slices to give an accurate 3D representation of the patient. The models are then used by the maxillofacial technologists to sculpt the implant, initially using wax. The wax is either cast to manufacture the final implant directly or used to manufacture a press tool to produce a shaped titanium plate. Alternatively, using metal additive manufacturing, a cranial plate can be built directly from the CT data—a technique that has already been successfully undertaken for a patient in a collaboration with Nottingham’s QMC and 3T RPD.
Guides: Surgeons are also using 3D CAD models to design drill and saw guides. The guides are custom designed using the CT data from the patient. They are then manufactured using additive manufacturing. This has improved the placement of implants and allowed the surgeons to accurately cut and place bone grafts. The models and guides also help surgeons plan surgery before entering theatre, debate the approach with colleagues and also discuss the surgery with the patient. A model produced by 3T RPD for a UK hospital recently resulted in a change of approach to surgery and a much improved outcome for the patient.
Prosthetics: With the development of photogammetry, the capture of a 3D surface is as quick and simple as taking a picture. Images taken by this process are automatically generated into 3D models. 3T RPD worked on a project to develop this application to create a new way to produce facial prosthetics—The Facemaker Project—with a number of UK partners including Nottingham University, Queens Medical Centre in Nottingham, and UK design software specialist Delcam. The outcome allows the data generated to be used in many applications that have revolutionised the manufacture of prosthetics. For example, the surface data can be offset and used to manufacture a formtool that can be used to manufacture burns masks.
A further example is to take the data of a healthy ear, then mirror it to produce a representation of a replacement ear that can be used to produce a mould tool for a prosthetic replacement ear. In a more extreme example, the process was used to capture data from the face of a father and son. The father had lost his nose and part of his face due to complications with a tumour. The son’s face was morphed to that of the father and the resulting model used to produce a successful prosthetic.
Biocompatible brands of additive manufacturing plastics tested to ISO10993 or USP Class VI
The following list, compiled by Sam Anson, managing editor of Medical Plastics News, provides examples of selected biocompatible additive manufacturing material brands tested according to ISO10993 or USP Class VI.
3D Systems
VisiJet EX200/Crystal—clear
VisiJet MP200/Stoneplast—clear
Accura ClearVue for SLA—clear and selective colouring, amber
VisiJet Clear for SLA—clear and selective colouring, amber
Accura Y-C 9300R—selective colouring, pink
Dreve Fototec SLA and SLE—hearing aid material, clear, skin tone, red, blue
DuraForm PA SLS nylon—white
DuraForm PRO SLS nylon—white
DSM SOMOS
Watershed XC11122—epoxy-based photopolymers
Protogen 18420—liquid ABS-like photopolymer
BIOCLEAR—clear tough polymer
EnvisionTec
E-shell 200—ABS-like low viscocity photopolymer, colours include pink, tan, beige, cocoa and mocca
E-shell 300—clear, sterilsable, tough. Colours include water, rose, red and blue
E-shell 600 Clear guide—Clear rigid
EOS
PA2200—nylon
Fortus
PC-ISO—clear sterilisable polycarbonate
ABS-M30i—ABS
Objet
MED610—PMMA
OPM
OXPEKK—similar to PEEK (long term implantable)