Making the impossible possible: the freeformer in 3D printing
Lukas Pawelczyk, Freeformer Sales Manager, Arburg explains how the APF process with the freeformer is particularly suited to AM in medical technology.
The medical plastics market was one of the first to make heavy use of additive manufacturing (AM) technology, creating complex and often custom-designed components and devices in relatively small numbers. This trend has accelerated as the imaginations of medical designers and new 3D printing equipment have made possible what was previously impossible. Today, a different type of system pushes those boundaries even further by allowing the use of the same plastic granules used in injection molding, including original biocompatible, absorbable, sterilizable and FDA approved materials.
Developed and built by Arburg, the German manufacturer of precision injection molding machines, the freeformer machine, with its Arburg Plastics Freeforming (APF) process, facilitates sophisticated medical applications that cannot be achieved with any other process.
“Open system” is the key
Similar to injection molding, the freeformer works by melting conventional plastic granules through a heated plasticizing cylinder. A high-frequency pulsed rigid nozzle then discharges tiny droplets of the molten plastic melt. The workpiece carrier, which can be moved along three axes, allows each individual drop to be deposited with precision. The applied droplet binds to the existing surrounding material so that, layer by layer, three-dimensional components with high mechanical strength are produced.
Part production starts from 3D CAD data in STL basic stereolithography format. Unlike conventional filament-fed AM systems, freeformers work with a wide variety of standard qualified plastic granules. Additionally, users can process their own custom compound materials with this “open system” and optimize droplet size and processing on their own. Alternatively, they can access Arburg’s material database and select certified plastic granules, such as ABS (acrylonitrile butadiene styrene), amorphous PA (polyamide) and PC (polycarbonate), elastomer TPU (thermoplastic polyurethane) and semi-crystalline PP (polypropylene), PLLA (poly-L-lactic acid) and other special and certified original materials, including the original biocompatible, absorbable, sterilizable and FDA approved materials.
An outstanding example of the use of the APF process in medical technology is the processing of Resomer LR 706 (composite of poly L-lactide-co-D, L-lactide and ß-TCP) from Evonik to create plaques of implants that are inserted directly into the body in case of bone fractures. The polymer composite, which is modeled on human bone, contains 30% ceramic additives, known as β-TCP. This makes the component stronger and releases calcium to promote bone regeneration. After a given time, the implant dissolves completely.
Absorbable cranial bones, cheekbones and finger bones were also made from medical PLLA (Purasorb PL18, Resomer LR 708). In addition, the plastic granules can be loaded with anti-inflammatory agents, for example, to minimize rejection.
Permanent implants are also produced using the APF process. For example, spinal implants were made using thermoplastic Bionate (polyurethane polycarbonate) PCU, and a multi-material meniscus (using different types of polyurethane) was developed within days, eliminating the tedious (and more complicated) development. of an overmolded part produced. by conventional overmolding.
The APF process is also used for medical devices and aids. The freeformer processes e.g. medically approved SEBS (styrene-ethylene / butylene-styrene) (Cawiton PR13576) with a hardness of 28 Shore A. This very soft material is dense and tear resistant and is suitable for the manufacture of articles such only functional bellows. Another typical example is PA saw jigs, which are used as personalized surgical aids. Flexible and electrically conductive strain gauges are an example of future developments. These are made of a soft TPU (Desmopan) material with carbon components and an inserted LED. The two-component functional part made with the freeformer is both flexible and electrically conductive. Depending on the voltage and therefore the electrical resistance, the LED lights up with a different brightness. Such strain gauges could be used in physiotherapy, emitting an acoustic signal as soon as an injured arm or an operated knee is stretched or under-stretched.
Fill level can be selectively changed
To date, the freeformer is the only AM system that can process the FDA approved Medalist MD 12130H TPE (hardness 32 Shore A) and, without changing the processing parameters, adjust the room fill level – proximity to droplets – to refine the mechanical properties and achieve different hardnesses. For example, at 100% fill level (i.e. drops as dense as possible) maximum mechanical strength and rigidity are achieved. At a fill level of 20% for example, the distance between the drops is greater and the part is more flexible. It is even possible to create different densities of materials in different parts of the same component.
An ongoing research project at the University of Belfast in Ireland is examining how vaginally inserted rings loaded with active ingredients can protect women against HIV infection. Using medical grade TPU, rings with different fill levels (100, 50 and 10%) were studied. The lower the filling level, the more porous the TPU ring and the greater the release of active principle. Result of the study: at a filling rate of 50%, approximately 60 mg out of a total of 111 mg of active principle are released over a period of 30 days. This compares to just five out of a total of 190 mg for an injection molded ring. In addition, the APF process is also smoother than injection molding, so there is less temperature degradation, less stress, and the active ingredient remains more stable.
Freeformers are suitable for clean rooms
With a few minor modifications, all freeformers are suitable for cleanroom use. They operate with low emissions, are virtually dust free, and their build chamber is typically made of stainless steel. An optional robotic interface automates the AM and integrates the freeformer into computer networked production lines. The quality of the process can be reliably documented and the components traced individually if necessary.
The APF process with the freeformer is particularly suitable for AM in medical technology. Geometric freedom combined with material freedom opens up entirely new plastic applications, including for use in the human body.