March 2009 - Issue #5
Plastic iron breaks the mould
Story by Penny Fannin
View articles in related topics: Advanced Manufacturing, Engineering
Lying on a benchtop in a laboratory at the Industrial Research Institute Swinburne (IRIS) is a human hand. Just 15 centimetres away is the lower jaw of a human skull. It’s not until you see white filaments curling from what would be the fleshy side of the fingers that the synthetic nature of the hand, and the accompanying jawbone, becomes apparent.
Both objects have been made with a fused deposition rapid prototyping machine and are examples of the complex plastic objects that can now be created when you combine some plastic filament, heat, a computer-aided design (CAD) program and a machine the size and shape of a refrigerator.
The machine uses a process called Fused Deposition Modelling (FDM) through which plastic parts are produced when layer after layer of plastic is rapidly deposited. Each layer is 0.05 to 1.25 millimetres thick and the part takes its form from a computer-generated model that is sent to the rapid prototyping machine.
Most of the parts fabricated using FDM are used for design verification or checking the form and fit of a part. Although the first FDM system was launched in the early 1990s, the applications of FDM have remained limited because the layer-by-layer approach it uses means that plastic is the only suitable raw material for creating parts.
That was before Professor Syed Masood from IRIS at Swinburne University of Technology came along. Professor Masood saw the potential for fused deposition rapid prototyping machines to be used to create complex metal parts, assuming the machines could handle metal as the raw material.
Because the machines need the raw material to be delivered in filament form, Professor Masood and two PhD researchers have spent the past few years developing metal composites that can be used in the FDM process.
“Any part of any shape can be created using fused deposition rapid prototyping machines,” Professor Masood says. “But nobody has tried to make parts or tooling with metal-based composites. The applications increase if you can use the same technology (FDM) with new material and make a stronger part. One of the main applications is in the development of injection moulds.”
Dr William Song spent the three years of his PhD developing and testing an iron–nylon composite that could be extruded into filaments and fed into the fused deposition rapid prototyping machine. The material is 40 per cent metal and 60 per cent nylon plastic, so it can withstand the heat from the molten plastic that is used in injection-moulding machines.
Injection moulding is one of the most widely used manufacturing processes for common plastic products. It involves producing a plastic part by injecting molten plastic into a closed steel mould cavity of the desired shape, allowing the plastic to cool and then ejecting the part. For each new plastic product the injection-moulding machine requires a new injection mould, generally called a tool die.
Most injection moulding dies are made of hardened steel, and can be used to make up to two million parts, Professor Masood says.
“For every new plastic product, a steel mould needs to be designed, machined and cut into shape. It is very time-consuming and very expensive. If the part is complex, creating the mould may cost millions of dollars. So why not use a technology that can develop the mould directly on a rapid prototyping machine?” he asks. “It saves a lot of time and money to create a part and reduces the cost of making the mould.”
Although steel moulds are desirable for high production runs, moulding dies can be made of aluminium or softer material if they are being used to make parts with smaller production runs in the hundreds or thousands.
“FDM is generally used to make 3D prototypes, but we are using it to make moulds,” Professor Masood says. “The injection mould produced from our iron–nylon composite material is as good as any full steel mould for short production runs.”
The mould has already been tested in an injection-moulding machine and plastic parts have been successfully produced using the mould, he says. The technology is ready to be taken to market if a suitable commercial partner can be identified.
PhD researcher Mr Mostafa Nikzad is now extending the range of composites available for use in FDM by developing and testing a composite of iron and the thermoplastic acrylonitrile butadiene styrene (ABS).
Although moulds made from the iron–polymer composite can be used in injection-moulding machines instead of steel or aluminium moulds, Professor Masood acknowledges that limitations remain. “It (the composite mould) is much stronger but it still contains plastic. When you do injection moulding into this mould it will eventually give way, so it’s only good for short-run production of, say, 100, 200 or 500 parts.”
Despite this limitation, moulds made from metal–polymer composites have a particular use in medical applications where parts – such as prosthetics – are only needed in small numbers as they are for a specific person or use, he says.
