Materials for Micromoulding
Introduction
The micromoulding process is highly value-adding due to the very small amounts of material involved and very fine detail possible on the final product. The process is thus well suited to use some of the more expensive engineering plastics and other exotic materials, as the material costs are a lower percentage of the production costs compared with conventional injection moulding. The high shear rates present also encourages the use of materials which exhibit high shear-thinning rheology to minimise the stresses within the melt. The list shown below gives some insight into the range of materials which have been processed at the University of Bradford as part of the EPSRC-funded micromoulding research programme.
Material types
Engineering Thermoplastics
The most common materials used in micromoulding are high-grade engineering thermoplastics such as Polyactal and PEEK resins which have a good balance of material properties and processibility. Work performed at Bradford University has mainly involved the study of Dow POM109 Polyacetal and various PA6 and PA6,6 materials.
Liquid Crystal Polymers boast both excellent processing behaviour and impressive mechanical properties which makes them well suited for the micromoulding process. These polymers have a high shear-thinning behaviour and very low shrinkage which assists in filling of regions of high detail on micromoulded products and ensures dimensional stability during cooling.
Nanocomposites
It is very difficult to use conventional glass or carbon fibre-based composite materials in the micromoulding process simply due to the (relatively) large dimensions of the filler particles. Nano-composite materials consist of nano-scale (10nm diameter) carbon platelets mixed within a polymer matrix and are currently a source of great interest within the polymer community due to the favourable gains in material, barrier and flame retarding properties possible with the addition of a low percentage (1-2%) of filler material. These materials appear to be ideally suited for micromoulding especially where applications demand a product with high modulii/impact strength etc.
Biosorbables
Biosorbable polymers are materials which can be readily broken down and absorbed by the body over a period of time. They are very useful for use in surgical implants or bodily repairs because they can assist in healing without requiring removal in a further operation. Examples include PGA (Polyglycolic Acid), PLA (Polylactic acid) and co-polymers of the two. The last two years have seen a surge in interest in micromoulding technology from the medical community as a means to create biosorbable implants with detailed structures and textures. The low residence time of polymer in a micromoulding machine plus the ability to handle very small amounts of material makes it ideally suited for processing of biosorbables which are very susceptible to thermal degradation.
Metal/Ceramic Powders
Metal and ceramic injection moulding (MIM/CIM) are very similar techniques used to form complex 3-d geometries from either metals such as tool steels and tungsten alloys or ceramic materials such as Alumina and Zirconia. In the process, fine powders of the desired material(s) are blended to form the desired alloy and are then compouded with a polymer binder which allows the feedstock to be injection-moulded. Following the moulding process, the binder is removed using either an incineration or solvent method and finally the porous material is annealed to produce the required product.
These materials are of great interest to the micromoulding community especially in the medical field where ceramic materials can be used for a variety of applications such as surgical instrument components and cochlea, pacemaker and other implants.
Novel Materials
The ability of micromoulding hardware to process small amounts of material at a time makes them ideally suited for processing small batches of novel polymers or high cost materials such as deuterated polymer blends. Materials such as these have been processed in batches of a few tens of grammes using the micromoulding machines at the University of Bradford, a feat which would have been impossible using conventional injection moulding technology.
Material Handling
Hydroscopic (Nylons etc.) and unstable polymers (PLA, PLGA) demand a material feed system with a tightly controlled atmosphere. The confined volume within the Microsystem50 negates the use of conventional commercial dryers and alternatives must be sought. Motan addressed these issues when they released the Micro-Dryer, the first materials handling unit designed specifically for the Battenfeld machine.
Motan Micro-dryer installation
The advent of micromoulding technology creates new demands from material manufacturers.
The narrow processing window and tiny volumes associated with micromoulded products requires that not only should variation between batches of material be considered, but also inter-batch and pellet to pellet consistency, which may have a significant influence on the variability of final product properties. These issues have the potential to create a new niche in the raw material market for the supply of low volumes of high performance materials with tightly controlled properties for a premium price.
The particle geometry of these feedstocks is also an issue because the plastication stage of the micromoulding process is currently limited by extrusion screws which are chosen to have small flight diameters to minimise melt volume, whilst still possessing deep enough channels to be able to process regular commercial feedstock (pellets). The optimum design to meet these requirements currently is a screw with a diameter typically 14-16mm with melt volume ~10g, which can pose problems when moulding very small shot volumes due to the associated high residence time of material in the screw. Smaller pellets tailored for the micromoulding process would allow the adoption of smaller screw diameters/volumes and thus reduce material residence time whilst still allowing the creation of a homogenous melt.
