Creating components with complex internal structures is a difficult business. Not only do you have to be able to define the necessary geometry, using a material that can withstand the conditions required for producing the component, but you’ve also got to be able to get rid of any unwanted material at the end of the process. The key to success is finding a suitable sacrificial material. In the ancient lost wax technique for producing cast metal components, wax was used, as the name implies. Now, the lost-wax technique is being updated for injection moulding with a new sacrificial material - an environmentally benign, water-soluble, biodegradable polymer known as ‘depart’.
What is Depart?
Manufactured by Environmental Polymers of Irlam, near Manchester, UK, ‘depart’ is a pelletised polyvinyl alcohol that is soluble in warm water but offers a range of mechanical properties that make it extremely useful in a number of applications - and in particular as a sacrificial material for making cores in the injection moulding process. For a start ‘depart’ melts at a higher temperature than many of the plastics used for making components via injection moulding.
How Does it Work?
This means that ‘depart’ can be used create complex cores that can withstand the heat of the incoming moulding material in the injection moulding process, while retaining their shape and definition. Melting temperatures for the various formulations of ‘depart’ being tested for injection moulding vary between 185°C and 210°C. Once the plastic material being used to create the component has set, the whole injection moulding can be washed using a jet of war water, and the core made from ‘depart’ dissolve and be removed, leaving the required internal geometry.
Advantages of Using Depart
Application trials are currently being carried out using ‘depart’ by a major injection moulding company, with the aim of refining the technique. Already it is clear that t new version of the lost-wax casting process could be used to make one-piece injection mouldings of components that currently have to be made in two halves, which are then joined together, owing to their complex internal structures. One example is gas manifolds - and this highlights a benefit of using the new technique. If two halves of a component have to be joined together, this inevitably introduces a risk of defects and potential leakages. Being able to manufacture such complex components as single mouldings should offer a significant increase in quality and, probably, a lowering of costs.
The version of ‘depart’ being developed for the injection moulding process is just one of the family of water-soluble, biodegradable polyvinyl alcohols being produced by Environmental Polymers. The properties of the basic material can be tailored to suit the application for which it is being developed, and this is one of the key differences between ‘depart’ and earlier biodegradable plastics - it has user-controllable solubility in water. Depending on the formulation used, materials can be designed that dissolve at preset temperature ranges centring on, for example, 20°C, 55°C or 80°C. After about two to twenty minutes immersion at the designated temperature, the ‘depart’ polymer dissolves in water leaving a harmless, non-toxic aqueous solution of polyvinyl alcohol with a small amount of glycerol. Once this comes into contact with micro-organisms such as those found in water treatment plants, biodegradation to carbon dioxide and water takes place within about 30 days.
Applications for Depart
As well as being developed for ‘lost-wax’ injection moulding, ‘depart’ thermoplastic materials are currently being processed using conventional machinery for the manufacture of plastic film, and will shortly also be available for direct injection and blowmoulding. This allows the water-soluble characteristics of the material to be usefully applied in other applications. Environmental Polymers has developed a patented extrusion process for making the materials that has overcome the stability problems encountered during previous attempts to make polyvinyl alcohol pellets.
Properties of Depart
In addition to their high melting temperature, the ‘depart’ materials offer other good mechanical properties. Tests have shown them to be strong - a formulation that becomes fully soluble at 60°C has a tensile strength of around 50-60 MN.m-2, which is about three times that of low density polyethylene (LDPE). Variations of ‘depart’ have been produced that have tensile strengths of up to 90 MN.m-2. The materials also have unusually high tear strength and puncture resistance, making them suitable for use in packaging. Data taken for a medium-temperature water-soluble formulation shows a puncture resistance of about 1050N/mm when dry, and of about 640N/mm when immersed in water for three minutes at 25°C - which is still about three times stronger than standard polyethylene film.
The materials also have good barrier properties, an important characteristic in many packaging applications. Tests have shown that at 23°C and 50%RH, 30 micron films of ‘depart’ have oxygen permeation rates ranging from 0.24-1.85ml per square metre per day depending on the formulation - versions of the material dissolving at the highest temperatures show the lowest permeation rates. Nitrogen permeation rates for film materials are so low that they could not be measured in laboratory tests, suggesting permeability similar to that of aluminium foil. Finally, depart also has natural anti-static properties, which suggests that it may find applications in the packaging and protection of delicate electronics components and assemblies.
Laundry Bags for Hospitals
These impressive properties open up a wide range of potential applications for the materials, but their biodegradability and water-solubility are key, as in the lost-wax injection moulding application. The sacrificial nature of the materials is crucial in their current areas of use and should prove extremely attractive to product and process designers in the future. One obvious current example is that of plastic film wrapping that dissolves when placed in hot water. Laundry bags are already available that make it possible for dirty laundry, particularly from hospitals, to be hygienically collected in closed plastic bags. These are then simply loaded into the laundry without the sheets and clothes having to be handled again, thereby eliminating possible infection risks to the laundry workers. Once in hot water within the washing machine, the bags dissolve without harming the fabrics (in fact, the polyvinyl alcohol acts as a surfactant to improve the effect of the detergent) and the bedding and clothing emerge washed and ready for drying and ironing without a trace of the plastic bags which enclosed them.
Agricultural and Horticultural Applications
Similarly, soluble bags can be used in the agriculture and horticulture industry, for example for the collection of garden waste and other compostable materials. The bags can be loaded into the composting system with some added water, and they then break down quickly, leaving no trace of the plastic material. Trials in Scandinavia at a commercial composting plant have already proved the concept works extremely well. Food packaging and containers could also be made from depart, which could then be washed away in a standard domestic dishwasher or, preferably, added to the compost heap!
In the longer term, other foreseeable uses of depart would enable much more efficient recycling of glass and extensively used plastics such as PET. If labels and closures on glass and PET bottles were made from depart, these could be washed away with hot water during the recycling operation. This would leave clean glass bottles, or pure PET, eliminating the costs of purifying the materials prior to reuse or recycling.
Environmental Polymers has already entered into several joint manufacturing and supply negotiations. It is currently looking to negotiate more partnerships worldwide with companies and governments seeking a foothold in the massive expansion that commercial environmentally friendly products are predicted to enjoy during the next decade. Unlike the materials themselves, the potential applications for depart don’t seem likely to disappear overnight.