Enric Sirera, sales director, Ultrasion, discusses injection moulding with no material degradation.
Enric Sirera, sales director, Ultrasion
For all manufacturers of plastic parts for medical applications, there is one problem that is a constant issue when using injection moulding technologies – that is material degradation.
All traditional injection moulding processes use the standard screw, barrel and heater band configuration, with material being melted well in advance of injection into the mould. It is this residence time that is inherent in all standard injection moulding technologies that leads to material degradation.
Ultrasion, headquartered in Barcelona, Spain, is now an established alternative injection moulding technology that is being used worldwide and which was created specifically to address the issue of material degradation. It does this by eliminating the reliance on screws, barrels, and heater bands and instead doses only the amount of material necessary per shot direct to the mould, where it is melted at the gate through the use of an ultrasonic horn is injected immediately into the mould.
The basics of the ultrasonic moulding process is shown in figure 1. Using a dosage system that delivers the correct quantity of standard pellets for every shot, the production cycle begins with the mould already closed and dosed with raw material at room temperature. The material is then contacted by an ultrasonic horn or ‘sonotrode’ which is lowered, and as well as melting the material forces the polymer to flow into the mould cavities. The sonotrode then returns to its original position, and the cycle begins again.
The ultrasound moulding technology is extremely precise, uses no heaters, and as already stated the process means that there is no material residence time, and no material degradation. In addition, as the energy needed in the process is only at the point when the ultrasonic horn contacts the raw material to induce melt, it uses upwards of 90% less energy than a traditional micro injection technology.
Material wastage, a problem in all sizes of injection moulding machines, is a massive issue in precision and micro moulding applications, where in some instances upwards of 99% of material processed will be scrapped. Where this material is expensive as is the case of some critical medical mouldings, this becomes an even bigger problem. In the Ultrasion process, only the material required is dosed, and so runner and sprue wastage is all but eliminated.
The nature of the ultrasonic moulding process is such that material melt characteristics are very different from those produced in injection moulding machines. The application of high intensity mechanical vibration that transmits energy directly into the polymer molecular structure results in an extremely fast and efficient melting process ‘inside out’ rather than ‘outside in’ which is how melting occurs in injection moulding via the electric heater bands. In addition, the new sprue concept in the Ultrasion technology means that it behaves as an energy director, orientating the waves in the flow direction meaning that molten material and waves travel together towards the mould cavities, which induces extremely low viscosity (almost as low as water) in the melted plastic.
The lower viscosity characteristics of polymers when melted via ultrasonics also means that the technology uses significantly reduced moulding pressures, typically not exceeding 350 bar as opposed to the more typical 2500 bar used in traditional micro injection moulding machines. This opens up an array of over moulding and insert moulding applications previously impossible, and also means that the technology can mould over intricate core pins or difficult core pin configurations which would either be destroyed or deflected using traditional moulding pressures.
The technology has been designed for ease of use, and requires no more than a short orientation and training session as the machine is installed. There are a few simple adaptations necessary to the tooling for the Sonorus 1G, which are covered in the installation training and in specification documents that the company issues to all customers.
There are no materials that cannot be processed using the ultrasonic moulding technology, with successful moulding projects using everything from standard polypropylene to high density polyethylenes. The Sonorus 1G machine — which has been designed specifically for precision and micro applications — can accommodate shot weights from 0.05 g to 2.0 g.
In all materials, the reduced viscosity allows for the attainment of especially long parts or parts with extremely thin walls. The machine can easily mould 15 mm long parts with wall thicknesses of 0.075 mm, and achievable tolerances are in the region of 0.01 mm.