Oil hydraulics has become firmly established as the drive system for the vast majority of injection molding machines and until recently was almost unchallenged as the power source. Put at its simplest, the injection molding machine contains a reservoir of hydraulic oil which is pumped by an electrically-driven pump at high pressure, typically at up to 2000 psi, to actuating cylinders and motors. High and low pressure linear movements are performed by hydraulic cylinders, and rotary movements for screw drive and other purposes are achieved by hydraulic motors. Hybrid machines, in which the screw is driven by electric motor while the linear movements remain hydraulically powered, are not uncommon.

In recent years, the supremacy of the hydraulic machine has been challenged by all-electric machines. These use new brushless servo motor technology to power the various machine movements. The capital cost of all-electric machines is higher than that of conventional machines but the energy consumption in production is much lower. This is because the electric motors run only on demand, and there are no losses due to energy conversion, pipelines, or throttling. The elimination of hydraulic oil makes the all-electric machine inherently cleaner, so these machines are attractive for sterile or clean room use. There is also evidence that all-electric machine movements can be resolved with a higher degree of precision and repeatability than hydraulic systems.

The Process

The process may involve either a thermoplast or a duroplast as the polymeric binder. With a thermoplast, solidification of the melt occurs on cooling; with a duroplast, a hardener is added to the feed mixture and solidification results from a binder-hardener reaction that occurs at elevated temperature. Figure 1 shows the viscosity-temperature relation for each type of binder. The reversibility of a thermoplast in terms of solidification makes recycling the reject a possibility but can lead to deformation of the compact during the subsequent burnout stage (see Sect. 3). For the duroplast process, solidification is irreversible and no deformation can occur during reheating, but the time for hardening is relatively long and the mold temperature is relatively high, and the reject cannot, of course, be recycled. The thermoplast process is the most widely used for ceramics and so this is discussed here.