Hydraulically powered injection axes have a major advantage over electro-mechanical versions in that they offer a high force and power density combined with a compact design. At the same time, the use of hydraulic pressure accumulators enables high injection speeds to be achieved. This has a knock-on effect on the dynamics, i.e. how quickly the speed can be reached. All these characteristics are decisive factors for the filling of moulded parts with an extremely broad range of wall thicknesses and long flow paths, or thin-walled moulded parts. When the so-called injection accumulator is combined with an electro-mechanical dosage drive, the result is a high-performance injection unit that combines a high plasticising capacity with rapid cycles. This is advantageous in the field of multi-cavity applications, for example, and for packaging items and thin-walled products.
 
However, the reproach is often made against hydraulic injection axes that they work less precisely than electro-mechanical versions. In addition, the hydraulic cylinder responsible for the injection movement is mostly only subjected to pressure at one end. An injection system of this kind can be compared to a car without active braking. The defined speed cannot be adhered to so rapidly or precisely, because here the only control options are “Apply pressure” and “Remove pressure”.
To improve the precision of hydraulic injection systems, ARBURG includes in its range an option, position-regulated screw (PRS), which is specified when injection accumulators are used. Here, the hydraulic cylinder responsible for the injection movement is subjected to pressure at both ends. With the help of pressure transducers and a dedicated, closed-loop control circuit (servo control), the injection axis can be accelerated and braked in a precise, targeted manner. Control parameters include the screw position (stroke measurement system) and the pressure difference between the two cylinder chambers. The pressure transducers and hydraulic valves are situated close to the hydraulic consumers. This results in very short oil columns and direct control of movements. Together with the regulation of the pressure difference, extremely accurate position, speed and pressure control becomes possible.

The achievable reproducibility of a hydraulic injection system with injection accumulator and PRS is comparable to that of the electro-mechanical version. But with a system like this, considerably higher injection speeds can be attained. This, in turn, leads to higher injection power, which is necessary for high-performance applications.
The auxiliary axes “Ejector”, “Nozzle Movement” and “Core Pull” were already available as hydraulic alternatives for the electric ALLROUNDER A. Now, thanks to the 1300 injection unit with hydraulic injection accumulator, the equipment of this series can become even more flexible. At the same time, its range of applications can increase.

In future, it will be possible to use the ALLROUNDER 570 A for applications with performance requirements that far exceed the capabilities of the largest electric, size 800, injection unit and with comparable precision. Nevertheless, the machine still remains energy-efficient, thanks to the other electric movement axes, such as for the clamping system or dosing axis, for example.