On hydraulic machines with only one pump, the injection process starts once the locking force has been achieved. The pressure built up in the hydraulic cylinder of the clamping unit for this purpose is generated via valves and only dissipated at the end of the remaining cooling phase. During the injection moulding process, no further control of mould locking is possible. This is only possible if the hydraulic drive is equipped with at least two pumps and, therefore, two functions can be performed simultaneously.
However, due to the independence of the electro-mechanical direct drive, the electric ALLROUNDER A machines are generally capable of performing several functions simultaneously. Because the clamping unit is designed as a toggle system, the locking force is always dependent on the mould installation height. This can change during operation due to heat expansion of the mould, so that the locking force also changes. In this case, clamping force control provides for fully-automatic compensation of the heat expansion, by adapting the mould installation height during the ongoing process via mould height adjustment.

But what advantage does that have for the injection moulding process? In addition to the “Extended clamping programme”, up to two locking forces can be programmed during the injection, holding and remaining cooling phases respectively. This enables micro-stamping or so-called “active breathing”, for example (photo, top). Here, the locking force is deliberately reduced at the end of the injection phase, permitting pressing open of the mould in the hundredths of a millimetre range with increasing internal mould pressure: In this way, the mould “breathes” slightly (curve point 1, photo, below).

This results in small overfeeds, as the boundary layer of the moulded part was already able to cool down and the plastic remains in the cavity. During the holding pressure phase, the locking force is increased again and the mould closes completely. The result is a minimal stamping stroke in which the holding pressure acts on the entire surface of a cavity. This enables the reduction of stresses in optical components and increases the flatness of planar moulded parts. In the case of moulded parts which are subsequently electroplated, improved adhesion and therefore higher quality can be achieved.
Whether active breathing can be applied, depends largely on the part geometry and the mould design. The process is particularly suitable for rotationally symmetrical moulded parts. A special stamping mould is not required. Due to the stamping process, the moulded parts are more tightly packed and the result is increased part weights. On the other hand, shrinkage of the parts is also reduced, which must be taken into account in the mould design.
Another interesting option offered by the “Extended clamping programme”, is the process-dependent support of mould venting. At the start of the process, a relatively low locking force is programmed so that when the mould is closed, the air in the cavity can escape as the mould fills. In order to exclude overfeeding, the locking force must however be increased again just before the cavity is completely filled with plastic and the internal mould pressure increases. In addition to improved mould filling, the so-called diesel effect, localised burning of the plastic, can be prevented. The overall cycle time is not increased with this type of venting.
The individual parameters for active breathing or venting can be perfectly harmonised via the freely configurable process graphics of the SELOGICA control system. This enables easy and quick optimisation of the entire process.