Meaning
Measurement instrumentation placed in direct contact with the polymer melt inside the mould cavity belongs to this category of hardware. A direct sensor captures the actual melt pressure and temperature without the frictional losses or dampening associated with force transfer pins. When these sensors operate correctly, they provide the injection controller with real-time feedback that reflects the true state of the plastic flow.
This setup is confined to areas of the cavity where a small pin mark on the finished part is acceptable.
Measurement Fidelity
Piezoelectric elements located within the sensor tip generate an immediate response as the melt front flows over them. The direct sensor avoids the friction that occurs when force travels through a sliding pin. This direct contact provides an accurate pressure curve that enables the moulder to detect minor changes in polymer viscosity.
Tooling Design
Integrating these instruments into a mould plate requires specialized machining of the cavity pocket and cable channels. The direct sensor must be installed flush with the cavity wall to prevent melt stagnation or flow front disruptions. Precise boring is necessary to prevent the polymer from squeezing around the sensor head and creating frozen blockages.
These requirements increase the initial tooling cost but reduce the long-term maintenance costs of mechanical ejector assemblies.
Surface Quality
Direct contact between the sensor and the melt leaves a subtle circular imprint on the outer layer of the moulded part. Moulders must position the direct sensor on a non-cosmetic face or behind an internal rib to prevent visual defects. When cosmetic appearance is the primary requirement, indirect methods or sensor placements in the runner system are preferred.
Despite this cosmetic limitation, the direct sensor remains the preferred choice for producing critical structural parts where safety and dimensional tolerances are paramount and any physical deviation in part performance can lead to field failure.
