Areal Surface Texture Parameter Measurement in Micro Injection Mould Cavities
Areal surface metrology in micro-cavities requires non-contact optical instruments with matched bandwidth filters to verify steel texture and polymer replication.
Cavity Pressure Transmission
Meaning
Physical behaviour during the packing phase of injection moulding represents the transfer of force from the injection piston through the molten polymer to the furthest points of the mould cavity. Effective cavity pressure transmission ensures that the polymer compresses uniformly to offset volumetric shrinkage as the melt cools. This transmission decays as the polymer freezes, particularly at the narrow gates of the cavity where the flow path is thinnest.
Melt temperature and polymer viscosity govern how much of the applied pressure actually reaches the end of the fill, meaning that a more viscous resin requires higher machine pressure to achieve the same result.
Hydraulic Loss
Frictional resistance along the flow channel reduces the peak pressure achieved in the cavity compared to the machine’s nozzle pressure. In a typical injection run, the molten polymer exhibits non-Newtonian flow behaviour, which means that shear thinning improves the flow initially but pressure drops rapidly once the flow slows. Sourcing high-molecular-weight resins can increase this flow resistance, causing a larger drop in pressure transmission.
The moulder must adjust pack settings to compensate for this difference.
Shrinkage Control
Uniform compression across the moulded part depends directly on the consistency of the pressure transferred throughout the runner and gate. Low cavity pressure transmission results in higher shrinkage near the end of the fill, leading to warpage and dimensional instability. This defect is particularly pronounced in crystalline polymers like polypropylene or polyamide.
Adjusting the melt temperature helps maintain a lower viscosity, allowing better pressure transfer.
Process Limit
Monitoring the peak pressure at the end of fill using transducer pins defines the point of gate solidification. Once the gate freezes, cavity pressure transmission drops to zero. Any further pressure applied by the machine’s screw only compresses the runner system without affecting the cavity itself.