Meaning
Process transition points define the moment when an injection machine switches from controlling the forward speed of the screw to controlling the pack pressure. Setting the velocity to pressure switchover ensures that the cavity fills almost completely before the system begins to compensate for volumetric shrinkage. This parameter governs the transition between the dynamic flow stage and the static packing stage of the moulding cycle.
It happens typically at ninety five to ninety eight percent of the total volume to avoid high pressure spikes at the end of the stroke. If the switch occurs too late, the tool is over pressurized, resulting in flash or structural damage to the press. Correct placement identifies the balance between a full part and a stable process that does not stress the steel.
Cycle Control
Consistency in the timing of the transition leads to repeatable part weights and predictable dimensional outcomes. The velocity to pressure switchover prevents the screw from slamming into the full cavity at maximum speed. By dropping into pressure mode just before completion, the machine gently pushes the final few drops of resin into the furthest reaches of the tool.
This strategy ensures that air can escape from the vents without being trapped by a fast moving melt front. Moulders monitor this switch position using screw distance or hydraulic pressure triggers. If the position drifts, it indicates that the resin viscosity or feed consistency has changed, which alerts the quality team to investigate.
Effective control relies on this point being held within a fraction of a cubic centimeter.
Flash Prevention
Excessive force at the moment of fill causes the mould halves to move apart, allowing plastic to seep into the gaps. The velocity to pressure switchover acts as a relief valve that limits the peak internal stress before it overcomes the clamp tonnage. When this point is reached, the machine limits its output torque and focuses on maintaining a steady hold.
This protection is especially important for tools with delicate cores or long, unsupported ribs. If the transition is skipped or set too late, the explosive arrival of the melt into the small cavities will warp the mechanical components. Setting the point early enough allows the inertia of the moving screw to subside safely.
Successful moulders refine this setting during the first few shots of any new product launch.
Weight Stability
Volumetric repeatability depends on finding the perfect spot where filling resistance meets holding demand. The velocity to pressure switchover is the most critical process setting for achieving six sigma levels of weight consistency. Variations here manifest as sinks in the part center or variations in the thickness of the final product walls.
Most advanced machines allow for switchover based on cavity pressure sensors, which provides a direct look inside the actual mould. This feedback loop eliminates the influence of screw variations and environmental temp shifts. Regular data logging of these events helps maintain the high standards required for critical components like gears or medical vials.
Identifying the limit of this transition is essential for any high stability manufacturing process.
