Injection Molding: Closing the Loop on Clamping Force and Shot Volume

On an injection molding machine, neither clamping force nor shot volume is a directly measured quantity inside the controller: clamping force is determined by platen position after the toggle mechanism is closed and by elastic elongation of the tie bars, and shot volume is converted from screw axial displacement multiplied by screw cross-sectional area. The stability ceiling of both closed loops therefore depends on the repeatability and update rate of the platen and screw position feedback, not on the grade of the pressure sensor. Magnetostrictive displacement sensors output absolute position, have no contact wear and retain zero on power loss, and are the mainstream feedback replacing potentiometer-type electronic scales.

Four position-measuring points on an injection molding machine

A standard horizontal injection molding machine usually needs four channels of position feedback: clamp opening/closing stroke (moving-platen position, longest range), injection-unit stroke (nozzle-to-mould seating, shorter range), screw metering/injection stroke (determines shot volume and the hold-pressure switch point; highest accuracy demand), and ejector stroke (ejector pins; short range, fast cycle). The common duty of all four is: continuous machine vibration, oil temperature rising with continuous production, barrel radiant heat spreading toward the injection unit, and long-term oil and mould-release mist. The typical failure of a potentiometer-type electronic scale in this duty is a local jump after wear — the reading flickers repeatedly over a section of stroke, and the controller misreads it as a position step and emergency-stops.

Clamp stroke: the conversion chain from “mould closed” to clamping force

Clamping force of a toggle clamp comes from elastic elongation of the tie bars, and the elongation is set jointly by mould-height position and the clamp end position. Position feedback has two tasks on this chain: to give the absolute position of the clamp end point, and to give the speed-curve datum of the low-pressure mould-protection section. Low-pressure mould protection requires deceleration to be completed before mould contact; the deceleration-point decision depends on consistency of the position reading. If the feedback has random jitter of tens of micrometres, mould-protection sensitivity must be dulled or nuisance trips become frequent. Repeatability matters more here than resolution; Germanjet magnetostrictive displacement sensors can reach repeatability of the order of ±0.002 mm, enough to support a stable mould-protection criterion.

Shot-volume closed loop: screw-displacement repeatability sets metering stability

Shot volume = screw displacement × screw cross-sectional area. A large part of shot-to-shot part-weight scatter comes from repeatability of the metering end point and the V/P (hold-pressure) switch point. The switch-point criterion is generally screw position; every 0.05 mm of position-feedback error means a significant volume error on a small-diameter screw. The screw side should therefore use a 1–5 μm resolution step and confirm the non-linearity figure (typically <0.02% FS); the update rate must also match the injection-section control cycle, so that the controller does not switch on the previous cycle’s stale position. Matching is in Matching Response Time and Refresh Rate to the Control Cycle.

Selection comparison of the four measuring points

Mounting form: external or in-cylinder

Legacy retrofits and standard machines mostly use the external type: the sensor body is fixed on the machine frame and the magnet moves with the platen or screw carriage, with no need to open a cylinder. If a new-machine design wants a clean machine and to avoid external knocks, the sensor can be integrated inside the cylinder; then the pressure rating (in-cylinder types by series: Series 16/16R is 350 bar working / 530 bar peak, Series 17/17EX is 350/600 bar, Series 19 in-cylinder is 300/600 bar) and the minimum piston-rod bore (generally ≥12.7 mm) must be calculated as in Why Choose Magnetostrictive Sensing for Hydraulic Cylinder Position Control. Mobile or frequently washed machines can use Series 13 mobile hydraulics, whose IP69K protection and 25 g vibration / 100 g shock ratings suit continuous machine vibration; in-cylinder needs can refer to Series 16 cylinder-integrated. Retrofit differences of two-platen and all-electric machines are in Two-Platen and All-Electric Injection Machines: Position Measurement Solutions; positioning of take-out robots is in Positioning Injection Molding Take-Out Robots.

Wiring and signal form

Dedicated injection-molding controllers mostly take analog, 0–10 V or 4-20 mA both being common; on long runs and in strong electromagnetic environments prefer a 4-20 mA current loop. If the machine uses a fieldbus architecture, a CANopen (DSP406 profile) type can be used so that multiple axes share one bus, reducing wiring and providing diagnostics. In either case the signal cable must run in a separate tray from heater-band and servo-drive power cables, with the shield earthed at one end; practice is in Installing Magnetostrictive Displacement Sensors in Practice.