Part Weight Control in Injection Molding: From Measurement to Correction

Record the point at which a part is weighed. Comparing freshly demolded parts with samples measured after different storage or conditioning periods can introduce an avoidable difference into the data. Use a consistent procedure for setting the reference and checking production.

Establish the molding window before enabling correction

An automatic adjustment needs limits established from acceptable parts. Define the target weight alongside critical dimensions, appearance and functional requirements. Then test how those characteristics respond as the proposed control variable changes.

Holding time deserves attention before pressure becomes the corrective lever. RJG's gate-seal guidance describes weighing parts at different hold times to determine whether material remains in the cavity. It also explains how post-gate pressure sensors can reveal discharge where a scale lacks sufficient resolution for very small parts. This is a setup study, distinct from measuring weight during every production cycle.

A practical commissioning plan should establish:

• The allowed correction range, with evidence that its boundaries still produce acceptable parts.
• The response to an out-of-range measurement, an absent part or a failed scale reading.
• The number of consecutive deviations that triggers an alarm or stop.
• The procedure for restart after a material change, mold intervention or maintenance.

These are acceptance criteria for the proposed cell, not assumptions about the behavior of any particular controller. Ask the supplier to demonstrate them using the intended mold and material.

A constant weight does not certify a complete part

Weight records total mass. It does not show where that mass sits or whether every feature meets its drawing. A part can meet its weight limit and still have flash, warpage, a misplaced insert or a damaged sealing surface. Treat each critical requirement as a separate inspection problem until trials establish a useful correlation.

Kistler's cavity-pressure overview describes how pressure curves capture conditions during filling, compression and holding. Such measurements provide process information that a final weight cannot recover. Whether they are needed depends on the part, mold and quality plan; adding sensors without defining the decision they support only adds data. For another announced application, see FANUC's planned Fakuma 2026 pressure-monitoring and automation cells.

For a connector housing, weight might flag packing drift while a dimensional check verifies the mating interface. For an insert-molded electrical part, placement and continuity checks answer questions that weighing cannot. The injection molding editorial section covers related process and automation decisions.

Specify the evidence you want from a supplier

When comparing injection molding equipment and automation suppliers, request cycle-level records connecting weight, controller adjustments and rejection decisions. Include cavity identity where relevant, and retain independent measurements of the critical part characteristics used to approve the process.

For an RFQ, supply the part drawing, resin grade, cavity count, weight tolerance and functional inspection requirements. Ask for a trial covering normal production, startup and an agreed material variation. Judge the system by whether it detects deviations, corrects within the validated window and keeps rejected parts out of the accepted stream. A flat weight chart is useful evidence, but the delivered part still has to meet its specification.