Last updated: 2026-09-04 · 5 min read
Why Weigh Parts on the Floor? — Shot Weight as the Fastest Early-Warning Gauge in Molding
Part weight is the most sensitive process-health gauge an injection molding shop owns, and it is nearly free: every gram in a molded part was metered, plasticated, injected, packed and held under the machine’s settings, so any drift in that chain — a softening check ring, a changing regrind ratio, a gate freezing a fraction earlier — shows up as grams before it shows up as millimeters.
Key Facts
<>
Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.
What Do Weight Changes Diagnose?
Five signatures, each pointing at a different cause:
| Process Change | Weight Signature | Downstream Risk |
|---|---|---|
| Holding pressure drop | Steady weight decline | Sink marks; undersize |
| Earlier gate freeze | Gradual decline, cavity-specific | Sink near gate; voids |
| Viscosity shift (moisture, additives) | Weight wanders shot to shot | Fill inconsistency; flash or shorts |
| Regrind ratio increase | Weight creeps up | Oversize; flash risk |
| Check-ring wear | Growing shot-to-shot scatter | Whole-process inconsistency |
The signatures differ, and the signature is the diagnosis — this is the skill worth training on the floor. A steady decline across all cavities reads as a packing-side cause: holding pressure or holding time has eroded, or the non-return valve is letting pressure decay. A decline isolated to one cavity is the gate’s story — that gate is freezing earlier than its siblings, from a cooler mold zone or a gate land that has worn or partly plugged.
Wandering weight, shot to shot, with no trend, is a viscosity story: moisture in a hygroscopic resin, a drifting melt temperature, or a regrind blend whose flow changes by the batch. Upward creep is the regrind row’s signature — lower effective viscosity packs more into the cavity — and it arrives with flash risk at the parting line.
A worn check ring widens the spread rather than moving the average, and rising standard deviation on the weight chart is often the first objective evidence that a seal needs replacement.
How Do You Run Weight as a Control Metric?
Four steps, in order:
- Baseline every cavity at process acceptance. The chart is worthless without its reference, so the baseline is set where the process is qualified: at trial acceptance, weigh a defined sample from each cavity — to the precision the part demands, commonly 0.01 g or finer — and record the mean and the spread per cavity. These become the target lines on the X̄ chart and the expected range on the R chart. For multi-cavity tools, the per-cavity baseline is the whole point — a single number for a four-cavity mold hides the imbalance that per-cavity baselines expose.
- Sample on a fixed plan and chart both average and spread. Discipline beats frequency: five consecutive shots weighed every thirty to sixty minutes — each cavity weighed separately, recorded to the chart — detects drift far better than occasional spot checks, because trends live in consecutive data. Plot the mean (X̄) and the range (R) per cavity, with the baseline-derived control limits drawn in. The X̄ chart answers “is the process centered where it was accepted?” and the R chart answers “is the process as repeatable as it was?”
- React to pre-agreed rules, not to feelings. The value of charting collapses if every wiggle triggers tinkering — the most destructive habit in molding is adjusting the process against noise. Agree the reaction rules in advance: a point outside control limits, a run of consecutive points on one side of the baseline, or an R-chart widening beyond its limit each trigger a defined response — first investigate-and-log, then act. The investigation reads the table above: which signature, which candidate cause, check the parameter log before touching anything.
- Wire weight into every documented change. Whenever anything changes — a parameter correction, a new resin lot, a regrind ratio adjustment, a maintenance intervention — the weight chart is the before-and-after evidence that the change landed as intended and the process returned to its accepted envelope. That habit closes the loop between the change log and the quality system: the chart shows the step change or its absence, and the record shows who changed what and why.
Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.
Weight does not tell you which cause is acting — but it tells you to look, and where, hours or days before the first visible defect. The technique costs a calibrated balance, a sampling discipline and thirty minutes of setup per tool.
Frequently Asked Questions
How accurate does the balance need to be?
Accurate enough that the readings sit far below the process variation you are trying to detect — the working rule is balance resolution around 0.1% of part weight or better, with practical floor choices being 0.01 g for most molded parts and 0.001 g for small precision components in the gram range.
The balance also needs repeatability and calibration discipline: a floor scale drifting with temperature defeats the whole exercise. One more often-missed requirement is conditioning consistency — parts weighed hot weigh differently than parts weighed at ambient because they hold moisture and are still shrinking, so the procedure fixes a cool-down interval before weighing.
How often should parts be weighed?
Densely at the start, sparsely but regularly in the run. At startup and after any change, weigh consecutive shots until the chart stabilizes — that is when you learn whether the process has settled onto its baseline. In steady production, a fixed sampling plan such as five shots every thirty to sixty minutes per cavity is the common working rhythm, tightened for new tools, new materials, or after any alarm.
The interval is a risk decision, not a ritual: a high-cavitation tool with known sensitivity, a moisture-sensitive resin, or a customer with tight weight specification justifies shorter intervals. What matters more than the exact number is that the plan is written, followed identically across shifts, and that the data lands on the chart the same hour it is collected.
Can weight monitoring replace dimensional checks?
No — it complements them, and understanding the division of labor is what makes both effective. Weight is the high-frequency, non-destructive, whole-part integral: it reacts to everything and localizes nothing, telling you the process changed but not which dimension moved. Dimensional inspection is the low-frequency, high-resolution map: it tells you exactly where the part stands against the drawing, but sampling it densely enough to catch fast drift would be prohibitively slow.
The two together form a rational system — weight charts run continuously as the tripwire, and when weight signals a change, dimensional checks concentrate on the features the change would plausibly move. Shops that try to run on weight alone fly blind on geometry; shops that run on dimensional audits alone discover drift after the parts are made.
Key Takeaways
<>
Over a tool’s life this produces the quiet dataset that separates mature shops from struggling ones — a weight history aligned with every intervention, capable of answering, years later, exactly when a drift began and what preceded it.
The return is catching process decay while it is still a cause and not yet a pallet of suspect parts.
Sources: