Jar Preform Mould: How Does It Decide Jar Gram Weight?

Pick up two seemingly identical PET jars—one feather-light, one with a reassuring heft—and you will feel the invisible hand of the Jar Preform Mould at work. Every 0.1 g it adds or subtracts from the preform ripples through freight bills, resin budgets, stacking strength and even consumer perception. Yet many brand owners still believe final jar weight is “fixed” by the blow-moulding machine. In reality, 90 % of the gram-weight destiny is cast the instant hot PET enters the injection cavity. Below, we decode the five controllable features inside a Jar Preform Mould that let processors dial in anything from a 25 g salad-dressing bottle to a 48 g mayo jar without ever touching the blower.
To begin with, cavity geometry is the heavyweight lever. By enlarging the preform’s outer diameter by 1 mm while keeping the neck finish constant, engineers increase sidewall thickness after blow-moulding by roughly 0.05 mm, adding 1.8 g to a 500 ml jar. Modern high-speed five-axis mills can interpolate this change into existing mould bases in under six hours, letting converters run A/B weight tests on the same injection press within a single shift. One European nut-butter brand saved 220 t of resin per year by accepting a 0.3 g lighter preform after Finite-Element-Analysis showed the thinner wall still survived a 1.2 m drop test.
Second, core-rod design micromanages material distribution. A rod milled with an 0.8 ° taper instead of a straight shaft pulls more PET into the jar’s shoulder, boosting top-load strength by 12 N without touching total weight. Conversely, a rod tip recessed 0.25 mm deeper into the gate area steals 0.5 g from the finish and re-deposits it in the base, eliminating the “rocker” defect on lightweight jars. Swiss mould-makers now laser-clad tungsten-carbide rings on high-wear zones, guaranteeing ±0.02 g shot-to-shot repeatability over 4 million cycles.

Third, valve-gate timing acts like a dimmer switch for resin flow. Delaying gate opening by 0.15 s short-packs the cavity by 1 g, ideal for down-gauging trials. The same actuator can be re-programmed to full stroke for good “heavy-base” jars during the next production lot, giving converters SKU flexibility without tool changes. A Californian co-packer uses this trick to alternate between 29 g and 34 g versions of the same 750 ml salsa jar on a 96-cavity wheel, trimming resin cost by 7 % on private-label orders while preserving the upscale feel of the branded version.
Fourth, integrated cavity pressure sensors close the feedback loop. Mounted 2 mm below the parting line, they graph real-time pvT (pressure-volume-temperature) curves and auto-adjust hold pressure to maintain a 280 bar cavity peak. If resin lot MFI drifts, the controller tweaks injection speed within 0.8 s, holding preform weight variance below ±0.15 g—one third of the PET industry standard. Over a year, that precision prevents 18 t of “giveaway” resin on a single 200-cavity production cell.
Fifth, neck-finish inserts play a surprising role. Switching from 33 mm standard thread to 38 mm deep-skirt steals 0.6 g of PET that would otherwise migrate into the jar body. One Asian coconut-water exporter leveraged this to hit an airline’s 20 g cabin-service limit without sacrificing bottle height, unlocking a six-million-unit contract.
Real-world math proves the mould’s dominance: a 0.5 g cut on a 1 billion-jar annual programme frees 500 t of resin, worth USD 700 000 at today’s PET prices. Achieving that cut on the blower side—by stretching the same preform thinner—would rupture the base or collapse the shoulder; doing it inside the Jar Preform Mould is risk-free.
So, next time you weigh two jars and wonder why one feels lighter, remember: the scale was tipped not in the blow-moulding aisle but inside the injection cavity, where micrometres of steel and milliseconds of timing quietly decide every gram you hold.