Shot Weight Control in Extrusion Blow Molding

Extrusion blow molding shot weight is the cheapest measurement available to a production team and the one most often skipped. It takes a calibrated scale, a labelled container and about ninety seconds per check, yet it detects screw wear, heater failure, moisture variation, regrind ratio drift and die-head temperature problems before any of them become a customer complaint. If you can only trend one number on an extrusion blow molding line, trend this one.

The reason it works so well is that shot weight is a total. It integrates every variable upstream of the mould — melt temperature, melt pressure, screw speed, die gap, parison swell, cycle timing — into a single figure that can be written on a board. When it moves, something upstream moved, and the direction of the move usually narrows the suspect list immediately.

Sailwin has delivered extrusion blow molding machines into 500+ installations across 60+ countries over 15+ years, manufacturing under ISO 9001:2015 with CE marking. Machines are FAT tested at full load before shipment, installation and commissioning on site takes 3–7 days, common wear parts ship within 48 hours, remote support runs 7×24, and the machine warranty is 2 years. This guide covers what shot weight physically is, how to measure it so the number is comparable shift to shift, what each drift pattern indicates, and how to run it as a control chart rather than an occasional spot check.

1. What Shot Weight Actually Measures

A shot is everything the machine extrudes for one moulding cycle. On a single-head machine that is one parison; on a multi-head machine it is the combined weight of every parison produced in that cycle. Shot weight is therefore the mass of molten polymer delivered in one extrusion interval, before trimming and before any flash is removed.

Two different numbers are commonly quoted and they must not be confused. Shot weight is the delivered mass; finished part weight is what remains after trimming, deflashing and any post-operations. The gap between them is scrap, and if you track only the finished part you are blind to a growing flash problem or a trimming change. Track shot weight as the process indicator and finished part weight as the quality and material-cost indicator.

Because shot weight is mass rather than dimension, it behaves differently from wall thickness measurements. A wall thickness reading at one point on one container tells you about that point. Shot weight tells you about the material actually delivered to the machine, which is why it responds to upstream faults so reliably and why it is the natural first number to check when a customer reports a weight or performance complaint.

2. How Shot Weight Relates to Wall Thickness and Part Weight

Shot weight, wall thickness and part weight are linked, but not by a single ratio. The relationship depends on how the parison is stretched, where the die gap is set and what the parison programmer is doing. The table below sets out how each move propagates through the process, which is the reasoning behind every correction you will make.

One consequence deserves emphasis: because several adjustments move shot weight in the same direction, weight alone cannot tell you which one changed. That is why shot weight is a detector rather than a diagnosis. It tells you that something moved and roughly when; the trend shape and the other readings on the machine tell you what.

3. Measuring Shot Weight So the Numbers Are Comparable

The measurement is simple, which is exactly why it is easy to do inconsistently. Four rules make readings comparable across shifts and machines.

• Weigh at the same point in the part’s life. A part weighed hot and a part weighed after it has cooled to room temperature do not have the same mass. Choose one convention — conditioned parts at ambient temperature is the practical choice — and put it on the process sheet.
• Define whether you weigh the shot or the trimmed part. Both are useful; mixing them is not. If the trimming operation changes, a shot weight series stays valid while a trimmed part series jumps for a reason that has nothing to do with the machine.
• Use the same scale, and calibrate it. A dedicated scale kept on the line, checked against a known mass weekly, removes the largest source of false drift. A scale that drifts upward produces a trend that looks exactly like screw wear.
• Weigh a defined sample, not one part. Three to five consecutive parts from the same cycle condition, averaged, absorbs normal cycle-to-cycle scatter. A single part reading will always look like drift because it includes the noise.
• Record the conditions with the number. Screw speed, melt pressure, melt temperature, die-head temperature and regrind percentage belong in the same line of the log. A weight without its conditions cannot be compared with anything.
• On multi-head machines, weigh heads separately. A single average over four heads will hide one head drifting badly. Per-head readings are what make the pattern diagnostic.

With those rules in place, shot weight becomes a genuinely comparable series. On a typical line a reading once per shift is enough for stability monitoring; during a grade change, a mould change or the first week after a new screw is fitted, readings every two hours repay the time.

4. Reading Drift: What Each Pattern Means

The shape of the trend is more informative than its absolute value. A gradual change, a step change and a repeating cycle each point at a different part of the machine.

Use action limits, not just targets. Write three numbers on the process sheet: the target shot weight, the point at which the process gets a second look, and the point at which it stops. Limits derived from your own historical scatter are more useful than any figure copied from another plant, because they reflect the repeatability your machine actually achieves.

When the second limit is crossed, the correct response is to check the other readings before adjusting anything. Adjusting the die to chase a weight that moved because a heater zone failed will simply move the defect somewhere else.

5. Separate Machine Drift from Material Drift

A reliable weight trend cannot distinguish between a machine that changed and a material that changed. Both look the same on the chart, and the remedies are completely different. Three additional readings separate them in a few minutes.

• Melt pressure at the head. Stable pressure with a changing weight points at the die, the head or the downstream cycle. A rising pressure with stable screw speed points at the material or at screw and barrel wear.
• Melt temperature. Grades with different flow behaviour, and material that has absorbed moisture, alter melt viscosity. Measure rather than assume that the displayed zone temperature equals the melt temperature.
• Bulk density and regrind share. Rerun a known-good virgin sample with the same settings. If the weight returns to target, the machine is fine and the material supply is the variable.
• Cycle time. A cycle that has crept shorter passes material through faster. Confirm the extrusion window on the machine controls rather than the label on the process sheet.
• Multi-layer and recycled content. Grades with barrier layers or higher recycled content can shift weight at unchanged conditions, because the layers do not flow identically. Accept that some drift is material-driven and re-baseline after a supply change instead of hunting a machine fault.

6. Where Shot Weight Sits on a Sailwin Machine

Monitoring works best when the machine can hold steady conditions and expose them. These are Sailwin’s confirmed specifications for extrusion blow molding machines.

• Visible parameters. The PLC monitors 40+ parameters in real time, so screw speed, melt pressure, zone temperatures and cycle data can be logged next to the weight reading instead of guessed at.
• Stable thermal control. PID control holds ±1 °C on heating zones, which is the condition that makes a weight trend readable. Thermal oscillation is one of the most common causes of scatter that looks like mechanical wear.
• Energy behaviour that changes with load. Servo drives cut energy consumption by up to 30%, and high-pressure exhaust recovery reduces compressor load by roughly 20%. Both mean the machine’s energy signature can be logged alongside weight as a secondary drift indicator.
• Range across container sizes. Machines cover 0.5 L to 1,000 L. Examples include the SW-2S1L at up to 0.5 L running 950 × 2 per hour, the SW-S30L at up to 30 L running 600 per hour, the SW-S160L at 300 per hour, the SW-S1000L at up to 1,000 L running 250 per hour, and all-electric models in the SW-60/70/80/90 range. Shot weight scales with container size, so the tolerance band must be set per mould rather than globally.
• Materials. PE, PP, ABS, EVA, PC and PA, plus multi-layer PE+PA+EVOH structures. Different grades swell and flow differently, so a material change should trigger a re-baseline of the weight target, not an attempt to keep the old number.
• Setup and support. Mould changes take under 30 minutes, machines are FAT tested at full load before shipment, installation and commissioning takes 3–7 days, common wear parts ship within 48 hours, remote support is available 7×24, and the machine warranty is 2 years.

7. Case Study: Locating an Intermittent Weight Shift

A manufacturer of industrial containers was rejecting parts for thin walls on one shift out of three, on a two-head machine producing containers in the 20–30 L range. Part weight had never been recorded, so the assumption was that the mould was at fault.

Composite scenario from blow molding line process support work, with no customer-identifying detail. Weight targets, tolerances and control limits are established per mould and per material from the plant’s own data.

8. Frequently Asked Questions About Shot Weight