Solid density for the weight, melt density for the stroke
A part with 21.5 cm3 of solid in it, moulded in a resin whose datasheet says 1.05 g/cm3, weighs 22.6 grams when it is cold. Four of them plus a 12 gram runner is a 102.3 gram shot, and that is what the shot pot weighs on a bench scale. Nothing about melt density enters that number.
The screw sees it differently. It has to push 102.3 grams of melt, and melt at 0.98 g/cm3 takes 104.4 cm3 of stroke rather than the 97.4 cm3 the same material occupies cold. Seven cubic centimetres does not sound like much until it is expressed the other way: the cold volume understates the stroke by 6.7 percent, always in the direction that makes the machine look roomier than it is.
What an ounce rating actually is
Machines are commonly rated in ounces, and the ounce is not an ounce of your resin. It is a mass of some reference resin the builder chose, and the only thing the number really fixes is a swept volume — screw area times available stroke. To get back to that volume you divide the rating by the melt density the rating was taken at, which is why this page asks for it instead of burying a constant.
An 8 ounce rating at 0.94 g/cm3 is 241.3 cm3 of stroke. Fill that with a resin at 0.98 g/cm3 and the same barrel holds 236.4 grams, or 8.34 ounces of what you are running. Change the assumed rating density from 0.94 to 1.04 and the swept volume drops to 218.1 cm3, so the barrel share of the same shot jumps by four and a half points. That is the whole reason the field exists.
Shots in the barrel
The last line divides the mass sitting in the barrel by the mass leaving it each cycle. At the values in the form it comes to 2.31 shots. That number is where residence time starts: multiply it by the cycle time and you have roughly how long a given pellet spends hot before it becomes a part. Small shots in a big barrel push it up, which is a different problem from a big shot in a small one and is not visible in a percentage on its own.
The page prints the figure and stops. Whether the residence that follows from it is acceptable for a particular resin is a question for the resin datasheet and the person setting the machine.
Where the arithmetic is thin
It assumes the shot is exactly the parts plus the runner. Real shots are set with a cushion in front of the screw that never leaves, and the stroke is set past the volume of the shot rather than to it, so the barrel share the machine is set to will read above what this page prints. It also assumes one melt density across the whole shot, when melt density moves with temperature and with pressure during pack. And it takes the CAD volume as the part volume, which ignores the shrinkage between the cavity and the cold part — that error runs the other way and is worth a percent or two on a semi-crystalline resin.
The way to settle all three at once is to weigh a full shot off the machine, sprue and runner together, and put that number in weight mode. After that the page is describing your press rather than a model of one.
Questions people ask
Is shot weight worked out from melt density or solid density?
Solid density, because the shot weight is the mass of the parts and the runner after they have frozen and that is what a scale reads. Melt density comes in one step later, when that mass is turned into the barrel stroke needed to deliver it. Both are on the form for exactly this reason, and using melt density for the weight understates the shot by the ratio between the two, which is commonly six or seven percent.
How do I convert a machine rated in ounces to my resin?
Divide the rating by the melt density it was taken at to get the swept volume of the barrel, then multiply that volume by the melt density of the resin you are running. An 8 ounce rating at 0.94 g/cm3 is 241.3 cm3 of stroke, which holds 236.4 grams of a resin at 0.98 g/cm3. The rating density is a field on the form because it belongs to the machine, not to the calculator, and the data plate or the builder is where it comes from.
What percentage of barrel capacity should a shot use?
This page does not answer that and will not. Moulders work to a band for reasons that depend on the resin, the screw geometry, how long the material tolerates being hot and how repeatable the job has to be, and none of that is visible in a volume ratio. What the page gives you is the percentage itself, plus the number of shots the barrel is holding, so you can take real figures to whoever is setting the machine.
Why is the melt volume larger than the solid volume?
Because polymer expands when it melts. The mass is the same going in and coming out, so the volume has to grow as the density falls. At 1.05 solid and 0.98 melt the growth is about seven percent, and that seven percent is barrel stroke you have to have. It is also why the same shot looks smaller than it is if you work the barrel share off a CAD volume.
Does the runner count as part of the shot?
Yes. The barrel has to deliver it whether or not you keep it, and it goes into the residence figure and the barrel share the same as the parts do. On the default job the runner is 11.7 percent of the shot, which is material bought and heated for every cycle. Whether it comes back as regrind is a separate question and it does not change the stroke.