Injection Molding Shot Size: Part Weight, Runner Weight, and Barrel Capacity

Add molded part mass across all cavities and the ejected runner and sprue to get nominal shot mass. Then convert the machine’s rated capacity to your resin, leave headroom, and compare the two. At 120 g per part, four cavities, and a 40 g runner, the nominal shot is 520 g.

What Is Shot Size in Injection Molding?

Shot size is the quantity of material the injection unit delivers per cycle. Shot volume is the space that melt occupies, and shot mass is what it weighs. Volume is what the screw physically meters. Mass is what the scale reads when you weigh the parts.

Part weight and total shot weight are different numbers. A 120 g part from a four-cavity cold runner tool does not ask the injection unit for 120 g. It asks for everything that leaves the nozzle in one cycle.

Machine specifications complicate the comparison because older ratings can state capacity in ounces of polystyrene. That is a reference resin, not your resin, and the figure has to be converted before it means anything for a polypropylene or nylon job.

How to Calculate Total Shot Weight

Nominal shot mass = Total molded part mass + Ejected runner and sprue mass

Part weight per cavity

Weigh a molded part if one exists. Otherwise, take CAD volume and multiply by the solid density of the resin grade at room temperature, since this is what the finished part will weigh.

Number of cavities

Multiply by the cavities that will run. A tool with one blocked cavity has a smaller shot until that cavity returns, which also changes barrel utilization.

Sprue and cold-runner contribution

A cold runner solidifies and ejects, so its full mass belongs in the shot every cycle. In a tool with a generous runner on small parts, this can be a large share of the total.

Hot runner systems and material remaining in the manifold

A hot runner complicates the picture in a useful way. Material in the manifold stays molten and is not ejected, so its mass is not part of the shot weight. Each cycle pushes new melt in and moves previous melt forward, so that inventory still affects how long material stays hot.

Capacity headroom is a separate line

A molded part’s weight already includes the material packed into it, so packing is not a further addition. Screw cushion is material retained ahead of the screw, and it is not ejected with every shot. Machine-capacity headroom and cushion requirements should therefore be evaluated separately from nominal shot mass. There is no universal figure for that headroom, so write it as its own named line instead of blending it into a rounded total.

Converting Material Volume to Weight

Resin density changes with temperature. Solid density at room temperature is what you use to get part mass from CAD volume. Melt density is lower, and it governs how much mass a given screw stroke delivers. A conversion that mixes the two will quietly misstate the available capacity.

State the conditions you used. A grade sheet can show solid density alone. BASF’s Ultramid B3E2G3 processing data sheet lists 1,230 kg/m³ tested to ISO 1183, a value for that one grade and no other. If you need melt density and do not have it, say so in the calculation instead of substituting a solid value without comment.

Comparing Shot Requirement With Machine Capacity

Compare against the manufacturer’s injection unit specifications, which can include these:

• Screw diameter, which determines the screw’s cross-sectional area, proportional to the square of the diameter
• Metering stroke, which sets the usable length
• Theoretical injection volume, the cross-sectional area multiplied by the stroke: V = (πD²/4) × S, where D is screw diameter and S is stroke length
• Plasticizing capacity, a rate that determines whether the screw can prepare the shot in the time available

Confirm the usable stroke and cushion requirements against the machine manufacturer’s specifications.

Machine size and injection unit size are not the same thing. Presses of the same tonnage can be offered with different screw and barrel options. The clamp calculation answers a separate question, and how to calculate clamp force from projected area covers it.

Why Barrel Utilization Matters

Barrel utilization is shot size as a share of barrel capacity. RJG, an injection molding training and process technology company, says in its Right Mold, Right Machine article that it recommends running between 20 and 80 percent of barrel capacity, ideally 40 to 60 percent. That is one organization’s recommendation, written with medical molding in view, and not a universal limit.

Very small shots and residence time

Residence time is how long a given piece of material stays heated before it is injected. With a small shot in a large barrel, several cycles pass before material reaches the nozzle. RJG links low utilization to longer residence time and a higher risk of degradation, which it says can show up as discoloration or black specks in polycarbonate, acrylic, and polyester.

Very large shots

Near the top of the range, the problem reverses. RJG warns that running above 80 percent may not allow enough time to melt the polymer adequately, and cycle time may become “recovery limited,” meaning the screw sets the pace instead of cooling.

Melt temperature and resin-specific windows

Limits on residence time depend on the resin and its temperature. BASF’s data sheet gives that one Ultramid grade a maximum residence time of 10 minutes at a melt range of 260 to 280 °C. That figure belongs to that grade, so it should not be applied to another polymer or even another nylon grade. Check the processing sheet for the exact grade you will run.

Worked Example

All values here are hypothetical. Capacity figures do not describe any real machine, and the density values are assumptions for this arithmetic.

Input	Example value
Part weight	120 g
Cavity count	4
Molded parts	480 g
Runner and sprue	40 g
Total nominal shot	520 g

Assume two presses rated in polystyrene ounces, with 1 oz at 28.35 g. The density ratio decides how much polypropylene that capacity holds, and the choice of ratio changes the answer. The values below are assumptions for this arithmetic: solid densities of 1.05 and 0.90 g/cm³, and melt densities of 0.945 and 0.74 g/cm³. The last column applies an assumed 10 percent capacity headroom to the 520 g shot, giving a 572 g sizing figure. That figure is not mass injected each cycle. A real rating rests on whatever basis its builder defined, so the 666 g and 1,332 g results are illustrations, not machine capacities.

Machine	Rating	PS capacity	PP capacity, solid ratio	PP capacity, melt ratio	Use at 520 g, melt	Use at 572 g sizing figure, melt
A	30 oz PS	850.5 g	729 g	666 g	78 percent	86 percent
B	60 oz PS	1,701 g	1,458 g	1,332 g	39 percent	43 percent

The solid ratio is the easier step, 850.5 × (0.90 / 1.05) = 729 g, and it errs in the unsafe direction here. Because PP’s melt density falls further below its solid density than this assumed PS pair does, the melt ratio gives 850.5 × (0.74 / 0.945) = 666 g. A solid-density result makes a barrel look roomier than it is, so use the melt ratio or, better, the manufacturer’s volumetric data.

The conclusion moves with it. On a solid ratio with no headroom, machine A sits at 71 percent and looks comfortable. On a melt ratio, its actual 520 g shot sits at 78 percent, just inside RJG’s 20 to 80 range, and the 572 g sizing figure reaches 86 percent, outside it. Machine B sits at 39 percent on the actual shot, just under the 40 to 60 band that RJG prefers, and at 43 percent on the sizing figure. Machine A leaves little room for recovery. Machine B holds material longer.

A rough residence estimate shows that difference. Dividing capacity by shot gives the shots held in front of the screw, and multiplying by an assumed 40-second cycle gives a time:

• Machine A: 666 / 520 = 1.28 shots, about 51 seconds
• Machine B: 1,332 / 520 = 2.56 shots, about 102 seconds

That estimate runs low. It counts only metering capacity and leaves out the plastic conveyed along the screw flights behind it, which Rosti, a molder, also counts in its residence time guide. Treat these figures as a comparison between machines, not as a number to hold against a grade’s limit. For a heat-sensitive resin close to its limit, get the screw and barrel inventory from the machine builder or run a color-change trial on the press. Cycle time itself has its own logic, explained in how the fill, pack, cool, and eject stages add up.

Final Injection Unit Compatibility Checklist

Check sheet: tick each point as you work through it

• Material: exact grade, density basis stated, and any drying or thermal limits noted
• Shot weight: parts and runner, with capacity headroom on its own line
• Residence time: compared with the grade’s published limit, using screw and barrel inventory rather than metering capacity alone
• Recovery capability: plasticizing rate versus the time available in the cycle
• Injection pressure and flow capability: enough for this flow length and fill time
• Shot consistency: enough stroke and cushion to repeat from cycle to cycle

Shot Size FAQ

Why might machine capacity be listed in ounces of polystyrene?

Polystyrene served as a reference resin, which gave buyers one comparable number across machines. The rating describes a volume expressed as polystyrene weight. Because density differs by resin, the same barrel delivers a different weight of polypropylene or glass-filled nylon, and metric specifications may state volume directly in cubic centimeters.

How to Decide Between Two Candidate Injection Units

Rank candidates by utilization on a melt-density or volumetric basis with headroom stated, then test the survivors against the grade’s residence limit and against recovery time within your cycle. A press in the 40 to 60 percent band is RJG’s stated preference, but a press at 75 percent can still be the right choice for a heat-sensitive resin if recovery fits the cycle. A press at 35 percent can be fine for a stable resin on a fast cycle. If no unit clears the residence limit and recovery together, the answer may be a different screw size rather than a different press. Confirm with the machine builder’s volumetric data before committing the job.