Shot size too small
- Short shot and incomplete fill in last regions
- Weak weld lines where pack is starved
- Unstable cushion and light cavities on multi cavity tools
Glossary
What Is Shot Size in Injection Molding? Part Weight, Runners, and Cushion Explained
Shot size is total melt per cycle, not net part weight. See the formula, three term definitions, and links to runners, gates, and cycle time.
Glossary
Shot size is melt delivered per cycle, not part weight alone
Short shots and flash both send buyers hunting the wrong root cause.
Shot size is the volume or weight of molten plastic the injection unit prepares and pushes forward in one cycle. It must cover every cavity, the runner system, the gate vestige allowance, and a stable cushion for packing.
A short shot is not always a tooling defect. Transfer position, shot size setting, and venting belong in the same troubleshooting thread.
On multi cavity tools, one cavity short while others fill usually points to runner balance, gate detail, or venting, not only machine shot capacity.
NetProto scopes shot size after part geometry, resin, runner type, and cavity count are fixed. A quoted part weight is not the same as a process shot size.
Process
Shot size on the production floor
The screw must deliver parts, runners, gates, and cushion every cycle.
- Part weight on a quote is not the same as process shot weight.
- Multi cavity imbalance shows up before you blame machine capacity.
- Runner type changes how much material each cycle consumes.
Melt delivery on a production injection press
Formula
How shot size adds up
Use geometry first, then convert to weight at process conditions.
Shot size = sum(part volume x cavities) + runner volume + sprue and gate volume + cushion.
Convert volume to weight with resin density at process temperature.
Machine rated shot capacity is the press limit. It is not the shot size you should set for every mold.
Terms
Part weight, shot weight, and shot capacity
Each number answers a different question on quotes versus the production floor.
| Term | Includes runner and system? | Used for |
|---|---|---|
| Part weight | No | Quoting and weighing the finished part |
| Shot weight | Yes | Process setup and material per cycle |
| Shot capacity | Machine maximum | Machine selection and barrel utilization |
Process
Too small versus too large shot size
Symptoms overlap with venting and tooling, but shot volume is often the first check.
Shot size too large
- Flash and overpack stress on shutoffs
- Longer melt residence time and degradation risk on heat sensitive grades
- Wasted material on runners and cushion
Machine selection
Sizing the press injection unit
Confirm with your molder. No fixed machine model or steel grade applies to every program.
Required shot is approximately total molded volume x cavities x density x a safety factor often in the 1.1 to 1.3 range for cushion and process headroom.
Choose a machine so required shot sits in a stable utilization band. Many plants start near roughly 30 to 80 percent of rated shot capacity, then tune for cushion stability.
Do not max out the barrel on every program. Residence time and mixing suffer when shot is tiny on a large screw.
Troubleshooting
RFQ and trial shot actions for short fill
Share this list with your molder when photos show incomplete fill or cavity imbalance.
Which cavity or region is short, with a photo if possible
Material lot and drying record for hygroscopic grades
Recent change to shot size, transfer position, or back pressure
Gate freeze off time relative to pack completion
Cushion position trend shot to shot on the same setup
Check ring and non return valve leakage check when cushion will not hold
Go deeper
Shot size and related terms
Cavity count, runners, gates, cycle time, and press size connect to fill and piece price.
Cycle time
Seconds per shot and resin per hour.
Cycle timeCavity count
Impressions per cycle multiply molded volume.
Cavity countRunner
Feed path volume is part of shot weight.
RunnerGate
Gate type and size affect balance and vestige.
GateShort shot defect
Fill troubleshooting when geometry is already qualified.
Short shot defectTonnage
Clamp rating pairs with shot capacity on the data sheet.
TonnageWall thickness (DFM)
Wall stack drives part volume in the shot sum.
Wall thickness (DFM)Sprue
Nozzle to runner volume in cold runner shots.
SprueVisual reference
Photos from molding and tooling programs
Images are selected from the NetProto Forge media library for this topic.
Cold runner parts and sprue on tray
Cold runner parts and sprue on tray
Short shot check on part drawing
Short shot check on part drawing
Multi cavity housings from steel mold
Multi cavity housings from steel mold
Runner layout and shot volume
Runner layout and shot volume
FAQ
Shot size questions
What is a good shot size versus machine capacity?
Many plants use roughly 30 to 80 percent of rated shot capacity as a starting check, then confirm cushion stability and fill balance on the actual mold.
Why does one cavity short in a multi cavity mold?
Usually runner balance, gate size or location, or venting, not a lack of machine shot capacity alone.
Can shot size be stated in grams and cubic centimeters?
Yes. Cubic centimeters come from part and feed path geometry. Grams require resin density at process temperature.
Does shot size equal part weight?
No. Part weight is only the molded component. Shot size includes the feed path and cushion.
Can shot size limit cavitation?
Yes. The injection unit must physically deliver the total molded volume each cycle.
Where do I learn runner and gate sizing rules?
Start with the design guide hub for gate location and runner design spokes.
Next step
Connect shot issues to DFM
Upload CAD when fill balance or short shots trace back to geometry, not just machine settings.