Shot Weight and Casting Volume: Selecting the Right Injection Unit
Clamping force is usually the first figure people compare, but a die casting machine also has to deliver the right quantity of metal at the right pressure on every shot. Shot weight, plunger diameter, shot sleeve volume and specific casting pressure are closely linked. Understanding these relationships helps to judge whether an injection unit really fits a die and part.
How Do You Check Whether the Injection Unit Is Big Enough?
Add up the complete shot: all castings plus runners, gates, overflows and biscuit. Convert this mass into a metal volume using the density of the liquid alloy. Then compare it with the usable shot sleeve volume for the planned plunger diameter and check the resulting fill ratio and the specific casting pressure the shot end can generate with that diameter.
A plunger that is too large lowers the achievable casting pressure; one that is too small may not hold enough metal. The right choice is a balance between both, confirmed by machine data and the die design.
What Belongs to the Shot Weight?
The shot weight is the entire mass of metal injected in one cycle, not only the weight of the finished casting. In practice, the gating system and overflows can represent a substantial share of the shot, especially for thin-walled parts or multi-cavity dies.
| Component | Function | Note |
|---|---|---|
| Castings | The actual parts, one or more per shot. | Use the gross weight before machining. |
| Runners and gates | Guide the metal from the sleeve to the cavity. | Defined by the die design. |
| Overflows and vents | Collect cold, oxidised metal and help vent the cavity. | Often underestimated in early calculations. |
| Biscuit | Remaining metal in front of the plunger that transmits pressure. | Its thickness varies with dosing accuracy. |
Hot chamber machines have no biscuit; the shot is fed through the gooseneck and nozzle, and the sprue takes its place in the calculation.
Converting Shot Weight into Metal Volume
The shot sleeve holds a volume, not a mass. The shot weight must therefore be divided by the density of the alloy in its liquid state, which is lower than the density of the solid casting. For aluminium alloys, liquid density near casting temperature is roughly 2.3 to 2.4 g/cm³; the exact value should be taken from alloy data.
Vmetal = mshot / ρliquid
Vmetal in cm³, mshot in g, ρliquid in g/cm³. Assumption: the full shot including biscuit is considered and the metal is fully liquid when poured.
Machine data sheets often state a maximum shot weight for each plunger diameter. Check which alloy and which reference conditions these values are based on before comparing them with your own calculation.
Fill Ratio: How Full Is the Shot Sleeve?
The fill ratio describes which share of the usable shot sleeve volume is occupied by metal after pouring. It strongly influences how much air is present in the sleeve and how the metal wave behaves during the slow first phase of the shot.
Fill ratio = Vmetal / (Aplunger × Leff)
Aplunger = π × d² / 4 in cm² (d = plunger diameter in cm); Leff = usable sleeve length in cm between plunger face at start position and the die-side end of the sleeve.
A low fill ratio leaves a large air volume above the metal, which makes wave formation and air entrapment more likely and requires a carefully tuned slow shot. A very high fill ratio reduces the air volume but leaves less margin for pouring and can increase the risk of metal spilling from the pour opening. Suitable targets depend on sleeve length, slow-shot profile and the part, and should be validated in the process. How the subsequent filling phases affect quality is explained in our article on mold filling and vacuum.
Specific Casting Pressure and Plunger Diameter
The shot cylinder delivers a force. The pressure acting on the metal results from this force divided by the plunger area. With the same shot force, a smaller plunger therefore produces a higher specific pressure, but holds less metal.
p [bar] = 100 × Fshot [kN] / Aplunger [cm²]
Since 1 kN/cm² equals 100 bar. Losses from friction and dynamic effects are neglected in this simplified relationship.
The specific pressure also determines the clamping force requirement, because the metal pressure acts on the projected area of the casting and gating system. Shot end and clamping unit must therefore always be assessed together.
A Simplified Calculation
The following figures are hypothetical and only illustrate the method. Assumed: an aluminium shot of 3.5 kg (castings, gating, overflows and biscuit), liquid density 2.4 g/cm³, usable sleeve length 50 cm and an intensification force of 400 kN.
| Plunger Ø | Plunger Area | Sleeve Volume | Fill Ratio | Specific Pressure |
|---|---|---|---|---|
| 70 mm | 38.5 cm² | 1,924 cm³ | approx. 76 % | approx. 1,040 bar |
| 80 mm | 50.3 cm² | 2,513 cm³ | approx. 58 % | approx. 800 bar |
Metal volume in both cases: 3,500 g / 2.4 g/cm³ ≈ 1,458 cm³. The comparison shows the trade-off: the smaller plunger gives more pressure and a fuller sleeve, the larger one leaves more volume reserve at lower pressure. Which option suits the part depends on its quality requirements and the die design.
Checklist for Evaluating an Injection Unit
- Complete shot weight determined, including overflows and biscuit
- Liquid alloy density taken from alloy data, not solid density
- Available plunger diameters and shot sleeves for the machine identified
- Usable sleeve length measured or taken from documentation
- Fill ratio calculated for each candidate plunger diameter
- Shot force and resulting specific pressure checked against requirement
- Clamping force verified for the resulting pressure and projected area
- Dosing accuracy of ladle or furnace sufficient for consistent biscuit thickness
- Condition of shot sleeve, plunger and shot cylinder inspected on used machines
Consistent dosing is a prerequisite for repeatable fill ratios. The differences between dosing, melting and holding furnaces are covered in our article Dosing, Melting or Holding Furnace?
Frequently Asked Questions
Can the plunger diameter be changed on an existing machine?
Often yes, within the range the manufacturer provides. Shot sleeve, plunger, sleeve mounting and possibly the die interface must match. Check which sizes are approved for the specific machine.
Why is liquid density used instead of solid density?
The sleeve is filled with liquid metal, which takes up more volume than the same mass in solid form. Using solid density would underestimate the volume required.
Is a larger injection unit always the safer choice?
Not necessarily. An oversized unit can force a low fill ratio or a larger plunger with lower specific pressure. The aim is a suitable match, not the maximum figure.
FISS Helps Compare Shot End Data with Your Part.
Send us the shot weight, alloy and die dimensions of your part. FISS can check which used cold chamber die casting machines offer a suitable combination of shot capacity, plunger sizes and clamping force.
