RIM Molding Parameters Calculator
Injection time, flow velocity and runner dimensions from the RIM machine size and gate geometry.
Inputs
Gate thickness, width, runner length and radii share one length unit. Do not mix units.
Results
Unrounded values
How this calculator works
In PDCPD reaction injection molding, the two liquid components are metered by the RIM machine at a fixed output and mixed in the head just before entering the mold. The shot rate therefore sets the injection time, and the gate and runner must be sized so the flow velocity stays low enough to avoid turbulence and trapped air.
This calculator takes the machine size (100, 150 or 200), the pump speed ratio and the part gross weight, and returns the actual shot rate, injection time, flow velocity through the gate and the recommended runner radii.
Formulas
- Nominal shot rate
machine size / 60- Actual shot rate
machine size × pump speed ratio / 60- Injection time
gross weight / actual shot rate- Length per segment
L / number of segments- Volumetric flow Q
gross weight / 1.03 / 1000 / injection time / 2- Flow velocity
gross weight / injection time / 1.03 / (t × L) × 1000- Reference r(round)
√√[2 × (8 × Q × 270 × segment / 3.1415926 × (6 × Q × 270 × segment / w × t × t × t))]- r(half) / R(round) / R(half)
r(half) = r(round) / 0.61; R(round) = 1.25 × r(round); R(half) = 1.025 × r(half)
Recommended flow velocity: wall 6–10 mm → ≤ 2 m/s; wall 3.2–4 mm → ≤ 4.5 m/s. No limit is given for other wall thicknesses.
Q is labelled m/s in the source sheet, but the formula describes a volumetric flow. Treat the unit as unconfirmed.
Worked example
A 10 kg part (all cavities) with 4 mm walls on a 150 machine running at 77.7% pump speed, with a 2 × 20 gate and 1000 total runner length split into 3 segments:
| RIM machine size | 150 |
|---|---|
| Part gross weight (all cavities) | 10 kg |
| Part wall thickness | 4 mm |
| Pump speed ratio | 0.777 ratio |
| Gate thickness t | 2 |
| Gate width w | 20 |
| Total runner length L (all cavities) | 1000 |
| Number of runner segments | 3 pcs |
| Actual shot rate | 1.943 kg/s |
|---|---|
| Injection time | 5.148 s |
| Nominal shot rate | 2.500 kg/s |
| Length per runner segment | 333.33 |
| Volumetric flow Q (per side) | 0.000942961 |
| Flow velocity | 0.943 m/s |
| Half-round runner radius r(half) | 28.238 |
| Round injection radius R(round) | 21.531 |
When to use it
- Choosing between a 100, 150 or 200 RIM machine for a new part
- Checking that gate flow velocity is within the limit for the part wall thickness
- Sizing half-round runners and the round injection throat for multi-cavity molds
Frequently asked questions
How is PDCPD injection time calculated?
Injection time equals the part gross weight divided by the actual shot rate. The actual shot rate is the machine size × pump speed ratio ÷ 60, in kg/s. A 150 machine at 0.777 delivers 1.943 kg/s, so a 10 kg shot takes about 5.15 s.
What flow velocity is allowed at the gate for PDCPD?
As a rule of thumb, keep flow velocity at or below 2 m/s for walls of 6–10 mm, and at or below 4.5 m/s for walls of 3.2–4 mm. The calculator flags the result against these limits.
What does the machine size number (100, 150, 200) mean?
In this formula the machine size divided by 60 gives the nominal shot rate in kg/s, so a 150 machine has a nominal output of 2.5 kg/s at full pump speed.
How are the runner radii related?
The half-round runner radius r(half) is the round radius r(round) ÷ 0.61, the round injection radius R(round) is 1.25 × r(round), and the half-round throat R(half) is 1.025 × r(half).
Related calculators
Results are estimates from empirical rules used in PDCPD mold development. Verify critical dimensions before cutting metal.