The Clamping Force Formula for Thin-Wall Parts
The fundamental clamping force equation is: F_clamp = A_projected x P_cavity x S_factor. Where F_clamp is the required clamping force in kN, A_projected is the total projected area of all cavities plus runners at the parting line in cm2, P_cavity is the average cavity pressure during the packing phase in MPa (or bar / 10), and S_factor is the safety factor (1.1-1.2 for thin-wall applications). For thin-wall parts, the cavity pressure is substantially higher than conventional molding due to the resistance of the narrow flow path. Typical cavity pressures for PP thin-wall packaging: 0.30-0.40mm wall yogurt cups 40-60 MPa, 0.40-0.50mm wall food containers 30-50 MPa, 0.50-0.70mm wall margarine tubs 25-40 MPa. These pressures are significantly higher than the 20-30 MPa typical for conventional parts with 1.5-3.0mm walls. The HWAMDA SPV5 series provides clamping forces from 2,700 kN (HMD 270M8-SPV) to 6,000 kN (HMD 600M8-SPV), covering the full range of thin-wall food packaging applications from small sauce cups to large containers.
Key Specs
- •Typical cavity pressures for PP thin-wall packaging: 0.30-0.40mm wall yogurt cups 40-60 MPa, 0.40-0.50mm wall food containers 30-50 MPa, 0.50-0.70mm wall margarine tubs 25-40 MPa.
- •These pressures are significantly higher than the 20-30 MPa typical for conventional parts with 1.5-3.0mm walls.
- •The HWAMDA SPV5 series provides clamping forces from 2,700 kN (HMD 270M8-SPV) to 6,000 kN (HMD 600M8-SPV), covering the full range of thin-wall food packaging applications from small sauce cups to large containers.
High-speed injection unit with linear guides
Calculating Projected Area for Common Container Shapes
The projected area is the shadow area of all cavities viewed from the clamping direction (along the mold opening axis). For round containers: A = pi x r2 x n_cavities. A 150ml yogurt cup with 85mm top diameter has a projected area of 56.7 cm2 per cavity. An 8-cavity mold: 56.7 x 8 = 453.6 cm2 total. For rectangular containers: A = L x W x n_cavities. A 500ml food container (170 x 120mm) has 204 cm2 per cavity. A 4-cavity mold: 204 x 4 = 816 cm2. Add the runner system projected area for cold runner molds (typically 10-20% of cavity area). Hot runner molds with valve gates have zero runner area at the parting line. For IML molds with side-entry robot access, the mold may be slightly larger than minimum, but the projected area remains based on cavity geometry only. Always use the maximum projected area (typically at the parting line, which corresponds to the largest cross-section of the container opening). Note that tapered container walls reduce the projected area below the parting line, but the calculation conservatively uses the parting line dimension.
Worked Example: 8-Cavity Yogurt Cup Mold
Product: 150ml yogurt cup, PP homopolymer MFI 60, 0.40mm wall, 85mm top diameter, 65mm bottom diameter, 70mm height. Step 1 - Projected area per cavity: A = pi x (42.5mm)2 = 5,674 mm2 = 56.74 cm2. Step 2 - Total projected area for 8 cavities: A_total = 56.74 x 8 = 453.9 cm2. No runner addition needed (hot runner with valve gate). Step 3 - Estimate cavity pressure: for 0.40mm wall PP with 85mm flow length and 368-442 mm/s injection speed, cavity pressure is approximately 45-55 MPa (from flow simulation or empirical data). Use 50 MPa as the working estimate. Step 4 - Calculate clamping force: F = 453.9 cm2 x 50 MPa x 1.15 (safety factor) / 10 = 2,609.9 kN. Divide by 10 to convert from cm2 x MPa to kN. Step 5 - Select HWAMDA machine: the HMD 270M8-SPV provides 2,700 kN, which is the minimum for this application. For production margin and future mold variations, the HMD 308M8-SPV at 3,080 kN or HMD 350M8-SPV at 3,500 kN provides better safety margin. HWAMDA recommends a 350-400 ton machine for 8-cavity yogurt cup production.
Key Specs
- •Product: 150ml yogurt cup, PP homopolymer MFI 60, 0.40mm wall, 85mm top diameter, 65mm bottom diameter, 70mm height.
- •Step 1 - Projected area per cavity: A = pi x (42.5mm)2 = 5,674 mm2 = 56.74 cm2.
- •Step 2 - Total projected area for 8 cavities: A_total = 56.74 x 8 = 453.9 cm2.
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Worked Example: 4-Cavity Food Container Mold
Product: 750ml rectangular food container, PP copolymer MFI 50, 0.55mm wall, 170 x 120mm opening, 60mm depth. Step 1 - Projected area per cavity: A = 170 x 120 = 20,400 mm2 = 204 cm2. Step 2 - Total projected area for 4 cavities: A_total = 204 x 4 = 816 cm2. Step 3 - Estimate cavity pressure: for 0.55mm wall PP with 170mm maximum flow length and 400-442 mm/s injection speed, cavity pressure is approximately 35-45 MPa. Use 40 MPa. Step 4 - Calculate clamping force: F = 816 x 40 x 1.15 / 10 = 3,753.6 kN. Step 5 - Select machine: the HMD 400M8-SPV at 4,000 kN provides the required force with adequate margin. For IML versions with slightly larger mold footprints, the HMD 418M8-SPV (4,180 kN) or HMD 428M8-SPV (4,280 kN) offer additional headroom. The platen size of the HMD 400M8-SPV (1,020 x 960mm with tie bar distance 680 x 630mm) must physically accommodate all 4 cavities. Verify that the mold outline fits within the tie bar spacing with minimum 25mm clearance on each side.
Common Mistakes in Clamping Force Calculation
Mistake 1: Using nominal injection pressure instead of cavity pressure. The HWAMDA HMD 400M8-SPV delivers 164-165 MPa injection pressure, but cavity pressure is only 30-60% of this due to pressure losses in the nozzle, hot runner, and gate. Using full injection pressure would over-specify the machine by 2-3x. Mistake 2: Ignoring cavity pressure variation with injection speed. Faster injection (increasing from 300 to 442 mm/s) raises cavity pressure by 15-25% due to increased shear in thin sections. Always calculate at the actual production injection speed, not a generic estimate. Mistake 3: Forgetting the cold runner projected area. Cold runner molds add 10-20% to the required clamping force. Hot runner molds eliminate this addition. Mistake 4: Using insufficient safety factor. Conventional molding uses 1.05-1.10 safety factor, but thin-wall applications require 1.10-1.20 because the narrow process window means small pressure spikes during packing can momentarily exceed calculated values. Mistake 5: Selecting a machine with exactly the calculated tonnage. Always choose one size up in the HWAMDA SPV5 range to provide margin for mold changes, material grade switches, and process optimization.
Key Specs
- •The HWAMDA HMD 400M8-SPV delivers 164-165 MPa injection pressure, but cavity pressure is only 30-60% of this due to pressure losses in the nozzle, hot runner, and gate.
- •Faster injection (increasing from 300 to 442 mm/s) raises cavity pressure by 15-25% due to increased shear in thin sections.
- •Cold runner molds add 10-20% to the required clamping force.
Toggle clamping unit — high rigidity for thin-wall molding
HWAMDA SPV5 Model Selection Guide by Application
Based on clamping force calculations and field experience, here are recommended HWAMDA SPV5 models by application. Sauce cups (2-4g, 30mm OD, 8-16 cavities): HMD 270M8-SPV (2,700 kN) for 8-cavity, HMD 350M8-SPV (3,500 kN) for 16-cavity. Yogurt cups (4-8g, 85mm OD, 4-8 cavities): HMD 308M8-SPV (3,080 kN) for 4-cavity, HMD 350M8-SPV to HMD 400M8-SPV (3,500-4,000 kN) for 8-cavity. Milk tea cups (8-12g, 95mm OD, 4-6 cavities): HMD 350M8-SPV (3,500 kN) for 4-cavity, HMD 418M8-SPV (4,180 kN) for 6-cavity. Food containers (15-25g, 170x120mm, 2-4 cavities): HMD 400M8-SPV (4,000 kN) for 2-cavity, HMD 480M8-SPV (4,800 kN) for 4-cavity. Margarine IML tubs (20-30g, 120mm OD, 2-4 cavities): HMD 400M8-SPV for 2-cavity, HMD 480M8-SPV to HMD 530M8-SPV (4,800-5,300 kN) for 4-cavity. Yogurt pails (30-50g, 140mm OD, 1-2 cavities): HMD 400M8-SPV for 1-cavity, HMD 530M8-SPV for 2-cavity. Contact HWAMDA engineering for calculation verification at sales@hwamdaglobal.com.
Frequently Asked Questions
For standard 150ml yogurt cups (85mm diameter, 0.40mm wall, PP) in an 8-cavity mold, calculated clamping force is approximately 2,500-3,000 kN. The recommended HWAMDA SPV5 model is the HMD 350M8-SPV (3,500 kN) or HMD 400M8-SPV (4,000 kN), providing 15-35% safety margin above calculated minimum. The extra margin accommodates pressure spikes during packing, material viscosity variations, and future mold modifications.
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