The average pressure acting on the cavity perpendicular to the parting line will define the amount of clamp force required to hold the mold shut and inhibit flashing.
Cavity pressures acting on the sides of the cavity are contained by the stiffness of the mould walls.
The average pressure across a mold cavity depends on a number of factors. Two of the most important factors are the material type and melt viscosity of the plastic. The other major factor is the thickness to flow length ratio (L/T ratio) of the cavity. This ratio defines the maximum flow length from the gate to the end of the part divided by the average thickness.
Cavity pressure is usually measured in pounds per square inch (psi, or lbs/in²). The cavity pressure for an average molding job tends to be around 6,000 psi. It is easier to calculate the required clamp pressure if the units are in tons/in²
To convert psi to tons/in², simply divide by 2,000 pounds per ton.
6,000 lbs/in² / 2000 lbs/ton = 3 tons/in²
Materials that are hard flowing (high viscosity) often require 4 to 6 tons/in². this means the average cavity pressure is in the range of 4 to 6 x 2000 lbs/ton = 8,000 to 12,000 psi. these inherently hard-flowing materials include plastics such as polycarbonate, polysulfone and ployether imide.
Specific clamp tonnage values for various grades of plastic can be obtained from the material supplier.
A good starting point in working out the clamping force of a mould is determined by multiplying the projected area of the parts by 4.5kN/cm²
One does not need to know the depth or sides of the parts to do this.
The average mold requires a force equal to 4.5kN/cm² (3 tons/in²) of projected area to contain the plastic pressure.
Area of Part #1 – 25 cm x 25 cm = 625 cm²
Area of Part #2 – 25 cm x 25 cm = 625 cm²
Area of Runner – 25 cm x 0.6 cm = 15 cm²
Projected Area of Mold = 1265 cm² or 196.075 in²
1265 cm² x 4.5 kN/cm² = 5700 kN approx clamping force.
196.075 in²x 3 tons/in² = 588.22 ton force.
A safety factor of 10-20% is added to the result to make sure the mold can be run without flashing.
Note: Excessive clamp tonnage causes a number of problems.
- Higher operating costs (requires more energy)
- Causes extra wear on the mold and machine.
- Decreases the venting capability of the mold.
- May even increase the molding cycle. (eg: A toggle clamp often takes longer to lock up at full tonnage than it does at a partial tonnage.
For the above reasons, the tonnage setting should be minimised on any press with an adjustable clamp tonnage.