Moulding Machine Clamp Force Calculations

<<< Back to information for moulders index >>>

Everyday information the injection moulder requires from Invotec Solutions.

The below article can be used for machine setters and machine operators of injection moulding machines.

Discover our full range of plasticising components or for the injection moulder.

View our other articles form our setter and operators guide for injection moulders. Taken from our moulders catalogue (Toolbox Edition)

How To Calculate The Clamping Force Required For Injection Moulding Tools.

The projected area of a moulding is the largest area that can be seen if the moulding is viewed from the direction of the clamp force application.

The total projected area for a multicavity, injection mould is given by the area of one cavity multiplied by the numbers of cavities in the mould plus the area of the feed system.

Information Required: To calculate the total projected area, for a given multi-impression mould, it is necessary to know the following:-

  1. The dimensions of one impression in the mould
  2. The number of impressions in the mould
  3. The layout of the impressions in the mould
  4. The dimensions of the feed system (if any) i.e. the diameter and length of runner branches.

The total area of the runner system is obtained by adding the sum of all the areas for each section of the runner configuration. The area for each section is the largest area seen when viewed from the direction of the clamp force application.

NB. The area of the gate is not usually considered because of its very small size. However, if a film, fan, ring, diaphragm or tab-type gate is used then the area should be included in the total projected area value.

To calculate the projected area of a moulding, the split or parting line for the component has to be decided or has already been selected.

Once this decision is made, then the component can then be arranged so that it is viewed from the direction of clamp force application or, in the line of extraction from the mould (i.e. its plan view).

To determine the clamping force, calculate the total projected area for the mould tool and then multiply this area by the required cavity pressure. The cavity pressure will vary for each thermoplastics material and is obtained from the material suppliers.

The applied clamping force must be sufficiently high so as to ensure that the mould tool remains fully closed during the filling and packing phases of the moulding cycle.

Consider the following example:

  • Moulding – Container
  • Material – Polypropylene
  • Hot runner mould tool of 8 impressions
  • Diameter of moulding at top = 70mm
  • Diameter of moulding at base = 50mm
  • Height of moulding = 48mm
  • Diameter of cavity at split line = 70mm
  • Depth of cavity = 48mm
  • Diameter of cavity at base = 50mm
  • Thermal gate type centrally located at base of mould cavity
  • Wall section of component is 1.6mm

Step One

Calculate total projected area = Area of one cavity x number of impressions

Area of one cavity = πD2/4 or πR2

(convert mms into cms)

π x 7 x 7/4 or π x (3.5)2 = 34.49cm2

Total projected area: area of one cavity x number of impressions: 34.49 x 8 = 307.92cm2

Step Two

Calculate the clamping force

The clamping force = total projected area (cm2) x cavity pressure (Kgf/cm2 or bar) / 1000

The cavity pressure value is obtained from various material suppliers using data provided for the wall section used, the flow length of the molten material from the gate point and the injection time employed. These pressure values are usually predicted, whereas the values obtained from a CAE analysis (although still predicted) are far more accurate.

In this instance the predicted cavity pressure is 380bar for this easy flowing (MFR 40) Polypropylene (PP) and component.

Predicted clamping force

Total projected area x predicted cavity pressure
= 307.92 x 380 / 1000
= 117 tonnes

Applying a factor of safety of 10% the anticipated clamping force to produce 8 flash free mouldings would be:

= 117 x 1.1 = 128.7 tonnes