I often hear feedback from some peers that there is a big gap between clamping force calculated by Moldflow and actual one. Main reason for this problem is that way Moldflow calculates clamping force is not well understood, and analysis is not combined with actual molding. This issue focuses on analyzing various variables that affect clamping force analysis, so that engineers can more accurately grasp direction and method of improving clamping force, solution is more targeted.

Clamping force, refers to maximum clamping force applied by clamping device of injection molding machine to mold. When melt fills cavity, force generated by injection pressure in cavity always tries to expand mold along parting surface. For this reason, injection molding machine must provide sufficient clamping force to offset pressure transmitted from inside of mold.

If clamping force is insufficient, plastic product is prone to flash (plastic is easy to flow out from mold gap)
If clamping force is insufficient, plastic product is prone to shrinkage (pressure cannot be increased to reduce shrinkage)
If clamping force is too large, machine will be worn out, production cost will be high, and unnecessary waste will be caused

Two variables of clamping force
Area: Area projected to XY plane (as shown below, Z direction is mold opening direction)
Pressure: Injection pressure distribution during molding process is uneven. If projected area is divided into multiple small segments, each projected area corresponds to a pressure. Clamping force should be sum of product of pressure and area generated on each projection.

 

Moldflow clamping force calculation formula
 
Where n=number of segments projected area is divided into A=area of each segment P=average pressure on each segment area

At the end of filling, product that has been injected has the largest projected area. At this time, the most direct impact on calculation accuracy of clamping force is average pressure distribution on product.

It is usually believed that the greater molding pressure, the greater clamping force. In fact, what needs to be paid attention to in analysis is not maximum pressure, but average pressure. As shown in figure below, for different gate position schemes, clamping force with high pressure is smaller. The most important thing is that yellow high-pressure area of right scheme occupies most of area on product, and the overall average pressure is higher.

 

 

The better filling balance, the smaller average pressure, which can significantly reduce clamping force. As shown in figure below, by adjusting gate position, not only the overall pressure is reduced, but also problem of excessive pressure concentration will not occur, effectively reducing clamping force.

 

 

Effective prediction of clamping force requires good process optimization to obtain analysis results close to actual situation. As shown in figure below, design has been finalized, but different injection times will produce different injection pressures, which will affect analysis results of clamping force, so the best injection time should be selected to minimize average pressure.

 

Usually, when plastic is close to the end of filling, pressure rises quickly and pressure is easy to reach peak. This is mainly because filling is more unbalanced towards end, which leads to a large change in clamping force, especially for large products, such as domestic TCL, shyworth and other companies. Back cover of flat-screen TV is more sensitive to pressure changes because of its large projection area. Therefore, large products pay more attention to filling balance, but even so, since absolute balance cannot be guaranteed, clamping force will still be too large. In this case, it can be solved by switching holding pressure in advance, such as V/P switching point can be set between 90% and 96% (adjusted according to actual conditions), as shown in figure below, pressure varies greatly at different switching points.

 

In actual molding, multi-stage speed is used for molding, and maximum pressure limit is set (as shown in figure below). Actual pressure will fluctuate with injection speed and pressure limit. Case shown in figure below is because machine has set a maximum pressure limit. When actual pressure reaches limit pressure due to high-speed filling, machine automatically reduces speed (different from actual set speed), and pressure decreases accordingly. Final clamping force will be smaller than result obtained without setting maximum pressure limit.

 

Maximum pressure limit can be set during Moldflow analysis, as shown in figure below. Combined with multi-stage speed setting analysis, a result closer to actual molding can be obtained. This type of analysis is relatively difficult because it involves multi-stage speed settings. It is generally more suitable for large products or parts with complex structures.

 

During actual molding, clamping force peak may appear at the end of filling or in holding pressure stage. It is more common to increase holding pressure in order to improve product quality, such as eliminating shrinkage marks and ensuring that product size meets standard. As shown below:

 

There are usually some undercuts on product, which will produce overlapping projection areas. Moldflow will take it into account when calculating clamping force, thus over-predicting clamping force, as shown in left figure. For this reason, we need to exclude calculation of these undercut areas in clamping force analysis, as shown in right figure.

 

To better grasp calculation of clamping force, we must understand principle of clamping force analysis, combine it with actual molding to find gap between analysis and reality, so that we can easily find direction and method to solve problem and improve application level of Moldflow.