For a given set of weld parameters, variations in force control that result in applying too little force reduce compression of the mating surfaces, reduce the heat generation needed for plastic melt, and result in cold or weaker welds. Similarly, force variations that result in applying too much force can cause part joints or energy directors to deform, deflect or break and may not provide enough time for proper melt flow and polymer entanglement to occur.

Applying just the right amount of force at just the right time results in quality welds with highly consistent characteristics and strength. Ideal force control requires rapid, dynamic changes in the clamp force/downspeed applied by the actuator following the melt of the plastic. This adjustment, called ‘dynamic follow through,’ enables each weld cycle to adapt to part-to-part variances and other factors such as the type of plastic, joint style, and part geometry.

As the speed and precision of force control and dynamic follow-through increase, the strength, quality and consistency of plastic welds follow. For example, the strongest ‘pull force’ for a part weld results from a controlled force profile that allows for complete and random polymer chain entanglement that makes the weld as strong as the parent material (As shown in figure 1).

As seen in the right-most illustration in figure 1, ideal force control adjusts downforce milliseconds after the melt, allowing polymer chains to extend vertically across the part interface and entangle with each other across the bond line as melt and compression occur before cooling. By contrast, weaker welds, characterised by partial or no polymer chain entanglement, show polymer chains that reassemble parallel to the bond line without entangling across the part interface. The centre weld shows the impact of inadequate force control, while the ‘cold’ weld at left could be caused by too little or too much downforce in too short a weld time.

More consistent and complete polymer chain entanglement and stronger welds are a direct result of technical improvements in force control. So, for even hard-to-weld shapes and small or delicate parts, it can provide superior weld quality and improved yields, characterised by uniform and consistent weld collapse depths, with minimal flash or part marking.