Open time and heat soak
Another paradigm shift occurs with this level of welder control as it relates to so-called ‘heat soak’. Traditional welders naturally require a longer dwell period during the melt cycle – not due to the resin characteristics necessarily, but because of the welder performance…or lack thereof. The long ‘open time’ associated with low performance welders naturally require that more heat be applied to the subject components. It is certainly true that sufficient heat must be transferred into the weld beads to create a plasticized zone or melt front to facilitate suitable mixing during the seal cycle, however, with platen speed on the processor’s side, this required and programmable ‘dwell’ period is now truly a product of the polymer thermal conductivity (PTC) and heat diffusivity properties, not due to the welder’s inability to facilitate process optimization.
Our comparative testing of a number of applications processed with traditional hot-plate welders prove again and again the relevance of reducing ‘open time’ with the resultant quicker cycle times and improved weld quality consistency.
The simple cross section depiction below shows two component weld beads in contact with tooling on the heat platen…this is the melt phase of the welder cycle. The red zone of the weld bead is semi-molten and somewhat displaced, the orange zone is softened material and the blue is the unaffected substrate surface… the red/orange into the blue transition is what is known as the Heat Affected Zone (H.A.Z.)
There’s Nothing Good About Open Time
Open time or the duration that the heated (plasticized) weld joint is exposed to the ‘environment’, is a bad thing. For the sake of clarity, this is the period in the weld cycle directly after the melt phase when the parts are retracted from the heated tooling allowing for the removal of the heat-platen. Obviously a necessary process but one that adds variables.
Reducing your open time will result in quicker cycle times and improved weld quality consistency.
As you can see below, heat losses of the H.A.Z. to the environment and even into the substrate itself – if excessive, could radically change the weld bead composition.
It is a fundamental understanding that reducing – to a minimum, this exposure and related weld bead ‘cooling’ is of significant benefit. The sooner the parts can be joined after the melt phase – the better, no question about it. Additionally, the quicker this transition can be executed the more practical a reduction of the so called ‘heat soak’ dwell period can be applied…simply because of less heat loss.
The servo controlled architecture of the Extol, Rapid Conductor welders provide for a programmable minimum open distance and lightning quick removal speed of the heat platen thereby reducing ‘open time’ substantially over conventional style welders.
With that same level of speed and control in the press platens, the parts can be brought together very quickly – with the element of velocity control. It may be easier to consider it as feed rate control – a decel rate as the surfaces come together is generally beneficial. It’s all about control.
An ideal mix of the weld bead surfaces is shown here. The molten surface as well as the softened ‘base’ of the weld beads are involved in the weld joint.