Rapid Conductor Series, Hot-Plate Welder
The rules have changed, hot-plate welding long known as a robust but slow and 'basic' process has undergone a major transition...at least here at Extol.
With years of experience manufacturing custom hot-plate welders performing complex functions - beyond the actual welding process, Extol has introduced a standard product line with many of those high-speed, precision attributes 'built-in'.
The servo-controlled platen motion coupled with our proprietary coatings and related higher temperature platen operation make consistent, high-speed hot-plate welding a reality.
It's Not Just About Speed
Most hot-plate welder manufacturers incorporate an obsolete method of platen travel control which incorporate "stops" in the welder tooling. Melt and seal stops have been utilized for decades and curiously, continue in use today on the majority of the major plastics-joining equipment suppliers welders.
It's understandable to a degree, pneumatic or hydraulic powered platens simply can't provide controlled and programmable feed rates with accurate melt and seal final positions without the use of stops.
So, the accepted practice was...and still is with most welders, to slam parts into the heated platen tooling until the melt stops are 'made' - which by the way may be impossible to truly detect, retract parts from the heated tooling, remove the heated platen and then drive the parts together until the seal stops are 'made'...you guessed it, who knows when they are truly 'made' or are in complete contact? We feel that this very dated approach simply has no place in a technology driven industry.
The major benefits offered by servo-controlled platen motion are true process control and speed...and the elimination of those archaic and clumsy stops! With the Extol Rapid Conductor series welders the user has control comparable to other true state-of the-art plastics joining processes with programmable:
- Platen speed and feed rates...actual velocity control
- Force control - independent control for both press platens
- Total dimensional control of platens
- Hi/Low alarm limits...that really matter
Speaking of Force...
Imagine not applying a controlled force through the re-solidification phase of a plastic welding process...any plastic welding or forming process. Not a sound approach. Experienced processors realize the benefit of force - especially a programmable force value that may differ from the force applied through the welding phase, as the material re-solidfies. The best ultrasonic and vibration welding machines offer this control...but most hot-plate welders do not - bogus!
As a matter of fact, when you analyze what is really happening through the hold or seal phase of traditional hot-plate welding when using tool 'stops', true hold force values actually drop off - right off the map, when the stops are contacted. Oh, the platens continue to push and force is generated - but against the hard stops, not the components welded!?
Take a look at the graph below that shows the force application through the weld cycle phases. As you can see, when tooling stops are contacted during the seal phase of the weld cycle there is a significant drop in force, how much is really not known...
We can almost hear our competition arguing that "...without hard stops you can push right past the melt front and cause a 'cold weld' and...blah, blah, blah". Yes, when you try to eliminate stops with pneumatic or hydraulic controlled welders you can generate all sorts of cold, bad, or otherwise nasty welds.
Exactly. They are stuck in a very old paradigm of acceptable practices and standards in hot-plate welding.
Get rid of the stops and start controlling the weld process by distance and force - with velocity control, and watch weld strength increase while overall cycle times decrease...in some applications, dramatic cycle time reductions can be realized.
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 comparitive 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 tranisition is what is know 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.
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 involoved in the weld joint.
A well known challenge in the hot-plate welding process is the tendency of certain polymers sticking to the heated tooling. Before we address how Extol and the features of the Rapid Conductor address this challenge, let's discuss what is actually causing the material sticking phenomenon. If you take another look at the basic weld bead cross sections that depict the H.A.Z. above, the red area of the weld bead has been heated to a molten state; the amber area is semi-molten, waxy or softened; the blue area shows the transition to an unaffected substrate condition. At the final stages of the melt phase, the adhesion properties and surface tension between the molten layer of the weld bead and the heated tooling may be greater than the temporary tensile strength of the resin through the cross section of the weld bead.
The result? Depending on the resin in question, a thin layer of the weld bead may separate from the parent material and stick to the tooling. A gradual material build-up on the heated tooling could take place that will ultimately negatively affect weld quality in a number of ways, bad news! Another scenario of polymer separation is in the form of a stringy (sometimes called 'angel hair') flash that can also build up on the tooling but may also attach itself to the components being assembled which is an unacceptable condition. In some cases a form of build-up on the heated tooling may be very slight and literally vaporize or decompose - after each melt cycle, which is actually a stable and desirable condition.
What to do about this sticking? Non-stick coatings applied to the heated tooling is the simple (theoretical) solution to this challenge. The problem is that the industry 'standard' non-stick coating will break down at or around 530 degrees F. (277C.) restricting the application of the optimum processing temperatures of many resins. Operating at higher platen temperatures without coating is an option and with certain polymers that respond in the vaporization fashion as mentioned above, this may be feasible. (Another method of hot-plate welding that precludes any polymer build up is ‘non-contact’ hot-plate welding, a method requiring considerable discussion - not covered in this summary.)
Extol set out years ago to find suitable hot-plate weld tool coatings which would offer good release properties at very high operating temperatures and would also provide high-wear resistance as many of our customers’ process mineral, 'glass or talc-filled resins with abrasive characteristics. One coating type we utilize is a proprietary ‘thin dense chrome’ application which provides excellent results when utilized above 550 degrees F. (288C.) and has an operating range of over 1000 degrees F. (537C.). So, gone are the platen temperature limitations driven by a coating performance! This is not to suggest, however, that all materials and applications can be processed with a single coating type. Extol conducts ongoing tests with various coating types and we offer recommendations for the most suitable tool materials and coatings based on the unique attributes of each application.
In addition, our patent pending 'stutter' and 'platen stir' technologies tackle the really stubborn materials that generate the stringy flash and material sticking/build-up issues even with the advanced coatings we implement. Because we utilize servo control on the heated platen axis as well as the press platen axis', the ability to perform some very creative machine actions exist. The stutter process is a method of press platen retraction from the heat platen that intentionally creates a brief dwell period intended to combat 'angel hair' type flash. Stated another way, as opposed to one smooth movement clearing the parts from contact with the heated surface, the deliberate pausing of the movement in proximity to the heated platen creates a condition that in essence vaporizes the stringy type 'angel hair' flash common with materials such as Polycarbonate. The distance the platens retract and the stutter duration are totally programmable.
Platen Stir is a process whereby the heated platen actually oscillates during the melt phase - it literally moves back and forth while the parts are in contact with the heated tooling. Naturally the part and weld joint geometries play directly into the feasibility of utilizing this feature but the simple logic applied can be quite beneficial with difficult applications.