Sticky Issues

Jan 20, 2015 | Page: HPW | 0 comments

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.

“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.”

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 proprietary ‘stutter’ technology tackles 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.