Cutout tools: To heat or not to heat

A seemingly simple practice could put your customers at risk

Q: When I’m on the road in the Northeast in the winter time, I [often] see installers using a torch to heat their cold knives and scrapers, to cut out windshields and scrape pinchwelds. When I ask them about it, they say it makes it easier in a cold-weather climate to remove the old adhesive. Does the heat have an adverse effect on the new bonding surface, assuming the torch heats the scraper to a temperature in excess of about 300 degrees Fahrenheit?

A: You pose an interesting question, and I do not think it has been addressed previously. Heating the [cold knife] blade for the removal of a piece of glass should pose no problem as long as the existing bead of urethane [on the pinchweld] that serves as the new bonding surface can be trimmed to 1-2 millimeters. We suspect any improved ease of removal resulting from the use of a heated blade would likely be anecdotal as the temperature of the vehicle and the ambient air temperature would quickly cool the heated blade and neutralize any effect it might have had.[However,] if the blade is too hot, it could cause the urethane bead to melt or even catch fire. Urethane combustion releases oxides in both cases, and should be avoided for health reasons.

To make the removal of glass parts easier when using a power tool, check with your adhesive supplier to determine what [product] is most appropriate for use with its system. For instance, at Dow Automotive, we recommend techs use the GC-800 glass cleaner as a cutout lubricant when using power tools. Others might recommend water or their own product.

Using a heated blade in the final preparation of the bonding surface—trimming the urethane bead to 1-2 mm—is not recommended as it could alter the structure of the existing bead and result in an unsafe installation. Use a sharp trimming blade for this application.
In summary, to ensure the safety of the technician and vehicle owner, we do not recommend heating the blade.

—Philip Jentoft 
Commercial manager, aftermarket
Dow Automotive, Dayton, Ohio

A: Good question. Most polyurethane adhesives, or PURs, have a limited thermal resistance of one to two hours to temperatures around 200 degrees Fahrenheit. Typically, PURs begin to depolymerize at this point and can become useless as adhesives and sealants. Warmer temperatures can be tolerated, but for shorter intervals.  

During the glass removal process, the technician cuts the urethane bead close to the glass and then trims the bead on the pinchweld to about 1-2 mm. I don’t suspect that [using a hot knife blade during cutout] would create any major problems because technicians make their cuts relatively quickly over any given area and the urethane should provide sufficient insulation between the hot knife blade and the 1-2 mm of urethane on the pinchweld. Although the knife blade might be quite hot initially, I would rationalize it cools rather quickly when it makes contact with cold urethane.  
Using a flame-heated blade for trimming down the urethane or contacting the bonding surface of the 1-2 mm base adhesive is not an acceptable practice. Heat can break down chemical bonds and weaken the adhesive, potentially creating an unsafe installation.  

Reciprocating tools also can produce significant amounts of heat energy by way of friction. While the amount of heat generated by a reciprocating tool is uncertain compared to flame-heating a cold knife, I have not heard of any urethane depolymerization or other issues using this equipment to date. 

The bottom line is: I would not recommend this practice because it has not been thoroughly tested, and any historical evidence of improved cutout performance is purely qualitative at this point. At worst, flame-heating a cold-knife blade could create an unsafe installation or a “leaker.” At best, it might slightly reduce the effort needed to perform the installation.

—Bob Stenzel
AGR application engineer
Sika Corp., Madison Heights, Mich.