How hot for how long?
Some Micro-Measurements adhesive systems require an elevated temperature cure. If your strain gage application would benefit from one of these adhesives, then it is critical to apply proper clamping pressure, achieve the proper cure temperature for the bond line, and maintain the proper cure temperature for the minimum required time.
Epoxy-phenolic adhesive systems are used in the highest performing strain gage sensor applications. All Micro-Measurements two-component, solvent-thinned epoxy-phenolic adhesives require a minimum recommended elevated cure schedule to perform properly. Not adhering to this requirement can result in poor performance, including excessive creep and poor zero return.
Every M-Bond adhesive kit includes an instruction bulletin, which includes a chart that indicates the recommended cure schedule for the adhesive. As shown in the example below for M-Bond 600, this epoxy-phenolic adhesive requires a bond line curing temperature of at least 175°F (80°C) and must dwell at that temperature for at least four hours. Exceeding the required minimum time is okay, but reducing this time can result in a partial cure, and therefore unstable glue line. For precision transducer applications requiring the highest possible stability and repeatability, the recommended bond line temperature is 225°F (107°C) for a minimum of four hours. This is to ensure the cured adhesive is sufficiently hard and will precisely follow the applied strains and fully transmit those strains to the sensor.
Oven temperature is often not a good measure of bond line temperature. Depending on the mass of the part being bonded, bond line temperature can significantly lag behind oven temperature. It is recommended that a temperature-measuring sensor, such as an RTD or thermocouple, is attached near the bond line to ensure the proper temperature has been achieved. Additionally, typically there is a heating ramp rate associated with solvent-thinned adhesives to allow time for the solvents to escape. As described in the instruction sheet, for M-Bond 600 the heating ramp rate is 5°F to 20°F per minute (3°C to 11°C per minute).
Finally, to achieve the highest precision from the installed sensor for transducer applications, the cured installation should be post-baked for at least one hour at 50°F (30°C) above the cure temperature, or max. operating temperature, whichever is higher. This relieves residual stresses that were cured into the glue line. To complete the bonding process, load the finished transducer at 150% of rated capacity to “exercise” the glue line and the sensor prior to performance testing and; last but not least to relieve stresses in the glue line in order to improve repeatability.
Micro-Measurements Strain Gage Technology Knowledge Base - Instruction Bulletins: http://www.micro-measurements.com/stress-analysis-strain-gages/instruction-list/.
Available Adhesives: http://www.micro-measurements.com/installation-accessories/adhesives/