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52 The PCB Design Magazine • November 2016 arate compartments of the pack. When you are ready to use the product, you simply remove the clip or other separating device between the compartments and 'massage' the result- ing pouch, ensuring that both components are completely mixed. For larger production jobs, resin and hard- ener are likely be supplied in separate bulk con- tainers and you will need to check the shot sizes for each component regularly to ensure that the correct amounts of resin and hardener are being dispensed and mixed correctly. When mixing bulk resin and hardener, it is important to avoid introducing excessive amounts of air, which will form micro-bubbles within the cured resin. These may expand with the temperature rise to such an extent that they may cause problems. Polyurethane resins are particularly sensitive to moisture, so humid air could also pose problems. If you are not happy with bulk materials mixing and incurring the potential problems of introducing too much air (and moisture) into the mix, then it might be more appropriate to use automatic mixing equipment, which will accurately mix resin and hardener in the cor- rect proportions, and usually in a controlled at- mosphere. Remember: Incorrect ratios will lead to a poor cure and the physical properties of the res- in will differ from those specified in the manu- facturer's data sheet. Viscosity As well as the viscosity of the mixed system, the viscosity of the two components needs to be considered. Often there is a large difference between the two components, particularly with filled systems, and this could place additional strain on the dispensing equipment's pumps and pistons. It should also be noted that vis- cosity of each component will vary depend- ing upon the temperature of the environment in which the mixing and dispensing is taking place. Usable Life/Gel Time The useable life is defined as the time that the mixed resin can flow and is still workable; the gel time is the interval between mixing and the point at which the resin has just set and cannot flow. In the latter state, however, the resin is still relatively soft and can often be re- shaped by applying slight pressure. Gel time is usually quoted for a sample size of 150g, and it is important to note that the larger the volume of mixed resin, the shorter the useable life and gel time. As a rule, all resins should be mixed and dispensed within their useable life. Fillers generally do not affect the gel time; however, gel time may be affected if the rate of cure or the cure reaction is influenced by fillers, so this should be ascertained if any minor working is required prior to the final cure. Cure Temperature Again, this is normally stated for a 150g sam- ple size. Once a resin and its associated hardener are mixed together, the reaction can be very fast but also very exothermic, which can lead to the possibility of a runaway reaction. The exotherm temperature can be controlled by adjusting the volume or by using a filled system, as the fillers absorb some of the heat as well as reducing the concentration of the hardener that promotes this rapid cure. It should be noted that the tem- perature at which the resin is cured will affect not only the rate of cure, but also the develop- ment of cure. Controlling the cure temperature is important because the components requiring potting or encapsulation might be adversely af- fected by raised temperatures. Although the volume of the resin applied is important, so is its depth and the surface area it covers. Every application is different, so resin users should take the curing temperature data RESINS: FIVE ESSENTIALS TO ACHIEVE THE RIGHT CURE " When mixing bulk resin and hardener, it is important to avoid introducing excessive amounts of air, which will form micro-bubbles within the cured resin. "

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