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JULY 2021 I DESIGN007 MAGAZINE 59 or sensors embedded in road surfaces. If large temperature swings are expected over a short timeframe, then the flexible nature of the resin is advantageous as there is a low probability of stress cracking occurring. Silicone resins have the widest operating temperature range (-50°C to +220°C) and when cured are the most flexible of the three resin chemistries. eir adhesion to certain substrates is poor, as is their chemical resis- tance, particularly to chemicals more com- monly encountered in everyday use, such as alcohols, aromatics, and ketones. While they do tend to differ widely in terms of performance characteristics, all resin types have excellent electrical insulation properties across their respective temperature ranges. Resin chemistries have advanced considerably in recent years and there are now resins avail- able that exhibit properties and performance criteria which is oen well beyond the normal boundaries expected for that resin type. How Much Resin Should You Apply? As a rule of thumb, the amount of resin that needs to be applied must be sufficient to cover the top of the highest component of the board, and the thickness of the resin layer must pro- vide the desired level of protection. While most customers will determine the minimum thickness of resin layers for their particular applications by trial and error, the relevant technical datasheet will provide good guid- ance and consulting your supplier will oen help resolve a problem. What Are the Key Factors for a Successful Mix Ratio? e mix ratio is quite possibly the most criti- cal aspect of resin mixing. Brace yourselves to face long-term adverse repercussions if you get it wrong. ere are two methods of mixing a resin with its associated hardener, either by hand or by using specialist dispensing equip- ment. If mixing by hand, then the ratio of the weight of the two components is the more use- ful method to employ. If mixing via dispensing equipment, then the volume ratio is used. If the job is reasonably small, then you are likely to use a resin pack, which provides the resin and hardener in precise quantities, in separate 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 resulting pouch, ensuring that both compo- nents are completely mixed. For further sup- port on mixing techniques, there is a how-to video in the Knowledge Base section of the Electrolube website. For larger production jobs, the resin and hardener are supplied in separate bulk containers, suitable for use with two-component metering and mixing systems. It is important to check the shot sizes for each component regularly to ensure that the correct amounts of resin and hardener are being dis- pensed. When mixing bulk resin and hardener, it is important to avoid introducing excessive amounts of air, which will form micro-bub- bles within the cured resin. e liquid resin and liquid hardener used for polyurethanes are particularly sensitive to moisture, so it is important to avoid humid conditions when potting to avoid micro-bubble formation. ese micro-bubbles may expand when hot and create voids. If you are not happy with It is important to check the shot sizes for each component regularly to ensure that the correct amounts of resin and hardener are being dispensed.

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