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82 DESIGN007 MAGAZINE I NOVEMBER 2018 into the dispensing equipment reservoirs and then leaving the material undisturbed for a couple of hours will allow the bulk of any air that has been trapped in the mix during the decanting process to be released naturally back into the environment. If the reservoir is equipped with a stirrer, then this should be set to a low speed not to incorporate air during the mixing process. Also, make sure that the lines of the dispensing equipment are fully bled of air before commencing production. It is good practice to check the shot weights of both components of the resin periodically during the day to ensure that the dispensing equipment is operating reliably. If the weights begin to drift significantly, this could be a sign that the pumps are not working correctly, such as air coming into the system from a split pipe or leaking coupling, or the pipe was not prop- erly bled before starting. Polyurethane resins are susceptible to mois- ture, which will react more readily with the hardener to release bubbles of carbon dioxide, adversely affecting the cure. Meanwhile, sili- cone resins are sensitive to certain chemicals that can inhibit the cure of silicone encapsu- lants. When using polyurethane and silicone resins, it is essential to ensure that the pipes and reservoirs are both clean and dry before filling with these materials. Desiccant towers or a supply of dry air or nitrogen should be used to keep the system as dry as possible. It is also important to ensure that material containers are kept closed at all times when not in use, and that desiccant tow- ers fitted to automated equipment are regularly monitored. The desiccant medium, which is usually either a molecular sieve or silica gel, should also be renewed as necessary. More- over, the relative humidity of the mixing, dis- pensing, and curing environments should be monitored and ideally kept below 70%; oth- erwise, the ingress of moisture can lead to blocked pipes, cured materials in the reservoir, and pumps seizing as a result of fouling by cured materials. In the case of heavily filled resin systems, these will always show signs of sedimentation over time; thus, it is best to operate a policy where the oldest material is used first in pro- duction to maintain good stock rotation. Make sure that the material is properly dispersed before use or as it is being decanted into a reservoir. Try to keep only one day's worth of material in the reservoir at a time. Also, use a stirrer operating at its lowest speed setting to keep the material agitated, which will ensure that any fillers contained within the resin dis- perse correctly. Heating the resin will help to lower its vis- cosity, allow the mixed system to flow more readily, and make it easier to process. A note of caution—heating will reduce the useable life and the gel times of the mixed resin sys- tem, and where polyurethanes are concerned, heating of the hardener is not recommended. If your production volumes are low, you are more than likely to use resin packs, which provide accurate weights of both hardener and resin. When using these handy packs, always make sure that the two components are cor- rectly mixed before use. In particular, pay care- ful attention to releasing any material that may linger in the corners of the pack; otherwise, you will have an incorrect mix ratio in the material being dispensed. If lots of small volume units are to be pot- ted by resin pack, it is often easier to use a smaller pack size and have better control over dispensing the material rather than a single large volume pack, which can be difficult to control when attempting to dispense the cor- rect amount of resin. Overall, pay attention to these basic do's and don'ts and you are likely to achieve the levels of reliability and long service life that will ensure happy relationships with your cus- tomers. DESIGN007 Alistair Little is global business/ technical director at Electrolube. To read past columns from Electro- lube, click here. To download your copy of Electrolube's micro eBook, The Printed Circuit Assembler's Guide to… Conformal Coatings for Harsh Environments, click here.

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