Maintenance of hot-melt adhesive equipment is usually recommended at one-year intervals. This does depend on the type of hot melt. Most modern polyolefin hot melts are very thermostable and hardly age at all. An annual maintenance seems a bit much in terms of cleaning, but there is no harm in inspecting the wear and tear on the moving parts.
Hot melt units working with highly ageing hot melt, such as a brown/yellow EVA adhesives usually really need annual maintenance and cleaning.
PUR hot-melt systems may also require more frequent maintenance and usually need to be flushed if there is extended downtime (several days or longer).
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