In plastics moulding, localized heaters are essential to keeping molten plastic at the correct temperature as the material flows through the mould base, sprue nozzle, and manifold and into a die head or through an injection barrel. Nevertheless, too often, heating of the mould system is a designer’s afterthought. The heater should instead be regarded from the start as an integral part of the system.
Many heater configurations are available, a fact reflected in the portfolio of specialty manufacturer Watlow. But besides considering the shape and construction of the heater, Watlow recommends thinking about its interior insulation with respect to the application, especially when elevated temperatures are involved. The material used for insulation provides the dielectric strength needed while the heater heats the part. Mould heaters with mineral insulation are the best performers, but mica and ceramic knuckle heaters also are common.
Each has distinctive performance capabilities and limitations. Part geometry, moulding temperature, and heat-up time requirements generally dictate the type of heater to use, but the insulation is a significant factor in heater lifetime and performance.
Mica, a natural silicate, offers excellent thermal, mechanical, electrical, and chemical properties. It conducts little heat. Mica’s resistance to erosion and arcing and its dielectric strength make it a solid insulation option. However, the mica temperature in a heater assembly should not exceed 600°C, because the binder deteriorates and dielectric strength weakens.
The typical mica band heater can accept many geometries and features such as holes and notches. It has design versatility, but does not tolerate a sheath temperature above 480°C, thus limiting its use. Many new processes require higher temperatures than that.
Ceramic knuckle band heaters are made from type L-5 steatite, which exhibits low electrical loss. They are designed to handle temperatures up to 760°C, a performance resulting directly from the excellent insulation properties of the knuckle segments that work in ball-and-socket fashion to create the heater diameter. Unfortunately, ceramic’s strong point is also its weakness: it stores heat generated by the element wire, which makes it difficult to control heater temperature. This can lead to unnecessary material scrap, particularly in early-stage processing.
Mineral-insulated heaters have magnesium oxide thinly layered between the resistance element and heater sheath. This compound offers high thermal conductivity and dielectric strength and an upper temperature limit approaching 1,100°C (though the mineral-insulated band should generally not exceed 760°C). The thin insulation layer’s ability to resist current flow yet allow quick heat transfer produces an efficient heater.
Only 5/32 in. (4 mm) thick, the Watlow mineral-insulated band heater provides faster heat-up and cool-down than mica and ceramic knuckle heaters, and allows precise control. Faster heat-up minimizes scrap upon machine start-up, besides shortening cycle time. Tubular, cable, and cartridge heaters from Watlow also employ mineral insulation.
John Pape, a Watlow product specialist, prepared the paper from which this article has been adapted.
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