Many mold systems require heat as part of the manufacturing process. In the plastics industry, heaters are key to maintaining the proper temperature of the molten plastic. The plastic flows through the mold base, sprue nozzle, manifold, into a die head, or through an injection barrel. Without heat, the mold or machine is useless.
Too often it seems that the heat in the system is an afterthought. The heater should be considered from the start, as it’s an integral part of the overall system. There are many heater configurations available. However, when looking at the heater from an insulation standpoint, there are three common heater types available in the industry: mica, ceramic knuckle and mineral-insulated.
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When considering heater type, understand the performance capabilities and limitations of each. The part geometry, temperature and heat-up time generally dictate the type of heater to use. Many system designers use a process of elimination when determining the heater type needed for a machine.
Each heater type has distinctive characteristics. The unique material that differentiates them is the interior insulation that provides the needed dielectric strength while the heater heats the part. The insulation in each heater plays a significant role in determining heater life and performance. The following illustrates the insulation properties and other features that make each heater type perform differently.
Mica comes from Paleozoic rocks and can be found in many areas around the world, including India, southern Africa and Russia, as well as in the American continents. Mica is used in toasters and microwave ovens, along with band and strip heaters. Mica falls into the aluminum silicates category, which means that chemically they all contain silica (SiO4). The insulation material used in mica heaters offers excellent physical characteristics such as thermal, mechanical, electrical and chemical properties. There are two primary types of mica. The first is muscovite, which contains large amounts of potassium that promote strong mechanical properties. The second is phlogopite, which contains various levels of magnesium that enable it to withstand higher temperatures than muscovite.
Mica has a unique characteristic in that one can obtain very thin flakes with a consistent thickness. Mica conducts low amounts of heat, especially perpendicular to its strata. In addition, it’s non-flammable, flame-retardant and doesn’t emit fumes when heated. From a heating perspective, mica is a solid option due its resistance to erosion and arcing and its dielectric strength. Additionally, mica is resistant to chemicals and water, and it has excellent compressive strength. It also holds up to bending stresses because of its high elasticity.
While some mica types can withstand temperatures in excess of 1,000°C (1,830°F), the mica temperature shouldn’t exceed 600°C (1,112°F) when used in a heater assembly. When temperatures exceed that level, deterioration begins in the binder and the dielectric strength weakens.
These features are important because the mica band heater is curved under perpendicular pressure to form a specific diameter. The typical mica band heater is approximately 3/16 in. thick and can accommodate many geometries and special features such as holes and notches. Its design versatility lends itself well for many applications and markets.
The greatest disadvantage to a mica band is the maximum temperature capability of 480°C (900°F) sheath temperature. An increasing number of processes require higher temperatures than mica heaters can withstand.
Of the ceramics, steatite is a type comprised primarily of aluminum oxide (Al2O3), silica (SiO2) and magnesium oxide (MgO). Steatite is formed when these materials are mixed in the correct proportion and fired at a certain temperature. L-3 and L-5 are the most common grades of steatite. L-3 is used in most applications. However, L-5 is recommended where low electrical loss is critical. The ceramic is formed using industry-specific processing methods and can be readily machined or net-shape sintered into a variety of designs.
Ceramic knuckle band heaters are made with the L-5 material because of its superior electrical characteristics. “A specific L-5 formula is prepared, which contains the correct proportions of Al2O3, SiO2 and MgO, along with binders, plasticizers, release agents and other additives to aid in the processing. The ingredients are then mixed for a specified period of time and the batch is sent to the presses,” says Jim Shaner of Saxonburg Ceramics Inc.
A 30-ton press forms the powder into its finished shape. The final step is to fire the ceramic to a temperature of 2,320ºF.
The ceramic knuckle heater can handle temperatures to 760ºC (1,400ºF). This level of performance is a direct result of the excellent insulating properties of the ceramic knuckle segments. The knuckles work together similar to a ball-and-socket in the knee or elbow to conform to the heater diameter. Unfortunately a ceramic’s strength is also its weakness because it stores heat generated by the element wire, which makes it difficult to control the heater temperature. This can lead to unnecessary scrap, particularly in the early stages of the plastic manufacturing process.
Mineral-insulated heaters dominate the market with respect to overall heater performance. Mineral-insulated heaters use MgO. Magnesium oxide or mineral insulation is a fine granular powder in bulk form. It’s layered between the resistance element and the heater sheath. In many mineral-insulated heaters, the MgO is compacted into a thin, solid layer, which offers excellent thermal conductivity and great dielectric strength.
MgO has an upper useful temperature limit of more than 1,094°C (2,000°F). This is usually never reached, because the heater’s Nichrome resistance wire has a much lower operating temperature of about 870°C (1,598°F). As guideline, the temperature of the mineral-insulated band shouldn’t exceed 760°C (1,400°F). The ability of a thin layer of insulation to resist current flow, yet allow quick heat transfer, supports an efficiently performing heater.
With a heater thickness of only 5/32 in., a mineral-insulated heater can provide rapid heat-up and cool-down compared to mica and ceramic knuckle heaters. The compacted insulation also allows for higher Watt densities that enable the unit to heat the part faster, which means a reduction in scrap during machine start-up. The mineral-insulated band’s thin construction and low mass is highly responsive to precise heat control. Less thermal lag and minimum temperature overshoot result in faster start-up and reduced cycle time. Other heaters that feature mineral insulation are tubular, cable and cartridge heaters.
Choosing the correct heater for an application is important, especially when operated at elevated temperatures. When characteristics such as temperature, performance and efficiency are important, the characteristics of these insulation material types can help guide the buyer to choosing the correct heater for their application.
John Pape is a product specialist at Watlow Electric Manufacturing Co. in St. Louis. For more information, visit www.watlow.com.