Many mold systems require heat as part of the manufacturing process. From the plastics industry, heaters will be the key ingredient to maintaining temperature from the molten plastic. The plastic flows throughout the mold base, sprue nozzle, manifold, into a die head, or through an injection barrel. Without heat, the mold or machine is useless.
The heater should be considered in the first place, since it is a fundamental part of the complete system. There are numerous heater configurations available. However, when examining the band heaters from an insulation standpoint, there are three common heater types available in the marketplace: mica, ceramic knuckle and mineral insulated.
When it comes to heater type, you need to know the performance capabilities and limitations of each and every heater type. The part geometry, temperature as well as heat-up time requirements generally dictate the sort of heater to use.
All of the three heater types has distinctive characteristics. The unique material that differentiates these heaters is definitely the interior insulation that provides the needed dielectric strength while the heater heats the part. The insulation in each heater plays a tremendous role in determining heater life and satisfaction.
Mica is primarily obtained from Paleozoic rocks and can be found in many areas all over the world, including India, southern Africa, and Russia, plus in the American continents. Mica is utilized in appliances, such as toasters and microwave ovens, together with band and strip heaters. Mica falls in to the aluminum silicates category, meaning chemically they contain silica (SiO4). The insulation materials used in mica heaters offers excellent physical characteristics such as thermal, mechanical, electrical and chemical properties. There are two primary types of mica: (1) muscovite, that contains considerable amounts of potassium promoting strong mechanical properties and (2) phlogopite containing various quantities of magnesium, which enables it to stand up to higher temperatures than muscovite.
Mica features a unique characteristic in that you can obtain very thin flakes having a consistent thickness. It conducts low amounts of heat, especially perpendicular to its strata. Furthermore, it can be non-flammable, flame-retardant and fails to emit fumes. From your heating perspective, mica is a solid option for its effectiveness against erosion and arcing, as well as its dielectric strength. Additionally, mica is resistant against chemicals and water, and possesses excellent compressive strength. Furthermore, it holds around bending stresses because of its high elasticity.
Even though some mica types can withstand temperatures in excess of 1000°C (1830°F), the mica temperature ought not exceed 600°C (1112°F) when utilized in a heater assembly. When temperatures exceed that level, deterioration begins inside the binder along with a weakening in the dielectric strength will occur.
These traits are essential since the mica band heater is curved under perpendicular pressure to make a specific diameter. The typical mica band heater is around 3/16-inch thick and might accommodate many geometries and special features for example holes and notches. Its design versatility lends itself well for a lot of applications and markets.
The mica bands’ greatest disadvantage is the maximum temperature capability of 480°C (900°F) sheath temperature. There are a lot more processes which require higher temperatures than mica heaters may offer.
Steatite the type of ceramic comprised primarily of aluminum oxide (Al2O3), silica (SiO2) and magnesium oxide (MgO). Steatite is actually created when these materials are mixed inside the correct proportion and fired in a certain temperature. L-3 and L-5 are the most common grades of steatite. L-3 can be used in many applications. However, L-5 is required where low electrical loss is vital. The ceramic is formed using industry specific processing methods and might readily be machined or net shape sintered into various designs.
Ceramic knuckle band heaters are produced with all the L-5 type of material due to its superior electrical characteristics. In accordance with Jim Shaner of Saxonburg Ceramics Inc., “A specific L-5 formula is ready, which contains the correct proportions of Al2O3, SiO2, and MgO, in addition to binders, plasticizers, release agents, and/or other additives to aid in the processing. The components are then mixed to get a specified period of time and also the batch is brought to the presses.” A press effective at pressures as much as 30 tons is utilized to press the powder into its finished shape. The very last step is to fire the ceramic to a temperature of 2320ºF.
The ceramic knuckle heater is designed to handle up to 760ºC (1400ºF). This amount of performance is really a direct outcome of the heaters’ excellent insulating properties in the ceramic knuckle segments. The knuckles interact just like a ball-and-socket inside the knee or elbow to produce the heater diameter. Unfortunately, the ceramic’s strength is additionally its weakness since it stores heat generated through the element wire, which creates difficulty in controlling the heater temperature. This can lead to unnecessary scrap, especially in the early stages of the plastic manufacturing process.
Mineral insulated heaters dominate the industry with respect to overall heater performance. Mineral insulated heaters contain magnesium oxide called MgO, which is the oxide of metal magnesium. Magnesium oxide or mineral insulation is really a fine granular powder in bulk form. It is layered between your resistance element and also the heater sheath. In several mineral insulated heaters, the MgO is compacted in a thin solid layer. The compacted MgO offers excellent thermal conductivity and great dielectric strength.
MgO posseses an upper useful temperature limit of more than 1094°C (2000°F). This is usually never reached, since the heater’s nichrome resistance wire includes a reduced operating temperature around 870°C (1598°F). Usually of thumb, the temperature of your mineral-insulated band must not exceed 760°C (1400°F). The capacity 96dexnpky a thin layer of insulation to resist current flow, yet allow quick heat transfer, creates an efficient performance heater.
With a heater thickness of only 5/32-inch, a mineral insulated heater provides rapid heat-up and funky down in comparison with mica and ceramic knuckle heaters. The compacted insulation also provides for higher watt densities which allow the thermocouple sensor to warm the part faster, which suggests a reduction in scrap upon machine startup. The mineral insulated band is extremely responsive to precise heat control for its thin construction and low mass. Less thermal lag and minimum temperature overshoot bring about faster startup and reduced cycle time. Other heaters that utilize mineral insulation are tubular, cable and cartridge heaters.