Many mold systems require heat in the manufacturing process. Within the plastics industry, heaters will be the key ingredient to maintaining temperature from the molten plastic. The plastic flows from the mold base, sprue nozzle, manifold, in a die head, or through an injection barrel. Without heat, the mold or machine is useless.
The heater is highly recommended from the beginning, since it is an integral part of the general system. There are lots of heater configurations available. However, when examining the heater from an insulation standpoint, there are actually three common heater types available in the business: mica, ceramic knuckle and mineral insulated.
When considering heater type, you have to comprehend the performance capabilities and limitations for each heater type. The part geometry, temperature and heat-up time requirements generally dictate the kind of heater to work with.
All of the three heater types has distinctive characteristics. The unique material that differentiates these heaters will be the interior insulation that gives the needed dielectric strength while the heater heats the part. The insulation in each heater plays a tremendous role in determining heater life and gratification.
Mica is primarily obtained from Paleozoic rocks and may be found in many areas worldwide, including India, southern Africa, and Russia, plus in the American continents. Mica is commonly used in appliances, including toasters and microwave ovens, along with band and strip heaters. Mica falls in to the aluminum silicates category, meaning that chemically they contain silica (SiO4). The insulation materials in mica heaters offers excellent physical characteristics including thermal, mechanical, electrical and chemical properties. There are two primary kinds of mica: (1) muscovite, containing large amounts of potassium promoting strong mechanical properties and (2) phlogopite containing various levels of magnesium, which enables it to withstand higher temperatures than muscovite.
Mica carries a unique characteristic in that one can obtain very thin flakes by using a consistent thickness. It conducts low amounts of heat, especially perpendicular to the strata. Moreover, it can be non-flammable, flame-retardant and fails to emit fumes. From the heating perspective, mica is a solid option for its resistance to erosion and arcing, and its particular dielectric strength. Additionally, mica is resistant to chemicals and water, and possesses excellent compressive strength. Furthermore, it holds approximately bending stresses due to the high elasticity.
While many mica types can withstand temperatures above 1000°C (1830°F), the mica temperature should never exceed 600°C (1112°F) when employed in a heater assembly. When temperatures exceed that level, deterioration begins within the binder as well as a weakening in the dielectric strength will occur.
These traits are important considering that the mica band heater is curved under perpendicular pressure produce a specific diameter. The typical mica band heater is around 3/16-inch thick and will accommodate many geometries and special features including holes and notches. Its design versatility lends itself well for most applications and markets.
The mica bands’ greatest disadvantage may be the maximum temperature ability of 480°C (900°F) sheath temperature. You can find a lot more processes that 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 created when these materials are mixed from the correct proportion and fired at the certain temperature. L-3 and L-5 are the most common grades of steatite. L-3 can be used generally in most applications. However, L-5 is usually recommended where low electrical loss is crucial. The ceramic is formed using industry specific processing methods and may readily be machined or net shape sintered into a variety of designs.
Ceramic knuckle band heaters are made together with the L-5 sort of material due to the superior electrical characteristics. In accordance with Jim Shaner of Saxonburg Ceramics Inc., “A specific L-5 formula is ready, which contains the right proportions of Al2O3, SiO2, and MgO, along with binders, plasticizers, release agents, and other additives to aid in the processing. The constituents are then mixed to get a specified period of time as well as the batch is sent to the presses.” A press competent at pressures as much as 30 tons is commonly used to press the powder into its finished shape. The ultimate step would be to fire the ceramic into a temperature of 2320ºF.
The ceramic knuckle heater is made to handle as much as 760ºC (1400ºF). This amount of performance is a direct outcome of the heaters’ excellent insulating properties of the ceramic knuckle segments. The knuckles work together similar to a ball-and-socket within the knee or elbow to produce the heater diameter. Unfortunately, the ceramic’s strength is likewise its weakness because it stores heat generated by the element wire, which creates difficulty in managing the heater temperature. This can lead to unnecessary scrap, particularly in the initial phases from the plastic manufacturing process.
Mineral insulated heaters dominate the market with respect to overall heater performance. Mineral insulated heaters consist of magnesium oxide called MgO, which is the oxide of metal magnesium. Magnesium oxide or mineral insulation is really a fine granular powder in mass form. It can be layered between your resistance dexppky61 along with the heater sheath. In numerous mineral insulated heaters, the MgO is compacted into a thin solid layer. The compacted MgO offers excellent thermal conductivity and great dielectric strength.
MgO posseses an upper useful temperature limit greater than 1094°C (2000°F). This is usually never reached, since the heater’s nichrome resistance wire carries a much lower operating temperature of around 870°C (1598°F). As a rule of thumb, the temperature from the mineral-insulated band ought not exceed 760°C (1400°F). The capacity of the thin layer of insulation to resist current flow, yet allow quick heat transfer, creates an effective performance heater.
By using 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 permits higher watt densities which allow the heater to heat up the part faster, which means a decrease in scrap upon machine startup. The mineral insulated band is extremely responsive to precise heat control because of its thin construction and low mass. Less thermal lag and minimum temperature overshoot lead to faster startup and reduced cycle time. Other heaters that utilize mineral insulation are tubular, cable and cartridge heaters.