Many mold systems require heat included in the manufacturing process. Inside the plastics industry, heaters would be the key ingredient to maintaining temperature in the molten plastic. The plastic flows with the mold base, sprue nozzle, manifold, in to a die head, or via an injection barrel. Without heat, the mold or machine is useless.
The heater is highly recommended in the first place, because it is a fundamental part of the overall system. There are several heater configurations available. However, when viewing the heater from an insulation standpoint, you will find three common heater types available in the business: mica, ceramic knuckle and mineral insulated.
When it comes to heater type, you have to know the performance capabilities and limitations of each and every heater type. The part geometry, temperature and also heat-up time requirements generally dictate the type of heater to use.
Each one of the three heater types has distinctive characteristics. The unique material that differentiates these heaters is the interior insulation that offers the appropriate 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 taken from Paleozoic rocks and are available in many areas around the world, including India, southern Africa, and Russia, as well as in the American continents. Mica is commonly used in appliances, including toasters and microwaves, 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 this heat band offers excellent physical characteristics such as thermal, mechanical, electrical and chemical properties. The two main primary forms of mica: (1) muscovite, containing large amounts of potassium promoting strong mechanical properties and (2) phlogopite containing various degrees of magnesium, which enables it to withstand higher temperatures than muscovite.
Mica includes a unique characteristic in that one can obtain very thin flakes by using a consistent thickness. It conducts low quantities of heat, especially perpendicular to its strata. In addition, it can be non-flammable, flame-retardant and will not emit fumes. From your heating perspective, mica is actually a solid option because of its effectiveness against erosion and arcing, and its dielectric strength. Additionally, mica is resistant to chemicals and water, and it has excellent compressive strength. Additionally, it holds up to bending stresses for its high elasticity.
Even though some mica types can withstand temperatures in excess of 1000°C (1830°F), the mica temperature should not exceed 600°C (1112°F) when utilized in a heater assembly. When temperatures exceed that level, deterioration begins from the binder plus a weakening from the dielectric strength will occur.
These functions are very important because 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 may accommodate many geometries and special features like holes and notches. Its design versatility lends itself well for several applications and markets.
The mica bands’ greatest disadvantage may be the maximum temperature ability of 480°C (900°F) sheath temperature. There are actually progressively more processes that require higher temperatures than mica heaters will offer.
Steatite is a kind of ceramic comprised primarily of aluminum oxide (Al2O3), silica (SiO2) and magnesium oxide (MgO). Steatite is actually created when these materials are mixed within the correct proportion and fired in a certain temperature. L-3 and L-5 are the most typical grades of steatite. L-3 is utilized in the majority of applications. However, L-5 is usually recommended where low electrical loss is crucial. The ceramic is created using industry specific processing methods and will readily be machined or net shape sintered into a number of designs.
Ceramic knuckle band heaters are made using the L-5 kind of material because of its superior electrical characteristics. According to Jim Shaner of Saxonburg Ceramics Inc., “A specific L-5 formula is prepared, containing the proper proportions of Al2O3, SiO2, and MgO, along with binders, plasticizers, release agents, and other additives to help in the processing. The components are then mixed for any specified time frame and the batch is delivered to the presses.” A press able to pressures up to 30 tons is commonly used to press the powder into its finished shape. The ultimate step is always to fire the ceramic to some temperature of 2320ºF.
The ceramic knuckle heater is designed to handle around 760ºC (1400ºF). This measure of performance can be a direct reaction to the heaters’ excellent insulating properties of the ceramic knuckle segments. The knuckles interact much like a ball-and-socket inside the knee or elbow to generate the heater diameter. Unfortunately, the ceramic’s strength is likewise its weakness as 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 early stages from the plastic manufacturing process.
Mineral insulated heaters dominate the industry with regards to overall heater performance. Mineral insulated heaters comprise of magnesium oxide referred to as MgO, the oxide of metal magnesium. Magnesium oxide or mineral insulation can be a fine granular powder in big amounts form. It is layered between the resistance dexppky61 as well as the heater sheath. In several mineral insulated heaters, the MgO is compacted in to a thin solid layer. The compacted MgO offers excellent thermal conductivity and great dielectric strength.
MgO has a upper useful temperature limit greater than 1094°C (2000°F). Normally, this is never reached, as the heater’s nichrome resistance wire includes a lower operating temperature around 870°C (1598°F). As a rule of thumb, the temperature in the mineral-insulated band should never exceed 760°C (1400°F). The power of any thin layer of insulation to face up to current flow, yet allow quick heat transfer, creates a reliable performance heater.
Having a heater thickness of just 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 suggests a reduction in scrap upon machine startup. The mineral insulated band is highly responsive to precise heat control due to the 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.