Many mold systems require heat as part of the manufacturing process. In the plastics industry, heaters are definitely the key ingredient to maintaining temperature of your molten plastic. The plastic flows with the mold base, sprue nozzle, manifold, into a die head, or via an injection barrel. Without heat, the mold or machine is useless.
The heater is highly recommended from the start, because it is a fundamental element of the complete system. There are several heater configurations available. However, when looking at the the heaters band from an insulation standpoint, there are actually three common heater types available in the market: mica, ceramic knuckle and mineral insulated.
When thinking about heater type, you must know the performance capabilities and limitations of every heater type. The part geometry, temperature and 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 the interior insulation that offers the needed dielectric strength even though the heater heats the part. The insulation in each heater plays a substantial role in determining heater life and performance.
Mica is primarily obtained 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 used in appliances, like toasters and microwave ovens, in addition to band and strip heaters. Mica falls into the aluminum silicates category, meaning chemically they contain silica (SiO4). The insulation material used in mica heaters offers excellent physical characteristics including thermal, mechanical, electrical and chemical properties. There are two primary forms of mica: (1) muscovite, containing a lot of potassium promoting strong mechanical properties and (2) phlogopite containing various levels of magnesium, which enables it to resist higher temperatures than muscovite.
Mica features a unique characteristic in that you can obtain very thin flakes by using a consistent thickness. It conducts low quantities of heat, especially perpendicular to its strata. In addition, it is non-flammable, flame-retardant and fails to emit fumes. From your heating perspective, mica is really a solid option because of its potential to deal with erosion and arcing, as well as its dielectric strength. Additionally, mica is proof against chemicals and water, and possesses excellent compressive strength. In addition, it holds up to bending stresses for its high elasticity.
While some mica types can withstand temperatures in excess of 1000°C (1830°F), the mica temperature should never exceed 600°C (1112°F) when utilized in a heater assembly. When temperatures exceed that level, deterioration begins within the binder as well as a weakening of your dielectric strength will occur.
These functions are very important considering that the mica band heater is curved under perpendicular pressure to make a specific diameter. The normal mica band heater is approximately 3/16-inch thick and can accommodate many geometries and special features including holes and notches. Its design versatility lends itself well for several applications and markets.
The mica bands’ greatest disadvantage is definitely the maximum temperature ability to 480°C (900°F) sheath temperature. There are actually progressively more processes that need higher temperatures than mica heaters may offer.
Steatite is a type of ceramic comprised primarily of aluminum oxide (Al2O3), silica (SiO2) and magnesium oxide (MgO). Steatite is created when these materials are mixed in the correct proportion and fired with a certain temperature. L-3 and L-5 are the most prevalent grades of steatite. L-3 is commonly used in most applications. However, L-5 is required where low electrical loss is vital. The ceramic is formed using industry specific processing methods and can readily be machined or net shape sintered into many different designs.
Ceramic knuckle band heaters are produced with all the L-5 type of material because of its superior electrical characteristics. According to Jim Shaner of Saxonburg Ceramics Inc., “A specific L-5 formula is ready, containing the appropriate proportions of Al2O3, SiO2, and MgO, together with binders, plasticizers, release agents, and/or other additives to help in the processing. The constituents are then mixed for the specified period of time along with the batch is brought to the presses.” A press effective at pressures as much as 30 tons can be used to press the powder into its finished shape. The very last step is to fire the ceramic into a temperature of 2320ºF.
The ceramic knuckle heater is made to handle up to 760ºC (1400ºF). This measure of performance is actually a direct result of the heaters’ excellent insulating properties in the ceramic knuckle segments. The knuckles work together similar to a ball-and-socket in the knee or elbow to generate the heater diameter. Unfortunately, the ceramic’s strength is additionally 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, especially in the initial phases from the plastic manufacturing process.
Mineral insulated heaters dominate the industry with regards to overall heater performance. Mineral insulated heaters consist of magnesium oxide generally known as MgO, the oxide of metal magnesium. Magnesium oxide or mineral insulation is a fine granular powder in big amounts form. It is actually layered involving the resistance element and the heater sheath. In lots of 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 reduced operating temperature of approximately 870°C (1598°F). Generally speaking of thumb, the temperature of the mineral-insulated band should never exceed 760°C (1400°F). The capability 96dexnpky a thin layer of insulation to face up to current flow, yet allow quick heat transfer, creates an efficient performance heater.
Having a heater thickness of only 5/32-inch, a mineral insulated heater provides rapid heat-up and cool down when compared with mica and ceramic knuckle heaters. The compacted insulation also allows for higher watt densities that enable the custom thermocouples to warm up the part faster, which suggests a decrease in scrap upon machine startup. The mineral insulated band is highly responsive to precise heat control due to 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.