Many mold systems require heat within the manufacturing process. Inside the plastics industry, heaters would be the key ingredient to maintaining temperature of your molten plastic. The plastic flows with the mold base, sprue nozzle, manifold, in to 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, as it is an important part of the entire system. There are numerous heater configurations available. However, when looking at the heater from an insulation standpoint, there are three common heater types available in the marketplace: mica, ceramic knuckle and mineral insulated.
When contemplating heater type, you must be aware of the performance capabilities and limitations of each and every heater type. The part geometry, temperature and heat-up time requirements generally dictate the particular 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 which offers 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 extracted from Paleozoic rocks and are available in many areas worldwide, including India, southern Africa, and Russia, plus in the American continents. Mica is utilized in appliances, like toasters and microwave ovens, in addition to band and strip heaters. Mica falls in to the aluminum silicates category, which means chemically they contain silica (SiO4). The insulation materials used in band heaters offers excellent physical characteristics such as thermal, mechanical, electrical and chemical properties. There are two primary types of mica: (1) muscovite, which contains considerable amounts of potassium promoting strong mechanical properties and (2) phlogopite containing various levels of magnesium, which enables it to stand up to higher temperatures than muscovite.
Mica has a unique characteristic in that you can obtain very thin flakes using a consistent thickness. It conducts low quantities of heat, especially perpendicular to the strata. Furthermore, it is non-flammable, flame-retardant and will not emit fumes. From a heating perspective, mica is really a solid option because of its resistance to erosion and arcing, as well as its dielectric strength. Additionally, mica is immune to chemicals and water, and features excellent compressive strength. In addition, 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 must 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 characteristics are important because the mica band heater is curved under perpendicular pressure produce a specific diameter. The typical mica band heater is roughly 3/16-inch thick and might accommodate many geometries and special features such as holes and notches. Its design versatility lends itself well for several applications and markets.
The mica bands’ greatest disadvantage is the maximum temperature ability to 480°C (900°F) sheath temperature. You can find progressively more processes that require higher temperatures than mica heaters will 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 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 suggested where low electrical loss is critical. 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 with all the L-5 kind of material because of its superior electrical characteristics. As outlined by Jim Shaner of Saxonburg Ceramics Inc., “A specific L-5 formula is prepared, containing the appropriate proportions of Al2O3, SiO2, and MgO, in addition to binders, plasticizers, release agents, and/or other additives to help in the processing. The components are then mixed for a specified length of time as well as the batch is shipped to the presses.” A press able to pressures approximately 30 tons is utilized to press the powder into its finished shape. The final step would be to fire the ceramic to some temperature of 2320ºF.
The ceramic knuckle heater was created to handle approximately 760ºC (1400ºF). This amount of performance is really a direct result of the heaters’ excellent insulating properties of your ceramic knuckle segments. The knuckles come together just like a ball-and-socket within the knee or elbow to generate the heater diameter. Unfortunately, the ceramic’s strength can also be its weakness since it stores heat generated with the element wire, which creates difficulty in controlling the heater temperature. This can lead to unnecessary scrap, especially in the early stages in the plastic manufacturing process.
Mineral insulated heaters dominate the market when it comes to overall heater performance. Mineral insulated heaters include magnesium oxide referred to as MgO, the oxide of metal magnesium. Magnesium oxide or mineral insulation is actually a fine granular powder in bulk form. It is layered involving the resistance dexppky61 and also 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). This is usually never reached, as the heater’s nichrome resistance wire features a lower operating temperature of around 870°C (1598°F). Generally speaking of thumb, the temperature of the mineral-insulated band ought not exceed 760°C (1400°F). The ability of any thin layer of insulation to face up to current flow, yet allow quick heat transfer, creates an efficient performance heater.
By using a heater thickness of just 5/32-inch, a mineral insulated heater provides rapid heat-up and cool down in comparison with mica and ceramic knuckle heaters. The compacted insulation also enables higher watt densities that enable the heater to warm the part faster, which suggests a decrease in scrap upon machine startup. The mineral insulated band is very responsive to precise heat control because of 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.