Many mold systems require heat included in the manufacturing process. In the plastics industry, heaters would be the key ingredient to maintaining temperature of your molten plastic. The plastic flows through the mold base, sprue nozzle, manifold, in a die head, or using an injection barrel. Without heat, the mold or machine is useless.
The heater is highly recommended in the first place, as it is an integral part of the overall system. There are several heater configurations available. However, when examining the band heater from an insulation standpoint, you can find three common heater types available in the marketplace: mica, ceramic knuckle and mineral insulated.
When thinking about heater type, one must know the performance capabilities and limitations for each heater type. The part geometry, temperature as well as heat-up time requirements generally dictate the sort of heater to make use of.
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 even though the heater heats the part. The insulation in each heater plays a tremendous role in determining heater life and gratification.
Mica is primarily extracted from Paleozoic rocks and are available in many areas worldwide, including India, southern Africa, and Russia, along with the American continents. Mica is utilized in appliances, for example toasters and microwaves, together with band and strip heaters. Mica falls to the aluminum silicates category, meaning chemically they contain silica (SiO4). The insulation materials used in mica heaters offers excellent physical characteristics for example thermal, mechanical, electrical and chemical properties. There are 2 primary varieties of mica: (1) muscovite, which contains large amounts of potassium promoting strong mechanical properties and (2) phlogopite containing various amounts of magnesium, which enables it to withstand higher temperatures than muscovite.
Mica has a unique characteristic in that one can obtain very thin flakes with a consistent thickness. It conducts low levels of heat, especially perpendicular to its strata. Furthermore, it is non-flammable, flame-retardant and will not emit fumes. From the heating perspective, mica can be a solid option due to its potential to deal with erosion and arcing, as well as its dielectric strength. Additionally, mica is resistant against chemicals and water, and features excellent compressive strength. Additionally, it holds up to bending stresses due to its high elasticity.
Although some mica types can withstand temperatures above 1000°C (1830°F), the mica temperature must not exceed 600°C (1112°F) when used in a heater assembly. When temperatures exceed that level, deterioration begins in the binder plus a weakening of your dielectric strength will occur.
These functions are essential since the mica band heater is curved under perpendicular pressure to make a specific diameter. The common mica band heater is approximately 3/16-inch thick and might accommodate many geometries and special features like holes and notches. Its design versatility lends itself well for many applications and markets.
The mica bands’ greatest disadvantage is definitely the maximum temperature capacity for 480°C (900°F) sheath temperature. You can find progressively more processes which require higher temperatures than mica heaters can offer.
Steatite is a kind of ceramic comprised primarily of aluminum oxide (Al2O3), silica (SiO2) and magnesium oxide (MgO). Steatite is created when these materials are mixed within the correct proportion and fired at the certain temperature. L-3 and L-5 are the most typical grades of steatite. L-3 is used in many applications. However, L-5 is suggested where low electrical loss is critical. The ceramic is created using industry specific processing methods and may readily be machined or net shape sintered into many different designs.
Ceramic knuckle band heaters are produced using the L-5 kind of material due to the superior electrical characteristics. As outlined by Jim Shaner of Saxonburg Ceramics Inc., “A specific L-5 formula is ready, that contains the right proportions of Al2O3, SiO2, and MgO, in addition to binders, plasticizers, release agents, and other additives to aid in the processing. The constituents are then mixed for the specified time frame as well as the batch is brought to the presses.” A press capable of pressures up to 30 tons can be used to press the powder into its finished shape. The ultimate step is usually to fire the ceramic to a temperature of 2320ºF.
The ceramic knuckle heater was created to handle as much as 760ºC (1400ºF). This level of performance can be a direct outcome of the heaters’ excellent insulating properties from the ceramic knuckle segments. The knuckles interact similar to a ball-and-socket from the knee or elbow to produce the heater diameter. Unfortunately, the ceramic’s strength can also be its weakness because it stores heat generated with the element wire, which creates difficulty in controlling the heater temperature. This can lead to unnecessary scrap, particularly in the initial phases in the plastic manufacturing process.
Mineral insulated heaters dominate the market when it comes to overall heater performance. Mineral insulated heaters contain magnesium oxide known as MgO, the oxide of metal magnesium. Magnesium oxide or mineral insulation is a fine granular powder in big amounts form. It can be layered between the resistance element and the heater sheath. In lots of mineral insulated heaters, the MgO is compacted into a thin solid layer. The compacted MgO offers excellent thermal conductivity and great dielectric strength.
MgO comes with an upper useful temperature limit of more than 1094°C (2000°F). This is usually never reached, as the heater’s nichrome resistance wire carries a much lower operating temperature around 870°C (1598°F). Usually of thumb, the temperature of the mineral-insulated band ought not exceed 760°C (1400°F). The capacity 96dexnpky a thin layer of insulation to face up to current flow, yet allow quick heat transfer, creates an efficient performance heater.
Using a heater thickness of just 5/32-inch, a mineral insulated heater provides rapid heat-up and cool down compared to mica and ceramic knuckle heaters. The compacted insulation also provides for higher watt densities which allow the thermocouple sensors to warm up the part faster, which suggests 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 bring about faster startup and reduced cycle time. Other heaters that utilize mineral insulation are tubular, cable and cartridge heaters.