Many mold systems require heat within the manufacturing process. Inside the plastics industry, heaters will be the key ingredient to maintaining temperature from the molten plastic. The plastic flows through the mold base, sprue nozzle, manifold, in a die head, or via an injection barrel. Without heat, the mold or machine is useless.
The heater should be thought about in the first place, because it is a fundamental element of the entire system. There are many heater configurations available. However, when viewing the band heater from an insulation standpoint, there are actually three common heater types available in the marketplace: mica, ceramic knuckle and mineral insulated.
When contemplating heater type, you need to comprehend the performance capabilities and limitations of every heater type. The part geometry, temperature and heat-up time requirements generally dictate the kind of heater to utilize.
All the three heater types has distinctive characteristics. The unique material that differentiates these heaters will be the interior insulation that provides the needed dielectric strength even though the heater heats the part. The insulation in each heater plays a tremendous role in determining heater life and gratifaction.
Mica is primarily obtained from Paleozoic rocks and may be found in many areas around the globe, including India, southern Africa, and Russia, along with the American continents. Mica is commonly used in appliances, including toasters and microwave ovens, along with band and strip heaters. Mica falls into the aluminum silicates category, which means that chemically they contain silica (SiO4). The insulation materials used in mica heaters offers excellent physical characteristics like thermal, mechanical, electrical and chemical properties. There are 2 primary forms of mica: (1) muscovite, that contains huge amounts of potassium promoting strong mechanical properties and (2) phlogopite containing various amounts of magnesium, which enables it to stand up to higher temperatures than muscovite.
Mica features a unique characteristic in that one can obtain very thin flakes having a consistent thickness. It conducts low levels of heat, especially perpendicular to its strata. Moreover, it is non-flammable, flame-retardant and is not going to emit fumes. From your heating perspective, mica is really a solid option due to the effectiveness against erosion and arcing, along with its dielectric strength. Additionally, mica is resistant against chemicals and water, and contains excellent compressive strength. Furthermore, it holds around bending stresses because of its high elasticity.
While many mica types can withstand temperatures in excess of 1000°C (1830°F), the mica temperature must not exceed 600°C (1112°F) when found in a heater assembly. When temperatures exceed that level, deterioration begins within the binder and a weakening of the dielectric strength will occur.
These traits are crucial because the mica band heater is curved under perpendicular pressure to create a specific diameter. The normal mica band heater is roughly 3/16-inch thick and might 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 is definitely the maximum temperature ability to 480°C (900°F) sheath temperature. You will find progressively more processes which require higher temperatures than mica heaters can provide.
Steatite the type of ceramic comprised primarily of aluminum oxide (Al2O3), silica (SiO2) and magnesium oxide (MgO). Steatite is actually created when these materials are mixed in the correct proportion and fired in a certain temperature. L-3 and L-5 are the most prevalent grades of steatite. L-3 can be used in most applications. However, L-5 is usually recommended where low electrical loss is vital. The ceramic is created using industry specific processing methods and might readily be machined or net shape sintered into a number of designs.
Ceramic knuckle band heaters are created with the L-5 form 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 correct proportions of Al2O3, SiO2, and MgO, along 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 and also the batch is delivered 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 last step is always to fire the ceramic to a temperature of 2320ºF.
The ceramic knuckle heater was designed to handle as much as 760ºC (1400ºF). This amount of performance can be a direct result of the heaters’ excellent insulating properties in the ceramic knuckle segments. The knuckles interact just like a ball-and-socket in the knee or elbow to make the heater diameter. Unfortunately, the ceramic’s strength is likewise its weakness since it stores heat generated from the element wire, which creates difficulty in managing the heater temperature. This can lead to unnecessary scrap, particularly in the initial phases of the plastic manufacturing process.
Mineral insulated heaters dominate the current market regarding overall heater performance. Mineral insulated heaters comprise of magnesium oxide called MgO, the oxide of metal magnesium. Magnesium oxide or mineral insulation is really a fine granular powder in mass form. It really is layered in between the resistance element and also the heater sheath. In several mineral insulated heaters, the MgO is compacted right into a thin solid layer. The compacted MgO offers excellent thermal conductivity and great dielectric strength.
MgO has a upper useful temperature limit in excess of 1094°C (2000°F). Normally, this is never reached, since the heater’s nichrome resistance wire includes a reduced operating temperature of about 870°C (1598°F). As a rule of thumb, the temperature of your mineral-insulated band should not exceed 760°C (1400°F). The ability 96dexnpky a thin layer of insulation to face up to current flow, yet allow quick heat transfer, creates a reliable performance heater.
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 enables higher watt densities which allow the thermocouple sensors to heat the part faster, which means a decrease in scrap upon machine startup. The mineral insulated band is highly responsive to precise heat control for its thin construction and low mass. Less thermal lag and minimum temperature overshoot result in faster startup and reduced cycle time. Other heaters that utilize mineral insulation are tubular, cable and cartridge heaters.