Many mold systems require heat included in the manufacturing process. In the plastics industry, heaters are the key ingredient to maintaining temperature in the molten plastic. The plastic flows throughout the mold base, sprue nozzle, manifold, right into a die head, or through an injection barrel. Without heat, the mold or machine is useless.
The heater is highly recommended in the first place, because it is an integral part of the overall system. There are many heater configurations available. However, when examining the mica heater from an insulation standpoint, you will find three common heater types available in the industry: mica, ceramic knuckle and mineral insulated.
When considering heater type, you need to comprehend the performance capabilities and limitations of each heater type. The part geometry, temperature and heat-up time requirements generally dictate the sort of heater to make use of.
Each of the three heater types has distinctive characteristics. The unique material that differentiates these heaters is the interior insulation that provides the appropriate dielectric strength as the heater heats the part. The insulation in each heater plays a significant role in determining heater life and gratification.
Mica is primarily obtained from Paleozoic rocks and are available in many areas worldwide, including India, southern Africa, and Russia, plus in the American continents. Mica is used in appliances, including toasters and microwaves, along with band and strip heaters. Mica falls in to the aluminum silicates category, meaning chemically they contain silica (SiO4). The insulation materials in mica heaters offers excellent physical characteristics like thermal, mechanical, electrical and chemical properties. There are 2 primary types 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 stand up to higher temperatures than muscovite.
Mica carries a unique characteristic in that you can obtain very thin flakes with a consistent thickness. It conducts low numbers of heat, especially perpendicular to its strata. Additionally, it is non-flammable, flame-retardant and will not emit fumes. From the heating perspective, mica is actually a solid option for its potential to deal with erosion and arcing, along with its dielectric strength. Additionally, mica is resistant against chemicals and water, and it has excellent compressive strength. It also holds around bending stresses due to its high elasticity.
While many mica types can withstand temperatures greater than 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 from the binder and a weakening from the dielectric strength will occur.
These functions are important since the mica band heater is curved under perpendicular pressure to create a specific diameter. The standard mica band heater is around 3/16-inch thick and may accommodate many geometries and special features such as 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 will find progressively more processes which 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 within the correct proportion and fired in a certain temperature. L-3 and L-5 are the most common grades of steatite. L-3 is utilized generally in most applications. However, L-5 is required where low electrical loss is essential. The ceramic is formed using industry specific processing methods and may readily be machined or net shape sintered into a variety of designs.
Ceramic knuckle band heaters are created with the L-5 type of material for its superior electrical characteristics. As outlined by Jim Shaner of Saxonburg Ceramics Inc., “A specific L-5 formula is prepared, which contains the correct proportions of Al2O3, SiO2, and MgO, in addition to binders, plasticizers, release agents, and/or other additives to assist in the processing. The constituents are then mixed for a specified period of time and the batch is delivered to the presses.” A press able to pressures around 30 tons is utilized to press the powder into its finished shape. The last step is always to fire the ceramic to your temperature of 2320ºF.
The ceramic knuckle heater was designed to handle approximately 760ºC (1400ºF). This level of performance is really a direct consequence of the heaters’ excellent insulating properties of your ceramic knuckle segments. The knuckles interact just like a ball-and-socket from the knee or elbow to create the heater diameter. Unfortunately, the ceramic’s strength is also its weakness as it stores heat generated with the element wire, which creates difficulty in managing the heater temperature. This may lead to unnecessary scrap, especially in the early stages of the plastic manufacturing process.
Mineral insulated heaters dominate the industry regarding overall heater performance. Mineral insulated heaters comprise of magnesium oxide known as MgO, the oxide of metal magnesium. Magnesium oxide or mineral insulation is a fine granular powder in large quantities form. It is layered between your resistance element 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 comes with an upper useful temperature limit of over 1094°C (2000°F). This is usually never reached, as the heater’s nichrome resistance wire includes a reduced operating temperature of approximately 870°C (1598°F). Usually of thumb, the temperature in the mineral-insulated band must not 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 a competent performance heater.
Having a heater thickness of only 5/32-inch, a mineral insulated heater provides rapid heat-up and funky down when compared with mica and ceramic knuckle heaters. The compacted insulation also enables higher watt densities which allow the temperature sensor thermocouple to warm up the part faster, which means a decrease in scrap upon machine startup. The mineral insulated band is extremely responsive to precise heat control due to the thin construction and low mass. Less thermal lag and minimum temperature overshoot cause faster startup and reduced cycle time. Other heaters that utilize mineral insulation are tubular, cable and cartridge heaters.