Many mold systems require heat within the manufacturing process. Inside the plastics industry, heaters are the key ingredient to maintaining temperature of the molten plastic. The plastic flows through the mold base, sprue nozzle, manifold, into a die head, or through an injection barrel. Without heat, the mold or machine is useless.
The heater is highly recommended from the start, because it is a fundamental part of the entire system. There are numerous heater configurations available. However, when looking at the heater from an insulation standpoint, there are actually three common heater types available in the business: mica, ceramic knuckle and mineral insulated.
When considering heater type, you need to understand the performance capabilities and limitations of every heater type. The part geometry, temperature and heat-up time requirements generally dictate the particular heater to use.
Each one of the three heater types has distinctive characteristics. The unique material that differentiates these heaters is the interior insulation that gives the appropriate dielectric strength even though the heater heats the part. The insulation in each heater plays a substantial role in determining heater life and satisfaction.
Mica is primarily obtained from Paleozoic rocks and can be obtained from many areas worldwide, including India, southern Africa, and Russia, also in the American continents. Mica is used in appliances, such as toasters and microwaves, along with band and strip heaters. Mica falls in the aluminum silicates category, which means that chemically they contain silica (SiO4). The insulation materials used in mica heater offers excellent physical characteristics for example thermal, mechanical, electrical and chemical properties. There are 2 primary types of mica: (1) muscovite, which contains considerable 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 includes a unique characteristic in that you can obtain very thin flakes having a consistent thickness. It conducts low levels of heat, especially perpendicular to the strata. Furthermore, it is non-flammable, flame-retardant and is not going to emit fumes. From your heating perspective, mica is a solid option due to the resistance to 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 up to bending stresses due to its high elasticity.
Although some mica types can withstand temperatures in excess of 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 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 normal mica band heater is approximately 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 capacity for 480°C (900°F) sheath temperature. There are progressively more processes that need higher temperatures than mica heaters may offer.
Steatite is a form 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 at the certain temperature. L-3 and L-5 are the most typical grades of steatite. L-3 is utilized in most applications. However, L-5 is suggested 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 various designs.
Ceramic knuckle band heaters are created with all the L-5 form of material due to the superior electrical characteristics. Based on Jim Shaner of Saxonburg Ceramics Inc., “A specific L-5 formula is ready, containing the proper proportions of Al2O3, SiO2, and MgO, together with binders, plasticizers, release agents, and other additives to assist in the processing. The constituents are then mixed for any specified length of time as well as the batch is delivered to the presses.” A press able to pressures approximately 30 tons is utilized to press the powder into its finished shape. The very last step is always to fire the ceramic to your temperature of 2320ºF.
The ceramic knuckle heater was designed to handle around 760ºC (1400ºF). This measure of performance can be a direct consequence of the heaters’ excellent insulating properties from the ceramic knuckle segments. The knuckles work together just like a ball-and-socket in the knee or elbow to generate the heater diameter. Unfortunately, the ceramic’s strength can also be its weakness mainly because it stores heat generated by the element wire, which creates difficulty in managing the heater temperature. This may lead to unnecessary scrap, especially in the early stages of your plastic manufacturing process.
Mineral insulated heaters dominate the industry regarding overall heater performance. Mineral insulated heaters comprise of magnesium oxide known as MgO, which is the oxide of metal magnesium. Magnesium oxide or mineral insulation is really a fine granular powder in big amounts form. It can be layered between your resistance dexppky61 and the heater sheath. In several 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 posseses an upper useful temperature limit in excess of 1094°C (2000°F). Normally, this is never reached, since the heater’s nichrome resistance wire features a reduced operating temperature around 870°C (1598°F). Generally speaking of thumb, the temperature of your mineral-insulated band ought not exceed 760°C (1400°F). The ability of a thin layer of insulation to face up to current flow, yet allow quick heat transfer, creates a competent performance heater.
By using a heater thickness of only 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 enables higher watt densities that enable the heater to warm the part faster, which means a reduction in scrap upon machine startup. The mineral insulated band is highly responsive to precise heat control because of its 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.