Many mold systems require heat in the manufacturing process. From the plastics industry, heaters are the key ingredient to maintaining temperature of the molten plastic. The plastic flows from 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 considered from the start, because it is an integral part of the general system. There are lots of heater configurations available. However, when viewing 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 have to understand the performance capabilities and limitations for each heater type. The part geometry, temperature and also heat-up time requirements generally dictate the type of heater to make use of.
All the three heater types has distinctive characteristics. The unique material that differentiates these heaters is definitely the interior insulation that gives the appropriate dielectric strength while 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 are available in many areas around the world, including India, southern Africa, and Russia, also in the American continents. Mica can be used in appliances, including toasters and microwaves, along with band and strip heaters. Mica falls to the aluminum silicates category, which means chemically they contain silica (SiO4). The insulation materials used in heater bands offers excellent physical characteristics including thermal, mechanical, electrical and chemical properties. There are 2 primary types 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 carries a unique characteristic in that you can obtain very thin flakes using a consistent thickness. It conducts low levels of heat, especially perpendicular to the strata. In addition, it really is non-flammable, flame-retardant and will not emit fumes. From the heating perspective, mica is really a solid option because of its effectiveness against erosion and arcing, as well as its dielectric strength. Additionally, mica is resistant to chemicals and water, and contains excellent compressive strength. Furthermore, it holds around bending stresses for its high elasticity.
While many mica types can withstand temperatures greater than 1000°C (1830°F), the mica temperature ought not exceed 600°C (1112°F) when utilized in a heater assembly. When temperatures exceed that level, deterioration begins inside the binder as well as a weakening of the dielectric strength will occur.
These features are very important considering that the mica band heater is curved under perpendicular pressure to make a specific diameter. The typical mica band heater is roughly 3/16-inch thick and will accommodate many geometries and special features for example holes and notches. Its design versatility lends itself well for many applications and markets.
The mica bands’ greatest disadvantage is definitely the maximum temperature ability to 480°C (900°F) sheath temperature. There are a lot more processes that need higher temperatures than mica heaters will 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 inside the correct proportion and fired with a certain temperature. L-3 and L-5 are the most prevalent grades of steatite. L-3 is used generally in most applications. However, L-5 is required where low electrical loss is vital. 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 the L-5 kind of material because of its superior electrical characteristics. According to Jim Shaner of Saxonburg Ceramics Inc., “A specific L-5 formula is ready, which contains the appropriate proportions of Al2O3, SiO2, and MgO, along with binders, plasticizers, release agents, or other additives to assist in the processing. The ingredients are then mixed to get a specified time frame and the batch is shipped to the presses.” A press able to pressures as much as 30 tons can be used to press the powder into its finished shape. The last step is to fire the ceramic to a temperature of 2320ºF.
The ceramic knuckle heater is designed to handle around 760ºC (1400ºF). This level of performance is actually a direct reaction to the heaters’ excellent insulating properties of the ceramic knuckle segments. The knuckles interact similar to a ball-and-socket in the knee or elbow to create the heater diameter. Unfortunately, the ceramic’s strength is also its weakness since it stores heat generated through the element wire, which creates difficulty in controlling the heater temperature. This may lead to unnecessary scrap, especially in the early stages in the plastic manufacturing process.
Mineral insulated heaters dominate the market with regards to overall heater performance. Mineral insulated heaters contain magnesium oxide generally known as MgO, which is the oxide of metal magnesium. Magnesium oxide or mineral insulation is actually a fine granular powder in big amounts form. It is actually layered between the resistance dexppky61 and the heater sheath. In several 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 greater than 1094°C (2000°F). Normally, this is never reached, since the heater’s nichrome resistance wire features a far lower operating temperature of around 870°C (1598°F). As a rule of thumb, the temperature of your mineral-insulated band should never exceed 760°C (1400°F). The ability of your thin layer of insulation to resist current flow, yet allow quick heat transfer, creates a reliable performance heater.
With a heater thickness of just 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 allows for higher watt densities which allow the heater to warm the part faster, which implies a reduction in scrap upon machine startup. The mineral insulated band is tremendously responsive to precise heat control due to the 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.