Many mold systems require heat included in the manufacturing process. From the plastics industry, heaters are the key ingredient to maintaining temperature from the molten plastic. The plastic flows through the mold base, sprue nozzle, manifold, in to 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, as it is an integral part of the complete 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 market: mica, ceramic knuckle and mineral insulated.
When considering heater type, one must know the performance capabilities and limitations of each and every heater type. The part geometry, temperature and heat-up time requirements generally dictate the type of heater to work with.
Each one of the three heater types has distinctive characteristics. The unique material that differentiates these heaters is definitely the interior insulation that gives the needed dielectric strength as 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 can be found in many areas worldwide, including India, southern Africa, and Russia, plus in the American continents. Mica can be used in appliances, for example toasters and microwave ovens, as well as band and strip heaters. Mica falls in the aluminum silicates category, which means that chemically they contain silica (SiO4). The insulation materials in mica heater offers excellent physical characteristics such as thermal, mechanical, electrical and chemical properties. The two main primary kinds of mica: (1) muscovite, which contains a lot of potassium promoting strong mechanical properties and (2) phlogopite containing various quantities of magnesium, which enables it to withstand higher temperatures than muscovite.
Mica carries a unique characteristic in that you can obtain very thin flakes by using a consistent thickness. It conducts low amounts of heat, especially perpendicular to the strata. Furthermore, it really is non-flammable, flame-retardant and does not emit fumes. From a heating perspective, mica is actually a solid option due to the effectiveness against erosion and arcing, along with its dielectric strength. Additionally, mica is proof against chemicals and water, and it has excellent compressive strength. It also holds as much as bending stresses for its high elasticity.
While some mica types can withstand temperatures greater than 1000°C (1830°F), the mica temperature should never exceed 600°C (1112°F) when employed in a heater assembly. When temperatures exceed that level, deterioration begins inside the binder along with a weakening of your dielectric strength will occur.
These functions are crucial since 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 can 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 capability of 480°C (900°F) sheath temperature. There are actually progressively more processes that 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 from the correct proportion and fired with a certain temperature. L-3 and L-5 are the most prevalent grades of steatite. L-3 is commonly used in the majority of applications. However, L-5 is suggested where low electrical loss is vital. The ceramic is created using industry specific processing methods and can readily be machined or net shape sintered into a number of designs.
Ceramic knuckle band heaters are manufactured with all the L-5 form of material due to the superior electrical characteristics. According to 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/or other additives to help in the processing. The components are then mixed for any specified period of time along with the batch is delivered to the presses.” A press competent at pressures up to 30 tons is utilized to press the powder into its finished shape. The very last step is to fire the ceramic to a temperature of 2320ºF.
The ceramic knuckle heater was created to handle up to 760ºC (1400ºF). This level of performance is really a direct consequence of the heaters’ excellent insulating properties of your ceramic knuckle segments. The knuckles come together much like a ball-and-socket within 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 controlling the heater temperature. This may lead to unnecessary scrap, especially in the initial phases of your plastic manufacturing process.
Mineral insulated heaters dominate the market regarding overall heater performance. Mineral insulated heaters contain magnesium oxide referred to as MgO, the oxide of metal magnesium. Magnesium oxide or mineral insulation is really a fine granular powder in mass form. It is actually layered in between the resistance dexppky61 and the heater sheath. In numerous 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 more than 1094°C (2000°F). Normally, this is never reached, as the heater’s nichrome resistance wire includes a much lower operating temperature of around 870°C (1598°F). Typically of thumb, the temperature in the mineral-insulated band ought not exceed 760°C (1400°F). The ability of your thin layer of insulation to face up to 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 compared to mica and ceramic knuckle heaters. The compacted insulation also provides for higher watt densities which allow the heater to warm the part faster, which means a reduction in scrap upon machine startup. The mineral insulated band is very responsive to precise heat control because of 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.