Many mold systems require heat in the manufacturing process. Within the plastics industry, heaters would be the key ingredient to maintaining temperature in the molten plastic. The plastic flows with 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 should be thought about from the start, as it is an important part of the overall system. There are lots of heater configurations available. However, when examining the heater from an insulation standpoint, you can find three common heater types available in the market: mica, ceramic knuckle and mineral insulated.
When contemplating heater type, you must understand the performance capabilities and limitations of every heater type. The part geometry, temperature and also heat-up time requirements generally dictate the sort of heater to make use of.
All the three heater types has distinctive characteristics. The unique material that differentiates these heaters is 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 globe, including India, southern Africa, and Russia, also in the American continents. Mica is used in appliances, for example toasters and microwave ovens, along with band and strip heaters. Mica falls in to the aluminum silicates category, meaning that chemically they contain silica (SiO4). The insulation material used in heater band offers excellent physical characteristics for example thermal, mechanical, electrical and chemical properties. There are 2 primary varieties of mica: (1) muscovite, which contains large amounts of potassium promoting strong mechanical properties and (2) phlogopite containing various quantities of magnesium, which enables it to stand up to 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. Moreover, it is non-flammable, flame-retardant and fails to emit fumes. From a heating perspective, mica can be a solid option due to the potential to deal with erosion and arcing, and its dielectric strength. Additionally, mica is resistant to chemicals and water, and possesses excellent compressive strength. In addition, it holds up to bending stresses for its high elasticity.
While many mica types can withstand temperatures more than 1000°C (1830°F), the mica temperature should not exceed 600°C (1112°F) when employed in a heater assembly. When temperatures exceed that level, deterioration begins in 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 produce a specific diameter. The normal mica band heater is approximately 3/16-inch thick and will 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 the maximum temperature capability of 480°C (900°F) sheath temperature. You will find progressively more processes which 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 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 frequent grades of steatite. L-3 is commonly used in most applications. However, L-5 is usually recommended where low electrical loss is essential. The ceramic is created using industry specific processing methods and can readily be machined or net shape sintered into various designs.
Ceramic knuckle band heaters are produced with the L-5 type of material due to the superior electrical characteristics. As outlined by Jim Shaner of Saxonburg Ceramics Inc., “A specific L-5 formula is ready, containing the appropriate proportions of Al2O3, SiO2, and MgO, in addition to binders, plasticizers, release agents, and other additives to aid in the processing. The ingredients are then mixed to get a specified time period as well as the batch is shipped to the presses.” A press effective at pressures up to 30 tons is utilized to press the powder into its finished shape. The final step is always to fire the ceramic to your temperature of 2320ºF.
The ceramic knuckle heater is made to handle up to 760ºC (1400ºF). This degree of performance is actually a direct outcome of the heaters’ excellent insulating properties of the ceramic knuckle segments. The knuckles come together similar to a ball-and-socket from the knee or elbow to make the heater diameter. Unfortunately, the ceramic’s strength can also be its weakness mainly because it stores heat generated through the element wire, which creates difficulty in controlling the heater temperature. This might lead to unnecessary scrap, particularly in the early stages from the plastic manufacturing process.
Mineral insulated heaters dominate the marketplace with regards to overall heater performance. Mineral insulated heaters contain magnesium oxide generally known as MgO, the oxide of metal magnesium. Magnesium oxide or mineral insulation can be a fine granular powder in big amounts form. It is actually layered 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 posseses an upper useful temperature limit of more than 1094°C (2000°F). Normally, this is never reached, because 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 the mineral-insulated band ought not exceed 760°C (1400°F). The power of the thin layer of insulation to face up to current flow, yet allow quick heat transfer, creates an effective performance heater.
Using a heater thickness of just 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 allows for higher watt densities that enable the heater to heat up the part faster, which implies a decrease in scrap upon machine startup. The mineral insulated band is tremendously responsive to precise heat control for its thin construction and low mass. Less thermal lag and minimum temperature overshoot bring about faster startup and reduced cycle time. Other heaters that utilize mineral insulation are tubular, cable and cartridge heaters.