Selecting the correct insulation for heaters an important consideration in molding systems
By John Pape | Watlow Product Specialist -- Plant Engineering, 11/3/2006 11:43:00 AM
Many molding systems require heat as part of the manufacturing process. In the plastics industry, heaters are the key ingredients to maintaining the temperature of the molten plastic. The plastic flows through the mold base, sprue nozzle and manifold, into a die-head or through an injection barrel. Without heat, the mold or machine is useless.
Too often it seems that the ‘heat’ of the system is an afterthought. The heater should be considered from the start, as it is an integral part of the overall system. Many heater configurations are available, however, when looking at the heater from an insulation standpoint, three common heater types stand out in the industry: mica, ceramic knuckle and mineral-insulated.
What type of heater do you need?
When considering heater type, one must understand the performance capabilities and limitations of each type. The part geometry, temperature and heat-up time requirements generally dictate the type of heater to use. Many system designers employ a process of elimination when determining the heater type needed for their machine.
Each of the three heater types has distinctive characteristics. The unique material that differentiates them is the interior insulation, which provides the needed dielectric strength while the heater heats the part. The insulation in each heater plays a significant role in determining heater life and performance.
Mica is primarily extracted from Paleozoic rocks and can be found in many areas around the world. Used in appliances such as toasters and microwave ovens, along with band and strip heaters, mica falls into the aluminum silicates category, which means it contains silica (SiO4 ). It appears in nature in two primary forms: muscovite and phlogopite. Muscovite contains large amounts of potassium, promoting strong mechanical properties; phlogopite contains various levels of magnesium, which enables it to withstand higher temperatures than muscovite. The insulation material used in mica heaters offers excellent physical characteristics such as thermal, mechanical, electrical and chemical properties.
Mica has a unique characteristic in that one can obtain very thin flakes with a consistent thickness. Mica conducts low amounts of heat, especially perpendicular to its strata. In addition, it is non-flammable, flame-retardant and does not emit fumes. From a heating perspective, mica is a solid option due its resistance to erosion and arcing, and its dielectric strength. Additionally, mica is resistant to chemicals and water, and it is has excellent compressive strength. It also holds up to bending stresses due to its high elasticity.
While some mica types can withstand temperatures in excess of 1,830 degrees F, the mica temperature should not exceed 1,112 degrees F when used in a heater assembly. When temperatures exceed that level, deterioration begins in the binder and a weakening of the dielectric strength will occur.
These features are important since the mica band heater is curved under perpendicular pressure to form a specific diameter. The typical mica band heater is approximately 3/16-inch thick and can 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 the maximum temperature capability of 900 degrees F sheath temperature. An increasing number of processes require higher temperatures than mica heaters can offer.
Steatite is a type of ceramic comprised primarily of aluminum oxide (Al2O3), silica (SiO2) and magnesium oxide (MgO). Steatite is formed when these materials are mixed in the correct proportion and fired at a certain temperature. L-3 and L-5 are the most common grades of steatite, with L-3 used in most applications. However, L-5 is recommended where low electrical loss is critical. The ceramic is formed using industry specific processing methods and can be readily machined or net-shape-sintered into a variety of designs.
Ceramic knuckle band heaters are made with the L-5 material due to its superior electrical characteristics. According to Jim Shaner of Saxonburg Ceramics, Inc., “A specific L-5 formula is prepared, which contains the correct proportions of Al2O3, SiO2, and MgO, along with binders, plasticizers, release agents and/or other additives to aid in the processing. The ingredients are then mixed for a specified period of time and the batch is sent to the presses.” A press capable of pressures up to 30 tons is used to press the powder into its finished shape. The final step is to fire the ceramic to a temperature of 2,320 degrees F.
The ceramic knuckle heater is designed to handle temperatures of up to 1,400 degrees F, a direct result of the heaters’ excellent insulating properties of the ceramic knuckle segments. The knuckles work together similar to a ball-and-socket in the knee or elbow to create the heater diameter. Unfortunately, ceramics’ strength is also its weakness as it stores heat generated by the element wire, creating difficulty in controlling heater temperature. This can lead to unnecessary scrap, particularly in the early stages of the plastic manufacturing process.
Mineral-insulated heaters dominate the market with respect to overall heater performance. They consist of magnesium oxide, a fine granular powder in bulk form. It is layered between the resistance element and the heater sheath. In many mineral insulated heaters, the MgO is compacted into a thin, solid layer. The compacted MgO offers excellent thermal conductivity and great dielectric strength.
MgO has an upper useful temperature limit of more than 2,000 degrees. This is usually never reached because the heater’s nichrome resistance wire has a much lower operating temperature, about 1,598 degrees. The temperature of the mineral-insulated band should not exceed 1,400 degrees F. The ability of a thin layer of insulation to resist current flow, yet allow quick heat transfer, creates an efficient performance heater.
Choosing the correct heater for an application is important, especially when operated at elevated temperatures. When characteristics such as temperature, performance and efficiency are important, the breakdown of these insulation material types will help guide the buyer into choosing the correct heater for their application.
John Pape has worked for Watlow Electric Manufacturing Company for 17 years. Throughout his career at Watlow he has been involved in many aspects of the plastics industry including manufacturing, design and sales. Pape earned a Bachelor of Arts degree in Business Administration at Lindenwood University and is a member of the Society of Plastics Engineers. His current duties as a technical support specialist include account management, pre- and post-sales support, project management, and he is the sales training coordinator for Watlow’s St. Louis heater division.
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