Plastics Knowledge: Heaters

HEATERS:

Plastics processing and heaters go hand in hand. Cylindrical band heaters are found on barrels of injection rnolding machines and extruders. Flat strip heaters are the heat source on flat surfaces of machines. Cartridge heaters are inserted in reamed holes of dies and other machinery parts. Tubular heaters may be cast in aluminium or strapped on barrels. When used in these applications, the heaters normally heat by conduction.

Radiant heaters are also used in plastics processing. They are found on roll leaf decorating machines, on thermoforming equipment, and in many packaging operations such as shrink packaging and heat sealing.

Heaters have a maximum manufacturers recommended watt density. Watt density is the watts/sq.in. of the active sheath area of the heater that is in contact with the machinery part or mold. Watt density of band heater is determined as follows:

WATTS DENSITY = watts/ID x 3.14 x width

Watt density of cartridge heater is determined as follows:

WATTS DENSITY = watts/(Length 0.05) x OD x 3.14

Maximum watt density of heaters should be used only if heat calculations indicate it is necessary. Excessive watts can waste energy, cause temperature problems and lead to shorter heater life.

Band heaters

Mica insulated band heaters are versatile and efficient. Many original equipment manufacturers pick these heaters since they are low cost, efficient, and easy to control American manufacturers as a rule choose narrow heaters for each zone. Reasoning that if one heater fails the machine probably can continue to operate. Offshore manufacturer choose one or two wide heaters per zone. Reasoning that the heat is more evenly distributed over the zone being heated. Both reasons have merit. As mica-insulated heaters heat by conduction is important they are properly installed in a clean, dry environment and tightened on the cylinder.

Ceramic insulated band heaters are energy efficient, offer long service life and easy to install. So they are the choice of many machinery builders and may be specified by the processor when ordering a new machine. They also may be chosen as replacement heaters for machines in the field. Ceramic insulated heaters incorporate a fibrous insulation that minimizes heat to the atmosphere and therefore, are more energy-efficient. , Many rebuilders of machine and energy conscious processors choose these heaters when retrofitting machine to meet current productivity demands. Interlocking ceramic insulators allow these heaters to flex and make installation easy. Ceramic-insulated heaters may be rated d with higher watt densities than many other heaters. Improved Ceramic-insulators now permit the manufacture of these heaters in the small sizes needed as nozzle band heaters on injection molding machines.

Mineral insulated heaters incorporate a refractory sheet insulation and similar in construction to mica-insulated heaters. They are, however, capable of higher watt density rating than are mica-insulated heaters. They are ridged in construction and are manufactured in close tolerance ID to match the OD of the cylinder being heated.

Refractory insulated heaters are also known as curved strip heaters. These heaters incorporate a compacted ceramic insulation and coiled resistance wire.

Cast in heaters are tubular heaters cast in aluminum and bronze and are used primarily on extruders. Cooling tubes can be cast in and connected to a chiller or heat exchanger where cooling is needed because of overheated cylinders caused by frictional heat from the screw. Cast-in heaters also have fins for air-cooling.

Tubular band heaters may be shaped and clamped on cylindrical shapes or placed in extruded aluminum profiles shaped and clamped on cylinders with straps. They are found on barrels of injection moulding machines and extruders.

Mica insulated nozzle band heaters continue to be choice of the injection. Molders, but their service life may be limited because of contamination, lead Breakage and high watt. Density. High heat requirements have contributed to the introduction of mineral insulated mini tubular and cartridge heaters for this application. Mini-tubular and cartridge heaters are used on nozzles and nozzle adaptors for hot runner mold systems as well.

1. Sheath material brass.

2. Max.operating temp 280°C.

3. Max.surface load 25w/sq. inch.

4. Fiberglass or Teflon insulated SS mixed wire terminals.

Nozzle band heaters may vary according to type, lockup. Flange, strap or key lock up wedge.

Many heater manufacturers will custom- build heaters to suit Specific applications. Custom heaters could be minor. Changes of standard heaters such as watts, volts, type lockup and electrical connections.

Versatile manufacturers will also build special shapes such as oval, rectangular,U shape and partial-coverage heater. Holes through heaters and cutouts can be incorporated in the heater design. Two-piece heaters for large diameters are a common request.

Recent developments include air-cool band heater Assemblies. These units are self-contained and include band heaters, a vent shroud, and a blower. They are available to OEMS or processors who need replacements for air-cooled extruders or water-cooling with air-cooling. Heat exchangers are not needed. Increased thermal and dielectric qualities of both mica insulation and mineral insulation allow higher ratings of heaters.

1. Plastic proof.

2. Sheathing material high grade steel or brass.

3. Operating temp brass 280°C - SS 350°C.

4. Max. surface load Brass 30w/sq. inch, SS sheath 45w/sq. inch.

5. Metal braided Fiberglas wire terminals.

Cartridge heaters are designed to fit into a reamed hole and are needed with many molds and platens. Packaging operations use them in heat-sealing of plastic containers. Stand diameters available are ¼,3/8, ½,⅝,3/4 and 1 inch. Metric sizes are also available. Minimum and maximum length vary depending on diameter and other tolerances.

Standard construction includes 12 in. leads exiting straight from one end of heater. Right-angle exit, longer leads, leads with metallic sleeving to minimize lead damage, or moisture-proof sleeving may be included in the heater design. Thermocouples may be built into the heater. They can incorporate fittings and be made moisture proof for immersion applications.

Strip and plate heaters are designed to be mounted (clamped on flat surfaces of machinery parts, molds, dies, etc.).Most heaters and their insulation medium can be constructed in flat shapes and called strip and plate heaters.

Installation can be via mounting bolts incorporated in the design, or via external clamping arrangements designed in.

Radiant heaters heat by radiation instead of conduction, as do most other heaters. Thermoforming and packaging machines usually incorporate radiant heaters.

Radiant panel heaters normally incorporate mica-insulated heat emitters evenly aligned in metal frame for mounting.Tubler heaters are used in many plastics applications calling for radiant heaters. They can be incorporated in panel heaters and all types of ovens. Flat or cylindrical ceramic- insulated heaters may also function as radiant heaters.

Leaf proof Nozzle Band Heaters

1. Plastic proof.

2. Sheathing material high grade steel.

3. Max.operating temp 600 °C.

4. Max.surface load 60 w/Sq. inch.

5. Insulation - high grade mineral insulation.

6. Metal braided fiberglass wire terminals.

Applications

1. Hot runner bushings.

2. Nozzles of plastic processing machineries.

Types of Terminals

1. 12" Lead or Post Terminals.