Band Heaters
A band heater is a heating device that clamps onto objects to provide external heat using radiant and conductive heating. The different mounting methods of band heaters makes it possible to secure them tightly and...
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This article contains everything you will need to know about flexible heaters and their use.
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A flexible heater is a heater made of material that can bend, stretch, and conform to a surface that requires heating. The various forms of flexible heaters include polyimide film, silicone rubber, tape, rope, tank wrapping, gas cylinder, and ones that are designed and engineered for special applications. The structure of a flexible heater consists of chemically etched, screen printed, or wire wound configurations that can conform to the contours of a surface.
The rugged, reliable, accurate, and efficient nature of flexible heaters makes them an ideal heating method that can bond and adhere to any form of substrate. They are an essential part of various industrial, commercial, and military applications due to their lightweight and ability to endure harsh and stressful conditions.
The need for flexible heaters arose from heating situations that were challenging and difficult that required a heater that could adapt to the conditions by having its shape adjusted. This particular problem led to the development of a wide range of heaters capable of heating any type of surface in any type of conditions or circumstances.
Every type of flexible heater is unique and different with additional features that allow it to conform to the conditions for which it is used. Regardless of the many variations of adaptable flexible heaters, there are certain aspects of flexible heaters that are common to all types such as single layer circuits. This aspect of flexible heaters is what differentiates them from other types of heating devices.
Flexible heaters provide heat transfer using highly conductive metals such as copper and resistive alloys like Inconel or Cupro-nickel. Each type of metal has high thermal conductivity and can conform to the flat thin shape of a flexible heater. Heat is generated by high resistance conductors that transfer heat to various substances, fluids, and materials. The width and thickness of the conductors are selected to achieve the desired resistance in accordance with the supplied voltage.
Although there are several types, styles, kinds, and shapes of flexible heaters, the configurations that are found in most flexible heaters are etched foil, screen printed, and wire wound. These configurations involve different manufacturing techniques and methods for supplying electrical energy to the heater.
Etched foil flexible heaters are made by precision etching on a very thin substrate. They are chemically etched with a specific pattern in order to form a circuit like structure with patterns designed for resistive heating elements. Etched foil flex heaters allow for precision control of heat distribution and temperature. They are used in applications where uniform, distributed, and efficient heating is required.
When current is applied to the foil, resistance causes the foil to heat up due to the joule heating effect. As the voltage gets higher, the resistance increases with the result being higher heat output. This principle allows for precision control of heat production in the resistive foil.
Etched foil heaters have a precision conductor layout that makes it possible for designers to create intricate heating patterns. The high precision ensures the placement of heat exactly where it is needed and prevents cold and hot spots. The main reason for the use of flexible heaters is due to their ability to provide uniform heat, which is a critical aspect of heating applications. Etched foil heaters are manufactured to meet and exceed this benefit of flexible heaters.
Wire wound flexible heaters consist of evenly spaced laminated wires placed between sheets of fabricate, which is typically silicone rubber. As with etched foil flexible heaters, wound wire flexible heaters are designed to evenly distribute heat across their enclosing fabric that serves as an insulator and protective covering.
Nickel chrome wire or nichrome wire is typically used for wire wound flexible heaters due to its high resistance to electricity, which makes it ideal for heating elements. The properties of nichrome wire enable it to endure the multiple flexures of a wire wound flexible heater.
Although wire wound flexible heaters are substantially durable and resilient, they take longer to warm up, which can delay the heating process. Wire wound flexible heaters are less expensive and can apply heat over a larger area, a characteristic that enhances their efficiency.
Screen printed flexible heaters are manufactured by screen-printing a conductive ink onto a flexible surface or substrate. The fabrics used for the screen-printing process are various types of polymers and include polyester (PET), polyimide (PI), polycarbonate, and others. The versions of screen-printed flexible heaters include self regulating ones that maintain and sustain a temperature when it is reached and fixed resistor ones.
The screen-printing process has existed for many years and is used to imprint images, patterns, or pictures on fabric. In the mid twentieth century, the use of screen printing was expanded to the creation of printed circuits. The key to the process is the mixing of the ink that is deposited onto the surface of various fabrics using a tool to force the ink through a pattern of a circuit imprinted on a stencil.
The stencil that has the pattern of the circuit determines heating traces and the distribution of heat. The design of the pattern allows heating in accordance with where it is needed. The power density is adjusted by changes to the stencil pattern.
The manufacture of screen-printed flexible heaters is very fast using flatbed screen printing, rotary screen printing, or roll to roll methods. How a screen-printed flexible heater is manufactured is determined by the type of heater and the materials used to produce it.
The main material used to produce a polyimide flexible heater is polyimide, which is a polymer. In order to understand a polyimide flexible heater, it is necessary to understand the characteristics of polyimide, a material that can endure temperatures as low as -200°C (-328°F) and as high as 260°C (500°F). The chemistry of polyimide combines all of the exceptional characteristics of flexible heaters with excellent mechanical properties.
Polyimide polymers can withstand intense heat and maintain excellent electrical insulation properties. The production of polyimides produces a highly flexible structure that is ideal for the manufacture of flexible heaters. The structure of polyimides make it possible to produce intricate and complicated geometries that provide uniform heating across the surface of the heater.
The main characteristics and features of polyimide heaters is their lightweight and versatility combined with outstanding thermal performance. They maintain their form while transferring heat to an application. The thin profile of polyimide heaters makes it possible to fit them into limited tight spaces.
Polyimide flexible heaters are customizable and can be adjusted to meet voltage, wattage, shape, and size requirements, which gives engineers the opportunity to tailor a polyimide heater to their exacting specifications. Additionally, polyimide flexible heaters have outstanding dielectric strength and chemical resistance, characteristics that give them longevity regardless of the conditions.
There are two types of silicone rubber flexible heaters, which are wire wound and etched foil. Wire wound flexible heaters have resistance wires that are laminated between rubber to provide uniform heating and physical strength. They are used with low volume large sized applications.
The heating capabilities of wire wound flexible heaters are determined by the type of wiring used to manufacture them. Nickel chrome or nichrome wiring is one of the more common forms of wiring for silicone rubber flexible heaters and can reach temperatures of 1250°C (2280°F). It is a type of wiring that has high resistivity and oxidation resistance.
As would be expected, copper nickel wiring is used to produce silicone rubber flexible heaters. It is a type of wiring that has medium range electrical resistivity with a low temperature coefficient of resistance. Copper nickel wired silicone rubber flexible heaters are normally used for flexible applications where the maximum temperature needs to reach 600°C (1110°F).
Another form of wiring for a silicone rubber flexible heater is iron chromium aluminum, which is a high resistance material with good form stability for long element life. Iron chromium aluminum wiring has an operating temperature of 1400°C (2550°F). Silicone rubber flexible heaters made with iron chromium aluminum wiring have high surface loading, high resistivity, and low density and can be produced using less material for weight savings.
The other form of silicone rubber flexible heaters are etched foil heaters that are made from chemically etched circuits that are laminated between thin sheets of silicone for quick and even heat transfer. Etched foil silicone rubber flexible heaters have wider widths with tight spacing between elements. The design of silicone rubber etched foil flexible heaters enables them to provide twice the output wattage with multiple heating zones on a single flexible heater. The etched structure of etched foil silicone rubber flexible heaters can be manufactured from aluminum, stainless steel, nickel chrome, or copper.
Polyester flexible heaters are known as polyester film heaters and polyester foil heaters. They consist of conductive ink placed between carrier layers of polyester with a maximum size of 600 mm x 1000 mm (2 ft x 3 ft). Polyester flexible heaters have a resistance to temperatures up to 90°C (194°F) with a maximum temperature resistance of 100°C (212°F). The unfortunate aspect of polyester flexible heaters is the inability to predict the temperature their heating element will reach because there are so many influencing factors in the heating process.
The use of conductive ink ensures even distribution of heat across the surface of the polyester layers. With the ink, it is possible to have low power small areas. The varieties of polyester flexible heaters include fixed self adhesive foil types or ones wrapped in PVC for use with liquids.
Polyester flexible heaters are manufactured using screen printing technology that includes the use of silver-based conductor tracks that are applied for contact with a layer of conductive ink. Part of the variations of polyester flexible heaters is due to the many types of conductive inks that are available with new recipes being continually perfected. Heating adjustments can be made by changing the heating ink, which helps avoid tooling costs.
There are several varieties of flexible heaters that come in multiple configurations, shapes, sizes, types, and designs. Regardless of the types and kinds of flexible heaters, their basic elements tend to be similar and include common features with the main goal being a durable and long-lasting heating device.
The three basic types of flexible heaters are etched foil, wire wound, and screen printed. The difference between the three methods is the materials used as resistors, how heat is transferred, and the materials or fabrics upon which the electrical resistance is placed. Each of the different methods has existed for many years and been improved and perfected over the years. Although their purpose and goal is the same, the production methods for each type are very different.
The substrate of etched foil flexible heaters is an important aspect of their production since it must be able to be easily bent, shaped, and formed. Polyimide and silicone rubber are the most common substrates that carry the foil and serve as dielectric layers that cover the heating element.
Stage One – The first stage in the manufacturing process is the selection of the type of foil and laminate, which are chosen in accordance with the type of heater resistance that is required.
Stage Two – The second step in the process is to select the type of substrate to which the foil will be adhered using a thermoset adhesive layer that is compatible with both materials and forms a tight bond. An essential part of the bond is that it forms a tight enough connection to be able to withstand the chemical etching process and the stresses of the final application.
Additional requirements for the bond include reduced outgassing, UL flame retardance, and mechanical flexing capabilities. To ensure a tight connection, the foil is bonded to the substrate at increased pressure and temperature with the final goal being flat unstressed laminates with high bonding.
Stage Three – To ensure that the layers remain aligned during fabrication, holes are drilled in the base laminate as a registration method for the different layers of the heater. This prevents shifting or movement during the stress of chemical etching.
Stage Four – The fourth step of the etching process is to place the pattern of the conductive element on the laminate, which begins with the application of a photo imageable resist to the foil and polyimide. A mask layer with the dimensions and shape of the heater element is placed over the photo imageable resist and exposed to UV light that cures the resist for chemical etching. The cured resist protects the heater element as the non cured resist is removed.
Stage Five – The fifth stage is the etching of the heater element into the foil by passing it through a series of chemical etching, stripping, and cleaning cycles that remove unprotected foil. The process requires precision control and accuracy to ensure that the heater element has the correct thickness and width. The types of chemicals used in the etching process vary in accordance with the type of foil.
Once the etched foil is cleaned and the chemicals are removed, the resistance of the heater is tested before the application of the top dielectric lamination is applied.
Stage Six – The final step in the process is the application of the top dielectric lamination that includes a dielectric film or coverlay that has a thermoset adhesive on one side and pre-drilled holes to align it with the laminate and access points. In order to provide for wire attachments, sensor attachments, and component mounting, access points are placed in the laminate along with the alignment holes.
The key aspect of the construction of a wire wound flexible heater is the selection of the type of resistance wire. The manufacturing process for wire wound resistance heaters is not as complicated or complex as the processes for screen printed and etched foil flexible heaters since there is no need for the use of chemicals. The main factors of the process are the wires and the overlay, which is normally some form of silicone rubber or other form of flexible and pliable material.
Wire – The types of wire used for wire wound flexible heaters are nickel chrome or nichrome wires, which are a brand of resistance wire made of 80% nickel and 20% chromium. The diameter of the wire is carefully planned and designed to ensure the highest efficiency from the heater. Nichrome wire is a high resistance wire with high resistivity and exceptional resistance to oxidation at high temperatures, which is the reason it is used in wire wound flexible heaters. Nichrome comes in several gauges, which gives engineers flexibility in flexible heater design.
Fiberglass Cord – The wire is placed in a fiberglass cord to add flexibility and support for it. The fiberglass cord serves as protection and insulation and is capable of withstanding temperatures as high as 801°C (1475°F), which is the reason it is used for wire wound flexible heaters.
Pattern – As with etched foil flexible heaters, the pattern for the wiring for wire wound flexible heaters is important for the even distribution of heat. The pattern is carefully planned and engineered for the placement of the wires in the enclosing material. It is designed in accordance with the diameter of the wire being used. Included in the pattern is the distance between the wires, which further influences the distribution of heat.
The wire is laid out in its pattern to ensure it will provide uniform heat and meets the requirements of size and shape for the material used to enclose it. The examination of the pattern helps in avoiding holes or cutouts and determines where the concentration of the heat profile as dictated by the design. Lead wires and cord sets are attached during the pattern examination using soldering.
Attaching to the Substrate – The examined and tested pattern is vulcanized into a neoprene substrate or silicone rubber that is reinforced with nylon. The thickness of the substrates, wires and lead wires is approximately 0.813 mm (0.032 in). As with all forms of flexible heaters, the thickness of the heater makes it possible to fit into tight spaces and restrictive areas.
There are aspects of the screen-printing flexible heater process that are similar to the methods used for the production of etched foil flexible heaters since both processes involve creating a pattern that will be overlaid on another material. Unlike the etched foil process, screen printing does not involve acids but relies on specially mixed inks to serve as the conductive element.
Step One – The shape and form of the substrate is determined by the geometric pattern of the engineering drawing’s shape and dimensions. The substrate is chosen for its ability to accept the imprint of the pattern and withstand the stress related to flexible heater use.
Step Two – The composition of the substrate for screen printing includes various types of polymers, such as polyimide and polyethylene terephthalate (PET), which are used in very thin sheets selected for their durability and resilience. The pattern for the electrical element is imprinted on the sheets using one of various screen-printing processes.
Step Three – The third step in the process is the selection and mixing of the ink, which is a complex process that involves choosing the right ingredients in the proper portions. Pastes or inks include carbon paste ink or silver paste ink that are mixed with a solvent to produce a paste-like consistency. Mixing uses microscopic dispersion to create a highly homogeneous mixture. The final result is a paste or ink that can be used for printing the conductive element onto the substrate.
Step Four – The stencil that has been designed with the dimensions of the pattern of the conductor is a template that allows ink to pass through to the substrate to imprint the conductor pattern. It is designed to block ink flow to areas that are not to be imprinted. Stencils can be framed or frameless and are repeatedly used to create screen printed flexible heaters. Framed stencils are used for high volume production of screen-printed flex heaters while frameless stencils are less expensive and easy to store.
The material for the stencil is a thin sheet of plastic or metal that has the pattern cut into it. The material of the pattern has to be sufficiently strong to withstand the printing process that is performed at high pressure.
Step Five – Printing of the pattern on the substrate can take several forms with the goal being the placing of the resistive conductor on the substrate. The screen-printing process is one of the oldest printing methods and is simple, flexible, and economical. Regardless of the method, screen printing involves forcing the paste or ink through the stencil onto the substrate. For flexible screen-printed heaters, the pattern is printed directly onto the substrate. The screen-printing process allows for control of the thickness and width of the heating element in order to accurately control heater resistance, wattage, watt density, and heating uniformity.
Step Six – The printing process places the heating element on the substrate. In this step of the process, the printed pattern is adhered to the substrate by heating it to remove any liquid. As the ink is heated, it is bonded to the substrate forming a permanent heating element. The bonding and curing process enhances the mechanical strength of the flexible heater and increases its flexibility.
Step Seven – The final step in the screen printing of flexible screened printed heaters is placing a dielectric insulated layer over the heating element, which can be another polymer or epoxy that is printed, coated or sprayed on. As with all forms of flexible heaters, terminations are connected with the dielectric layer using epoxy or silicone.
Flexible heaters have gradually become an essential component of many industries due to their adaptability, durability, and efficiency. Their ability to be shaped and fitted to meet the needs of any application makes them a tool for heating large and small applications. The materials, substrate, and elements of flexible heaters are designed to withstand any environment and conditions.
In certain industries, equipment moves rapidly from warm to cold conditions, which causes parts to shrink. As the parts move back to warm conditions, they expand. The fluctuation in temperature creates physical changes such as cracks, stress, and wear to the point that parts fail and suffer damage. In those conditions, flexible heaters provide thermal control to keep parts at a steady, even, and warm temperature to avoid expansion and contraction, a factor that helps prolong the life of parts.
Security cameras, night vision equipment, surveillance equipment, and mirrors encounter humidity, moisture, temperature changes, fog, and mist that clouds their lenses and prevents them from working effectively. Under those conditions, flexible heaters are installed directly on a device to remove moisture and prevent lens from fogging. The surface of equipment lenses is kept warm and dry if a component is stationary or in motion.
Equipment that is placed outside has moving parts that are essential to their operation. Inclement weather causes moisture to build up on mechanisms of such equipment, which causes malfunctions. In climates where there are low temperatures, the built up moisture can freeze and make equipment inoperable by forming ice clogs. In order to prevent expensive repairs, flexible heaters are used to prevent moisture build up and damage. They are ideal for this type of work due to their flexibility, size, and precision temperature control.
The stabilization of product viscosity is one of the widest uses for flexible heaters since normal heating elements are incapable of being moved to or with a product. Chemical producers, food processors, cosmetic companies, and oil companies make use of flexible heaters to keep their products or the ingredients for their products free flowing such that they can be easily dispensed.
The problem with many fluids is the change in their viscosity depending on the environments through which they are traveling, production processes, and their basic characteristics. Flexible heaters are able to stabilize the viscosity of liquids and control their thickness.
During the processing of products, changes may occur that affect the characteristics and texture that influences the quality of the final product. This can occur with melted chocolate for candy and pastry batter for doughs and confections. Significant changes affect the flow, texture, and properties of the raw materials to the point that they damage products.
A key factor for most products is the maintenance of a steady and reliable environment to help maintain product quality. Prepared foods have to have a safe temperature to prevent bacteria growth while medical laboratories have to maintain constant temperatures for specimens and cultures.
In all these examples, flexible heaters are used to provide even, stable, and steady temperatures that prevent damage and harm. By establishing temperature control, they help with cost savings, prevent waste, ensure quality, and avoid the acquisition of unwanted properties.
Sensors have become a stable part of many environments as means of detecting and responding to various forms of input. They are detection devices designed to protect special locations and resources. Part of the functioning of sensors involves overseeing outdoor activity and indoor activity at remote locations, which subjects sensors to low temperatures.
Flexible heaters are used to keep sensors warm without interfering with their function. By supplying sufficient heat, sensors can continue to examine external conditions without signal failure.
The finishing process for products can involve the use of glue, sealant, laminate, or bonding agents that are required to seal or cure an item. During mass production, allowing products to set while the glue or sealant sets is not possible especially when there are large batches. Flexible heaters are used to maintain a steady temperature on products for quick and efficient curing and bonding processes without any temperature drop.
Electronics can become damaged by cold temperatures, even without the presence of moisture. Semiconductors and circuits experience signal interruption and potential failure in such conditions. Batteries that experience excessively high temperatures degrade and fail while extreme cold can have the same effect. Flexible heaters ensure consistent temperatures and help avoid radical and damaging temperatures.
In essence, vacuum environments do not have any atmospheric conditions and do not experience temperature changes for the safe operation of equipment, which components may require. Since flexible heaters are direct heaters, they can provide the stability that is required by equipment in vacuum conditions such as clean rooms and vacuum chambers.
One of the abilities of screen printed flexible heaters is to heat fabrics such as seats and clothing. They are used in cars to provide heated seats and for heating instruments. Heating pads, blankets, and therapeutic equipment are used to provide patients with warmth during recovery and treatment for injuries. Flexible heaters are used in incubators to provide steady, constant, and consistent heat for newborns.
The output wattage of a flexible heater is an important factor in its design. The correct output wattage ensures that a flexible heater has enough resistance, voltage, and current to generate sufficient heat. The mathematics for determining output wattage involves multiplying the volts by the amps, which equals the wattage.
Wattage is expressed as watts per square inch or wpsi. The use of Ohm’s law ensures that a flexible heater is generating heat for an application without going over allowable tolerances for an application or heater. Each type of flexible heater has limitations, which has to be determined prior to selecting a flexible heater to ensure that the chosen heater matches the needs of the application for which it is purchased.
Ohm’s law shows the relationship between voltage, wattage, amperage, and resistance. It is used to calculate the load that will be switched with a thermostat or digital controller. It is used to ensure the correct application of charge for the given resistance since resistance cannot be measured in an open circuit. Knowledge of voltage, current, and resistance make it possible for any two factors to be used to calculate the third factor.
The use of Ohm’s law makes it possible to validate the static values of circuit components, current levels, voltage supplies, and voltage drops, knowledge of which are necessary for proper use of a flexible heater and matching it to the correct applications.
The formula for Ohm’s law involves current equal to voltage divided by resistance. In flexible heaters, determining the strength of the current by amperage based on operating power of a flexible heater is wattage divided by voltage. Ohm’s law is an essential part of flexible heater design in order to determine the amount of resistance that will be created measured in ohms. In a flexible heater, resistance is transformed into heat, which is represented by watt density. In normal electronics, the goal is to minimize heat to avoid damage to circuit boards, circuitry, and other components. With flexible heaters, the goal is to have higher resistance in order to create more heat that will pass through the element of a flexible heater to the application.
Since flexible heaters are made from silicon rubber or polyimide materials, they are able to withstand the temperatures created by the electrical resistance. Silicon rubber can withstand temperatures of 221°C up to 260°C (430°F up to 500°F) while polyimide can withstand temperatures of 149°C up to 199°C (300°F up to 390°F).
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