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Advanced materials are replacing metal parts in everything from planes to sporting goods because they are lighter weight while maintaining the strength required of the product. The materials — liquid resins, fiber cloths impregnated with resin (prepreg) and bonding adhesives, among others — are engineered to have specific curing regimes to maximize strength and durability. In many instances, the resins and adhesives will harden at room temperature; however, heat may be required to complete or speed up the necessary chemical reactions.

This article will examine how surface heaters are used in manufacturing environments to cure these materials. Determining the heater requirements, selecting the heater and applying proper control are among the first steps to successful curing. In addition, this article will explore specific applications to illustrate how heaters and controls are used in manufacturing processes.

Bonding Thermoset Resins

Thermoset resins are polymers used when products require the high structural strength created by molecular cross-linking of the material. They typically will withstand higher temperatures than thermoplastic materials, and once cured, they will not melt or flow. Examples of thermoset resins are polyester, epoxies and phenolics.

Using adhesives to bond structures transfers loads more efficiently than traditional fasteners because more surface area is bonded compared to traditional fasteners with unbonded surfaces. Adhesives may include thermoset resin material and often require heat for complete curing.

Hot bonders are specialized controllers used for ramp/soak applications that may require vacuum for debulking. Typically, they are used for repairs but also may also be used co-bonding parts during the manufacturing process.

Heater Selection for Advanced Materials Bonding

Manufacturers of advanced materials will provide instructions for using their products. Typical instructions may require mixing the resin or adhesive, for instance, or they may specify application conditions — material film thickness or environmental requirements — as well as curing cycles defined by time and temperature.

Surface heaters often are used to cure composite resins and adhesives because direct contact provides efficient heat transfer. Users should follow instructions as written to obtain the full benefit of the material. Heaters that will meet the manufacturer’s requirement for curing should be selected.

When considering which heater to select, choose one based on:

  • Temperature Limits. Blankets should be able to withstand a temperature range beyond what is required.
  • Flexibility. Blankets should be flexible enough to fit tightly against the material to be cured.
  • Construction. Blankets should be constructed for durability and uniformity and suitable for the environment.

The primary factor in heater selection is determining the wattage required for curing the materials to be processed. Heater wattage should be calculated by determining the power needed to raise the temperature from ambient to the maximum required for curing. The specific heat and density of the materials must be considered as well as the volume of material in the structures to be heated.

Insulation placed over the heater increases energy efficiency and temperature uniformity. Using insulation will reduce wattage requirements and is recommended. It is best to add 20 percent additional capacity to provide a built-in safety/buffer factor.

In addition, the material to be cured will dictate acceptable heat levels. Most thermoset resins can be cured at temperatures below 450°F (232°C), making flexible silicone heaters a good choice. Some blankets can conform to a 0.25" (6 mm) radius under vacuum, and silicone usually is moisture and chemical resistant, which eases cleanup. Blankets with smooth surfaces are recommended if the blankets will be in close contact with a resin surface. Otherwise, pressure from debulking could result in a pattern from the heater being transferred to the surface being cured.

2 PH 0222 Briskheat Industrial Heating and Control Cloth Heater

Cloth heaters can be used for curing resins with higher temperature requirements. Photos credit: BriskHeat Corp. (Click on the image to enlarge.)

Curing thermoplastic resins may require temperatures exceeding 450°F (232°C). In such applications, flexible silicone blankets cannot be used. High temperature cloth blankets are capable of higher setpoints and are a good high temperature heater choice if they will not be exposed to moisture.

When determining heater locations, keep in mind that the heaters should allow for heat distribution to the entire area to be cured. They must have intimate contact with the surfaces to be heated. Gaps between the surface heaters and the surface to be heated can cause the blankets to overheat, material to scorch due to poor heat transfer, or imperfections in the cured part. Pressure-sensitive adhesive allows the heaters to be affixed directly to tooling for injection molding of liquid resin parts, reducing the potential for gaps.

Other considerations for heater placement include:

  • The entire surface area of the material being cured must be heated, either through direct contact with a heater or by heat transferred from a heated mold.
  • Heat should be applied from multiple sides for double-sided curing of thick parts.
  • Irregular shapes may require multiple temperature sensors and controllers.

In addition, several heaters can be used to apply heat uniformly such that the materials will cure at about the same rate.

3 PH 0222 Briskheat Industrial Heating and Control On Mold

Heating blankets are secured to the mold, with each blanket having its own controller. Photos credit: BriskHeat Corp. (Click on the image to enlarge.)

Heater Temperature Controllers

Almost all industrial heaters require some type of temperature control. Manufacturers requiring accurate process control temperatures usually select digital temperature controllers with discrete setpoints and alarms. There are three common control schemes: on/off, PID and ramp/soak.

On/off types operate as the name would imply. Without a limiter feature, once the temperature is set, the controller will drive the heater at 100 percent capacity until the setpoint is reached. Next, the controller will stop supplying power to the heater until a low threshold is reached. Then, the heater is supplied with 100 percent power again.

A PID (proportional-integral-derivative) controller uses a control loop with feedback to reduce the output power as the heater approaches the setpoint. This controls heat much more accurately with less overshooting of the setpoint temperature.

Ramp/soak temperature controllers are most often used to cure advanced materials. Manufacturers often specify a ramp cycle to prevent materials from being heated too quickly. As an example, a material may require a ramp speed of 5°F (2.8°C) per minute from ambient to a specific temperature such as 250°F (121°C). The controller automatically increases the setpoint temperature every minute and powers the heater appropriately. Once the material being cured reaches 250°F, the soak feature of the controller will hold the temperature of the product for the prescribed time by cycling the power to the heater as required.

4 PH 0222 Briskheat Industrial Heating and Control Control Panel for 36 Zones

A control panel for 36 zones is used by a manufacturer of fiberglass-reinforced plastic. Photos credit: BriskHeat Corp. (Click on the image to enlarge.)

Surface Heaters in Action

A few brief histories illustrate how surface heaters can be used to provide process heating.

A manufacturer of fiberglass-reinforced plastic vessels and structures uses the spray-up method as one of its manufacturing processes. A reusable mold is covered with silicone blankets placed end-to-end for equal heat distribution. Temperature sensors are placed on each blanket to monitor and control the temperature during curing. Each blanket and sensor are connected to a control panel with separate controllers for each blanket. The controllers are programmed for a two-stage ramp/soak schedule as prescribed by the resin manufacturer. By using individual blankets and sensors, areas requiring more or less power during the cycle can operate independently from other sections.

In another example, electronics manufacturers use secondary bonding processes to perform operations such as potting or gap-filling when accessories are added to a product. Adhesives are used to join two pre-cured parts. The bond requires localized heat to cure the adhesive while protecting the pre-cured parts from damage that could be caused by heat. This process may require a special 3D molded heater constructed to the profile of the joint to be cured.

5 PH 0222 Briskheat Industrial Heating and Control Reusable Vacuum Heater SRV

Blankets should be flexible enough to fit tightly against the material to be cured. Photos credit: BriskHeat Corp. (Click on the image to enlarge.)

One method used to make these heaters is to coat a form with a release agent or film. Liquid silicone is applied to the form and allowed to dry. Resistance heating wire is laid over the silicone in a precise pattern to provide the required wattage for the curing application. Lead wires are added to provide power to the heater. A second layer of silicone is applied over the heating wire.

In conclusion, advanced materials are replacing metal parts in everything from planes to sporting goods because they are lighter weight while maintaining the strength required of the product. Electric surface heaters with accurate temperature control are used to obtain the maximum properties of the material. One manufacturer uses a variety of surface heating products throughout its manufacturing process. See the sidebar for more information.