Focused Infrared Solutions - Thermal Curing, Preheating,
Annealing of Extruded Plastic Pipe, Hose, and Profiles

Background
Quartz halogen lamps are effective in many industrial heating applications including preheating, curing and annealing of plastic extruded materials. Quartz halogen lamps with focusing reflectors were originally developed for applications in the United States Space Program. These applications often required rapid heating rates, precise control, and the ability to turn the heat on and off instantly. The same benefits also apply in plastics processing, as noted below:

• Focused infrared can heat the extruded shape 5 to 10 times faster than with gas convection, resulting in greater production throughput, or a smaller footprint.
  
  
• Penetrating short wave infrared heats directly into the depth of the extruded material, giving a more uniform heating and cure.
  
  
• Instant on and off capability of a quartz lamp/chilled aluminum reflector system offers process and operator safety in the event of a line stoppage

Specific applications that have benefited form focused infrared heating that will be discussed include:

• Curing of silicone medical tubing
  
  
• Preheating and curing of silicone braided hose
  
  
• Annealing of multi-layer plastic extrudate on wire and cable

Quartz Halogen Lamp Description
Quartz halogen lamps are similar to the incandescent lamps used in everyday lighting applications. The similarities include the ability to instantly turn on and off, and the ability to focus the energy. The differences are that a quartz envelope is used (instead of glass) and the filament if larger, which improves lamp life and increases the emitting area. The quartz halogen lamp (see Figure 1) consists of a helically wound tungsten filament encased in a quartz envelope. Tungsten is a fast responding resistive elements that is capable of surviving temperatures in excess of 2750 °C. The quartz envelope allows transmission of IR energy, while protecting the filament from convective cooling and corrosion. The quartz is evacuated and filled with inert gases. A small amount of halogen gas is added to promote long life of the emitter. Rated life for most T3 lamps is 5000 hours. Actual life will depend upon the application and environment and may be greater of less than rated.


Figure 1: T3 Quartz Halogen Lamp

Quartz halogen lamps were originally designed for lighting applications. In fact, there are many applications today for quartz lamps in area, accent and entertainment lighting. Of the energy emitted when a lamp is at 2227 °C, less than 6% of the energy is in the visible spectrum, which is the energy used in lighting applications. Over 94% of the energy is in the IR range of the spectrum, which has a useful propose in industrial heating.

Ability to Focus Infrared
Quartz halogen lamps have small filaments, which optically allows the energy to be focused towards a target. The lamps are available as linear emitters (T3 lamps) or point source emitters. Using a T3 lamps and an elliptical reflector shape, (see Figure 2), the energy of the lamp can be


Figure 2: Focusing Infrared with a Polished Elliptical Reflector

redirected to a fine line. An ellipsoidal reflector can be used with a point source lamp to focus the energy to a point. A parabolic reflector generates parallel rays (rays that do not spread), resulting in narrow strips of heat. The reflector may be made of specular aluminum, ceramic, or other metals. The advantage of the aluminum reflector system is that it can be continually cooled and does not become a significant emitter if IR. In additions that aluminum is polished to reflect and focus over 90% of the incident energy. Aluminum has several advantages including the ability to water cool, it is highly relatively low cost.



Figure 3: Chamber Heater

Multiple focused heaters can be arranged to heat radically inward, to surround the product with IR energy. Using this technique, products such as wire and tubing can be uniformly heated around their circumference. High speed drying, curing, and annealing of continuous tubular materials is achieved effectively with this heating arrangement. Figure 3 displays four elliptical line heaters arranged to heat small diameter continuous specimens with high density IR. Multiple parabolic reflectors can be used in a similar arrangement to effectively heat tubular shapes up to 23.5 centimeters in diameter.

The ability to focus energy can result in a number of benefits. Energy savings can be achieved by taking advantage of the capability to focus the IR energy to a point, Line, strip, small area, or radially inward. The saving occurs since the heat is directed mainly at the area that needs to be heated, and not the entire specimen. Cooling requirement for the product being heated are also reduced, as the entire product is not heated. By focusing the IR, very high heating densities can be achieved in a small area resulting in high line speeds or a small oven footprint. For non-circular profiles, the chambers can be zoned to vary the heat around the circumference. Thus profiles such as window molding may be uniformly heated by varying the zone setpoints.


Figure 4: Lamp Emission at 2227°C

Rapid Heating with Focused, Short Wave IR
Heating rates to the extrudate are 5-10 times faster than with conventional systems. There are several reasons for this. First, the ability to focus infrared increases the heating density on the product surface (see previous discussion). Second, quartz halogen lamps operate at a very high temperature and have a high resulting heat flux (as compared with conventional systems). And third, the emission spectrum of a quartz halogen lamp products a high percentage (see Figure 4)
of short wave infrared (.78 to 1.5 microns). This energy will penetrate into many plastic materials, which allows thermal energy to arrive instantly into the depth of the material being heated. With convection (or medium wave IR emitters), the heating is done form the surface. The material then must thermally conduct the heat into the middle to fully cure. This process is slow, as most plastics are not good thermal conductors. This, quartz halogen lamp systems may heat much faster, as the thermal energy is arriving instantly into the depth, and not rely on thermal conduction.


Figure 5: Short Wave Heating Penetration

Instant on/Instant off
There are two components of heating the influence the system's heating response. The response of the quartz halogen lamp is nearly instant. Keep in mind the comparison to a light bulb. When power is applied, the resulting light (and thus heat) occurs very rapidly. Quartz halogen lamps have a response time of approximately one second. Compare this to other emitters with response time of minutes and even hours.
The seconds component of the heater's response is the reflector. In the case of the chilled aluminum reflector, there is no warm-up time involved, as the reflector is continuously cooled. Thus, the heating and cooling response of the system is the same as the lamp.

The instant on/off feature of the T3 lamp/aluminum reflector system yields a potential safety benefit to operators and to the product. In an extrusion application, conventional hot wall heaters will burn the product in the event of a line stoppage. This is due to the high heat capacity and slow response. A quartz halogen lamp/aluminum reflector system does not have this problem, as the aluminum is continuously cooled and the T3 lamp dissipates it energy instantly. This type of system can be controlled to not burn the product in a line shut down situation.
Maintenance can also begin within minutes (versus hours) in this system due to the rapid cool down.

Energy savings may also be achieved using a halogen lamp/aluminum reflector system. This ia possibly as the system requires virtually no warm-up time (and nearly instantly cools as well). The system can also be turned off during breaks or between shifts. This is in stark contrast to hot wall type systems that require hours to heat to equilibrium and perhaps days to cool the system down for maintenance.

Other Benefits
Quartz halogen lamp/aluminum reflector systems share other benefits evident in all IR systems. Since IR is a non-contact heat transfer, there is no surface disturbance on the heated specimen. IR heat is direct between the source and the target. This allows high system efficiencies of 60 to 70 percent in many applications. IR energy can also be shielded from temperature sensitive areas through masking.

Applications - Curing of Silicone Medical Tubing (reference Research Inc. Application Note 5527-A-01-B)
A manufacturer of extruded silicone medical tubing previously cured the extrudate with a 30 foot (9.1m) gas convection oven. Some of the problem areas for this manufacturer are listed:

• Change over from one size of tubing to another required moving the existing furnace to gain access of the extruding head
  
  
• The slow response time of the furnace (over 2 hours) meant excessive downtime when warming up or changing oven profiles. This long warm up/change over time also created energy wastage.
  
  
• The product quality was often variable, due to limitations in the control of the existing oven
  
  

Applications - Preheating/Curing of Silicone Braided Hose (reference Research Inc. Application Note 5527-A-03-A)
A manufacturer of braided hose previously preheated the hose with low density infrared resistance heaters. Some of the issues of the previous system included:

• The low heating density of the non-focused resistance heaters limited the line speed to 25 feet per minute (7.6m/min), which did not meet the required production through put.
  
  
  Figure 6: Application-Preheating/Curing Silicone
  
  
• The slow response of the resistance heaters caused fires when the line stopped. The heaters' high thermal mass cooled slowly, and continued to provide heat to the products even after the power was shut down.

The solution used six focused infrared chamber. Two of the heaters were used prior to the extruder to preheat the raw, braided silicone. Four units were mounted vertically after the extruder to cure the silicone. The manufacturer was able to increase its speed to 80 feet per minute (24.4 m/min). the line was controlled with a speed detection system to immediately shut off the lamps in the event of a line stoppage with out causing a fire. The even heating provided by the chamber heaters along with the precise control also improved the surface finish and reduced scrap.

Application - Annealing PVC Molding (reference Research Inc. Application Note
5506-A-07-A)
A manufacturer of extruded PVC automobile molding used low density, non-focused infrared heaters to anneal the PVC molding after the folder unit. The material needed to be heated to 180° F (82 °C) to be annealed. The previous system could run at 23 feet per minute (7.0 m/min), which did not meet the production requirement. The solution was to mount four infrared parabolic strip heaters above and below the molding (refer to Figure 7). The line speed was increased by 39%, and met the production requirement. The product quality improved due to the enhanced control capability. Power savings were also realized, as the heaters could now be shut off in between production runs.


Figure 7: Application-Annealing PVC

Summary
Quartz halogen lamp/aluminum reflector systems offer unique features including the ability to focus heat, turn heat instantly on and off, and the ability to heat rapidly. Users of this type of system realize many benefits including higher line speeds, reduced floor space requirements, lower energy consumption, and product and operator safety.

Reference: Cox, N., "Benefits of Quartz Halogen Lamp/ALuminum Reflector Systems:, Process Heating, June, 1998.