A new concept for microwave/hot air curing systems.

The Halstead Innovation Center commissioned Gerling Laboratories to build a microwave/hot air vulcanizing system for extruded sponge products using several new concepts.

The decision to try a new system approach was based on Halstead's desire to develop a microwave/hot air system which would produce a product having the same quality as that produced by the salt bath (LCM) method. It is Halstead's long term objective to minimize or eliminate the need for salt bath systems and their attendant environmental control problems.

This new system has been delivered and is currently undergoing performance trials. This article describes the system and summarizes the performance results obtained to date.

Microwave system Power generation

The microwave power is generated by six microwave power sources, each delivering 3kW to the cavity (figure 1). Five of these power sources are fixed power" power sources whose output can be adjusted by means of switches on the control console from completely off to one-half power or to full power. The sixth power source (which is located nearest to the exit port of the microwave cavity) is a variable power" power source. The power can be adjusted by means of a potentiometer on the control panel from zero power output to full power or by a process control system through a 4-20m-Adc control loop.

The magnetron used in the power sources is a custom adaptation of a magnetron made by Hitachi. This adaptation has been used for a number of years and has been proven in production applications. It has the advantages of low cost and guaranteed availability. The magnetron, when operated in a properly designed and installed system, gives a nominal life of 5,000 to 8,000 hours and has a user replacement cost of $400 per tube.

Power delivery system

The microwave power is delivered from the power sources to the cavity through WR284 aluminum waveguide. At the output of each power source is a 3-port circulator with dummy load which serves to protect the magnetron in the power source from the life-reducing effects of reflected power. The power is injected into the microwave heating cavity by means of tunable irises. The basic orientation of each iris is such that the E-field polarity of adjacent irises is rotated 90 degrees, thus minimizing cross-talk. The irises have an angular adjustment capability which permits precise impedance matching of each power source to die microwave cavity.

Heating uniformity system

Associated with each pair of irises is a high performance mode stirrer. The three mode stiffers are independently driven by motors located outside the cavity. The mode stirrers work on the principle of angular reflection of the energy as it enters the cavity with the angular position of the reflecting plate continually changing to minimize microwave hot and cold spots on a time averaging basis. The mode stirrers completely shield die product from first pass" energy. Should this be permitted to occur, the general result is a top to bottom heating non-uniforniity. By preventing first pass energy from reaching the product, the energy absorption by the product is reasonably uniform around the circumference of the product.

Control console

The control console contains all of the controls and status indicators required to operate the microwave portion of the system.

The interlock status panel provides information on the status of each microwave power source, cooling water flow and interlock status of the access doors.

The access doors have redundant interlock switches to insure that the microwave system cannot be operated until the doors are closed and latched.

The control console contains individual controls for each of the five fixed power sources. The control system permits any one of them to be either turned off or set at half or full power. The indicators on the control panel provide information as to the condition of the power source, its interlock status and performance of the power source during standby and operate modes.

The variable power source control allows the same functions to be achieved and provides the same status information and in addition has the circuitry necessary to permit the power source to be continuously adjusted from zero to full power.

The master control panel turns the entire system off or puts it into a standby mode and turns the microwave power on or off utilizing the power generating conditions established for each of the individual power sources. Pulse operation With present systems, and with many formulations where the loss factor of the material increases with temperature, when dual extrusions are processed one of the extrusions generally cures faster than the other.

The pulse operation was designed to possibly control this problem. It is hoped that dual extrusions will rise in temperature uniformly by turning the microwave power off for a period of time allowing the temperature of each extrusion to equilibrate, particularly with die jet velocity heat transfer system from the hot air.

This mode of operation has not yet been investigated by Halstead and its ability to provide this performance advantage is yet unknown.

The pulse system permits the operator to cause the power output of any selected power source or all of the power sources to be pulsed on and off at any preset power level.

The pulse conditions are set up at the control console. The operator has two basic choices. The first choice is to set the pulse timebase. The pulse timebase is the interval between the leading edge of successive pulses. The pulse timebase can be set from 1 second to 99 seconds in 1 second increments.

The second choice is to set the amount of time the power is pulsed on as a percent of the pulse timebase. The operator can select any percentage from 1% to 100% in 1% increments.

The pulse operation is accomplished by switching the magnetic field of the magnetron from the operate level to a level such that no electrons can flow in the magnetron even though the high voltage is present. The pulse turn-on and turn-off is accomplished quite smoothly with the rise and fall time of each pulse taking eight line cycles or approximately 0.1 seconds. There is no overshoot or ripple on either the leading edge or trailing edge of the pulse. Hot air system The hot air delivery system constitutes the principal innovative concept in this system. Conventional microwave hot air systems have hot air introduced into the microwave cavity at one end and have it exit from the cavity at the other end causing the air to flow through the cavity to the other and past the product being vulcanized. This type of airflow has poor thermal transfer characteristics of energy from the hot air to the product being processed because of the laminar air barriers around the product.

The new system has high velocity air jets impinging on the rubber at a velocity such that the laminar air barrier is broken and more efficient thermal transfer is achieved between the hot air and the product.

Figure 2 shows a detail of the air impingement ring and the arrangement of the rings within the microwave cavity. The rings are on six-inch centers and are adjustable in height to compensate for different product diameters. A 30 horsepower blower delivers high velocity pressurized air to an air plenum which delivers the air to the impingement rings. A 60kW electric heater is in series with this air system which brings the air up to a preset temperature.

An exhaust port with control valve is located in the air delivery side of the blower which permits some of the recirculating air to be bled off with the makeup air coming in through the entrance and exit ports of the microwave cavity. This controls the buildup of contaminants in the air system which are given off by the product as it is brought up to vulcanizing temperature. The air heater consists of exposed electric resistance heater elements which assist in the cleanup of the recirculating air by incineration of some of the airborne contaminants.

The complete system including the microwave cavity, the blower and all of the ducting are thermally insulated with two inches of rigid fiberglass insulation with a K factor of 0.3 covered with an aluminum shell.

Transportation system

The transportation system is unique in that the material being processed is conveyed through the microwave cavity without resting on a belt refer to figure 3).

This design approach was virtually forced after the decision was made to use a high velocity jet air thermal transfer system of a ring type. This design permits the jet air thermal system to provide uniform heat transfer to the complete circumference of the material being processed.

The system used is a series of driven rollers in between the jet air impingement rings which transport the material through the microwave cavity. The rollers are located on six inch centers which means that even with a ho% soft extrusion very little sag of the material being processed is encountered. The sag is further reduced by the levitation action of the jet impingement rings which are located on alternate six inch centers.

The rollers are covered with heat-shrink teflon sleeving to eliminate any tendency of the material to stick to the rollers. The rollers themselves are connected by a shaft to an outside chain drive system.

The material is brought into the cavity and caused to exit from the microwave cavity by means of belt systems which transport it through the entrance and exit microwave leakage control tunnels. The belt drive and return rollers are made with a spiral helical self-tracking system which is extremely effective. The entrance belt drive system and the exit belt drive system can be independently driven by drive motors furnished by the customer.

In addition to permitting complete circumferential jet air impingement on the product, this drive system has the additional advantage of eliminating any flat spot or belt marking which normally degrades the product in the standard microwave system and has the further advantage of permitting different sections to be driven at different speeds. This is an important advantage when processing sponge products because it permits easy adaptation of the transport system to the change in linear velocity of the product as it expands.

In the prototype system installed at Halstead the entrance belt system is tied to the product speed as it emerges from the extruder and the output belt system is tied to the speed of the take-away conveyor. The chain-driven rollers are broken into two independent drive sections. The first section, approximately two-thirds of the rollers, is driven by the input drive system and the second section, the last third of the rollers, is driven by the exit drive system. Performance trial results

Installation

The prototype system is installed at the Halstead Innovation Center. It is located at the output of a Troester 3-1/2 inch vacuum extruder with the barrel having a 17:1 L/D. Immediately following the microwave system is a conventional salt bath system, a triple pass hot/air post cure oven and a washing stage for rubber that might have passed through the salt bath system. This equipment arrangement allows Halstead to try combinations of microwave only, salt-bath only, and microwave/salt-bath combined curing.

Compounds

Halstead has significant experience in producing sponge rubber products in microwave systems and has extensive compounding experience for this type of production. The compounds used for the trials with the jet air system are the same as the compounds used by Halstead in its conventional microwave/hot air production facilities. The basic compounds which have been investigated to date include the following:

* NBR/PVC Blends

* Polyacrylate

* EPDM Blends

* EPDM

* Polychloroprene

* Chlorosulfanated polyethylene

Shapes

The trials to date include a wide variety of shapes from very intricate shapes having thin long sections to basic round tubes.

Hot air system

During all of the trials die hot air system has been adjusted to run at the maximum blower speed with approximately 510% of the air bled off into a pollution control system with the makeup air coming in through the entrance and exit end loads. The air temperature has been set from 300'F to 375OF depending on the formulation.

Performance results

The microwave system has been able to completely process, in conjunction with the triple pass hot air post cure oven, all compounds and shapes which have been tried to date. No salt bath cure cycle has been required. All products have met product sales specifications.

The jet velocity system provides a better skin (smoother, softer) with all products. It has also been noticed that the sponge production has been taken to the very outer edge of the product. Similar products made in conventional microwave/hot air systems have shown a slight tendency to a thicker, denser outer section.

Production rates

The production rate of any microwave system depends on the "loading" of the system in terms of amount of material within the microwave cavity and its degree of microwave receptivity. The smaller the load and the less receptive the compound, then the lower percentage of microwave energy absorbed by the product For this system, with normal size products and reasonably receptive formulations, production rates are up to (and can probably exceed) 250 kg per hour. With heavy loads and receptive compounds, the production rate could go as high as 500 kg per hour. Line speeds depend somewhat on the extruder and die take away conveyors. Line speeds up to 30 meters per minute can be achieved.

Economics

A complete economic survey has not yet been accomplished and numeric comparisons between the jet air/ microwave system and a salt bath system are not yet available. Some work has been done to identify the factors which would be a part of such an analysis.

The major operational comparative cost factors relate to electric power costs, system maintenance costs and replacement parts/raw material costs. In addition to these, the following factors have been identified as being in favor of the microwave system with respect to the salt bath system:

* Less scrap during change-over/start up;

* Less costly environmental pollution control system;

* Cleaner product/no residual salt;

* Less floor space;

* Safer system for operators;

* Less thermal energy delivered to plant environment;

* Infinite power control;

* Faster thermal response time.