Continuous production of rubber profiles.

This is the second of a two-part series. Part I of this article appeared in the September 2000 issue of Rubber World. High quality requirements and cost pressures on the profile processing industry require profile manufacturers to pursue constant improvement of compounds, profile geometries and production lines. We would like to show some of the options available for further development of the process from the point of view of the processor and the machine supplier.

The goal of every new development must be to increase productivity and production quality. Cost-saving developments must not affect the quality in any way.

Designs in which the connection between the screw and feed roll occurs through a coupling that can be adjusted for torque do not deliver the advantages claimed for them.

The main problems that can occur in extruder feeding are over- and underfeeding. With strip feeding, the circumference speed of the screw determines the maximum intake speed of the strips. Underfeeding is caused when the compound volume given by the circumference speed and strip dimension is not equal to or greater than the throughput volume of the screw.

When the feed roll runs empty, it runs at full speed. It cannot compensate for the underfeeding. The only solution is to increase the strip volume.

Overfeeding the extruder creates compound backpressure in the feed opening. The compound, already heated, swells up as a ball. To avoid this, the too large feed strips must be braked. The torque-controlled feed roll can only create this braking effect by decreasing speed if the ball presses on the feed roll. Experience shows that in most cases, the ball still grows upward and does not press on the feed roll.

Therefore, the solution is to give the feed roll its own separately controlled drive. Then it is possible to generate the braking effect directly through pre-setting of the speed.

In the temperature control zones of the extruder, the design of the water guiding for heating and cooling of the barrel is important.

Within the system, water is directed through an inner coil of the outer cylinder around the barrel liner. The advantage is that more water can be sent through the system with a lower risk of clogging. The alternative solution with a peripheral bore in the cylinder does not offer any technological advantages, but is more prone to clogging.

The temperature control units needed to operate an extruder should have the right heating and cooling capacity for the size of the extruder.

The influence of the extruder accuracy with temperature control is often overestimated. The temperature, set in the individual zones, should be made as even as possible with the temperature control unit. Nevertheless, an incorrect temperature setting can do far more harm than slight temperature variations in the temperature control system.

Temperature control should be viewed mainly to support the functioning of the extruder. The temperature should be set to support friction-depending transport to the extent that the needed throughput is achieved with the lowest screw speed and thus the lowest and most even compound temperature.

The screw temperature can have the most influence on the extrusion process. This is possible because the screw surface is the surface that has the most contact with the material.

It is possible to make the throughput more even and improve the extrusion against high resistance using a gear pump downstream from the extruder. The use of the gear pump was adopted from plastic extrusion.

The forced transporting of the gear pump makes the decrease in throughput against high extrusion resistance much lower, as with friction-dependent transporting with a screw.

This is clear in energy consumption. Energy consumption is lower with the gear pump and extruder as resistance increases than with the extruder alone.

Other advantages of the gear pump extrusion are a shorter start-up process and better tolerances.

Head

An extrusion head should be easy to heat and cool, as well as to disassemble. The smallest possible compound volume is advantageous.

The special extrusion heads for multi-component profiles, with and without steel inserts, are developments often based on a profile die system existing at the customer.

A crosshead is used for profiles with steel inserts. The main compound flows over a hollow-bored pintle through which the steel profile is fed. Other extruders are connected in the die area. The 180 [degrees] arrangement of the main extruders is recommended as the ideal design.

An interesting variation is to arrange the extruders on one side with a common connection to the head. An extruder can also be arranged on the opposite side or in a vertical design.

The use of screens is an important point in the subject of extrusion heads. There is a distinction between safety screens and straining screens.

The safety screen prevents the individual dirt particles from destroying narrow areas on the profile. The safety screen should only be changed when a die or compound change is done.

Straining screens are generally not recommended in production. As the screen becomes dirty, the extrusion parameters, and thus the profile dimensions, change. This problem is not solved by using a screen changer, either. The advantage of the screen changer is that it makes it possible to change screens without opening the head and, depending on the profile dimension and speed, possibly also without interrupting the profile, thus creating a flaw.

The disadvantage of a screen changer is that it increases the compound volume between the screen and the head and makes it more difficult to change compounds and disassemble the head.

New designs have minimized this problem as much as possible by reducing the volume before and after the screen.

Screen changers are used when the screen must be changed several times over a long continuous production time.

Another option for compound straining is to use a gear extruder in front of the profile extruder as a strainer. The temperature increase of the compound is relatively low with the gear extruder. The strips extruded in this manner are then fed into the profile extruder without cooling. The loop control regulates the strip feeding.

Vulcanization units

Saltbath units

Today's saltbath units need to have low energy costs and correspond to environmental requirements regarding wastewater. The low energy costs are achieved by using gas to heat the salt melt. Cascading wash units with salt reclamation guarantee the purity of the wash water.

The technology of the process calls for a combination of salt spray and immersion. Complex profiles with only one or no way to lay them on a conveyor belt require spray in the first section. This makes the surface begin to vulcanize and stabilizes the profile lips before immersion. This system prevents deformation or surface marks.

The profile is then immersed using a steel belt or immersion rollers. The current trend is toward immersion rollers, which are less delicate mechanically and which improve the vulcanization results. This improvement is due to the salt exchange between rollers on the upward profile surfaces or profile chambers. Sectional driven rollers in a saltbath with immersion rollers allow the system to compensate for profile elongation.

At the end of the saltbath, a hot air blow-off station with a vibrating unit helps to retain the salt melt. The cascade wash unit consists of a station with three reservoirs. The water mixed with salt flows against the profile direction and is pumped into the salt reclamation station in a specified concentration. After the cascade, the final wash and drying of the profile occurs.

The predominant heating method for saltbaths in Europe is gas. Gas is favored for cost reasons. This saltbath design heats air with gas and uses it as a heat carrier to heat the salt tank.

Other features of the new saltbaths include good insulation, a separate drive station, an effective spray section and ease of immersion roller removal and easy maintenance.

Microwave units

Microwave units have been the state of the art for several years. High-volume channels and resonator chambers are now being used.

Microwave energy increases in efficiency with profile volume. Good microwave units are distinguished by even profile heating, safe, easy to maintain electrical systems and an easy to clean treatment channel with minimal radiation.

An up-to-date microwave line consists of a hot air shock channel, 4 x 6 kw microwave part and hot air section with a length of 12 to 18 m (40-60 ft.).

Hot air units

The hot air unit is one of the oldest machines for vulcanizing rubber profiles. Since their efficiency is dependent on the heat transfer from air to rubber, hot air units were mainly used as a downstream section for a microwave unit or shear head.

The units can be made with one belt for single profile transport or with up to three belts to run the profiles through three times. It is also possible to use a combination of the two-level hot-air channel with a cooling channel below it.

The shock channel is a common addition to microwave or hot-air lines. The channel, 3 m (10 ft.) long, for example, is located directly after the extruder. It uses a very high air temperature to allow the surface to begin to vulcanize very quickly.

This has advantages such as prevention or reduction of belt markings, stabilization of profile lips and formation of a closed skin with sponge rubber.

The hot air temperature can be up to 600 [degrees] C (1,110 [degrees] F). The temperature is generated only through gas heating or infrared radiation. The profile runs over smooth-running metal rollers, which are sometimes driven.

Automation

Automation systems are now fully established. This applies not only to the profile lines described here, but also to extrusion lines in the wire and cable industries.

The drive toward these systems came from the demands of the automobile industry to prove that the process is working properly. This proof requires data capture that is available in the automation system.

The most important function of an automation system should still be product improvement. In this point, the system can be seen as an aid to the machine operator. This means that it must be clear that the data capture is also positive for him, since it shows that he has produced the profiles correctly.

Summary

Product quality is dependent on many factors. In the trend toward high-quality automation systems, it is especially important to have a solid base. The operating personnel must be well trained and the units must be kept clean. Regular maintenance must not be sacrificed to the current cost pressures.

The understanding of process technology contributes to the farthest reaching use of the possibilities of automation. Cooperation among chemistry, compounding and processing is needed for good process technology.