Zero pressure extrusion.

For the last 20 years there has been steady development in extruders and extrusion systems. Extruder manufacturers, spurred by their clients, have continuously upgraded the performance of their systems. Optimized screw and barrel configurations, with highly developed head and die designs, utilize the laws of laminar flow and fluid dynamics. Use of 3D modeling and finite element analysis has accelerated the improvement cycle. All this is done to achieve a relatively simple objective - just make the extrusions the same, every hour, every day, every week - exactly the same.

Where the extrusion system manufacturers and the rubber product manufacturers have differed is in just what constitutes "the same."

New manufacturing processes in the tire and robber industry, combined with generally tighter tolerances in manufacturing overall, and the elimination or minimization of random variables, have made production variables all the more visible and undesirable. In short, the bar has been raised.

The refinement of the old processes has returned less and less in terms of product improvement. New technologies and processes are required to ensure the desired product improvements and tolerances can be achieved.

Objectives

Develop an extrusion system which will:

* Improve dimensional accuracy of profiles;

* improve productivity, especially in a short run length environment;

* reduce or eliminate uncontrollable variables, i.e.,

- die swell;

- rotational moment;

- screw throw effect; and

- internal stress in extrudate;

* maintain or improve flexibility;

* reduce tooling costs; and

* provide all of the above in either a single extrudate or a co-extruded compound environment.

Extrusion design principles

The extrusion system should have a reliable, predictable and consistent feed system with an optimized screw and barrel configuration. The head should be designed for (figure 1):

* Uniform flow across the head cavity opening;

* laminar flow within the head cavity;

* temperature profiling of the compound through the system; and

* precise pressure control.

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The preform die is designed for:

* Uniform and laminar flow through the die;

* intermediary between the head cavity opening and the final die where the compound is banked; and

* provides a pressure relief point for the compound prior to the final shaping die.

For the final die and roller design:

* The roll rotation pulls the banked compound in the preform die past the final die to produce the desired shape;

* profile control across the roll face;

* simplicity of die change-over;

* low cost final dies;

* final profile is easily cut and can be modified quickly during die trials;

* consistent roller characteristics with independent variable speed; and

* temperature controlled roll.

Applying the principles

Feed mechanism - screw and barrel

We would be remiss in not mentioning the importance of the optimum extruder design. Starting with a state of the art feed mechanism and selecting the best screw and barrel configuration is vital to a successful operation. Over the past 80 years, we at RMS have designed and manufactured just about every conceivable configuration of extruder from hot feed to cold feed smooth barrels to transfermix designs to pin types. Our most recent deliveries have centered on the pin barrel extruder configuration.

The zero pressure single roll roller die head

Successful head design requires an intimate understanding of the flow characteristics of the polymers, combined with the application of laminar compressing flow theory. Critical to the successful head design is equalized flow across the head opening; such flow must be equalized under varying velocities and pressures. In the RMS system, the preform die is an extension of this head cavity and can be changed to suit the desired product. The preform die is open to the atmosphere at its junction with the roller. The product, therefore, is presented to the final die in a relaxed state substantially free of internal stresses and pressure. The rotation of the roller carries the compound past the final die, which renders the final dimensions of the extrudate. Final shaping takes place with zero back pressure.

Die swell, as well as the effects of screw dynamics, are eliminated as the final shaping takes place well away from the screw and barrel, and after the pressure of the compound is relieved. This substantially eliminates variables in dimensions associated with pressure pulsations or rotational moments. Extrusion surface quality and tack values are exceptional with the single roll roller die. Surface scorch caused by friction acting in conventional pressure head dies simply does not happen with the SRRD.

Final dies are designed for quick change operation taking less than one minute to changeover for a single compound system, and up to three minutes for a multiplex system. Final die designs fall into two main categories:

* Flexible die system - consists of a single preform die and a family of final dies. This system allows the production of a number of final profiles using the same preform die. Trim knives are provided to remove the small amount of flash (0.010" to 0.015") which may be either left on the roll or returned to the feed hopper, depending on the application.

* Trimless die system - consists of a preform die dedicated to a single final die width. This system is more suited to longer run length environments with few changeovers.

Both die systems provide substantial flexibility in allowing:

* Gauge changes (without width or die changes);

* variable production rates while maintaining profile accuracy;

* compensation for differing compounds without changing dies;

* production of sheets (0.030" thickness minimum);

* production of undercut sections;

* production of feather edges down to 0.015";

* ability to produce thick to thin sections; and

* consistent production tolerances of [+ or -] 0.002" and better.

Profile two-roll calenders versus single roll roller die

On the calender (figure 2), pressure is built up from the rolling bank to the nip of the rolls. This distance is short and the pressure is usually low. Low pressure equals low die swell, which produces a dimensionally accurate strip. Advantages of profile calenders include high line speed; ability to calender thin sections; controlled tack; and dimensional stability.

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Disadvantages of profile calenders include costly profile rolls; limitation of calendering thick sections with blistering resulting; costly down time of roll profile changes; limited profile capability (i.e., undercuts); cannot produce multiple compound components; and temperature variations and cold flow depending on calender feeding and bank control.

The zero pressure single roll roller die offers comparable line speed (in some cases); extrusion of both thin and thick sections at the same time; control of thickness using roll speed; configurations with undercuts; inexpensive dies; short die change time; multiple compound components; and controlled temperature with no cold flow.

Pressure style heads versus single roll roller die

Pressure heads

Pressure is built up from the hopper of the extruder to the exit of the die (figure 3). This distance is long and pressure is usually high. High pressure equals high die swell, which produces a dimensionally inaccurate strip. Complicated die profiles using empirical theory and history are used to compensate for this die swell situation.

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Advantages of pressure heads include lower final die costs; average line speed usually depending on extrudate temperature; and a short time for die changes. Disadvantages of pressure heads include: limited thin sections; uncontrolled die swell; limitations on extrusion straightness, especially on sections with varying thickness; and loss of tack due to high die exit temperatures.

The zero pressure SRRD offers the following: Higher line speed/increased output; controlled thickness through roll speed adjustment; reduced extrusion temperatures due to low extruder head pressures; simple die plates and comparable die changeover time; and adaptability to hot and cold feed extruder systems.

Advantages of RMS zero pressure single roll roller die

Head pressure is built up the same way as in an extruder with a conventional head. The difference is in the specially designed preform die and roll combination. The preform allows the high head pressure to be released when the compound comes to the roll. The roll, acting as the first conveyor section, then wipes the compound past the final die in a short distance, similar to the calender. Low pressure equals low die swell which produces very accurate strips. The SRRD also offers the versatility of the extruder and head with its variety of shapes and profiles that can be produced with low tooling cost.

The applications as a primary extrusion system are: Single components of various shapes; multiple components of the same profile; multiple components with slightly different profiles; two, three and four compound applications with patented early junction preforming system; thick to thin profiles; back angle profiles; sheet sizes down to 0.030" thick; and near net shape blank production for product molding.

As a secondary extrusion system, it offers a profile to hot laminate to a primary strip from another system and is used in strip feeding special calendering applications.

Conclusion

Today's manufacturing processes with their ever-tightening tolerances and demands for product uniformity over a wide range of line speeds, have created a demand that cannot be satisfied by conventional pressure head extrusion. The best solution to this problem is a carefully engineered system incorporating controlled feed to a modern cold feed extruder and a head which by its design provides laminar flow with equal velocity and pressure distribution, combined with the "zero pressure" feature of the single roll roller die.

Conclusions as they relate to the original objectives include:

* Dimensional accuracy - using the roll to pull compound from the preform bank past the final shaping die results in zero pressure extrusion and a dimensionally accurate product.

* Improved productivity - zero pressure extrudate results in lower temperatures and high tack levels. Lower extrudate temperature results in increased speed and higher production levels.

* Uncontrollable variables

- Die swell is virtually eliminated when the extrudate is produced under zero pressure conditions;

- rotational moments and screw "throw effect" caused by screw revolutions are eliminated by the preform banking and pressure relief area behind the final shaping die; and

- internal stress in the compound due to the extrusion process is relieved when there is no pressure to distort the product.

* Flexibility of the process

- Profile shapes for most products can be handled using the SRRD system; and

- die changes are easy and repeatable.

* Reduced tooling costs

-- in many cases, costs will be lower than any comparable system. With a family of final dies, using the same preform die reduces tooling costs further.