Setting up a modern mixing facility.

By Lawrence R. Gooch, Gooch Engineering Associates, Michael V. Musco, Musco Engineering Associates and Frank A. Pascuzzo, Hancock Engineering Inc.

The title implies that there have been changes over the last few years that make setting up a modem mixing facility today somehow significantly different than it might have been, say, twenty years ago. Certainly the regulatory climate has changed considerably, and the amount Of collateral industrial side effects (noise, odor, late night truck traffic, bright lights on the outside receiving yard) that those living near our plants will tolerate, has greatly decreased. The cost of doing business has increased, and, in market areas with high labor and manufacturing costs, more companies are finding themselves in high value added niches than in commodity markets.

For nearly two decades, quality has been an issue of ever increasing importance, of which the current focus on the ISO 9000 quality management certifications is only the latest embodiment. The increasingly global nature of the marketplace has made every aspect of doing business more competitive. Fluctuations in currency rates result in overseas investments, in order to allow companies to remain competitive in their overseas markets. The investment by major Japanese auto and tire companies in North American plants is but one example. This pattern of investment results in a broader technology base, and wider dissemination of ideas and methodology. Investment in developing countries by major manufacturing firms to take advantage of reduced labor costs and gain an early toehold in emerging markets, and the purchase of technology and equipment by emerging economies has had a similar result.

While all of this was going on, the microchip spawned a revolution in controls and information systems. It is now possible to know almost anything we want to about nearly any pail of process, and in real time. We are now faced with the dilemma that we still do not know what it is we need to know, and once knowing the information, what to do with it. This is particularly true in the process industries, where much of why things happen, or what makes one formula extrudable and another not, is still more art than science.

Another, quieter revolution has been going on in laboratories and the companies that make instruments for them. We can see things we could not see twenty years ago -- we can measure with much greater precision nearly anything it is that we want to measure. We can make those measurements with tools that will perform calculations for us, and deliver the results of the measurement and calculations in both traditional units and graphical displays. The tools that let us do this are becoming more and more "user friendly", so that increasingly sophisticated testing is becoming ever more readily available.

What are the implications of these developments and trends for the modem mixing facility? We think it means a higher profile for the mixing/compounding facility. The emphasis on quality as part of the process has helped focus attention on the compounding operation as a key component, of product quality, bringing compounding and mixing out of the dark comers of the plant, where it has been for many years hidden from visitors and, senior management.

We think it also means the following: More emphasis on the control of process variables. More emphasis on regular maintenance, to keep equipment performing consistently. A cleaner work place -- cleaner because it is better for the product, better for the instrumentation, better for the worker, and more conducive to regular maintenance. An emphasis on the attainment of consistency, and for many, an emphasis on flexibility -- the ability to meet tomorrow's challenge with today's equipment.

Does it mean sophisticated control systems? Perhaps, if they are tools which will assist in achieving the agreed upon goals and objectives for the facility. Does it mean fully automatic weighing and feeding equipment? Not necessarily. Does it mean new machinery? Only if new adds an important capability, one which is part of the previously defined goals and objectives, that is not available in old, used or remanufactured equipment.

We think that the process of developing and setting up a modern mixing facility follows the same path that it always has. There are more options, and more people have an interest and a stake in the outcome, but the route is a familiar one. Here, then, is a review of some of the things we think it is important to keep in mind as the planning gets under way for a modem mixing facility. Definitions are given in table 1.

Table 1 - definitions

Compounding: The entire process of producing rubber
compound in a form suitable for subsequent operations.
Includes the measuring or weighing of raw materials, the
feeding to the mixer or fluxing unit, the subsequent milling
or pelletizing operation, and the stock cooling and placing
in in-process storage.

Blending: The process of stirring or mixing together several
ingredients, without a state change. Usually means dry
ingredient blending, but may include a minor liquid component.
In our industry, blending usually does not involve liquid - liquid
mixing.

Low intensity blending: Ribbon blenders, tumble blenders,
twin shell blenders, double cone blenders.

Medium intensity blending: Plow blenders.

High intensity blenders: Bowl type blenders with high
speed propeller style or open mixing arms. Henschel style
mixers.

Mixer/Mixing: High intensity internal mixer, which imparts
sufficient energy to the feed stock to result in a change in
viscosity or a state change, usually from a solid to a plastic
or viscous pseudo Newtonian fluid. Banbury[R] Mixers, twin
screw extruders, twin and single rotor continuous mixers.
Sometimes called the fluxing unit, or fluxing, usually in
PVC compounding.

The starting point

A new or upgraded mill room does not begin as an engineering, maintenance, or production project -- it flows out of the vision, goals, and objectives of the parent company or corporation. With the exception of those whose business is the production of compound as product, or who compound for others, the mill room is a component part of a larger manufacturing operation. It exists to supply compound to a subsequent operation or operations.

The present and future requirements of that operation or operations dictate what the output of the mill room must be. Determining those requirements requires planning and decision making on the part of the senior management of the company.

There are six basic parameters which must be determined in order to begin the planning work for a new mill room. They are:

* The design capacity, in terms of finished compound, net of scrap and other losses, which the mill room must deliver per year.

* The number of shifts which the mill room will operate in order to meet the design production rate.

* The average or design output per unit time of the primary compounding equipment.

* The number of different formulations or compounds which must be produced and the minimum and average run size for representative compounds.

* The basic technology which will be employed to masticate the rubber.

* The form in which the compounded rubber is required for subsequent processing.

The first three parameters are all related to determining the size or capacity of the basic compounding unit. The first parameter is the cornerstone. Management must determine what the current and projected requirements for compound will be for the design life of the installation. In addition, management must decide if all of these requirements are to be met by the captive compounding installation, whether some of the requirements will be satisfied by purchasing compound, or whether some will be met by buying production time at other plants (having certain product lines or portions of product lines manufactured by others, for example).

Management must also decide how many shifts the compounding facility will operate to meet production requirements. Must compounding and production be essentially in balance, or can compounding produce material on one shift for two or more shifts of production capacity? If so, is the plant to be designed to permit operation in that mode?

The third parameter, taken together with the first two, will establish the hourly production rate for the primary compounding machine or mixer. What is sought here is what is called cycle time in a batch mixer, and the actual average instantaneous output on the design basis compound for a continuous mixer or compounder.

For a batch mixer, hourly rate is the average number of batches per hour multiplied by the batch size in pounds or kilos (notes 1 and 2). The answer is fairly straight forward if the compound makes a single pass through the mixer. If two or more passes are required, or if a component of the product is always trimmed and returned to the process through the mixer, adjustments must be made for these factors in determining the hourly production rate.

At this point, information must be established about the mixing process. If actual data are not available, then reasonable assumptions must be made, based on published data, input from machinery manufacturers or previous experience on similar compounds. Data are generally well known on mixing procedures and average cycle times for tire tread stocks, hose coverings, and insulation stocks, for example.

Adjustments to the expected hourly rate must be made for the usual lost time. If work crews are allowed break time, clean-up time, etc., and the machine will be idle during these periods, the available hours must be adjusted downward to account for this. Other factors having an impact include: Compound development work, production trials, scheduled and unscheduled maintenance downtime, and cleanout time, if products or the process will require a machine clean down for certain product changes.

If the starting point is the required annual production rate, adjustment must be made for shutdown periods, holidays, and other non-workdays.

Once the required hourly production rate is established, the size of the primary compounding machine can be established. The development of other specifications for this machine (or machines) requires further work, which we leave to the section on the mixer proper.

Having established the throughput requirements and the mixer size, the fourth parameter is concerned with what the new facility is going to have to do. Will it run a few, several, or many different compounds? Will it have to make small runs, with frequent product changes? A related concern is the number of ingredients in each formula, and the total number of ingredients which must be handled for all formulae combined.

This information affects the area required for the receipt, storage, and handling of raw materials, the amount of in-process storage required, and the degree to which the compounding process might be automated or mechanized.

The fifth parameter is dependent to some degree on the fourth. If the number of compounds is low, and the base polymer is common to most or all of the formulations, it may be possible to consider continuous compounding, using one or more of the available technologies. The questions to be answered here include:

* Batch or continuous (see the next section)?

* Single or multiple pass mixing?

* Strict batch traceability, or cross blending for improved uniformity?

The sixth parameter is dictated some what by the subsequent process. Injection molding machines can be strip or pellet fed, compression presses require preforms, transfer presses may be fed with preforms (slabs cut to size), pellets or granules. Extruders may be cold fed, generally requiring strip feed, or hot fed from a warm-up mill, in which case slab feed is usually used.

Rubber stocks which will be solvated in the next process step are usually chopped or pelletized. Calendering operations are normally fed from a mill in pigs or strips, and require slab stock from the mill room. (More recently, in large volume applications, strip or slab fed pin barrel extruders have been used, either alone or in conjunction with mills.

Batch or continuous?

This issue has been discussed at great length in connection with rubber processing. The attractiveness of a continuous process is the steady state nature of the processing conditions. Steady state conditions are susceptible to on-line monitoring, and rapid adjustments can be made to correct for changes in the condition or quality of the feed stock.

The drawbacks are that short runs can become inefficient, as the relationship between start-up and changeover time to run time becomes larger; there is usually no batch traceability; feed stock requirements are more stringent.

Continuous mixers operate on small volumes of material at a time, and therefore require that the feed stream be rather precisely metered into the feed throat of the mixer. This dictates that all of the feed stocks be either fairly finely divided particles or liquids, and that a quality feeding system be installed. This investment is mandatory with continuous mixing equipment.

Up to this point, the energy costs associated with converting rubber in bale form into particles or granules for feed to a continuous mixer have not been justified by process or quality improvements gained through continuous compounding.

Certain synthetic polymers are now available in crumb form. Some synthetic rubber producers offer their product in crumb form with carbon black and other additives added in solution during the final stages of manufacture. A compounding installation which will produce a particular hose cover stock or family of stocks, or a group of extrusion compounds from the similar formulations based on such a polymer available in crumb form, would be excellent candidates for continuous mixing.

If the compound will be processed in two steps, and runs are fairly long, producing the first or breakdown pass as a pellet would allow for continuous processing of subsequent or final pass compound.

The advantages of batch mixers are: A run can be as short as single batch; rubber can be fed in bale form; multiple ingredients can be weighed or measured off-line and fed as a single charge or shot, manually or automatically; process conditions can be adjusted on the fly to "steel" the batch to the desired end point, using either operator knowledge and control or so called "intelligent" control systems. While elaborate automated feeding and weigh systems may be employed for a variety of reasons, they are not essential in order to get started.

For the balance of this article, we will discuss the mill room in terms of a batch mixer. While it is our conviction that continuous compounding will play an increasing role in rubber compounding in the coming years as more polymers become available in crumb form and solution, addition of structural modifiers becomes more prevalent, the practical reality is that the vast majority of today's polymers remain available only in bale form, or in chips too large for small volume continuous compounding.

Additional planning considerations

Quality. Consistency. Repeatability. OSHA. EPA. Product liability. ISO 9002. SPC. SPQC. JIT. TQM. Quality Circles. Kanban. NFPA. BOCA. NEC. Industry is rife with these buzzwords. Every trade journal, technical magazine, and industry news weekly contains some reference to one or more of these terms.

Those who are or plan to be suppliers to the automotive industry now are, or soon will be, certified under one of the ISO 9000 programs. This supplier quality assurance program is being demanded by more and more buyers in other industries as well.

OSHA issues concerning health and safety in the work place, and federal, state, and local EPA regulations concerning our contribution to the external environment that our plants occupy, are issues that one ignores at serious peril and financial risk.

NFPA, BOCA, SBC, the NEC, as well as our insurance underwriters, local building codes and officials, are concerned with the protection of life and property in the working environment of our plants and factories, and in the reduction of loss due to catastrophic events. The NFPA, BOCA, SBC, and, the NEC contain proscriptions for building egress, storage and containment of flammable materials, access around electrical storage panels, minimum lighting levels, and other factors which must be taken into account in the design of the compounding facility.

Quality, consistency, repeatability, and product liability/performance are all issues that are critical to any business. They are economic issues. Poor quality may be costly, but, contrary to the title of Philip Crosby's book, quality is not free. The essential issue in planning the modem compounding facility is to establish what tools will be used to assure that the quality of the product will be within the targets required by both the end product (performance as measured in the finished product and by the manufacturing steps which will follow the compounding operation (processability, for want of a better term).

Once the tools of measurement are determined, the process control points can be established, and the facility planned to provide for both. The modem compounding facility recognizes the need for determining and controlling quality in the process itself, not simply "testing it in" at the end of the line.

This does not necessarily mean extensive electronic controls. It does mean understanding how process variables affect product quality, and being able to measure and control those variables to keep product quality within acceptable limits. It means knowing what the quality of the material inputs to the process are, and how to manage or control the compounding process to maintain or improve that quality.

Task analysis -- process flow

How will the material flow through the compounding facility? Is raw materials receipt and storage for compound ingredients dedicated to the compounding area, or will all incoming material to the plant pass over the same receiving dock? Are there dedicated raw materials storage areas for different parts of the manufacturing process, or do all functions (compounding, fabrication, assembly, finishing, and packaging) share a common raw materials storage area?

Will the mixing process be single, double, or multiple pass? In what form will intermediate processed material be stored, and how will it be handled? Will the compounding facility run to inventory, or to keep pace with the manufacturing process? What are the shelf life and aging considerations for compounded material in intermediate and finished storage?

In general terms, the flow of material through the compounding area should be integrated into the flow through the plant. Two types of flow diagrams should be prepared to be sure that the flow of material through the compounding area is understood and agreed to.

The first is a process flow diagram, which documents in schematic form the flow of material from raw materials storage through to the staging or storage of finished compound. This diagram focuses on the how of the process -- how material will be transported, weighed, cooled, etc., and what will be required to accomplish those tasks (bins, scales, conveyors, cooling tanks, etc.). This diagram becomes the basis for design of the facility, sizing of vessels, conveyors, development of control systems, etc. Normally, as design progresses, process flow diagrams are expanded into piping and instrumentation diagrams, and decisions about measurement and control parameters and locations, quality control points, and machine safety and operation interlocks are documented on those drawings.

The second is a physical flow diagram, showing in schematic form on the proposed floor plan, how the actual movement of material will take place -- what doorways it must pass through, where it will be elevated, stored, etc. This diagram looks primarily at material transport, without too much concern for what is happening to it as it is passing through the process. From this work, the final plant plan and equipment arrangement drawings are developed.

Raw materials receipt and storage

Determining how raw materials will be received is an important consideration in defining the plant layout. These days, natural rubbers are delivered in a standard sized 33.3 kilogram bale, and the majority of synthetic rubbers are shipped in bales of similar proportions. Some elastomers are shipped in chip form (polychloroprenes), some in crumb, and some in slab. All are shipped in some sort of box on a pallet. The irregularly shaped 250 pound smoked sheet bale shipped loose in trucks and rail cars is pretty much a thing of the past. Except for very large volume users, who may receive rubber via rail in box car lots, most plants will receive rubber via highway in truck load lots or less.

Carbon blacks, and for that matter, most dry ingredients, are shipped in bags on pallets, and are handled by forklift truck at the plant site. Carbon black and other large volume non-black structural modifiers or fillers may also be shipped via rail in special tank cars, or by highway, in especially designed trailers, or in intermediate bulk containers.

Bulk deliveries are transferred to on site bulk storage vessels or silos by means of pneumatic or mechanical conveying methods (note 3).

Intermediate bulk containers come in several variants: Rectangular metal containers or bins of various designs, usually referred to as Tote bins, after the company which first popularized the concept; flexible rubber containers first marketed by US Rubber/Uniroyal, most recently, several forms of what are loosely called bulk bags, or some times SuperSacks. Intermediate bulk containers were originally intended to be reusable shipping and storage containers, which would be delivered to the plant site full, and returned to the distributor or manufacturer for refilling.

From a shipping cost standpoint, collapsible containers offer an attractive concept when compared with metal bins, due to the reduced shipping volume and lower empty weight on the return trip. Metal bins offer improved durability, ease of stacking and handling in the plant, and good weather resistance. The current cloth super sack is usually fitted with a plastic liner to contain times and exclude moisture. Most are intended to be returnable, but are considered packaging, and are generally not owned by the buyer, unlike portable bins.

Liquid ingredients (extenders, process aids, internal lubricants) may be received either by bulk truck or in drums on pallets, depending on the usage volume and the economics of delivery and storage. Changes in regulations regarding underground storage tanks, and increased stringency in spill containment requirements and monitoring systems, have affected design and construction costs for bulk liquid storage facilities. Careful planning is required to insure regulatory and underwriting compliance.

Some of these concerns an offset by the equally daunting prospect of dealing with drum recycling and disposal. Provisions must be included in the design of the facility to allow for proper storage, recycling, and disposal of used drums.

A determination of the best method for receiving and handling each ingredient requires an analysis of usage patterns, a determination of the economic order quantity, the cost of shipping, the cost of containers, and the cost of lost material (spillage, residue remaining in bags, material lost to the atmosphere at transfer points, etc.).

One of the considerations affecting the decision on the form in which raw materials are received is the control of fugitive dust emissions, both within the plant, and at its exterior. The majority of the ingredients in rubber compounds are do not disperse readily in relatively still air, and will fall to the floor or other flat surfaces, and can be vacuumed or swept up.

On the other hand, carbon black, fumed silica, and certain of the more finely divided talcs readily become airborne, and will migrate into electrical enclosures and the air workers must breath. They will travel extensive distances before settling out on the surrounding surfaces. In some instances, these materials may be handled in intermediate bulk containers simply to eliminate the bag break operation.

Plants have been designed with captive intermediate bulk operations, where carbon black received in bags is de-bagged by means of an enclosed mechanical bag opening station and transferred into portable bins for movement around the plant. This accomplished three objectives: It put the black transfer operation in a separate room, isolating this operation from other functions in the plant, thereby reducing the possibility of contaminating finished compound; it allowed for the possibility of automatically feeding more blacks with fewer feeders, by swapping bins at the feeder inlets; it cut down on the open transfer of blacks, and reduced the opportunity for carbon black to escape into the plant environment.

Normally, most compounding ingredients are received in boxes, bags, drums, or Gaylords, delivered on pallets, and will be received over the same dock as all other incoming material in the plant. Because many of the ingredients in rubber compounding are in powder form, and some readily become airborne (see above), it is desirable to segregate compound raw materials storage from other storage areas in the plant.

Storage areas should be arranged to minimize handling, and facilitate first in first out usage patterns. Intermediate and finished compound storage areas should be separate from raw materials storage and weigh up areas, to reduce the likelihood of contaminating product with free chemicals.

Thought should be given to providing storage areas where humidity and temperature can be controlled within reasonable ranges. Variations in the temperature of the rubber feed stock affect mixing cycle times and the work input required to reach a target temperature. Variations in the moisture content of dry ingredients also affect mixing cycle times.

Last, but certainly not least, in planning for the receipt and storage of incoming raw materials, thought should be given to how the quality of incoming materials will be checked. Staging and holding areas may be necessary, and access for sampling may be required.

The mixer

An entire paper could be written on the application analysis which should go into specifying the mixer in a new compounding facility. These are the high points, after capacity:

* Adequate power. Appropriate rotor speed(s). The ability to control ram pressure (batch machines).

* The ability to control the temperature of those parts of the machine in contact with the product.

* The ability to inject a liquid into the machine after initial breakdown has occurred, and preferably in an active zone of the chamber. Materials of construction appropriate to the material being mixed. Rapid and reliable charging and discharge door operation (again, batch mixers). A rugged drive train with a high service factor -- 1.75 AGMA on the gear reducer, or better, and a motor rated for mixer duty.

* The ability to monitor batch temperature and motor power over time.

The other major issues which will have a lasting impact on the productivity of the compounding facility are: Feeding, discharge and take away, and maintenance.

It is essential that the installation design recognize that the mixer will require regular, routine maintenance at frequent intervals, major maintenance at regular extended intervals, and provide for the ready accomplishment of both.

Maintenance personnel, the operator, or both, should perform a walk-around inspection daily, to check fluid levels, look for leaks, and to clean up the machine by wiping up any material spills. The gear box, motor, and couplings should be checked for any unusual noises, higher than normal temperatures and the proper flow of ventilating air. The hydraulic system should be checked for leaks, as well as the dust stop lubrication system. This will only happen if access around the machine is easy, does not require stooping, and is well lighted.

Sooner or later, the machine will require a body change, the motor new bearings or a thorough servicing, and the gear box a complete inspection. Provision for accomplishing this work can b designed into the installation in the form of properly located beams to support trolleys and chain falls, and adequate space to remove covers and major subassemblies and set them on the floor.

Attention to the details of the piping and electrical installation will avoid the necessity of removing extensive pipe and wire in order to perform major service on the mixer. Thought in the arrangement of the feeding and weighing equipment will also help out in this area.

The installation of the mixer should be designed to minimize material handling and the number of open transfer points. To this end, the installation should provide for a gravity charge of dry ingredients into the mixer, and a gravity discharge to the mill or extruder below. This will allow for making material transfers through closed chutes (or in the case of a mill, at least a controlled transfer zone where appropriate dust and fume collection can minimize the effect of the open transfer).

Feeding and weighing

The proportions of the batch or feed stream must be correct when they enter the mixer, or they will not be correct in the finished product. It may be possible, depending on the subsequent process, to do some cross blending to level off or average out minor inconsistencies, but the objective should and must be to avoid introducing those inconsistencies in the feeding and weighing process. We know that there will be inconsistencies in the raw materials -- and should reserve pre-and post-mixing cross blending for the purpose of averaging out those variations.

The next important point is to establish what is required, and what we are trying to do. Formulatons, and the tolerances for variation in the target weight of the component ingredients in those formulations, form the basis for specifying the weighing and feeding system. It is important that those tolerances be realistic, and that they are limits outside of which the performance of the end product becomes unacceptable or unpredictable. Like quality, accuracy in weighing costs money, and has an impact on both the cost of production and the investment which the sales of the product must support.

It is also important to understand the difference between mechanization and automation. Mechanization provides machinery to assist the operator or worker in the accomplishment of his task. A screw feeder is a substitute for a scoop, a charging conveyor eliminates lifting and dumping -- these are examples of mechanization. A scale which starts and stops a series of feeders to weigh up a multi-component batch of dry ingredients without intervention from the operator, once the weighing cycle is initiated, is automation.

It is not necessary to either mechanize or automate to attain accuracy -- the most accurate weighments in the world are attained by human operators using a precision balance, adding or removing material until the correct weight is reached. We mechanize to reduce operator fatigue and increase the consistency and reliability of the operator's judgment, to increase speed by eliminating hard work, to improve precision by increasing the ability to control. We automate to relieve the operator of repetitive, boring tasks, and to reduce reliance on the operator's skill, attention span, and performance.

An objective often voiced by clients approaching the specification of a feeding and weighing system is the desire to eliminate reliance on the operator. This is never totally possible -- but a great deal of money can be spent trying to get to make it so.

An example from a project some years back serves to illustrate the point: The customer wanted to eliminate operator error in making up the bags of minor chemicals that are to be charged to the mixer after the initial breakdown pass. The process of making up the bags, which was done off-line, required the operator to push a cart fitted with a small scale down an aisle of scoop bins. A batch counter controlled the number of cycles he had to make through the aisle to fill bags for a particular compound formulation, depending on the number of batches scheduled for the running later in the day or week.

A computer based system programmed the scale with the weights of each successive ingredient in the bag, and sequenced the operator through the aisle by illuminating a light on the bin from which the next ingredient was to be drawn. If the operator opened the wrong bin, the scale on the cart was disarmed.

At the mixer, a small bench scale fitted with a tilting pan was mounted adjacent to the charging hopper, and arranged so that at the appropriate time in the mix cycle, the pan would tilt, dumping the previously placed bag of small chemicals into the mixer throat. Before the next batch could be initiated, the computer would check the scale to be sure that a bag of the correct weight was sitting on it. The system worked well, and the customer was pleased.

But note: There was no check on what material was actually placed in each bin. Similarly, in that same operation, liquid additives were drawn from an oil dispensing drum rack system, with each new drum connected into the system by means of self closing hose couplings. The weigh system controls had no way of checking whether the correct drum was in each position -- they only controlled which position the liquid was drawn from. The system still relied on an operator making the correct decision about what material went where.

The message is: Know what it is that wants and needs to be accomplished, and specify wisely to achieve what is most important to the success of the operation. Some of the legitimate objectives of feeding and weighing systems include:

* Safety.

* Elimination or reduction of spillage and waste.

* Repeatability.

* Consistency.

* Reducing labor.

* Productivity -- reduced charging time.

* Cleanliness.

* Reduced contamination.

Some of the concerns associated with design and specification of feeding and weighing systems include:

Scale accuracy -- in net weight scales usually a percentage of total scale capacity. You cannot make a very small weighment with the same accuracy with which you can make a large weighment in a scale whose capacity is based on making large weighments.

Materials handling characteristics -- does it flow readily or does it tend to pack? How does it come out of a bin? Is flowability affected by humidity? Will it fluidize and flood once it starts to flow? If the material is a blend, does it tend to segregate when it flows? How much head can be placed on the ingredient before it packs too tightly to flow?

How much time is available to make the weighment in? How many different ingredients comprise the weighment?

How will the use or surge bins be re-supplied? Is an elevator required, or can material be moved by pneumatic or mechanical conveyor?

What type of feeder will meter the material without degrading it or segregating it? Will the feeder work at normal and dribble speeds to allow inching up to weight, or must a second, lower volume, feeder be installed to provide that ability?

How can the charging sequence and the system be designed to minimize the loss of powders through the mixer charging door? How will the system isolate the weighment function from the movement of air in the charging chute during the mixing cycle? How will it keep fumes and condensables from affecting system performance?

Cooling and take away

For batch mixing installations, the most common machine to follow the mixer remains the two roll mill. Mills provide a great deal of flexibility. They can be used to finish the mixing started in the internal batch mixer; in some instances cures can be added on the mill while the next batch is in process above. They provide a method of quickly knocking the heat out of a batch, and in fact in certain operations, two mills have been installed in tandem for just that purpose. The first mill is used primarily to drop the temperature of the batch as quickly as possible, while the second is used for cross blending and converting to strip or slab stock.

Mills may be single or variable speed, fitted with drilled, cored, or bored rolls, have high or low friction ratios, and be installed with the fast or slow roll toward the operator. Each option has a process impact, and needs to be evaluated in terms of ease of operation and control of the process. Mills must be correctly sized for the process throughput rate or the mixer batch size, depending on the application.

When it is desirable to change the process from batch to continuous at the discharge of the mixer, or if the compound must be strained, an extruder is employed, either instead of or after the mill.

Extruders mounted directly under batch mixers vary widely in design, depending on the function intended. For a number of years, mixer dump extruders were fitted with closed hopper chutes, hopper sections with feed undercuts below the screw, reciprocating pushers to force the batch into the screw, and tapered screws. Later on, dump extruders were built with straight screws, to avoid did necessity of disassembling the extruder to pull the screw.

Where the primary function of the extruder is to form a continuous slab, the extruder is fitted with a pair of rolls to convert the discharge to a sheet or slab. There are several variations on this design, depending on the density and surface finish desired for the slab, and the viscosity of the stock. Farrel alone has fielded three distinctly different designs.

In the last few years a post mixer sheet former originally marketed in the United States by Moriyama has found considerable favor, with most major suppliers now offering some variation of it. This machine is a twin screw extruder with large tapered screws taking the place of the machined under cut and the pusher. The twin screws force the batch into the nip of a pair of rolls, continuously producing a slab.

Extruders under mixers may also be used to strain the product to remove foreign objects, un-dispersed agglomerates, and lumps of un-processed polymer. This is commonly done with electrical insulation stocks, whether for cable or molded goods, and other applications where the presence of a defect in the finished product is not allowable.

Straining is accomplished by forcing the rubber stream through a screen pack supported by a breaker plate. Breaker plates are removable for cleaning, and for renewing the screen pack. Extruders intended for straining are fitted with heads which may be swung open to allow access to the breaker plate. Many extruders have been fitted with dual heads, either to decrease change over time, or to allow the extruder to perform different functions. Head clamping mechanisms vary from simple swing bolts, to C-clamps, to breech lock assemblies, with varying degrees of mechanical assist systems for locking and unlocking.

Other types of extruder discharge heads employed on post mixer extruders include: End discharge pelletizers, externally and internally supported cone heads, pork chop heads, and slot dies. Cone heads produce a tube whose thickness is varied by changing the clearance between the cone and the head discharge annulus. A knife is arranged to slit the tube, which then. opens into a slab of very uniform profile. A pork chop head discharges the extrudate through a round opening, where a slowly rotating knife supported off the extruder centerline slices off pieces. The name is derived from the appearance of the cut pieces. The discharge of a pork chop" extruder is normally fed to a mill for conversion to a slab.

If the mill, extruder, or mill-extruder combination produce a slab, the stock is normally coated with an anti-tack agent, cooled, and then stacked on pallets for transfer to second pass mixing or to the fabrication or molding operation. If strips are required for feeding to extrusion lines, injection molding machines, or performers, the slitting may be done either at the exit of the anti-tack dip, or at the exit of the cooler, depending on the slitting technology and the method of storing the strip for transfer to the next operation.

One of the primary methods of slab cooling is by means of batch-off machines, which are available in a variety of constructions to suit the space and the process flow. Batch-off machines are normally sized to provide thirty minutes to an hour of cooling time, depending on the thickness of the slab being formed. After the initial dip in the antitack solution, they rely on air cooling, with air flow assisted by banks of fans mounted on one side of the rack. Some variations have a short spray section at the front end, to obtain additional conductive and evaporative cooling.

Where green strength is an issue, or where the stock will take a set if allowed to hang over a bar, cooling conveyors are sometimes used. These are usually multi-pass affairs, arranged for chilled water spray, or sometimes chilled air, if water contact is an issue. Operation of both cooling conveyors and batch-off machines varies, depending on whether the device is being batch fed from a mill, or fed continuously, as from an extruder, and whether batches are to be kept separate, or made as continuous as possible.

Where strip production is the primary objective, and production volumes are low, special rotary strip coolers are used. These machines are large drums on which a strip of three to four inches in width is wound in a spiral path as it comes off the mill, and allowed to cool in the tank until the next strip is fed into the machine.

There are a lot of variations in this area, depending a great deal on the specific needs of the production department and the nature of the compound itself. Cooling equipment and conveyors can be arranged to deliver material to either the first or second floor, depending on whether the material is first, second or final pass. Straining can be accomplished on line or off line, strip can be left hooked together by tabs to be tom off at the extruder, or stored in individual lengths in compartmented boxes.

Pelletizing is sometimes done with first pass stock, to allow cross blending, and automatic feed of second pass stock. It is also done to provide a pellet for feeding to transfer presses, injection molding machines, and cold feed extruders. Pellets can be cut in anti-tack slurry, or in a dry talc air stream, and are normally cooled in some form of fluidized bed cooler.

Intermediate and finished compound storage, handling

The primary considerations in this area are the previously mentioned first in first out arrangement, and the need to be sure that compound is not contaminated by spillage or airborne particles. The amount of material to be in storage at any time must be determined, depending on the aging which may be required, and the number of hours of compound inventory required to provide a cushion for unexpected shutdowns in the compounding area.

For those compound facilities producing formulations with hot cure systems, refrigerated, or at least temperature controlled, storage may be required to provide control over shelf life and processability. Depending on the local climate and the requirements for subsequent processing steps, humidity control may be important as well.

Storage and transport of pellets requires careful design to prevent blocking. Distributed storage in containers, as well as various forms of live storage have been used to overcome this problem. Pellet handling systems must be carefully shrouded to prevent pellets from escaping and rolling on the floor. Back interlocking should be utilized to insure that all parts of the transfer system are working before material is fed in to it.

Process support systems

Process support systems include plant and instrument air systems, cooling tower water systems, chilled water systems, and where required, steam and condensate or hot water systems. Plant power distribution systems, industrial ventilation (dust and fume collection), and motor ventilation are integral parts of the process support system.

The proper design, specification, and installation of these systems is vital to the success of the compounding facility. If they are not stiff enough, the operation of the facility will not be consistent under load. This is even more critical for the one or two mixer compounding facility than it is for a multi mixer facility, as load concentrations will represent a much larger percentage of the total load.

It is also important to consider some redundancy in the design of these systems, or at least alternate ways of keeping the facility running in the event of failure of one or more key components. Having the entire facility down due to a pump failure in the cooling tower water system, for example, might not be cost effective, compared to designing a system that uses two smaller pumps, or even three that run in stages.

Industrial ventilation is becoming more of an issue than it was seven years ago. The added dimensions of EPA permitting and OSHA concern for health in the work place have made the design of these systems more critical. Thought should be given to using smaller self contained subsystems that will also recover material, as well as keep the environment clean and capture fugitive emissions.

Notes

(1.) Output is always talked about in terms of pounds or kilos per hour, or tons per year. In fact, the mixer capacity is volume, not weight (or mass) related. The specific gravity of the compound has a significant impact on the production rate when measured in weight per unit time. It is important to know on what basis the machinery manufacturer states production capacities, and adjust the figures to suit the proposed compound(s). (2) Manufacturers of intensive internal batch mixers have taken to stating mixer size in nominal volumes which the mixer would hold if filled to the top of the chamber with water. In fact, the mixer cannot actually hold that volume and work properly. The actual working volume must be adjusted downward by a coefficient sometimes called the fill factor. This factor varies with the design of the mixer (intermeshing versus non-intermeshing, variable versus fixed clearance, two wing versus four wing rotors) and with the type of compound. Manufacturers can provide charts or guidance on selecting an appropriate fill factor based on either particular machine and the type of compound being mixed. Ordinarily, theses factors are adjusted based on knowledge gained during operation, but the initial coefficients will be adequate for sizing the mixer. (3.) For many years, the conventional wisdom in North America was that carbon black could not be transported successfully by pneumatic conveying, and that attempting to do so would destroy the spheroidized agglomerate particles the manufacturers had so carefully created. Despite this wisdom, carbon black has been successfully transferred by pneumatic means for many years in Europe. In more recent times, the idea of pneumatically handling carbon black has gained wider acceptance.