arrangement of the hoppers should probably provethe most rational.

ConclusionIn conclusion, let us turn aside from the purely

technical aspect of the problem and raise the questionof the economic and social aspects of the automaticfactory. A great deal has been said about theelimination of the human factor, but this does notmean the exclusion of manpower from the wholeworking process. It would be a great mistake tothink that in an automatic factory no workers areemployed and all the machines and installations arestarted and stopped by pressing a button on a centralcontrol desk. We shall never reach the point whereeverything is done without human intervention.Automation merely aims at eliminating manpowerfrom processes which can be done better and more

rapidly automatically than by hand. We also wantto render automatic the jobs which subject theworkers to bad hygienic conditions, such as the dustclouds raised in mixing rooms. Manufacturers haveevery reason to release manpower where it can bereplaced by other means and thus make it availableof manpower and one of the aims of technology mustfor other tasks. Nowadays there is an acute shortagebe to find new ways and means of relieving thiscritical shortage.

Industry is forced to develop automatic methodsbecause there is no other way to produce enoughgoods to satisfy humanity's innumerable needs.Therefore, automation is not mainly a means ofsaving manpower, but rather a means recently dis-covered after long years of experiment and still in itsinfancy to replace the manpower engaged in otherprocesses.

PLASTICS MATERIALS,

PROCESSES AND PLANT

by D. C. NICHOLAS.

Presented to the Eastern Counties Section of the Institution, 10th December, 1954.

Mr. Nicholas was educated at the City of London School and trained asan engineering pupil at a large locomotive works. He entered the plasticsindustry thirteen years ago as an extrusion engineer and has since beenintimately concerned with the subject. He has also been concerned withdesign and development of research and production equipment for thermo-plastics in many other fields.

He is the co-author of a plastics manual and was awarded the Silver Medalof the Plastics Institute, 1954, for a paper dealing with the extrusion of sheetand layflat tube.

Mr. Nicholas is now Head of the Engineering Department at B.X. PlasticsResearch Station, Manningtree, Essex. Mr. Nicholas

THE object of this Paper is to provide members, inthe limited time available, with as much interest-

ing information as possible on the subject of plasticsengineering and the plastics industry. As, however,the field has become very extensive, it was thoughtadvisable to limit the scope to the thermoplastic fieldin the first instance and to subdivide this into theprocesses and plant used for the production of the

intermediate materials such as individual sheets, con-tinuous sheets, moulding compound and extrudedproducts.

In order to bring the subject of plastics productioninto perspective with other materials, some recentdata may be of interest. The total production of allplastics in 1953 in the U.K. was about 200,000 tonsand this figure is almost double that of the production

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for 1949. Production for 1954 will be considerablyhigher than 1953. The proportion of thermoplasticand thermosetting materials may be assumed to beroughly equal. It will be appreciated that this tonnagerepresents a significant contribution to our nationaleffort in terms of volume, when allowance is madefor the fact that the average density of plasticmaterials is about 1.35 compared with about 7 formetals, to draw a very rough comparison.

Plastics MaterialsThe materials which command the greatest atten-

tion today are :-1. Polyvinylchloride.2. Polyethylene.4. Cellulose acetate.3. Polystyrene.

These materials constitute the major part of ourthermoplastics production.

Polyvinylchloride, or P.V.G., is available in twomain forms, rigid and flexible, and these materials areavailable either as sheets, extruded products of varioustypes, moulding or extrusion compounds.

Polyethylene is available in various hardness gradesas sheet, film, layflat tube, extrusions and mouldingcompound. It is essentially a flexible material in thethinner sections and film form, but in thick sectionsit exhibits a unique toughness and semi-rigidity.

Polystyrene, a hard glass-like substance, is availablein the form of moulding compound, sheet, film andextrusion, all of which may be termed rigid materials.

Cellulose acetate, a tough rigid material, is alsoobtainable in the form of moulding compound, sheet,film and extrusions.

The plastics manipulator may subject the materialsdescribed to a variety of processes which may includeextrusion injection or compression moulding, drawmoulding, vacuum moulding, blow moulding, welding,machining, or stamping.

Dealing now with these four main products in turn,P.V.C. may be regarded as the most important bothfrom the tonnage aspect and its ultimate suitabilityfor a variety of applications. It is capable of beingmanufactured into products which may be very softand rubbery, or by suitable adjustment of ingredientsand processing conditions it may be made quite rigid.The colour range is complete, from clear transparentto black, in both hard and soft ranges. It is resistantto water and many acids, alkalis, oils, and solvents, isdimensionally stable and has very good ageingproperties.

Typical uses for the flexible grades are raincoats,handbags, baby pants, upholstery for cars and furni-ture, curtains, cable coverings, etc. In the rigid gradeslampshade coverings, draw moulded articles, chemicalresistant pipes and fittings, tank linings, and mathe-matical instruments are representative of the uses.

Polyethylene, a more recent addition to the thermo-plastic range, has several properties which make ita useful complementary material to P.V.C. Itpossesses remarkable toughness, even at temperaturesbelow zero, has superior chemical resistance to certainsubstances and zero water absorption. The outstand-ing electrical properties enable it to be used for a

multitude of applications in the cable and electricalindustries, where high dielectric strengths and lowpower factors are essential. The colour range is fromnatural milky white to black in the thick sections,but thin films in the natural colour are almosttransparent.

The comparative ease with which polyethylene maybe injection moulded allows it to be used for manyimportant industrial articles, such as large chemicalcontainers. In the cosmetic field polyethylene hasfound a ready market in bottles and containers of allkinds for a wide variety of uses.

Perhaps the most interesting application for thismaterial lies in the packaging field, particularly forfood in view of the complete absence of taste or smell.More recently, polyethylene has been available in theform of thin flat film and tubular film, the latterbeing known as layflat tube. This tube may be easilyconverted into bags by cutting and welding trans-versely. Polyethylene possesses the further advantagethat it may be applied hot as a very thin coating topapers or fabrics to render them moisture proof, at thesame time enhancing the mechanical strength.

Polystyrene, a hard glass-like substance havingexceptionally good clarity, may be regarded as theprimary injection moulding material in use today.Since the War, its popularity has grown rapidly andit has displaced cellulose acetate in many fields. Thefactors which contributed mainly to its success includeease of manipulation, dimensional stability, superiorclarity and colour range, cheapness and availability.Apart from the attributes already mentioned, it hasoutstanding electrical properties and zero waterabsorption. It is, therefore, becoming increasinglypopular for H.F. electronic applications and for thispurpose is supplied in the form of very thin flexibleoriented film. Such film is used almost exclusively atpresent for the manufacture of wound capacitorshaving a low loss factor and exceptional capacitancestability.

Cellulose acetate, a tough rigid material, is usedprimarily for injection moulding. In the form ofsheet used for manipulative purposes it has beenuntil recent years the most useful complementarymaterial to flexible P.V.C. However, the advent ofrigid P.V.C. and its application to those fields hithertocovered by acetate sheet, together with the com-petition of polystyrene in the moulding trade hascaused cellulose acetate to decline in popularity.There are however, certain specialised applicationsfor cellulose acetate which cannot be met by othermaterials.

ProcessesPolyvinylchloride

Turning now to the manufacture of plasticsmaterials, P.V.C. again commands the closest atten-tion, both from the point of view of tonnage and theequipment necessary to convert the basic raw materialinto a useful product. The raw vinylchloride polymeris obtained from the manufacturer in the form of afinely divided flour and is derived, in the first place,by the polymerisation of vinylchloride monomer inwater.

677

CONNECTION

FOR EXHAUST FAN

TO REMOVE DUST

FEED HOPPER DOOR

HAND OR POWER OPERATED

SIDES AND ROTORS

CORED FOR

CIRCULATION OF

COOLING WATER

OR STEAM

SLIDING DISCHARGE DOOR

POWER OPERATED

SINGLE SLOPE

FLOATING WEIGHT

Fig. 1. Diagram of Bridge Banbury mixer.(By courtesy of David Bridge & Co. Ltd.)

The manufacture of this polymer is now carriedout in most of the major countries, although the U.S.has the greatest capacity. It is available in manygrades for specific applications and is always suppliedin the natural colour, white. In the powder form asmall batch is first mixed with pigments, lubricants,

heat stabiliser and plasticiser to form a master batch.This batch is then dispersed thoroughly in a furtherquantity of dry powder by stirring in a bowl mixer orenclosed type paddle mixer. A small amount of heatmay be applied during this stage to ensure adequatepenetration of the polymer by the plasticiser. Theplasticisers used are involatile oils or soft solids,which are compatible with the polymers, and theproportion of plasticiser to polymer will largely deter-mine the hardness of the final product.

The pre-mixed powder may then be transferred toeither a pair of hot milling rolls or to an enclosedtype Banbury mixer. During this stage of the process,the material is subjected to considerable shearingaction at a closely controlled temperature for a timesufficient to ensure complete gelling of the mix. Thefinal properties of the product will depend to a largeextent upon the control of this mixing operation.

The load of hot gelled material may then besheeted out and fed to rotary granulating cutters ifthe product required is moulding or extrusion com-pound. For the production of continuous sheet,however, the hot gelled material may be fed from thesheeting rolls to a calender in order to produce asmooth web of even gauge.

The calendering operation depends firstly upon thecontinuous feeding of the hot gelled material evenlyto the first nip. The considerable shearing actionupon the plastic mass causes it to emerge from thenip in a moderately even fashion, spread over aconsiderable area of the heated bowl surface. Thesheet then passes through one or two subsequent nipsin the same manner, depending upon the type ofcalender, and each of these nips will have at theentry faces a small rolling bank of hot plastic.

Fig. 2.Bridge Banbury andsheeting rolls installation.(By courtesy of David

Bridge & Co. Ltd.)

678

Fig. 3. Four bowl vertical Bridge calenderand wind up.

(By courtesy of David Bridge & Co. Ltd.)

Successive reductions in thickness and improvementsto gauge and surface texture occur at each pass untilthe sheet is finally stripped from the last bowl, stillquite hot but as a continuous web. The surfacetexture of the sheet may be made either polished ormatt, depending upon the surface finish of the finalpair of bowls.

Before final winding up occurs the sheet passesover several cooling drums and through an edgetrimmer. The rough edges which arise during thecalendering operation are previously trimmed at ornear the last bowl.

Quite a large proportion of flexible P.V.C. sheet isrequired with an embossed surface and the embossingprocess may be carried out in certain circumstancessimultaneously with the calendering operation. Thehot plastic sheet, after stripping from the last bowl ofthe calender, is passed immediately into the nip of afurther pair of rolls, one of which is embossed withthe design and the other consisting of a rubberpressure roller. The limitation of this method ofembossing depends, however, upon the length of therun, the design to be embossed and the thickness ofthe sheet. A considerable amount of embossing istherefore carried out as a separate operation, whenit becomes necessary to reheat the sheet before passingit into the nip of the embosser. During this operationit is also possible to apply certain inked effects to theheated sheet which will further enhance itsappearance, or the sheet may be printed with adesign prior to embossing.

The thickness range of sheet produced by thecalendering operation is approximately .003" to .030"max. for flexible material, and .005" min. and .020"max. for rigid material. The surface finish of thesheet becomes progressively more difficult to maintainas the gauge increases, due mainly to the rubber-likerheological properties of the plastic and thedecreasing pressures and shear rates at the roll nips.Thus, in order to produce P.V.C. sheet in thickergauges in either flexible or rigid grades it becomesnecessary to resort to a laminating process, usuallyachieved by hot pressing in a multi-platen press.In this operation, the previously calendered sheet iscut into lengths and laid between polished plates toreproduce the surface required. The ' nips' thusmade up are placed between the platens of the pressand pressure and heat applied. After the requisitetime has elapsed for complete welding of the sheets,the platens are cooled and the pressure released.Typical sizes for such sheets are 24" X 54" and48" X 96" approximately. Thicknesses may be 1"or more and perhaps the most common example ofthis method of manufacture is P.V.C. sheet for shoesoles.

The production of P.V.C. sheet is at presentuniversally achieved by the calendering process andit is without doubt the most economic and speedymethod. Thin sheet in polyethylene, acetate orpolystyrene may, however, be produced by extrusionprocesses which are comparatively recent develop-ments. It is worth noting here that none of thesematerials is capable of being calendered satisfactorily

679

due to their respective rheological properties. WhilstP.V.G. can be sheeted by extrusion if required, itstendency to decomposition upon prolonged heatingin an extruder renders it a difficult material to handleby this method.

Space does not permit of a more detailed analysisof the essential differences between the productionof rigid and flexible P.V.C., but in general it may bestated that although the equipment used issubstantially similar, the techniques employed differconsiderably. The unplasticised grades are moredifficult to process, due in part to greater heatsensitivity and a reduced degree of plastic flow at theprocessing temperatures. The pressures required onrolling mills and calenders are higher and the powerrequired to drive these units is also greater.

PolystyreneWhereas the production of P.V.C. necessitates a

relatively large amount of equipment to convert thebasic polymer into an end product suitable for furthermanipulation, polystyrene is produced by one of anumber of chemical processes which provide a finishedpolymer suitable for immediate use for injectionmoulding, filming by extrusion or compressionmoulding. Apart from the addition of colour, poly-styrene requires no further processing. It cannot beplasticised in the normal manner, although fairlyrecent advancements have been made with modified

polymers containing rubber compounds, renderingthe materials less brittle.

Briefly, the manufacture of polystyrene dependsupon subjecting the basic raw material, styrenemonomer, to a chemical reaction under closely con-trolled conditions of heat and pressure. The styrenemonomer is derived initially by the dehydrogenationof ethyl benzene and before processing is a clear lowviscosity liquid, highly inflammable and toxic. Duringthe reaction the molecules of styrene monomer arecaused to link up in a continuous manner to formmuch longer chains of higher molecular weight. Thelong chain molecules of polymer thus formed causethe mass to emerge as a highly viscous transparentplastic at the processing temperature but which, whencooled, becomes hard and transparent.

PolyethyleneThis material, a description of which was given

in an earlier section, is derived initially from the gasethylene. As with polystyrene, the final polymerrequires no further processing other than colouring,before injection moulding, compression moulding orextrusion into various forms. The manufacture ofpolyethylene is also essentially a chemical processinvolving the polymerisation of ethylene gas at veryhigh pressures in the region of 1,000 atmospheres ormore. The resultant polymer, when cool, is a toughwax-like substance having a milky appearance. The

Fig. 4. Continuous roller embossing machine.(By courtesy of Dornbusch & Co., Germany.)

680

degree to which the polymerisation is carried andconsequently the molecular chain length affects thehardness and softening point of the polymer.

Cellulose AcetateThe manufacture of this material resembles in some

respects the initial stages of P.V.G. compounding.The cellulose acetate flake, derived from theacetylation of cotton linters or cellulose pulp, is mixedwith certain proportions of plasticisers and pigmentsin a bowl mixer or enclosed type paddle mixer. Themixed powder is then transferred to a Banbury mixerand subjected to severe shearing action at a controlledtemperature. The powder is thus gelled and the massis removed and immediately transferred to sheetingrolls. The rough sheet is then stripped from therolls in a narrow band and fed to a rotary cutter forgranulation. In this form the material is ready forinjection moulding or extrusion.

ExtrusionProcesses

Mention has already been made of the extrusionmachine and it may be of interest to dwell upon thisversatile item of equipment. The machines referredto and used extensively today in the plastics industryoriginated some 75 years ago, possibly for clay orrubber. The essentials of such a machine are :-

(a) a screw having a relatively deep profilerotating in a cylinder having a feed openingat the rear end;

(b) means for heating the cylinder;(c) a die head for forming the product;(d) a means for driving the screw.

Machines of this type have been in use in therubber industry for about 75 years, but the use ofthe screw extruder for plastics did not commenceuntil just prior to World War II. At that time thesemachines, in their rather crude form, were called uponto handle new materials requiring new techniquesand this they did with varying degrees of success.However, the need for close temperature control,corrosion resistant screws and cylinders and improvedheating to replace the orthodox steam heating resultedin the gradual evolution of a plastics extruder suitablefor a variety of materials. This development haslargely occurred since World War II and today oneis able to choose from quite a startling range ofmachines made in Great Britain, U.S.A., Germany,Italy, Belgium, and France. A further recent develop-ment has been the use of two or more intermeshingscrews within a common cylinder which, it is claimed,gives superior results to the single screw machine.

The obvious attraction of a continuous method ofprocessing material from the granule stage to thefinished product has caused much thought to be givento the application of this process to a host of differentproducts. These processes include the colouring ofmoulding powder, mixing of plastics ingredients forinjection moulding, monofilaments, solid rods, rigidand flexible tubes, rigid and flexible sections, rigidand flexible thick sheet and slabs, thin films up to100" wide, stretched foils, layflat tube in continuousrolls, coated paper, and covered wires, ropes or rods.

HEATTM6 ZONES

Fig . 5 . D iagram of screw extruder.

The extrusion machine would appear, at first sight,to be extremely simple both in principle andoperation, but in fact proves to be one of the mostcontroversial items of equipment manufactured forplastics manipulation. The successful operationdepends to a large extent upon the exercising of agood deal of art and individual experienced users ofthese machines have preferences for certain featuresof design calculated to make the extrusion machinemanufacturer's life very difficult. Therefore, it willbe readily deduced that the precise behaviour of theplastic material between the time of entry and exitfrom the machine is not an exact science.

Turning now to a brief survey of some of thesemachines, the production of plastic sheet by theextrusion method has received much attention inrecent years and several different methods have beenevolved. The processes depend upon either theextrusion of a tube which is subsequently slit openlongitudinally, or the direct extrusion of the sheetfrom a straight slot die.

The advent of polythene as a wrapping materialcreated a demand for this sheet up to 100" wide fora multitude of purposes. As polythene is a relativelyheat stable polymer and its flow properties extremelygood, wide sheet is most conveniently extruded bymeans of a straight wide manifold type die. Diesof this type consist of a solid bar of steel boredthroughout the length and having a narrow slotalso machined longitudinally to communicate with thebored hole. A central feed hole is usually providedand this communicates with the front fitting on theextruder cylinder. The extrusion slot is formed bytwo plates bolted on either side of the feed slot andthese plates are made adjustable in order that thegauge of the sheet may be varied during extrusion.The body of the die is heated electrically in narrowsections and the temperatures controlled electronicallyat each section. The relatively low viscosity ofpolythene at the extrusion temperature makes itnecessary to pass the sheet immediately into waterin order to set and cool it prior to winding up.

The most interesting and useful application ofpolythene as a wrapping medium is perhaps thelayflat tube. Polythene lends itself admirably to theextrusion of a tube having a gauge between 30microns and 250 microns (.0012" - .010"). The tube isinflated with air in order to support it at the dieand cooling is applied externally to set the film close

681

Fig. 6 (left).Sheet extrusion die.

Fig. 7 (below).Sheet extrusion unit.(By courtesy of The

National RubberMachinery Co.—U.S.A.)

682

to the die head. The tube is passed through con-verging rolls in order to flatten it and subsequentlythrough a pair of pull rolls which are driven at therequired speed. The control of gauge and widthis achieved largely by varying the amount ofentrapped air in the tube and the speed of the pullrolls. The range of widths may be from 1" up to 60"in any thickness within the specified range and thecontinuous envelope is available in roll form. It willbe appreciated that to produce bags from such tubeonly requires transverse heat welds before severingfrom the roll.

The superior moisture resistance and toughness ofpolythene makes its application to paper an obviouschoice when considering the moisture proofing ofbags, etc. A process for this purpose was developedin the U.S.A. several years ago and is now operativein this country. The extruder and die aresubstantially the same as that already described andshown in Fig. 7. However, the die in this processis required only for the purpose of extruding a thin

layer of hot polythene into the nip of a pair of rollsthrough which the paper to be coated is passing.The rolls are chrome-plated brass and rubberrespectively and they are forced together by pressurescrews or hydraulic rams. After coating the paper iscooled, trimmed and batched up in the usual manner.The thickness of the coating may be as low as 6microns (.00025" approx.) and speeds of coating maybe as high as 1,000 ft./min. with an extruder of 8"screw diameter.

The processes illustrated, although only touchinglightly upon the field of utilisation of the extruder,indicate the varied duties of this item of equipment.

MachinesIt is generally accepted that stepless speed control,

either by variable speed motor or constant speedmotor with a variable speed gear box, is a primenecessity for extruders required for a wide range ofplastics. The variable speed A.G. motor is widelyused for extruder drives, but where close speed control

Fig. 8. Vertical layflat tube extrusion unit.(By courtesy of The National Rubber Machinery Co.—U.S.A.)

683

Fig. 9 (above). Polythene paper coating plant.(By courtesy of The John Waldron Corporation—U.S.A.)

Fig. 10 (below). Close up of extruder and die.(By courtesy of The John Waldron Corporation—U.S.A.

684

is essential, the D.C. Ward-Leonard system, althoughmore expensive, is to be preferred. The D.C. drivehas the added advantage of a wider speed rangethan the customary 3 or 5 :1 of the A.C. motor.The h.p. required will, of course, vary considerablywith the type of extrusion being carried out, but arough estimate would be j$ h.p./lb./hr. This figuretakes into account the frictional loss of the gearbox and screw and the frictional heat produced inthe plastic. The lowest h.p. available is always inexcess of the normal requirements in order to avoidstalling during the starting period.

Most modern extruder manufacturers build thereduction gear box integral with the cylinder andmain frame in view of the high torques involved onthe larger machines and this design feature is verydesirable, in that misalignment between screw anddrive shaft is minimised. The types of gears used callfor little comment and both spur gears and wormdrives are in common use. The worm drive isperhaps more economical in space, but has the dis-advantage that the drive motor and variable speedgear box must be placed separately to the rear orside of the machine. The double spur reductionmay be designed in such a way that it is possible tomount the drive motor within the base of the frame,giving a particularly clean appearance to the machine.

Screw thrust bearings may be called upon towithstand pressures of the order of 2 tons/sq. inch ofthe screw area. Thus a 3" screw may generate aback thrust of 14 tons, but on occasions this figuremay be considerably exceeded due to cold startingand poor plastic flow from the die. The thrust racesused are of the tapered roller or ball type and inpractice little trouble is experienced in this direction.

The extrusion cylinder may either be of the steamjacketed type having an inserted hardened liner or itmay be of the solid variety, usually Nitralloy, nitridedto give the best resistance to wear.

The screw, being the only operative part of the

machine commonly believed to affect materially theextruded product, has been the object of muchingenious thought on the part of the extrudermanufacturer.

It is no exaggeration to say that almost everypossible combination of number of starts, constant andvariable pitch, constant and variable depth in varyingdegrees has been evolved and advocated for efficientextrusion. Experience has shown, however, that forthe majority of plastic materials almost any screwcan be made to perform satisfactorily by those skilledin the art, providing that the correct combination ofspeed, temperature, pressure and die design for thematerial is attained. Certain specific cases are found,however, where careful attention must be paid to thescrew characteristics, for satisfactory results. Thematerials used for the screw may be mild steel,hardened tool steel, Nitralloy, stainless steel or forspecial non-corrosive purposes, Z-nickel or similaralloys.

CalendersIn the field of thermoplastics the P.V.C. calender is

probably the most important item of equipment,since the tonnages produced in the form of sheetoutweigh the extrusion and injection compounds usedfor other purposes. Although calenders have beenin use in the rubber industry for about 100 years for avariety of purposes, it was not until just prior toWorld War II that P.V.C. in the form of continuoussheet began to appear as a commercial product,produced on standard rubber calenders. By presentday standards the sheet thus produced was of poorquality, but much was to be learned in the ensuingyears of the chemistry of P.V.C. compounds in orderthat satisfactory processing might be achieved.

The type of calender used for the early work wasthe three-bowl steam heated vertical machine and atypical example is shown in Fig. 13.

Fig. 11. 4\" Plastics Ex-truder with variable speed

motor.{By courtesy of Francis Shaw

& Co. Ltd.)

685

In later years, the four-bowl vertical calender wasused by some in order to obtain improved surfaceand reduced thickness with improved gauge control.

The difficulties encountered when feeding thevertical type calender, and the fact that adjustmentof the inner nips required accurate tandem movementof the top or bottom pair of bowls, encouragedcalender designers to evolve more suitable arrange-ments. The inverted L and the Z types are themost favoured designs in use at present.

The major factors which have governed theevolution of these calenders are :-

1. favourable positioning of the feed nip to ensureeven feeding;

2. efficient method of heating the bowls to ensureclose temperature control;

3. the provision of improved means for circulating

Fig. 12. 4" Plastics Ex-truder with P.I.V. drive.(By courtesy of Hermann

Berstorff, Germany.)

the heating medium through the bowls to ensurea more constant temperature of the surface;

4. independent precision adjustment of the nipsfor fine tolerance working and the thinnestpossible gauges;

5. the provision of compensation for the deflectionof the bowls under load to ensure that withplastics of different hardnesses, calenderingspeeds and thicknesses, an even gauge may bemaintained within ±0.0001" across 54" widthof sheet;

6. universal shaft drive to all bowls to eliminatethe influence of gear loads on the roll necks;

7. increased accuracy of bearing manufacture inorder to eliminate roll ' float' within thebearing under load;

8. Stepless variable speed drive covering a widerange of speeds.

Fig. 13. Three-bowl vertical rubbercalender.

(By courtesy of The Adamson UnitedCompany—U.S.A.)

5 BOWL-RUBBER 4-BOWL-RUBBER iNygRrep l_- PLASTICS 2.- PLASTICS

SOME TYPES OF CALENDERS

Fig. 14. Four types of calenders used for plastics.

686

FeedIt is generally accepted that horizontal centres for

the first pair provide improved working conditions.The stock is fed either by conveyor from a sheetingmill or by extruder in a continuous rod which maybe traversed slowly across the nip.

HeatingIncreased speeds of calendering necessitated the

use of fluid heating rather than steam. High pressurehot water up to 200 lb./sq. inch allows closetemperature control and since with high calenderingspeeds considerable frictional heat must be dissipatedfrom the plastic mass, the hot water is capable ofremoving this excess heat.

Bowl CirculationWhereas the orthodox rubber calender bowl was

provided with a single bored chamber concentric withthe surface, this type of bowl was considered topossess poor heat transfer properties. An improved

method of boring the bowl was therefore developed,consisting of longitudinal holes near the workingsurface, communicating with a central flow and returnbore. This type of bowl allows far more efficientheat transfer in both directions, together with areduced differential temperature between the centreand the ends.

Nip AdjustmentThe pressure is applied to each roll bearing

independently by means of precision built enclosed-type worm reduction gears, driven from independentmotors. The motor may have two speeds, giving aration of 5 to 1 between fast and slow adjustments.Provision is made for adjusting pairs of bearings oneach roll in tandem or separately and safety devicesensure that excessively high frame stresses areavoided.

The calendering of film .003" thick at high speedsmay produce roll separating forces of the order of75 - 90 tons. Thus, the nip adjusting jack screws

Fig. 15. Four-bowl Z calender showing feed nip.(By courtesy of The Adamson United Company—U.S.A.)

087

////////////AV//////////////7ZZ77,

A—1

CONVENTIONAL CHAMBERED ROLL

OR1LLEO ROLL

Fig. 16. Chambered bowl anddrilled bowl.

are liable to develop a high frictional resistance whenadjustments are made during the run. In recentyears, systems have been developed for applying thehigh pressures to the rolls by hydraulic rams operatedfrom a remote control power driven hydraulicintensifier. Each bearing is supplied separately froma bank of cylinders driven from separate motorsprovided with push button control. This system hascertain advantages over the direct application ofpressure by screw, in that the pressures applied maybe directly read off on the gauges andelectro-hydraulic cut-off may also be provided toprotect the main frames.

Crown CompensationIn order to obtain flat lying sheet from rubber or

plastics calenders it has hitherto been necessary togrind and hone the contour of the bowl surface witha convex camber, in order to compensate for bowldeflection under load and variations in temperatureacross the surface of the bowl. Bowl contour is, how-ever, related closely to speed, temperature, nip pres-sure, and plastic flow properties, with the result thatflat sheet could only be produced within certain

Fig. 17. Z calender, showing nip adjusting gear.(By courtesy of Farrell Birmingham & Co.—U.S.A.)

specific limits of these conditions. A method was, there-fore, evolved whereby one or two of the bowls couldbe crossed axially in relation to the adjacent bowl inorder to produce the necessary deflection compensa-tion. This effect is produced by displacing eachbearing housing an equal amount in oppositedirections by screw adjustment, the axis of the rollscrossing in the centre. This design feature providesthe calender operator with considerable latitude inspeeds, temperature and plastic hardness but ensuresthat extreme accuracy may be maintained with thegauge of the sheet. The graph in Fig. 19 illustratesthe equivalent crown thus produced.

Roll Drive ShaftsThe introduction in recent years of universal shaft

drive to all bowls from a separate pinion stand wasnecessitated by the inclusion of compound adjustmentof the cross axis bowls and to eliminate the reactionof the gearloads upon the journals. This system alsohas the additional advantage that all drive gearsmay be totally enclosed and mounted in rollerbearings, lubricated by a separate circulating system.

The universal shafts are also provided with splinedcouplings to allow for the expansion of the bowls atthe running temperatures.

BearingsIn order to produce sheet in widths of 5 feet or

more to an accuracy of ±.0001", calender designershave given much thought to the problem of bearingaccuracy and design. One leading calendermanufacturer in the U.S.A. has specialised in the useof roller bearings for several years and these bearingshave been developed specially for this type of work.The problems of maintaining accurate fits attemperatures of 350°F., providing allowances forlongitudinal expansion of bowls and lubricationpresents no mean task, but it is felt that theadvantages to be gained are worth the extra expenseand effort.

The conventional sleeve bearing is still favoured bymany calender builders and the problem of journalfloat within the bearing has been given muchattention.

688

Fig. 18. L calender with hydraulic nipadjustment.

(By courtesy of Joseph Eck & Sohne—Germany.)

In order to ensure that the bowl journal assumesthe same position within the bearing, irrespective otworking conditions, pre-loading devices have beendeveloped which pull the journal against the bearingface at the correct point. These devices are eitherspring or hydraulically actuated.

Journals and bearings are machined to a very highdegree of accuracy and flood lubrication to eachbearing from an independent circulating system withoil cooler and filter ensures trouble-free running. Oilseals are usually of the metallic labyrinth type andmust be entirely reliable to ensure that no oil canreach the working face of the bowl.

DrivesThe wide range of speeds required necessitates

a stepless variable speed drive of the D.C.Ward-Leonard type. These generator sets are nowfitted to most precision calenders and the output maybe up to 700 h.p. The D.C. supply is also used forthe actuating motors on the various adjusting devices.

In order to provide a drive to each bowl which issmooth and free from gear tooth influence, somethought has been given to the independent driveof each bowl by a separate D.C. motor. One calendermanufacturer has built such a machine and it isclaimed that it has the added advantage that the

Fig. 19. Graphs showingcrown compensation and

roll separating force.

x.66 ROLL

• / • / . / ' S , / .. / .-/

.cf*

So

Cwoss AXIS FILM THICKNESS

689

Fig. 20. Inverted L calender with universal drive andpinion stand.

(By courtesy of The Adamson United Co.—U.S.A.)

Fig. 21. Inverted L calender, 32" X 92" with pre-loadedtop rolls.

(By courtesy of The Adamson United Co.—U.S.A.)

speed relationship between each bowl may be set tosuit individual calendering requirements, thusproviding a greater latitude of adjustment ofconditions.

ConclusionThis Paper has touched upon but a few of the

many facets of the rapidly expanding plastics industryin this country. The plant required for the processingand manipulation of plastics materials is, to say theleast of it, highly specialised. The evolution of newmaterials at fairly frequent intervals, requiring newprocessing techniques and equipment, represents acontinual challenge to the plant manufacturer tospend more on development projects for improvingexisting designs or devising new methods.

AcknowledgmentsThe Author wishes to thank the Directors of Messrs.

B.X. Plastics Ltd., for permission to publish thisPaper and to acknowledge the generous help providedby the following Companies :-

1. The Adamson United Company, Akron 4, U.S.A.2. Hermann Berstorff, Maschinenbau, Hanover, Germany.3. David Bridge & Co. Ltd., Castleton, Rochdale.4. Dornbusch u. Co., Krefeld, Germany.5. Joseph Eck u. Sohne, Dusseldorf, Germany.6. Farrell Birmingham & Co. Incorporated, Ansonia,

U.S.A.7. The National Rubber Machinery Co., Akron 8, U.S.A.8. Francis Shaw & Co. Ltd., Manchester 11.9. The John Waldron Corporation, New Brunswick, U.S.A.

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