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`must be incorporated into the pattern and mold sizes. Thermoplastic materials
`expand and shrink when subjected to temperature changes andwill therefore
`experience shrinkageaftercooling. For the controlofsize reduction after form.
`ing, the molds are purposely built oversized tocompensate for shrinkage. Once
`the maker is armed with the correct shrinkage rate for the particularplastic,
`building oversized moldswill eliminate most surprises from resulting shrink.
`age. This is particularly important when producing mating components that
`mustfit together correctly; for example, containers and their lids must have a
`snug, snap-actionfit to serve their purpose.
`Each particular type of plastic displays its own shrinkage rates, which
`should be specified by the specific resin manufactureror supplier. Theshrink.
`age rate and its range depend largely on the type of resinand its molecular
`weightor blend. For general purposes, a wide range of shrinkage rate can be
`used. However,forcritical dimension controls, those numbersshouldbe nar-
`rowed down to the actual shrinkage values. Table 1 provides the wide shrink-
`age ranges of some of the most popular thermoplastic materials used in
`thermoforming.
`The shrinkageratesfora critically importantfit should be specified and
`when material changesare anticipated, the material shrinkage levels must be
`matched betweentheold and new resin supplies. Discrepancies among shrink-
`age rates are one of the main causes of improperfitting results between pre-
`viously well-fitted parts and put a damper on quick, hastily made material
`substitution. It should be noted that some thermoplastic materials can have
`such a great shrinkage change thatit may affect the product fill capabilities or
`containercapacity of a thermoformed receptacle. Such variationsin shrinkage
`mustbe investigated beforefull-scale production is attempted.
`In the pattern-making procedure,the patteritself usually represents an
`intermediate form of tooling where the pattern should be converted into a
`more permanenttype of tooling. Patterns are generally made of wood, such
`as gelutong, mahogany, basswood,or sugarpine. These high-quality, uniform-
`
`Table 1 Thermoplastic Shrinkage Rates
`Shrinkage range
`(in.)
`
`Material
`
`ABS
`Polyester
`Polyethylene
`Polypropylene
`Polystyrene
`PVC
`
`0.004-0.009
`0.015-0.025
`0.015-0.050
`0.010~-0.025
`0.002-0.006
`0.001-0.006
`
`
`
`grained woodsare idealto workwith for shaping andpiecing togethera pattern
`or mold form. Other woodsarejustas usable, but due to their grain irregulari-
`ties, the maker must be extremely selective as to what portions of the lumber
`are usable. Using a choice wood quality usually provides some saving in the
`laborphase of pattern-making projects. Local wood material availability is also
`a guiding factor as to the selection of one type of wood over another.
`The wood pattern byitself can be used as a temporary or sample-making
`mold. Using wood as a mold material can only be considered temporary. The
`wood itself does not keepits original dimensions accurately. Ambient humidity
`conditions may decreaseor increase the size of the wood. Since a pattern or
`wood mold is made up from several pieces of different grain-line direction,
`each piece may acquire a changein its own dimension,causing “stepdown”or
`overhangs among the pieced-together components. For that reason, wood
`molds should be considered only interim structures. It is also true that wood
`materials provide poorheattransfer qualities. In repeated thermoforminguse,
`each surface contact between the heated thermoplastic sheet will eventually
`raise the wood mold’s temperature to the same heatlevel as thatof the plastic
`sheet itself. Since the mold’s retained heat does notallow sufficient cooling
`for the plastic to set, the mold will not render any useful quality for further
`thermoforming past a certain point. At this point, the operation hasto be shut
`downuntil the heat in the wood mold dissipates. This poor heat transfer quality
`will never allow reasonably fast cooling times, thus causing long periods of
`waiting.
`When wood patterns are made for temporary molding purposes only,
`they are madelarger than actual size to compensateforthe plastic’s shrinkage.
`Whenthe patternsare used for the purposeofcasting to produce the perma-
`nent metal molds, additional shrinkage factors must be incorporated. This
`additional shrinkagefactoris built into the patterns to accountfor the shrink-
`age that takes place when the molten metal cools down. Pattern makers often
`use “shrink rules” for this purpose. These special rulers resemble standard
`measuring rulers except that a shrinkage factoris built into their calibration.
`For example, with styrene materials together with aluminumtool casting, a
`“fz-in. shrink ruler is used. This ruler has ’/s2 in. of shrinkage per foot incor-
`poratedintoits calibration, The "/s2-in. shrinkage designation combinesa '/,6-
`in. allowance for the styrene shrinkage and a */32-in. allowance for the alumi-
`num castings shrinkage. Withthis total shrinkage allowance,the actual pattern
`is built to oversized with proportionalsize increases that have been distributed
`overall of its dimensions.
`Whenthe proper amountofshrinkageis notincluded into the mold, the
`outcome of the moldingwill be a proportionally undersized part. This is true
`especially when copying an existing item where the newly created mold shape
`is duplicated directly from an already thermoformedarticle.
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`C. Basic Mold Constructions
`Oneof the most beneficial reasons for choosing the thermoforming process
`over other manufacturing methodsis to make useful plastic products with sub.
`stantially lower mold costs. The mold can be made much more easily and from
`less costly materials, Both of these factors can be so overwhelmingly advanta-
`geousover the tooling costs of other manufacturing methodsthat this alone
`often determinesthe choice of the thermoforming process over other process
`techniques. The reduction in tooling cost, however,is not the only reasonfor
`this choice; the higher output production rates create equally advantageous
`benefits.
`
`1. Mold Materials
`Making molds for the thermoforming process can be accomplished using a
`wide range of materials. Thefirst thing the user has to establish is the size of
`the production run. The thermoforming process can be applied feasibly toa
`short production run and a “temporary” mold can be used for the job. Long
`production runs that produce a high volumeof products need a more durable
`and “permanent”type of mold. For either purpose, different mold materials
`should be used, and with them, matching related tooling cost can be obtained,
`Theless costly tooling materials are easier to work with and assemble; however,
`their life expectancy and dimensionalstability make them useful only for tem-
`porary or short-term molds. On the other hand, specially treated metallic
`molds can have substantial “permanence” and years of production longevity
`unless premature damageorlack of product sales force them to be shelved.
`As mentioned earlier, wood is a popular tooling material for thermo-
`forming. Mostpatterns and temporary molds are made of wood; they can be
`shaped and workedby all common woodworking tools. Complicated ners
`can be put togetherin sections or segments rather than as a single piece, mak-
`ing them easier to build. The pieces can be glued, nailed, and screwed to one
`another, creating the final structure.
`When the woodworking proceduresarefinished, a sealing compound
`can be used to coat and protect their outer surfaces. Vacuum holes and air
`channels can be built into the wood structures togetherwith metallic threaded
`mounting inserts. The use of threaded metal inserts is a good idea a
`frequent screw removalwill wear and damage mounting holes used repeatedly.
`In addition to all the aforementioned advantages, wood molds will usually be
`subjected to frequent modifications. The ability to make alterations, such as
`adding or removing surface patterns, makes wood the ideal material for ex-
`perimental mold making.
`7
`Oneof the drawbacks to wood moldsis their reaction to humidity, which
`causes dimensional changes which affect the highly accurate and sensitivé
`
`mold dimensions. Another, more serious inadequacy is that the wood itself
`will retain the heat placed into it by thermoforming. A wood mold quickly
`becomesunusable dueto its noncooling tendency. This will force the thermo-
`forming operations to shut downuntil the wood mold can cool down. Since
`wood is an insulating material, cooling may take several hours and may stop
`the operation entirely for the day. This heat buildup in a wood moldis obvi-
`ously the direct result of the molding frequency. With fewer moldingcycles,
`the heat buildupwill not be as severe or start as soon.
`To improve on wood mold qualities and make molds with improved du-
`rability, the wood patterns can be convertedinto plaster or epoxy form. These
`higher-grade materials provide better dimensional permanency and maintain
`their quality over a longerperiod ofuse. The mold’slife expectancy is expanded
`togetherwith the length ofoperation. The higherdensities ofplaster and epoxy
`make the molds stronger and give them more heat absorbency. The plaster
`mold is usually madeof “hydrocal,” a cementlike grade of plaster compound.
`The white powdery hydrocalis mixed with waterto create a slurry, which dis-
`plays a good liquefied, pourableviscosity. The slurry is poured over the wood
`pattern, which is preparedwith a retaining frame surroundingthe pattern
`body.Prior to the casting, all exposed surfaces of the pattern and the accom-
`panying frames mustbe coated with sometype ofrelease agent. Wax or lacquer
`mixed with oil compoundsorsilicon greases provide excellent release after the
`hydrocal has set. To avoid a large amountof air entrapment or air bubble
`collection, the slurry viscosity is substantially thinned. After pouring the mix-
`ture, the entire setup is subjected to vigorous shaking on a vibrator table to
`force the bubblesout of the slurry. It might also be placed in a vacuum chamber
`to undergo air bubble evacuation. With either method, the trapped and en-
`capsulatedair is reduced, making the pouredcasting internally more solid and
`with fewer surface imperfections that must be patcheduplater. After the hy-
`drocal has set, the pattern and the hydrocal are separated. The hydrocalwill
`display a reversed impression ofthe originalpattern.Ifduplication ofthe origi-
`nal pattern is desired, a repeated hydrocalcasting is made. Two things must
`be remembered whenusing hydrocal. Oneis that a moisture-resistant release
`agent must be usedto avoid adhesion ofthe casting to the original pattern or
`previously made pattern. The surface bonding can be so severe without the
`properrelease agent that any attempt at separationof the two sideswill result
`in the destruction of both. The second factor to rememberis that hydrocal
`molds must be completely dry before thermoforming is attempted. The heat
`generated by the process converts the retained moisture into steam within the
`castings and will develop enoughinternal pressure to cause cracking or even
`rupture of the molds. After complete drying, the mold can be sealed with a
`sealant compound and,if necessary, have pieces inserted or other materials
`glued to it. A broken or chipped hydrocal mold can easily be mended or
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`patched. Thesize of repairable damage depends onthe repairman’s skills and
`the thermoformingcriteria encountered. The repairwork may take moreef.
`fort than that needed to make another hydrocal casting.
`There are toolmakers and pattern shops that prefer to work with epoxy
`casting materials. In this type of casting, the water-based castings of the hy-
`drocal method are replaced by a non-water-based two-part epoxy chemical
`substance. Since wateris not used to create a pourable media,the wood pattern
`will not experience the pattern reaction to moisture that it does with hydrocal,
`Epoxy will also not produce grain raising and dimensional moisture-related
`changes. On the other hand, it may take longerto set up and will generate a
`much higherlevel of heat in the setting phase. The higherlevel of heat gen-
`eration can be just as destructive or damaging to the pattern as moisture. If
`low-melting-point components such as wax filetsare used in the original pat-
`terns,they will be melted away,creatingan alterationin the final configuration,
`All these factors must be worked outprior to casting. Without the use of a
`properrelease agent compound,the epoxy casting material can also lock
`against the casting surfaces, and removalwill inevitably result in disaster.
`Epoxy molds offer much improved surface details, dimensional stability,
`and structuralstrength. Their density and durability make themideal materials
`for making low- to medium-production-volume molds. Their dimensionalsta-
`bility is excellent and their heat absorbencies are not poor, but they are, of
`course, no match for metallic molds. The heat-transferring quality of the basic
`epoxy materials can be substantially improved by blending in small metallic
`particles. Aluminum powder,pellets, shots, and even needles can be blended
`into the epoxy mass. Their purposeis to improve the epoxy’s heat-transferring
`qualities and at the sametime, boostits strength. The closely packed metallic
`particles tend to allow better heat channeling between the embedded pieces.
`Further improvementscan be obtained by embedding cooling lines of metal
`tubing. Mold coolantcan be circulated through the tubing to enhance greatly
`the material’s cooling ability.
`The two-part epoxy materials are usually purchased in premeasuredcans.
`When the two compounds are mixed together, they react to form the epoxy
`compound. Each epoxy material has a different “potlife,” which is the amount
`of time the material takes to set up after mixing. Shorter pot life makes the
`casting harden more quickly butallowsless time for the mixing andblending
`of the metalfillers. A longer potlife is more advantageousfor getting rid of
`the entrapped air bubbles that have worked themselves into the mixture.
`Working with epoxy castings does require someskill to achieve choice mold
`replicas; such skills are improved when working often with this material. Epoxy
`suppliers are usuallyvery helpful and will cometo the aid ofthe unexperienced
`practitioner. Epoxy molds, with their durability, can be considered as produc-
`tion-type molds, ideally suited to short to medium-sized production runs. A
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`well-constructed epoxy mold can be expected to perform well for up to several
`million production units without major breakdown orfailure.
`The last group of mold materials are the metallic molds. Metal molds
`can be producedout of a wide range ofavailable basic metals or their alloys.
`There are two basic methods for shaping metals into a mold configuration.
`Metals can beheated,liquefied, and then poured into castings, or they can be
`mechanically cut, milled, and turned into the desired shape on regular metal-
`working equipment. Metal tools have substantial permanency and therefore
`qualify as “permanent” molds. With additional surface treating they can be
`made virtually indestructible. Of course, like anything else, if they are sub-
`jected to abnormaluseor abusive practices, they can be destroyed. Metal
`molds offer the most outstanding heat channeling andheattransferqualities.
`This makes them idealas cooling surfaces for the forming of hot thermoplas-
`tics. The most popular metal used in the mold making for thermoforming is
`aluminum. This metalis ideal becauseit has specifications thatfit the thermo-
`forming process best. Aluminum is lightweight and easy to work with on any
`common toolmaking equipment.It has one ofthe best heat transfer qualities
`among the inexpensive materials. On top of this, aluminum doesnot corrode
`easily like steel does, Aluminum’s only shortcomingis thatit is soft and easily
`damaged by nicking, scratching, or banging. Otherwise, it is the most ideal
`mold material in use today and for that reason is the most commonlyused.
`There are other metals used for mold productionthatare not as popular
`as aluminum but are demandedbyspecific conditions or situations. Brass or
`titanium alloys provide a much stronger mold composition, making them far
`more damageresistant than aluminum.Theyarealso less vulnerable to cor-
`rosive environments, and tooling made out of them will probably outlast the
`life expectancy of most product designs. Tooling madeofsteel or brass alloys
`can be chromedornickel plated if the product criteria demandit. They can
`also be Teflon coated for helping in the release of the formed articles. The
`possible variations of metal mold composition can be as complex and wide
`ranging as the thermoforming process itself. Many different materials based
`on different concepts and intended purposes canbeused.Innovation in mold-
`making technology is a perpetualprocess. In time, more thermoforming ideas
`will be developed together with manyvariations of tool-material combinations.
`
`2. Mold-Making Methods
`With all the mold materials available, the mold maker has several options.
`First, a decision has to be made as to whether a male or female moldis the
`correct choicefor the final mold configuration. Second, plans should be made
`as to how best to achieve the specific mold features and design as well as the
`mold arrangements. Whenall this information has beencollected, the third
`critical factor to be determined is how many thermoformedarticles will be
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`needed. Having answered all three basic questions, the tooling-up eee
`can begin. The numberofarticles neededwill dictate whether the on = to
`be used fortemporary, short-term, orlong-term production. For= =ume
`production, molds made ofwood,plaster,or epoxy would be the
`st es
`With atleast somebasic pattern-makingskills, simple-configuration molds can
`easily be constructed and ifnecessary, converted from male to ae orvice
`versa. Sufficient sidewall angles and large radii on the mating su —
`minimize the release problems whencasting is produced. It is not possi
`le to
`create negative or reversed draft angles or undercuts in any of the casting’
`and attempts to do so will lock the casting and the original Seene
`prohibiting their separation. All epoxy and most plaster casts ae 0 : eir
`originalsizes after casting; however, there are some unique expandingp ri
`compounds which,after casting and cast removal, tend to grow in ua a
`materialsmaybe capable ofexpandingaftercastingtogain some oft As = .
`age allowancesrequiredin plastic production. However, the mo
`mmaker
`should not place too high an expectation on the outcome, and the — ing
`plaster should be used with somecaution. The expansion this material expe-
`riencesis not in true proportion and some areas maygain satisfactory ae,
`of shrinkage compensation, whereas others will not, resulting in some design
`Seeecielearlier,short-run production moldscanbemadeofwood,plas-
`ter (hydrocal), or epoxy materials. A cautionary note: These materials are=
`intended to make molds for pressure-forming techniques,as they do notoffer
`the structural strength andstability to take theforce usually induced uyLs
`sure-forming methods. Pressurized moldsreceive on the average 15 to ; -
`ofpressure; this could turn these moldsinto potential bombs, ae em .
`rupture and even explode. Noattemptoo everbe madeto use theseweak-
`" ——vs for sheet-fedthermoforming operations can also be made
`outoffiberglass materials. This type ofmold isusually madeby the one
`fiberglass lay-up technique. In this mold-making method, a pattern te
`pling shape is used for the lay-up or spray-up ofthe fiberglass mere : oe
`rally, the pattern or modelsurfacesare treated with release agents forS .
`separation after the fiberglass sets. Fiberglass molds are generally ma ;boi
`thick gel coats to provide strong, solid surface coating; the fiberglass itse a
`provide the structural strength. Depending on the moldsize, fiberglass a
`can have from '/,- to 3/,-in.-thick sidewall members to give sufficient mol ‘a
`tegrity for the thermoforming process. The fiberglass material itselfa ,
`good heat-absorbing qualities when making surface contact with the hea "
`thermoplastic sheet.Itscoolingabilitiescan be improved furtherbyfan- ries
`aircoolingbetween the formingcycles.Theuseoffiberglassmoldsis= i ;
`mostly when large products are thermoformed,such as spas, boats, and
`larg
`
`for pressure-forming methods.
`
`ials
`
`tubs or showerenclosures. The manufacturing speeds and production modes
`of these products are well suited to this type of mold structure. Fiberglass
`mold-making methodsare also oneofthe mostcost-effective ways to produce
`moldsand to obtain duplicate molds from existing articles, For this particular
`mold-making procedure, no knowledge or equipmentis needed otherthan the
`customary fiberglass technology, whichis easily acquired. Numerousarticles
`and booksareavailable on fiberglass form-producing procedures.
`Moldsfabricated from anyofthe popular tooling metals can be produced
`by one oftwo basic fabricating methods, Metals can either be cast into shapes
`or machinedoutof a solid metalblock. In both cases the metal material content
`will afford the most durability in strength and long-term servicelife. The high
`density of metals provides the best heat exchange rate and therefore the best
`efficiency in mold cooling. Of course, depending onspecific thermoforming
`criteria, one metal could outperform other metals, which certain metals may
`notfunction wellatall. Higher costs can also precludethe use ofcertain metals.
`Actual metal moldsconsist oftwo basic body components: the mold body
`and its mounting plates. With metal moldsit is most commonto produce the
`mold body out of a softer metal such as aluminum whileits mounting plates
`or surrounding frames are made from harder metals, such as steel. However,
`it is not inconceivable to choose aluminum for the entire mold construction.
`Also, there are molds for which most of the components are made outof steel,
`with portions containing hardenedtool steel segments,
`The actual body of the mold, which performsthe thermoforming func-
`tion, can consist of just female cavities or just male mold structures orboth.
`The complete molding tool consists ofthe mold body, the mounting structures,
`and possibly, elevating “stand-off” legs. Each componentof the mold serves a
`specific purpose. ‘The mounting plates provide the mounting surfaces for the
`individual mold body or bodies when multi-up mold formats are concerned;
`they are also used for mounting the mold to the thermoforming machineplat-
`ens. The stand-offs fill and compensatefor distancesthatare left between the
`sheetline and the platen’s daylight opening, so the mold can close right at the
`sheetline. The two halves of the mold body must meet when the platensclose
`and mustdo soin close proximity to the sheetline for ideal forming conditions.
`If the mold halves mate out of the sheet’s stretch range from the sheetline,
`they will cause unduestrain on the outside edgesof the sheet and may pull it
`out of the holding or even causeit to tear. Under- or oversized mold stand-offs
`always renderincorrect mold matings and therefore prevent thermoforming.
`The correct mold surface elevation to the sheet line should be a part of the
`specifications for any thermoforming machine.This distance is established by
`Subtracting the combined height of the complete mold body when closed, in-
`cluding the base and mountingplate thickness,from the total platen movement
`distance. For the purpose of mold weight reduction,the stand-offcomponents
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`can be made of aluminum insteadofsteel or can contain a numberoflarge
`cess of thermoforming production. Properly made castings will be consistent
`to one another in dimensionsand will require minimalfinishing work. There
`cutouts.
`;
`;
`In making any metallic mold body, there are two basic manufacturing
`are improved metal-casting methods andcasting variations that could further
`techniques. Thefirst consists of making the mold configuration by metal cast-
`improve the basic sand-casting methods described here. Such casting methods
`ing. In the casting ofany metal, a premanufactured pattern is used. The pattern
`use “plastic/rubber”casting techniquesor “permanentmold casting” methods
`and in their own way, offer unusual design and quality benefits. However, with
`for any metal-casting procedure can be made with any number of design intri-
`each improved casting method,the costof castingwill rise. The choice ofcast-
`cacies together with the necessary shrinkage allowances but cannot contain
`ing method should therefore be predicated on the level of quality sought for
`undercutor reversed dart angles. As described in Section I.B, the pattern itself
`is usually made larger to compensate for both the natural metal cast shrinkage
`the particularjob in order to help keep costs down. Some thermoformed prod-
`ucts, because of their material makeup,will display, from onecasting to an-
`and the plastic material shrinkage, which occurafter the forming cycle of the
`other, higherlevels of postforming distortions within themselves than those of
`thermoforming.
`sand-casted metal molds.In suchacase, the cast mold body tolerance variation
`The varioustypes of casting procedures used for making thermoforming
`would befar less than the postthermoforming results, which would indicate
`moldsclosely follow the customary metal-casting methods. The most popular
`that the casting method to producethe moldsis within a satisfactory tolerance
`casting method is the sand-casting procedure.In this type of metalcasting, the
`level. More often than not, the casting method is well within the range of the
`pattern is inserted into fine-grained casting sand within a metal core box and
`thermoforming mold-making criteria and should be given full consideration.
`then rammed down tightly to compact the sand around the pattern into an
`Molds for thermoforming are generally cast of aluminum,butnot exclu-
`almost solid form. The procedure is performed in such a waythat either the
`sively so. Other high-quality metals, such as brass or beryllium alloys, are also
`pattern or the core box is made with separating halves—when the pattern is
`used. Aluminum is the most popular metal becauseofits noncorrosive prop-
`detailed on both sides—orthe pattern is placed adjacent to a core-box surface
`erties, light weight, ease of shaping, and excellent heat conductivity. It is also
`plate—whenthe pattern is flat on one side. In this way the compacted sand
`readily available at reasonable prices. The most popular aluminum alloy used
`will duplicate only oneside of the pattern. After the sand isrammed, the core
`in thermoforming mold making for casting purposesis No. 356 aircraft alloy.
`box is opened upandthepattern is carefully removed,creating a female cavity
`This alloy is easy to work with becauseit is neither soft nor overly rigid. This
`impression of the pattern. Another core boxis similarly rammed up to carry
`alloy does not require agingorstress relieving and does not readily gum up on
`eithera flat surface or the details of the remainingside of a two-sided pattern.
`or adhere to the metal-working tools’ cutting edges. There are other metal
`The two core boxes are then assembled face to face. Pour-in funnels are cre-
`alloys, with different characteristics, which can serve other purposes and may
`ated in the compacted sandforfilling purposes. When molten metal is poured
`be more advantageousto use. The choiceofalloys and base metals is entirely
`into this hollow mold through thefilling funnels, the flowing metalwill fill the
`cavity. After several hours of cooling, the poured metalwill solidifyin the
`up to the thermoforming practitioner, who should maintain a close collabora-
`cavity. After removal, the metalwill retain the details of the sand cavity and
`tion with the mold maker,
`duplicate the shape ofthe original pattern. The quality of the resulting sand
`Any cast mold components, whether female or male, require some ma-
`chining andfinishing. The mold arrangementis produced in either a one-up
`castis, of course, directly related to the ability of the maker. The use of fine-
`grained sandwill make the surfaces and the details ofthe castingmore refined.
`or a multi-up configuration. Each set of mold units is cast as individual parts
`Skillful placementof the pour funnel andthe risers used to eliminate “sink-
`and then fitted and assembled into the common mold format. Eachcasting
`holes”is very important for minimizing warping and distortion of the casting.
`represents a single mold unit, and if in the matched mold arrangement, each
`will consist of a female casting andits separate corresponding malecastings.
`Somecastings, depending on their various configurations, will develop more
`To ensure properseating on the mountingplates and properside-to-side align-
`shrinkageat their heavier, bulkier points, while thinner areas undergosubstan-
`tially less shrinkage. Addingrisers or body extensions to the heavier areas will
`ment betweenthe individualcastings, the mating surfaces must be prepared
`and machinedflat and square—as well as parallel to the other mating sur-
`draw the shrinkage from theriseritself instead of at critical body areas. Neu-
`face—in orderthat they can be stacked andfitted together to create the com-
`tralization of shrinkage and warpage dependson the foundry operator’sskill.
`bined mold setup. Since the castings do comeoutfrom the foundry with some
`Somefoundries are capable ofproducing uniform and flawlesscastings, others
`degree of distortion and surface flaws,it is always good practice to order two
`produce a high rate of unacceptable castings. Quality castings free of distor-
`or three morecastings than the actual mold configuration requires. In this way,
`tions, casting flaws, and surface irregularities are vitally important to the suc-
`
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`MacNeil Exhibit 2153
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`MacNeil Exhibit 2153
`Yita v. MacNeil IP, IPR2020-01139, Page 49
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`280
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`Chapter §
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`Molds for Thermoforming
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`281
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`the toolmakerwill have a numberofcastings from which to select. The castings
`will be checkedfor quality and those that contain higher levels ofdistortion or
`critical surface flawswill be rejected.It is also good practiceto finish up one
`extra casting for the specific mold configuration and thus make the individua}
`castings interchangeable. If any of the castings are damaged in the thermo.
`forming process, the available ready-to-mount extra casting could becomea
`real “lifesaver.” If any of the nonessential or rejected castings later become
`obsolete, they can always be sold back to the foundry for their materialvalue,
`Whenanyofthe casting procedures are used to make molds for thermo.
`forming, the cast surfaces require some preparation. Even with the finest-qual-
`ity sand, the surface of the castwill retain some ofthe roughness of the sand
`grains. The surface of the casting must receive some smoothing and surface
`refinements.If the casting surface is left as is, the forming plastic will pick up
`the surface imperfections and display the same roughness.However, the cast-
`ing surface, whenrefined, should not need a complete polishing job. Giving a
`high-quality polish to the surface could be just as harmful as leaving it un-
`touched. A highly polished mold surface is not only time and laborintensive
`and adds unnecessarily to the tooling cost, but actually has an adverse effect
`on the mold release of the formed article. After the plastic article is formed,
`a highly polished mold surface will result in almost perfect surface contact
`between the mold andthearticle. Both surfaces are smooth enoughto create
`a contactthatwill not permit air to penetrate between; this creates a vacuum-
`like adhesionthat will interfere with release of the plastic article. This inter-
`ferencewill remain in effect until the mold surfaceis altered by sandblasting
`or other meansofsurface roughening.It is always embarrassing whena highly
`polished andvisually attractive mold has to undergo destruct