Patent Application
20120101174
The invention relates to an improved method for cold forming a shaped foam article having a shape with a high degree of draft having improved aesthetic appearance, specifically reduced surface cracking. The improvement comprises the use of a mold having a reduced-slip cavity surface.
Various methods and techniques are currently known and employed in the industry for shaping articles from a thermoplastic foam material, such as extruded polystyrene (XPS) foams. For example, shapes such as toys and puzzles can be die cut from foams that are formed by extruding a thermoplastic resin containing a blowing agent. There are also examples of foam sheet being shaped into articles such as dishes, cups, egg cartons, trays, and various types of food containers, such as fast food clam shells, take out/take home containers, and the like. More complex shaped foam articles can be made by thermoforming thermoplastic foam sheet. These methods lend themselves to the manufacture of relatively simple shaped articles from typically thin foams which are easily extracted from the molds used to produce them.
Recently, there have been significant advances in shaping more complex, and in particular, thicker thermoplastic foam (i.e., foams greater than 1 mm thick), shaped articles by pressing, or sometimes referred to as cold forming, unique foam compositions and/or structures, for example see USP Publication 2009-0062410. However, it has been found that simple and/or complex shaped foam articles having shape with a high degree of draft are prone to cracking when produced by the aforementioned cold forming method. Cracking is undesirable. Depending on the article and/or application cracking can be aesthetically as well as functionally unacceptable. It would be desirable to have an improved method for forming thicker shaped foam articles having a shape with a high degree of draft which minimizes and/or eliminates cracking.
The present invention is such an improved method for forming thicker shaped foam articles having a shape with a high degree of draft which minimizes and/or eliminates cracking.
In one embodiment, the present invention is a method to manufacture one or more shaped foam article comprising the steps of:
Another embodiment of the present invention is a method to manufacture one or more shaped foam article having a maximum draft angle (θ) comprising the steps of:
μ≧tan(θ).
Preferably, the reduced-slip cavity surface in either embodiment of the present invention disclosed hereinabove is produced by applying sandpaper to the cavity surface; adhering sand directly to the cavity surface; chemically etching the cavity surface; electro eroding the cavity surface; coating the cavity surface with rubber, silicon, plasma, textured paint, or a sticky coating; texturing the cavity surface; sand blasting the cavity surface; media blasting the cavity surface; embossing the cavity surface; scratching the cavity surface; milling the cavity surface; forming protrusions on the cavity surface, forming indentations on the cavity surface; forming micro perforations on the cavity surface; forming ribs on the cavity surface; forming needles on the cavity surface; forming serrated blades on the cavity surface; heating the foam and/or the pressing surface of the mold to a point where the foam's pressing surface becomes sticky; vacuum applied through the pressing surface of the mold; or combinations thereof. Most preferably, each cavity surface is textured
Preferably, in either embodiment of the method of the present invention disclosed hereinabove, the foam has a cell gas pressure equal to or less than 1 atmosphere.
Preferably, in either embodiment of the method of the present invention disclosed hereinabove, the thermoplastic polymer is polyethylene, polypropylene, copolymer of polyethylene and polypropylene; polystyrene, high impact polystyrene; styrene and acrylonitrile copolymer, acrylonitrile, butadiene, and styrene terpolymer, polycarbonate; polyvinyl chloride; polyphenylene oxide and polystyrene blend.
Preferably, in either embodiment of the method of the present invention disclosed hereinabove, the blowing agent is a chemical blowing agent, an inorganic gas, an organic blowing agent, carbon dioxide, or combinations thereof.
Another embodiment of the present invention is an article made by either of the methods disclosed hereinabove.
In processes to make shaped solid and/or foam articles, conventional mold making wisdom dictates the mold forming surface be as smooth as possible. There are many reasons for a smooth finish on the mold surface, for example, to improve part release from the mold after a part is formed, better replication of the mold shape into the molded article, glossy part finishes, longer mold life expectancy and lower maintenance to name a few. State of the art molds are often made from stainless steel or aluminum which is further polished, or coated with a low friction coating such as chrome plating, TEFLON™, electroless nickel boron nitride or nickel-PTFE. Unexpectedly, it was found that when cold forming shaped foam articles, as a step change (or draft angle) in the shape of the shaped article increased, the probability that the shaped foam article would develop cracks during forming was increased dramatically. Extensive study of the frictional forces encountered by a foam plank during the process of shaping into a shaped foam article surprisingly led us to the process of the present invention that can dramatically reduce, if not all together eliminate cracks in shaped foam articles. The present invention is a method of forming, preferably cold forming a shaped foam article wherein the static friction coefficient between the pressing surface of the mold and the pressing surface of the foam is equal to or greater than the tangent of the greatest draft angle of the shaped foam article.
The foamed article of the present invention can be made from any foam composition. A foam composition comprises a continuous matrix material with cells defined therein. Cellular (foam) has the meaning commonly understood in the art in which a polymer has a substantially lowered apparent density comprised of cells that are closed or open. Closed cell means that the gas within that cell is isolated from another cell by the polymer walls forming the cell. Open cell means that the gas in that cell is not so restricted and is able to flow without passing through any polymer cell walls to the atmosphere. The foam article of the present invention can be open or closed celled. A closed cell foam has less than 30 percent, preferably 20 percent or less, more preferably 10 percent or less and still more preferably 5 percent or less and most preferably one percent or less open cell content. Conversely, an open cell foam has 30 percent or more, preferably 50 percent or more, still more preferably 70 percent or more, yet more preferably 90 percent or more open cell content. An open cell foam can have 95 percent or more open cell content. Unless otherwise noted, open cell content is determined according to American Society for Testing and Materials (ASTM) method D6226-05.
Desirably the foam article comprises polymeric foam, which is a foam composition with a polymeric continuous matrix material (polymer matrix material). Any polymeric foam is suitable including extruded polymeric foam, expanded polymeric foam and molded polymeric foam. The polymeric foam can comprise, and desirably comprises as a continuous phase, a thermoplastic or a thermoset polymer matrix material. Desirably, the polymer matrix material has a thermoplastic polymer continuous phase.
A polymeric foam article for use in the present invention can comprise or consist of one or more thermoset polymer, thermoplastic polymer, or combinations or blends thereof. Suitable thermoset polymers include thermoset epoxy foams, phenolic foams, urea-formaldehyde foams, polyurethane foams, and the like.
Suitable thermoplastic polymers include any one or any combination of more than one thermoplastic polymer. Olefinic polymers, alkenyl-aromatic homopolymers and copolymers comprising both olefinic and alkenyl aromatic components are suitable. Examples of suitable olefinic polymers include homopolymers and copolymers of ethylene and propylene (e.g., polyethylene, polypropylene, and copolymers of polyethylene and polypropylene). Alkenyl-aromatic polymers such as polystyrene and polyphenylene oxide/polystyrene blends are particularly suitable polymers for of the foam article of the present invention.
Desirably, the foam article comprises a polymeric foam having a polymer matrix comprising or consisting of one or more than one alkenyl-aromatic polymer. An alkenyl-aromatic polymer is a polymer containing alkenyl aromatic monomers polymerized into the polymer structure. Alkenyl-aromatic polymer can be homopolymers, copolymers or blends of homopolymers and copolymers. Alkenyl-aromatic copolymers can be random copolymers, alternating copolymers, block copolymers, rubber modified, or any combination thereof and my be linear, branched or a mixture thereof.
Styrenic polymers are particularly desirably alkenyl-aromatic polymers. Styrenic polymers have styrene and/or substituted styrene monomer (e.g., alpha methyl styrene) polymerized in the polymer backbone and include both styrene homopolymer, copolymer and blends thereof. Polystyrene and high impact modified polystyrene are two preferred styrenic polymers.
Examples of styrenic copolymers suitable for the present invention include copolymers of styrene with one or more of the following: acrylic acid, methacrylic acid, ethacrylic acid, maleic acid, itaconic acid, acrylonitrile, maleic anhydride, methyl acrylate, ethyl acrylate, isobutyl acrylate, n-butyl acrylate, methyl methacrylate, vinyl acetate and butadiene.
Polystyrene (PS) is a preferred styrenic polymer for use in the foam articles of the present invention because of their good balance between cost and property performance.
Styrene-acrylonitrile copolymer (SAN) is a particularly desirable alkenyl-aromatic polymer for use in the foam articles of the present invention because of its ease of manufacture and monomer availability. SAN copolymer can be a block copolymer or a random copolymer, and can be linear or branched. SAN provides a higher water solubility than polystyrene homopolymer, thereby facilitating use of an aqueous blowing agent. SAN also has higher heat distortion temperature than polystyrene homopolymer, which provides for a foam having a higher use temperature than a polystyrene homopolymer foam. Desirable embodiments of the present process employ polymer compositions that comprise, even consist of SAN. The one or more alkenyl-aromatic polymer, even the polymer composition itself may comprise or consist of a polymer blend of SAN with another polymer such as polystyrene homopolymer.
Whether the polymer composition contains only SAN, or SAN with other polymers, the acrylonitrile (AN) component of the SAN is desirably present at a concentration of 1 weight percent or more, preferably 5 weight percent or more, more preferably 10 weight percent or more based on the weight of all polymers in the polymer composition. The AN component of the SAN is desirably present at a concentration of 50 weight percent or less, typically 30 weight percent or less based on the weight of all polymers in the polymer composition. When AN is present at a concentration of less than 1 weight percent, the water solubility improvement is minimal over polystyrene unless another hydrophilic component is present. When AN is present at a concentration greater than 50 weight percent, the polymer composition tends to suffer from thermal instability while in a melt phase in an extruder.
The styrenic polymer may be of any useful weight average molecular weight (MW). Illustratively, the molecular weight of a styrenic polymer or styrenic copolymer may be from 10,000 to 1,000,000. The molecular weight of a styrenic polymer is desirably less than about 200,000, which surprisingly aids in forming a shaped foam part retaining excellent surface finish and dimensional control. In ascending further preference, the molecular weight of a styrenic polymer or styrenic copolymer is less than about 190,000, 180,000, 175,000, 170,000, 165,000, 160,000, 155,000, 150,000, 145,000, 140,000, 135,000, 130,000, 125,000, 120,000, 115,000, 110,000, 105,000, 100,000, 95,000, and 90,000. For clarity, molecular weight herein is reported as weight average molecular weight unless explicitly stated otherwise. The molecular weight may be determined by any suitable method such as those known in the art.
Rubber modified homopolymers and copolymers of styrenic polymers are preferred styrenic polymers for use in the foam articles of the present invention, particularly when improved impact is desired. Such polymers include the rubber modified homopolymers and copolymers of styrene or alpha-methylstyrene with a copolymerizable comonomer. Preferred comonomers include acrylonitrile which may be employed alone or in combination with other comonomers particularly methylmethacrylate, methacrylonitrile, fumaronitrile and/or an N-arylmaleimide such as N-phenylmaleimide. Highly preferred copolymers contain from about 70 to about 80 percent styrene monomer and 30 to 20 percent acrylonitrile monomer.
Suitable rubbers include the well known homopolymers and copolymers of conjugated dienes, particularly butadiene, as well as other rubbery polymers such as olefin polymers, particularly copolymers of ethylene, propylene and optionally a nonconjugated diene, or acrylate rubbers, particularly homopolymers and copolymers of alkyl acrylates having from 4 to 6 carbons in the alkyl group. In addition, mixtures of the foregoing rubbery polymers may be employed if desired. Preferred rubbers are homopolymers of butadiene and copolymers thereof in an amount equal to or greater than about 5 weight percent, preferably equal to or greater than about 7 weight percent, more preferably equal to or greater than about 10 weight percent and even more preferably equal to or greater than 12 weight percent based on the total weight or the rubber modified styrenic polymer. Preferred rubbers present in an amount equal to or less than about 30 weight percent, preferably equal to or less than about 25 weight percent, more preferably equal to or less than about 20 weight percent and even more preferably equal to or less than 15 weight percent based on the total weight or the rubber modified styrenic polymer. Such rubber copolymers may be random or block copolymers and in addition may be hydrogenated to remove residual unsaturation.
The rubber modified homopolymers or copolymers are preferably prepared by a graft generating process such as by a bulk or solution polymerization or an emulsion polymerization of the copolymer in the presence of the rubbery polymer. Depending on the desired properties of the foam article, the rubbers' particle size may be large (for example greater than 2 micron) or small (for example less than 2 micron) and may be a monomodal average size or multimodal, i.e., mixtures of different size rubber particle sizes, for instance a mixture of large and small rubber particles. In the rubber grafting process various amounts of an ungrafted matrix of the homopolymer or copolymer are also formed. In the solution or bulk polymerization of a rubber modified (co)polymer of a vinyl aromatic monomer, a matrix (co)polymer is formed. The matrix further contains rubber particles having (co)polymer grafted thereto and occluded therein.
High impact poly styrene (HIPS) is a particularly desirable rubber-modified alkenyl-aromatic homopolymer for use in the foam articles of the present invention because of its good blend of cost and performance properties, requiring improved impact strength.
Butadiene, acrylonitrile, and styrene (ABS) terpolymer is a particularly desirable rubber-modified alkenyl-aromatic copolymer for use in the foam articles of the present invention because of its good blend of cost and performance properties, requiring improved impact strength and improved thermal properties.
Foam articles for use in the present invention may be prepared by any conceivable method. Suitable methods for preparing polymeric foam articles include batch processes (such as expanded bead foam steam chest molding processes), semi-batch processes (such as accumulative extrusion processes) and continuous processes such as extrusion foam processes. Desirably, the process is a semi-batch or continuous extrusion process. Most preferably process comprises an extrusion process.
An expanded bead foam process is a batch process that requires preparing a foamable polymer composition by incorporating a blowing agent into granules of polymer composition (for example, imbibing granules of a thermoplastic polymer composition with a blowing agent under pressure). Each bead becomes a foamable polymer composition. Often, though not necessarily, the foamable beads undergo at least two expansion steps. An initial expansion occurs by heating the granules above their softening temperature and allowing the blowing agent to expand the beads. A second expansion is often done with multiple beads in a mold and then exposing the beads to steam to further expand them and fuse them together. A bonding agent is commonly coated on the beads before the second expansion to facilitate bonding of the beads together. The resulting expanded bead foam has a characteristic continuous network of polymer skins throughout the foam. The polymer skin network corresponds to the surface of each individual bead and encompasses groups of cells throughout the foam. The network is of higher density than the portion of foam containing groups of cells that the network encompasses.
Complex articles or blocks may be produced by steam chest molding. Blocks may be further shaped by cutting, for example by CNC hot wire, to a sheet of uniform thickness. A structural insulated panel (SIP) is an example of a steam chest molded block foam cut into uniform thickness sheet.
The foamed article can also be made in a reactive foaming process, in which precursor materials react in the presence of a blowing agent to form a cellular polymer. Polymers of this type are most commonly polyurethane and polyepoxides, especially structural polyurethane foams as described, for example, in U.S. Pat. Nos. 5,234,965 and 6,423,755, both hereby incorporated by reference. Typically, anisotropic characteristics are imparted to such foams by constraining the expanding reaction mixture in at least one direction while allowing it to expand freely or nearly freely in at least one orthogonal direction.
An extrusion process prepares a foamable polymer composition of a thermoplastic polymer with a blowing agent in an extruder by heating a thermoplastic polymer composition to soften it, mixing a blowing agent composition together with the softened thermoplastic polymer composition at a mixing temperature and mixing pressure that precludes expansion of the blowing agent to any meaningful extent (preferably, that precludes any blowing agent expansion) and then extruding (expelling) the foamable polymer composition through a die into an environment having a temperature and pressure below the mixing temperature and pressure. Upon expelling the foamable polymer composition into the lower pressure the blowing agent expands the thermoplastic polymer into a thermoplastic polymer foam. Desirably, the foamable polymer composition is cooled after mixing and prior to expelling it through the die. In a continuous process, the foamable polymer composition is expelled at an essentially constant rate into the lower pressure to enable essentially continuous foaming. An extruded foam can be a continuous, seamless structure, such as a sheet or profile, as opposed to a bead foam structure or other composition comprising multiple individual foams that are assembled together in order to maximize structural integrity and thermal insulating capability. An extruded foam sheet may have post-extrusion modifications performed to it as desired, for example edge treatments (e.g., tongue and groove), thickness tolerance control (e.g., via planning or skiving the surface), treatments to the top and/or bottom of the sheet, such as cutting grooves into the surface, and the like.
Accumulative extrusion is a semi-continuous extrusion process that comprises: 1) mixing a thermoplastic material and a blowing agent composition to form a foamable polymer composition; 2) extruding the foamable polymer composition into a holding zone maintained at a temperature and pressure which does not allow the foamable polymer composition to foam; the holding zone having a die defining an orifice opening into a zone of lower pressure at which the foamable polymer composition foams and an openable gate closing the die orifice; 3) periodically opening the gate while substantially concurrently applying mechanical pressure by means of a movable ram on the foamable polymer composition to eject it from the holding zone through the die orifice into the zone of lower pressure, and 4) allowing the ejected foamable polymer composition to expand to form the foam. U.S. Pat. No. 4,323,528, hereby incorporated by reference, discloses such a process in a context of making polyolefin foams, yet which is readily adaptable to aromatic polymer foam. U.S. Pat. No. 3,268,636 discloses the process when it takes place in an injection molding machine and the thermoplastic with blowing agent is injected into a mold and allowed to foam, this process is sometimes called structural foam molding. Accumulative extrusion and extrusion processes produce foams that are free of such a polymer skin network.
Suitable blowing agents include one or any combination of more than one of the following: inorganic gases such as carbon dioxide, argon, nitrogen, and air; organic blowing agents such as water, aliphatic and cyclic hydrocarbons having from one to nine carbons including methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, neopentane, cyclobutane, and cyclopentane; fully and partially halogenated alkanes and alkenes having from one to five carbons, preferably that are chlorine-free (e.g., difluoromethane (HFC-32), perfluoromethane, ethyl fluoride (HFC-161), 1,1,-difluoroethane (HFC-152a), 1,1,1-trifluoroethane (HFC-143a), 1,1,2,2-tetrafluoroethane (HFC-134), 1,1,1,2 tetrafluoroethane (HFC-134a), pentafluoroethane (HFC-125), perfluoroethane, 2,2-difluoropropane (HFC-272fb), 1,1,1-trifluoropropane (HFC-263fb), 1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea), 1,1,1,3,3-pentafluoropropane (HFC-245fa), and 1,1,1,3,3-pentafluorobutane (HFC-365mfc)); fully and partially halogenated polymers and copolymers, desirably fluorinated polymers and copolymers, even more preferably chlorine-free fluorintated polymers and copolymers; aliphatic alcohols having from one to five carbons such as methanol, ethanol, n-propanol, and isopropanol; carbonyl containing compounds such as acetone, 2-butanone, and acetaldehyde; ether containing compounds such as dimethyl ether, diethyl ether, methyl ethyl ether; carboxylate compounds such as methyl formate, methyl acetate, ethyl acetate; carboxylic acid and chemical blowing agents such as azodicarbonamide, azodiisobutyronitrile, benzenesulfo-hydrazide, 4,4-oxybenzene sulfonyl semi-carbazide, p-toluene sulfonyl semi-carbazide, barium azodicarboxylate, N,N′-dimethyl-N,N′-dinitrosoterephthalamide, trihydrazino triazine and sodium bicarbonate.
The amount of blowing agent can be determined by one of ordinary skill in the art without undue experimentation for a given thermoplastic to be foamed based on the type thermoplastic polymer, the type of blowing agent, the shape/configuration of the foam article, and the desired foam density. Generally, the foam article may have a density of from about 16 kilograms per cubic meter (kg/m3) to about 200 kg/m3 or more. The foam density, typically, is selected depending on the particular application. Preferably the foam density is equal to or less than about 160 kg/m3, more preferably equal to or less than about 120 kg/m3, and most preferably equal to or less than about 100 kg/m3.
The cells of the foam article may have an average size (largest dimension) of from about 0.05 to about 5.0 millimeter (mm), especially from about 0.1 to about 3.0 mm, as measured by ASTM D-3576-98. Foam articles having larger average cell sizes, of especially about 1.0 to about 3.0 mm or about 1.0 to about 2.0 mm in the largest dimension, are of particular use when the foam fails to have a compressive ratio of at least 0.4 as described in the following few paragraphs.
The compressive strength of the foam article is established when the compressive strength of the foam is evaluated in three orthogonal directions, E, V and H, where E is the direction of extrusion, V is the direction of vertical expansion after it exits the extrusion die and H is the direction of horizontal expansion of the foam after it exits the extrusion die. These measured compressive strengths, CE, CV and CH, respectively, are related to the sum of these compressive strengths, CT, such that at least one of CE/CT, CV/CT and CH/CT, has a value of at least 0.40, preferably a value of at least 0.45 and most preferably a value of at least 0.50. When using such a foam, the pressing direction is desirably parallel to the maximum value in the foam.
The polymer used to make the foam article of the present invention may contain additives, typically dispersed within the continuous matrix material. Common additives include any one or combination of more than one of the following: infrared attenuating agents (for example, carbon black, graphite, metal flake, titanium dioxide); clays such as natural absorbent clays (for example, kaolinite and montmorillonite) and synthetic clays; nucleating agents (for example, talc and magnesium silicate); fillers such as glass or polymeric fibers or glass or polymeric beads; flame retardants (for example, brominated flame retardants such as brominated polymers, hexabromocyclododecane, phosphorous flame retardants such as triphenylphosphate, and flame retardant packages that may including synergists such as, or example, dicumyl and polycumyl); lubricants (for example, calcium stearate and barium stearate); acid scavengers (for example, magnesium oxide and tetrasodium pyrophosphate); UV light stabilizers; thermal stabilizers; and colorants such as dyes and/or pigments.
A most preferred foam article is a shaped foam article which may be prepared from a foamed polymer as described herein above in the form of a foam plank and further shaped to give a shaped foam article. The use of the term plank, herein, is merely used for convenience with the understanding that configurations other than a flat board having a rectangular cross-section may be extruded and/or foamed (e.g., an extruded sheet, an extruded profile, a pour-in-place bun, etc.). A particularly useful method to shape foam articles is to start from a foam plank which has been extruded from a thermoplastic comprising a blowing agent. As per convention, but not limited by, the extrusion of the plank is taken to be horizontally extruded (the direction of extrusion is orthogonal to the direction of gravity). Using such convention, the plank's top surface is that farthest from the ground and the plank's bottom surface is that closest to the ground, with the height of the foam (thickness) being orthogonal to the ground when being extruded. As defined herein, shaped means the foamed article typically has one or more contour that create a step change (impression) in height 32 of at least 1 millimeter or more in the shaped foam article 10 having thickness 17 as shown in
The forming of the shaped foam articles is surprisingly enhanced by using foam planks 1 that have at least one direction where at least one of CE/CT, CV/CT and CH/CT is at least 0.4 said one of CE/CT, CV/CT and CH/CT (compressive balance), CE, CV and CH being the compressive strength of the cellular polymer in each of three orthogonal directions E, V and H where one of these directions is the direction of maximum compressive strength in the foam and CT equals the sum of CE, CV and CH.
After the foam plank 1 is formed, a pressing surface is created 30, for example by removing a layer from the top or bottom surface of the foam plank or by cutting the foam plank between the top and bottom surface to create two pressing surfaces opposite the top and bottom surface. Suitable equipment useful for preparing a pressing surface are band saws, computer numeric controlled (CNC) abrasive wire cutting machines, CNC hot wire cutting equipment and the like. When removing a layer, the same cutting methods just described may be used and other methods such as planing, grinding or sanding may be used.
Typically, after removing a layer from the top and/or bottom surface of the foam plank and/or cutting the plank, the resulting plank with pressing surface is at least about several millimeters thick to at most about 60 centimeters thick. Generally, when removing a layer, the amount of material is at least about a millimeter and may be any amount useful to perform the method such as 1.2, 1.4, 1.6, 1.8, 2, 2.5, 3, 3.5, 4, 5 millimeters or any subsequent amount determined to be useful such as an amount to remove any skin that is formed as a result of extruding the thermoplastic foam, but is typically no more than 10 millimeters. In another embodiment, the foam is cut and a layer is removed from the top or bottom surface opposite the cut surface to form two pressing surfaces.
In a particular embodiment, the foam plank 1 having a pressing surface, has a density gradient from the pressing surface 30 to the opposite surface of the foam plank 34. Generally, it is desirable to have a density gradient of at least 5 percent, 10 percent, 15 percent, 25 percent, 30 percent or even 35 percent from the pressing surface to the opposing surface of the foam plank. To illustrate the density gradient, if the density of the foam at the surface (i.e., within a millimeter or two of the surface) is 3.0 pounds per cubic foot (pcf), the density would be for a 10 percent gradient either 2.7 or 3.3 pcf at the center of the foam.
In one embodiment of the present invention, the shaped foam article 10 may be formed in a foam plank 1 and in a subsequent and separate step, the shaped foam article is separated, or trimmed from the continuous unshaped foam plank 16. In another embodiment, the plank 1 may be cut to fit into a forming tool prior to contact with the tool, the cut foamed plank is sometimes referred to as a foam blank. In another embodiment, the final shape maybe cut from the pressed plank, for example, the foam plank 1 may be pressed to form a shape into the pressing surface and the shaped foam article subsequently cut from the pressed foam plank. When cutting the foam, any suitable method may be used, such as those known in the art and those described previously for cutting the foam to form the pressing surfaces. In yet another, preferred embodiment, the shaped foam article is trimmed from the continuous unshaped foam plank by a trimming rib 51 simultaneously as the shaped foam article is formed. In addition, methods that involve heat may also be used to cut the foam since the pressed shape has already been formed in the pressing surface.
The method of the present invention may use a molding machine, sometimes referred to as a press, to form the shaped foam article of the present invention. This process is often referred to as discontinuous as it consists of a cycle where a foam plank is placed in an open mold, the mold closes to form an article, then after the article is formed the mold opens. The shaped foam article is removed form the mold, a new foam plank is inserted into the mold and the process repeated.
Typically, a press has a stationary platen and a movable platen to which a forming tool may be affixed. The pressing surface(s) of the plank is contacted with a forming tool such as a die face or mold. Herein die face and/or mold means any tool having an impressed shape and/or cavity that when pressed into the foam plank will cause the foam to take the shape of the die face. That is, the material making up the forming tool is such that it does not deform when pressed against the foam plank, but the foam plank deforms to form and retain the desired shape of the forming tool, die face, and/or mold cavity. Typically, a mold comprises a cavity portion, or cavity half and a core portion, or core half. The cavity half of the mold may be affixed to the stationary platen, but more often is affixed to the movable platen. Hereinafter, when the mold half with a cavity is affixed to the movable platen is referred to as the movable forming surface and the stationary platen is referred to as the stationary forming surface. The stationary platen may or may not have a mold half with a core affixed to it.
Alternatively, the method of the present invention may use a calendaring machine, sometimes referred to as a roll press, to form the shaped foam article of the present invention. This process is often referred to as continuous as it consists of a foam plank passing through one or more continuously circulating roll which impress the shape into the shaped foam article (not pictured).
In roll forming, the pressing surface(s) of the plank is contacted with a roll face of a forming roll. Herein roll face means any roll having a defined shape that when pressed into the foam plank will cause the foam to take the shape of the roll face. That is, the material making up the roll face is such that it does not deform when pressed against the foam plank, but the foam plank deforms to form and retain the desired shape of the roll face.
The roll speed will vary depending on the specifics of the foam plank being shaped, for example the composition of the foam, the thickness of the foam plank, the shape being imparted onto the foam plank, etc. Preferably, the roll speed is as fast as possible to provide acceptable shaped foam articles. Preferably the roll speed is equal to or greater than 5 feet per minute (fpm), more preferably equal to or greater than 10 fpm, even more preferably equal to or greater than 25 fpm, and even more preferably equal to or greater than 40 fpm.
The roll diameters, especially the forming roll diameter, is equal to or greater than the thickness of the foam plank. Preferably, the rolls are independently equal to or greater than 2 times the thickness of the foam plank, more preferably 4 times, even more preferably 6 times, even more preferably 8 times, even more preferably 10 times the thickness of the foam plank. The rolls can be even larger than 10 times the thickness of the foam plank, the size of the roll diameter is limited only by any practical limitations of the roll forming equipment.
The roll gap is set such that the gap is less than the thickness of the foam plank. Preferably the gap is set such that the applied strain to compression set ratio is equal to or less than 10, more preferably equal to or less than 2.5, even more preferably equal to or less than 1.5, and even more preferably equal to or less than 1. From a practical stand point, the roll gap should not be set at a thickness that results in a forming pressure on the surface of the foam plank of greater than 1,200 pounds per square inch (psi).
In either process, discontinuous and/or continuous, both sides of the foam plank may be shaped. In this embodiment both the mold half with the cavity and the mold half with the core impart shape to the shaped foam article or both rolls impart shape to the shaped foam article. In another embodiment, only one surface of the foam plank is shaped. In this embodiment, the foam article is shaped only on one surface pressed by the platen having the half of the mold with the cavity or by a single roll. In this embodiment, for the discontinuous process, the foam plank may be pressed directly against the other platen or against a mold half with a core affixed to the other platen.
Typically when pressing, at least a portion of the foam is pressed such that the foam is compressed to a thickness of 95 percent or less of the to-be-pressed foam thickness 17 as shown in
The forming tool or roll, because a shape is most often desired, typically has contours that create an impression (step change) in height 32 of at least a millimeter in the foam 10 having thickness 15 as shown in
The step change, surprisingly, may be formed where the foam undergoes shear. For example, the foam may have a shear or draft angle 33 (θ) of about 45° to about 90° from the press surface 30 of the foam 10 in a step change of height 33. It is understood that the shear angle θ may not be linear, but may have some curvature, with the angle in these cases being an average over the curvature. The angle surprisingly may be greater than 60°, 75° or even by 90° while still maintaining an excellent finish and appearance. The draft angle at any point along the mold surface is defined as the tangent of the angle taken at that location of the mold.
In another aspect of the invention, a foam having a higher concentration of open cells at a surface of the foam than the concentration of open cells within the foam is contacted and pressed to form the shape. In this aspect of the invention the foam may be any foam, preferably a styrenic foam such as the extruded styrenic polymer foam described above. It may also be any other styrenic polymeric foam such as those known in the art including, for example, where the blowing agent is added to polymer beads, typically under pressure, as described by U.S. Pat. No. 4,485,193 and each of the U.S. patents cited hereinabove.
With respect to this open cell gradient, the gradient is as described above for the density gradient where the concentration of open cells if determined microscopically and is the number of open cells per total cells at the surface.
Generally, the amount of open cells in this aspect of the invention at the surface is at least 5 percent to completely open cell. Desirably, the open cells at the surface is at least in ascending order of 6 percent, 7 percent, 8 percent, 10 percent, 20 percent, 30 percent, 40 percent, 50 percent, 60 percent, 70 percent, 80 percent, 90 percent and completely open cell at the surface.
The foam may have the open cells formed at the surface by mechanical means such as those described above (e.g., planing/machining or cutting) or may be induced chemically, for example, by use of suitable surfactants to burst closed cells at the surface.
The foam surface with the higher concentration of open cells is contacted with a forming tool and pressed as described above. In a preferred embodiment for such foams, one or both sides of the forming tool, e.g., both sides of the die face and/or mold are heated, but the foam is not (ambient 15-30° C.) and the foam is pressed. Surprisingly, heating the die faces with the foams having open cells at the surface results in superior surface contour and appearance as compared to doing the same with a foam without such open cells at the surface, in this case, the appearance of the foam is degraded.
In another embodiment of the present invention, the shaped foam article may be perforated. Such an article may have a plurality of perforations. Perforation is defined herein to mean one or more hole which passes through the foam plank/shaped article one surface to another, i.e., from the top surface to the bottom surface. Perforation may occur at any time, in other words, it may be done to the foam plank prior to shaping, to the shaped foam article, or a combination of the two. The perforations extend through the shaped foam article, for instance for a shaped foam article made from a foam plank, through the depth of the foam plank. The foam may be perforated by any acceptable means. Perforating the foam article may comprise puncturing the foam article with a one or more of pointed, sharp objects in the nature of a needle, pin, spike, nail, or the like. However, perforating may be accomplished by other means than sharp, pointed objects such as drilling, laser cutting, high-pressure fluid cutting, air guns, projectiles, or the like. The perforations may be made in like manner as disclosed in U.S. Pat. No. 5,424,016, which is hereby incorporated by reference.
When pressing with a heated forming tool, the contact time with the foam is typically from about 0.1 second to about 60 seconds. Preferably, the dwell time is at least about 1 second to at most about 45 seconds.
When pressing with a heated forming tool, the temperature of the forming tool is not so hot or held for too long a time such that the foam is degraded. Typically, the temperature of the forming tool is about 50° C. to about 200° C. Preferably, the temperature is at least about 60°, more preferably at least about 70° C., even more preferably at least about 80° C. and most preferably at least about 90° C. to preferably at most about 190°, more preferably at most about 180°, even more preferably at most about 170° C. and most preferably at most about 160° C.
The forming tool provides the shape to the shaped foam article. The forming tool or roll comprises the forming cavity (shape) and all the necessary equipment for temperature control, trimming, ejection, etc. The most frequent case, the forming tool, such as a mold, comprises two halves, one which may be the stationary platen 60 or which is mounted to a stationary platen (sometimes referred to as the core side or stationary forming surface), the other mold half 50 to a moveable platen 70 (sometimes referred to as the cavity side or movable forming surface) and moving with it. The shape of the article will dictate the design and complexity of the forming tool. In the simplest case, the mold half with the cavity is affixed to the movable platen and the stationary forming surface is the stationary platen itself 60
In one embodiment of the present invention the shaping/trimming step of the present invention, the surface of the foam plank 34 opposite the pressing surface(s) 30 of the foam plank is placed on a stationary forming surface, such as a stationary platen 60. A movable platen 70 which can move toward or away from the stationary platen on which the plank is placed comprises a movable forming surface of the forming tool 50 for example, a single cavity mold or optionally a multiple cavity mold. To shape the foam, the movable platen moves towards the stationary platen such that the pressing surface(s) of the plank 30 is contacted and pressed with the movable forming surface of the forming tool 50. For a multi-cavity mold, each cavity may be identical in shape or there may be as many different shapes as cavities or there may be a combination of multiple cavities with the same first shape in combination with multiple cavities with one or more shapes different than the first shape. The layout of cavities in a multi-cavity mold may be side by side, in tandem, or any other desirable configuration. A multi-cavity mold produces more than one shaped article in a plank per molding cycle.
We have found in the process of forming shaped foam articles having one or more large draft angle, during the step of pressing the foam, the pressing surface of the foam may slip relative to the mold pressing surface causing failure in the structural integrity of the foam pressing surface. Failure results when the lateral forces experienced by the pressing surface of the foam exceed the ultimate tensile strain of the foamed thermoplastic. Failure may manifest it self in cracking, shearing of the foam, foam surface separation, or combinations thereof, any of which result in an aesthetically and/or functionally unacceptable shaped foam article.
The frictional forces encountered by a foam plank during the process of shaping into a shaped foam article are described in the free body diagram
Fc compressive force
Ff frictional force
W weight
N normal force
μ static frictional coefficient between the foam plank and mold surface
θ shear (inclination) angle
g acceleration due to gravity.
The static friction coefficient (μ) between two solid surfaces is defined as the ratio of the tangential force (Ff) required to produce sliding divided by the normal force between the surfaces (N)
μ=Ff/N (1)
For a horizontal surface the horizontal force (Ff) to move a solid resting on a flat surface
F
i=μ·mass of solid·g. (2)
If a body rests on an incline plane the body is prevented from sliding down because of the frictional resistance. If the angle of the plane is increased there will be an angle at which the body begins to slide down the plane. This is the angle of repose (θ) and the tangent of this angle is the same as the coefficient of friction (μ):
μ=tan(θ). (3)
Therefore, for the body not to slide down the plane, the static coefficient of friction must be equal to or greater than the tangent of the angle:
μ≧tan(θ). (4)
In one embodiment of the process of the present invention, the static coefficient of friction between the pressing surface of the mold surface and the pressing surface of the foam is preferably equal to or greater than the tangent of the angle. The purpose for increased friction between the foam and the pressing surface of the mold is to reduce lateral forces. When a tool is pressed into foam with a low friction in non-flat regions, the normal force acting from the tool to the foam is not in the direction of desired forming. This creates lateral forces in the transverse direction and we believe one of the, if not the major contributing factor to propagating cracks. A high friction interface between the foam and pressing surface of the mold, creates frictional forces that counteract lateral forces, eliminating forces in the transverse direction and thus cracking.
The static coefficient of friction is categorized as a “system property”. For a specific mold and a specific foam, it depends on system variables like mold/foam temperature, press/roll velocity, mold/foam surface roughness, foam density, foam cell size, and the like. The static coefficient of friction can be measured experimentally, it cannot be calculated. Rougher surfaces tend to have higher effective values. Empirically, the static coefficient of friction must be equal to or greater than the tangent of the greatest draft angle in the shaped foam article. Practically, to achieve foam pressing surface integrity, slippage of the foam plank in the mold must be minimized. If the foam pressing surface slides along the tool surface interface, lateral strains develop which may result in ultimate failure of the foam and produce surface cracking. If foam pressing surface slippage is reduced or eliminated lateral strains would be reduced and ultimate failure may be remedied. Preferably, the imparted tensile strain when the pressing surface of the mold is being pressed into the foam plank is less than the ultimate tensile strain of the foam thermoplastic. Any means to reduce surface slippage such as, but not limited to, increased friction, mechanical interlocking, chemical adhesion, or combinations thereof between the foam pressing surface and the tool pressing surface may be employed.
For the process of the present invention, slippage between the pressing surface of the foam and the pressing surface of the mold must be minimized. In other words, the mold must have a reduced-slip or non-slip surface which effectively ‘grabs’ the pressing surface of the foam such that slippage relative to the mold is reduced or eliminated. If the imparted tensile strain when the pressing surface of the mold is being pressed into the foam plank is less than the ultimate tensile strain of the thermoplastic foam, slippage is minimized and cracking in the shaped foam article is reduced or eliminated.
For a PS styrenic foam plank, a SAN styrenic foam plank, and/or an impact modified styrenic foam plank of PS or SAN, the imparted tensile strain when the pressing surface of the mold is being pressed into the foam plank is preferably less than or equal to the ultimate strain, more preferably less than or equal to 80 percent of the ultimate strain, and even more preferably less than or equal to 60 percent of the ultimate strain.
Any method to impart a reduced-slip or non-slip surface, e.g., a rough cavity surface, into the pressing surface of the mold cavity which sufficiently reduces slippage of the foam plank during pressing is acceptable. For example, sandpaper may be applied to the mold forming surface or sand or an equivalent directly adhered to the surface; the mold forming surface may be chemically etched or electro eroded; the mold forming surface may be coated with rubber, silicon, plasma, textured paint, any kind of sticky (friction causing) coating; surface roughness such as achieved by texturing, sand blasting, media blasting, embossing, scratching, milling, or the like; the mold forming surface may be made with protrusions, indentations, micro perforations, ribs, needles, serrated blades; heating the foam and/or the forming surface of the mold to the point where the foam's pressing surface becomes sticky; vacuum is applied through the pressing surface of the mold; or combinations thereof. Preferably, the pressing surface of the mold is textured.
As defined herein, a smooth cavity surface is such that negligible mechanical interlocking occurs between foam and tool cavity. Conventionally, a smooth cavity surface is as machined, a polished surface, chrome coated, TEFLON coated, and the like. When a mold with a smooth cavity surface is used to press a foam of the present invention into a shaped foam article having a shear angle of 30° or greater (because the cavity surface is smooth this is not an example of the present invention), slippage of the foam plank during pressing may occur producing cracks in the shaped foam article.
Preferably, the cavity surface of a mold of the present invention has a surface roughness sufficient to reduce cracking in the formed shaped foam article by at least 50 percent versus the formed shaped foam article pressed by a cavity with a smooth cavity surface, more preferably at least 60 percent, even more preferably at least 70 percent, even more preferably at least 80 percent, even more preferably at least 90 percent, even more preferably at least 95 percent, and most preferably at least 98 percent versus the formed shaped foam article pressed by a cavity with a smooth cavity surface.
The density profile through the thickness of each foam blank was tested using a QMS Density Profiler, model QDP-01X, from Quintek Measurement Systems, Inc. Knoxville, Tenn. The High Voltage kV Control was set to 90 percent, the High Voltage Current Control was set to 23 percent and the Detector Voltage was approximately 8 v. Data points were collected every 0.06 mm throughout the thickness of the foam. Approximate thickness of the foam samples in the plane of the x-ray path was 2 inches. Mass absorption coefficients were calculated for each sample individually, based on the measured linear density of the foam part being tested. The skin density, ρskin, was reported as a maximum value whereas the core density, ρcore, was averaged within an approximate 5 mm range.
The density gradient, in units of percentage, was then computed in accordance with the following equation:
The compressive response of each material was measured using a Materials Test System equipped with a 5.0 displacement card and a 4,000 lbf load card. Cubical samples measuring the approximate thickness of each plank were compressed at a compressive strain rate of 0.065 s−1. Thus, the crosshead velocity of the MTS, in units of inches per minute, was programmed in accordance with the following equation:
Crosshead Velocity=Strain Rate*Thickness*60
where the thickness of the foam specimen is measured in units of inches. The compressive strength of each foam specimen is calculated in accordance with ASTM D1621 while the total compressive strength, CST, is computed as follows:
C
ST
=C
SV
+C
SE
+C
SH
where CSV, CSE and CSH correspond to the compressive strength in the vertical, extrusion and horizontal direction respectively. Thus, the compressive balance, R, in each direction can be computed as shown below:
R
V
=C
SV
/C
ST
R
E
=C
SE
/C
ST
R
H
=C
SH
/C
ST
Open cell content was measured by using an Archimedes method on 25 mm×25 mm×50 mm samples.
While certain embodiments of the present invention are described in the following example, it will be apparent that considerable variations and modifications of these specific embodiments can be made without departing from the scope of the present invention as defined by a proper interpretation of the following claims.
Percent crack reduction Cr can be determined from the ratio of the rough crack value Rcv to the smooth crack value Scv by the following formula:
C
r=(1−Rcv/Scv)*100
Wherein crack values are manually calculated for a shaped foam article pressed by a mold with a smooth cavity surface Scv by first measuring the length of each crack in the shaped foam article (or a specified portion thereof) made from a mold with a smooth cavity surface and then adding each of the individual crack lengths together to get an overall smooth crack value Scv in units of length. Crack values are manually calculated for a shaped foam article pressed by a mold with a reduced-slip cavity surface Rcv by first measuring the length of each crack, if any, in the shaped foam article (or the same specified portion as used in the shaped foam article pressed from the mold with a smooth cavity surface) made from a mold with a reduced-slip cavity surface and then adding each of the individual crack lengths together to get an overall reduced-slip crack value Rcv in units of length.
For Comparative Example A and Example 1 an IMPAXX™ 300 Foam Plank, available from The Dow Chemical Co., Midland, Mich. is used. The IMPAXX 300 Foam Plank is an extruded polystyrene foam with dimensions measuring 2,200 mm by 600 mm by 110 mm in the length, width and thickness directions respectively. The IMPAXX 300 Foam Plank has a density gradient of about −18.6 percent, an open cell content of about 4.9, and a cell gas pressure of about 0.6 atmosphere (atm). About 7 millimeters (mm) layer is removed by planing from the top and the bottom of an IMPAXX 300 Foam Plank. The planed IMPAXX 300 Foam Plank is then cut to render a foam blank having a planed surface (top or bottom) opposite a cut surface (core) measuring approximately 355 mm by 241 mm by 50 mm, in the length, width and thickness directions respectively. The cut, or core, surface of the foam blank is then compressed against the movable forming surface comprising a mold cavity in the shape of Spanish roofing tiles (
The foam blank is pressed by an aluminum compression fixture (mold) with a pressing surface milled in the shape of Spanish roofing tiles. The resulting shaped foam article is a panel with the appearance of Spanish roofing tiles measuring 997 mm×600 mm×78 mm. The periphery of the mold cavity/panel is defined by a trimming rib measuring about 0.38 inch (in.) wide and about 1,125 in. long. The fixture is mounted to the movable platen of a MTS Millutensil Spotting Press. The Millutensil is programmed for a crosshead velocity of 12 inch per minute (in./min.) and the foam sample is compressed 2.25 in. (i.e., the movable platen is 0.75 in. from the stationary platen). For Comparative Example A, the pressing surface of the mold cavity is machined from a solid billet of aluminum resulting in a smooth machined surface. This was then taken to Sun Coating Co. in Plymouth Mich. to be TEFLON coated resulting in a smooth cavity surface. For Example 1, the pressing surface of the mold cavity is textured with a media blasted with a Wheelabrator 48 inch Spin Blast media blaster loaded with SN-460 Steel Nugget media. The tool was processed within the Wheelabrator for a couple of minutes to sufficiently texture the surface. The cracking is reduced by 80 percent.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US10/40787 | 7/1/2010 | WO | 00 | 12/13/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61223741 | Jul 2009 | US |
