ETHOD FOR THERMOFORMING CONTOURED PRODUCTS BY USING TEMPERATURE DIFFERENTIAL
BACKGROUND OF THE INVENTION
The present invention relates to thermoforming and more particularly to a process for thermoforming contoured articles from planar substrates.
It is commonplace to form thermoplastic articles from planar sheets of thermoplastic substrate. Perhaps the most conventional method for thermoforming a contoured article is to begin with a planar thermoplastic substrate cut to the appropriate size.
The substrate is uniformly heated in a conventional radiant, convection, or infrared heating oven. After the entire sheet has become uniformly heated and consequently formable, it is transferred to a vacuum mold. A vacuum applied to the sheet conforms the sheet to the contours of the mold.
Unfortunately, conventional thermoforming presents a variety of structural and aesthetic problems. In areas where the planar substrate is pulled excessively, for example, in forming a comer or other substantially contoured portion of article, the substrate tends to stretch and become more thin in that area than in areas where the substrate is uncontoured or primarily planar. Thus, there is an inconsistency in the thickness of the resultant thermoformed article. This can lead to substantial structural integrity issues in the finished article. For example, the corners of the article may be too thin, making them overly susceptible to breaking and crumpling.
The associated problems are even more acute with contoured articles that are used as injection molding inserts. There has been a dramatic increase in the use of formed thermoplastic materials as inserts with injection molded articles. Typically, these inserts are used to provide the molded article with a finished, aesthetically pleasing surface. For example, in the automotive industry, thermoplastic inserts are used to provide molded bumper, trim, instrument panel control buttons, and other components with a finished outer surface that does not require painting or other treatment after the molding process. Typically,
a thermoplastic substrate having a painted, coated or otherwise finished surface is formed using one of a variety of thermoforming methods. The formed insert is placed into the mold and the article is formed by injection molding the structural plastic behind or in front of the insert to form the bumper, trim, button or other article. Another application for formed inserts is in the manufacture of housings, such as computer monitor housings, printer housings, cellular phone housings and other plastic injection molded housings. In this application, the insert can form the outer surface of the housing or it can form the inner surface of a translucent housings and include images, text or other graphic material to enhance the housings' appearance. The images, text or other graphic material printed on the insert is visible through the translucent housing when the insert is disposed on the interior of the housing, or directly viewable when the insert is disposed on the exterior of the housing.
Unfortunately, the above discussed problems with conventional thermoforming processes present additional issues in insert applications. In insert applications, the formed insert is contained in the mold during forming of the structural portion of the article. As a result, the insert is subjected to the force of molten material being injected into the mold. If the insert is too thin, the force of the injected molten material will cause the insert to wrinkle or crease. An example of a thermoplastic image-carrying insert used to enhance the appearance of translucent computer housing formed with unacceptable wrinkling is illustrated in Figs. 1 and 2. As can be seen in Fig. 1, the completed insert 10 has wrinkles 14 near contours 16 and 18. These wrinkles are the result of the substrate in regions near those contours being too thin and consequently folding over on itself. Fig. 2 illustrates a cross section of the wrinkles. In regions 22 and 24, the substrate is of a uniform thickness Tl. In regions 26 and 28, which are adjacent, the contour 16 of the article, the substrate is of thickness T2 and T3, which are markedly thinner than Tl. Accordingly in areas 26 and 28,
wrinkles 14 are formed easily because the substrate is thin enough that it folds over on itself under the force of molten plastic during injection forming of the structural portion 29 of the article. This wrinkling, particularly in the computer and automotive industry where the appearance of the finished article is important, is unacceptable. In applications where an ink or other coating has been applied to the surface of the substrate or mixed with, or embedded in the thermoplastic before thermoforming, the appearance and consistency of the ink or coating are likewise altered in the contoured regions of the formed article. For example, where a colored ink image overlaps a contour, the stretching of the substrate near the contour will consequently lighten the color there due to the stretching and thinning of the substrate and ink coloring layer. Accordingly, the formed article will be non-uniformly colored.
Several methods are used to reduce stretching, and consequently the attendant problems, during thermoforming. In a first method, a thicker substrate is used so that even in extremely contoured areas where stretching is most severe, the substrate does not thin beyond a desired thickness to induce wrinkling or compromise structural consistency. The added thickness of the substrate, however, increases the amount of the material and thus the cost of thermoformed articles.
In a more complicated method of reducing stretch, typically referred to as the "zonal heat method," heat is focused on different zones of the substrate before it is vacuum molded. Zones that are heated more consequently stretch more than regions heated less. In the zonal heat method, non-contoured areas of the substrate, including excess regions that will be trimmed from the final product, are heated more than contoured regions. Accordingly, the excess regions and flat regions "give" and stretch so the integrity of the substrate around the contoured portion is not compromised. This method, while somewhat effective, is difficult to set up because the positioning of heating elements must be extremely
precise to properly heat specific regions of the substrate. For example, when heating zones of individual heating elements overlap, the amount of heat added to the substrate in those regions of overlap cannot be satisfactorily controlled. Accordingly, the method is relatively imprecise, expensive and time consuming. In another even more complicated method, an articulated frame is used to bend a thermoplastic planar substrate. In this method, a thermoplastic planar substrate is positioned over an articulated frame that corresponds substantially to the shape of the article to be formed. The substrate is then physically bent around the contours of the frame to conform the contoured article. Unfortunately, this method requires an expensive articulated frame that is specifically configured to the contours of the resultant article.
SUMMARY OF THE INVENTION The aforementioned problems are addressed in the present invention wherein a thermoforming method is provided in which heat absorption control coatings are used to control the stretching of a substrate in various regions. More particularly, heat-absorptive or heat-reflective coatings are deposited on specific regions of a thermoplastic substrate to minimize or maximize stretching of the substrate in those regions when forming a contoured shape.
In general, a preferred embodiment of the present invention includes the steps of: (a) providing a sheet of thermoformable material including contoured regions and non- contoured regions; (b) selectively coating at least a portion of one of the regions with a heat- absorption controlling material; (c) heating the sheet; and (d) forming the sheet into a contoured article such that the heat -absorption controlling material controls the amount of stretch in various regions of the substrate.
Preferably, the regions that correspond to primarily planar regions of the contoured substrate or the excess regions subsequently trimmed from the substrate, that is,
the "non-contoured regions," are coated with a heat-absorptive material so that they absorb a substantial amount of heat during the heating step and stretch to facilitate forming the sheet into contours during the forming step. Consequently, portions of the sheet corresponding to contours of the article, that is, the "contoured regions," resist stretching and therefore maintain a more consistent, uniform thickness. Portions of the non-contoured region may also be coated with a heat-absorptive material to prevent stretching of the contoured region. Additionally, specific regions of the contoured region may be coated with a heat-reflective material to prevent stretching of the sheet in those regions during forming.
The thermoforming process of the present invention incorporates some of the conventional thermoforming steps. The sheet of thermoplastic material described above is heated, vacuum molded into a contoured shape, and then optionally trimmed to remove an excess area of the sheet that does not form part of the contoured shape. This excess area provides a gripping surface to hold the sheet during the thermoforming process.
In the present invention, the non-contoured area, preferably the excess area, is coated with a black ink, or other heat-absorptive material. In the preferred embodiment, when the sheet is heated, the excess area heats more than the contoured area and becomes more pliable and stretchable than the contoured area. Consequently, when the sheet or substrate is formed into a contoured shape, the excess area stretches in the mold to a greater degree than the contoured area. Accordingly, the contoured area does not become thin and wrinkle. Similarly, portions of the non-contoured area that are not excess and form part of the contoured article may be coated with a heat-absorptive material. These portions will stretch to a higher degree than uncoated contoured areas during molding. Moreover, the uncoated contoured areas are stretched to a lesser degree than the coated non-contoured areas and therefore resist thinning and wrinkling.
In another aspect of the invention, portions of the contoured area are coated with a heat-reflective or heat dispersive material. Those areas absorb less heat than uncoated areas as a result. Consequently, those areas are very resistant to stretching when the substrate is molded into a contoured article. Moreover, the heat-absorptive coatings and heat-reflective coatings may be combined on a single substrate to maximize stretch control.
In a third aspect of the invention, the thermoplastic substrate includes a margin area between the contoured area and the excess area. The margin area is of the same color or pattern as the contoured area. The margin area may separate the contoured area from a coated excess area. Accordingly, the excess area, which may have been stretched during molding, is prevented from becoming part of the completed contoured article.
The present inventive process is useful in manufacturing a variety of contoured encasements, instrument panels, structural or body panels, containers, and the like. The present invention provides a process for thermoforming these contoured articles without compromising structural or esthetic quality of the substrate from which it is formed. By minimizing stretch in contoured regions of the completed article, the structural integrity and appearance of the article are not flawed by the formation of wrinkles or exaggerated distortion of images printed on the substrate. Additionally, the inventive stretch control process reduces the requisite thickness of the raw substrate, thereby reducing the total amount of material used to thermoform an article. Accordingly, the overall cost is reduced. Further, the process of the present invention may be used to thermoform a contoured article of a more uniform thickness or of region-specific thickness as required in various electromagnetic, magnetic, conductive, and electrical applications.
These and other objects, advantages and features of the invention will be more readily understood and appreciated by reference to the detailed description of the preferred embodiments and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a computer housing thermoformed according to a process of the prior art;
Fig. 2 is a sectional view of the prior art taken along line 2-2 of Fig. 1; Fig. 3 is a perspective view of a computer housing having a thermoformed insert manufactured according to the present invention;
Fig. 4 is a plan view of a substrate of the present invention; Fig. 5 is a plan view of the substrate with a reflective coating; Fig. 6 is a perspective view of the substrate being positioned in a mold; Fig. 7 is a perspective view of the thermoformed substrate;
Fig. 8 is a sectional view taken along line 8-8 of Fig. 7; Fig. 8a is an enlarged sectional view of Fig. 8; Fig. 9 is an elevational view of the thermoformed substrate; Fig. 10 is a top plan view of the thermoformed substrate; and Fig. 11 is an exploded view of the thermoformed substrate and a computer housing.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In the preferred embodiment, the present invention is described in connection with the manufacture of decorative inserts that interfit within or over translucent computer housings or encasements. With reference to Fig. 3, an insert 10 includes images 4 printed thereon and is contoured to conform to a computer housing 50. Given the translucence of the computer housing 50, the images 4 on the insert 10 are viewable through the translucent computer housing 50. Alternatively, for inserts that fit over the outside of the computer casing (not shown), the images are viewable directly. As will be appreciated by those skilled
in the art, the present invention may also be used to thermoform automobile parts, containers, building materials, and many other structures.
The inserts of the present invention are formed from a thermoformable sheet material, a preferred embodiment of which is illustrated in Fig. 4 and generally designated 10. The substrate 10 includes an insert region 2 comprising contoured areas and non- contoured areas. Excess area 6 makes up a part of the non-contoured area of the insert. As used herein, "contoured area" means a region of the thermoformable substrate that corresponds to the contours of the completed contoured insert. "Non-contoured area" means a region of the thermoformable substrate that does not correspond to the contours of the completed contoured insert, the excess area of the substrate that is trimmed subsequent to the molding step, and/or the primarily planar region of the substrate that is included in the insert. "Excess area" is a subset of the non-contoured area that is trimmed from the insert after molding. Although the excess area is defined herein as a subset of the "non-contoured area," the excess area may in fact be contoured; however, those contours do not typically form the contours of the finished article, that is, in most cases they are trimmed from the article. In the preferred embodiment, the excess area 6 is pre-selectively coated with a heat-absorptive coating. As depicted, the sheet 10 includes opposed layouts for two inserts, 2a and 2b. Adjacent the excess area 6 is margin 8. Printed on the insert region 2 are images 4.
In the preferred embodiment of Figs. 4 and 5, any thermoformable material may be used to form the substrate. "Thermoformable material" is defined to include any material that becomes formable when heat and pressure are applied. Examples include polycarbonate, polyvinyl chloride, styrenes or any derivatives thereof, acrylonitrile butadiene styrene (ABS), polyethylene terephthalate (PET), styrene, polystyrene, thermoplastic olefin (TPO), or any other thermoformable material as will be appreciated by those skilled in the art. Thermoformable materials may also include materials which contain thermoplastic
resins and thermosetting resins. Thermoplastic resins are defined as materials that soften or fuse when heated and of hardening and becoming rigid again when cooled. Thermosetting resins are defined as materials that become permanently hard and rigid when heated to a critical temperature. The thermoformable material may also include other substances as desired to enhance certain characteristics of the substrate. For example, the thermoformable material may be impregnated with fibers to enhance the structural strength of the substrate. The substrate may also include conductive materials to enhance the electrical conductivity of the substrate. Alternatively, the substrate may be coated or impregnated with metals or alloys or other materials that enhance the magnetic shielding capabilities of the substrate as will be appreciated by those skilled in the art. Moreover, the physical, chemical, and electrical characteristics of the substrate may be enhanced uniformly throughout the sheet, or in specific regions of the substrate.
As depicted in Figs. 4 and 5, in the excess region 6, the substrate is coated with a heat-absorptive material. "Heat-absorptive material" includes any coating, covering or pigmentation in or on the substrate that causes the substrate or the material itself to retain certain wavelengths of radiation or any type of energy incident upon it, followed either by an increase in temperature of the substrate or the material or by a compensatory change in the energy state of their molecules. The heat-absorptive material may include any heat- absorptive ink such as black, blue, purple or any other dark color that tends to absorb radiant, convection or other energy. Inks may be printed on or impregnated in the substrate using conventional offset printing techniques, screening techniques, gravier techniques or any other conventional ink printing method as will be appreciated by those skilled in the art.
The dimensions of the excess region 6 in Figs. 4 and 5 are dependent on the size and number of contours of the computer casing to which the completed insert will be
secured. For larger casings or casings having more contours, the excess region will need to be larger than that required for smaller casings or casings having fewer contours.
With reference to Figs. 4 and 5, the substrate is produced from planar sheets of uniform thickness. This thickness may vary from about 0.001 thousandths of an inch to about lA inch or greater as the application requires.
With further reference to Figs. 4 and 5, the insert region 2 is generally adjacent the excess region 6. At the boundary of the excess region 6 is margin region 8 that may be of any width. The specific width depends on the anticipated stretch of the excess region 6 during vacuum forming. For example, if it is anticipated that the excess region 6 will undergo substantial and differential stretching, which is dependent on the contours of the completed insert, the margin region must be quite large. This is so that the excess region coated with the heat-absorptive material does not stretch and form part of the completed, contoured insert.
The insert region 2 of the preferred embodiment includes images 4 which may include textual or graphic information. The text or graphics are printed with conventional inks using conventional printing techniques such as offset printing, screen printing, or gravier printing or any other printing as will be appreciated by those skilled in the art. Additionally, non-contoured areas included in the insert area 2 that do not correspond to contours of the completed insert may include heat-absorptive material. Accordingly, these portions will stretch during forming to a greater degree than contoured areas.
With particular reference to Fig. 5, the insert area 2 may also include portions coated with or including a heat-reflective material 26 and 28. "Heat-reflective material" means any coating, covering, or pigmentation in or on the substrate that prevents the substrate or the material itself from absorbing any energy or radiation incident upon it. Typically, the material 26 and 28 is positioned over contoured areas which, for example,
correspond to a contour such as contour 27 to the completed insert depicted in Fig. 3. It will be appreciated by those skilled in the art that the reflective material may be configured in any shape corresponding to the desired contour of the thermoformed insert. The reflective material may be a chrome foil or highly reflective ink, such as silver or chrome colored ink. Alternatively, the reflective material may be a peelable, reflective tape that is temporarily positioned over the formable region of the substrate corresponding to the anticipated contour 27 of the completed insert. As used herein, "heat-absorption controlling material" means heat-absorptive material and/or heat-reflective material, as defined above.
In the preferred embodiment of Figs. 4 and 5, portions of the substrate, may be coated with a silicone or other material to enhance slippage of the substrate in the mold. The silicone prevents the substrate from gripping the mold via friction and unnecessarily stretching the substrate in contoured areas.
Method of Manufacture
The preferred process of the present invention is used to form an insert that corresponds to the contours of a computer encasement. This process includes the steps: (1) providing a sheet of thermoformable material including a contoured area and a non-contoured area as described above; (2) heating the sheet; (3) vacuum molding the sheet to form a contoured insert; and (4) trimming a portion of the non-contoured area, particularly the excess area, after the insert has been formed. The contoured insert may then be affixed to a computer housing where the contours of the computer casing correspond to the contours of the insert.
More particularly, a sheet of thermoplastic material as described above and illustrated in Figs. 4 and 5 is provided. This sheet, or substrate, is heated with heat lamps 20 as depicted in Figs. 4 and 5. Any heating method such as convection heating, blown air heating, infrared heating, or electromagnetic heating may be used to heat the substrate.
Because the excess region 6, and where desired other non-contoured areas, is pre-selectively coated with the heat-absorptive material and the non-contoured areas are not coated with such material, the excess region 6 heats up at a rate greater than the non-contoured areas in the insert area 2. Accordingly, the excess area becomes more pliable and prone to stretching during subsequent steps. With further reference to Fig. 5, regions of the substrate coated with the heat-reflective material 26 and 28 heat up at a rate slower than the rest of the insert area 2 and the excess area 6. Accordingly, the heat-reflective material covered regions are less prone to stretching than the remainder of the insert area and the excess area. The rate of heating the excess area 6 may also be increased by strategically placing the lamps 20 directly over the excess area 6.
Next, the substrate is removed from the heating elements 20 and transferred to vacuum mold 100 in frame 120 as illustrated in Fig. 6. The vacuum mold 100 of Fig. 6 is conventional, and the process by which the insert is vacuum molded will be appreciated by those skilled in the art. Generally, the male mold 120 descends downward as indicated. As can be seen, the male mold includes molding portion 134, shaped corresponding to the contours of two opposing inserts for computer housings. The molding portion 134 also includes a plurality of vacuum holes 132 through which a vacuum is applied during molding.
The molding portion 134 descends until it pushes downward against the sheet
10 held in frame 120. The molding portion subsequently projects through the frame 120 so that the sheet 10 conforms substantially to the molding portion 134. Vacuum box 140 is lifted upwards until it abuts frame 120. A vacuum is applied through the vacuum holes 132 to form the remainder of the sheet 10 to the molding portion 134.
The male mold 130 may then be lifted away from the frame 120. The sheet 10, now formed into opposing contoured inserts, may be removed, as will be appreciated by those skilled in the art.
Fig. 7 depicts the thermoplastic substrate after it has been vacuum formed. The excess area 6 is trimmed from the insert area 2. Any conventional cutting techniques such as laser cutting, matched metal trim die cutting, water jet cutting or router cutting, may be used to trim the excess region 6. Depending on the contours of the formed insert, certain portions of the excess region 6 may have been stretched more than other regions. Because the margin region 8 is provided adjacent the excess area 6, this uneven stretch is compensated for. Accordingly, the completed contoured insert may be trimmed along the margin region 8 without the excess region 6 forming part of the contoured insert. It will be appreciated that portions of the excess region alternatively may form part of the contoured insert, and need not be trimmed completely therefrom. For example, if the excess region, including a heat absorptive material is not visible to a consumer, such as the case where the coating is on the inside of an opaque or colored insert, the excess region may form part of the insert without aesthetic detriment. If any reflective materials 26 and 28 (see Fig. 5), such as peelable reflective tape, remain on the formed insert 10, that material may be removed before further processing.
Figs. 8 and 8a depict the uniformity in thickness of the formed inserts. As can be seen, the insert area 2 is of a uniform thickness particularly around the contoured areas, and has resisted stretching—unlike excess area 6. With particular reference to Fig. 8a, the thickness of the excess area 6, that is thicknesses T8, T10, T12, and T20 are substantially less than the thicknesses T14, T16, and T18 of the insert area 2. This is the result of the excess region absorbing and retaining more heat, and thus stretching more than the insert region, in particular, in contoured areas, during the vacuum molding process. Accordingly, the completed contoured insert is, as desired, of a more uniform thickness throughout its cross section.
Figs. 9 and 10 illustrate the insert 10 after trimming it. As can be seen, the insert corresponds to the contours of a computer housing. Next, the insert may be secured to a computer housing. As illustrated in Fig. 11, the computer housing 50 has the same contours as the insert 10. The housing 50 is preferably made from translucent plastic, thus, any images on the insert 10 are viewable therethrough. The insert is preferably adhered with a permanent high-tack adhesive 60 to the interior of the housing 50 as illustrated in Fig. 11. The insert 10 may be trimmed manually along its edges 11, 12 should it not correspond exactly to the perimeters of the computer housing 50. Alternatively, the thermoformed contoured article may be adhered to the exterior of the computer housing. In this case, it is not necessary that the computer housing is translucent, rather, it may be opaque. Moreover, the computer housing may be constructed of other materials such as opaque plastic, metal, alloy or other non-translucent material as desired.
In an alternative method of securing the insert to the computer housing, a conventional injection molding process is used. In this process, the insert is placed in a mold. Plastic material is then injected over the insert. Due to the excessive heat, the injected material bonds to the thermoformed insert. This material forms the computer housing. Moreover, due to the consistent and uniform thickness of the insert in areas of contour, as well as the lack of stretch in the contoured areas of the insert, the injected material does not wrinkle or deform the insert. As will be appreciated by those skilled in the art, the material may be injection into the mold so that the insert is on the interior or the exterior of the completed injection molded housing. Once the injection is complete, the computer housing and the insert may be removed from the mold.
Alternative Embodiments In an alternative embodiment, the contoured areas and/or the insert areas may be coated with a heat-absorptive material to promote stretching of these areas during
molding. For example, in Fig. 4, insert area 2 may be coated with a dark ink while excess region 6 may be left transparent. During thermoforming, the insert area will heat up more than the excess region and therefore stretch more than the excess region when molded.
In another alternative embodiment, the non-contoured areas may be coated with a heat-reflective material. Thus, during when the substrate is heated, the contoured areas will heat more than the non-contoured areas. Consequently during molding, the contoured areas will stretch more than the non-contoured area when molded.
The above descriptions are those of the preferred embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. Any references to claim elements in the singular, for example, using the articles "a," "an," "the," or "said," is not to be construed as limiting the element to the singular.