Powder slush molding process and equipment

A powder slush molding process and associated equipment utilize a flow restrictor disposed between a powder box and a mold surface to deposit powdered resin onto the mold surface in a controlled manner, thereby forming skins having decreased variation in thickness.

TECHNICAL FIELD

The present disclosure relates generally to powder molding processes and related equipment.

BACKGROUND OF THE INVENTION

Powder slush molding processes can be used to form polymer-based skins that are useful as decorative coverings in automobile interiors or other applications. For example, such decorative coverings can be used on interior door panels and instrument panels in automobiles. In some applications, it can be desirable to minimize variation in or otherwise control the thickness of the skin.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a powder slush molding tool that includes a mold having a mold surface, a powder box having an open end, and a grate having one or more openings. The open end of the powder box permits flow of powdered resin from the powder box to the mold surface when the powder box and mold are connected together. The grate extends at least partially across the open end and restricts the flow of powdered resin through the open end.

In accordance with another aspect of the invention, there is provided a method of powder slush molding. The method includes the steps of providing a molding tool having a mold surface and having a powder box at least partially filled with a powdered resin, and depositing at least some of the powdered resin onto the mold surface through a flow restrictor that extends at least partially across an open end of the powder box.

In accordance with another aspect of the invention, there is provided a method of powder slush molding. The method includes the steps of: providing a molding tool that includes a powder box containing a powdered resin and a mold having a mold surface; heating the mold surface to a level sufficient to melt the powdered resin; attaching the mold to the powder box so that the molding tool is in a starting orientation with the mold surface above the powder box and opposing an open end of the powder box; rotating the molding tool about an axis so that the molding tool is inverted during the rotating and so that the molding tool returns to the starting orientation; depositing at least some of the powdered resin onto the mold surface while restricting the free flow of the powdered resin from the powder box to the mold surface during the rotating step so that powdered resin is deposited onto the mold surface in a controlled manner; melting the powdered resin that is deposited onto the mold surface to form a molten layer of resin; and solidifying the molten layer of resin to form a skin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Powder slush molding generally includes depositing powdered resin onto a heated mold surface of a mold so that the resin melts when it comes into contact with the heated mold surface. Excess powder that is far enough from the mold surface to remain in powder form can then be removed from the mold for use in a subsequent molding cycle, while a layer of molten resin remains on the mold surface. The layer of molten resin can then be solidified to form a skin.

Referring toFIGS. 1 and 2, a powder slush molding tool10is shown according to an exemplary embodiment. The molding tool10includes a mold12, a powder box14, and a flow restrictor16. In this embodiment, mold12includes a frame18, a shell20, temperature control lines22, and fence24. The frame18is a structure provided to support the shell20and various other components such as temperature control lines, control line connectors, attachment points for a rotating shaft for turning the molding tool, etc. The frame18can also provide a structure to attach or connect the mold12to the powder box14.

In the illustrated embodiment, shell20is a structure having a generally uniform wall thickness and includes a mold surface26. The mold surface26includes a contour that is formed in the desired final contour of the skin. The shell20is formed from a material having sufficient strength and thermal conductivity so that it can be heated and cooled rapidly and is preferably a metallic material such as nickel, a nickel-based alloy, or another material that includes a plated layer of nickel on the mold surface side of the shell. Materials not including nickel or nickel layers may also be used. Mold surface26can be a smooth surface, or it can be textured so that a skin formed thereon will include a matching textured surface when removed from the mold surface26. As shown in the cross-sections ofFIG. 2, the mold can include more than one mold surface26; e.g., two mold surfaces26are shown inFIG. 2.

Temperature control lines22are shown in cross-section inFIG. 2and are conduits that can facilitate the flow of a fluid for heating and/or cooling. In this embodiment, the lines22are positioned adjacent the surface of the shell20that opposes the mold surface26through the wall thickness of the shell, and they extend longitudinally to reach all of the regions of the shell that require heating and/or cooling. The lines22can be a plurality of discrete lines, each having its own temperature control, or they can be a single line in a serpentine arrangement running back and forth along the shell and ultimately attached to a single temperature controller. Or the lines22can be divided into any number of separate temperature control regions, each region having its own temperature control. The lines22are preferably formed from thermally conductive materials to efficiently transfer thermal energy between the shell20and the fluid or fluids contained within the lines. The heating and/or cooling fluid can include oil, water, or some other fluid, and not all lines22necessarily include the same temperature control fluid. Of course this is only one example of temperature control lines. For example, temperature control lines22could be formed as conduits completely contained within the shell20or could be arranged to run in a different direction than that shown in the figures. Skilled artisans will also appreciate that providing a shell20and temperature control lines22is only one of many ways to provide a mold surface26that can be heated and/or cooled. For instance, shell20can be heated and cooled using a variety of methods such as hot air heating, infrared heating, fluidized bed heating, induction heating, and/or cooling with cold air or other cooling fluids in direct or indirect contact with a surface of shell20. Alternatively, mold surface26can be provided as a surface formed on a solid piece of material similar to a mold surface used in a mold for plastic injection molding, rather than a shell. But the reduced mass of shell20compared to a solid piece of material can allow relatively faster heating and cooling of mold surface26.

Fence24is an optional component included with mold12to surround or circumscribe the desired portions of the mold surface side of shell20to help define mold surface26and so that powdered resin is conserved. The fence24, shown inFIG. 2, can prevent powdered resin from reaching portions of the mold surface side of the shell20that are not part of the mold surface26; i.e., some portions of the mold surface side of the shell20are included to allow the shell to be continuous and are not used to help form the skin. The fence24can also provide a surface against which the powder box14can seal.

Powder box14is a container for holding powdered resin (depicted as P inFIG. 2A) before, during, and/or after the powder slush molding process. Powder box14may include a powder reservoir28, an open end30, a seal32, and a flange34. Powder reservoir28is generally a storage area for powdered resin when powder box14is in the orientation shown inFIG. 2A. During a slush molding process, powdered resin can be transferred to and from the powder reservoir28through open end30.

Open end30broadly includes an end of powder box14through which powdered resin can flow to and from mold12during the slush molding process. As depicted in the embodiment in the figures, open end30can be in the form of one or more chutes that extend toward the mold12when powder box14and mold12are connected together, but can also be in any variety of shapes, including a simple two-dimensional opening in an otherwise continuous surface or more than one of such openings. Powder box14may include, for example, a powder reservoir28in the form of a box with no top, in which case the open end30is defined by the area where the top would be if the box was closed-off.

Seal32is an element that can be disposed between the mold12and powder box14to prevent powdered resin from escaping from the molding tool10during the slush molding process. In this embodiment, seal32is attached to powder box14and is located at an outer periphery that circumscribes each chute of open end30. The seal32can be formed from an elastomeric material having sufficient sealing properties such as, for example, silicone rubber, polyurethane-based elastomers, rubber-based materials, thermoplastic elastomers, or other polymeric or non-polymeric materials. Seal32could be attached to mold12, or more specifically to fence24where included, or it could be an entirely separate component attached to neither the mold12nor powder box14. It could also be omitted entirely in embodiments in which sufficient sealing surfaces are formed on mating surfaces of mold12and powder box14.

Flange34is a structure provided to attach or connect the mold12to the powder box14, among other uses. The flange34can be formed integral with the powder reservoir28, or it can be a frame-like component to which other powder box components can be attached. It is an optional component of powder box14, as other means can be used to connect the mold12to the powder box.

Flow restrictor16is an element for restricting flow of powdered resin from powder box14to mold12during the slush molding process. One embodiment of a flow restrictor includes a grate, as shown inFIGS. 1-3. Grate16generally includes one or more openings36through which powdered resin can flow and is configured to extend at least partially across open end30of powder box14of molding tool10. As will be further discussed below, a variety of types, shapes, and arrangements of openings36is possible, but any opening36is smaller in projected area than the open end30such that the opening36will restrict the free flow of powdered resin through the open end30. In the embodiment of the molding tool10shown inFIGS. 1 and 2, the grate16extends across substantially the entire open end30and is attached to powder box14. More specifically, grate16is disposed within the open end30, which includes a pair of chutes in this example. The grate16may be attached to powder box14such that at least a portion of its margins have substantially the same contour as at least a portion of the outer periphery of the powder box that circumscribes the open end30. The examples inFIGS. 1 and 2include such an arrangement in that the grate16is generally planar in contour, and the plane in which the grate lies is generally parallel to the plane in which the outer periphery lies (in this case, generally the same as the plane in which seal32lies). The contour of one or more portions of the outer periphery and/or the grate16can be shaped such that the contour is at least in the same general direction as the contour of mold surface26, thereby minimizing the variation in distance from grate16to mold surface26. In some embodiments, grate16may only extend partially across open end30. The flow restrictor16may alternatively be attached to the mold12or may be a separate element altogether and attached to neither the mold12nor the powder box14and disposed between the two components temporarily during the slush molding process. Flow restrictor16is not limited to grates. It may be any element that has the effect of restricting the flow of powdered resin from powder box14to mold12or to mold surface26, such as a series of funnels or other three-dimensional shapes or other types of restrictors that do not use physical barriers or mechanical restriction and instead use other methods of flow restriction, such as controlled air pressure. Flow restrictors may also or alternatively use electrical, electrostatic, magnetic, or other properties of the powdered resin to restrict its flow.

Referring now toFIGS. 3A-3I, several different embodiments of portions of grates16are shown including different types, shapes, and arrangements of openings36. Openings36may be of at least two types, including apertures38and cut-outs40. As used herein, a cut-out40is an opening that is larger in projected area than any aperture38included in the same grate16. A cut-out40has a projected area at least 4 times that of any other opening in the same grate, excluding other cut-outs. Every opening that is not a cut-out is referred to as an aperture38. Some examples of grates having both apertures38and cut-outs40will be described below.

In the embodiment ofFIG. 3A, the openings of grate16include a plurality of apertures38. The apertures38in this example are arranged in a pattern. Each aperture38is circular in shape and the distance from the center of any one aperture to the center of another adjacent aperture is equal for all apertures. Each aperture38in this example has the same diameter or width. This type of pattern may be commonly found in commercially available perforated stainless steel plate, but other materials may of course be used to form the grate. This or other exemplary grates may be formed such that opposing faces of the grate are smooth; i.e., the grate16does not include any appreciable standing features, such as burrs around the apertures or any other raised features. Smooth grate faces can allow powdered resin to move freely from one end of the grate to the other, as will be described in more detail below.

In the embodiment ofFIG. 3B, the openings of grate16also include a plurality of apertures38arranged in a pattern. In this example, each aperture38is generally diamond-shaped and has the same dimensions. This type of pattern may be commonly found in commercially available expanded metal products.

In the embodiment ofFIG. 3C, the openings of grate16again include a plurality of apertures38arranged in a pattern. In this example, each aperture38is generally square-shaped and has the same dimensions. This type of pattern may be commonly found in commercially available wire mesh products in which strands of metal wire are interwoven with one another in a grid pattern.

In the embodiment ofFIG. 3D, the openings of grate16include a plurality of apertures38arranged in a pattern. In this example, each aperture38is elongated in shape or slot-shaped, each aperture38having a length greater than its width. The apertures are arranged side-by-side with the length-wise ends of each aperture38aligned with one another.

In the embodiment ofFIG. 3E, the openings of grate16include a plurality of apertures38arranged in a pattern. In this example, each aperture38is elongated in shape or slot-shaped, each aperture38having a length greater than its width. The apertures are arranged end-to-end in side-by-side rows, and the length-wise ends of each aperture38are aligned with the length-wise ends of apertures in alternating rows.

In the embodiment ofFIG. 3F, the openings of grate16include a plurality of apertures38arranged in a pattern the same as the pattern of the apertures inFIG. 3A. The grate16in this example also includes a cut-out40that is elongated or slot-shaped. Whereas grates having only patterned apertures38, such as the grates inFIGS. 3A-3E, theoretically allow a uniform flow of powdered resin through them over the entire grate area, the grate ofFIG. 3Fincluding cut-out40allows an increased flow of powdered resin (relative to the flow through the apertures) through the grate in the region that includes the cut-out. This can be useful in slush molding processes that would otherwise produce skins having thin areas; i.e., the cut-out can allow additional powdered resin to be deposited onto the mold surface in desired locations corresponding generally to the location of the cut-out.

In the embodiment ofFIG. 3G, the openings of grate16include a plurality of apertures38. The apertures38in this example are arranged in a pattern. Each aperture38is circular in shape. As is apparent from the figure, some of the apertures38in this example have different diameters or widths than some of the other apertures. More particularly, the aperture sizes in this exemplary grate include an increasing gradient from the left side of the grate to the right side of the grate (as oriented in the figure). This type of arrangement of grate openings can be useful in applications in which a variation in powdered resin flow is desired along the length or width of the grate. The variation in this case would theoretically include a gradual increase or decrease in flow along the length or width of the grate.

In the embodiment ofFIG. 3H, the openings of grate16include a plurality of apertures38. The apertures in this example are arranged in at least two patterns. One pattern includes circular apertures in one portion of the grate, and another pattern includes elongated apertures in another portion of the grate. This type of arrangement of grate openings can also be useful in applications in which a variation in powdered resin flow is desired along the length or width of the grate. The variation in this case would theoretically include a more step-like increase or decrease in flow along the length or width of the grate, as opposed to the gradual one described in connection withFIG. 3G. This type of arrangement of openings may also be useful where increased flow of powdered resin is desired through a particular portion of the grate, but where a cut-out having the required size would compromise the structural integrity of the grate or otherwise be undesirable.

In the embodiment ofFIG. 3I, the openings of grate16include a plurality of apertures38having a plurality of sizes that are not arranged in any particular pattern. The openings also include exemplary cut-outs40. This example demonstrates the completely customizable nature of grate16. The openings can be sized and arranged in a limitless number of ways to achieve the desired flow and restriction characteristics for the grate such that the desired amount of powdered resin can be allowed to flow from powder box14to the appropriate portions of mold surface26during the slush molding process. Skilled artisans will appreciate that while grate16may preferably be formed from metal materials, it may be formed from any material or combination of materials.

Grate16can be further characterized by an openness percentage and/or by the sizes of openings36. The openness percentage of a particular portion of a grate can be calculated by first determining the actual surface area of one face of the portion of the grate in question. A second determination of the surface area of the same face of the same portion is then made, this time as if there are no openings in it. The difference represents the open area in that portion of the grate, and the openness percentage is the difference divided by the result of the second determination and multiplied by100. For example, if a portion of a solid sheet of material has a surface area of 3 in2, and a 1 in2opening is then formed therethrough, leaving behind 2 in2of actual grate surface area, the openness percentage of that portion of the resulting grate is 33%. In one embodiment, grate16includes a plurality of openings arranged in a pattern in at least a portion of the grate, and that portion of the grate has an openness percentage from about 30% to about 65%, and preferably about 40%. This same range and preference for the openness ratio may be applicable to the entire grate16as well, exclusive of portions having cut-outs. Grate openings36may range in size from about 5 mm to about 15 mm in width or diameter, and preferably have an average size of about 10 mm. In one embodiment, the grate16includes a plurality of circular apertures38formed therethrough and arranged in a pattern, each aperture having a diameter of about 10 mm, such that the openness percentage across the entire grate is about 40%. Of course, skilled artisans will appreciate that openness percentages outside the above range, as well as opening sizes outside the above range, may be used.

An exemplary method of powder slush molding can best be described with reference toFIGS. 2A-2D. The method includes providing a powder box14having an open end30, as described above, that is at least partially filled with powdered resin (depicted as P inFIG. 2A). The powdered resin can be any one of a variety of powdered resins, but is generally a polymer-based resin. Some exemplary polymer-based resins include resins comprising polyvinylchloride (PVC), thermoplastic urethanes (TPU), thermoplastic olefins (TPO), and/or thermoplastic elastomers (TPE), to name a few. Of course, any polymer-based material that can be produced in powdered form may be used in this exemplary process. Amorphous thermoplastic materials are preferred for their broad softening and melting ranges, but semi-crystalline thermoplastic materials such as TPOs or others may be used. Additionally, it is possible to use thermosetting polymer-based materials that soften or melt with increased temperature then cross-link with additional exposure to elevated temperatures. The method further includes providing a mold12, such as one described above, having a mold surface26. The mold surface26can be heated to a temperature that is sufficient to melt the particular powdered resin, generally at or above the melting or softening point of the resin or of the component of the resin having the highest melting point. The heating can be accomplished in a variety of ways as previously described. In the embodiment ofFIG. 2, it is accomplished by flowing a fluid that is at a temperature at or above the desired mold surface temperature through one or more of the temperature control lines22. A flow restrictor16is also provided that extends at least partially across open end30of the powder box14. In this embodiment, the flow restrictor includes a grate that is disposed within and attached to open end30. With the open end30of powder box14facing in an upward direction, the mold12can be securely connected to the powder box with mold surface26facing in a downward direction, above and opposing open end30of the powder box. The secure connection between powder box14and mold12can be formed by any suitable means such as mechanical, electrical, or fluid powered clamps, by disposing pins or dowels in aligned apertures of the respective frame components of the powder box14and mold12, or by other means.

Molding tool10has a starting orientation, as shown inFIG. 2A, and can progress through other orientations as shown inFIGS. 2B-2D. The other orientations progressively cause at least some of the powdered resin to be deposited onto the mold surface26through flow restrictor16. In this exemplary method, molding tool10is rotated (clockwise as oriented in the figures) about an axis such that the powdered resin moves across a powder box side of the grate16from a first end42to a second end44(labeled only inFIG. 2C) so that powdered resin passes through one or more openings36adjacent the first end42before passing through one or more openings36adjacent the second end44. The molding tool10can be further rotated so that it is inverted, then further rotated to be returned to its starting orientation. Rotation of the molding tool can be in any direction and can include more than one rotation.

As shown in the progressiveFIGS. 2A-2D, the flow restrictor16has the effect of restricting the free flow of powdered resin from the powder box14to mold surface26during rotation of the molding tool10. For example, if flow restrictor16was omitted, all of the powdered resin from powder box14would be quickly “dumped” onto mold surface26during rotation of the tool. Flow restrictor16restricts the flow of the powdered resin, allowing the resin to be more gradually and evenly distributed over the mold surface26during rotation of the molding tool. In embodiments in which the flow restrictor includes a grate, the powdered resin is also “conditioned” as it first contacts then flows through openings in the grate. In other words, any clumps of resin that may sometimes be formed in the bulk powdered resin due to moisture, temperature variations, static build-up, etc. can be sifted by the grate such that the clumps are either broken up into small enough pieces to pass through openings in the grate or prevented from reaching the mold surface26, thereby preventing localized regions of excess material on the mold surface that could cause unwanted variations in final skin thickness.

As powdered resin is deposited onto mold surface26during rotation of the molding tool, the resin that comes into contact with the mold surface melts to form a molten layer of resin over the mold surface26. The adhesive properties of the molten resin allow the molten layer to remain on the mold surface as the molding tool continues through a complete rotation to return to its starting orientation, while excess unmelted powdered resin returns to the powder box14through openings in the grate16. When a sufficient amount of powdered resin has been deposited to form the desired molten layer thickness, the molten layer can be solidified by cooling or by cross-linking or curing of the resin. Cooling may be performed by flowing a cooling fluid through one or more temperature control lines22to cool mold surface26. Cooling can either solidify the molten layer of resin to transform it into a skin that can be removed from the mold surface after mold12and powder box14are separated, or it can simply cool an already solidified layer of resin so that it can be removed from the mold surface. Of course, the molten layer or solidified layer can be cooled using other methods such as direct forced air cooling of the molten layer or other methods.

The method steps outlined above do not necessarily have to be performed in the order presented. For example, the powder box14can be separated from the mold12any time after a molten layer of sufficient thickness has been formed on mold surface26. Heating of the mold surface26can continue even after the separation, for example to cure the resin. In some cases, such as cases where curing or cross-linking of the resin occurs with additional heating, the skin is formed prior to any cooling of the mold surface. As another example, the mold surface26may be heated after mold12and powder box14are connected together. In some embodiments, additional molding tool rotation steps can be included after the powder box14and mold12are separated to use gravity to more evenly distribute the molten layer of resin on the mold surface26during heating or cooling. Successive rotations can be performed in opposite directions as well. Additionally, the use of a flow restrictor16, such as a grate, to restrict the free flow of powdered resin can allow some non-traditional powder molding techniques. For example, a flow restrictor such as one described herein could be placed above a heated mold surface, and a sufficient amount of powdered resin could be moved across the top side of the flow restrictor, such as in a wiping motion, so that powdered resin is deposited onto the mold surface through openings in the flow restrictor in an evenly distributed manner.