Insert Molded Bezel and Method of Manufacturing

The invention relates to molding of plastic parts, and more particularly to insert molding using preprinted sheets.

To reduce the complexity and cost of manufacturing plastic parts, "in-mold decorating" has been used to eliminate secondary printing steps, and to avoid multiple injection steps using conventional insert molding. For instance, an automotive instrument bezel may require printed legends, back-lit translucent legends, and clear display windows in a single molded plastic part otherwise appearing opaque black or any other color. Using in-mold decorating, a plastic applique sheet is preprinted with the desired background color, legends using colored or translucent inks, and clear portions. The applique is thermoformed to the approximate shape of the finished bezel (if the bezel is not substantially flat), and is trimmed using a die cutter or steel ruled die to trim the periphery and to form any windows or switch apertures. The formed applique is inserted into an injection mold, and clear plastic is injected behind the applique, forcing it to take the shape of the mold, so that the applique forms the external surface of the bezel.

Using this conventional method, the edge of the applique does not perfectly correspond to the edge of the bezel. Mechanical tolerances of applique cutting and other steps require that some clearance be provided between the edge of the applique and the mold surface that defines the edge of the finished part. In the absence of such clearance, the edge of the applique may be crushed and buckled by the closing mold, yielding defective parts. In addition such crushing leads to molding tool damage or premature wear. In practice, the clearance gap is uneven due to the unpredictability of material flow and applique cutting and stretching during molding. This causes a wavy black edge visible in the clear molding material which is unattractive, particularly at corners of a bezel where stretching is most unpredictable. For applications in which the edge of the bezel is concealed, such as an instrument panel recessed into an automotive dash board, the wavy edge is concealed, and is of little concern. However, for applications such as hand held instrument housings where all surfaces are visible, and a precision appearance is required, current in-mold decorating techniques are inadequate.

The embodiment disclosed herein overcomes these limitations by providing a method of manufacturing a plastic shell by providing a printed applique having a central portion with a peripheral skirt portion having an edge, then inserting the applique into the recess on a first side of a mold. The skirt of the applique has a length greater than the depth of the recess, so that the edge of the skirt extends beyond the parting line. The second side of the mold is closed to enclose the recess, and has a second recess that receives the edge of the skirt on the second side of the parting line. Plastic material is injected into the closed mold pressing the applique against the first side, and providing a bezel with an applique that covers the exterior of the front portion, and which extends to cover the skirts beyond the parting line.

FIG. 1 is a sectional side view of a hand held instrument case according to a preferred embodiment of the invention.

FIG. 2 is an enlarged sectional view of the embodiment of FIG. 1.

FIGS. 3, 4, and 5 are sectional side views of a sequence of manufacturing steps for producing the embodiment of FIG. 1.

FIG. 6 is an isometric view of an applique portion of the embodiment of FIG. 1.

FIGS. 7 and 8 are enlarged sectional views of a corner portion of the embodiment of FIG. 1 at a first manufacturing step.

FIGS. 9 and 10 are enlarged sectional views of a corner portion of the embodiment of FIG. 1 at a second manufacturing step.

FIG. 1 shows a hand held instrument with a housing 10 having a front bezel 12 and a rear cover 14 enclosing a chamber 16 containing electronic circuitry and components. Although a multitude of housing configurations may be used without departing from the principles of the invention, the illustrated housing has a rectangular box-like shape.

The front bezel has a generally flat central panel portion 20, with an integral side skirt panel 22 depending downward at each edge. The upper surface of the central panel portion 20 is covered with a thin plastic film applique 23 that extends over the edges to cover each of the side skirts. The applique may be tinted to provide the desired color of the finished housing, and may have printed legends, or unprinted portions providing visible text or legends against the printed background. As shown, the applique may define a window aperture 24 that reveals the bezel material behind; in this case, a transparent thermoplastic material such as polycarbonate provides a window for viewing an electronic display 26 in the chamber 16. Alternatively, windows may be created without an aperture in the applique by leaving an unprinted window portion of the applique, and by coating the exterior surface with a scratch resistant coating to preserve transparency of the applique material. A hole 30 through both the applique and the bezel material provides mechanical access for a switch 32. In the preferred embodiment, the applique has been vacuum formed to approximate the finished shape, and trimmed to provide relatively even edges.

FIG. 2 shows details of the joint or seam between the front bezel and the rear cover. The bezel side skirt has a major exterior surface 34 that may be nearly vertical or perpendicular to the central panel portion, but which has a slight draft angle so that it faces slightly upward from a lateral direction. The surface 34 joins an angled second surface 36 at an edge 40 coinciding with a parting line 42. The second surface 36 is angled laterally and downwardly away from the parting line, at about a 45 degree angle as illustrated. A third surface 44 extends downward more vertically, and terminated at a downward-facing edge face 46.

The rear cover 14 mates with the bezel, and defines a ledge 50 and inner wall 52 forming a step receiving the edge of the bezel and contacting the third surface 44 and edge 46. A rim 54 covers the third surface and part of the second surface of the bezel skirt, and has an angled surface 56 that faces upwardly and laterally at about a 45 degree angle. Together, the cover surface 56 and bezel second surface 36 define a V-shaped groove 57 providing an aesthetic reveal feature that makes slight dimensional variations undetectable relative to the width of the groove, analogous to a conventional rectangular channel reveal feature.

The edge of the rear cover reveals only part of the sloped bezel second surface 36, covering the portion of the bezel that extends beyond a coverage line 58 at the vertex of the V-shaped groove. The applique 23 has an edge 60 that extends beyond the coverage line 58 so that the edge is hidden from view. Because the applique is likely to have dimensional variations along its edge and between different parts, the edge extends by more than the expected manufacturing tolerance. In addition to providing a concealed edge for aesthetic benefits, the applique effectively covers edge 36 at the parting line 42 around the entire periphery of the bezel. This captures and prevents any molding flash from forming at the edge, reducing the need for trimming, even as a molding tool wears.

FIGS. 3 through 5 show the process of insert molding the front bezel of the preferred embodiment. A molding tool having a first part 62 and a second part 64 are movable toward and away from each other along an axis 66 between an open position (shown) and a closed position. The first tool part defines a cavity 70 having a depth corresponding to the depth of the bezel beyond the parting line 42. The second tool part 64 defines a channel 72 shaped to form the bezel portion below the parting line. The channel surrounds a core 74 that largely fills the cavity 70 when the tool is closed, and which forms the chamber in the finished bezel. Spaced apart at intervals along the channel are a plurality of sloped guide elements 76 having a sloped surface facing upwardly and medially, coplanar with a mold surface 36' that forms surface 36 in the finished product. A corresponding number of closely sized pockets 80 are defined in the opposite surface of the first tool part 62. While the guide elements might be elongated to extend about the entire channel without spaces in between to provide a sloped parting line, this is undesirable because it would require a knife edge at the parting line of the upper tool portion, creating a tool maintenance and life disadvantage.

The applique 23 has been vacuum formed to provide an open box shape that fills the cavity 70 at the upper surface, but which does not flare outward to as great of a draft angle. The lower edge 60 of the applique extends laterally only as far as it is intended to be position in the finished product, which avoids the crushing or buckling of the edge that might occur in the corners of the bezel otherwise if the sheet edge were forced medially during processing. The edge 60 extends beyond the parting line 42 of the first tool portion 62 so that it first contacts the sloped surface 36 as the tool is closed. If the sides of the applique have bowed slightly, the sloping surface acts as a wedge to position the applique edge upon closure. The guide elements 76 serve the same purpose to correct any significant bowing that may occur along a long straight edge of the applique, preventing the applique edge from being crushed between the tool halves.

As shown in FIG. 4, the tool halves have been shifted to the closed position, and the applique edge is pressing on mold surface 36'. As shown in FIG. 5, a molten transparent plastic material 82 has been injected into the space between the applique and the mold core 74. The pressure and heat of the injected plastic and the flexibility of the applique causes the applique to be forced laterally against the mold surfaces, and to fill the angled space at the parting line. Along the intermediate portions of the straight sides of the bezel, the applique behaves as shown, bending only in two dimensions without stretching or compressing over its area.

However, at the corners, or over any portion of a periphery having a rounded profile as might be required in alternative designs, the applique must either stretch or compress from its original vacuum formed shape to conform to the shape of the mold. Particularly, the applique must flare outward as it conforms to the skirt above the parting line, but flare inward below the parting line. With the rapid process of injection molding, the applique does not have sufficient time to stretch readily to generate the outward bulge at the parting line at the corners. To ensure that the applique edge extends low enough to be covered by the edge of the lower cover, the lower edge of the applique is formed initially to the dimensions intended in the finished part. Thus, the edge will not be appreciably stretched or compressed. However, the intermediate portions of the skirt at the corner 84 will require stretching to extend fully at the parting line. Because this is beyond the capabilities of the applique material under normal molding conditions, the skirt at the corner 84 is provided with several vertical pleats or flutes 86. These unfold into a smooth surface sized to fill the mold corner without substantial stretching. The flutes are deepest at the parting line, allowing greatest stretching where needed, and taper toward the edge and central panel portion of the applique. FIGS. 7 and 8 illustrate the pleats and their appearance before the plastic material is injected, and FIGS. 9 and 10 illustrate how the pleats are unfolded to a flat condition.

In the preferred embodiment, the applique is formed of polycarbonate material, with a thickness of 0.015 inch (0.38 mm.) The non conductive nature of the material ensures compliance with safety standards for electrical instruments, and the thickness permits conformal flexing not possible with more rigid material alternatives. The plastic molding material is preferably also polycarbonate material. The bezel is 3.25 inches (83 mm) by 6 inches (152 mm) by 0.85 inches (22 mm) overall, with a typical wall thickness of 0.10 inch (2.5 mm.)

The applique and molding materials are selected to be compatible. Under the heat and pressure of molding, a thin layer of the applique melts, creating a bond that provides effective adhesion. In the preferred embodiment, the materials are the same formulation of polycarbonate, although other paired materials may be used. Non-identical but compatible materials such as polycarbonate-ABS may also be used in conjunction with polycarbonate. When the rear surface of the applique is printed with legends and the like, the printing ink also becomes part of the melted layer, and should be selected for compatibility to avoid compromising mechanical integrity. When non-compatible ink is required, it should be printed on the external surface of the applique.

While the disclosure is made in terms of a preferred embodiment, the invention is not intended to be so limited.