Background of the Invention In an attempt to increase vehicle fuel efficiency, many automotive engineers have focused on vehicle weight reduction. To this end, many components which traditionally were made from stamped steel are now made, either in part, or in full, from lighter weight materials such as aluminum and plastic. One category of vehicle components which is particularly well-suited to incorporate the use of plastics are nonstructural panels such as interior door and dashboard members and exterior trim components such as bumper facia panels.

Although the use of plastic components has greatly contributed to vehicle weight reduction, traditional methods have proved only marginally satisfactory for producing a plastic component where a high quality surface finish is important. The shortcomings associated with traditional forming methods is due primarily to the inability to remove air which is trapped between the plastic sheet and the tooling which is used to form the plastic sheet. In applications where it is imperative that a plastic part possess a high quality finish, removing trapped air from the surfaces of the forming tools is critical. If trapped air is allowed to remain between the surface of the forming tools and the plastic sheet, the presence of the trapped air produces surface irregularities in the finished part thereby making it difficult, or impossible, to produce a smooth, high quality surface on the plastic component.

Summarv of the Invention The method of the present invention includes forming a sheet of plastic material by first placing the plastic sheet between first and second mold portions and heating the plastic sheet. When the sheet has achieved the desired temperature the sheet is clamped between the two mold portions and a vacuum is applied to a void which is formed between the sheet and one of the mold portions. Thereafter, a second vacuum is applied between the void formed between the sheet and the remaining mold portion. Lastly, a pressure is applied in the first void wherein the pressure is of sufficient magnitude to urge the sheet material away from the first mold portion and toward the second mold portion. By combining the use of vacuum, pressure, and temperature in the above-referenced sequence, it is possible to form plastic parts which have a high quality surface finish.

Brief Description of the Drawings Figures 1 through Figures 5 depict, schematically, the method of the present invention.

Detailed Description of the Preferred Embodiment Now referring to Figure 1, in step one of the present method, upper mold 10 is displaced 12 from lower mold 14. Plastic sheet 16 is manipulated between upper mold 10 and lower mold 12 and is ready to be acted upon by molds 10,12. Preferably plastic sheet 16 has been preheated (preheating step not shown) in an oven, or the like to a temperature which is suitable for forming plastic sheet 16. Although it is contemplated that heating plastic sheet 16 in an oven is the most preferable method of heating, it is contemplated that other forms of heating plastic sheet 16 are equally suitable (such as heating plastic sheet 16 with molds 10,14, or the like). In an actual application of the method of the present invention, a automotive grade TPO was used,

wherein, proper temperature for forming the plastic sheet range between 325° and 400° F. However, the precise temperature ranges are largely dictated by the composition of plastic sheet 16 and such temperature ranges are well known to those skilled in the art of forming plastics.

Upper mold 10 and lower mold 14 include a plurality of passages 18,20 which emanate from a respectively associated manifold 22,24.

Now referring to Figure 2, in step 2, upper mold 10 and lower mold 14 are manipulated towards each other such that plastic sheet 16 is hermetically trapped therebetween. This hermetic sealing is accomplished by way of lid 26 which is urged against shoulder 28 of lower mold 14. The clamp pressure exerted by lid 26 against shoulder 28 should be sufficiently high enough to resist the leakage of pressurized air which is to be introduced between upper mold 10 and lower mold 14. Thus, the clamp pressure is a function of the size of lid 26 as well as a function of the air pressure used in the method of the present invention. Clamp forces between 100,000 and 150,000 pounds-force would not be unusual.

Now referring to Figure 3, in step 3, vacuum V, is applied to port 30 while simultaneously port 32 is blocked. Manifold 22 in conjunction with passages 18 are effective for distributing vacuum Vl across the entire surface area of upper mold 10.

This vacuum urges plastic sheet 16 against the outer surface of upper mold 10 while simultaneously pulling sheet 10 away from the outer surface of lower mold 14.

During this vacuum stage, port 34 is preferably vented to atmosphere so that any movement of sheet 16 will not be impeded by air trapped within passages 20 or manifold 24. In the preferred embodiment upper mold 10 and lower mold 14 are sized, relative to one another, such that a thin air space 36 resides between outer surface 38 of plastic sheet 16 and concave surface 40 of lower mold 14. This air space assists in minimizing the heat transferred from sheet 16 to the surrounding environment. In a preferred embodiment, upper mold 10 is comprised of materials, or material coatings having very low thermal conductivity and diffusivity to impede the heat loss from plastic sheet 16. Such materials are wood, ceramic, plastic or the like.

Maintaining plastic sheet 16 at the proper, elevated temperature is essential for achieving a plastic part having a high quality surface finish.

Now referring to Figure 4, in step 4, a vacuum V2 is applied to port 34 of lower mold 14. This vacuum is maintained for sufficient duration to evacuate the air present in air space 36. In a preferred embodiment, V, is greater than V2 to prevent sheet 16 from moving toward lower mold 14 until a pressure is applied to port 32.

Premature movement of plastic sheet 16 to lower mold 14 can cause uneven cooling of plastic sheet 16 thereby resulting in surface defects in the finished plastic part.

Now referring to Figure 5, vacuum V, (which in Figure 4 was applied to port 30) is turned off (blocked) and pressure P, is applied to port 32 of manifold 22.

Vacuum V2 is maintained. Pressure Pl is distributed from port 32 along the surface of upper mold 10 by way of manifold 22 and passages 18. Pressure Ps is effective for urging plastic sheet 16 away from upper mold 10 and forcing plastic sheet 16 against concave surface 40 of lower mold 14. As plastic sheet 16 is maintained in this position against lower mold 14, it begins to cool and harden thereby taking on the form of the finished plastic part. Preferably, the rate of cooling of the plastic part is accomplished by controlling the temperature of lower mold 14. Those skilled in the art are well versed in methods for controlling mold temperatures using water, oil, or other cooling mediums. Mold pressures (P) ranging between 80 and 100 PSI have been used to generate finished plastic products having excellent high quality surface qualities. However, it is recognized that mold pressures greater or less than this range may be appropriate depending on the size, geometry, composition, and thickness of the plastic part being formed. In a preferred embodiment, diameter 42 for passages 20 is in the range of 1/10,000th of and inch. It is critical that the diameter of these passages be small (at least where they intersect concave surface 40 of mold 14) so that they do not form a void large enough that plastic sheet 16 is deformed therein.

Although a plurality of passages 20 is shown in lower mold 14, it is possible to construct it using only a single passage.

The foregoing detailed description shows that the preferred embodiments of the present invention are well-suited to fulfill the objects of the invention. It is

recognized that those skilled in the art may make various modifications or additions to the preferred embodiments chosen here to illustrate the present invention, without departing from the spirit of the present invention. Accordingly, it is to be understood that the subject matter sought to be afforded protection hereby should be deemed to the extent of the subject matter defined in the appended claims, including all fair equivalents thereof.