Static molds – Including means within surface to confine heat exchange medium – Mold part of particular material
Reexamination Certificate
1998-09-10
2001-04-03
Mackey, James P. (Department: 1722)
Static molds
Including means within surface to confine heat exchange medium
Mold part of particular material
C249S111000, C249S114100, C264S338000, C065S374110
Reexamination Certificate
active
06209847
ABSTRACT:
FIELD OF THE INVENTION
The present invention broadly relates to the mechanical locking and constrainment of an active layer on a given solid surface, such as a metal surface, wherein the active layer is formed of a metal matrix composite by infiltration of a powder with molten infiltrating alloy. The mechanical locking and constrainment is achieved by modifying an otherwise smooth solid surface to contain selectively positioned regularly defined geometrical features including but not limited to shapes such as rectangular grooves, channels, cylinders, boxes, pyramids and/or triangular types features. Such regularly defined geometrical features, which can be more broadly considered as protrusions or indentations, uniquely contribute to the enhanced locking and constrainment of the active layer as described herein. Accordingly, molds for different production applications, including plastic injection molds, die casting molds, glass forming molds, blow molds etc. can be uniquely prepared according to the present invention and are made to more efficiently lock an active layer on their associated surfaces. In addition, the present invention relates to metal matrix composites, mainly on a carbide base, with sharply improved wear and corrosion resistance, high hardness and mechanical strength. The present invention also discloses multi-layered structures made from different metals or alloys for fabrication of articles with a specific distribution of properties within an article. Finally, the present invention relates to improved mold designs, particularly, the design of a conforming heat-transfer system of molds.
BACKGROUND OF THE INVENTION
Many products required by today's markets are made by molding parts of plastics and other materials. One of the most costly and time-consuming steps in the manufacture of these products is the preparation of molds. A most common mold design generally consists of working inserts of a core and cavity, which contain a working surface with critical requirements of dimensional tolerance and surface finish. Also, inserts contain parting lines and a mold frame, which provides structural integrity to the mold element. In order to prepare molds having an active surface that provides accurate reproduction of the products with every manufacturing cycle, the active mold surface must have a hard detailed finish which in most processes requires complex machining. Such molds must be long lasting so that their cost can be amortized over as large number of manufacturing cycles as possible. All of the problems of mold production are exaggerated when the product has intricate well-defined surfaces to replicate.
In today's world of computer aided design and modeling almost every step in the manufacturing process has been upgraded to accommodate high-speed operations. This presents a real dilemma for toolmakers, as the extended periods required for mold manufacture remains a serious problem. Mold making therefore remains the only low speed operation in the manufacturing process. For example, a typical tooling operation may be optimized to about 14-18 weeks preparation time.
As will be seen, improvements in tooling and surface finish thereof represents one of the purposes of this invention, which is to provide a high-speed method of mold making with a savings in time of about 20 to 30 percent. In addition, mold lifetime is an important factor for many tooling applications, particularly for die-casting or glass forming processes. The present invention is intended to improve significantly such tool life due to the use of new effective tooling materials and new techniques to improve adhesion of the active layer of the mold to the mold substrate surface. Furthermore, in many tooling applications, the function of heat removal is critical as it defines the productivity of the manufacturing process and quality of some cooling-sensitive materials like glass, nylon etc. The present invention, therefore, has the additional object to offer solutions to such issues, which leads to improvement to injection mold heat transfer.
In order to create fine detailed mold elements, and to minimize finish machining, powdered metal casting has become a popular process. The use of powdered metals to make a tools is well established as shown in U.S. Pat. No. 4,327,156; 4,431,449 and 4,455,354. In the methods of the prior art, as disclosed by these patents, a mold or tool element is constructed of a skeleton made of a first metallic powder preform having microscopic interconnected pores filled with a second infiltrating metal. Tool elements of this type are strong, reliable, and can be used to form many complex and detailed shapes. The choice of metal used depends on the application of the tool and can include almost an infinite number of combinations.
A process of manufacturing mold elements is disclosed in the above referenced '354 patent, and is worthy of detailed review. This method consists of constructing a master model of a product to be replicated and forming a mold of rubber or other flexible material therefrom. Appropriate metal powders are mixed together with a heat fugitive thermoplastic binder and then the powder-binder mixture is applied into a rubber mold, and allowed to chill at room temperature. Following low temperature heating of the powdered-binder the preform burns off binder and forms a porous skeleton to be filled with an infiltrating alloy. At a next higher temperature heating, the porous preform is filled with molten infiltrating alloy under capillary forces. At both stages of heating, the powder preform is surrounded and supported with another powder, which does not wet and infiltrate with the molten infiltrating alloy. Therefore an infiltration front stops at a border between wettable and non-wettable powders. Due to very fine particles used in this powder system, a smooth surface with any type of detail can be obtained. Such surface requires a polishing only to be accorded the requirements of a tool.
The above mentioned infiltration method provides the ability to compose materials with different useful properties. For instance, applying carbide powders allows preparation of a high wear resistance tool. The use of corrosion-resistance-infiltrating alloy allows combination of a high wear and corrosion resistance etc. Therefore, the '354 disclosure and related patents completely accord to the requirements of toolmakers from the point of view of surface quality and specific properties of tooling materials. However, these methods do not satisfy a third critical requirement: dimensional accuracy of infiltrated articles. This requirement is so critical that its omission overcomes the above mentioned advantages. The dimensional accuracy problem has therefore stimulated efforts to increase tolerances and dimensional predictability of infiltrated products. Nevertheless, in spite of a number of attempts, there has not yet been achieved a successful result. In fact, a dimensional threshold of acceptable tolerance at +/−0.1% from a total size has yet to be reported.
Because the overcoming of this dimensional threshold is one of the important objects of the present invention, a detailed analysis of sources of inaccuracy, by way of background, is reviewed below.
First, there are several technological steps at the fabrication of infiltrated articles, which are reflected in '354 and similar patents with some variations: making a rubber mold, preparation of a powder preform, burning off a temporary binder and infiltration followed by solidification and cooling. Each link of this technological chain brings its effect on total inaccuracy.
The loss of accuracy starts from a dimensional non-stability of the rubber mold during its fabrication. Then, thermal instability of the rubber mold and non-uniformity of shrinkage of the powder-binder mixture also generates an inaccurate replication of the powdered tool preform. Furthermore, the evacuation of binder by heating the powdered preform in a non-wettable powder medium, according to '354 patent, present
Brookfield Innovations Inc.
Hayes, Soloway, Hennessey Grossman & Hage, P.C.
Mackey James P.
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