Heating – Processes of heating or heater operation – Treating an article – container – batch or body as a unit
Reexamination Certificate
2001-05-16
2003-11-25
Wilson, Gregory A. (Department: 3749)
Heating
Processes of heating or heater operation
Treating an article, container, batch or body as a unit
C432S018000, C264S040500, C264S040600, C425S144000, C425S149000
Reexamination Certificate
active
06652270
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for impregnating porous articles with a curable impregnant composition. More particularly, this invention relates to a process for heat curing impregnated porous articles which includes making successive incremental temperature and pressure increases, each of which are sustained during the curing process. The invention includes a heat curing system having a chamber for curing impregnated porous articles and, desirably, a heat transfer medium such as a liquid for immersing the impregnated porous part to be heat cured.
BACKGROUND OF THE INVENTION
It is often desirable to form parts from lightweight metals in order to reduce the weight of a component or system and correspondingly reduce energy consumption as well as the costs of manufacture and maintenance thereof. With the advent of new machining technologies and emphasis on the environmental impact of power usage, more and more lightweight metals are being machined for more and more uses requiring these metals to perform multiple functions simultaneously. Examples of such metals include but are not limited to zinc, copper, brass, iron, aluminum, and various alloys. The terms “porous part” and “porous article” are used synonymously herein to refer to components made from such metals.
An inherent problem with the use of lightweight metals is the presence of micropores which inhibit commercial viability. The occurrence of micropores is especially prevalent in components formed from metal powder. Porosity of porous parts is particularly problematic when such porous parts are utilized in fluid power systems or other liquid applications, where entrance of fluid in micropores can cause premature deterioration and fracture of the part. Other problems include the introduction of air and gas which may create processing or finishing difficulties as well as difficulties in the end use of the porous member.
In response to these problems, impregnation sealing technology emerged as a way to eliminate the micropores inherent in lightweight metal components yet retain the desirable performance characteristics thereof. During an impregnation sealing process, the porosity of porous articles is impregnated with a curable sealant composition, or “impregnant”. Upon curing of the impregnant, the resulting sealed part is suitable for use in fluid exposure applications, as well as facilitating plating, coating and further processing of formed articles. The structural integrity of a porous part can also be enhanced through impregnation sealing. Sealing of porous parts maintains many advantages, including: rendering the parts leak-resistant or leak-proof; preventing or minimizing the incidence of internal corrosion in metal castings and sintered parts; increasing density to make the article capable of withstanding liquid or gas pressure during use; improving its strength; and preparing the surface of the article for a subsequent painting or plating operation.
The practice of using a liquid impregnant for the purpose of sealing the porosity of porous articles is a well-known and highly utilized process. Often an impregnation process is followed by an independent curing process. The curing process is conducted independently of the impregnation process to initiate and/or accelerate polymerization of an impregnant composition. Although cure can be accelerated by several factors, such as instantaneous temperature increases and removal of ambient air, the present disclosure is specifically concerned with heat curing processes and problems posed thereby.
A typical impregnation process is shown and described in U.S. Pat. Nos. 3,672,942, 4,416,921 and 5,273,662, all of which are incorporated by reference herein. To execute the conventional steps in the impregnation of a given part, the part is initially degreased and cleaned, then the cleaned part is subjected to vacuum aspiration in a vacuum tank, thereby attempting to remove entrapped air from the minute pores in the part. During immersion of the part in a bath of a curable liquid impregnant, such as an anaerobic or heat curable impregnant, the part is maintained in a vacuum. Subsequently, the immersed part is exposed to atmospheric pressure, thereby causing the liquid impregnant to permeate the minute pores of the part. Any residual liquid impregnant is returned to a storage reservoir and the part which has undergone the impregnation is centrifuged to expel any excess impregnant adhering to the surface thereof. Thereafter, the part is generally cleaned with detergent to remove liquid impregnant remaining on the surface of the part while leaving the impregnant within the pores.
The impregnated part is then conventionally subjected to a curing process, usually at elevated temperatures, to initiate and/or accelerate cure of the impregnant. This curing process is conventionally conducted at a standard temperature for the cure of the chosen impregnant. The impregnated part is placed in a curing chamber wherein curing temperature and pressure levels are pre-set in accordance with selection of part configuration, type of impregnant, end use of the part and other factors. The pre-set temperature and pressure levels are maintained for a time interval sufficient to initiate and achieve a desired level of cure, after which the parts are removed from the curing chamber and often subjected to post-cure treatments such as plating, painting and the like. Impregnation and curing processes are conventionally conducted in separate vessels, however, a common vessel may be employed during an impregnation-curing sequence.
Curing processes can and often are executed after completion of any of several types of impregnation processes. Conventional impregnation processes are accomplished generally by three methods: wet vacuum impregnation, wet vacuum/pressure impregnation or dry vacuum/pressure impregnation. Among these impregnation methods, wet vacuum impregnation techniques are generally employed more frequently than the dry vacuum/pressure method described herein. However, the steps required to complete each of these processes are similarly executed. To effectively illustrate the conventional impregnation processes, examples of such processes are schematically depicted in the flow diagrams of
FIGS. 1 and 2
. The numbers assigned to
FIGS. 1 and 2
are indicative of the different operations or steps performed sequentially on a single containment vessel which is stationary.
During a conventional wet vacuum impregnation procedure (hereinafter “WV process”) as shown in
FIG. 1
, porous parts are placed in a single container or basket at Block
10
. The parts and the vessel are then inserted into an impregnation chamber at Block
12
where both parts and basket remain stationary for the duration of the impregnation process. At Block
14
, the parts are submerged into a vacuum tank substantially filled with a flowable sealant composition. While the parts are in the vacuum tank, a short term vacuum cycle removes air from the porosity of the parts at Block
16
. The duration of the vacuum cycle is dependent upon the material characteristics of the part being treated and the type of sealant used as an impregnant. In this arrangement, the goal is to remove air from the pores of the part to allow impregnant to follow thereinto once the pressure is normalized to ambient pressure. The chamber is then returned to ambient pressure so that sealant penetrates the evacuated porosity of the parts. At Block
18
, the parts may then be spun briefly in the basket to eliminate excess sealant from the part surfaces and to prevent undesirable surface curing of the impregnant thereon during the cure cycle.
The conventional wet vacuum/pressure impregnation process (hereinafter “WVP process”) has many common steps to the WV process shown in
FIG. 1
with the difference being shown at Block
17
where the impregnation chamber is pressurized after the completion of the vacuum cycle at Block
16
. Pressurization forces the sealant further into small porosity passages. The centrifuge step at Block
18
Bauman Steven C.
Henkel Loctite Corporation
Wilson Gregory A.
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