Plastic and nonmetallic article shaping or treating: processes – Vacuum treatment of work – To degas or prevent gas entrapment
Reexamination Certificate
1998-12-15
2002-02-05
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Vacuum treatment of work
To degas or prevent gas entrapment
C264S219000, C264S236000, C264S299000, C264S313000, C264S317000, C264S319000
Reexamination Certificate
active
06344160
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of molding composite, structural plastics and the objects fabricated thereby. In particular, this invention relates to a method of casting plastic commercial components in conventional metalcasting molds without the need for injection or compression molding. This invention further relates to the rapid fabrication of prototypes in composite, structural plastics using conventional soft tooling or rapid prototyping techniques.
BACKGROUND OF THE INVENTION
The use of molds to create parts of varying size, quality and implementation pervades the industrial landscape. Over 4000 metal casters cast over 32 billion pounds of metal annually in metal foundries all over the world. In the foundries, metals are processed into commercially viable shapes by melting and pouring a molten metal into a mold. In this manner, structural items can be fabricated from steel, iron, copper, aluminum and like materials for a virtually limitless variety of applications.
Selection of a specific metalcasting process depends upon several factors, including, without limitation, the metal or alloy selected, casting size and complexity, surface finish, dimensional tolerances, production quantities and cost constraints. In addition, selection of a mold must anticipate whether the process uses expendable molds which are used only once and then discarded (i.e. in sand casting operations) or metal molds intended for repeated use (i.e. as used in permanent molding and diecasting). No matter what metalcasting process is used, however, all processes share two main objectives: the pattern must be removable from the mold without damage, and the casting must be removable from the mold or die without damage to either of the die or the casting (see 1996 CD&A Reference handbook). Various metalcasting processes are described hereinbelow.
1. Conventional Metalcasting Processes:
a. Sand Casting
Sand casting metal is the backbone of several predominant industries, such as the automotive industry, because the materials and the tooling used in the process are inexpensive and rapidly produced. More than 80% of all castings made in the United States are produced by green sand moldings (see 1996 CD&A Reference handbook). The term “green sand” denotes a mixture of raw sand and a binder that has been tempered with water.
Sand molding is a multipurpose metal-forming process in which a pattern is made of wood, metal or plastic based upon the design specifications of the casting. In a conventional sand casting process illustrated in
FIG. 1A
, a pattern
10
is usually constructed in two parts which include a bottom part
10
a
and a top part
10
b
to allow ease of pattern removal from a mold. Parts
10
a
and
10
b
are aligned with each other using a plurality of registration pins
13
.
Referring to
FIG. 1B
, bottom part
10
a
of the pattern is placed upside down on a molding board
16
. In this way, the pattern defines a desired shape within a bottom half
18
a
of a mold
18
. The bottom half
18
a
of mold
18
is then filled with green sand
19
as shown in FIG.
1
C. Sand
19
is compacted firmly around and over the pattern by manual or mechanized compression means, such as a ram
21
. The bottom half
18
a
of mold
18
is then inverted and set on a board or pallet
24
, and molding board
16
is removed therefrom, as seen in FIG.
1
D. Top part
10
b
of the pattern is aligned with bottom part
10
a
and set using a plurality of alignment pins
26
. The separation of the pattern parts defines a recognizable parting plane
27
therebetween such that a shape is defined in the top half
18
b
of mold
18
that is substantially symmetrical to that defined in the bottom half .
As shown further in
FIG. 1E
, the top half
18
b
of mold
18
is filled with sand
19
which is then compacted over and around the top part
10
b
of pattern
10
with ram
21
. A vertical channel or sprue
27
is cut into the top half
18
b
of mold
18
to provide an ingress for pouring molten metal into a mold cavity. Mold
18
is then parted along parting plane
27
, and pattern
10
is removed therefrom.
As depicted in
FIG. 1F
, a horizontal channel or runner
29
is cut in the lower half
18
a
of mold
18
so as to be in communication with sprue
27
to accommodate flow of molten metal therethrough. If it necessary to compensate for metal shrinkage during the process, one or more risers
31
can also be cut into the mold. As further illustrated in
FIG. 1G
, a sand core
33
is set in place and positioned using core prints
14
that are created in the mold by pattern
10
(shown in FIG.
1
A). Mold
18
is finally closed thereafter and ready to produce a casting.
Sand casting can be used to mold a wide range of materials having considerable complexity. The low tool and die costs associated with this method, coupled with the ability to produce varying lot sizes of materials (i.e. a few pieces or huge quantities can be produced) make sand casting desirable for a wide range of applications. However, a significant disadvantage of this type of molding process is that the mold is a single use mold which inhibits high volume production. Furthermore, the use of binder within the sand anticipates the release of toxic substances into the environment upon removal of the binder and disposal thereof. The low tool and die costs are compromised by high labor and finishing costs which are incurred during the production cycle.
b. Permanent Molding
In permanent mold casting (also known as “gravity diecasting”), a metal mold consisting of at least two parts is repeatedly used for components that require high volume production. A conventional permanent mold arrangement is illustrated in FIG.
2
. Up to 99% of such molds currently in use are made of steel or plaster; however, these mold molds may also be constructed of cast iron, graphite, copper or aluminum.
Molten metal is poured into a mold
41
having mold halves
41
a
and
41
b
and a core
43
. Mold
41
is a permanent mold wherein the metal cools more rapidly than in a sand mold and produces a finer grain structure with enhanced mechanical properties and tighter dimensional tolerances. As can be seen from
FIG. 2
, mold
41
emulates the sand casting procedure described hereinabove, except that a material such as metal or plaster is used in place of sand. In this process, a mold configuration
44
is formed in the mold which corresponds to the desired configuration of the cast product. A sprue
45
is defined for pouring of molten metal into the mold cavity defined by mold configuration
44
.
Although the permanent mold process has moderate labor costs and low finishing costs, the problems associated with this procedure include limitations on casting size coupled with high initial tooling costs, which make the process prohibitively expensive for low production volumes. In addition, several alloys and shapes are not amenable to permanent mold casting due to part line location, complex undercuts in the design or difficulty in removing the casting from the mold. Lot size is limited to large quantities, making the process untenable for small scale molding. Furthermore, mold coatings which are often required to protect the mold from erosion, cracking and other forms of metal degradation can deleteriously effect surface finish.
c. Diecasting
Diecasting is a permanent molding process is primarily for high production of intricately-designed components cast from zinc, lead, tin, aluminum, copper or magnesium. There are two types of diecasting machines: cold chamber (illustrated in
FIG. 3A
) and hot chamber (illustrated in FIG.
3
B). In either method, a molten alloy
51
is manually or automatically poured into a shot well
53
A or
53
B and injected into a die
55
A or
55
B under pressure. The locking force in diecasting machine operation keeps the die halves firmly closed against the injection pressure exerted by a plunger
57
A or
57
B as the plunger injects the molten metal.
As further shown in
FIG. 3A
, during cold chamber diecasting, molten metal is hel
Compcast Technologies, LLC
Hoffman & Baron LLP
Vargot Mathieu D.
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