Method and mold assembly for producing a molded object

Details Method and mold assembly for producing a molded object Method and mold assembly for producing a molded object

2000-09-13

2004-03-09

Drodge, Joseph (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Preform reshaping or resizing means: or vulcanizing means...

By fluid pressure actuated flexible diaphragm

C425S405100, C425S555000

Reexamination Certificate

active

06702565

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This document claims priority and contains subject matter related to Japanese Patent Application No. 11-258192 filed in the Japanese Patent Office on Sep. 13, 1999, the entire contents of which being incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and mold assembly for producing a molded object of plastic optical elements such as a lens, a mirror, and a prism.
2. Discussion of the Background
Metallic mold assemblies that employ an injection molding method to produce plastic optical elements such as plastic lenses with high accuracy are known. Known methods of producing a molded object
21
of a plastic optical element using a mold assembly
1
are shown in
FIGS. 8A through 8C
. As illustrated in
FIG. 7
, the molded object
21
has mirror surfaces
22
and
23
as optical surfaces, as well as non-optical surfaces
24
and
25
. Referring to
FIG. 8A
, the mold assembly
1
includes a fixing-side metallic mold
2
providing a cavity piece
8
(i.e., a molding piece for forming a cavity) and a moving-side metallic mold
3
providing a cavity piece
9
. The cavity pieces
8
and
9
have transfer surfaces
8
a
and
9
a
, respectively. A pair of porous members
26
is arranged opposing each other in the mold assembly
1
. A cavity, which is filled with molten resin during molding, is defined by the cavity pieces
8
and
9
and the porous members
26
. After the molten resin is loaded into the cavity, the molten resin is cooled under a controlled pressure. In the cooling process, when air is guided to the cavity via the porous members
26
, the sides of the resin in the cavity corresponding to the non-optical surfaces
24
and
25
of the molded object
21
are pressed. This presses the resin in the cavity against the transfer surfaces
8
a
and
9
a
. As a result, the transfer surfaces
8
a
and
9
a
are transferred to the resin, and thereby the mirror surfaces
22
and
23
of the molded object
21
are formed.
In another mold assembly
1
illustrated in
FIG. 8B
, a vent hole
27
is provided at a side surface other than the transfer surfaces
8
a
and
9
a
in the cavity. Furthermore, a communication path
28
is provided to place the vent hole
27
in fluid communication with the exterior of the mold assembly
1
. When the molten resin is loaded into the cavity, the communication path
28
is filled with compressed air in the cavity. Then, a differential pressure is generated between portions of the resin contacting the transfer surfaces
8
a
/
9
a
and a portion of the resin contacting the vent hole
27
. This forms a sink only at the portions of the resin that contact the vent hole
27
. Thereby, a sink can be prevented from occurring at the mirror surfaces
22
and
23
of the molded object
21
.
In still another background mold assembly
1
illustrated in
FIG. 8C
, a slide cavity piece
29
is provided to form a side surface of the cavity other than the transfer surfaces
8
a
and
9
a
. When the molten resin in the cavity is cooled to a temperature lower than a softening point of the resin, a gap
30
is forcibly formed between the resin and the slide cavity piece
29
. This is done by sliding the slide cavity piece
29
in a direction away from the resin so as to form a sink only at a portion of the resin facing the gap
30
. Thereby, a sink is prevented from occurring at the mirror surfaces
22
and
23
of the molded object
21
.
In the above-described mold assembly
1
, the molded object
21
of a highly accurate optical element can be obtained by forming a sink in a portion of the molded object
21
other than mirror surfaces
22
and
23
which are to be used as optical surfaces. This allows mirror surfaces
22
and
23
to be shaped by transfer surfaces
8
a
and
9
a
. This is further accomplished by reducing internal distortion of the molded object
21
. However, because a portion of the resin where the sink is formed is separated from a piece of the mold assembly
1
at a temperature above the softening point of the resin, the thermal conductivity of the above-described portion of the resin where the sink is formed becomes extremely low. As a result, a significant cooling time is required to cool the resin.
Furthermore, when the sink is formed asymmetrically at a surface of the cavity other than transfer surfaces
8
a
and
9
a
as illustrated in
FIGS. 8B and 8C
, the temperature distribution of the resin becomes uneven. This is because the temperature of the portion of the resin where the sink is formed is relatively high and the temperature of other portions of the resin is relatively low. As a result, after the molded object
21
is removed from the mold assembly
1
, the molded object
21
may deform due to differences in contraction rates. In order to prevent the deformation of the molded object
21
, it is necessary to sufficiently cool the resin and to lower the temperature of the molded object
21
before removing the molded object
21
from the mold assembly
1
. However, the process of cooling the resin requires a significant amount of time.
Furthermore, in the mold assembly
1
illustrated in
FIG. 8C
, when the slide cavity piece
29
is slid away from the resin, the molded object
21
may be deformed due to a change in the contact-force between the slide cavity piece
29
and the resin during the process of cooling the resin to a temperature lower than a softening point of the resin.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-discussed and other problems, and an object of the present invention is to address these and other problems.
The present invention provides a novel method and mold assembly for producing a molded object with a high accuracy wherein a time of cooling a resin can be reduced.
The present invention also provides a novel method and mold assembly for producing a molded object with a high accuracy wherein deformation of the molded object can be prevented.
These and other objects are achieved according to the present invention by providing and using a novel mold assembly for molding a resin. The novel mold assembly includes a cavity configured to be filled with resin so as to form at least one surface of the molded object into a predetermined shape with at least one transfer surface of the cavity, a molding insertion member arranged to form another surface of the molded object other than the surface formed in the predetermined shape by the at least one transfer surface of the cavity, and a holding insertion piece configured to hold the molding insertion member. The holding insertion piece includes a communication path that places a surface of the molding insertion member at a side opposite to the cavity in fluid communication with the exterior of the mold assembly. This is done to guide atmospheric air outside of the mold assembly toward the surface of the molding insertion member. When the resin in the cavity is cooled, the resin in the cavity shrinks and a pressure in the cavity becomes lower than a pressure of the atmospheric air guided through the communication path in the holding insertion piece. Thus, atmospheric air is drawn into the communication path toward the molding insertion member. This deflects the molding insertion member toward the cavity so as to form a sink at the other surface of the molded object and presses the at least one surface of the molded object against the at least one transfer surface of the cavity.
According to an embodiment of the present invention, a mold assembly for molding a resin, and a method of using the mold assembly, are described. The mold assembly includes a cavity configured to be filled with resin so as to form at least one surface of the molded object in a predetermined shape with at least one transfer surface of the cavity, a molding insertion member arranged so as to form another surface of the molded object other than the surface formed in the predetermined shape by the at least one transfer surface of the cavity, and a holding

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