Molding method of air bag cover

A molding method is used to produce a module cover for an air bag device which has undercut portions in mounting edge portions thereof. A lower mold disposed inside the module cover to be produced is of the split type so that is includes a central mold disposed at the center and a plurality of lateral molds for molding of the undercut portions. The mating face of the central mold with each lateral mold is oblique so that its bottom is closer to the edge of the module cover than its top. Thus, the module cover produced can be detached from the lower mold by moving the lateral molds mutually inward.

FIELD OF THE INVENTION 
This invention relates to a method of molding a cover for housing an air 
bag device capable of inflating and expanding upon detection of impact, 
collapse and the like resulting from a vehicle collision. 
RELATED ART 
Air bag device comprises an air bag which expands rapidly in case of 
emergency, for example, when a vehicle collides. As shown in FIG. 16, an 
air bag 2 is normally folded and covered with a module cover 1. 
Conventionally, the air bag 2 together with an inflator 4 is supported by 
a mounting plate 3 called "retainer", and the module cover 1 is attached 
to the mounting plate 3 by rivets 5, screws or bolts. 
As shown in FIG. 17, the air bag device is generally attached to a central 
section of a steering wheel 10. 10A designates a spoke of the steering 
wheel 10. 
The module cover 1 was generally made from ream urethane by one-layer 
molding. Recently, two-layer molding has been practiced to make module 
covers composed of a hard layer or core layer 1a and a soft layer or skin 
layer 1b. The module cover 1 shown in FIG. 16 is of the two-layer molding 
type. The module cover 1 is formed with a groove-like rupture-initiating 
line or tear line 1A which causes the cover to rupture when the air bag 2 
expands. The tear line 1A is a portion whose mechanical strength is 
smaller than that of the other, or is made thin to the order of 0.5 to 1.0 
mm in thickness, whereby the module cover 1 can rupture along the tear 
line 1A when the air bag 2 expands. 
In FIG. 16, 1B designates a decorative line, and 6 designates a body cover. 
Conventionally, the module cover for the air bag is made from urethane 
foam-integral skin foam or thermo-plastic resin by injection molding using 
a metal mold assembly of given configuration. 
However, when producing the module cover for the air bag device from 
synthetic resin, if the module cover is to be formed with undercut 
portions, an excessive force tends to be imposed on products when 
detaching from the mold assembly, resulting in cover deformation. 
OBJECT AND SUMMARY OF THE INVENTION 
Is is an object of the present invention to provide a molding method of an 
air bag cover which allows detaching of the molded cover from a mold 
assembly without cover deformation. 
To accomplish the foregoing object, the present invention provides a method 
of molding a module cover for an air bag device, the module cover having 
undercut portions in mounting edge portions thereof, which is 
characterized in that a lower mold disposed inside the module cover to be 
produced is of the split type so that it is composed of a central mold 
disposed at the center and a plurality of lateral molds for molding of the 
undercut portions, and the mating face of the central mold with each 
lateral mold is oblique so that its bottom is closer to the edge of the 
module cover than its top, whereby the module cover produced can be 
detached from the lower mold by moving the lateral molds mutually inward. 
As will be appreciated, according to the present invention, by moving the 
lateral molds mutually inward, the module cover produced can be detached 
from the metal mold assembly without any hindrance being caused by the 
undercut portions. That is, the module cover having the undercut portions 
produced can be detached from the metal mold assembly without imposing any 
excessive force on the cover; thus, the cover undergoes no deformation in 
manufacture and can be produced efficiently.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present inventions will now be described in greater 
detail with reference to the drawings. 
FIGS. 1 through 10 illustrate an embodiment of the present invention. As 
shown in FIGS. 1 through 3, a module cover 1 will be molded which has 
undercut portions 1C in opposing edge portions thereof. As shown in FIGS. 
4 through 6, a lower mold 11 forming a part of a metal mold assembly for 
use in this embodiment is composed of a central mold 12 for molding of a 
hollow central section of the module cover 1, first and second lateral 
molds 13 and 14 for molding of the undercut portions of a hollow lateral 
section, and a frame mold 15 for molding of a peripheral section. The 
central mold 12 is made integral with the frame mold 15. The mating face 
of the central mold 12 with each of the first and second lateral molds 13 
and 14 is oblique so that its bottom is closer to the edge of the module 
cover 1 than its top. On the other hand, the mating face of the frame mold 
15 with each of the first and second lateral molds 13 and 14 is 
substantially vertical. 16 designates an upper mold. 
As shown in FIGS. 5 and 6, the module cover 1 is molded by clamping the 
upper mold 16 and the lower mold 11 together, injecting a resin material 
into a cavity 50 defined between these molds, and hardening/curing the 
injected resin material. Specifically, to form a core layer, a first upper 
mold of relatively small size is clamped to the lower mold 11, a resin 
material is injected, and the injected material is hardened/cured. Then, 
to form a skin layer, the first mold is removed, a second upper mold of 
relatively large size is clamped to the lower mold with the core layer 
left thereon, another resin material is injected into a different cavity 
defined between them, and the injected material is hardened/cured. When 
detaching the molded cover from the metal mold assembly, as shown in FIGS. 
7 and 8, the upper mold 16 is moved up. Then, as shown in FIGS. 9 and 10, 
the first and second lateral molds 13 and 14 are moved up along the mating 
faces of the central mold 12. As described above, the mating face of the 
central mold 12 with each of the first and second lateral molds 13 and 14 
is oblique so that its bottom is closer to the edge of the module cover 1 
than its top, and the mating face of the frame mold 15 with each of the 
first and second lateral molds 13 and 14 is substantially vertical; 
therefore, the first and second lateral molds 13 and 14 can readily be 
moved up without imposing any extra force on the module cover 1. After the 
first and second lateral molds 13 and 14 are moved up, they are moved 
mutually inward in the direction of the arrows A in FIG. 9; therefore, the 
undercut portions 1C of the module cover 1 come out of contact with the 
lateral molds 13 and 14. Accordingly, the module cover 1 produced can 
readily be detached from the metal mold assembly without undergoing any 
deformation. 
FIGS. 11 through 15 illustrate another embodiment of the present invention, 
which intends to mold a module cover having undercut portions in all edge 
portions thereof. As shown in FIGS. 11 through 13, a lower mold 21 forming 
a part of a metal mold assembly for use in this embodiment is composed of 
a central mold 22 for molding of a hollow central section of the module 
cover, first, second, third and fourth lateral molds 23, 24, 25 and 26 for 
molding of the undercut portions of a hollow lateral section, and a frame 
mold 27 for molding of a peripheral section. The central mold 22 is made 
integral with the frame mold 27. The mating face of the central mold 22 
with each of the first through fourth lateral molds 23 through 26 is 
oblique so that its bottom is closer to the edge of the module cover (not 
shown) than its top. On the other hand, the mating face of the frame mold 
27 with each of the first through fourth lateral molds 23 through 26 is 
substantially vertical. 
In the embodiment, the module cover (not shown) is molded by clamping the 
upper mold (not shown) and the lower mold 21 together and injecting a 
resin material. When detaching the molded cover from the metal mold 
assembly, as shown in FIGS. 12 and 13, the upper mold (not shown) is moved 
up. Then, as shown in FIGS. 14 and 15, the central mold 22 integral with 
the frame mold 27 is moved down. In connection with this downward 
movement, since the mating face of the central mold 22 with each of the 
first through fourth lateral molds 23 through 26 is oblique so that its 
bottom is closer to the edge of the module cover than its top and the 
mating face of the frame mold 27 with each of the first through fourth 
lateral molds 23 through 26 is substantially vertical, the central mold 22 
together with the frame mold 27 can readily be moved down without imposing 
any extra force on the module cover. After the central mold 22 is moved 
down, the first and second lateral molds 23 and 24 are moved mutually 
inward in the directions of the arrows A in FIG. 15, and the third and 
fourth lateral molds 25 and 26 are also moved mutually inward in the 
directions of the arrows A in FIG. 14; therefore, there is no fear that 
the undercut portions of the module cover will come into contact with the 
lateral molds 23 through 26. Then, the lateral molds 23 through 26 are 
moved down. Accordingly, the module cover produced can readily be detached 
from the metal mold assembly without undergoing any deformation. 
The foregoing processes are illustrative and are not to be construed as 
limiting the method of the present invention. For example, the manner of 
splitting the lower mold, the mold configuration, the degree of 
inclination of the mating face of the central mold or lateral mold, etc. 
may be modified without departing from the spirit of the present 
invention. Further, the mating face of the lateral mold with the frame 
mold may be made oblique.