Molding part of thermoplastic resin and method of fabricating same

A protector made of thermoplastic resin to which a foaming agent is added is adapted such that no foams are present in the thermoplastic resin forming a predetermined volume area including lock portions mounted on a protector body for integrating the protector body and a lid element but foams are present in the thermoplastic resin forming the area other than the predetermined volume area, whereby a molded part of thermoplastic resin is provided which accomplishes weight reduction and an insured strength at a strength-required area.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a molded part made of foamed thermoplastic 
resin employed as a protector or the like for an automotive wire harness 
and a method of fabricating the same. 
2. Description of the Prior Art 
A wire harness for wiring in an automotive electric system is protected by 
a protector 100 shaped as shown in FIGS. 8A and 8B, for example, which has 
an opening and closing lid 101 on one side surface thereof so as not to be 
damaged by a flash formed by punching in a press molded vehicular body. 
Such a protector 100 includes lock portions 102 for fixing the opening and 
closing lid 101 in a closed state after the wire harness is housed in the 
protector 100, and a bolting portion 103 for fixing the protector 100 to 
the vehicular body. 
There has been a need for weight reduction of automotive vehicles for 
various reasons, and the protector 100 is no exception. In general, the 
above-mentioned protector 100 is fabricated by injection molding using 
thermoplastic resin such as polypropylene. One of the approaches to reduce 
the weight of a molded part of thermoplastic resin is to foam the molded 
part. Such a foamed molded part is achieved by adding a foaming agent to 
thermoplastic resin, plasticizing the thermoplastic resin, and injecting 
the plasticized thermoplastic resin into a mold. 
Although required to be reduced in weight, the protector 100 shown in FIGS. 
8A and 8B includes local areas required to have a strength higher than 
that of the other areas, such as the lock portions 102 and bolting portion 
103. However, when the molded part of resin is foam molded in the 
conventional manner such as the protector 100, foams are present in the 
resin forming the lock portions 102 and the bolting portion 103, resulting 
in an insufficient strength of the lock portions 102 and the bolting 
portion 103. 
SUMMARY OF THE INVENTION 
The present invention is intended for an injection molded part made of 
thermoplastic resin to which a foaming agent is added. According to the 
present invention, the molded part comprises: a predetermined selected 
area formed of the thermoplastic resin containing no foams; and a 
non-selected area formed of the thermoplastic resin containing a group of 
foams dispersed generally uniformly therein. 
According to the molded part of thermoplastic resin, the presence of foams 
in the thermoplastic resin forming the area other than the selected area 
allows the reduction in weight of the area other than the selected area. 
The absence of foams in the thermoplastic resin forming the selected area 
prevents the strength of the selected area from lowering. 
The molded part of thermoplastic resin according to the present invention 
ensures the strength of the required area as well as providing weight 
reduction. 
The present invention is also intended for a method of fabricating a molded 
part of thermoplastic resin. According to the present invention, the 
method comprises the steps of: feeding a thermoplastic resin material and 
a foaming agent into a cylinder of an injection molding machine having a 
nozzle communicating with an injection gate; rotating a screw disposed in 
the cylinder at a predetermined rotational speed to knead and plasticize 
the thermoplastic resin and the foaming agent in the cylinder; injecting 
the plasticized thermoplastic rein from the injection gate into a cavity 
in a mold; and cooling the thermoplastic resin in the cavity and then 
removing the molded part of thermoplastic resin from the mold, the 
injection gate being coupled to a portion of the cavity corresponding to a 
strength-required area of the molded part of thermoplastic resin, the 
rotational speed of the screw being controlled such that the thermoplastic 
resin injected into the cavity is rendered unfoamed only in a region 
corresponding to the volume of the strength-required area at an outlet of 
the injection gate. 
In the method, as the rotational speed of the screw increases when the 
thermoplastic resin is plasticized, the volume of the unfoamed portion at 
the outlet of the injection gate increases. By injecting the thermoplastic 
resin into the cavity from the injection gate coupled to the portion 
corresponding to the strength-required area of the molded part, with the 
rotational speed of the screw adjusted to a predetermined speed, the 
unfoamed portion having a volume corresponding to the volume of the 
strength-required area is formed only at the outlet of the injection gate. 
According to another aspect of the present invention, the method comprises 
the steps of: feeding a thermoplastic resin material and a foaming agent 
into a cylinder of an injection molding machine having a nozzle 
communicating with an injection gate; rotating a screw disposed in the 
cylinder, with a predetermined back pressure applied to the screw, to 
knead and plasticize the thermoplastic resin and the foaming agent in the 
cylinder; injecting the plasticized thermoplastic resin from the injection 
gate into a cavity of a mold; and cooling the thermoplastic resin in the 
cavity and then removing the molded part of thermoplastic resin from the 
mold, the injection gate being coupled to a portion of the cavity 
corresponding to a strength-required area of the molded part of 
thermoplastic resin, the back pressure being controlled such that the 
thermoplastic resin injected into the cavity is rendered unfoamed in a 
region corresponding to the volume of the strength-required area at an 
outlet of the injection gate. 
In this method, as the back pressure applied to the screw increases when 
the thermoplastic resin is plasticized, the volume of the unfoamed portion 
at the outlet of the injection gate increases. By injecting the 
thermoplastic resin into the cavity from the injection gate coupled to the 
portion corresponding to the strength-required area of the molded part, 
with the back pressure adjusted to a predetermined pressure, the unfoamed 
portion having a volume corresponding to the volume of the 
strength-required area is formed only at the outlet of the injection gate. 
The foregoing aspects of the method of the present invention provide the 
molded part fabricated such that foams are absent in the thermoplastic 
resin forming the strength-required area but foams are present in the 
thermoplastic resin forming the other area. 
An object of the present invention is to provide a molded part of 
thermoplastic resin which is light-weight and has an increased strength in 
a required portion, and a method of fabricating the same. 
These and other objects, features, aspects and advantages of the present 
invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a molded part of thermoplastic resin in the form of a 
protector 50 for an automotive wire harness according to the present 
invention. 
Similarly to the protector 100 of FIGS. 8A and 8B, the protector 50 
comprises a protector body 51 of a U-shaped cross-sectional configuration, 
and a plate-like lid element 52 for closing an opening side surface of the 
protector body 51. The protector body 51 is coupled to the lid element 52 
with a thin-walled hinge 53. Lock portions 54 are formed integrally with 
the protector body 51 at upper ends of one side surface of the protector 
body 51, and lock portions 55 are formed integrally with the lid element 
52 on one side of the lid element 52. The lid element 52 is fixed in such 
a manner as to close the opening side surface of the protector body 51 by 
engagement of the lock portions 54 with the lock portions 55. Referring to 
FIG. 2, in the protector 50, the thermoplastic resin forming a selected 
area 56 including the lock portions 54 contains no foams, and the 
thermoplastic resin forming a non-selected area other than the selected 
area 56 contains foams 57 which are dispersed generally uniformly so as to 
provide a predetermined expansion ratio. 
Each of the lock portions 54 of the protector body 51 has a through 
aperture 54a of a rectangular cross-sectional configuration extending 
vertically therethrough between the lock portion 54 and a side surface of 
the protector body 51. The lock portions 54 are elastically deformable so 
as to deform the through apertures 54a. Each of the lock portions 55 of 
the lid element 52 includes a projecting piece 55a projecting in a 
direction orthogonal to the surface of the lid element 52 and having a 
thickness generally equal to the width of the through aperture 54a, and an 
engaging projection 55b bulging outwardly from the outer surface of the 
projecting piece 55a. 
When the lock portions 55 are fitted into the through apertures 54a of the 
lock portions 54, the engaging projections 55b pass through the through 
apertures 54a while deforming the through apertures 54a. The lock portions 
54 are elastically recovered after the engaging projections 55b pass 
through the through apertures 54a. Consequently, the engaging projections 
55b are locked by the lock portions 54 and are not capable of being 
withdrawn from the through apertures 54a. In this state, the lid element 
52 closes the opening side surface of the protector body 51. 
In the protector 50 as above constructed, the selected area 56 in the 
protector body 51 has a relatively high strength since no foams are 
present in the thermoplastic resin forming the selected area 56. The lock 
portions 54 included in the selected area 56 are not easily damaged by 
relatively large load applied thereto by the engagement with the engaging 
projections 55a of the lock portions 55 of the lid element 52. In the 
protector 50, the whole non-selected area other than the selected area 56 
is formed of the thermoplastic resin containing the foams 57. The presence 
of the foams 57 allows the protector 50 to be lighter in weight than a 
protector of the same volume made of thermoplastic resin containing no 
foams. As described above, the protector 50 achieves weight reduction and 
the increased strength of the relatively greatly loaded lock portions 54. 
The above-mentioned protector 50 is fabricated in a manner to be described 
below by using an injection molding machine 10 shown in FIG. 3. 
The injection molding machine 10 shown in FIGS. 3 and 4 will now be 
described. The injection molding machine 10 comprises a cylinder 11, a 
screw 12, a hopper 13, a mold 14, a sprue 15, and a runner 16. 
The cylinder 11 has a nozzle 11a at its front end. 
The screw 12 is housed in the cylinder 11, with a screw head 12a thereof 
directed toward the nozzle 11a of the cylinder 11. The screw 12 is rotated 
by the drive power of a driving source 12A such as a motor. The driving 
source 12A rotates the screw 12 at a speed indicated by a controller not 
shown. The driving source 12A is movable in an axial direction of the 
screw 12 and is fixed to a piston rod 12Ba of an actuator 12B such as a 
hydraulic cylinder. The actuator 12B applies thrust to the screw 12 to 
move the screw 12 toward the nozzle 11a of the cylinder 11 (in the 
direction of the arrow F) and, when the screw 12 moves toward the driving 
source 12A (in the direction of the arrow R), applies back pressure 
against the movement. The actuator 12B can adjust the thrust and back 
pressure to any magnitude by the indication of the controller not shown. 
The hopper 13 has a discharge opening connected to the rear end of the 
cylinder 11. 
The mold 14 includes a plurality of divided molds. The divided molds are 
fastened together to define a cavity 14a of a predetermined configuration, 
i.e. the same configuration as the protector 50 herein, inside of the mold 
14 (See FIG. 4). 
The sprue 15 is a tapered passage for coupling the nozzle 11a of the 
cylinder 11 to the runner 16 to be described later as shown in FIG. 5 and 
has a seat for fixing the nozzle 11a to a predetermined position at the 
small-diameter end thereof. 
The runner 16 is branched from the large-diameter side of the sprue 15 
toward two portions 14b and 14c of the cavity 14a which correspond to the 
lock portions. Injection gates 16A and 16B are formed at the front ends of 
the runner 16 and communicate with the portions 14b and 14c corresponding 
to the lock portions, respectively. 
Using the injection molding machine 10 as above constructed, the protector 
50 shown in FIG. 1 is fabricated through the following procedure. 
Referring to FIG. 6A, a thermoplastic resin material and a foaming agent 
are fed into the hopper 13, with the screw 12 advancing to the forefront 
position (in the direction of the arrow F). The quantity ratio of the 
thermoplastic resin material to the foaming agent is appropriately 
determined so as to ensure the required strength of the foamed portion in 
the molded part of thermoplastic resin to be fabricated by this method. 
The driving source 12A is actuated, with the thermoplastic resin material 
and the foaming agent fed in the hopper 13, to rotate the screw 12 in the 
direction to retract the helical channel of the screw 12 at a rotational 
speed controlled in a manner to be described later. At that time, a 
predetermined back pressure greater than zero which is controlled in a 
manner to be described later is applied to the screw 12. The rotation of 
the screw 12 under such conditions permits the thermoplastic resin 
material and foaming agent in the hopper 13 to be fed into the cylinder 
11. The thermoplastic resin material and foaming agent fed into the 
cylinder 11 are kneaded by the rotating screw 12 and plasticized into a 
molten resin M1. The molten resin M1 is transferred to the front of the 
screw head 12a along the helical channel of the screw 12 as shown in FIG. 
6B. The screw 12 is retracted toward the actuator 12B (in the direction of 
the arrow R) by the pressure of the molten resin extruded into the front 
of the screw head 12a. The rotation of the screw 12 is interrupted when 
the screw 12 retracts a predetermined amount. That is, the operation of 
the driving source 12A is interrupted when a predetermined amount of 
molten resin is accumulated in the front of the head of the screw 12. 
The actuator 12B is actuated, with the rotation of the screw 12 
interrupted, to move the screw 12 forwardly (in the direction of the arrow 
F) at a predetermined speed. This permits the plasticized thermoplastic 
resin or the molten resin M1 accumulated in the front of the screw head 
12a to be fed into the injection gates 16A, 16B through the sprue 15 and 
runner 16 and then into the cavity 14a of the mold 14, as shown in FIG. 
6C. The predetermined amount of molten resin accumulated in the front of 
the screw head 12a is entirely injected into the cavity 14a, which is then 
filled with the plasticized thermoplastic resin. 
The thermoplastic rein in the cavity 14a is cooled by cooling the mold 14. 
The mold 14 is then opened, and the molded part of thermoplastic resin or 
the protector 50 is provided which is of the predetermined configuration. 
In the method of fabricating a molded part of thermoplastic resin described 
herein, the protector 50 is fabricated wherein the resin forming the 
selected area 56 contains no foams but the resin forming the other area 
contains foams. For this purpose, the rotational speed of the screw 12 
and/or the back pressure against the screw 12 are controlled in the 
following manner in the step of plasticizing the thermoplastic resin. The 
control method will be described below. 
The solid curve I of FIG. 7 shows experimental values of the volume of the 
unfoamed portion formed in portions of the molded part of thermoplastic 
resin which correspond to the outlets of the injection gates 16A, 16B 
versus the rotational speed of the screw 12 where the back pressure is 
zero. The curve I indicates that the volume of the unfoamed portion formed 
in the portions corresponding to the outlets of the injection gates 16A, 
16B where the back pressure is zero is about 1 cm.sup.3 at 80 r.p.m. 
rotational speed of the screw 12, about 2.5 cm.sup.3 at 100 r.p.m., and 
about 3 cm.sup.3 at 120 r.p.m. This indicates that, if the volume of the 
lock portions 54 is less than 3 cm.sup.3, the 120 r.p.m. rotational speed 
of the screw 12 ensures that at least the selected area 56 of 3 cm.sup.3 
in volume including the lock portions 54 is rendered unfoamed at the back 
pressure of zero. 
As above described, the thermoplastic resin forming a strength-required 
area such as the lock portions 54 can be achieved in the form of unfoamed 
resin containing no foams by controlling only the rotational speed of the 
screw 12 in the step of plasticizing the thermoplastic resin. 
For achieving the thermoplastic resin forming the strength-required area in 
the form of the unfoamed resin containing no foams, the back pressure may 
be controlled. The dashed-and-dotted curve II and dashed-and-double-dotted 
curve III of FIG. 7 show experimental values of the volume of the unfoamed 
portion formed in portions of the molded part of thermoplastic resin which 
correspond to the outlets of the injection gates 16A, 16B versus the 
rotational speed of the screw 12 where the back pressure is 8 Kg/cm.sup.2 
and where the back pressure is 12 Kg/cm.sup.2, respectively. It is 
apparent from FIG. 7 that, if the rotational speed of the screw 12 is 
fixed to 100 r.p.m., the volume of the unfoamed portion is about 3 
cm.sup.3 at the back pressure of zero, about 7 cm.sup.3 at 8 Kg/cm.sup.2 
back pressure, and about 9 cm.sup.3 at 12 Kg/cm.sup.2 back pressure. Thus, 
at 100 r.p.m. constant rotational speed of the screw 12, the back pressure 
is adjusted to 12 Kg/cm.sup.2 for execution of the plasticizing step when 
the volume of the lock portions 54 is less than 9 cm.sup.3. 
In the foregoing description, one of the back pressure and rotational speed 
is fixed while the other is controlled. It is, however, apparent from FIG. 
7 that both of the back pressure and the rotational speed may be 
controlled simultaneously so that the volume of the unfoamed portion is a 
volume corresponding to the strength-required area. In the FIG. 7 case, 
for example, the back pressure is adjusted to 8 Kg/cm.sup.2 and then the 
rotational speed is adjusted to 100 r.p.m. when the volume of the unfoamed 
portion is 7 cm.sup.3. When the volume of the unfoamed portion is 8.5 
cm.sup.3, the back pressure is adjusted to 12 Kg/cm.sup.2 and then the 
rotational speed is adjusted to 90 r.p.m. 
As hereinabove described, control of the rotational speed of the screw 
and/or the back pressure in the step of plasticizing the thermoplastic 
resin provides the molded part fabricated such that no foams are present 
in the thermoplastic resin forming the strength-required area such as the 
lock portions 54 but foams are present in the thermoplastic resin forming 
the area other than the strength-required area. 
It should be noted that the values on the abscissa and ordinate of the 
graph of FIG. 7 are inherent in a predetermined device, and the present 
invention is not limited to these values. 
While the invention has been shown and described in detail, the foregoing 
description is in all aspects illustrative and not restrictive. It is 
therefore understood that numerous modifications and variations can be 
devised without departing from the scope of the invention.