Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Title: Process for Making Metallized Plastic Molding Pellets for
Shielding Electromagnetic Interference
Background of the Invention:
There are many methods provided for shielding
electromaqnetic interference (F:MI ) . However, shields made from
metal are cumbersome, heavy and complex in forms to thereby be
unsuitable for electronic industries. The metal coating on the
surface of plastic articlQ for making ~MI shield may be easily
scratched to partially lose its shielding efficiency. Once the
scratched scraps, which are electrically conductive, drop onto a
printed circuit board of an electronic product, a short-
circuiting may be caused to deteriorate the electronic product.
Meanwhile, the metal coating or plating may increase the problems
of environmental protection.
Recently, several plastic molding processes were disclosed
by incorporating metal materials into the resin compositions for
making ~MI shields.
U. S. Patent 4,474,685 entitled "High Performance Molding
Compounds for Shielding ~lectromagnetic Interference" to Myron C.
Annis discloses a molding composition comprising a thermosetting
resin binder and an electrically conductive filler comprising
particles of carbon black, graphite and a conductive metal for
achieving a shielding effect to the emissions of electromagnetic
interf erence ~
However, when blending the resin with the particulate
f illers for molding processing for making ~MI shields, the
particulate fillers may be easily clustered to cause
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unhomogeneous dispersion of the electrically conductive fillers
in the resin matrix, thereby influencing the shielding
effectiveness of the molded products.
Other fillers such as fiber fillers and flake fillers may
also be considered to substitute the particulate fillers as
abovement ioned .
However, the process by using fiber fillers is costly and
uneconomic for a commercial production.
The flake fillers, when used in the processing steps of
resin blending, pelleting and injection molding, may be easily
broken to reduce the electrical conductivity, thereby possibly
decreasing the EMI shielding effect of the molded product.
The present inventors have found the drawbacks of the
conventional methods for making EMI shields and invented this
process for making homogeneously metallized plastic pellets for
molding effective EMI shields.
Summary of the Invention:
The object of the present invention is to provide a process
for making metallized plastic molding pellets comprising: first
metallizing a laminated plastic sheet by sandwiching an
electrically conductive metal foil in between two plastic films;
secondly slicing the metallized laminated plastic sheet into a
plurality of metallized plastic strips; thirdly wetting and
binding the metallized plastic strips, which have been radially
arranged, with a thermoplastic resin matrix to fo~m a metallized
plastic bar by pultrusion processingi and finally cuttin~ the
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pultruded metallized bar to obtain homogeneously metallized
plastic pellets for making effective EMI shields.
Brief Description of the Drawings:
Figure 1 is a flow sheet diagram showing the process of the
present invention.
Figure 2 is a perspective view of a f irst guiding mold of
the present invention.
Figure 3 is a perspective view of a second guiding mold of
the present invention.
Figure 4 shows a pultruded pellet in accordance with the
present invention.
Figure 5 shows two curves for determining the shielding
effectiveness (SE) of an ABS plastic material without being
metallized versus frequency (Fr~.
Figure 6 shows two curves for determining the shielding
effectiveness (SE) of the shield made by the present invention
versus frequency (Fr).
Detailed Description:
A process for making metallized plastic molding pellets in
accordance with the present invention as shown in Figures 1 - 3
comprises: metallizing a laminated plastic sheet by sandwiching
an electrically conductive metal foil M in between two
thermoplastic plastic films F by a sheet metallizing means 1 to
form a metallized laminated plastic sheet S; slicing the
metallized laminated plastic sheet S by a slicing means 2 into a
plurality of metallized plastic strips S1; wetting and binding
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the metallized plastic strips S1, as radially arranged, with a
thermoplastic resin matrix R to form a metallized plastic bar B
as pultruded and cooled by a pultrusion means 3; and cutting the
pultruded metallized bar B by a cutting means 4 to obtain a
plurality of metallized plastic pellets P.
The sheet metallizing means 1 includes: a metal-foil spool
11 for unrolling a metal foil M such as an aluminum foil, a pair
of agent applicators 12 disposed on two opposite surfaces of the
metal foil M for homogeneously coating a coupling agent A on the
two opposite surfaces of the metal foil M, a pair of plastic-film
spools 13 unrolling two plastic films F each plastic film F
guided by a film guiding roller 14 adjacent to the metal foil M
for sandwiching the metal foil M in between the two plastic films
F as bonded by the coupling agent A to form a metallized
laminated plastic sheet S, and a pair of hot-press rollers 15
operatively pulling and rotatably compacting the laminated
plastic sheet S having the metal foil M stably sandwiched and
metallized in the two plastic f ilms F .
The thermoplastic plastic film F may be selected from:
Acrylonitrile-butadiene-styrene (ABS) copolymer, Polyethylene
(PE) and other suitable thermoplastic plastic materials. The
meterial of f ilm F may be the same material of resin matrix R or
be compatible with the resin matrix.
The electrically conductive metal foil M may be selected
from: aluminum, copper, silver, nickel and other electrically
conductive metals. The coupling agent A may be selected
from: titanium coupling agent, zirconium-aluminum coupling agent
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such as Zircoaluminate, and other suitable coupling agents.
The slicing means 2 includes: at least a slicing guiding
roller 21 for guiding the metalli2ed laminated plastic sheet S
from the sheet metallizing means 1 to a feed port 22 of the
slicing means 2 to be sliced in the slicing means 2 , and a
discharge port 23 formed on a downstream side of the slicing
means 2 for discharging a plurality of metallized plastic strips
S1 having a generally linear arrangement from a side view
thereof .
The pultrusion means 3 includes: a feeding roller 31 having
a feeding roller axis 311 generally perpendicular to the
plurality of metallized plastic strips S1 delivered from the
slicing means 2 for juxtapositionally guiding the plastic strips
S1 towards an orienting roller set 32 consisting of at least four
orienting rollers rotatably mounted on a frame Inot shown) to be
generally rectangular or parallelogram shaped for divergently
developing the plurality of plastic strips S1 from the feeding
roller 31 to form a cross section of generally rectangular shape
Co; a first guiding mold 33 having a plurality of first radial
apertures 331 radially slotted in the first guiding mold 33 to
define a circular area A1 for convergently guiding the plurality
of plastic strips S1 having rectangular shape Co to be a f irst
cone-shaped core member C1 from the orienting roller set 32; a
second guiding mold 34 juxtapositionally positioned after the
first guiding mold 33 for continuously converging the plurality
of plastic strips S1 through a plurality of second radial
apertures 341 radially slotted in the second guiding mold 34 with
the second radial apertures 341 defining a circular area A2
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smaller than the area A1 of the first mold 33 for forming a
second cone-shaped core member C2 smaller than th3 f irst cone-
shaped core member C1; a pultruder 35 having an inlet die 351 and
an outlet die 352 disposed on two opposite end portions of the
pultruder 35 to be aligned with each center 330, 340 of the first
and second guiding molds 33, 34, with the inlet die 351 radially
formed with a plurality of slits ~not shown) in the inlet die 351
for converging the second cone-shaped core member C2 from the
second mold 34 to form a cylindrical core member C in the
pultruder 35; and a heater 353 mounted in the pultruder 35 for
heating the dies 351, 352 and keeping a constant temperature in
the pultruder for melting a resin matrix R delivered from a resin
feeder 36 mounted on the pultruder 35 for wetting the cylindrical
core member C as convergently guided by the inlet die 351 to form
a metallized plastic bar B as released from the outlet die 352;
a cooling means 37 for cooling and curing the metallized plastic
bar B as released from a central opening of the outlet die 352 of
the pultruder 35; and a pair of puller rollers 38 positioned at a
downstream of the cooling means 37 for tensioning and pulling the
metallized plastic bar B to the cutting means 4.
The cutting means 4 includes: a cutter 41 and an anvil
roller 42 disposed on two opposite sides of the metallized
plastic bar B for cutting the bar B to be a plurality of
metallized pellets P of predetermined length as shown in Figure
4.
Each guiding mold 33, 34 may be made of Teflon or other
suitable materials.
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.
The pultruder 35 has each die center of the inlet die 351
and the outlet die 352 aligned with a longitudinal axis X of the
cylindrical core member C passing through the pultruder 35 and
aligned with each center 330, 340 of the first and second guiding
molcls 33, 34.
The resin feeder 36 includes: a hopper 361 surrounded with a
pre-heater 362 for charging and preheating a resin matrix R which
is loaded into a screw extruder 360, and a resin applicator 363
for receiving the resin from the extruder 360 for delivering a
molten resin R1 as heated in the pultruder 35 for wetting and
binding the cylindrical core member C consisting of a plurality
of metallized plastic strips which are radially arranged to be
released from the outlet die 352 for cooling and curing by the
cooling means 37.
The cooling means 37 includes: a plurality of spray nozzles
for spraying cooling water onto the metallized plastic bar B, as
pulled by the puller rollers 38 made of rubber.
The preferred examples for performing the process of the
present invention are described hereinafter:
EXAMPLE 1:
By using the method and process equipments of the present
invention as above-mentioned, metallized plastic molding psllets
may be made by coating Titanium coupling agent, 1.2 phf (parts
per one hundred parts of resin) of Neoalkoxy, Tri-
( dioctylpyrophosphato ) Titanate produced by }tearich
Petrochemicals Inc., U. S. A. (Lica 38), having a formula of:
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o o
RO - Ti - [ O - P - O - P - ( OC8Hl7)2]3
,OH
on two opposite surfaces of the aluminum foil 20 ,~m thickness;
rotatably compacting and sandwiching the ~ l llmi nl~m foil in between
two 40~m ABS plastic films at 105 C to form metallized plastic
laminated sheet; slicing the laminated sheet to be 16 metallized
pla~tic strips each strip having a width of 1 mm; gradually
converging the 1~ strips through the two guiding molds 33, 34 for
forming a core member having the metallized strips radially
arranged in the core member; wetting and binding the core member
in the pultruder 35 by a molten ABS resin matrix at 220 - 230 C
to form a metallized plastic bar when keeping the pultruder at a
constant temperature of 230 C; pultruding the plastic bar through
the central opening of the outlet die 352 of 3 mm diameter;
cooling and hardening the pultruded plastic bar with 25 C water
sprayed from the nozzles of cooling means 37; and cutting the bar
to be pellets P each having a length of 5 mm by a cylindrically
shaped roller cutter 41. The metallized plastic strips
substantially contain a plurality of metal thin pieces Mf
radially distributed in the resin matrix about a longitudinal
axis in each pellet P.
The pellets thus produced may be provided for molding
electronic or computer products by plastic molding processes for
shielding electromagnetic interference.
EXAMPLE 2:
The pellets obtained from Example 1 is provided for forming
testing specimens by plastic molding process for performing test
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of electromagnetic shielding effectiveness. The test method of
Dual Chamber of ASTM ES7-~33 may be applied for testing the
shielding effectiveness of the molded product of the present
invention to obtain the shielding effectiveness (SE~ in decibels
between the two curves as shown in Figure 6 versu~ the frequency
~Fr~ from O to 1000 MHz in comparison with a co~trol test by
measuring the shielding effectivenesg (SE~ of an ABS plastic
molding material without being metallized versus frequency (Fr)
as shown in Figure 5.
Several setting values and test data are summarized as
follows:
1. Distance between antenna and the test specimen: 5 mm
2. Spectrum Analyzer:
Frequency range: 30 MHz - 1. 5 GHz
BW = 300 KHz
3. Tracking Generator:
0 dBm - 10 dBm; 30 MHz - 1. 5 GHz
4. Amplifier:
100 KHz - 1. 3 GHz
Gain 226 dB
5. TEM Cell:
DC - 200 MHz
6. Dummy Load:
50 ohm, 500 ll
. Close Field Probe:
Pmax = 0.5 W
Rdc = lS . 97 ohm
As shown in Figure 5, the ABS plastic product without being
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metallized substantially shows no shielding effect.
Comparatively, the metallized molding product of the present
invention indicates an average dB attenuation about 30 - 35
decibels between the frequency range of O - 1000 MHz (Figure 6).
Accordingly, the present invention may impa~t a shielding
effect of a metallized plastic product for shielding
electromagnetic interference.
Since the electrically conductive metal thin pieces Mf have
been firmly radially disposed in each plastic pellet P, the metal
pieces will be homogeneously dispersed in the resin matrix phase
during the molding processing without being clustered or broken
so as to ensure a better shielding effectiveness of a molded
product by the present invention.
As shown in ~igure 4, the length of each metal thin piece
Mf, the density or number of the metal thin pieces Mf distributed
in the resin matrix R may be Yaried or adjusted depending upon
the practical requirement, such as a commercial rating of EMI
sh i e lding e f f ect ivenes s .
The present invention may be modified without departing from
the spirit and scope of this invention.
