CA1126457A - Apparatus and method for injection molding of certain plastics to maximize dispersion of color and carbonization in product - Google Patents

Abstract

APPARATUS AND METHOD FOR INJECTION
MOLDING OF CERTAIN PLASTICS TO
MAXIMIZE DISPERSION OF COLOR AND
CARBONIZATION IN PRODUCT

Abstract of the Disclosure A method of injection molding for plastics taken from the family of polyolefins and particularly polyethylene, polypropylene, linear polyethylene, copolymers, and styrene, in which the plastic material is forced through a screen pack including at least two screens immediately in advance of an injection nozzle. This system maximizes uniform dis-persion of color in the polyolefins and substantially elimin-ates the inclusion of optically (human) observable black particles, such as carbonized pieces of the polyolefin or other ingredients. The apparatus consists essentially of an injection nozzle, a screen pack retainer to which the nozzle is secured, and a screen pack between the nozzle and retainer. The screen pack comprises a disk having a circular recess opening onto one face thereof, a fine mesh screen in said recess, and a breaker plate covering the screen and clamped to the disk.

Description

The present invention relate~s to the injection molding of certain plastics and is concerned primarily with a method and apparatus for injection molding in which color dispersion is maximized and inclusion of observable carbon-ized particles is minimized. This invention breaks up agglomerated color particles, i.e., pigments or dyes, and carbonized particles.

Background of the Invention At the present time a large number of plastic products are formed by the injection molding of that family of plastics known as polyolefins. Included in this category are polyethylene, polypropylene, linear polyethylene, copolymers, and styrene. The injection molding of such plastic products results in the generation of a large amount of scrap material which, under the now prevalent methods and apparatus used in some molding,is discarded as waste because of either one or both of two factors which would 50 impair the appearanc~ of the products made from such scrap material as to render them unsalable.

The first such factor is color or dye dispersion~
Should the scrap material contain pigments or any material having a color that is noticeably different from the main color of the product, the injection molding causes streak-ing, cloudiness, spotting, and blurring in the final product.
Such color dispersion impairs the appearance of ~he product to a degree which renders it unsalable.

The second factor is carbonization or the presence of small particles of carbon which also impairs the appear-ance of the ultimate product to a degree which renders it unsalable.

~fi~S~

While the present invention is founded on the basic concept of closely associating a screen pack of a particular character with an injection nozzla, there are many examples in the known art of using filters, screens and breaker plates with an injection nozzla. ~11 such known devices and matexials readily distinguish from the present invention either by the plastic of which they are made, the particular character of the screen pack, or both of these factors.

A patentability search conducted before the preparation of this application brings to light some seven United States patents which are believed to come the closest to the instant concept of the patents reviewed on the search.
These patents are cited as follows:

2,253,089 Nydegger 2,514,390 Hagen 2,880,460 Monett 2,895,167 Paggi

3,077,636 Reters 3,762,850 Werner et al.
3,947,202 Goller et al.

A brief comment is made on these references as follows:

N~deg er This patent is directed to a method and apparatus for spinning filamPnts. It discloses in Figures 1 and 7 screen packs or filters 11. Lines 58-60, column 2, page 1, state that it has been found advantageous to pass the molten material through these screen packs or filters, but no reason is given for it being advantageous and none is apparent.
Clearly there is no advantage relating to color dispersion or carbonization.

Hagen This patent is directed to an injection molding device for ground asbestos and bitumen. It discloses a per-forated disk or breaker plate at 22. The materials on which the device is used are characteristically different from those of the subject method and apparatus.

Monett .
This patent discloses a screen pack that is quite close to that of the present device. Note the apertured disks 26 and 27 and the fine mesh screen 29 clamped between the disks 26 and 27. However, this screen pack 28 is used with apparatus for molding rubber products. The problem of color dispersion and carbonization does not arise in connection with the molding of rubber products.

This patent discloses a filter used with injection molding. Plastics are the materials on which it is used and polymide is specifically mentioned in line 65 of column 2.

Peters __ A breaker plate i~ shown at 21. The injection molding of plastic material is mentioned.

.57 Werner et al --A spinning head having a sieve part S i9 disclosed.
Nylon filaments are specifically mentioned.

Goller et al.
This is the only patent found in the search that deals with discoloration and carbonization (lines 17 and 18, column 1).

While dispersion of color is an important reason for impairing products to a degree that entails a rejection, the pxesence of carbon particles or specks are the main cause of product rejection.

The test of whether or not carbon particles are of a size to result in product rejection is whether or not they are visible to the naked eye. If they are visible, then they are of a size which results in product rejection. It is estimated that carbon particles or specks above 0.010 square inch would be visible and therefore objectionable.

Objects of the Invention With the foregoing conditions in mind, the present invention has in view the following objectives:

1. To provide a method for the injection molding of plastics of the family of polyolefins in which the plastic is forced through a screen pack immediately in advance of an injection nozzle to enhance the dispersion of color in the molded products.

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2. To provide, in a method of the type noted, the step of forcing plastic through a screen pack to filter out or break up any carbon particles which may be visibly present in the molded product.

3. To provide, in a method of the character afore-said, the step of forcing the plastic through a screen pack including a breaker plate having relatively large apertures therein and a fine screen which overlies the breaker plate.

4. To provide, in a method of the kind described, the step of forcing the plastic through a screen pack com-prising a pair of breaker plates formed with comparatively large orifices and a fine screen clamped between the breaker plates.

5. To provide a method of the type noted in which the family of polyolefins includes polyethylene, polypropylene, linear polyethylene, copolymers, and styrene.

6. To provide an injection molding no~zle assembly for use on plastics of the family of pol~olefins which includes an injection nozzle having a inlet for t~e plastic material and a screen pack in advance of or on the upstream side of said inle~.

7. To provide, in an injection -molding nozzle assembly of the type noted, a screen pack including a breaker plate having relatively large apertures and a fine screen overlying the ~reaker plate.

8~ To provide, in an injection molding nozzle assembly oE the character aforesaid, a screen pack comprising a first breaker plate having a recess opening onto a face thereof, a fine screen snugly received .in said recess, and a second breaker plate overlying said screen and marginal por-tions of said first breaker plate, together with means for clamping said breaker plates about said fine screen.

SU~ARY OF THE INVENTION
.

The present method consists essentially of a method and apparatus for aiding color dispersion and eliminating the presence of carbon particles in the products of injection molding. The plastic materials which are molded are fre-quently scrap and are discarded as wastes under present injection molding conditions because in many instances color dispersion and ~he presence of carbon particles in the ulti-mate products made from this scrap material would render them unsalable.

The plastic materials which are susceptible of use with the instant method and apparatus are plastics of the family of polyolefins and particularly polyethylene, poly-propylene, linear polyethylene, copolymers, and styrene.

The method is carried out on apparatus which includes as a characteristic and essential element a novel nozzle assembly for the injection molding. This assembly comprises a noz21e having a cylindrical bore extending throughout a portion of its extent and terminating at one end in a jet orifice and at its other end in a frusto-conical recess. The end portion of the nozzle at the larger end of this recess is screw-threaded and the nozzle is formed with wrench-engaging means adjacent to the threaded portion.

A screen pack retainer is formed with a socket, an end portion of which is scxew-threaded and into which the threaded portion of the nozzle is screwed. The screen pack retainer is formed with a bore of reduced diameter as compared to the socket, with a shoulder between this bore and the socket. This bore communicates with an inlet for molten plastic by way of a frusto conical recess and a second bore of smaller diameter.

Mounted in the screen retainer between the shoulder therein and the screw-threaded end of the nozzle is a screen pack. This screen pack comprises a first breaker plate having a circular recess in the face confronting the nozzle, a fine screen snugly received in this recess, and a second breaker plate overlying the fine screen and marginal portions of the first breaker plate.

Mounted in the marginal portions of the first breaker plate and extending forwardly, that is, toward the nozzle, are a pair of diametrically opposed dowel pins which are received in openings in the marginal portions of the second breaker plate. These dowel pins result in correct alignment of the breaker plates and fine screen when the nozzle is screwed into the screen retainer to clamp the breaker plates 5~

together, with the ~ine screen sandwiched therebetween.

Each breaker plate is formed with 121 .062 inch diameter holes that allow flow through the screen pack.
While the fine screen is subject to variation in the number and size of the openings therein, it is noted that a screen pack which has been found accessible is one that has two 20 x 20 mesh screens with one 24 x 110 mesh dutch-weave screen sandwiched therebetween. The dimensions of 20 x 20 and 24 x 110 refer to the number and arrangement of filtering openings per square inch in each of these screen elements.

In starting a method using the above injection nozzle assembly, it is important that the filter nozzle be brought to a barrel temperature that has been found to be suitable for the particular plastic being operated upon.
Moreover, -the pressure which is applied to the molten plastic, which is called the injection pre~sure, should aIso be brought to a degree which is commensurate with the particular plastic materials being used.

For a full and more compiete understandiilg of the invention, reference may be had to the following description and accompanying drawing, in which: -Figure 1 is a perspective of the elements of theinjection nozzle assembly which i3 used in the method of this invention as illustrated in exploded relationship; and Figure 2 i9 a longitudinal section illustrating the elements of Figure 1 as assembled.

Referring now to the drawings and first more particu-larly to Figure 1, the instant nozzle assembly is shown as comprising an injection nozzle referred to in its entirety by the reference character 10, a screen pack retainer designated generally 11, a first breaker plate 12, a fine screen 13, and a second breaker plate 14.

Referring now to Figure 2, which may be considered along with Figure 1, the injection nozzle 10 is shown as comprising a barrel 15 which preferably is cylindrical and which is formed with a cylindrical bore 16 extending partially throughout its length. Barrel 15 terminates at one end in a round nose 17 formed with a jet orifice 18 which communicates with the cylindrical bore 16. Barrel 15 is enlarged by conical shoulder 19 to provide a thickened portion 20 which in turn is integrally connected to a wrench-engaging portion 21 that is formed with flats 22. At the end remote from nose 17, nozzle 10 termin~tes in an end portion 23 which is e~ternally threaded.
A frusto-conical recess 24 extends from the end of bore 16 remote from nose 17 through threaded end portion 23.

A screen pack retainer 11 comprises a cylindrical body 25 that is formed with a pair of diametrically opposed flats 26 for wrench-engaging purposes. Body 25 has a first end at 27 and a second end at 28. Opening onto the end surface at 27 is a socket 29, the end portion of which is adjacent to end-27 being internally threaded as indicated at 30.
Secured to end 28 of body 25 is a nipple 31. Only a portion of this nipple is illustrated but it i5 noted it is connected to a source of supply of molten plastic. Nipple 31 has an internal passage 32 which communicates with a cylindrical bore 33 which communica~es with a larger cylindrical bore 34 by way of a frusto-conical passage 35. A shoulder 36 separates socket 29 from bore 34.

Referring now more particularly to Figure 1, first breaker plate 12 is shown aR having a circular recess 37 opening onto its forward face. The rear face 38 of breaker plate 12 engages shoulder 36 when breaker plate 12 is positioned in socket 29. Recess 37 is defined by a marginal portion 39 in which two dowel pins 40 are anchored in diametrically opposed relation. Extending between the rear face 38 and the front face of breaker plate 12 within the marginal portion 39 are a plurality of apertures 41. Purely by way of example, it is noted that there are 121 of these apertures 41 and each of them is .062 inch in diameter. Needless to say, they are equi distantly spaced apart throughout the area within the marginal portion 37.

Fine screen 13 tak~s the circular form illustra~ed and is snugly received in recess 37. From their very nature, the fllter opening~ provided by screen 12 are smaller in size and more closely arranged than are the apertures 41 in breaker plate 12. This fine screen 13 may be made up so that it is suitable for different job applications. Thus, it is noted that i~t may in~lude two 20 x 20 mesh screens with one 24 x 100 mesh dutch-weave ~creen sandwiched between the two 20 x 20 mesh sc~eens. In this connection it i5 noted that the numbers 20 x 20 and 24 x 110 identify the number and arrange-ment of the filter openings of tne screen per s~uare inch.

In further relation to fine screen 13, it is noted that the 3-layered construction above described is not illus-trated in the drawing~ because of the fact that this fin~ screen or screen pack i9 susceptible of being made Up in variou~
combinations to suit particular jobs.

Second breaker plate 14 is also circular and has an outside diameter the same as the outside diameter of first breaker plate 12. It is also Eormed with a plurality of apertures 42 which correspond in numher and size to the aper-tures 41 of the irst breaker plate 12. Thu~, it also contains 121 .062 inch diameter holes in the sam~ area which corres-ponds to the area of first breaker plate 12 within marginal portion 39. It i~ also formed with two diametrically opposed passages 43 which receive dowel pins 40 and thus accurately position second breaker plate 14 with respect to the first breaker plate 12.and fine 3creen 13.

To show the manner in which the subject method and apparatu~ minimizes and eliminates dispersion of color and ~he presence of carbon particles, the following is a table oE
digests taken from evaluation reports Test No. 1 Material: High Density Polyethylene Hours of Test: Every one hour for five hoursO
Results: - .
Molding Without E~ilter Nozzle: During a 24-hour production run, it recorded 5.2% scrap.due to cylinder spots.
Molding With Filter Noæzle~ Scrap rate was reduced for first hour 0.7%, 2nd, 3rd and ~th hours - 0~; 5th hour - .7%.

omments of Reporter:
It was our very first test applying a screen-pac to the machine which was showing a high scrap rate. The screen-pac immediately eliminated cylinder spots. As it was our first test, we opened the nozzle every one hour to replace the screen-pac.

Test No. 2 Material: Polypropylene Hours of Testo One Results:
Molding Without Filter Nozzle: A scrap rate of over ~0%
was recorded due to color shade and color spots.
nozzle with a 0.06" orifice type disc tried but to no avail.
Molding With Filter Nozzle: Screen-pac #1 (20 x 200 x 20) was placed but did not help. Screen-pac #2 (20 x 200 x 200) was then tested. As result, the color ~hade became lighter but it did not compl~tely elimlnate the color shade nor color spots.
Comments of Reporter:
The problem was caused by the design of the pellets which had pits. Titanium pigments adhered to the pits.

Tes~ l~o. 3 Material: High Den~ity Polyethylene Hours of Test: 12 hours test x 2 (24 hours) Results:
Molding Without Filter Nozzle: During a 24-hour produc-tion run, it recorded 5.2% scrap due to cyllnder spots.

Molding With Filter Nozzle: During the first 12 hours of operations, the scrap rate was lowered to 0.45~
and only a few foreign substances were found in the screens. During the second 12 hours, the scrap rate decreased to 0.~7% and the size of carbon spots found in the screens were all in similar ~ize of approximately 0.04".
Comm2nts of Reporter:
Thi~ test was conducted to check the conditions of the screen-pac every 12 hours.

Test No. 4 .

Material: ~igh Density Polyethylene Hours ~f Te~t: 24 hours ~ i2 hours (36 hours) Results:
Molding Without Filter Nozzle: At Monday start-up, when some material purged, it appeared almost brown due to cylinder spot~.
Molding With Filtar Nozzle: After starting the machine with a screen-pac, it first produced 32 pieces of the item with some cylinder spots but it did not last and, during a 24-hour run, the screen-pac produced an excellent result by eliminating cylin-der spots~ The scrap rate for this run was 0.72~.
Comments of Reporter:
After ~ 24 hour run, the screen-pac was taken out.
~nly several pieces of tiny spots found on the screen.
Without clean1ng the screw, we s~arted the same machine and ran it for 8 hours without screen-pac. The scrap rate for this 8-hour rur. record~d 8.;%. Then, a screen-pac was put irl. Durir.y a 12-hour run after putting the screen-pac in, the scrap rate was only 0.82%. The screen-pac and the screw both were checked then and found the former very claan and the latter covered with a thick layer of burned material.

Test No. S

Material. High Density Polyethylene Hours of Test: 1 hour Results:
i~slding Without Filter Noæzle: During a 2~hour produc~
tion run, prior to the placement of a screen-pac, it recorded 22.6% scrap due to color spots. The stock temperature was 358F (average of 3 readings).
Molding With Filter Nozzle: By placing a screen-pac~ it immediately lowered scrap to 0%. The stock tempera-ture was 363F (average of 3 readings)O
Comments of Reporter:
Use o the screen-pac was effective 100% against pigmént lumps of average size 0.04".

Test No. 6 Material: Polypropylene Hours of Test: 3 Results:
Molding Without Filter Nozzle: Recorded color shades due to poor dispersion~ A dispersion disc of 0.06"
orifice was placed but did not eliminate this problem.
The back pressure was raised up to 565 lb/sq.in. with screw rotation of 85 rpm. The stock temperature was 375F ~average).

Molding With Filter Noz~le: With screen-pac #1 (20 x 20 x 20) and stock temperature 380F (average), results as follows: Back pressure 0 lb/sq.in., screw 90 rpm.
SLightly better than using dispersion disc. Back pressure 425 lb/sq.in., screw 130 rpm much better.
Light color shades appeared on the wall side of the items. Back pressure 565 lb/sq.in., screw 130 rpm -excellent, only occasionally light color shade was noticed. With sc.reen-pac #2 (20 x 200 x 200) and stock temperature 385F, ths result3 iden~ical as using screen-pac. However~ with Back pressure 565 lb/sq.in., screw 130 rpm, it almo~t eliminated color shades com~letely.
Comments of Reporter:
In comparison with a 0.06" orifice dispersion disc, the screen-pac produces much better result~. By placing a screen-pac, we were able to eliminate color shades up to the point that it only appear~d occasionally.

.
Test No. 7 Matarial: ~igh Density Polyethylen~
Hour~ of Te~t: 72 Resultsc Molding Without Filter Nozzle~ During two separate 12-h~ur production runs, it recorded, due to cylinder spots, 3.75% scrap and ~.68% scrap, respectively.
Molding With Filter Nozzle:
By placing a screen-pac, it immediately lowered scrap rate as specified below: 1st 12 hours -0.68%, 2nd 12 hours - 0.15%, 3rd 12 hours - 0.45%
(numerous black substances found in the screens), 4th 12 hours - 1.29%, final 24 hours - 0.36%, ~ --Comments of Reporter:
Every 1~ hours the screen-pac was replaced for inspec-tion. Up to 3rd 12~hour test, the scrap rate due to cylinder spots were very low. During the 4th 12-hour test, slightly carbonized cylinder spots were noticed. During th~ final 24-hour test, a yellow cylinder and carbon spots were noticed.

Test No. 8 Material: Polypropylene Hours of Test: 1 Results:
Molding Without Filter Nozzle: The production run could not be continued due to carbonized cylinder spots.
Molding With Filter Nozzle: 9y placing a screen-pac, the carbon spots were broken into granules; however, he item~ were still unacceptable.
Comments of Reporter:
The screen-pac was inspected and found carbonized materials fully packed in the screens. Also, the screw was disassembled for inspection. A black burned material of approximately 0.04" in thickness adhered-to the screw in the area approximately 2. 5 11 f rom the front end. With carbonized materials, w were unable to obtain desirous results.

Test No~ 9 Material: High Density PolyethylenP
Hours of Test: 120 s~

Re~ults:
Molding Without Filter Nozzle: During the week-end shut down, numerous cylinder spots were noticed and at the ~onday start-up, brown cylinder spots were ob-served in purged lumps of materials.
Molding With Filter No2zle: By placing a screen-pac, the scrap rate for 120-hour production run were as follows: Monday - 0.02%, Tue~day - 0.03%, Wednesday - 0.02%, Thursday - 0.02%, Friday - 0.02~.
Commenks of Reporter:
The use of the screen-pac recorded a very good result.
When the screw was disassembled for inspection, inside the cylinder head, a layer of dark brown and yellow burned materials were noticed. The screw head was covered with a layer of buxned materials ~ut none was observed on other parts of the screw. Tha screens were filled with burned materials of ~ize approximately 0.08" ^ 0.12".

Test NoO l_ Material: High Density Polyethylene Hours of Test: 32 Results:
Molding Without Filter Nozzl~: During an 8 hour pro~uction run, it recorded 1.5~ scrap due to carbon spots. Of this 4~-hour run, the final 16-hour production run was made without the ~creen-pac. The scrap rate, then, recorded 6.8~.
Molding With Filter Nozzle: A screen-pac was placed for 32 hours out of a 48-hour production run, lowering the ccrap rate to 1.1~. However, at about the end fi~.~7 of production run, suddenly the scrap rate raised due to cylinder spots and contaminations.
Comments of Reporter:
The screens were chec~ed and found to be in good order. The 200 mesh screen, however, was filled with carbon. It is believed that at an early stage of carbon-izins ~he screen-pac is very effec~ive but once carbonization goes too far the material becomes too tough or the screen-pac to handle.

Test No. 11 Material: High Density Polyethylene Hours of Test: 96 Results:
Molding Without Filter Nozzle: During a previous 96-hour production r~n, it recorded a scrap rate of approxi-mately 4.0% due to cylinder spots of early stage.
~olding With Filter ~ozzle: By placing a screen-pac, it lowered scrap rate as specified below: 1st 24 hours - 0~13%, 2nd 24 hours - 0.4~, 3rd 24 hours -2~8~, 4th 24 hours ~ 0.4 Comments of Rsportero The screen-pac eliminated cylinder spots remarkably.
~he domes~ic screens were used on this test. The effec-tiveness of the screens are same as the U~S. screens but they lack durability. The 200 mesh screen failed during the 3rd 24-hour test run.

Test No. 12 Material High Den~ity Poly thylene Hours of Test: 120 ,J , _ 5~

Results:
Molding ~ithout Filter Nozzle: Every once in a while, scrap due to contaminatlons and color spots came out continuously.
Molding With Filter Noz21e: By placing a screen-pac, it recorded scrap rate as ~pecified below;
1st 24 hours - 0O2%~ 2nd 24 hours - 0.1%, 3rd 24 hours - 0.7~, 4th 24 hours - 0.3~, 5th 24 hours -0.9%.
Comments of Reporter:
This test was conducted with a set of domestic screens to check their durability. The 200 mesh screen failed just before the 120-hour production run was com-pleted. The domegtic screens lack durability when compared to the U.S. screens.

Test No. 13 Material: Polypropylene Hours of Test: 112.
Results:
Molding Without Filter Nozzle: During a 96-hour produc-tion run~ it recorded 6.8% scrap due to cylinder and color spots.
Molding With Filter Nozzle: By placing a screen-pac, the scrap rate was lowered as follows:

May 2 May 3 ~ May_~ Ma~ 6 1st Shift: - 2.5% 1.5% 0.6% 0.5%
2nd Shift: 0.7% 1.2% 0.6% 0.6% 0.5%
3rd Shift- 0.3% 0.6% 0.1% 0.2% 0.3 Comment~ of Reporter:
During this 112-hour production run, the scrap rate due to contaminations and cylinder SpO~5 averaged less than 1.0%, and absolutely no problems of color spots.
Although the screens were used for a continuous 5 days run, they remained in a good condition. When the screens were checked at the end of this run, many black substances were found on the #200 screen.

Test No. 14 Material: High Density Polyethylene Hours of Test: 48 hours Results:
Molding Without Filter Nozzle: The scrap rate was 7.9%
due to cylinder spots and contaminations.
Moldi~g ~ith Filter Noæzle: During the first 24 hours of the operations, the scrap rate was lowered to an average of 2.0% and during the second 24-hour run, the scrap rate averaged only 1.5%.
Comments of Reporter:
Since numerous carboni~ed substances were found in the screens it i-~ believed that the scrap rate would have been much higher without a screen-pac.

Operation While the method of using the apparatus illustrated in the drawings is believed to be obvious from the description of parts and operation set forth, it is brie~ly described as ~ollows:
A fine screen or screen pack assembly such as indicated at 13 i5 first selected to meet ~he conditions of a particular job, such conditions being the particular plastic material being used, the temperature of the molten plastic, and 5~

the pressure which i9 applied thexeto. Nozzle 10 is unscrewed from screen pack retainer 11 and first breaker plate 12 inserted in socket 29 so that it abuts shoulder 36. Fine screen 13 is then positioned in rece~s 37. Second breaker plate 14 is then assembled over fine screen 13 and marginal portions 39 oE first breaker plate 12 by causing the dowel pins 40 to be received in openings 43. Threaded end portion 23 of nozzle 10 is then screwed into the threaded portion of socket 29 and nozzle 10 and screen pack retainer 11 rotated relative to one another to cause the screw threads to clamp the breaker plates together with the fine screen 13 therebetween.

.
While a preferred specific embodiment of the invention is hereinbefore set forth, it is to be clearly understood that the invention is not to be limited to the exact materials, steps and devices illustrated and described because various modifications of these details may be provided in puttin~ the invention into practice.

Claims (14)

WHAT IS CLAIMED Is:

1. In the injection molding of an article from unaided human eye observable carbon particle-contaminated plastics of the family of olefins including polyethylene, polypropylene, linear polyethylene, copolymers, and styrene, the method of aiding dispersion of color and generally breaking up the observable carbon particles into smaller human eye non-observable carbon particles in the molded product, including the steps of:
(a) forcing the plastic in a molten condition and under pressure through a screen pack comprising a breaker plate having relatively large apertures therein and a fine screen means having filter and carbon particle breaking openings smaller than said apertures; and (b) then immediately passing the plastic through an injection nozzle.

2. The method of Claim 1 in which the plastic is also forced through a second breaker plate having apertures larger than the filter openings of said fine screen.

3. The method of Claim 2 in which one of said breaker plates is formed with an opening on one face thereof, the fine screen is received in said opening, and the other breaker plate overlies the fine screen and marginal portion of the first breaker plate, together with the step of clamping said breaker plates together with the fine screen therebetween.

4. In a nozzle assembly for the injection molding of an article from unaided human eye observable carbon particle-contaminated plastics of the olefin family including polyethylene, polypropylene, linear polyethylene, copolymers and styrene:
(a) a nozzle having a barrel formed with a bore termina-ting in an ejection orifice at one end and an inlet for molten plastic at the other end;
(b) a screen pack retainer having means for connecting it to a source of supply of molten plastic under pressure;
(c) screen pack means received in said retainer for breaking up the observable carbon particles to particles of smaller size generally not observable in the molded article by the unaided human eye;
(d) said screen pack means including a breaker plate having relatively large apertures therein and a fine screen means having filter and carbon particle break-ing openings smaller then the apertures in the breaker plate; and (e) cooperating means on said nozzle and screen pack retainer for clamping said fine screen means against said breaker plate.

5. The injection nozzle of Claim 4 together with a second breaker plate having apertures larger than those in the filter openings of said fine screen and clamped against said fine screen and marginal portions of the first said breaker plate by said cooperating means on said nozzle and screen pack retainer.

6. The injection nozzle assembly of Claim 5 in which said cooperating means on the nozzle and screen pack retainer comprises internal threads on said screen pack retainer and external threads on said nozzle.

7. The injection nozzle of Claim 6 together with wrench-engaging means on said nozzle and said screen pack retainer.

8. The injection nozzle of Claim 4 in which said bore is cylindrical throughout a portion of said barrel and terminates in a frusto-conical recess at its inlet end.

9. The injection nozzle of Claim 6 in which the screen pack retainer is formed with a socket having said internal threads and a radially inwardly extending shoulder at the end of said socket which is engaged by the first of said breaker plates.

10. The injection nozzle of Claim 9 together with means for aligning said breaker plates relative to one another so that the apertures in one breaker plate align with the apertures in the other breaker plate.

11. The injection nozzle of Claim 10 in which said means for aligning the breaker plates comprises a pair of diametrically opposed dowel pins secured to one of said breaker plates and detachably received in openings in the other of said breaker plates.

12. The injection nozzle assembly of Claim 5 in which there are 121 apertures in each of said breaker plates with each aperture having a diameter of .062 inch.

13. The injection nozzle of Claim 4 in which said fine screen comprises two 20 x 20 screens with a 24 x 110 screen sandwiched therebetween, with said numbers identifying the number and arrangement of filter openings per square inch of each screen.

14. The injection nozzle of Claim 5 in which the circular recess in said first breaker plate is defined by a ring-like marginal portion and the number and arrangement of apertures in the second breaker plate correspond in number and arrangement with the apertures in the first breaker plate within said marginal portion.