GB2050929A - Injection molding method utilizing a low shear screw - Google Patents

Abstract

An injection molding method is described wherein the forward portion of a screw is so designed that mixing and shear of plastics material is minimized; this is advantageous when processing glass reinforced materials because high shear processing would break the fibers. Low shear processing of the material in the forward portion of an injector barrel (10) is accomplished by providing a screw (22) having a fully flighted rear portion (36) and a flightless forward portion (38); this gives an unobstructed, annular flow passage (52) formed between the outer periphery (50) of the flightless portion (38) and the inner surface (34) of the barrel (10). The material is softened by the rear portion and is forced forwardly through said passage by the pressure developed as the screw continues to rotate. <IMAGE>

Description

SPECIFICATION Injection moulding method utilizing low shear screw The present invention relates to a method for injection molding plastics material under low shear conditions in the forward portion of the barrel. More specifically, the invention relates to a method and apparatus for producing articles of glass reinforced materials.

The type of injection molding apparatus in wide use today comprises an elongated barrel having a screw which extends longitudinally therein. The screw has a helical land on its surface which moves relative to the internal surface of the barrel so as to work the plastics material and feed it toward the outlet end of the barrel. In order to inject the material, the screw is advanced forwardly toward the discharge opening so as to force the softened material into the mold cavity. When molding glass reinforced materials, such as BMC, breakage of the glass fibers is a problem when excessive shear is performed on the material.Although a certain degree of shear is inevitable if the material is to be conveyed forwardly by the rotating screw, if a low shear environment can be provided for the material after sufficient pumping pressure has been developed, then an improved product would result.

Although the use of a short screw with a length to diameter ratio of 13 would tend to minimize shear, the excessive pressures within the intermediate portion of the barrel during the injection stroke may cause the barrel to burst. Furthermore, this necessitates stocking an abnormally short barrel, which may not be used frequently.

The present invention overcomes the abovediscussed disadvantages and problems of the prior art apparatus and methods by providing an injector unit wherein low shear processing of the material takes place in the forward portion of the screw so that the unit can be used for glass reinforced materials. This is achieved by a modified screw having a fullyflighted rear portion and a forward portion which is unflighted and preferably of a smaller diameter than the root diameter of the flighted section of the screw. Consequently, the material undergoes very little shear in the unflighted section so that the condition of the material leaving the unflighted section is very similar to that entering it, except that its temperature could be raised or lowered by heating or cooling, depending on the requirements dictated by the particular application.

The ability to transfer heat to or from the barrel in the unflighted section is advantageous, because the desired temperature of the material as it exits the barrel can be very precisely controlled without the complicating factors resulting from a screw in the forward portion performing mechanical work on the material.

By utilizing in a conventional barrel a screw having a substantial portion of its length, for example, twenty-five percent to fifty percent or greater, which is unflighted, the screw behaves like a shorter screw vet without the problems of excessivelv high iniection pressures rearwardly of the forward section of the barrel. As discussed in the previous section, barrels are conventionally constructed such that they will tolerate the high injection pressures only in the forward portion of the barrel, and if a shorter screw is used, the area of high pressure is extended rearwardly to a position in the barrel that is not capable of withstanding it.

Such an injector unit is particularly advantageous when processing two or more materials of diverse colors in a conventional barrel because the heating and shear imparted to the materials as it is conveyed forwardly by the flighted, rear portion of the screw is sufficient to soften the materials and partially blend them so that distinct color patterns are present. As the partially blended, softened materials are pumped forwardly through the annular chamber between the flightless section of the screw and the inner wall of the barrel, little additional mixing will take place, although the material can be further heated or cooled by the extraction from or giving up of heat to the heated or cooled barrel.Since the screw in total length is matched to the length of the barrel using conventional design criteria, the high injection pressures developed forwardly of the screw are confined to the forward portion of the barrel, which is constructed so as to be capable of withstanding them.

Specifically, the present invention contemplates using an injection molding machine comprising a barrel having an inner wall and a discharge opening at the forward end of the barrel, a screw longitudinally received in the barrel, and rotation means operably connected to the screw to rotate the same.

The screw includes conveying means on the rear portion thereof to displace the material forwardly as the screw is rotated, and low shear means on the forward portion of the screw to permit the material displaced forwardly by the conveying means to flow between the inner wall of the barrel and the screw forwardly of the screw with a minimal amount of mixing of material and with a minimal amount of shear energy being imparted to the material by the screw. The low shear means comprises an unflighted portion of the screw defining an annular passageway between the barrel inner wall and the screw forward portion and extending along a substantial portion of the length of the barrel. Translation means operatively connected to the screw drives the screw forwardly relative to the barrel to displace the softened material through the discharge opening and into the mold cavity.

The apparatus described above is utilized to process glass reinforced materials such as glass reinforced polyester theremosets. The conveying zone of the screw, which is the flighted portion, accepts material from the feed opening and generates sufficient pressure to force material along the unflighted section of the screw and through the non-return valve. In the unflighted, low shear zone, areas of localized high shear are absent so that a minimum amount of fiber degradation occurs. The relative movement between the rotating screw and the inner wall of the barrel produces, however, a gentle circumferential mixing which aids wet-out of the material. The invention permits a long screw to process glass reinforced polyesters and other material, since the flighted conveying zone can be kept short.Thus, conventional length injection molding barrels can be utilized for the injection molding of this type of material.

Specifically, the present invention contemplates a method for injection molding particles of fiber reinforced plastics material comprising: providing a barrel having a discharge opening at the forward end thereof and an inner wall, providing a screw in the barrel comprising a rearflighted portion and a forward unflighted portion adjacent the flighted portion, feeding glass reinforced plastics material into the barrel to the flighted portion of the screw, rotating the screw to displace the materials forwardly into an annular passage defined between the barrel and the unflighted portion of the screw, flowing the material through the annular passage and forwardly of the screw with a gentle, circumferential mixing action and with a minimum amount of shear so that breakage of the fibers in the material is minimized, and translating the screw forwardly to displace the material in front of the screw through the discharge opening.

Figure 1 is a longitudinal sectional view of part of an injection molding machine used in the method of the present invention; Figure 2 is an elevational view of a modified screw adapted for use in the invention; and Figure 3 is a sectional view taken along line 3-3 of Figure 2 and viewed in the direction of the arrows.

Referring now to Figure 1, there is illustrated an injection molding machine constructed in accordance with the present invention and particularly adapted for injecting glass reinforced materials into a mold cavity (not shown). The machine comprises an injector barrel 10 having connected thereto a material feed unit 14 comprising a feed chamber 16 and a hopper 18. A large lock nut retains barrel 10 and feed unit 14 in assembled position.

Longitudinally received within barrel 10 for rotational and translational movement is a screw 22 modified in accordance with the present invention.

The rear portion 24 of screw 22 extends through ram stop 25 into injection ram 26, and is keyed to ram 26 so that it can be rotated thereby. A nozzle 28 is secured to the forward end of barrel 10 and includes a discharge passage 30 therein through which the molten plastics material is injected into the mold cavity (not shown). Heating or cooling bands 32 encircle barrel 10 at selected locations and serve to maintain the inner wall 34 of barrel 10 at the desired temperature so as to control the temperature of the plastics material before it is injected into the mold.

For certain types of materials, it is necessary to cool barrel 10 rather than heat it, in which case cooling fluid would be circulated through barrel 10. With the exception of the modification to screw 22 which will be described below, the injection molding machine described above is of conventional construction.

Screw 22 is of integral construction and comprises an unflighted rear portion 24 having a diameter approximately equal to the inner diameter of barrel 10, a fully flighted feed, transition and metering section 36, and an unflighted, low shear section 38.

The fully flighted section 36 includes a feed section 40 having generally constant channel depth and communicating with the throat 42 of feed chamber 16. Adjacent feed section 40, is the transition section 44 wherein the channel depth decreases in a continuous fashion up to the shallow channel depth present in the metering section 46. In conventional injection molding machines, a screw having the feed, transition and metering sections just described is not uncommon, except that these sections would together constitute the entire screw rather than just the rear portion as in the case of the present invention.

At the forward end of the metering section 46 is a tapered shoulder 48 which provides an abrupt transition from the larger diameter root of the metering section to the smaller diameter outer surface 50 of unflighted, low shear section 38. The outer surface 50 of section 38 and the inner wall 34 of barrel 10 together define an annular passageway 52 extending from metering section 46 to the forward end 54 of screw 22. Since the material pumped forwardly by the flighted section 36 of screw 22 flows through a generally unobstructed annular passageway 52, it undergoes very little shear so that mechanical working of the materials are held to a minimum. It is desirable that the diameter of the unflighted, shaft section 38 of screw 22 be as small as possible without exceeding the compressive strength limits dictated by the injection pressure.

In order to support the unflighted section 38 within barrel 10, it may be desirable to provide bearings 56, which are in the nature of radially extending ribs having tapered forward and rear ends 58 and 60, respectively. In some cases, and depending on the length of unflighted section 38, bearings 56 may not be necessary. It is desirable that bearings 56 be kept as short and narrow as possible within the screw structural constraints so that shearing of the material flowing past them will be minimized. Even with the presence of bearings 56, however, the annular passageway 52 between metering section 46 and the forward end 54 of screw 22 is generally unobstructed.

Attached to the forward end 54 of screw 22 is an annular non-return valve 62, which is designed to prevent the back-flow of material during injection.

The non-return valve 62 comprises a retainer 64 threadedly secured to the forward end 54 of screw 22, a check ring 66 received over retainer 64, and a retainer pin 68 extending through retainer 64 and received within a pair of slots 70 in check ring 66. By virtue of slot 70, check ring 66 can reciprocate relative to retainer 64 so that the nose 72 of retainer 64 will seat against the correspondingly tapered seat 74 of check ring 66 during the injection stroke, thereby preventing rearward flow of material through valve 62. During the feeding cycle, check ring 66 will be pushed forwardly by the forward pressure of the material pumped by screw 22, so that material can flow through valve 62.

In operation, plastics material reinforced with glass fibers is fed into barrel 10 by feed unit 14, and is conveyed forwardly by the threaded, rear section 36 of screw 22. As the material is pumped forwardly by the feed, transition and metering sections 40, 44 and 46, respectively, it is mechanically worked and, in the case of thermoplastic material, melted to a softened state.

The barrel 10 is substantially longer than the flighted section 36 of screw 22 and, to avoid excessively high pressures within the forward portion of barrel 10, a screw the length of the flighted section 36 is not practical. In order to apply injection pressure to the materials further forward in barrel 10, screw 22 includes unflighted section 38 so that the material can flow forwardly in barrel 10 through annular passageway 52 under low shear conditions.

The materials are pumped forwardly due to the forward pressure produced by additional material being pumped by the flighted section 36 of screw 22.

Except for the relatively small amount of agitation produced by bearings 56, the only mechanical working of the materials is that caused by the relative rotation between the outer, cylindrical surface 50 of unflighted section 38 and the circular inner wall 34 of barrel 10. Since this action is relatively gentle, very little additional mixing of the materials results.

As screw 22 rotates, the material is worked and heated so as to bring the materials to the desired final temperature and viscosity. Because the forward portion of screw 22 is not flighted, the additional complicating factor of additional heating of the materials produced by shear is not present, and easier control of the final melt temperature and viscosity can more easily be controlled, simply by controlling the temperature of the barrel 10.

The materials flow around the forward end 54 of screw 22, through valve 62 and build up pressure in front of screw 22 thereby causing it to retract. When the desired charge has been accumulated from the screw 22, it is translated forwardly by injection ram 26 thereby injecting the material through injection passage 30 into the mold cavity.

Although the dimensions of screw 22 will depend on the particular materials being injected and on the size of barrel 10, the following dimensions may be considered exemplary. Screw 22 has a length to diameter ratio of 26:1, a feed section 40 which is 35.0 in. in length, a transition section 44 which is 10.5 in.

in length, and a metering section 46 which is 3.5 in.

in length. The channel depth for feed section 40 is .433 in., and for metering section 46 is .25 in. The length of unflighted section 38 is 44.7 in. and the outer diameter thereof is 2.38 in. Bearings 56 are 2.5 in. in length, .62 in. in width, and the outer diameter of the bearing portion is 3.49 in.

For certain glass reinforced materials where fiber breakage is especially troublesome, even less shearing will be necessary. A screw 76 suitable for this purpose is illustrated in Figure 3 and comprises a long feed section 78 having a generally constant channel depth, which is slightly deeper than the channel depth of screw 22. The unflighted section 80 of screw 76, like screw 22, has a cylindrical outer surface 82 and includes bearings 84 integral therewith. With the exception of the fact that the flighted + 70 ;##i. inol +#,,,#;+;,,,, ~nrl mufWrinn sections 44 and 46, respectively, screw 76 is essentially identical to screw 22. Of course, the various dimensions, including the length of the unflighted section 80 relative to the flighted section 78, may be modified depending upon the particular application.

Due to the fact that the unflighted section 80 produces Due to the fact that the unflighted section 80 produces very little shear on the thermoset material, breakage of the glass fibers can be minimized, and wet-out will be enhanced by the gentle circumferential mixing which takes place at the inner wall 34 of the barrel 10.

As the material is pumped forwardly, screw 76 will retract, and when the desired charge has been accumulated, the screw 76 will be translated forwardly so as to displace the thermoset material into the mold (not shown).

While this invention has been described as having a preferred design, it will be understood that it is capable of further modification. This application is, therefore, intended to cover any variations, uses, or adaptations of the invention following the general principles thereof and including such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and fall within the limits of the appended claims.

Claims (8)

1. A method for injection molding articles of fiber reinforced plastics material comprising: providing a barrel having a discharge opening at the forward end thereof and an inner wall, providing a screw in the barrel comprising a rear flighted portion and a forward unflighted portion adjacent the ffighted portion, feeding glass reinforced plastics material into the barrel into the flighted portion of the screw, rotating the screw to displace the materials forwardly into an annular passage defined between the barrel and the unflighted portion of the screw, flowing the material through the annular passage and forwardly of the screw with a gentle wiping action and with a minimal amount of shear so that breakage of the fibers in the material is minimized, and translating the screw forwardly to displace the material in front of the screw through the discharge opening.

2. The method of Claim 1 wherein the diameter of the unflighted portion of the screw is less than the diameter of the root of the flighted portion of the screw.

3. The method of Claim 1 wherein the unflighted portion is at least twenty-five percent as long as the flighted portion.

4. The method of Claim 1 wherein the unflighted portion is at least fifty percent as long as the flighted portion.

5. The method of Claim 1 wherein the screw unflighted portion includes a bearing portion comprising a plurality of circumferentially displaced rnrlinl rihs .-d"nte d to nartiallv suooortthe screw in the barrel.

6. The method of Claim 1 wherein the material is fed into the barrel through a feed inlet communicating with the flighted portion of the screw, the flighted portion is immediately adjacent the unflighted portion, and the flighted portion has a substantially constant root between the feed inlet and the unflighted portion.

7. A method for injection moulding of fiber reinforced plastics material, said method being substantially as herein before described with reference to the accompanying drawings.

8. Any features of novelty, taken singly or in combination, of the embodiments of the invention hereinbefore described with reference to the accompanying drawings.