WO2006021113A1

WO2006021113A1 - Method and device for preparing plastic melts

Info

Publication number: WO2006021113A1
Application number: PCT/CH2005/000477
Authority: WO
Inventors: Andreas Lind; Roger Schirmer; Robert Weinmann
Original assignee: Netstal-Maschinen Ag
Priority date: 2004-08-27
Filing date: 2005-08-18
Publication date: 2006-03-02
Also published as: EP1802434A1; WO2006021113B1

Abstract

The invention relates to a method and to a device for preparing melts by means of an injection screw (4) and a heated cylinder (3), in addition to the shooting-like transfer of the melt by a cylinder head (1) and an injection nozzle (2) which can be closed, preferably, in the cavity of an injection mould (12). For each injection, all points of surfaces of the cylinder head (1) and the injection nozzle (2), which touch the melt flow, are rinsed for at least a minimum amount of time by producing a sufficiently high shear speed of the melt. The plastic melt is prepared by means of an injection screw (4) and a heated cylinder (3), and the melt is transported over the cylinder head (1) and an injection nozzle (2) which can be closed, preferably, in an injection mould (12).

Description

Process and device for the treatment of plastic melt

Technical area

The invention relates to a method for the treatment of melt by means of an injection screw and a heated cylinder and shot-wise transfer of the melt via a cylinder head and a preferably closable spray nozzle in the cavity of an injection mold. The invention further relates to a device for processing plastic melt by means of an injection screw with a heated cylinder and transfer of the melt via a cylinder head and a preferably closable spray nozzle in an injection mold.

State of the art

In the injection molding of plastic parts, the raw material in a heated cylinder is usually treated by means of an injection screw as a liquid melt and cyclically or shot fed to the injection mold. The cycle time depends on many factors. Two of the key factors are the melt temperature and the time required to cool the cast mold to sufficient strength to remove the parts from the injection mold. From the point of view of productivity, it is advantageous not to increase the melt temperature too much in order to limit the cooling effort. In most cases, an optimum in terms of melt flow properties, temperature and melt quality is sought to fill the melt into the cavity. In conventional injection molding, the melt is heated to a temperature of 20O ⁰ C - 450 ⁰ C. For the production of optical data carriers, for example CDs, polycarbonates are used as the raw material. The characteristic in the production of CD's is the extremely short cycle time of about 2 - 3 seconds. CD's demand the highest quality surface. This is generated in a special embossing step and the desired information is imprinted directly in many cases. In order to obtain the unimaginable density of the impressed information for a layman, highest demands are placed on the quality of the melt. Recent practice has shown that with respect to the injection molding of optical discs on the Based on polycarbonate higher melt temperatures up to 35O ⁰ C bring great benefits. Investigations have shown that best surface qualities are achieved close to the thermal load limit. This is for example polycarbonate in the range up to 400 ⁰ C. It is very important that the melt during the processing and the transfer is uniform in the casting mold and to local temperature variations are below 3 ⁰ C. According to investigations by the manufacturer of the polycarbonate raw material, a substantial material degradation already takes place in the melt after two hours at 400 ^° C. As part of the injection molding process with the corresponding residence times, according to the most recent practical tests, an upper limit of about 37O ⁰ C has been found. With 35O ⁰ C to 37O ⁰ C Although best injection molding qualities in terms of surface finish could be achieved, but with the disadvantage that in this temperature range in the finished CD's inclusions of degraded material are detectable, which represent an unacceptable loss of quality and the finished injection molding for Make committee.

The invention has now been set the task of finding ways and means to obtain high and highest melt temperatures maximum qualities for the finished injection molded products, especially for optical media.

Presentation of the invention

The method according to the invention is characterized in that, for each shot, all points of the surfaces of the cylinder head contacted by the melt flow and of the spray nozzle are rinsed at least briefly by generating a sufficiently high shear rate of the melt.

The inventive device is characterized in that the melt channel from the cylinder to the annular gap of the nozzle closure needle is formed as dead volume as possible.

Systematic investigations with solutions of the prior art showed that the required manufacturer's maximum local temperature difference has not been exceeded, for example, 3 ⁰ C at any point in the melt preparation of the melt and transfer into the mold. Nevertheless, a pronounced material degradation took place at highest melt temperatures. There were pronounced encrustations in individual areas of the surface in the cylinder head and at the spray nozzle. Repeated experiments with solutions of the prior art showed that these are only the two sites with strong cross-sectional extensions.

It was also interesting to observe with the prior art solutions that over the entire length of the injection screw, including the area of the screw tip, no deposits of degraded material, either on the heated cylinder or on the injection screw, were detectable. Based on the current state of knowledge, this is due to the following factors: In the area of the injection screw, the mechanical movement of the screw produces a high shear rate and thereafter prevents any adhesion of melt. The residence time in the area of the injection screw is still within the permissible time limit, in which there is still no material degradation. The basic requirement of the new invention lies in the fact that the processed melt must be continuously transferred from the heated cylinder into the casting mold at a high shear rate. According to the device, this means that at high melt temperatures, the melt channel from the cylinder tip to the annular gap of the nozzle closure needle should be designed as free of dead volume as possible. For a good rinsing effect it is sufficient if at least once per shot an intensive rinsing of all of the melt flow touched inner surfaces is performed. An important goal of the new solution is that the wall flow is at least once per shot as high as possible for a short time.

It follows that the purging action in the region of the cylinder outlet and in the region thereafter must indeed be achieved with the same basic idea of the at least temporarily high shear rate, but structurally with different means. During dosing, the amount of melt required for a shot is prepared in the injection screw antechamber. During the dosing process, no movement takes place in the injection screw antechamber. It is created as it were a big "dead space". During injection, the prepared quantity of melt is moved forward at a considerable speed in the direction of the spray nozzle by the linear movement of the injection screw. An important point here is that at the end of the dosing process only a small residue remains, so that between the screw tip and an adapted shape of the cylinder outlet the smallest possible gap. Due to a special design of the screw tip as well as the cylinder outlet can with support of the mechanically moved screw tip in the cylinder outlet an intensive flushing after injection - A -

and at the beginning of dosing the respective wall-related areas.

The focus is on the new knowledge that in the area of the cylinder outlet, a mechanical movement of the screw tip near the wall is carried out and then, with high flow velocities, an optimum effect is achieved which prevents any deposition of hot melt. Deposits mean a multiplication of the residence time for the deposited material. The experiments showed that of the deposited material in great irregularity smallest "shreds" are entrained with the melt stream. As a result, these inclusions in the form of already degraded material in the finished injection-molded part occur and render it useless.

According to a particularly advantageous embodiment, the transfer of the melt from the cylinder outlet to the spray nozzle during the injection phase is carried out continuously with high shear rate of the melt flow. In this case, a distinction is made between the melt in the heated cylinder and the melt path after the cylinder outlet. As soon as the melt has left the cylinder outlet, there are no moving parts in the melt flow. So only with high shear rate the necessary rinsing effect can be achieved.

Preferably, the melt is guided while avoiding locally increased residence times from the cylinder outlet via kinked torpedo bores into the annular gap of the nozzle closure needle. The melt rate is selected in the transition region from the torpedo bores to the annular gap of the nozzle closure needle so that the shear rate does not fall near the wall. The melt rate can be increased in the transition region, for example by at least 30%. By choosing a large number of, for example, six torpedo bores, the sum of the partial cross sections results in a larger cross section. Due to the relatively small diameter, the shear rate near the wall is very high, as the average flow velocity is 1 - 2 m / sec. aimed and a shear rate of greater than 200, preferably 500 to 1000 m / min, and more.

In the previous experiments with the new invention was processed as a raw polycarbonate and the melt heated to a temperature of about 35O ⁰ C for the production of optical data carriers, such as CD's. The new invention allows a number of particularly advantageous embodiments, to which reference is made to the claims 2 to 8 and 10 to 21. The previous experiments were carried out with the raw material polycarbonate, wherein the melt was heated to a temperature of about 35O ⁰ C. However, the new solution is not limited to polycarbonate but can be used anywhere where the shortest possible residence time of the melt in the cylinder head and in the spray nozzle is advantageous.

According to a particularly advantageous embodiment of the method, it is proposed that the melt, while avoiding locally increased residence times, be guided from the cylinder outlet via torpedo bores into the annular gap of the nozzle closure needle. The transition of the melt channel at the cylinder outlet in the torpedo bores has a short cylindrical constriction. The torpedo bores are transferred in the inlet area into the nozzle tip into an annular gap flow. The torpedo bores are continuously transferred into the annular gap near the annular gap in the direction of the nozzle closure needle. The cylinder outlet has a similar cross section. The cross sections are based on the smallest flow cross section known in the prior art. The kinked torpedo bores consist of two groups of outwardly bent and symmetrically arranged bores, which are separated and fed together at an acute angle and continuously transferred into the annular gap around the nozzle closure needle. Preferably, the free flow area of the cylinder piece A in the ratio 0.7 to 2 corresponds to the sum of the torpedo bores B.

The transition of torpedo bores is smaller than the annular gap at the beginning of the nozzle closure needle and results in an acceleration of the melt flow in the direction of the nozzle closure needle.

Most preferably, the torpedo bores are formed from two groups of outwardly bent and symmetrically arranged bores. The torpedo bores thus guided give sufficient free space for the mechanical parts for the nozzle closure needle, without affecting the flow path of the melt. This results in a very advantageous embodiment in that the cylinder head from the cylinder outlet to the spray nozzle forms an assembly which has an annular gap-like inlet opening and an annular gap-like outlet opening. The nozzle lock needle and the engagement point for the valve needle with an actuating lever are arranged in the continuation of the injection screw axis and the actuating lever between the two groups of torpedo bores in the region of the kinks of the bores. Preferably, the flow cross section between the cylinder tip and the nozzle closure needle is made approximately constant, wherein in particular the transitions between the torpedo bores and the two annular cross sections are formed with constant flow cross sections.

With the new invention, a channel-like flow with high shear speed is generated at least temporarily over the entire melt flow path during each shot.

In a further advantageous embodiment of the cylinder inlet is transferred into the cylinder outlet, such that the entire screw tip penetrates in the frontmost position in the cylinder head. The cylinder head corresponds with its outer dimensions on the inlet side of the outer cross section of the heated cylinder and has on the outlet side a reduced cross section corresponding approximately to the outer cross section of the transition piece of the spray nozzle. This allows to heat the cylinder head at least in the first part. The spray nozzle can be screwed in on the outlet side of the cylinder head. In this case, the transition from the outlet opening of the cylinder head in the screw part is annular.

Brief description of the invention

The invention will now be explained with reference to some embodiments with further details. Show it:

Figure 1 a a cylinder head with lockable spray nozzle of the prior

Technology; Figure 1 b is a section I of Figure 1 a; Figure 2 shows the structural design of the mechanical actuator for the

Nozzle valve pin; FIG. 3 shows an intermediate piece of the cylinder head according to FIG. 1 a with recognizable encrustations on the inner wall; FIG. 4 shows a micrograph of inclusions of degraded material in a finished injection-molded part; Figures 5 and 6 show a comparison of a solution of the prior art

(Figure 5) and the new solution (Figure 6); Here are the figures 5b and 5c resp. 6b and 6c each two different perspective

representations; Figure 7 shows an example of a novel spray head with spray nozzle; FIG. 8 schematically shows the area of a screw tip with the transition into the torpedo bores; FIG. 9 also schematically shows the course of the flow from the cylinder to the cylinder

Area of the spray nozzle.

Ways and execution of the invention

Figures 1 a and 2 show a section through a cylinder head 1 and a spray nozzle 2 of the prior art. The cylinder head 1 is attached to the heated cylinder 3. It can be seen within the cylinder 3, the injection screw 4 with its screw tip 5 and the injection screw axis 39. The cylinder 3 and a transition part of the cylinder head 1 are provided with a heating mantle 6. The cylinder head 1 has on the left side, in the region of the actuating mechanism 8, a reduced outer diameter d. The spray nozzle 2 is screwed via an external thread 9 in the front part of the cylinder head 1 and is brought over a hexagon 10 to a tight fit. The spray nozzle 2 has a slender tip 1 1, which penetrates into the injection mold 1 2. This is only hinted at. In the spray nozzle 2, a nozzle closure needle 1 3 extends in the annular gap 37 over its entire length and protrudes with a pin 14 to the contact point with an actuating lever 1 5. The operating lever 1 5 is connected via a rod 28 by a drive, not shown in the clock of Injection process activated. Depending on the position of the nozzle closure needle 1 3, the nozzle opening 16 is closed or released for injecting the melt via the injection channel into the cavity of the injection mold 1 2. For the construction of the front part of the injection unit, there are two difficult transition areas , The first region is the transition region 18 between the screw tip 5 and the torpedo bores 19. The second region is the transition 20 from the torpedo bores 19 into the inlet 21 of the spray nozzle 2. The transition region 18 is in the form of a cylinder 22 and the transition 20 as a cone 23 educated. The investigations have shown that the area of the cylinder 22 and the area of the cone 23 with increasing melt temperature at over 300 ⁰ C problems arise by deposits occur in both areas on the corresponding conical or cylindrical walls in particular at the cylinder outlet 27 after a certain period of operation , In the conical part, the deposit is at one hardened plug 24 shown as a photographic photograph in the upper left of Figure 1 a. The white part is hardened, perfect material. The black part consists of degraded, unusable material. If only the smallest parts of the mined material in the mold cavities are entrained by the melt stream, the injection molding part in question is rejected.

FIG. 3 shows an opened cylinder head with a view in the direction of the worm tip. Very interesting here is that the area 25 near the injection screw is clean, whereas the central area 26 is encrusted.

FIG. 4 is a micrograph of an encrusted particle 29.

FIG. 5a shows the solution of the prior art according to FIGS. 1 a, 2 and

3. Here are the three areas

Area 20 with the conical inner shape,

- Area 1 8 with the cylindrical inner shape as well

- Area of torpedo holes 1 9 dotted highlighted.

Figures 5b and 5c show the figure 5a in a different perspective view. It can clearly be seen that the torpedo bores 1 9 are arranged in parallel.

Figure 6a shows the new solution in the same schematic structure. The region 20 with the conical inner shape is completely replaced by a continuous transition 30 of kinked torpedo bores 35 into an annular outlet cross section 31. The area 1 8 with the cylindrical shape of Figure 5 is replaced in the new solution according to Figure 6 a) by a conical portion 32 with a conically tapered shape 33, which follows approximately the outer contour of the screw tip 5, b) by an annular transition 34 to the kinked torpedo bores 35. Both transitions 33 and 34 before and after the kinked torpedo bores 35 are designed so that there is a nearly constant and the largest possible flow velocity. It can be clearly seen from FIGS. 6b and 6c that in the new solution, preferably bent torpedo bores 35 are used. Each bore has an outwardly directed kink 36. This allows the torpedo bores 35 on the input side and output side merge on a much smaller ring cross-section, without reducing the free space for the shutter mechanism for the nozzle closure needle 1 third FIG. 7 shows a highly schematic view of the new solution. On the right is still hinted way the heated cylinder 3 with an outer diameter d.zyl. and an inner diameter d.sch. (for the injection screw diameter). Clearly, the contours are recognizable by reference numeral 40 as a transition from the inner diameter of the cylinder in the conical shape 32 and in the cylinder outlet and the point 41 as Torpedobohrungseintritt or as a transition from the conical shape 32 in the kinked Torpedobohren 35 with the kink 36, which lies approximately halfway along the kinked torpedo bores 35. The subsequent transition is the torpedo borehole exit 42. The decisive factor is that not only the torpedo bores but also the transitions have a small cross-section and no sudden cross-sectional jumps are present. The cylinder head is strictly adapted to the functionality. The operating lever is arranged in the widened part of the cylinder head 1. The same applies to the kinks of the torpedo wells. This has been created for the actuating mechanism for the nozzle closure needle 1 3 more space. Another important aspect is that with the new solution improved and more comfortable cleaning could be achieved both for the cylinder head 1 and for the spray nozzle 2.

Figure 8 shows schematically the area of the screw tip, approximately in the frontmost position with respect to the cylinder. A decisive point in this case is the gap "S", the region of an outlet cone 50 of the inner cylinder 51 and the cylinder 22. The solution shown is typical of an injection molding machine for the production of CD's. The foremost part of the injection screw 4 is formed by a return flow lock 52, which is screwed into the melting part via a thread 53. The return flow lock 52 consists of a shaft 54 which has the thread 53 at the rear end and the worm tip 55 at the front end. The tip of the screw 55 has spacer wings 56, which are similar in shape to the front cone with the exit cone 50. The actual tip 57 has a slight chamfer 58, which in the front position at the end of the injection or at the beginning of the dosing process into the transition region between the Cylinder exit 40 and the torpedo bore entrance 38 penetrates. Depending on the position and movement of the injection screw for opening or closing the return flow lock, which is assumed to be known, a return flow blocking ring 61 which is freely movable between a front stop 59 and a front stop 60. It can be seen from FIG. 8 that the entire area of the outlet cone 50 is flushed particularly well by the movement of the wings in the foremost position of the screw tip 5. The spacer wings 56 ensure their proximity to the inner Wall of the outlet cone and thus for a high shear rate. The shear rate is defined by the relative velocity of two moving parts (V) and the distance "S" of the two parts:

V m / sec 1

^ ^~ "S" m sec.

The chamfer 58 causes a swirl flow in the Torpedobohrungseintritt

38, so that even at this critical point a good flushing effect can be achieved.

FIG. 9 schematically shows the flow path from the end of the cylinder 3 into the region of the spray nozzle. For the illustrated flow cross sections (black) different sections were chosen without regular subdivision.

Oz = = => is a cross-section in the foremost region of the backflow preventer,

Ok = = => is approximately in the middle of the exit cone 50.

A = = => is the transition from the exit cone 50 into the torpedo bore entrance 38.

B = => is the kink of the torpedo bores 35.

C = => is an area of transition of the torpedo bores into the annular transition to the spray nozzle. D = => is the ring cross-section in the nozzle lock needle.

The cross sections, which are designated I, are relatively large and have a relatively large flow cross-section. However, this is due to the fact that in the illustration, the screw tip is not yet in the front position and thus the gap "S" is still relatively large.

In the figures Il, one already recognizes the chamfer 58 on the left. The image II left outside shows the free, full passage cross-section at the point A.

The cross sections III show the transition from the point A to the auseinander¬-going holes 35th

The cross section IV shows the kink of the torpedo bores, on which the actuating lever 1 5 engages. The reference symbol D denotes the greatest distance between the two groups of holes on the right (35r) and on the left 35I). The cross sections V show the merging of the torpedo bores 35 up to a pure ring cross-section, the last image, left outside. From Figure 9 it can be seen that the flow cross section in the range O - A by the longitudinally displaceable injection screw 4, respectively. the screw tip 55 is changed. In the area A - C, the torpedo boring cross sections must be selected so that a sufficiently high shear rate is obtained.

It is an optimization with increasing pressure loss with increasing flow velocity and the bore diameters. The area of C - D is no longer heated. By accelerating the flow rate, an optimal rinsing effect is achieved.

Only two kinked torpedo bores 35 are shown in FIG. 10, which can be produced by a simple drilling process. The exit end of all torpedo bores is adapted by a cylindrical cutter to the shape of the cylinder outlet 40.

Claims

claims

1. A method for the preparation of melt by means of an injection screw (4) and a heated cylinder (3) and shotwise transfer of the melt via a cylinder head (1) and a preferably closable spray nozzle (2) in the cavity of an injection mold (12), characterized in that, for each shot, all points of the surfaces of the cylinder head (1) contacted by the melt flow and of the spray nozzle (2) are rinsed at least briefly by generating a sufficiently high shear rate of the melt.

2. The method according to claim 1, characterized in that the transfer of the melt from the cylinder outlet (27) to the spray nozzle (2) takes place during the injection phase continuously with high shear rate of the melt flow.

3. The method according to claim 1 or 2, characterized in that the melt is guided while avoiding locally increased residence times of the cylinder outlet (27) via kinked torpedo bores (35) in the annular gap (37) of the nozzle closure needle (13).

4. The method according to any one of claims 1 to 3, characterized in that the melt in the transition region from the torpedo bores (35) to the annular gap (37) of the nozzle closure needle (13) is transferred with a constant change in speed.

5. The method according to any one of claims 1 to 4, characterized in that processed as a raw polycarbonate and the melt is heated to a temperature of about 35O ⁰ C, for the production of optical data carriers, such as CD's.

6. The method according to any one of claims 1 to 5, characterized in that the cylinder outlet (27) and the transition region in the torpedo bores (35) cyclically after injection or embossing at start dosing, supported by the rotational movement of the injection screw (4) , is rinsed briefly.

7. The method according to claim 6, characterized in that the screw tip (5) and the cylinder outlet (27) is conical, wherein the screw tip (5) at the end of the injection process to close to the conical cylinder outlet (27) moves, in such that with the blades (56) arranged on the screw tip (5) a short and strong flushing action is produced in a gap "S" by a correspondingly high shear rate.

8. The method according to any one of claims 6 or 7, characterized in that the screw tip (5) is moved at the end of the injection process into the Übergangs¬ or inlet region of the torpedo bores (35), wherein the screw tip (5) with bevelled tip (58) is formed and generates a swirl for the melt in the transition region.

9. Device for processing plastic melt by means of a Einspritz¬ screw (4) with a heated cylinder (3) and transfer of the melt via a cylinder head (1) and a preferably lockable spray nozzle (2) in an injection mold (12), characterized in that the melt channel from the cylinder outlet (27) to the nozzle closure needle (13) is formed as dead volume free as possible.

10. The device according to claim 8, characterized in that the melt channel between the Zyinderausgang (27) up to an annular gap (37) in the spray nozzle (2) is formed as a bent torpedo bore (35).

11. The device according to claim 10, characterized in that the transfer from the cylinder outlet (27) in the torpedo bores (35) is formed at least approximately as a short cylinder piece (41) with a circular cross-section.

12. Device according to one of claims 5 to 11, characterized in that the annular gap (37) near the nozzle closure needle (13) and the short

Cylinder piece in front of the torpedo bores (35) have approximately the same cross sections.

13. Device according to one of claims 9 to 12, characterized in that the kinked torpedo bores (35) are formed of two groups of outwardly bent and symmetrically arranged holes, merged at an acute angle and steadily in the annular gap (37) the nozzle lock needle are transferred.

14. Device according to one of claims 9 to 13, characterized in that the free flow area of the cylinder piece (41) - A in the ratio 0.7 - 2 to the sum of the torpedo bores is fi.

15. Device according to one of claims 9 to 14, characterized in that the transition C of the torpedo bores is smaller than the annular gap D, wherein the cross-sectional change is continuous.

16. Device according to one of claims 9 to 15, characterized in that the cylinder head (1) from the cylinder outlet (27) to the spray nozzle (2) a

Assembly forms, which has a short cylindrical inlet opening (38) and an annular gap-like outlet opening (31).

17. Device according to one of claims 9 to 16, characterized in that the injection screw (4) with a spacer wings (56) formed conical tip and a chamfer (58), such that the sweeter shape of the spacer wings (56) a conical cylinder outlet is adapted and the chamfer (58) in the foremost position of the injection screw (4) extends into the short cylindrical transition piece (41).

18. Device according to one of claims 9 to 17, characterized in that the nozzle closure needle (13) and the engagement point for the closure needle (13) has an actuating lever (15) in continuation of the injection screw axis (39) and the actuating lever (15) between the two groups of kinked torpedo bores (35) in the region of the kinks (36) of the bores (35) is arranged.

19. Device according to one of claims 9 to 18, characterized in that the cylinder head (1) integrates the cylinder outlet, such that the entire screw tip (5) penetrates in the frontmost position in the cylinder head (1).

20. Device according to one of claims 9 to 19, characterized in that the cylinder head (1) on the inlet side corresponds to the outer dimensions (d.zyl.) Of the heated cylinder (3) and the outlet side a reduced cross-section corresponding approximately to the outer diameter (d) of Having spray nozzle (2).

21. Device according to one of claims 9 to 20, characterized in that the spray nozzle (2) screwed on the outlet side of the cylinder head (1) and the transition from the outlet opening of the cylinder head (1) in the screw member (9) is annular.