INJECTION UNIT FOR A MOLDING MACHINE

Abstract

An injection unit includes an injection cylinder and a plasticizing screw in the injection cylinder. The plasticizing screw is rotatable about a longitudinal axis for plasticizing plastic raw material and is movable linearly along the longitudinal axis for injecting molten plastic raw material. The injection cylinder has an infeed and plasticizing zone for the plastic raw material with a circular-cylindrical inner wall with a constant diameter along the longitudinal axis, a metering zone in front of the infeed and plasticizing zone along the longitudinal axis in the injection direction with a circular-cylindrical inner wall with a constant diameter along the longitudinal axis, and a nozzle head in front of the metering zone in the injection direction with a nozzle-shaped inner wall. The diameter of the circular-cylindrical inner wall of the metering zone is smaller than the diameter of the circular-cylindrical inner wall of the infeed and plasticizing zone.

Claims

1. Injection unit for a molding machine, in particular for an injection-molding machine, with an injection cylinder and a plasticizing screw arranged in the injection cylinder, wherein the plasticizing screw is rotatable about a longitudinal axis for plasticizing plastic raw material and is movable linearly along the longitudinal axis for injecting molten plastic raw material, wherein the injection cylinder has an infeed and plasticizing zone for the plastic raw material with a circular-cylindrical inner wall with a diameter which remains constant along the longitudinal axis, a metering zone lying in front of the infeed and plasticizing zone along the longitudinal axis in the injection direction with a circular-cylindrical inner wall with a diameter which remains constant along the longitudinal axis, and a nozzle head (4) lying in front of the metering zone in the injection direction with a nozzle-shaped inner wall, wherein the diameter of the circular-cylindrical inner wall of the metering zone is smaller than the diameter of the circular-cylindrical inner wall of the infeed and plasticizing zone.

2. The injection unit according to claim 1, wherein the diameter of the circular-cylindrical inner wall of the metering zone is at most 90%, preferably at most 70%, particularly preferably at most 50%, of the diameter of the circular-cylindrical inner wall of the infeed and plasticizing zone.

3. The injection unit according to claim 1, characterized in that the injection cylinder has a cylinder main body, in which the infeed and plasticizing zone is formed.

4. The injection unit according to claim 3, wherein the injection cylinder has a cylinder front body which is separate from the cylinder main body, arranged in front of the cylinder main body in the injection direction and detachably connected, preferably screwed, to the cylinder main body and in which the metering zone is formed.

5. The injection unit according to claim 4, wherein in the cylinder front body has a flange element, in which most of the metering zone is formed, and the nozzle head which lies in front of the flange element in the injection direction and is connected, preferably screwed, to the flange element.

6. The injection unit according to claim 1, wherein the plasticizing screw has a screw main body with at least one screw flight and at least one screw channel, wherein the screw main body is arranged for the most part in the infeed and plasticizing zone.

7. The injection unit according to claim 6, wherein the maximum outside diameter of the screw main body corresponds to the diameter of the inner wall of the infeed and plasticizing zone.

8. The injection unit according to claim 6, wherein the plasticizing screw has a screw front body which lies in front of the screw main body in the injection direction and is preferably formed separate, wherein the screw front body is arranged at least partially in the metering zone.

9. The injection unit according to claim 1, wherein the plasticizing screw, preferably the screw front body thereof, has a screw tip in the form of a substantially cylindrical plunger, and the cylindrical plunger is arranged in regions in the metering zone.

10. The injection unit according to claim 9, wherein the cylindrical plunger has a lateral surface which corresponds to the inner wall of the metering zone.

11. The injection unit according to claim 9, wherein the cylindrical plunger has a, preferably conical, front face facing towards the nozzle head.

12. The injection unit according to claim 9, wherein the plasticizing screw, preferably the screw front body, has a plain bearing arranged behind the cylindrical plunger in the injection direction, wherein this plain bearing is arranged in the infeed and plasticizing zone.

13. The injection unit according to claim 12, wherein the plain bearing lies against the circular-cylindrical inner wall of the infeed and plasticizing zone with a clearance, preferably with a clearance of from 0.05% to 5% of the nominal diameter of the plasticizing screw.

14. The injection unit according to claim 12, wherein the plasticizing screw, preferably the screw front body thereof, has a feed front side arranged between the plain bearing and the cylindrical plunger and facing towards the metering zone.

15. The injection unit according to claim 12, wherein the plain bearing has several melt channels, preferably formed helical in regions, for allowing molten plastic raw material to pass through from the infeed and plasticizing zone into the metering zone.

16. The injection unit according to claim 1, wherein the injection cylinder has a transition region—preferably formed on the cylinder front body—between the circular-cylindrical inner wall of the infeed and plasticizing zone and the circular-cylindrical inner wall of the metering zone, wherein the transition region has an inner wall in the form of a lateral surface of a rotary truncated cone formed around the longitudinal axis.

17. The injection unit according to claim 14, wherein the feed front side is formed—at least in regions—as a lateral surface of a rotary truncated cone which corresponds to the transition region.

18. The injection unit according to claim 1, wherein the plasticizing screw, preferably the screw front body thereof, has a non-return valve, preferably formed in the cylindrical plunger.

19. The injection unit according to claim 1, wherein the nozzle-shaped inner wall of the nozzle head is formed in the shape of a lateral surface of a rotary truncated cone arranged around the longitudinal axis.

20. The injection unit according to claim 1, wherein the nozzle head has an opening region with a circular-cylindrical inner wall adjoining the nozzle-shaped inner wall in the injection direction.

21. The injection unit according to claim 1, wherein the portion of the plasticizing screw arranged in the infeed and plasticizing zone has a maximum outside diameter of 500 mm, preferably an outside diameter of between 5 mm and 450 mm.

22. The injection unit according to claim 1, wherein the portion of the plasticizing screw arranged in the metering zone has a maximum outside diameter of 400 mm, preferably an outside diameter of between 5 mm and 350 mm.

23. The injection unit according to claim 1, wherein during injection a stroke movement of the plasticizing screw relative to the injection cylinder is effected, wherein the relative stroke movement lies in a range between 0.2 times the nominal diameter of the plasticizing screw and 5 times the nominal diameter of the plasticizing screw, preferably between 0.5 times and 1.2 times the nominal diameter of the plasticizing screw.

24. The injection unit according to claim 1, wherein at least one buffer device for buffering plastic raw material during the injection is attached to the injection cylinder.

25. An injection unit for a molding machine, in particular for an injection-molding machine, with an injection cylinder and a plasticizing screw arranged in the injection cylinder, wherein the plasticizing screw is rotatable about a longitudinal axis in a conveying direction for plasticizing plastic raw material and is movable linearly along the longitudinal axis in the injection direction for injecting molten plastic raw material, wherein, during injection, at the same time as the plasticizing screw moves linearly in the injection direction the plasticizing screw is rotatable in a return direction counter to the conveying direction.

26. The injection unit according to claim 25, wherein a control or regulation unit for controlling or regulating a rotational movement and a linear movement of the plasticizing screw is provided, wherein the control or regulation unit is formed, for injecting the molten plastic raw material into a cavity of a molding tool, to actuate the plasticizing screw at the same time to move linearly in the injection direction and to rotate in the return direction.

27. The injection unit according to claim 25, wherein the movement in the return direction starts at the same time as or before the linear movement in the injection direction.

28. The injection unit according to claim 25, wherein the movement in the return direction ends at the same time as the linear movement in the injection direction.

29. The injection unit according to claim 28, wherein the volume reduction due to the linear movement in the injection direction substantially corresponds to the volume returned due to the movement in the return direction.

30. A molding machine with the injection unit according to claim 1.

31. The molding machine according to claim 30, wherein the molding machine has a clamping unit, wherein a molding tool is installed in the clamping unit and, in the closed state, at least one cavity is formed in the molding tool.

32. A cylinder front body for retrofitting on a cylinder main body of an injection cylinder, wherein the injection cylinder has a circular-cylindrical inner wall with a diameter which remains constant along the longitudinal axis, wherein the cylinder front body has an insertion projection protruding in the direction of the cylinder main body for inserting the cylinder front body into the cylinder main body, wherein the insertion projection has a, preferably circular-cylindrical, outer surface with an outer surface diameter which corresponds at least in regions to the diameter of the circular-cylindrical inner wall of the injection cylinder, wherein the cylinder front body furthermore has a circular-cylindrical inner wall with a diameter which remains constant along the longitudinal axis, and a nozzle head with a nozzle-shaped inner wall lying in front of the insertion projection and the circular-cylindrical inner wall in the injection direction, wherein the diameter of the circular-cylindrical inner wall of the cylinder front body is smaller than the outer surface diameter of the insertion projection.

33. The cylinder front body according to claim 32, wherein the cylinder front body has connection means, preferably in the form of screws, and a flange element, wherein the cylinder front body can be detachably connected, preferably screwed, to the cylinder main body via the flange element and the connection means.

34. A retrofitting set comprising, the cylinder front body according to claim 32 and a screw front body for retrofitting on a screw main body which is or can be arranged in the injection cylinder, wherein the screw front body has a screw tip in the form of a substantially cylindrical plunger, a plain bearing arranged behind the cylindrical plunger in the injection direction and a connection region arranged behind the plain bearing in the injection direction for detachably connecting the screw front body to the screw main body.

35. The retrofitting set according to claim 34, wherein the cylindrical plunger has a lateral surface which corresponds to the circular-cylindrical inner wall.

36. The retrofitting set according to claim 34, wherein the plain bearing has a contact surface which corresponds to the circular-cylindrical inner wall of the injection cylinder.

Description

[0107] Further details and advantages of the present invention are explained in more detail below with the aid of the description of the figures with reference to the embodiment examples represented in the drawings. There are shown in:

[0108] FIG. 1, schematically, a molding machine with an injection unit and a clamping unit,

[0109] FIG. 2, schematically, the injection unit in two extreme positions and a schematic section A-A,

[0110] FIG. 3 a cross section through a front region of the injection unit,

[0111] FIG. 4 a perspective representation of the screw front body,

[0112] FIGS. 5 & 6, in cross sections, a comparison of the volume in front of the feed front side in the extreme positions,

[0113] FIG. 7 a cross section through the front region of the injection unit with a ring non-return valve,

[0114] FIG. 8 a perspective representation of an alternative cylinder front body, and

[0115] FIG. 9 a cross section as in FIG. 7 with altered details.

[0116] A molding machine 100 is schematically represented in FIG. 1. This molding machine 100 has an injection unit 1 and a clamping unit 14, which are arranged on a machine frame 15.

[0117] The clamping unit 14 has a stationary platen 16, a movable platen 17 and an end plate 18.

[0118] In contrast to the horizontal three-platen machine represented, the clamping unit 14 could also be formed as a two-platen machine or as a vertical machine.

[0119] The movable platen 17 is movable relative to the machine frame 15 via a drive device 19. Such a drive device 19 can have a toggle lever mechanism, for example.

[0120] The mold halves of a molding tool 13 are clamped or installed on the platens 16 and 17. At least one cavity C is formed in the molding tool 13 represented closed in FIG. 1. An injection channel 20 leads to the cavity C.

[0121] The injection unit 1 has an injection cylinder 2 and a plasticizing screw 3 arranged in the injection cylinder 2. This plasticizing screw 3 is rotatable about the longitudinal axis L in the conveying direction F and in the return direction R and movable axially along the longitudinal axis L in the injection direction I.

[0122] These movements are initiated via a schematically represented drive device 21. This drive device 21 preferably comprises a rotary drive for the rotational movement and a linear drive for the axial injection movement.

[0123] The injection unit 1 (or the drive device 21 thereof) is connected to a control or regulation unit 12 by means of signaling. Control commands are output by the control or regulation unit 12 to the injection unit 1.

[0124] The control or regulation unit 12 can be connected to an operating unit or can be an integral component of such an operating unit.

[0125] If the injection unit is fitted with a retrofitting set, then the injection unit 1 can be operated in a retrofitting operating mode via the control or regulation unit 12.

[0126] The functional sequence of the injection unit 1 (and the entire molding machine 100) is as follows:

[0127] Plastic raw material K—preferably in the form of recycled plastic material—is poured into the hopper 22 and enters the interior of the injection cylinder 2. In the infeed and plasticizing zone E of the injection cylinder 2, the plastic raw material is melted and compressed and conveyed further in the injection direction I through a rotational movement of the plasticizing screw 3.

[0128] The molten plastic raw material K gradually accumulates in the metering zone M of the injection cylinder 2, wherein this metering zone M also comprises the space in front of the screw.

[0129] As soon as enough molten plastic raw material K has accumulated, this is injected into the cavity C via the nozzle head 4, an opening region O in the nozzle head 4 and the injection channel 20 through an axial movement of the plasticizing screw 3 in the injection direction I.

[0130] In the cavity C, the plastic hardens to form the molding part. After the molding tool 13 has been opened, the molding part thus formed can be ejected or removed.

[0131] In FIG. 1, it can already be seen that the diameter D.sub.M of the circular-cylindrical inner wall W.sub.M of the metering zone M is smaller than the diameter D.sub.E Of the circular-cylindrical inner wall W.sub.E Of the infeed and plasticizing zone E.

[0132] The transition region U is located between the metering zone M and the infeed and plasticizing zone E and delimits these zones from each other. An annular step of the inner wall of the injection cylinder 2 forms this transition region U.

[0133] A cross section through the injection cylinder 2 is schematically represented in FIG. 2. The position represented in the lower image corresponds to the end of the plasticizing and conveying process, whereas the position represented in the upper image corresponds to the end of the injection movement.

[0134] The injection cylinder 2 which has the cylinder main body 2.1 and the cylinder front body 2.2—comprising the nozzle head 4—is represented in each case in both images.

[0135] The injection cylinder 2 is divided into the infeed and plasticizing zone E and the metering zone M. The transition region U is located in between.

[0136] The plasticizing screw 3 has a rear region with a screw flight 6 and a screw channel 7.

[0137] In the front region, the plasticizing screw 3 has a substantially cylindrical plunger 8. A non-return valve 11 in the form of a ball or check non-return valve is formed in this plunger 8.

[0138] In the lower image, the ball 24 of the ball or check non-return valve is located in the region on the left, with the result that the channel 23 is opened. Through the rotation of the plasticizing screw 3 in the conveying direction F, the plastic raw material K melted in the infeed and plasticizing zone E is conveyed in the injection direction I and via the channel 23 enters the space in front of the screw 25 of the metering zone M through the non-return valve 11. Due to the plastic melt entering the space in front of the screw 25 the plasticizing screw moves axially backwards against the injection direction I.

[0139] As soon as enough plastic melt has accumulated in the space in front of the screw 25, the injection process can start. For this, the plasticizing screw 3, as represented in the upper image of FIG. 2, is moved in the injection direction I. As this movement starts the ball 24 of the ball or check non-return valve moves to the right into the closed position, in which the channel 23 is sealed. As a result, no more plastic melt can flow back out of the space in front of the screw 25. At the same time as the plasticizing screw 3 moves in the injection direction I this plasticizing screw 3 is also rotated in the return direction R. The screw tip formed as a cylindrical plunger 8 pushes the plastic melt located in the space in front of the screw 25 via the nozzle head 4 into the cavity C (not represented here).

[0140] A cross section through the injection cylinder 2 and the plasticizing screw 3 in the region of the non-return valve 11 and the channel 23 thereof is represented in the section A-A of FIG. 2.

[0141] A buffer device 30 is represented both in this cross section and in the variant represented at the top in FIG. 2. This buffer device 30 can be provided optionally. This means that the injection unit 1 also functions without this buffer device 30.

[0142] This buffer device 30 has a carrier 31 connected to the injection cylinder 2, a piston 32 movably mounted in the carrier 31 and an energy storage mechanism 33 (e.g. in the form of a spring, a hydraulic pressure accumulator or a pneumatic pressure accumulator) attached on the one hand to the carrier 31 and on the other hand to the piston 32.

[0143] The injection unit 1 can be provided with one (or more) melt storage device (schematically represented buffer device 30), which receives the compressed material in the intermediate space during the injection. The displacement work is absorbed by an energy storage mechanism 33, which discharges during the metering and returns the melt to the space in front of the screw again. The piston surface of the piston 32 finishes flush with the injection cylinder 2 (see section A-A), thus completely cleaning the latter. In addition, the piston 32 can be hydraulically or pneumatically controlled or regulated.

[0144] The buffer device 30 is shown in another embodiment example in the section A-A, in which the size and the alignment/position of the buffer device 30 are different from the upper cross section of FIG. 2. The dimensions of the injection cylinder 2 also differ from those in the upper cross section of FIG. 2.

[0145] A section through the front region of an injection cylinder 2 including plasticizing screw 3 is represented in FIG. 3.

[0146] The injection cylinder 2 has the cylinder main body 2.1, in which the infeed and plasticizing zone E is formed. Moreover, the injection cylinder 2 has the cylinder front body 2.2 which is separate from the cylinder main body 2.1, arranged in front of the cylinder main body 2.1 in the injection direction I and detachably connected, preferably screwed via the connection means 26, to the cylinder main body 2.1 and in which the metering zone M is formed.

[0147] The cylinder front body 2.2 in turn has a flange element 5, in which most of the metering zone M is formed, and the nozzle head 4 which lies in front of the flange element 5 in the injection direction I and is connected, preferably screwed, to the flange element 5.

[0148] It is preferably provided that the cylinder front body 2.2 has an insertion projection 34 protruding in the direction of the cylinder main body 2.1 for inserting the cylinder front body 2.2 into the cylinder main body 2.1. The insertion projection 34 has a, preferably circular-cylindrical, outer surface N with an outer surface diameter D.sub.N which corresponds at least in regions to the diameter D.sub.E of the circular-cylindrical inner wall W.sub.E of the injection cylinder 2.

[0149] The cylinder front body 2.2. can also be formed for retrofitting on a cylinder main body 2.1 of an injection cylinder 2. This means that this cylinder front body 2.2 can be retrofitted in the case of an injection cylinder 2 already being used (via the sleeve-shaped insertion projection 34 and the flange element 5 including connection means 26), as a result of which the injection unit 1 is suitable and converted for micro injection molding, for example.

[0150] The plasticizing screw 3 has a screw main body 3.1 (with at least one screw flight 6 and at least one screw channel 7; not represented here), wherein the screw main body 3.1 is arranged for the most part in the infeed and plasticizing zone E.

[0151] The maximum outside diameter A.sub.E of the screw main body 3.1 corresponds to the diameter D.sub.E, which remains constant along the longitudinal axis L, of the inner wall W.sub.E of the infeed and plasticizing zone E.

[0152] The plasticizing screw 3 has a screw front body 3.2 which lies in front of the screw main body 3.1 in the injection direction I and in this case is formed as a separate component, wherein the screw front body 3.2 is arranged at least partially in the metering zone M.

[0153] The screw front body 3.2, together with the cylinder front body 2.2, can form a retrofitting set for retrofitting on an injection cylinder 2 and a plasticizing screw 3.

[0154] The plasticizing screw 3, in the embodiment example represented in FIG. 3 the screw front body 3.2 thereof, has a screw tip in the form of a substantially cylindrical plunger 8, wherein the cylindrical plunger 8 is arranged in regions in the metering zone M.

[0155] The cylindrical plunger 8 has a lateral surface M.sub.8 which corresponds to the inner wall W.sub.M of the metering zone M.

[0156] The cylindrical plunger 8 has a circular front face S.sub.8 facing towards the nozzle head 4. In the embodiment example represented in FIG. 3, this front face S.sub.8 is formed on a front component 3.3 which is separate from the remaining screw front body 3.2 and is screwed to it. This separate formation ensures that during assembly the ball 24 of the non-return valve 11 can correspondingly be built into the interior of the plunger 8. The flat, circular front face S.sub.8 is aligned at right angles to the longitudinal axis L.

[0157] In contrast to the embodiment represented in FIG. 3, the front face S8 is formed conical. Ideally, the front face S.sub.8 corresponds to the nozzle-shaped inner wall W.sub.4 or nestles up against it.

[0158] The plasticizing screw 3 (in the embodiment example represented in FIG. 3 the screw front body 3.2 thereof) has a plain bearing 9 arranged behind the cylindrical plunger 8 in the injection direction I, wherein this plain bearing 9 is arranged in the infeed and plasticizing zone E. The plain bearing 9 lies against the inner wall W.sub.E with a slight clearance via the contact surface G. The melt channels formed in the plain bearing 9 can also be seen to some extent in FIG. 3.

[0159] The plasticizing screw 3 (in the embodiment example represented in FIG. 3 the screw front body 3.2 thereof) has a feed front side V arranged between plain bearing 9 and cylindrical plunger 8 and facing towards the metering zone M.

[0160] The injection cylinder 2 has a transition region U—preferably formed on the cylinder front body 2.2—between the circular-cylindrical inner wall W.sub.E of the infeed and plasticizing zone E and the circular-cylindrical inner wall W.sub.M of the metering zone M. The transition region U in turn has an inner wall W.sub.U in the form of a lateral surface of a rotary truncated cone formed around the longitudinal axis L.

[0161] In FIG. 3 it can clearly be seen that the feed front side V is formed as a lateral surface of a rotary truncated cone which corresponds to the transition region U.

[0162] The non-return valve 11 is formed in the plunger 8 of the plasticizing screw 3. Besides the ball 24, this comprises the channel 23, which is divided into an infeed-side section 23a and an injection-side section 23b. The infeed-side channel 23a in turn has a central channel Z formed along the longitudinal axis L as well as at least one transverse channel Q branching off it. Specifically, in the embodiment example shown, three transverse channels Q branching off the central channel Z are provided which are arranged at regular intervals (i.e. in each case offset by 120°) around the central channel Z.

[0163] The nozzle-shaped inner wall W.sub.4 of the nozzle head 4 is formed in the shape of a lateral surface of a rotary truncated cone arranged around the longitudinal axis L.

[0164] Moreover, the nozzle head 4 has an opening region O with a circular-cylindrical inner wall W.sub.O adjoining the nozzle-shaped inner wall W.sub.4 in the injection direction I.

[0165] The portion of the plasticizing screw 3 arranged in the metering zone M has a maximum outside diameter A.sub.M which corresponds to the diameter D.sub.M of the circular-cylindrical inner wall W.sub.M which remains constant along the longitudinal axis L.

[0166] FIG. 3 thus clearly shows that the diameter D.sub.M of the circular-cylindrical inner wall W.sub.M of the metering zone M is smaller than the diameter D.sub.E of the circular-cylindrical inner wall W.sub.E of the infeed and plasticizing zone E.

[0167] If the relative dimensions represented in FIG. 3 are regarded as being true, then the diameter D.sub.M of the circular-cylindrical inner wall W.sub.M of the metering zone M is less than 50% of the diameter D.sub.E Of the circular-cylindrical inner wall W.sub.E of the infeed and plasticizing zone E.

[0168] In the embodiment example represented in FIG. 3 the plasticizing screw 3 has a nominal diameter (corresponds to the maximum outside diameter A.sub.E) of 18 mm. In contrast, the plasticizing screw 3 in the region of the plunger 8 has a diameter A.sub.M of 8 mm. In the case of a stroke movement H which is 10 mm as represented, a maximum shot weight of from approximately 0.4 to 0.5 gram results over the surface area of the plunger front side of approximately 0.5 cm.sup.2.

[0169] Only the screw front body 3.2 of the plasticizing screw 3 alone in a three-dimensional drawing is represented in FIG. 4. In the region on the right this screw front body 3.2 has a connection region 27, preferably in the form of a thread.

[0170] In the central region of FIG. 4 the plain bearing 9 is represented together with the helically arranged melt channels 10. These melt channels 10 serve to convey the molten plastic raw material K into the metering zone M. This region corresponds to the maximum outside diameter A.sub.E Of the plasticizing screw 3. The contact surface G is located between the melt channels 10.

[0171] Moreover, the substantially cylindrical plunger 8 of the plasticizing screw 3 is visible in the region on the left in FIG. 4. This plunger 8 has a convex region in the middle portion. Moreover, the outlets of the transverse channels Q and of the injection-side sections 23b of the channel 23 are visible in the region of this plunger 8. This plunger 8 ends with the front component 3.3 on the injection side. The plunger 8 has a maximum outside diameter A.sub.E in the region of the metering zone M.

[0172] A region of the injection cylinder 2 including plasticizing screw 3 is represented in cross section in FIG. 5, wherein the general components correspond to those of FIG. 3. In FIG. 5, the plasticizing screw 3 is in the position at the end of the plasticizing process before the start of the injection process, such as is also represented in FIG. 2.

[0173] There is a particular quantity of molten plastic raw material K between the plain bearing 9 and the feed front side V thereof and the inner wall W.sub.M of the injection cylinder 2. Based on the dimensions already specified further above, this plastic raw material K has a volume of approximately 2.5 cm.sup.3 (plastic quantity K.sub.1).

[0174] If a stroke movement H—from the distance x.sub.1 to the distance x.sub.2 (corresponds to approximately 10 mm)—is carried out in the injection direction I by the plasticizing screw 3 (in order to inject the plastic melt of approximately 0.5 cm.sup.3 located in the space in front of the screw 25), then this volume is reduced due to the smaller diameter D.sub.M in the metering zone M (e.g. to a plastic quantity K.sub.2 of just 0.5 cm.sup.3), such as is represented by way of comparison in FIG. 6.

[0175] As this excess volume (difference between the plastic quantities K.sub.1 and K.sub.2 of approximately 2 cm.sup.3) can only escape backwards because the non-return valve 11, preferably check valve, is closed during injection, this excess volume must be returned through the melt channels 10 against the injection direction I.

[0176] In order to support this return and make it possible, it is provided according to the invention that, during injection, at the same time as the plasticizing screw 3 moves linearly in the injection direction I the plasticizing screw 3 is rotated in a return direction R counter to the conveying direction F. As a result the excess volume is, as it were, sucked back into the region of the screw flight 6 of the plasticizing screw 3.

[0177] FIG. 7 shows a cross section through an injection cylinder 2 including plasticizing screw 3, wherein in this embodiment example the non-return valve 11 is formed not as a ball or check non-return valve, but rather as a ring non-return valve with a blocking ring 28 including blocking wings 29. Moreover, in FIG. 7 the plunger 8 is formed with a conical tip.

[0178] FIG. 8 shows a perspective representation of a screw front body 3.2. In contrast to FIG. 4, the plunger 8 has a conical screw tip. The non-return valve 11 has a pin 29a and the blocking ring 28.

[0179] The non-return valve 11 or the entire cylinder front body 3.2 with reduced diameter can be retrofitted on any conventional plasticizing or injection-molding screw.

[0180] Ridges 35 are attached to the feed front side V. These support the pump effect during the backwards rotation and ensure a self-cleaning effect through directed flow conditions.

[0181] Further details can be seen in the cross section according to FIG. 9: A pressure sensor 36 is provided in the transition region U.

[0182] This pressure sensor 36 can be connected to a control or regulation unit 12, with the result that the current pressure in the transition region can be determined, whereby the current injection pressure can in turn be deduced.

[0183] Screw flights 37 are formed in the region of the plunger 8. These screw flights 37 support the conveying of the plastic raw material K.

[0184] Moreover, it can be seen in FIG. 9 that the gap in the transition region U is very small when the plasticizing screw 3 is in the foremost position.

[0185] Moreover, it can be seen in FIG. 9 that the cylinder front body 2.2—in particular the flange element 5 thereof—has a gradation 38 after the insertion projection 34 in the injection direction I. This gradation serves to center the cylinder front body 2.2 on the injection cylinder 2 during retrofitting. The insertion projection 34 and the gradation 38 together form an insertion region 39.

[0186] Finally, it should also be mentioned that large machines can be operated with lower injection forces if a stepped cylinder is used. As a result, the large machine can also be constructed much more simply and conveniently.

LIST OF REFERENCE NUMBERS

[0187] 1 injection unit [0188] 2 injection cylinder [0189] 2.1 cylinder main body [0190] 2.2 cylinder front body [0191] 3 plasticizing screw [0192] 3.1 screw main body [0193] 3.2 screw front body [0194] 3.3 front component [0195] 4 nozzle head [0196] 5 flange element [0197] 6 screw flight [0198] 7 screw channel [0199] 8 cylindrical plunger [0200] 9 plain bearing [0201] 10 melt channels [0202] 11 non-return valve [0203] 12 control or regulation unit [0204] 13 molding tool [0205] 14 clamping unit [0206] 15 machine frame [0207] 16 stationary platen [0208] 17 movable platen [0209] 18 end plate [0210] 19 drive device [0211] 20 injection channel [0212] 21 drive device [0213] 22 hopper [0214] 23 channel [0215] 23a infeed-side section of the channel 23 [0216] 23b injection-side section of the channel 23 [0217] 24 ball [0218] 25 space in front of the screw [0219] 26 connection means [0220] 27 connection region [0221] 28 blocking ring [0222] 29 blocking wing [0223] 29a pin [0224] 30 buffer device [0225] 31 carrier [0226] 32 piston [0227] 33 energy storage mechanism [0228] 34 insertion projection [0229] 35 ridges on feed front side V [0230] 36 pressure sensor [0231] 37 screw flights in the region of the cylindrical plunger 8 [0232] 38 gradation [0233] 39 insertion region [0234] 100 molding machine [0235] K plastic raw material [0236] K.sub.1 (larger) plastic quantity [0237] K.sub.2 (smaller) plastic quantity [0238] L longitudinal axis [0239] E infeed and plasticizing zone [0240] W.sub.E inner wall of the infeed and plasticizing zone E [0241] D.sub.E diameter of the infeed and plasticizing zone E [0242] I injection direction [0243] M metering zone [0244] W.sub.M inner wall of the metering zone M [0245] D.sub.M diameter of the metering zone M [0246] W.sub.4 nozzle-shaped inner wall of the nozzle head 4 [0247] A.sub.E maximum outside diameter of the plasticizing screw in the infeed and plasticizing zone E [0248] A.sub.M maximum outside diameter of the plasticizing screw in the metering zone M [0249] M.sub.8 lateral surface of the plunger 8 [0250] S.sub.8 front face of the plunger 8 [0251] V feed front side [0252] U transition region [0253] W.sub.U inner wall of the transition region U [0254] O opening region [0255] W.sub.O inner wall of the opening region O [0256] H stroke movement [0257] F conveying direction [0258] R return direction [0259] C cavity [0260] Z central channel [0261] Q transverse channel [0262] N outer surface [0263] D.sub.N outer surface diameter [0264] G contact surface [0265] x.sub.1 distance (before stroke movement H) [0266] x.sub.2 distance (after stroke movement H)