METHOD FOR THE PRODUCTION OF PLASTIC MOULDED PARTS

Abstract

A method is described for producing fibre-reinforced plastic moulded parts, wherein endless fibre strands are fed by a fibre braking device and/or cut fibres via a gravimetric metering device, and a plastic material to be melted, by a volumetric metering device are fed to a single-screw plasticizing unit. Plastic material which is molten and is mixed with fibre material is injected into a moulding tool by an injection stroke of the plasticizing screw. According to the application, the ACTUAL mass flow of the plastic material is calculated from the ACTUAL volume flow of the plastic material and from the ACTUAL mass flow of the fibre material. The ACTUAL mass flow of the plastic material is compared to a TARGET mass flow of the plastic material, and the rotation speed n.sub.d of the rotary drive of the metering element of the metering device is adapted.

Claims

1. A method for the production of plastic moulded parts, with a plasticizing unit (3), which has a cylinder (4) and a plasticizing screw (5) able to be driven rotatably and linearly in the cylinder (4), wherein a plastic material (34) which is to be melted is fed into the cylinder (4) via an opening (8) in the cylinder (4), wherein the plastic material (34) is fed into the cylinder (4) by means of a volumetric metering device (30), wherein the volumetric metering device (30) has a storage container (31) for the receiving of plastic material (34), a rotatable metering element (32) and a rotary drive (33) for actuating the metering element (32), and wherein molten plastic material is injected into a moulding tool (2) by an injection stroke of the plasticizing screw (5), wherein the ACTUAL mass flow of the plastic material (34) is calculated, wherein the ACTUAL mass flow is calculated from the return speed v.sub.screw,back,n of the plasticizing screw (5) during a melt metering process, the nominal diameter of the plasticizing screw (5) and the melt density, that the ACTUAL mass flow of the plastic material (34) is compared with a TARGET mass flow of the plastic material (34), and that with a predeterminable difference value between the ACTUAL mass flow of the plastic material (34) and the TARGET mass flow of the plastic material (34) the rotation speed n.sub.d of the rotary drive (33) of the metering element (32) is changed in such a way that the difference value is reduced.

2. The method according to claim 1, wherein fibre material is added into the cylinder (4) and a fibre-reinforced plastic moulded part is produced, wherein endless fibre strands (10a, 10b, 10c, 10d, 10e, 10f) are fed via a fibre braking device and/or cut fibres (35) are fed via a gravimetric metering device (50) to the plasticizing unit (3), and wherein plastic material which is molten and is mixed with fibre material is injected into a moulding tool (2) by an injection stroke of the plasticizing screw (5).

3. The method according to claim 2, wherein via a first opening (8) in the cylinder (4) the plastic material which is to be melted is fed as granulate (34) into the cylinder (4), that on the conveying remote side from the first opening (8) the endless fibre strands (10a, 10b, 10c, 10e, 10e, 10f) are fed via a second opening (9) and/or the cut fibres (35) are fed via the first opening (8) or via a third opening into the cylinder (4) and are drawn in by the plasticizing screw (5) through rotation.

4. The method according to claim 1, wherein the ACTUAL mass flow of fibre material on the feeding of endless fibres or on the feeding of cut endless fibres is calculated as follows:
dm.sub.f/dt=v.sub.f*n.sub.f*n.sub.tex wherein dm.sub.f/dt: actual mass flow of fibre material v.sub.f: fibre feed speed n.sub.f: number of fed fibre strands n.sub.tex: thread fineness of a fed fibre strand

5. The method according to claim 1, wherein a change to the metering rotation speed n.sub.d is carried out from injection moulding cycle to injection moulding cycle, or that a PI controller is used for a change to the metering rotation speed.

6. The method according to claim 1, wherein the ACTUAL mass flow of plastic material is averaged over several injection moulding cycles, and the thus formed mean value is used for a change to the rotation speed n.sub.d of the rotary drive (33) of the metering element (32).

7. The method according to claim 1, wherein the ACTUAL mass flow of plastic material is calculated as follows:
dm.sub.k/dt=D.sub.s*π*v.sub.screw,back,n*ρ.sub.s(p,T)−dm.sub.f/dt wherein dm.sub.k/dt: actual mass flow of plastic material D.sub.s: screw nominal diameter v.sub.screw,back,n: screw return speed during the plasticizing ρ.sub.s(p,T): melt density p: pressure T: temperature dm.sub.f/dt: actual mass flow of fibre material

8. The method according to claim 1, wherein for an injection moulding cycle the ACTUAL mass flow of the plastic material is calculated and that for one of the subsequent injection moulding cycles, for the injection moulding cycle immediately following therefrom, an adaptation of the ACTUAL mass flow to the TARGET mass flow is carried out by changing the rotation speed n.sub.d of the rotary drive (33) of the metering element (32).

9. The method according to claim 1, wherein a metering screw (32) or a metering disc is used as metering element.

10. The method according to claim 1, wherein the endless fibre strands (10a, 10b, 10c, 10d, 10e, 10f) are withdrawn from a fibre gate (18) equipped with fibre spools (15).

11. The method according to claim 1, wherein the fibre braking device (40) imparts an adjustable, slip-free speed to the fibre strands (10a, 10b, 10c, 10d, 10e, 10f).

12. The method according to claim 1, wherein the cut fibres are fed as chopped glass fibres or as a component of a plastic granulate.

13. The method according to claim 1, wherein the plastic material (34) which is to be melted comprises at least one of granulate, powder, bars and/or liquid silicone.

14. The method according to claim 1, wherein the difference value is reduced to zero.

Description

[0040] The invention is to be explained further below with the aid of example embodiments and with reference to the figures. There are shown:

[0041] FIG. 1 First embodiment of an injection moulding machine with a single-screw plasticizing unit according to the invention;

[0042] FIG. 2 Second embodiment of an injection moulding machine with a single-screw plasticizing unit according to the invention.

[0043] FIG. 3 Third embodiment of an injection moulding machine with a single-screw plasticizing unit according to the invention

[0044] FIG. 1 shows the use of a single-screw plasticizing unit according to the invention as component of an injection moulding machine. The injection moulding machine 1 illustrated in FIG. 1 comprises substantially a clamping unit 2, indicated only diagrammatically here, and a single-screw plasticizing unit 3 according to the invention. The clamping unit 2 and the single-screw plasticizing unit 3 are arranged in a manner known per se on a machine bed, which is not illustrated here. The single-screw plasticizing unit 3 comprises a cylinder 4 with a plasticizing screw 5. On the outer side of the cylinder 4, several heating elements 19 are arranged. The rear end of the screw 5 is operatively connected with a rotary drive 6 and with a linear drive 7. By means of the linear drive 7, the screw 5 can be moved axially in the cylinder 4, i.e. the screw 5 is configured as a reciprocating screw and is provided for the injecting of plastic melt, mixed with fibres, into an injection moulding tool, not illustrated here, situated in the clamping unit 2. In the rear end region of the screw threads, a first opening is provided as filling opening 8 for the feeding of a plastic material which is to be melted.

[0045] The plastic material is present as granulate and is delivered to the filling opening 8 by a volumetric metering device 30. The metering device 30 comprises a storage container 31 to receive granulate, a rotatable metering element 32 and a rotary drive 33 for actuating the metering element. The fact that plastic material is used in the form of granulate is to be indicated by the dots in the storage container 31. The reference number 34 is given for one of the dots.

[0046] On the conveying remote side from the first opening 8, a second opening is provided as filling opening 9 in the cylinder 4 for the feeding of a fibre material. The fibre material is preferably introduced via a fibre braking device 40 into the opening 9 in the form of fibre bundles 10a-10f, separated spatially from one another. A fibre bundle can also be designated as a roving. At the front end, the screw 5 has a backflow barrier 11, and has at the conveying remote side from the backflow barrier 11 a mixing part 12 connected in a rotationally fixed manner with the screw 5 and corrugating with the latter. FIG. 1 shows a situation as is present at the end of an injection process. The screw 5 is situated in its front end position. The conically tapering head of the mixing part 12 lies seated in a fitting conical recess in the cylinder 4.

[0047] The feeding of the fibre material and the mode of action of the fibre braking device 40 is to be described in further detail with the aid of the fibre bundle 10a. For a better overview, the fibre feed device, designated as a whole by reference number 13 and only illustrated diagrammatically, is illustrated on a greatly enlarged scale in relation to the plasticizing unit 3. The fibre feed device 13 comprises a fibre storage container 14 with one or more fibre spools 15, from which respectively a fibre bundle can be drawn off. In the present example embodiment according to FIG. 1, a total of six (6) fibre bundles 10a to 10f are to be fed to the screw 5, so that in the fibre storage container 14 consequently six (6) fibre spools are provided. Only one single fibre spool 15 for the fibre bundle 10a is illustrated here. The fibre bundle 10a is guided through a tube 16, configured as an antistatic tube, which has an inlet opening 16a and an outlet opening 16b. The inlet opening 16a is arranged at a suitable location at or in the fibre storage container 14. The fibre braking device 40 is arranged downstream of the outlet end 16b.

[0048] The fibre braking device designated as a whole by reference number 40 permits the determining of the ACTUAL mass flow of fibre material. The fibre braking device 40 comprises substantially at least one deflector roller 17 and at least one brake roller 18, driven in a braking manner, wherein endless fibres are guided in a slip-free manner via both rollers. Through the slip-free guidance, the fibre feed speed v.sub.f can be determined from the rotation speed of the brake roller n.sub.bw.

[0049] When the fibre bundle 10s is caught by the screw 5 so that through the rotation of the screw 5 the fibre bundle 10a is drawn into the melt and is thereby withdrawn from the fibre spool 15, the fibre feed device 13 acts as a brake, wherein the braking effect and thereby the braking force is distributed to the various components of the fibre feed device 13 as described below.

[0050] A first braking force is provided on the fibre spool 15. The fibre spool 15 is rotatably mounted and is braked by the friction of the mounting (not illustrated) in its rotation so that the fibre bundle 10a is prestressed with approximately 10 Newton.

[0051] A second braking force is achieved by means of the tube 16, wherein the tube 16 is installed such that it has one or more circular segments. This leads to a second braking effect with a second braking force through the effect of rope friction in accordance with Euler-Eytelwein. This second braking force generates approximately 70 Newton of the fibre drawing-in force, so that the fibre drawing-in force at the end of the tube 16 is in total approximately 80% of the fibre drawing-in force.

[0052] A third braking force is generated by means of the brake roller 18. The fibre bundle 10a is guided around a freely rotatably mounted deflector roller 17 and subsequently around a speed-regulated brake roller 18 and from there to the screw 5. The drive of the brake roller 18 takes place preferably by means of a gear motor. By means of the brake roller 18, the final 20% of the pre-stressing force is applied. The arrangement of the deflector roller 17 and brake roller 18 is such that both the deflector roller 17 and also the brake roller 18 will rotate respectively with 180°.

[0053] The ACTUAL mass flow of the fed fibre material is known from the conditions of the fibre braking device 40 and can be determined as follows:

dm.sub.f/dt=v.sub.f*n.sub.f*n.sub.tex

[0054] dm.sub.f/dt: actual mass flow of fibre material

[0055] v.sub.f: fibre feed speed

[0056] n.sub.f: number of fed fibre strands

[0057] n.sub.tex: thread fineness of a fed fibre strand

[0058] With reference to the volumetric metering device 30, firstly a calibration to the material which is used is to be carried out. This calibration provides an initial metering capacity P.sub.D,0, which corresponds to the granulate throughput in grams per rotation of the metering element 32.

[0059] In the following production operation, the ACTUAL mass flow of plastic material of an injection moulding cycle n can be determined as follows:

dm.sub.k/dt=D.sub.s*π*v.sub.screw,back,n*ρ.sub.s(p,T)−dm.sub.f/dt

[0060] dm.sub.k/dt: actual mass flow of plastic material

[0061] D.sub.s: screw nominal diameter

[0062] v.sub.screw,back,n: screw return speed during the plasticizing

[0063] ρ.sub.s(p,T): melt density

[0064] p: pressure

[0065] T: temperature

[0066] dm.sub.f/dt: actual mass flow of fibre material

[0067] A comparison of TARGET mass flow and ACTUAL mass flow or respectively of TARGET metering capacity and ACTUAL metering capacity provides a specification for the adaptation of the rotation speed n.sub.d of the rotary drive for the metering element, in particular a metering screw. This adaptation can take place for example from cycle to cycle. However, a PI controller can also be used.

[0068] Furthermore, it is possible to average the ACTUAL mass flow of plastic material or respectively the ACTUAL metering capacity over several cycles and to carry out an adaptation of the rotation speed n.sub.d on the basis of this mean value.

[0069] By means of the method according to the invention, it is possible to use volumetric metering devices and to thereby save costs, because these are distinctly more reasonably priced than gravimetric metering devices. Nevertheless, fluctuations in the bulk density can be detected and compensated. Consequently, as constant a ratio as possible of fibre material and of plastic material in the finished fibre-reinforced plastic moulded parts can be maintained during the production of these parts.

[0070] FIG. 2 shows a second embodiment of an injection moulding machine with a single-screw plasticizing unit according to the invention. In contrast to FIG. 1, here the fibre material is not fed in the form of endless fibres, but rather as cut fibres. Unlike in FIG. 1, the fibre material and the plastic material are fed into the cylinder through the same bore. The cut fibres 35 are fed into the cylinder 4 via a gravimetric metering device 50. Owing to the load cell 51, the ACTUAL mass flow of cut fibres 35 is known. Consequently, here also as constant a ratio as possible of fibre material and of plastic material in the finished fibre-reinforced plastic moulded parts can be maintained during the production of these parts. Chopped glass fibres are preferably used as cut fibres.

[0071] FIG. 3 shows a single-screw plasticizing unit according to the invention, which is operated without the addition of fibre material with only one plastic component. In this case, only the volumetric metering device 30 is used. By means of the method according to the invention, as constant an ACTUAL mass flow of plastic material as possible can be maintained. Thereby, it becomes possible that also in the case of plastic material which is critical with regard to dwell time, the desired ACTUAL mass flow is maintained. In particular, in a preferred embodiment the screw can be operated in an underfed manner. A preferred field of application is the production of plastic moulded parts for optical purposes.

[0072] Further variants of the invention are not illustrated. For example, it is also possible to work both endless fibres and also chopped glass fibres into the melt.

LIST OF REFERENCE NUMBERS

[0073]

TABLE-US-00001  1 injection moulding machine  2 clamping unit  3 single-screw plasticizing unit  4 cylinder  5 plasticizing screw  6 rotary drive  7 linear drive  8 first filling opening  9 second filling opening 10a-10f individual fibre strands or respectively rovings 11 backflow barrier 12 mixing part 13 fibre feed device 14 fibre storage container/fibre gate 15 fibre spool 15a-15f fibre spools 16 antistatic tube 16a inlet opening 16b outlet opening 17 deflector roller 18 brake roller 19 heating element 30 volumetric metering device 31 storage container 32 metering screw 33 rotary drive 34 granulate 35 cut glass fibres 40 fibre braking device 50 gravimetric metering device 51 load cell