DE102017123721A1 - Process for the production of fiber-reinforced plastic molded parts - Google Patents

Abstract

Disclosed is a method for the production of fiber-reinforced plastic molded parts, wherein endless fiber strands (10a, 10b, ...) or cut glass fiber strands of a plasticizing unit (3) are supplied. The plasticizing unit (3) has a cylinder (4) and in the cylinder (4) a rotationally and linearly driven screw (5). A plastic material to be melted is input via a first opening (8) in the cylinder (4). The endless fiber strands (10a, 10b,...) Or the cut glass fiber strands are fed into the cylinder (4) via a second opening (9) and retracted by the worm (5) by rotation away from the first opening (8). Molten and mixed with fiber material plastic material is injected by an injection stroke of the screw in a mold (2). To improve the homogeneity of the fiber concentration in the screw flights and thus also in the mold, the invention provides that the screw (5) is rotationally driven during the execution of the injection stroke and at the same time endless fiber strands or cut glass fiber strands are drawn into the screw flights.

Description

The invention relates to a method for producing fiber-reinforced plastic molded parts according to the preamble of patent claim 1.

The WO2011 / 066917A2 discloses an injection molding machine for producing fiber-reinforced plastic molded parts, comprising a cylinder and a screw rotatably and linearly driven in the cylinder, wherein in the cylinder a first opening is provided as a filling opening for the supply of a plastic material to be melted, and wherein freibabseitig of the first Opening in the cylinder, a second opening is provided as a filling opening for the supply of one or more fiber bundles. The fiber bundles can be pulled off from one or more fiber coils. In the production of a fiber-reinforced plastic molding, the fibers are drawn in by the screw during its rotation and mixed into the melt. The amount of retracted fiber bundles depends on the screw speed and the axial movement of the screw. The fiber feed produced by the screw rotation during the plasticizing a much higher fiber concentration in the flights than the collection by the axial movement in the execution of the injection stroke. This leads to an inhomogeneous fiber concentration in the screw flights, which in turn can lead to inhomogeneous fiber concentration in the fiber-reinforced plastic molding. In addition, an inhomogeneous fiber concentration leads to fluctuations in the plasticizing time.

From the WO2014 / 04866A1 is an injection molding machine for the production of fiber-reinforced plastic molded parts is known in which the fiber material is input in the form of cut fibers in the plasticizing. During the execution of the injection stroke, the fiber concentration in the melt is reduced compared to the plasticizing phase. This leads to an inhomogeneous fiber concentration in the screw flights, which in turn can result in inhomogeneous fiber concentration in the fiber-reinforced plastic molded part. In addition, an inhomogeneous fiber concentration leads to fluctuations in the plasticizing time.

In order to compensate for an inhomogeneous fiber concentration to some extent, the screw shaft used can optionally have one or more mixing zones or be carried out very long, so that a disproportionately high, spiral-bound convective mixing effect arises. A combination of long worm shaft and mixing zone is conceivable. The disadvantage of these two possibilities is increased production costs of an extended plasticizing unit and the increased shortening of the fiber length associated with mixing operations.

Proceeding from this, the object of the invention is to specify a method with which fiber-reinforced plastic molded parts can be produced on an injection molding machine equipped with a single-screw plasticizing unit, wherein the injection-molded plastic molded parts have improved homogeneity of the fiber concentration and, consequently, improved mechanical properties , Another object of the invention is to achieve a higher process stability through reproducible plasticizing times.

This solution is achieved by a method having the features of claim 1. Advantageous embodiments and further developments can be found in the dependent claims.

Characterized in that the screw is also rotated during the execution of the injection stroke, wherein endless fiber strands or cut fiber strand sections are drawn into the screw threads. Fluctuations in the fiber concentration in the melt and thus also in the injection-molded plastic molded part can be avoided. It can be dispensed with a distributive acting mixing part. A dispersively acting mixing part can be provided in order to dissolve any agglomerates that may occur.

Moreover, instead of a screw with a relatively long metering zone for the purpose of homogenizing the fiber concentration for the method according to the invention, a screw having a metering zone which is shorter in comparison can be used, which results in a reduction of the costs for the screw. This applies regardless of whether a mixing part is present or not.

This results in an improved homogeneity of the fiber concentration and, as a result, improved mechanical properties are also present in the molded part. In addition, a higher process stability can be achieved by reproducible plasticizing times. The fiber material can be in the form of endless fiber strands be supplied. However, it is also possible to use cut fiber strand sections, preferably cut glass fiber strands, as fiber material.

According to a preferred embodiment, during the execution of the injection stroke, the screw speed can be controlled in such a way that the fiber content in the screw flights is kept constant.

c:

Proportionalitätsfaktor, empirisch ermittelt oder aus Schneckengeometrie berechnet.

n_inj:

Drehzahl während des Einspritzens

v_inj:

Einspritzgeschwindigkeit

n_pl:

Schneckendrehzahl während des Plastifizierens

In this case, the screw speed can preferably be determined or regulated according to the following formula: $$n_{inj}=c\cdot \frac{v_{inj}\cdot n_{pl}}{v_{screw.back}}$$

c:

Proportionality factor, empirically determined or calculated from screw geometry.

n _inj :

Speed during injection

v _inj :

Injection speed

n _pl :

Screw speed during plasticizing

According to one embodiment of the invention can be provided that the Granuleindosierung is interrupted during the execution of the injection stroke and / or during the holding pressure phase.

The screw _speed (n _inj .) _{May take on} different values during an injection stroke.

The plastic material to be melted can preferably be entered as granules, wherein the input of the granules can be done as Granuleindosierung in the first opening with a gravimetrically or volumetrically operated Granulatdosiergerät.

The granule dosage can be interrupted during the execution of the injection stroke and / or during the holding pressure phase.

c_faser:

Gewünschter Fasergehalt im Bauteil

ṁ_faser :

Fasermassestrom während des Einspritzens (Bekannt, da durch Faserzuführvorrichtung aufgeprägt)

The granulate mass flow dm _granules / dt, which is to be metered into the screw thread during injection, can preferably be determined or regulated in accordance with the following formula, ie the following formula can be used for the granulate metering: $${\dot{m}}_{\text{granules}}=\frac{1-{\text{c}}_{faser}}{{\text{c}}_{faser}}\cdot {\dot{m}}_{faser}$$

c _fiber :

Desired fiber content in the component

ṁ _fiber :

Fiber mass flow during injection (Known as imprinted by fiber feeder)

Hereinafter, the invention with reference to an embodiment and with reference to the 1 and 2 be described in more detail.

The in the 1 illustrated injection molding machine 1 essentially comprises a presently only schematically indicated closing unit 2 and a plasticizing unit 3 , which is designed as an injection unit. closing unit 1 and injection unit 3 are mounted in a conventional manner on a machine bed, not shown here. The injection unit 3 includes a cylinder 4 with a snail 5 , On the outside of the cylinder 4 are several heating elements 19 appropriate. The back end of the snail 5 is with a rotary drive 6 and a linear drive 7 operatively connected. In the rear end of the screw flights is a first opening as a filling opening 8th provided for the supply of a plastic material to be melted. Delivery side of the first opening 8th is in the cylinder 4 a second opening as a filling opening 9 for the supply of a fiber material 10 intended. The fiber material can be used as endless fiber strands 10a to 10f (also called rovings) in a suitable fiber container 18 to be provided. The fiber reservoir 18 may be formed as a fiber gate with a plurality of fiber coils, of which the endless fiber strands 10a - 10f can be deducted. But it can also cut fiber strand sections, preferably cut glass fiber strands, used as fiber material and in the filling opening 9 be entered. At the front end, the snail points 5 a backflow stop 11 on. Optionally, downstream of the Rückströmsperre 11 one with the snail 5 rotatably connected and co-rotating with this, dispersive and / or distributive mixing part 12 be provided, which should be designed so that there is a fiber-friendly mixing effect as possible.

The 2 shows a situation after the execution of an injection stroke, shortly after the beginning of plasticizing again. Between two sections 20 and 21 with fiber material is a section 22 with a reduced concentration of fiber material. For the sake of simplicity, the section 22 represented as if no fiber material were present. In practice, however, it is the case that fiber material is also present in the screw flights. It just so that over the entire snail 5 seen, downstream of the feed opening 9 Overall, there is an inhomogeneous fiber concentration in the screw flights for the fiber material. The sections 20 and 21 form zones with a comparatively high fiber concentration, whereas the section 22 represents a zone with a comparatively low fiber concentration.

According to the invention, it is now provided that the single-screw plasticizing unit is operated in such a way that the screw 5 is also rotationally driven during the execution of the injection stroke, wherein endless fiber strands or cut fiber strand sections are drawn into the screw threads. During the injection stroke thus fiber material is nachgefördert. As a result, fluctuations in the fiber concentration in the flights and thus also in the injection-molded plastic molding can be avoided. This results in an improved homogeneity of the fiber concentration and, as a result, improved mechanical properties are also present in the molded part. In addition, a higher process stability can be achieved by reproducible plasticizing times.

Tables 1 and 2 show different variants of a method according to the invention. Table 1: Process flow variant 1 Process phase Worm turns (→ fiber intake) Granulateindosierung inject Yes Yes reprint Yes Yes Decompression stroke 1 No No plasticizing Yes Yes Decompression stroke 2 No No Residual cooling time No No Table 2: Process flow variant 2 Process phase Worm turns (→ fiber intake) Granulateindosierung inject Yes No reprint Yes No Decompression stroke 1 No No plasticizing Yes Yes Decompression stroke 2 No No Residual cooling time No No

c:

Proportionalitätsfaktor, empirisch ermittelt oder aus Schneckengeometrie berechnet.

n_inj:

Drehzahl während des Einspritzens

v_inj:

Einspritzgeschwindigkeit

n_pl:

Schneckendrehzahl während des Plastifizierens

The determination of the screw _speed n _inj during the injection, which is necessary in order to keep the fiber concentration constant, takes place according to the formula shown below: $$n_{inj}=c\cdot \frac{v_{inj}\cdot n_{pl}}{v_{screw.back}}$$

c:

Proportionality factor, empirically determined or calculated from screw geometry.

n _inj :

Speed during injection

v _inj :

Injection speed

n _pl :

Screw speed during plasticizing

c_faser:

Gewünschter Fasergehalt im Bauteil

ṁ_faser :

Fasermassestrom während des Einspritzens (Bekannt, da durch Faserzuführvorrichtung aufgeprägt)

The granulate mass flow dm _granules / dt, which is metered into the screw flight during the injection, is calculated in accordance with the formula shown below, ie for the granule dosing: $${\dot{m}}_{\text{granules}}=\frac{1-{\text{c}}_{faser}}{{\text{c}}_{faser}}\cdot {\dot{m}}_{faser}$$

c _fiber :

Desired fiber content in the component

ṁ _fiber :

Fiber mass flow during injection (Known as imprinted by fiber feeder)

LIST OF REFERENCE NUMBERS

1

injection molding machine

2

closing unit

3

plasticizing

4

cylinder

5

slug

6

rotary drive

7

linear actuator

8th

First filling opening - plastic material

9

Second filling opening - fiber material

10a-10f

fiber strands

11

backflow

12

mixing section

18

Fiber reservoir or fiber gate with multiple fiber bobbins

19

heating elements

20, 21

Zone with high fiber concentration

22

Zone with low fiber concentration

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2011/066917 A2 [0002]
• WO 2014/04866 A1 [0003]

Claims (7)

Process for the production of fiber-reinforced plastic molded parts, wherein endless fiber strands (10a, 10b, 10c, 10d, 10e, 10f) or cut fiber strand sections, preferably cut glass fiber strands, are fed to a plasticizing unit (3), wherein the plasticizing unit (3) comprises a cylinder (3) 4) and in the cylinder (4) has a rotatably and linearly driven screw (5), wherein via a first opening (8) in the cylinder (4) a plastic material to be melted is input, wherein the downstream of the first opening (8) via a second opening (9), the endless fiber strands (10a, 10b, 10c, 10d, 10e, 10f) or the cut fiber strand sections in the cylinder (4) and fed by the screw (5) by rotation in the screw flights, and wherein molten and plastic material mixed with fiber material is injected into a mold (2) by an injection stroke of the screw, characterized in that the S screw (5) is rotated during the execution of the injection stroke, wherein endless fiber strands or cut fiber strand sections are drawn into the screw threads. Method according to Claim 1 , characterized in that the screw _speed (n _inj ) is _chosen during the execution of the injection stroke such that the fiber concentration in the _flights is kept constant. Method according to Claim 1 or 2 , characterized in that the screw _speed (n _inj .) assumes different values during the injection stroke. Method according to one of the preceding claims, characterized in that for the screw _speed (n _inj .): $$n_{inj}=c\cdot \frac{v_{inj}\cdot n_{pl}}{v_{screw.back}}$$

where: c: proportionality factor, empirically determined or calculated from screw geometry. n _inj : speed during injection v _inj : injection speed n _pl : screw speed during plasticizing Method according to one of the preceding claims, characterized in that the plastic material to be melted is input as granules, wherein the input of the granules is carried out as Granuleindosierung with a gravimetrically or volumetrically operated Granulatdosiergerät Method according to Claim 5 , characterized in that the Granuleindosierung is interrupted during the execution of the injection stroke and / or during the holding pressure phase Method according to Claim 5 or 6 , characterized in that for the granule dosage applies: $${\dot{m}}_{\text{granules}}=c\cdot \frac{1-{\text{c}}_{faser}}{{\text{c}}_{faser}}\cdot {\dot{m}}_{faser}$$

where: c _fiber : Desired fiber content in the component ṁ _fiber : fiber mass _flow during injection (Known as imprinted by fiber feeder)

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