DE102011105764A1 - Apparatus for injection molding of plastic molded components, has fluid barrier element that is movable between two stop positions at which two abutment surfaces are provided - Google Patents

Abstract

The apparatus (1) has an injection unit (5) for injecting a fluid at an axial injection position of a screw cylinder (2). The injection unit has a nozzle head and two fluid guiding channels. A receiving chamber is provided in the nozzle head for arranging fluid barrier element. The fluid barrier element is movable between two stop positions at which two abutment surfaces are provided.

Description

The invention relates to a device for injection molding of plastic molded parts made of thermoplastic material, the device comprising: a worm cylinder with therein, both rotationally and translationally movable plasticizing and injection screw for producing thermoplastic melt and means for injecting a fluid an axial injection position of the worm cylinder, at which at least temporarily worm threads of the plasticizing and injection screw are located.

Such a device is for example from the DE 198 48 151 A1 known. Here, the structural foam spraying with physical gassing of the melt is used for the production of plastic molded parts from thermoplastic melt. In this process, blowing agent-provided melt is injected into the cavity of an injection mold. The gas pressure presses the melt during the cooling against the cavity walls, so that cooling volume contraction can be compensated and the molding surface is free of sink marks.

Hereby advantageously foamed moldings can be produced with a compact skin and a foamed core. In particular, in the present use of a physical blowing agent (fluid, in particular gas), it is advantageous that a higher gas pressure than when using a chemical blowing agent can be constructed to process higher-viscosity plastics well and to produce moldings with low wall thicknesses. This results in a uniform distribution of the gas in the melt.

However, due to the differences in viscosity between the gas and the melt, the gas tends to form one or more large bubbles. In order to avoid this, both the gas volume must be metered in very accurately and the gasified melt must be intensively mixed, whereby the melt must be kept under a sufficiently high pressure to avoid unwanted foaming in the screw cylinder.

For injecting the fluid (the gas) in the screw cylinder usually injectors are used for the fluid, which are screwed into the screw cylinder. These injectors must fulfill the following functions: First, the fluid must be injected into the plastic melt reproducible with them. It must then be ensured that, with each injection molding cycle, a blow-in, if necessary, of the melt which has penetrated into the injector occurs in order to ensure the functioning of the system. Furthermore, it is important to design the means for injecting a fluid so that the high melt pressures during injection can be withstood.

The previously known gas injectors fulfill these tasks usually only partially or incompletely.

The invention is therefore based on the object, a device of the generic type so that the fluid input, in particular the gas input, while avoiding the disadvantages mentioned in a reproducible manner and so a stable injection molding process is executable. Furthermore, the device should be able to easily withstand the high injection pressures in the melt, in particular with regard to the means for the input of the fluid. Finally, emphasis is placed on the fact that a simple and thus cost-effective design implementation is possible.

The solution of this problem by the invention is characterized in that the means for injecting a fluid comprise a nozzle head having two fluid passageways, wherein in the nozzle head a receiving chamber for a fluid barrier element is arranged, wherein the two fluid passage channels are each fluidly connected to the receiving chamber wherein the fluid-blocking element is reciprocally movable in the receiving chamber between a first stop position and a second stop position and wherein the receiving chamber has a first abutment surface for the fluid-blocking element in the first stop position and a second abutment surface for the fluid-blocking element in the second stop position.

The receiving chamber preferably has at least sections of a cylindrical contour, wherein the fluid passage channels are at least partially formed as a bore in the nozzle head. The cylindrical receiving chamber and formed as a bore fluid passage channels are preferably arranged concentrically to a common center axis.

The first contact surface is preferably formed as a circular ring surface, which is located at the transition between the first bore having a diameter to the receiving chamber with a diameter which is larger than the diameter of the bore. The fluid barrier element is preferably designed to be able to rest gas-tight on the first contact surface and thus prevent fluid flow through the nozzle head.

However, the second contact surface is preferably formed as a conical surface which is formed in an axial end portion of the receiving chamber, wherein on the conical surface, a transition between the diameter of the receiving chamber and the second bore having a diameter which is smaller than the diameter of the receiving chamber, is created.

In the region of the second bearing surface for the fluid-blocking element, at least one flow channel is preferably arranged, which produces a fluidic connection between the receiving chamber and the second fluid passage channel, even if the fluid-blocking element rests on the second bearing surface. A preferred embodiment of the invention provides that two flow channels are arranged in the region of the second contact surface, which are formed as recesses in the second contact surface, wherein the two flow channels with respect to the central axis are arranged opposite one another. Alternatively it can be provided that four flow channels are arranged in the region of the second contact surface, which are formed as cross-shaped recesses in the second contact surface.

The fluid barrier element is preferably a ball.

The proposed nozzle head is installed in the wall of the worm cylinder and has a fluidic connection to the interior of the worm cylinder.

In contrast to known gas injection valves with actively opened and closed Eingasungsöffnung (by hydraulic, pneumatic or electrical means) such a complex valve control is avoided according to the proposed solution. The proposed concept is therefore very cost-effective. The opening and closing of the fluid passage can rather be accomplished alone by suitable pressure conditions.

Accordingly, the invention proposal provides a Eingasungsdüse that is particularly small, no external operation needed and yet self-cleaning.

A gas injector is used, which essentially consists of a bore connected to the gas source (reservoir) through the screw cylinder. Between this bore and the opening to the screw is the pressure-controlled fluid barrier element, which acts only partially as a check valve.

The preferably formed as a ball fluid barrier element has an upper stop (first contact surface), in which the ball seals on a preferably linear surface to the gas source. This sealing position is achieved when the melt pressure is higher than the gas pressure.

For the injection of the gas, it is necessary that the gas pressure is higher than the melt pressure. In this case, the ball is in the opposite position, i. H. to the second contact surface, printed. Here comes the ball to the plant. Flow channels arranged next to the contact surface allow the gas to flow past into the melt. As soon as the gas pressure is reduced or turned off, the ball closes the fluid guide channel again. It can be provided a spring element which presses the fluid barrier element against the first contact surface and the nozzle head so when no external pressure forces acting shoots.

Accordingly, the proposed device has a closure element (fluid barrier element), which comes by pressure difference between the two said end positions to the plant and closes gas-tight at the end position directed to the gas source.

The fluid barrier element is - as mentioned - preferably a ball, but it may also be a torpedo or a differently designed body.

As mentioned, between the closure element and the gas supply, a biasing element, such as. B. a coil spring or disc spring, can be arranged.

The stroke of the fluid barrier element is preferably very small; it already sufficient few tenths of mm, z. B. 0.1 to 0.3 mm.

Another advantage of this arrangement is that the fluid barrier element can be located very close to the inner cylinder bore, z. B. less than 50 mm. As a result, the amount of melt which penetrates into the closure area during the plasticization without gas injection is very small and can be blown free without any problems with renewed gas injection.

Another advantage is that cooling and freezing of the penetrating melt is reliably avoided by the aforementioned short distance from the cylinder bore.

To ensure that the required gas pressure is already established at the start of gas injection (due to possible pressure drops due to the volumes of connection hoses and tubes), a release pressure gas valve can be placed immediately after the nozzle head.

In the drawings, embodiments of the invention are shown. Show it:

1 schematically a device for injection molding,

2 a side view of a nozzle head, which is part of means for injecting a fluid, according to a first embodiment of the invention,

3 the nozzle head according to 2 in section AB according to 4 .

4 the nozzle head according to 2 in the view C according to 3 .

5 in the side view of the nozzle head according to a second embodiment of the invention,

6 the nozzle head according to 5 in section DE according to 7 and

7 the nozzle head according to 5 in the view F according to 6 ,

In 1 schematically is an apparatus for injection molding 1 , So an injection molding machine, shown in a process stage in which a melt-gas mixture is produced. Later, the plastic melt produced in the injection mold 11 injected, that is a corresponding cavity 12 having. In the worm cylinder 2 is a plasticizing and injection screw 3 rotatably arranged and axially displaceable. For the production of thermoplastic polymer melt 4 the snail rotates 3 initially without axial movement in the worm cylinder 2 ,

Fluid in the form of a gas 6 is in a storage container 13 saved. It becomes a compressor 14 fed, which brings it to a desired pressure. From the compressor 14 From the gas passes via a line to a center 5 for injecting the fluid, namely to an injection nozzle on the screw cylinder 2 is appropriate. Not shown is a volume control device with which the volume of the injected gas can be regulated. Through the nozzle 5 can the gas 6 controlled or regulated in the screw cylinder 2 and thus into the plastic melt 4 be injected. However, this succeeds only if the gas pressure p _{F is} greater than the pressure in the melt p _S , ie the pressure difference between the pressure in the fluid and in the plastic melt Delta p = p _F - p _S must be positive.

The gasification of the gas 6 over the nozzle 5 takes place at an axial injection position G, at least at times - especially during the plasticization of melt - the screw flights 7 the snail 3 are located.

If enough melt-gas mixture is produced for a shot, the mixture can enter the cavity 12 of the injection mold 11 be injected, which is not shown.

To create a stable injection molding process care is taken that the pressure difference delta p remains largely constant, at least in a predetermined tolerance range, the z. B. may be 2 bar. The device is therefor with a pressure sensor 15 for measuring the pressure p _S in the melt 4 , with a pressure sensor 16 for measuring the gas pressure p _F and with a difference image 17 for determining the pressure difference delta p = p _F - p _S equipped. The measured pressure difference is the control 18 fed to the injection molding machine, which can ensure according to a program that this value remains within a predetermined tolerance band. As intervention options are those injection molding parameters that are familiar to those skilled, for. As the speed of the screw and the axial force during injection of melt. It is evident that a reduction of the injection force reduces the melt pressure, which provides an opportunity for intervention. On the other hand, the gas pressure p _F and / or the gas volume can be regulated accordingly in order to maintain the desired pressure difference delta p.

Of course, instead of a gas, another fluid, ie a liquid, may also be added to the melt.

The addition of fluid ensures that a molded part is produced from a plastic-gas mixture which has no sink marks and a foam structure inside. In particular, large, foamed plastic molded parts such as plates and molded parts with three-dimensional geometry, even with different wall thicknesses and ribs, can thus be manufactured in a process-reliable and cost-effective manner.

To prevent premature foaming of the plastic-gas or plastic-liquid mixture in the cavity, a back pressure can be built up before the injection of the mixture in the cavity, which is reduced only gradually with the entry of the mixture; the melt flow front is thus opposed by a gas cushion. The foaming can be controlled or regulated.

In 2 . 3 and 4 is a nozzle head in three different views 8th represented, the (main) part of the means 5 for injecting the gas 6 is. For orientation, it should be noted that in the 2 and 3 respectively. 5 and 6 connects at the top a line section leading to the reservoir 13 leads. Below would be the snail cylinder 2 lie in the wall of the nozzle head 8th is mounted, the lower end of the nozzle head 8th with the inside of the worm cylinder 2 fluidically connected.

The nozzle head consists of a substantially cylindrical body with three different diameter sections. The nozzle head has a central center axis a. As it is best in 3 can be seen, are two fluid passageways 9 and 10 in the form of concentric holes in the nozzle head 8th brought in. In the lower area of the nozzle head 8th the holes open 9 and 10 - From different axial ends of the nozzle head 8th coming - in a receiving chamber 19 , The reception chamber 19 has, like the holes 9 and 10 , also in a substantially cylindrical shape and is arranged coaxially to the center axis a.

It should be noted that the nozzle head 8th for the sake of simplicity is outlined as an integral part; With regard to the production and installation of the nozzle head, a split design may be necessary.

The holes 9 . 10 have diameter d and d ₀ , the receiving chamber 19 has a larger diameter D.

In the reception room 19 is a fluid barrier element 20 arranged in the form of a steel ball.

For the ball 20 are in the axial ends of the receiving chamber 10 two contact surfaces 21 (above) and 22 (below) formed.

The upper contact surface 21 arises as a conical annular surface, due to the transition of the receiving chamber 19 in the upper hole 9 with smaller diameter d. The ball can thus be in a first, upper stop position on the contact surface 21 place the fluidic connection between receiving chamber 19 and bore 9 to prevent.

The lower contact surface 22 arises as a conical surface formed at the transition of the receiving chamber 19 in the lower hole 10 is formed with a smaller diameter d ₀ . The cone opening angle here is about 120 °. The ball can thus in a second, lower stop position on the contact surface 22 lay. In this position, however, is the fluidic connection between the receiving chamber 19 and bore 10 not stopped. Rather, are in the lower contact surface 22 two flow channels 23 incorporated as a groove-shaped depressions in the contact surface 22 are formed. The flow channels 23 are - as in 3 can be recognized - formed as oblique incisions, which are at a steeper angle than the conical surface 22 fall off. Accordingly, gas can also be when the ball 20 on the second support surface 22 rests, flow.

The stroke which the ball has to cover in order to pass from the first to the second contact surface is preferably only about 0.1 to 0.4 mm.

In the 5 . 6 and 7 an alternative embodiment of the nozzle head is shown. In principle, the same applies as for the solution 2 . 3 and 4 was executed.

The transition between the receiving chamber 19 and the upper hole 9 is here formed sharp-edged; the receiving chamber 19 is slightly domed in the upper area, so that a narrower first contact surface 21 results.

In the lower end of the receiving chamber 19 are four cross-shaped flow channels 23 (Cross grooves) formed (see synopsis of 6 and 7 ). Accordingly, here too, a gas flow can take place when the ball 20 on the second contact surface 22 rests.

Accordingly, it is ensured in all described solutions that when the gas 6 , coming from above, with a higher gas pressure p _F than the melt pressure p _S in the screw cylinder acts, the ball 20 on the lower contact surface 22 is pressed, so it due to the flow channels 23 to an influx of gas in the screw cylinder comes.

However, if the melt pressure p _{S is} greater than the gas pressure p _F , the ball 20 up to the first contact surface 21 pressed, causing it to fluidly shut off the upper bore 9 comes. Accordingly, no melt from the screw cylinder in the upper hole 9 penetration.

If another gas is injected into the screw cylinder at a sufficient pressure during the next shot, the lower section of the nozzle head is blown open. It is advantageous that the distance between the ball center and the lower end of the nozzle head 8th is relatively short, z. B. between 5 mm and 50 mm, so that this area remains warm and can not freeze melt.

LIST OF REFERENCE NUMBERS

1

Device for injection molding

2

screw cylinder

3

Plasticizing and injection screw

4

Plastic melt

5

Means for injecting a fluid

6

Fluid (gas)

7

snail's pace

8th

nozzle head

9

Feed-through channel

10

Feed-through channel

11

injection mold

12

cavity

13

reservoir

14

compressor

15

Pressure sensor for melt

16

Pressure sensor for gas

17

difference images

18

Control / regulation

19

receiving chamber

20

Fluid barrier element (ball)

21

first contact surface

22

second contact surface

23

flow channel

G

Injection position for fluid (gas)

D

diameter

d

diameter

d ₀

diameter

a

central axis

p _F

gas pressure

p _s

melt pressure

Delta p

Pressure difference: p _F - p _S

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

• DE 19848151 A1 [0002]

Claims (10)

Contraption ( 1 ) for injection molding of plastic molded parts made of thermoplastic material, the device comprising: a screw cylinder ( 2 ) with a rotatably movable plasticizing screw ( 3 ) for producing thermoplastic melt ( 4 ) and means ( 5 ) for injecting a fluid ( 6 ) at an axial injection position (G) of the worm cylinder ( 2 ), at which, at least at times, screw flights ( 7 ) of the plasticizing screw ( 3 ), characterized in that the means ( 5 ) for injecting a fluid ( 6 ) a nozzle head ( 8th ) comprising two fluid passageways ( 9 . 10 ), wherein in the nozzle head ( 8th ) a receiving chamber ( 19 ) for a fluid barrier element ( 20 ), wherein the two fluid passage channels ( 9 . 10 ) each with the receiving chamber ( 19 ) are fluidically connected, wherein the fluid barrier element ( 20 ) in the receiving chamber ( 19 ) is arranged to move back and forth between a first stop position and a second stop position, and wherein the receiving chamber ( 19 ) a first contact surface ( 21 ) for the fluid barrier element ( 20 ) in the first stop position and a second contact surface ( 22 ) for the fluid barrier element ( 20 ) in the second stop position. Apparatus according to claim 1, characterized in that the receiving chamber ( 19 ) has at least sections a cylindrical contour and that the fluid passage channels ( 9 . 10 ) at least in sections as a bore in the nozzle head ( 8th ) are formed. Apparatus according to claim 2, characterized in that the cylindrical receiving chamber ( 19 ) and formed as a bore fluid passage channels ( 9 . 10 ) are arranged concentrically to a common center axis (a). Apparatus according to claim 3, characterized in that the first contact surface ( 21 ) is formed as a circular ring surface, which at the transition between the first bore ( 9 ) with a diameter (d) to the receiving chamber ( 19 ) having a diameter (D) greater than the diameter (d) of the bore ( 9 ) is located. Apparatus according to claim 4, characterized in that the fluid barrier element ( 20 ) is designed to be gastight on the first contact surface ( 21 ) and so fluid flow through the nozzle head ( 8th ) to prevent. Device according to one of claims 3 to 5, characterized in that the second bearing surface ( 22 ) is formed as a conical surface which in an axial end region of the receiving chamber ( 19 ), wherein over the conical surface, a transition between the diameter (D) of the receiving chamber ( 19 ) and the second bore ( 10 ) having a diameter (d ₀ ) smaller than the diameter (D) of the receiving chamber ( 10 ) is created. Device according to one of claims 1 to 6, characterized in that in the region of the second contact surface ( 22 ) for the fluid barrier element ( 20 ) at least one flow channel ( 23 ) is arranged, which a fluidic connection between the receiving chamber ( 19 ) and the second fluid passageway ( 10 ), even if the fluid barrier element ( 20 ) on the second contact surface ( 22 ) rests. Apparatus according to claim 7, characterized in that two flow channels ( 23 ) in the region of the second contact surface ( 22 ) arranged as recesses in the second contact surface ( 22 ), wherein the two flow channels ( 23 ) are preferably arranged opposite one another with respect to the center axis (a). Apparatus according to claim 7, characterized in that four flow channels ( 23 ) in the region of the second contact surface ( 22 ) are arranged as cross-shaped recesses in the second bearing surface ( 22 ) are formed. Device according to one of claims 1 to 10, characterized in that the fluid barrier element ( 20 ) is a ball.

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