DE3833220A1 - Injection mould for the processing of thermoplastics - Google Patents

Abstract

A description is given of an improved injection mould, comprising a needle shut-off nozzle, a hot-runner and temperature-controlling system and also a plunger drive, effecting the needle guidance, as well as a monitoring device for controlling the injection-moulding operations. It is essential for the invention that the susceptibility of the injection mould to wear has been reduced by special sealing measures of the plungers (11, 14) with respect to the valve housing (13) by special coating measures on the parts rubbing against one another and that the material throughflow through the hot-runner and the needle shut-off nozzle has been optimised. A sensor system (36) in conjunction with an evaluation unit causes defectively produced mouldings to be ejected (Figure 2). <IMAGE>

Description

The invention relates to an injection mold for molded parts, consisting of at least one electrically heated Nadelver closure system, wherein the front needle part a sprue periodically opens and closes, the rear needle part is connected to an axially displaceable piston which sealed in a cylinder slides, the cylinder with air or hydraulic oil to generate the piston stroke can be opened and a heating duct system for the feed of the thermoplastic and with a control system for the course of the injection molding process and a tempering system for quick heat dissipation.

Injection molds of the type mentioned above are commercially available in numerous designs. In Fig. 1, such an embodiment is shown schematically in section. It consists essentially of a in a mold block (2) built-in needle valve nozzle (1) which is electrically heated, and wherein the front conically tapering part (7) an axially displaceable needle (8) has a sprue opening (9) periodically opens and closes, through which the thermoplastic material coming through the hot runner ( 3 ) is injected into the cavity ( 15 ) at approximately 220 ° C. under high pressure. The separable tool block ( 10 , 11 ) forming the cavity is cooled via a temperature control system ( 12 , 12 ', 12 ''). From DE 32 45 571 it is known to arrange a pre-centering body in the needle valve nozzle adjacent to the gate opening, which ensures an exact guidance of the needle. A poor heat transfer must be feared by the type of heating via heating tapes and by the material properties of the pre-centering body.

Furthermore, there is a hot runner system ( 2 ) composed of several levels, in which hot runner ( 3 ) are machined, and where, for example, 6-32 molded parts are injected at the same time when using multiple tools in one operation. Methods for producing such temperature control and hot runner systems are known, for example, from DE-OS 36 32 574 and 36 32 640 of the applicant.

In the case of tempe generally cooled with tap water riersystemen by aggressive additions of water Cor rosion occur, due to the lime content at län Add the cooling channels to lower use, thereby reducing the use tool life is reduced.

The needle control is generally done by a piston drive, which is accommodated in a tool plate ( 5 ), consisting of a piston ( 14 ), which is slidably seated in a cylinder ( 13 ) with air or hydraulic oil. Such a piston drive is known for example from DE 32 49 486. In this patent, the sealing problems of the piston are discussed because of the high pressures required for the hydraulics, and it is proposed to solve the problems by means of two interacting pistons that are operated with compressed air. However, the proposed structure becomes very complicated and should cause wear problems with prolonged use.

Furthermore, the connection from the piston to the needle is unsatisfactorily solved in many injection molds, so that after prolonged use - many injection molds bring well over a million injection molding processes - considerable wear and tear is observed and exact guidance of the needle is no longer guaranteed.

After all, most of the commercially distributed ones Injection molds have the disadvantage that they are the control Check the valve pin only imperfectly and thereby produce a high scrap of molded parts what the Machine economy reduced or respectively expensive quality controls of the end products are required.

Therefore, the task was to create an injection mold at the beginning to find the generic species mentioned, which

• - a low-friction flow of the thermoplastic to the sprue openings in the cavity guaranteed
• - Avoids corrosion and clogging of the temperature control channels
• - the wear of the parts causing the needle guide belittles
• - The optimal sliding and sealing ability within the Ven tilgehäuses guaranteed
• - the output of incorrectly produced molded parts automatically prompted.

According to the invention the object was achieved with a spray Casting tool with in the characterizing part of claim 1  mentioned features. Details of the invention go out the dependent claims, the description and the drawings forth.

The invention will now be explained in more detail with reference to the drawings and show it

Fig. 1 shows a section through an injection mold conven tional type,

Fig. 2 shows a section through an inventive spray casting tool in the upper (open) position of the closure needle,

Fig. 3 is a partial view of a spray nozzle according to the prior invention.

1. Needle valve nozzle

With reference to FIGS. 1 and 3, the present invention also aimed to optimize the needle valve nozzle 1 . This is a combination of features that are known as well as newly found and introduced features.

For the purpose of good heat transfer, the pre-centering body should be made in one piece with the nozzle body ( 6 ), and the heating ( 4 ) of the nozzle ( 1 ) should be accomplished by a thermally conductive material, for example copper, into which the heating coils ( 28 ) are cast will.

On the other hand, the heat transfer from the nozzle body ( 6 ) to the tool block ( 10 ) should be as low as possible, which is caused at the transition point ( 41 ) by titanium or ceramic sealing insulation. Likewise, the nozzle body is thermally separated at its upper part against the tool block ( 23 ) by an analog insulation ( 42 ).

With regard to a rapid filling of the cavity and an optimal pre-centering of the needle ( 8 ), it was found that the cone angles ( α , b ) of the front and rear conical taper ( 7 ) of the needle ( 8 ) should not exceed 15 °, as is the case seen from FIG. 3. In this figure, the upper stroke state (open) of the needle and the closed position (lower stroke state) is shown in broken lines.

2. Hot runner and temperature control system

The hot runner or hot runner distributor ( 2 ) is constructed, as is known, for example, from DE 36 32 640 already mentioned. The flow channels are milled in such a way that blind spots and edges are avoided, in order in this way to enable a uniform flow of the thermoplastic and to avoid material deposits which lead to the faulty condition of the molded parts. A similar objective is described in EP 01 97 181.

In an analogous manner, the tempering system ( 12 , 12 ', 12 '') arranged in the tool block ( 10 ) is constructed as described in DE 36 32 574, which enables intensive cooling of the injection mold and ensures optimum coolant throughput. The temperature control channels ( 12 , 12 ', 12 '') in the assembled state of the temperature control system, which is accomplished by diffusion welding be protected by chemical nickel plating against corrosion. Here, nickel is deposited as a nickel-phosphorus alloy using the reducing agent sodium hypophosphite by pumping a corresponding reaction solution with pumps through the temperature control system until a deposition thickness of approximately 200 μm is reached. In this way, the corrosion, which caused considerable problems in conventional temperature control systems after a long period of use, was reliably avoided.

3. Piston drive for needle valve nozzle

The essential part of the present invention lies in the improved piston drive ( 5 ). The drive unit consists, as shown in Fig. 2, from a Ventilge housing ( 13 ) with a cover ( 16 ) in which a piston ( 11 ) is axially displaceable. The piston is firmly connected by a screw connection ( 32 ) to a further piston ( 14 ), which is also located in the valve housing. To drive the piston ( 11 , 14 ) are compressed air feeds ( 19 ) which, depending on the position of the piston in the open position of the needle, the upper part of the valve housing ( 33 ) or in the closed position of the needle, the lower part of the valve housing ( 20 ) apply air. The sliding seal of the piston ( 11 ) is produced by a sealing ring ( 21 ) with a rectangular cross section (trade name "Quad ring"), which is made of silicone rubber, and which is seated in a cylindrical wall recess of the piston. The sliding seal of the piston ( 14 ) causes a Hutman collar ( 22 ) which sits in a cylindrical recess of the valve housing and is pressed against the piston by a cylinder ring ( 12 ) which is screwed to the housing. Furthermore, the screw connection ( 32 ) is secured against loosening due to the frequent lifting processes, which are associated with considerable alternating loads, by means of a so-called Schnorrscheibe ( 29 ) as a washer. This disc is provided on both sides with obliquely radial corrugations. Alternatively, the screw can be permanently connected to the piston with a suitable adhesive, for example Loctite screw locking lacquer.

The connection of the piston ( 14 ) to the needle ( 8 ) is created by expanding it cylindrically at its upper end, and this extension ( 15 ) sits in a T-groove of the piston, into which it is inserted radially. The mutual fit of the T-slot and cylindrical extension is such that the needle ( 8 ) has an axial play of at most 5 µm, but at the same time compensates for displacements between the piston drive ( 5 ) and the hot runner distributor ( 2 ) due to thermal influences in the radial direction . A further voltage equalization can be achieved by means of a plate disk ( 17 ) known per se, the resilient edges of which are bent upwards bear against the piston drive and the underside of which rests on the hot runner ( 2 ).

In order to reduce the wear resistance, the parts of the valve housing or piston which rub against each other are coated, and indeed the pistons ( 11 ) and ( 14 ) made of hardened steel are preferably coated on the sliding surface with titanium nitride, the layer thickness being 2-4 μm, likewise the inside of the valve body, the needle ( 8 ) and the coupling pieces. The latter parts can also be plasma nitrided, the coating thickness being 10-50 µm. The permanent tightness and high lubricity between the needle ( 8 ) and the needle guide bush ( 18 ) is achieved by an additional plasma nitriding of the inside of the needle guide bush. Alternatively, these parts can also be chrome-plated.

4. Control system for the injection molding process

For this purpose, a sensor ( 36 ) is attached to the outside of the valve housing ( 5 ), which builds up an inductive field which is influenced by the piston ( 11 ) when it is adjacent to the sensor ( 36 ), for example in the upper stroke position. An electronic evaluation unit, not shown, registers whether there is a difference between the time of the piston control by the compressed air in the upper or lower stroke position and the time of the effective position of the piston in connection with the injection parameters. If a specified limit of the time difference for the corresponding stroke when ejecting the molding in question is exceeded, this is discarded as a committee. In the case of multiple tools, the procedure is analogous, since each valve housing has such a sensor, which are all connected to the evaluation unit, so that, if appropriate, all the molded parts of a cycle can be rejected as faulty.

Claims (7)

1. Injection mold for the production of molded parts made of thermoplastic materials, consisting of at least one electrically heated needle, the nozzle, the lower, partly tapered needle part periodically opens and closes a gate and is guided through a pre-centering body, the upper needle part with an axially displaceable piston is connected, which slides in a sealed manner in a valve housing, the valve housing being pressurized with air or hydraulic oil for the axial displacement of the piston, a hot runner system for the supply of the thermoplastic plastic to the valve gate nozzle and a control system for the course of the injection molding process, characterized in that the upper part of the needle ( 8 ) ends in a cylindrical extension ( 15 ) which is radially displaceable in a T-groove of an axially displaceable piston ( 14 ) which by means of a screw connection ( 32 ) m it with another piston ( 11 ) is firmly connected, the stroke of which is effected by two pressurized feeds ( 19 ) which open into air chambers ( 20 , 33 ), the piston ( 11 ) in the valve housing ( 13 ) is sealed by a sealing ring ( 21 ) with a rectangular cross-section, which is located in a cylindrical recess of the piston ( 11 ) and the piston ( 14 ) is sealed by a cap sleeve ( 22 ) which is in a cylindrical recess of the valve housing ( 13 ) sits and through a cylinder cylinder ( 12 ), which is screwed to the housing ( 13 ), is pressed onto the piston ( 14 ) and the control of the injection molding process is effected via a sensor ( 36 ), which detects the stroke state of the piston determines in which the piston ( 11 ) influences the inductive field of the sensor ( 36 ) and wherein the ejection of the molded part produced as defective via an evaluation unit at the wrong time of the piston control is caused or in the case of a multiple tool with a corresponding registration the ejection of all molded parts is effected as faulty. 2. Injection molding tool according to claim 1, characterized in that the screw connection ( 32 ) is secured against loosening by means of a screw disc ( 29 ) or by an appropriate adhesive. 3. Injection molding tool according to claim 1, characterized in that the cone angle ( α , β ) of the needle ( 8 ) have at their ends adjacent to the sprue an angle of at most 15 ° to the central axis. 4. Injection molding tool according to claims 1 to 3, characterized in that the temperature control channels ( 12 , 12 ', 12 ') of the temperature control system ( 10 ) are passivated by chemical deposition of a nickel-phosphorus alloy. 5. Injection mold according to at least one of claims 1 to 4, characterized in that the sliding surfaces of the pistons ( 11 , 14 ), the inside of the valve housing ( 13 ), the needle ( 8 ) and the coupling connection ( 15 ) are titanium nitride coated or chromed. 6. Injection mold according to claims 1 to 4, characterized in that the sliding surfaces of the pistons ( 11 , 14 ), the inner surface of the valve housing, the needle ( 8 ), coupling connection ( 15 ) and the needle guide bushing ( 18 ) are plasma nitrided. 7. Injection mold according to claims 1 to 6, characterized in that the nozzle body ( 6 ) is sealed against the clamping tool blocks ( 23 , 10 , 11 ) by insulating seals ( 41 , 42 ) consisting of titanium or ceramic.