WO2003008172A1

WO2003008172A1 - Method for the injection moulding of plastic workpieces

Info

Publication number: WO2003008172A1
Application number: PCT/DE2002/002451
Authority: WO
Inventors: Jürgen KÖBELIN
Original assignee: Siemens Aktiengesellschaft
Priority date: 2001-07-16
Filing date: 2002-07-04
Publication date: 2003-01-30

Abstract

The invention relates to a method for the injection moulding of plastic workpieces, in particular the upper, lower or exchangeable housings for mobile telephones, with simultaneous application of a surface decoration on the plastic workpieces, by the In-Mould-Decoration method or the In-Mould-Foiling method. A physical propellant is added to the plastic in order to reduce the injection pressure.

Description

description

Process for injection molding plastic workpieces

The invention relates to a method for injection molding plastic workpieces - in particular upper, lower or interchangeable shells for mobile phones - with simultaneous application of a surface decor to the plastic workpieces according to the in-mold decoration process or the in-mold foiling process according to the Preamble of claim 1.

Plastic workpieces are often given an imprint. This can take place, for example, after the plastic workpiece has been produced, that is to say after injection molding in a device, by means of one or more pressure stamps. Overprints are also created using screen printing technology.

Another method of applying an imprint is the IMD method (in-mold decoration method). Here, the imprint (decor) is first applied to a film that is inserted between the halves of an injection mold. The imprint is on the side of the film that faces the future surface of the finished plastic workpiece. The unprinted side of the film faces the injection mold surface that embodies the outer surface of the workpiece. The imprint on the film is therefore applied in mirror image.

If the plastic is now injected into the injection mold after the foil has been inserted, the foil will lay against the surface of the injection mold that will later be the printed surface of the workpiece. The injection point for the plastic must be selected so that the plastic that shoots into the injection mold hits the printed side of the film. If it were different, the plastic could see the slide and later slide surface of the finished workpiece.

After and while the injection mold is filling with hot plastic, the foil print is detached from the foil and stored on the surface of the workpiece. The imprint is now correctly aligned on the surface of the workpiece. The film itself is generally not absorbed by the workpiece. There are also methods in which the film is taken up by the workpiece as a whole. This process is called insert technology or in-mold foiling. The film remaining on the workpiece becomes part of the wall thickness. According to the prior art, such PC films are between 0.3-0.5 mm thick.

After a workpiece has been sprayed, the tool opens, the workpiece is ejected, the old film is removed, the new film is inserted in the plane of the mold-separating edge of the tool and the injection mold can move together again to spray the next workpiece. In the method described, the foils can consist of long webs with a large number of imprints, which are then fed to the injection mold synchronously with the spray cycle, likewise in cycles. With the insert technique, there are foils that are not designed as tapes, but as foils with only a single print. If strongly curved workpiece surfaces are to be imprinted, these foils can either be preformed by deep drawing prior to insertion into the injection mold or only in the injection mold. These foils are also partially bordered by punching the injection mold before insertion.

Problems arise with the prior art when spraying workpieces with a wall thickness of less than 1.5 millimeters in connection with the IMD or IMF process. Due to the toughness of the plastics, their temperature and the * pipe friction "of the plastic with the injection mold wall - and thus also with the film - there are distortions, washouts, streaks and the so-called yardage of the print in the area of the injection point. The smaller the wall thickness, the greater the shear forces - due to the * pipe friction "- of the plastic and the greater the risk of blurred printing.

For many workpieces, a wall thickness significantly smaller than 2 to 2.5 millimeters is acceptable for reasons of strength and dimensional stability. Wall thicknesses of 0.6 to 1.0 millimeters are desired. However, in the prior art, the tipping with conventional injection molding of these parts in connection with the IMD process or IMF process cannot be combined with one another in an acceptable quality, since the residual wall thickness when using foils, combined with low housing wall thicknesses, can no longer be filled , When the boundary wall thickness is reached with conventional injection molding, sink marks and visible weld lines are created due to pressure drop in the tool. This also applies to housing shells and decorative interchangeable shells for mobile phones.

It is therefore an object of the invention to provide a method which allows wall thicknesses of less than 2 millimeters to be sprayed in good quality in conjunction with the IMD or IMF method.

This object is solved by the feature of the independent claim. Advantageous developments of the invention are the subject of the subordinate claims.

Accordingly, the inventor proposes to significantly reduce the shear forces of the plastic injecting into the injection mold. This reduction takes place by means of a physical blowing agent in the plastic to be processed. Such a plastic is then also referred to as structural foam. By reducing the shear forces at the desired wall thickness, the injection pressures of are also reduced currently 1200 to 1500 bar by more than 50 percent. Surface tensions on the plastic part, which result from high shear forces during injection, can be identified with the aid of a polarizing film and can only be detected weakly around the injection point in such parts.

In addition, sink marks and visible weld lines are reduced because the plastic melt expands further after injection into the cavity and fills corners and unfavorable wall thickness ratios independently without emphasis.

Nitrogen, for example, can be used as the physical blowing agent. One of the advantages of this blowing agent is that it is available at low cost. C02 is also a common blowing agent.

In the case of a physical blowing agent, the gas is in dissolved form (super-critical state) during the addition to the plastic melt. When spraying, the gas is released again, at least in part. This also significantly reduces the density of the plastic material. Since the density is lower, the shear forces cannot be as great as with a plastic that contains no physical blowing agent (in contrast to physical blowing agents, chemical blowing agents work through a chemical reaction).

By means of the method according to the invention, it is now possible, for example, to inject housing shells and / or exchangeable shells for mobile telephones with a wall thickness that is significantly less than 2 millimeters. In general, wall thicknesses of 1.5 to 0.6 millimeters can now be sprayed without blurring an imprint using the IMD process or the IMF process. The wall thickness is preferably between 1.2 and 2.0 millimeters.

The method according to the invention also significantly reduces the specific injection pressure (by 30-50%). He is carries according to the invention only between 300 and 1000 bar. The injection pressure is preferably 500 to 800 bar or 600 to 750 bar.

The invention is described in more detail below on the basis of an exemplary embodiment with the aid of the figure. It shows:

1 shows a schematic cross section through an injection mold.

A cross section through a closed injection mold 1 can be seen in FIG. The injection mold 1 consists of a first mold half 5 and a second mold half 6. Further components of an injection molding machine have been omitted here for reasons of clarity.

A film 3 with an imprint lies in the plane of the mold-separating edge 2 of the injection mold 1 and on the inner contour of the first mold half 5. The surface with the imprint 8 of the film 3 faces the upper surface of the plastic workpiece 4.

A sprue 7 serves to fill the injection molding tool 1. These sprue 7 are provided with a nozzle-like constriction at their end facing the plastic workpiece 4. A needle valve 9 doses the amount of plastic melt. Without this, the plastic would expand into the cavity in an uncontrolled manner. As a result of this narrowing, the plastic undergoes a corresponding heating when it is injected and, as a result, additional liquefaction. The narrowing also makes it easier to separate the plastic workpiece 4 from the sprue 7 because of the small cross section and the associated minimal holding forces between the finished plastic workpiece 4 and the sprue when the finished workpiece is ejected. It goes without saying that the features of the invention mentioned above can be used not only in the respectively specified combination, but also in other combinations or on their own, without leaving the scope of the invention.

Overall, the invention thus proposes a method for injection molding plastic workpieces - in particular upper, lower or interchangeable shells for mobile telephones - with simultaneous application of a surface decor to the plastic workpieces using the in-mold decoration method or in-rαould foiling -Procedure, wherein a physical blowing agent is added to the plastic to reduce the injection pressure.

Claims

claims

1. Process for the injection molding of plastic workpieces (4) - in particular upper, lower or interchangeable shells for mobile telephones - with simultaneous application of a surface decor to the plastic workpieces (4) according to the in-mold decoration process or the in-mold - Foiling process, characterized in that at least one physical blowing agent is added to the plastic in order to reduce the injection pressure into the injection mold (1).

2. The method according to the preceding claim 1, characterized in that the physical blowing agent consists of a nitrogen dissolved in plastic.

3. The method according to any one of the preceding claims 1 or 2, characterized in that the wall thickness of the plastic workpieces (4) is between 0.5 to 1.5 millimeters.

4. The method according to the preceding claim 3, characterized in that the wall thickness of the plastic workpieces (4) is between 0.6 to 1.0 millimeters.

5. The method according to the preceding claim 3, characterized in that the wall thickness of the plastic workpieces (4) is between 0.7 and 0.8 millimeters.

6. The method according to any one of the preceding claims 1 to 5, characterized in that the injection pressure in the injection mold (1) is between 300 and 1000 bar.

7. The method according to the preceding claim 6, characterized in that the injection pressure in the injection mold (1) is between 500 and 800 bar. Method according to the preceding claim 6, characterized in that the injection pressure into the injection mold (1) is between 600 and 750 bar.