IE68438B1

IE68438B1 - A process for preparing moldings from thermoplastic long fiber granules

Info

Publication number: IE68438B1
Application number: IE178691A
Authority: IE
Inventors: Juergen Henning; Peter Maschkowski
Original assignee: Hoechst Ag
Priority date: 1990-05-26
Filing date: 1991-05-24
Publication date: 1996-06-12
Also published as: DE4017081A1; JPH04232006A; DK0459321T3; IE911786A1; ES2077715T3; JP2989931B2; DE59106346D1; PT97759B; EP0459321A1; PT97759A; EP0459321B1; GR3018186T3

Abstract

A process for producing articles starting from pellets of fibre-reinforced thermoplastic material, in which process the pellets are introduced in any, in particular random, arrangement into a heating station, the fibre-reinforced material having been heated by a heating process to a temperature above the softening point of the fibre-reinforced material, an article cavity, which has been heated to a temperature below the softening point of the fibre-reinforced material, is charged with the material and a complete filling of the cavity is effected under high pressure, and, after cooling, the article is released from the article cavity, has the advantage that even relatively complicated mouldings can be produced without defects, that the mechanical properties of the moulding are particularly favourable, and that the process can be carried out in a technically uncomplicated way.

Description

The present invention relates to a process for producing long fiber reinforced moldings from a thermoplastic material.

The latest developments in the field of plastics include l 5 efforts to widen the spectrum of available materials by incorporating reinforcing fibers into engineering and high performance thermoplastics. As well as improved strength and stiffness characteristics, the objectives include production cost minimization. Owing to the necessary fabrication costs, the cost cutting is concerned less with the pure raw material costs than with the cost reductions achievable through reduced wall thicknesses or higher strengths, through simplified production methods or else through the replacement of metallic materials. It is as well to consider in this context that not only the attainable mechanical properties but also the processing techniques depend crucially on the maximum possible length of the reinforcing fiber. This development can be subdivided as follows: a) short fiber reinforcement with fiber lengths of around 0.2 mm, b) long fiber reinforcement with fiber lengths from around 4 mm, c) mat reinforcement with fiber lengths of a few cm.

(The stated fiber lengths always relate to the average lengths in the ready-produced molding.) Short fiber reinforced thermoplastics (a) typically have poor properties in respect of impact toughness and shock f loading. Elastomer modified short fiber composite mate30 rials make good this disadvantage to a certain extent, * but a reduction in tensile and flexural strength and also in the modulus of elasticity is unavoidable.

By long fiber reinforcement (b) it is possible to obtain distinct improvements in all the mechanical properties of 8 43 8 - 2 the thermoplastic material, which is why long fiber reinforcement is attracting increasing interest.

In contradistinction to mats, rovings and fabrics as reinforcement (c), if a thermoplastic reinforced with short fibers is stressed in the fiber direction the stress is likely to become concentrated at the fiber ends in the adjoining matrix.

There are in principle several suitable ways of producing long fiber reinforced moldings: With the widely used injection molding process the disadvantage is that the fiber content is restricted and that, depending on the nature and consistency of the molding composition, the processing conditions and the geometry of the mold cavity, a fiber length reduction occurs, which has an adverse effect on the strength of the molded piece.

Possible causes for the fiber damage are the following production steps: mechanical size reduction of granules during feeding, size reduction following jamming due to mutual impedance of the granular particles, friction between metallic parts and fibers, size reduction of fibers due to shearing between screw flight land and cylinder wall, shearing and bending of fibers in the melt, mutual impedance of fibers owing to an increase in the shear rate due to flow path diversion.

Concerning details of the injection molding of long fiber t reinforced thermoplastics, reference may be made to B. Schmid (Kunststoffe 79, (1989), 7, 624-630, Carl · Hanser Verlag).

Long fiber reinforced moldings can also be produced by coa^ression molding of preplasticated material. This process (as described for example by A. Youngs in Thermoplastic Matrix BMC - Materials and Processing, Polymer Composites Incorporated Winona, Minnesota, SPIConference, February 1986, Cincinnatti, Ohio, USA, Proceedings) has the disadvantage that, first, plastication and metering results in a fiber length reduction; secondly, that the fibers are highly intertwined, as a result of which the flowability is distinctly reduced, in particular at high fiber contents; and, thirdly, that it is highly equipment and time intensive.

A further process for producing moldings from thermoplastic compositions is described in German Standard Specification DIN 16770 (Part 1, February 1989). Said process involves compression molding a granular material which, however, needs to be introduced evenly into the compression mold since no flowing of the thermoplastic material is involved. This method can only be used to produce very single moldings from fiber-reinforced thermoplastics, for example rectangular sheets.

Moldings composed of thermoplastics which contain relatively long fiber sections are also obtainable starting from glass mat reinforced materials. In this case, the glass mat reinforced thermoplastic is first made into a semi-finished material from which the desired molding is made by heating to beyond the melting point of the matrix material and subsequent flow molding in positive molds. The disadvantage with this process is that moldings with complex elements (fine ribs, stays, etc.) can be produced satisfactorily only with difficulty and that the composi30 tion of the material in these sensitive areas of the molding differs distinctly from that in other (thicker) areas. The formability of the mat-like reinforcement limits the flowability of the thermoplastic material (see for example Kunststoffe 79 (1989), 12, 1372 and S. Kupper in GMT - Glasmattenverstarkte Thermoplaste, 20th Open Annual Meeting of the German Symposium for Reinforced Plastics, Freudenstadt, October 1-3, 1985) and also J. Six in GMT - ein Werkstoff und eine Verarbeitungstechnologie setzt sich durch (21th Open Annual Meeting o£ the German Symposium for Reinforced Plastics, Mainz, November 1987).

EP-A-170245 discloses the production of fiber-reinforced chips from thermoplastic material. These chips can be injection-molded into bars or sheets. However, it is difficult to fill complicated molds with this material at one and the same time.

GB 1439327 discloses the production of a molded article from fiber-reinforced thermoplastic material. In this process, the unheated granular material is introduced into a mold cavity heated to 170°C. Since, after cooling, a molded article is removed from the cavity, the granular material must have been melted in the cavity in the meantime.

It is an object of the present invention to provide a process for producing long fiber reinforced moldings from a thermoplastic material, which on the one hand is very economical and requires little by way of apparatus and on the other makes it possible to produce defect-free molded pieces with favorable mechanical properties for the particular practical use to which they are put.

This object is achieved by a process for producing a molding starting from rod-shaped granules of a fiber reinforced thermoplastic material, by completely filling a mold cavity with the molten granules under elevated pressure, cooling the mold cavity and then removing the molding, introducing rod-shaped granules having a fiber content of from 30 to 80% by weight and a length of from 10 to 100 mm into a heating-up station where they are heated until all the material has a temperature above the softening point, charging the heated material into the mold cavity which has been heated to a temperature below the melting point of the fiber reinforced material without using a screw machine, and then causing the cavity to be filled.

The granular starting material for the production process comprises rod-shaped particles preferably from 0.1 to Π 50 mm, in particular from 0.1 to 5 mm, in diameter and preferably 20 - 55 mm, particularly preferably * 25-50 mm, in length.

As the basic thermoplastic component it is possible to use a range of materials, in particular polypropylene, polyethylene, polyester, polyamide (for example eNylon), polycarbonate, polyurethane, polyoxymethylene, polyether ketone, polyethylene terephthalate, polyphenylene sulfide and styrene-containing polymers such as acrylic-butylenestyrene or styrene-maleic anhydride. In principle, it is also possible to use copolymers (for example of various structural units of the compounds mentioned) .

As fiber for reinforcing the thermoplastic material it is possible to use the customary reinforcing fibers, in particular glass, carbon and aramid fibers. However, in principle it is also possible to use ceramic fibers, such as silicon carbide fibers, or metallic fibers. Hybrid fibers (for example carbon/aramid) are also usable in principle.

The weight proportion of fiber in the fiber reinforced thermoplastic material may be varied within wide limits.

On the one hand, sufficient thermoplastic base material (matrix) should be present to ensure encapsulation of the entire fiber material and flowability, οή the other the desired mechanical reinforcement requires a certain y minimum level of fiber. The percentage fiber content depends in particular on the nature of the fiber material and on the thermoplastic matrix. Preferably, from 40 to 60% by weight, in particular from 45 to 55% by weight, of fiber material is present in the fiber reinforced thermoplastic .

The rod-shaped granules are Introduced in any desired arrangement into a heating-up station where they are heated to a temperature which is above the softening point (melting point) of the fiber reinforced material. By any desired arrangement is meant for example a random arrangement, brought about by scattering the granules into the heating-up station, or an ordered arrangement, produced for example hy introducing the granules on a grooved or shaking tray.

The temperature to which the thermoplastic material is raised will in general be within the range from 100 to 500“C; it depends in particular on the thermoplastic material used. The hot thermoplastic material is .then charged into a mold cavity. This may be done manually or mechanically. Of particular importance is that the cavity has a temperature below the melting point of the thermoplastic material. The material is pressed into the cavity at a pressure of preferably from 10 to 500 bar, preferably from 100 to 300 bar. This ensures complete filling of the cavity. Owing to its process specific favorable flow properties, the fiber reinforced material can flow into all parts of the mold.

The closing speed of the mold cavity should in general be not less than 500 rnm/s, while the molding speed should be not less than 5 mm/s, preferably more than 10 mm/s.

After molding, the molded piece is cooled down in the cavity, preferably within a period of from 0.1 to 5 minutes. In principle, it is also possible to employ longer cooling times, for example in the case of large moldings and correspondingly heavy cavity blocks, but a longer period is economically less advantageous. The temperature to which the molding is cooled must always be below the melting point of the thermoplastic material. In general, a molded piece temperature of from 20 to 250*C is suitable for extracting the molded piece from the cavity. A demolding temperature which is far below the solidification point is usually less advantageous, since in this case the time required is high.

The moldings produced by the process just described have particularly favorable mechanical properties. .

' Suitable materials for these moldings, as mentioned earlier, include a wide range of thermoplastic matrix I materials and fibrous reinforcing materials. It is particularly advantageous to use a glass fiber reinforced thermoplastic containing from 30 to 80% by weight, preferably from 40 to 60% by weight, in particular from 45 to 55% by weight, of glass fiber. The glass fibers generally have a thickness of from 400 to 15,000 tex, in particular a thickness of from 5 to 30 /an, and a length of from 10 to 100 mm, preferably from 20 to 55 mm.

In a particularly preferred embodiment of the present invention, the thermoplastic matrix material is polyethylene or polypropylene and the glass fibers are from 25 to 50 mm in length.

The Examples which follow illustrate the invention: 1. Production of a molding The starting material used in the present example was a glass fiber reinforced thermoplastic material from PCI (Polymer Composites Incorporated, 5152 West Sixth Street, P.O. Box 30010, Winona, Minnesota 55987) . The commercial rod-shaped granular material bears the designation TM Compel 2 Pellets11 and is 50% by weight glass fiber having a length of about 50 mm and 50% by weight a polypropylene. The rods or pellets are about 50 ma in f length and about 2 mm in diameter. 1150 g of this granular material are convection heated in a hot air through circulation oven in which the air temperature is 190-220°C. The heating-up process is continued until the entire material has a temperature (above the melting point) of 170-210°C. The polypropylene in the granules has a melting point of 165°C.

The material thus heated is introduced by hand into a positive mold, temperature controlled to 120°C, and is compression molded at a closing speed of 800 mm/s and a molding speed of 15 mm/s. The applied pressure is 220 bar (based on the projected molding surface) . The material had beforehand been introduced over a surface of about 250 mm x 450 mm, so that the complete filling-out of the cavity takes place by material flow during the coapression molding process.

Figure 1 is a diagrammatic representation of the shape and the dimensions (length (L) = 530 mm, width (W) 300 mm, height (H) 80 mm) of the resulting molding. After the molding has cooled down in the cavity to a temperature of 120° (cooling time 60 sec), the cavity is opened and the molding is demolded by ejector pins. As Figure 1 shows, the molding produced is fully formed. The molding has a uniform fiber distribution and the following mechanical properties: impact flexural strength (determined in accordance with DIN 53 453) 85 kg/m2; flexural strength (in accordance with ISO 178, but specimen width 50 mm instead of 10 mm (on account of glass fiber length) and specimen thickness between 2.4 and 5.3 mm instead of 4 ± 0.2 mm) = 192 N/mm2. 2. Production of a molding by a comparative process.

Starting from 1150 g of the granules of Example 1 the same mold cavity as in Example 1 is charged with the material after it had been preplasticated in screw machines. The molten material (temperature 190°C) is placed in the cavity and then compression molded.

Owing to the intensive mixing and intermeshing of the fibers in the screw machine and owing to the considerable fiber length, the flowability of this material is distinctly lower than in Example 1, so that, even under the same or under a very much higher pressure, the molding produced is not fully formed.

The molding produced by the comparative process (screw 1 plastication) has the following material properties (determined as in Example 1) : Impact flexural strength = 78 kg/m2 Flexural strength = 168 N/mm2 Figure 2 shows a molding produced by this process. The central thin rib (R) (diameter about 1 mm) and the edge areas of the molding (R') have not been completely filled with material.

Owing to the poor flow properties, therefore, it is necessary in the comparative process to apply the fiber reinforced thermoplastic material uniformly in all areas of the mold cavity, which is possible in practise only with primitive molds. Furthermore, a higher percentage of reject moldings is likely.

The process of the present invention is more suitable for producing low-defect or defect-free moldings, the moldings produced have more favorable mechanical properties, and the process is simple and inexpensive to carry out.

Claims

1. A process for producing a molding starting from rodshaped granules of a fiber reinforced thermoplastic material, by completely filling a mold cavity with the molten granules under elevated pressure, cooling the mold cavity and then removing the molding, characterized in that rod-shaped granules having a fiber content of from 30 to 80% by weight and a length of from 10 to 100 mm are introduced into a heating-up station where they are heated until all the material has a tesqperature above the softening point, the heated material is charged into the mold cavity which has been heated to a tesqperature below the melting point of the fiber reinforced material without using a screw machine, and then causing the cavity to be filled.

2. The process of claim 1, characterized in that the rod-shaped granules comprise a fiber reinforced thermoplastic having a fiber content of from 40 to 60% by weight and the rod granules are from 20 to 55 mm in length.

3. The process of claim 1, characterized in that the rod-shaped granules comprise a fiber reinforced thermoplastic having a fiber content of from 45 to 55% by weight and the rod granules are from 0.1 to 50 mm in diameter and from 25 to 50 mm in length.

4. The process of claim 1, characterized in that the rod-shaped granules are introduced into a heating-up station in a random arrangement, and the fiber ί reinforced material is brought in the heating-up station to a tesqperature of from 100 to 500°C which *is above the softening point.

5. The process of claim 1, characterized in that the rod-shaped granules are introduced into a heating-up station in an ordered arrangement, and the fiber reinforced material is brought in the heating-up station to a temperature of from 100 to 500°C which is above the softening point. 5

6. The process of claim 1, characterized in that the heated fiber reinforced material is pressed into the mold cavity under a pressure of from 10 to 500 bar.

7. The process of claim 1, characterized in that the heated fiber reinforced material is pressed into the 10 mold cavity under a pressure of from 100 to 300 bar.

8. The process of claim 1, characterized in that the molding is cooled down in the mold cavity to a temperature between 20 and 250°C in the course of freon 0.1 to 5 minutes and then removed from the 15 cavity.

9. The process of claim 1, characterized in that the molding comprises a fiber reinforced thermoplastic having a fiber content of from 30 to 80% by weight, the fibers being on average from 10 to 100 mm in 20 length.

10. The process of claim 1, characterized in that the thermoplastic material used is a fiber reinforced thermoplastic selected from the group consisting of polyester, polyamide, polyethylene, polyurethane, 25 polyoxymethylene, polycarbonate, polyether ketone, polyethylene terephthalate, acrylic-butylene-etyrene, polyphenylene sulfide, polypropylene and polystyrene.

11. The process of claim 1, characterized in that the thermoplastic material used is a fiber reinforced 30 thermoplastic copolymer.

12. The process of claim 1, characterized in that the fibrous reinforcement of the thermoplastic material comprises glass, carbon, arsmid, metal or a hybrid of these fibers.

13.

14. A process as claimed in claim 1, for producing a molding, substantially as hereinbefore described and exemplified. A molding whenever produced by a process claimed in a preceding claim.