GB2171418A

GB2171418A - Facing of cylinders and worms in plastics worm extruders to enhance wear resistance

Info

Publication number: GB2171418A
Application number: GB08601241A
Authority: GB
Inventors: Peter Stommel; Stephan Weilandt; Hans Reifenhauser
Original assignee: Reifenhaeuser GmbH and Co KG Maschinenenfabrik
Current assignee: Reifenhaeuser GmbH and Co KG Maschinenenfabrik
Priority date: 1985-02-26
Filing date: 1986-01-20
Publication date: 1986-08-28
Also published as: IT8619312A1; GB8601241D0; IT8619312A0; CH669555A5; IT1188333B; DE3506726A1; GB2171418B; DE3506726C2

Abstract

The wear resistance of plastics worm extruders having a cylinder-and-worm shaft consisting basically of steel with a prescribed toughness and hardness is enhanced by facing with WC. Both the cylinder and the worm shaft are provided with the facing, at least in opposing areas, the contours being preserved on and between opposing areas within clearance tolerances of between 0.15 and 1% of the nominal diameter. The substrate prior to facing has a surface roughness of 2 to 4 mu m and a relative load-carrying area of between 40 and 80% at a depth of 0.25 mu m. The facing may be applied by plasma spraying or explosive spraying.

Description

SPECIFICATION Facing cylinders and worms in plastics worm extruders to enhance wear resistance This invention relates to plastics worm extruders having cylinders and worm shafts consisting basically of steel, and is concerned with enhancing the wear resistance of such plastics worm extruders by facing with tungsten carbide (WC). Both cylinders and the worms can consist of one or more sections.

The method of facing with WC for enhanced wear resistance is known (UCAR company brochure "Metall & Keramik Coatings", Union Carbide Deutschland GmbH, 77.09.06 CS.G). It is chiefly applied to tool components such as deep-drawing punches and dies, cutting tools, milling cutters, punches or hardmetal toolbits, and for injection moulds. It relies on the fact that it eliminates or very greatly reduces the incidence of phenomena which would otherwise damage the tools, such as edge build-up, cold welding, cold wear and cracking in the cutting edges of the tools. The facings are applied by plasma spraying or explosive spraying. In the plasma spraying process, a plasma gun controls an inert-gas jet at a high temperature around 20,000 C for example and at extremely high velocities. No combustion is involved in this process.On the contrary, an untransferred electric arc of high energy density transfers its heat potential to the stream of insert gas flowing through it. The powdered spraying material melts in this stream of hot gas. At the same tim, it is greatly accelerated before reaching the surface of the target component requiring facing. The incident material particles form an extremely close-packed facing, which is securely bonded within itself and to the surface of the prepared substrate. In the explosive spraying process, use is made of a detonation gun. The detonation gun consists substantially of an elongated tubular reaction chamber which protrudes like a gun barrel from a control block for gases, powdered material and electronic equipment.Exactly measured amounts of oxygen, acetylene and powdered spraying material are introduced into the reaction chamber, and the mixture is then detonated by means of an electrical spark. The resulting jet of hot gases easily melts the material particles and accelerates them in the lengthy tubular barrel to may times the velocity of sound. The particles arrive at the workpiece surface at this velocity, and become bonded to the surface by a micro-welding effect which produces a tough bonding to the substrate. A rapid succession of detonation cycles can produce facings of thicknesses between 0.03 and 0.5 mm on a workpiece surface which is usually traversed under automatic control, in accordance with the facing requirements.The temperature of the workpiece surface, apart from a thin transition layer, remains at or below 1 500C in both plasma spraying and explosive spraying processes. The workpiece undergoes no serious distortion.

The methods of enhancing wear resistance by facing with the aforesaid materials have also been proposed (as a practical measure) for application to the cylinders and worm lands on worms for plastics worm extruders, though the proposed facing materials are titanium nitride or titanium carbide rather than tungsten carbide. In this connection, the facings have been applied either to the cylinders or to the worm lands in various areas where wear phenomena have been observed on the cylinders and worm lands respectively.

The results have not been satisfactory. The durability of the worm-and-cylinder unit and thus the useful life of the extruder as a whole has not been adequately extended by these means. Consequently, other means are adopted in practice for prolonging the life of the worms and cylinders in plastics worm extruders. More specifically, surface hardening means such as nitriding are adopted, or the cylinders are lined with sleeves made from special metals, more particularly hard metals.

It is also known to adopt so-called bimetal cylinders, which are centrifugally lined with a suitable material.

The object of the invention is to improve substantially the durability of a plastics worm extruder, and hence the useful life of the entire machine.

According to the present invention a method of enhancing the wear resistance of a plastics worm extruder by the facing with tungsten carbide (WC) of a cylinder and a worm shaft consisting basically of steel with a surface layer hardness when ready for use of 40 to 70 HRC, in conformity with DIN 50 103, is characterised in that both the cylinder and the worm shaft are provided with the facing, at least in opposing areas (or over their entire length), the contours are preserved on and between opposing faced areas within clearance tolerances of between 0.15 and 1% of the nominal diameter, and moreover the substrate prior to facing has a surface roughness of 2 to 4 ,am and at a depth of 0.25 m a relative load-carrying area, as defined in DIN 4765, of between 40 and 80%, preferably about 60%.The basic idea of the invention consists in that, within the scope of the means employed by the invention, both the cylinder and the worm, or at least the worm lands, are faced over opposing areas, more particularly areas where wear would occur in the absence of any facing. This does not exclude the facing of other areas, not necessarily opposing areas.

The cylinder and the worm shaft are preferably surface hardened, or are through hard ened. However, the two alternatives can be combined, using for example a worm shaft in the through hardened condition and a cylinder in the surface hardened condition-or vice versa. If surface hardening alone is adopted, the depth of hardening should be between 0.3 and 3 mm. The facing layer should have a thickness of about 0.1 mm and a roughness depth of about 10 pm. The facing can be applied with the aid of a plasma spraying process, more particularly on a through hardened substrate. However, the facing can alternatively be applied with the aid of an explosive spraying process, in which case the substrate is through hardened. It is usually unnecessary to repeat the hardening treatment after applying the facing.

The accruing advantages are to be seen in that a surprising improvement is obtained in the durability and hence the useful life of the entire plastics worm extruder. Compared with otherwise identical combinations of materials, the durability or useful life is improved by more than 25% by providing the facing as such. The machine can continue to operate after the facing has worn off, because the through hardened basic components remain useable for a longer period.

The basic materials for worm shafts and cylinders can be the alloy steels already in common use. Particularly good results are obtained with the materials identified by the numbers in the following table: 1.2379~DIN 17350 1.7225 DIN 1654117200 1.7707~DIN 17200 1.2344~DIN 17350 1.8550~DIN 17211 1.8515~DIN 17211 1.7735~"Steels Key" (1983), p. 389, Ver lag Stahlschlussel Wegst The cylinder can alternatively consist of a main body and an inserted liner; in this case, the various requirements relate to the liner.

Claims

1. A method of enhancing the wear resistance of plastics worm extruders by the facing with tungsten carbide (WC) of cylinders and worm shafts consisting basically of steel with a surface layer hardness when ready for use of 40 to 70 HRC, in conformity with DIN 50 103, characterised in that both the cylinder and the worm shaft ar provided with the facing, at least in opposing areas, the contours are preserved on and between opposing faced areas within clearance tolerances of between 0.15 and 1% of the nominal diameter, and prior to facing the substrate has a surface roughness of 2 to 4 #m and at a depth of 0.25 Am a relative load-carrying area, as defined in DIN 4765, of between 40 and 80%, preferably about 60%.

2. A method as in Claim 1, wherein the cylinder and the worm shaft are surface hardened.

3. A method as in Claim 1, wherein the cylinder and the worm shaft are through hardened.

4. A method as in any one of Claims 1 to 3, wherein the facing layer has a thickness of about 0.1 mm and a roughness depth of about 10 Am.

5. A method as in any one of Claims 1 to 4, wherein the facing is applied with the aid of a plasma spraying process or an explosive spraying process.

6. A method as in any one of Claims 1 to 5, wherein the basic materials for the worm shafts and cylinders are the alloy steels identified by the numbers in the table.