GB2191442A

GB2191442A - Producing thermoplastic parts having a structure on their surface

Info

Publication number: GB2191442A
Application number: GB08704175A
Authority: GB
Inventors: Dr Ing Prof G W Ehrenstein; Dipl In C Maertin
Original assignee: Lindauer Dornier GmbH
Current assignee: Lindauer Dornier GmbH
Priority date: 1986-06-14
Filing date: 1987-02-23
Publication date: 1987-12-16
Also published as: GB8704175D0; JPS62299319A; BE906159A; FR2600006A1; DE3620081A1; IT8767147A0

Abstract

In a process for making a rod-like part of thermoplastics material having a profiled surface, for example a gripper rod with a toothed profile as used in a shuttleless loom, heated synthetic resin (3) is introduced into a master mould comprising a matrix (1) and a plug (2) by an injection operation. Formation of the profile and subsequent cooling of the moulded material (3) are carried out in the same matrix (1), using a profiled hobbing plug (2b) under pressure. The molecular chains which have already been oriented in the main direction of stress of the part during the injection operation are made to conform to the surface profile at the embossing stage of the process. This process results in a part having great rigidity and strength due to the increase in its intrinsic strength. <IMAGE>

Description

SPECIFICATION Process for the production of rod like parts of thermoplastic synthetic resins having a structure on their surface This invention relates to a process for making rodlike thermoplastics parts having a structure on their surface, for example, a gripper rod with a toothed profile as used in a shuttleless loom.

A gripper rod of this kind is described, for example, in DE-PS No. 1 710 293, according to which, a gripper rod has an inner plastics part which has the toothed profile and has been made, for example, by a pressure moulding process, and a supporting part of Ushape enclosing the inner plastics part on three sides and bonded to it. The gripper rod has a high flexural strength due to the bond between the hollow-sectioned supporting part and the inner part, and also due to the non-cutting process by which the inner part was made. This flexural strength is further increased by providing continuous webs on the two lateral surfaces of the inner part approximately equal in height to the teeth and lying in contact with the internal surface of the Ushaped supporting part.

In a gripper rod of this kind it is important to obtain a sound bond between the inner part and the supporting part and to ensure that the inner part has sufficient longitudinal strength, since it must take up all the forces acting in the direction of movement of the gripper rod when the rod is in operation.

One disadvantage of these known toothed rods is the complicated process by which they are made. They must be made from two parts which are subsequently joined together, e.g by glueing. The inner part, i.e. the toothed rod, is normally made of a plastics material while the outer part which supports it on three sides is a U-shaped hollow section made of metal.

Another form of gripper rod is disclosed in German Patent Application No. P 35 27 202.

In this application, the toothed rod is made of a stack of thermoplastic, fibre-reinforced synthetic resin bands which are joined together by pressure and heat treatment, with the fibres running in the longitudinal direction of the bands. The purpose of this construction is to improve the absorption of forces produced when the gripper rod is driven. The required toothed profile is obtained by punching different shapes in the individual bands according to the height at which they are situated. This toothed rod is therefore also obtained without any cutting work during manufacture.

Although this gripper rod has certain advantageous properties compared with the gripper rod mentioned in DE-PS No. 1 710 293 since it is not made of two parts and is also substantially lighter in weight, its longitudinal strength is no longer sufficient for the increasing demands made on gripper rods. Also the process for manufacturing these gripper rods is relatively complicated.

It is an object of the present invention to provide a process for making gripper rods with a toothed profile which are strong without being heavier than those known in the art, and which can be made more simply.

According to this invention there is provided a process for making rod-like thermoplastics parts having a structure on their surface, for example a gripper rod with a serrated profile as used in a shuttleless loom, the process comprising: a) injecting a thermoplastics material heated to just above its crystallization temperature into a smooth mould closed on all sides and having a matrix and a plug; b) further shaping under pressure in the same matrix by means of hobbing plug of the required form after removal of the injection unit; and c) cooling while pressure is maintained. The process of the invention includes the known process of injection moulding which, combined with the subsequent embossing step, results in an intrinsically stronger material.Since the temperature of the melt is only slightly above the crystallization temperature of the synthetic resin, the molecule chains which become oriented in the longitudinal direction of the material when it is injected soon begin to crystallize in the same direction. During the subsequent embossing step, another structure produced by localized flow by the action of the hobbing plug is superimposed on the basic structure and the melt crystallizes to a socalled Shish-Kebab structure which is distinguished by great rigidity and strength due to its self-generated reinforcement.

An embodiment of the invention will now be described by way of example, with reference to the drawings, in which: Figure 1 is a view in perspective of a matrix and plug; Figures 2a to 2c are cross-sections through the matrix and plug at various stages of the process; and Figures 3a and 3b are a simplified longitudinal section through a toothed rod and the cross-section through a toothed rod.

Figure 1 shows a matrix 1 and a plug 2.

The injection moulding machine with injection nozzle and the sprue have been omitted for simplicity. The plug 2 serves as plug holder for a plug 2a which closes the matrix 1 from above during injection of molten plastics and for a hobbing plug 2b having the appropriate surface shape required for the next step of the process.

The process is illustrated in Figures 2a to 2c. Injection moulding is carried out in known manner in a first step illustrated in Figure 2a.

At this stage, heated thermoplastic synthetic resin 3 which is at a temperature just above its crystallization temperature is injected into a master mould comprising the U-shaped matrix 1 and the smooth plug 2a which closes the open side of the matrix 1. The injection nozzle and the sprue are so designed that they ensure a continuously accelerated stream of molten plastics 3 which promotes orientation of the macromolecules. Due to the flow of material in the process of injection, the macromolecules undergo orientation in the longitudinal direction of the injection mould whereby the strength properties of the material are improved. The high pressures required for injection must be taken up by the fixed matrix 1 and the movable plug 2 with a force equal to Fs.

Figure 2b shows how in a second step the plug 2 is lifted from the matrix 1 and shifted by an amount equal to the distance S between the smooth plug 2a and the hobbing plug 2b so that the hobbing plug is brought into the position required for the embossing step illustrated in Figure 2c.

During the shaping brought about by the hobbing plug 2b acting on the molten plastics 3 with an embossing pressure Fp, the macromolecules which have already been brought into a preferential direction of orientation by the injection moulding process now become orientated in directions conforming to the surface profile which depends on the form of the hobbing plug 2b used.

In the third step of the process, the molten plastics 3 cools down while the pressure exerted by the hobbing plug 2b is maintained.

The continued action of this pressure promotes crystallization of the material and a series structure is produced which has a positive effect on the mechanical properties of the material in the direction of orientation.

Depending on the chosen surface profile of the hobbing plug 2b it would be possible to modify the process so that all its stages are carried out by a single plug, namely the hobbing plug 2b. It would not then be necessary to change plugs nor to lift the plug holder 2 from the matrix 1 so that there would be no risk of the orientated molecule chains becoming disorientated when the pressure is relieved during plug change. Figure 3a is a simplified view across a toothed rod 4 made by the process of the invention. The orientation 5 of the molecular chain following the surface profile is indicated schematically. The orientation 5 follows the form of the teeth 6 of the toothed rod 4, thereby reinforcing those regions of the rod 4 which are put under more severe stress in use.

Lateral webs 7 (Figure 3b) may be provided to increase the strength of the rod 4 as a whole. This requires only a slight modification of the form of the hobbing plug 2b.

Although this process has been described with reference to a toothed gripper bar as an example, it will be seen that it is widely applicable to other fields where rods or similar parts are required to be produced with a surface profile.

The advantages of the process lie not only in its simplicity and consequently simple production of the parts, but in its capacity to produce lightweight workpieces of increased intrinsic strength.

Claims

1. Process for making rod-like thermoplastics parts having a structure on their surface, for example a gripper rod with a serrated profile as used in a shuttleless loom, the process comprising: a) injecting a thermoplastics material heated to just above its crystallization temperature into a smooth mould closed on all sides and having a matrix and a plug; b) further shaping under pressure in the same matrix by means of a hobbing plug of the required form after removal of the injection unit; and c) cooling while pressure is maintained.

2. Process according to claim 1, wherein the injection nozzle and sprue provide a continuously accelerated stream of molten plastics material during injection.

3. Process according to claim 1 or claim 2, wherein the sprue is so positioned that the orientation of the melt in the mould substantially follows the subsequent direction of stress in the rod-like part.

4. Process according to claim 1, wherein the temperature of the melt during injection lies in the region of the crystallization temperature of the material.

5. Apparatus for carrying out a process according to claim 1, wherein a smooth plug closes a matrix during injection and another, hobbing plug conforming to the required profile are arranged on a common plug holder which is displaceable in relation to the matrix is such manner that the smooth plug and the hobbing plug alternately face the matrix.

6. Process for making rod-like thermoplastics parts, the process being substantially as herein described and shown in the drawings.

7. Apparatus for making rod-like thermoplastics parts, the apparatus being constructed and arranged substantially as herein described and shown in the drawings.