GB2025838A

GB2025838A - Injection Moulding Optical Reflectors

Info

Publication number: GB2025838A
Application number: GB7925380A
Authority: GB
Original assignee: Societe de Signalisations Automobiles SEIMA
Current assignee: Societe de Signalisations Automobiles SEIMA
Priority date: 1978-07-25
Filing date: 1979-07-20
Publication date: 1980-01-30
Also published as: DE2928582C2; IT7923836A0; FR2432137A1; IT1121915B; FR2432137B1; ES8101749A1; PT69866A; DE2928582A1; ES482375A0

Abstract

Process and apparatus for the production of an optical reflector, particularly for a motor vehicle headlight in which a first plastics material is injected into a mould (1), the volume of said material corresponding to only part of the mould volume and said first plastics material is made to flow in the form of a skin (51 and 52) against the mould walls through the immediate injection of a second plastics material (61) which, by displacing the first material, is positioned between the two skins formed by said first material against the mould walls. The apparatus includes pressure sensitive valves for controlling the flow of the first and second plastics materials from respective injection moulding machines. <IMAGE>

Description

SPECIFICATION Process for the Production of an Optical Reflector, Particularly for a Motor Vehicle Headlight and Apparatus for Performing this Process.

Technical Field The present invention relates to a process for the production of an optical reflector, particularly for a motor vehicle headlight, and apparatus for performing this process.

Reflectors in motor vehicle headlights or other directed lighting devices must have specific mechanical, thermal and optical properties in addition to a very precise quadric base (regular, truncated or staggered paraboloid).

Thus, reflectors must have an excellent mechanical rigidity, must not deform under the action of heat and must have a perfect reflecting power which provides a reflecting surface of a strict polyoptic, with no defect of material, shape or coating which could produce a diffusion of the light beam.

Background Art In order to reduce the manufacturing cost of such reflectors, which have hitherto been produced from metal by stamping, it has been envisaged to manufacture them by injection moulding a plastics material. Such an injection moulding process makes it possible to directly obtain very good optical surfaces, which reflect after vacuum metallisation.

However, such reflectors have a limited mechanical strength and are subject to deformation resulting from expansion produced by the heat given off by the light bulb located at the centre of the reflector.

In order to improve the robustness of such reflectors made from plastics material, it has been proposed to assemble with the plastics material defining the optical surface of the reflector another material having very good thermal resistance or effective heat dissipation which will provide the mechanical reinforcement of the reflector or at least reinforce its most exposed functional part (where the metal or plastics envelope or holder ring for the bulb is joined to the reflector by screws, tabs, grips, hooks, adhesion, welding, etc.). The various assembly processes used for these materials based on moulding from a casting have not effectively solved the problems of cohesion between these materials of different types, thermal conductivity, fluidity, compressibility and contraction.

Statement of Invention and Advantages It is an object of the present invention to alleviate the above disadvantages of plastics optical reflectors, and there is accordingly provided a process for the production of an optical reflector, particularly for a motor vehicle headlight, characterised in that a first plastics material is injected into a mould, the volume of said first plastics material corresponding to only part of the mould volume, and immediately thereafter injecting into the mould a second plastics material which forces the first plastics material against the opposed mould walls to form a skin on each of the opposed mould walls with the second plastics material sandwiched between said skins.

Further according to the present invention there is provided a plastics optical reflector, particularly for a vehicle headlight, when moulded by the process described in the immediately preceding paragraph.

In the process of the present invention the first plastics material may be chosen to give the inner surface of the reflector a good quality polyoptic, while the second plastics material is selected for its mechanical characteristics. The process makes it possible to obtain an optical reflector having good mechanical characteristics and resistance to thermal action, due particularly to the fact that the two materials are simultaneously shaped within the mould, i.e. when they are both in the pasty state. After cooling this can create a cohesion between the materials which will not be destroyed by the heat given off by the light bulb.

The two plastics materials are preferably injection moulded from separate machines, and there is still further provided according to the present invention apparatus for performing the process which comprises two injection moulding assemblies connected by respective passages to the gate of a mould, characterised in that each passage has a valve which is sensitive to the pressure of the injected material.

Preferably according to the process a sufficiently small quantity of the first plastics material is injected into the mould to ensure that the skins formed on the opposed mould walls do not reach the most remote points of the mould defining the edge of the reflector opening, said points of the mould being filled only by the second plastics material.

Furthermore, conveniently after injecting the second plastics material a quantity of a first plastics material is immediately injected in order to terminate the flow of the second material towards the most remote points of the mould.

The first and second plastics materials may be of the same nature, with an additive being incorporated in the second material. The additive in the second material may comprise, for example, glass fibres or mineral particles.

Figures in the Drawings One embodiment of a process and apparatus in accordance with the present invention, will now be described by way of example only, with reference to the accompanying drawings, in which:~ Figures 1, 2 and 3 illustrate the successive phases of the process; Figure 4 is a longitudinal sectional view of the apparatus; Figure 5 is a detail showing in section the injection part of the apparatus of Figure 4, and Figure 6 is a cross-sectional view of one of the valves in the apparatus.

Detailed Description of the Drawings The apparatus and process described hereinafter provides the rapid and uncomplicated manufacture of an optical reflector made from plastics material, which has a high mechanical resistance, particularly to thermal action.

The process utilises an injection moulding machine whose mould cavity 1 has the precise shape of the reflector to be made. The mould cavity 1 is defined by the narrow space formed between a female mould part 2 and a male mould part 3.

According to the process a first plastics material 5 is injected into the mould cavity 1 through a gate 4. The characteristics of this material are such that it is able to give the inner surface of the reflector a good quality polyoptic.

This first material 5 is displaced from the interior of mould cavity 1 by means of a second plastics material 6 which is selected as a function of the mechanical characteristics which it can give the reflector.

During the injection of the second material 6 the first material 5 flows towards the points of mould cavity 1 which are furthest from the gate and forms a skin against the opposed mould walls.

After injecting the second material a reflector is therefore obtained, whose inner and outer surfaces are constituted by the first material, said two surfaces being connected by a sandwiched layer of the second material.

The reflector strength is partly due to the cohesion provided when the two plastics materials solidify together from their semi-liquid or pasty states. This reflector strength also results from the fact that the first plastics material forms an inner skin 5, and an outer skin 52, which are of limited thickness and thereby reduce the internal stresses which may occur at the junction of the skins 51 and 52 with the core 6, formed by the second material.

The first plastics material 5 for the reflector surface material can be selected from plastics materials such as polycarbonates, polysulphonated ethers, polysulphones, and PBT.

The second or reinforcing material can be chosen from among the same materials, but it preferably contains an additive in order to improve its mechanical characteristics or its heat dissipation properties. This additive may, for example, be constituted by glass fibres or mineral particles, which will in particular reduce the co-efficient of expansion and retraction of the material by improving its rigidity.

During the performance of the process the quantity of the first material 5 injected is determined as a function of the fluidity of said first material, as well as the fluidity of the second material in order to obtain, in a mould system 1, skins of desired thickness on the reflector walls.

However, the quantity of the first material 5 can be reduced in such a way that the skins 5, and 52 formed against the walls of the mould system 1 do not reach the points thereof which are most remote from the gate 4 (see Figure 3) and in such a way that said points defining the edge 7 of the reflector opeing are only constituted by the second material 6, which has better mechanical qualities for the application in question.

Furthermore, after injecting the second plastics material 6 a quantity of the first plastics material 5 may be immediately injected in order that it completes the flow of the second material towards the most remote points of the mould.

This also has the effect of maintaining the injection pressure on the first material throughout the injection time and the start of cooling in order to prevent contraction phenomena which could occur locally and lead to a surface defect on the reflector.

In addition, this further injection of the first material has the effect of removing the second material from gate 4 and thus prepares the machine for the injection moulding of a new reflector.

T#his process makes it possible to obtain a reflector having a good optical surface constituted by a homogeneous skin 51 which firmly adheres to a reinforcement which is simultaneously produced and which also corresponds as regards shape to the shape of the desired reflector and can be obtained by using a press as shown in Figures 4 and 5.

This machine has two injection systems 8 and 9, each containing a supply screw 10 axially movable for the injection under the action of a plunger 11 located in a cylinder 12 supplied by hydraulic fluid pipes 13. Each screw is supplied with material from a hopper 14 and it is rotated by means of a hydraulic motor 15.

A press plate 17 supporting mould part 18 is fixed to frame 16 of said press. The core or male mould part 19 is fixed to plate 20 which is mounted to slide on columns 21 towards and away from the mould part 18, under the action of a piston 22 of a jack 23 supplied with hydraulic fluid by pipes 24.

As can be seen in Figure 5 mould part 18 is fixed to the press plate 17 by an arrangement comprising a plate 25 equipped with heating members 26, said heating plate 25 being inserted between two thermal insulating plates 27 and 28.

The injection machines 8 and 9 are connected to mould 1 by passages 29 and 30, respectively, which are joined at 31 at the intake of a nozzle 32. The channel 32, in nozzle 32 is continued by a channel 33, which defines the sprue in the moulded article, said channel 33, being produced in a part 33 of mould part 18.

The passages 29 and 30 have respective check valves 34 and 35 which are sensitive to pressure in such a way that one of the valves closes as soon as the material injection pressure in one passage exceeds that in the other.

In addition, valves 34 and 35 are positioned upstream of the junction 31 in such a way that the control of one of the valves is effected by the pressure reaction of the material in the other passage, said pressure being transmitted to the one valve via junction point 31.

This arrangement makes it possible to obtain a regular injection of each of the two materials, with the change of the injected material occurring with virtually no mixing of the materials. This is important in view of the small quantities of materials used in the construction of the reflector.

In the represented embodiment valves 34 and 35, which are shown positioned at the ends of passages 29 and 30 adjacent to the injection moulding machines 8 and 9, each comprise a ball 36 which moves within the valve body. Ball 36 can either be applied to a sealing valve seat 37 having an opening issuing into the respective injection moulding machine 8 or 9, or can be applied to a valve seat 38 issuing into the respective passage 29 or 30 in the direction of the mould cavity 1.

Each valve seat 38 (Figure 6) has a central orifice 38t against which the ball 36 can be applied. Orifice 38, has lateral openings 382 for the passage of the injected plastics material. The sum of the cross-sections of openings 382 corresponds to the respective cross-section of passages 29 and 30, in order to prevent a local modification of the injection moulding pressure of the material.

During the injection moulding of a reflector the cycle of pressures of each of materials 5 and 6 is determined automatically be the plungers 11 of each of the machines 8 and 9, whilst the temperatures are obtained by heating collars 39.

These temperatures are also determined by heating elements 26 in plate 25, whose operation is controlled by temperature reading pyrometers 40. Tubes 41 for the flow of a cooling liquid are provided in the plate 25 between the two passages 29 and 30 in order to respect the different injection temperature conditions of the two materials supplied to mould cavity 1 through the passages.

Claims

1. A process for the production of an optical reflector, particularly for a motor vehicle headlight, characterised in that a first plastics material is injected into a mould, the volume of said first plastics material corresponding to only part of the mould volume, and immediately thereafter injecting into the mould a second plastics material which forces the first plastics material against the opposed mould walls to form a skin on each of the opposed mould walls with the second plastics material sandwiched between said skins.

2. A process according to claim 1 characterised in that a sufficiently small quantity of the first plastics material is injected into the mould to ensure that the skins formed on the opposed mould walls do not reach the most remote points of the mould defining the edge of the reflector opening, said points of the mould being filled only by the second plastics material.

3. A process according to claim 1 or claim 2 characterised in that after injecting the second plastics material a quantity of the first plastics material is immediately injected in order to terminate the flow of the second material towards the most remote points of the mould.

4. A process according to any one of claims 1 to 3 characterised in that the first and second plastics materials are of the same nature with an additive being incorporated in the second material.

5. A process according to claim 4 characterised in that the additive in the second material comprises glass fibres.

6. A process according to claim 4 characterised in that the additive in the second material comprises mineral particles.

7. A process according to any one of the preceding claims characterised in that the plastics of the first and second plastics materials are selected from polycarbonates, poiysulphonated ethers, polysulphones and PBT.

8. A process for the production of an optical reflector, particularly for a motor vehicle headlight, substantially as herein described with reference to the accompanying drawings.

9. A plastics optical reflector, particularly for a vehicle headlight, when moulded by the process claimed in any one of the preceding claims.

10. Apparatus for performing the process according to any one of claims 1 to 8, said apparatus comprising two injection moulding assemblies connected by respective passages to the gate of a mould, characterised in that each passage has a valve which is sensitive to the pressure of the injected material.

11. Apparatus according to claim 10 characterised in that each valve comprises a valve body provided with a valve seat for a movable sealing ball, the seat having an opening issuing into the respective injection moulding assembly.

12. Apparatus according to claim 11 characterised in that each valve body has a second valve seat issuing into the passage towards the gate of the mould, said second valve seat also being provided with lateral openings, the sum of their cross-sections corresponding to the cross-section of the passage.

13. Apparatus according to any one of claims 10 to 12 characterised in that the valves are positioned at the ends of the passages adjacent to the injection moulding assemblies.

14. Apparatus according to any one of claims 10 to 13 characterised in that the passages are provided in plate means having heating means, cooling pipes also being provided in said plate means between the two passages.

15. Apparatus for the production of an optical reflector, particularly for a motor vehicle headlight, substantially as herein described with reference to the accompanying drawings.