GB2133738A

GB2133738A - Improvements in or relating to disc record preparation

Info

Publication number: GB2133738A
Application number: GB08301265A
Authority: GB
Inventors: Hermann Strausfeld
Original assignee: EMI Electrola GmbH
Current assignee: EMI Electrola GmbH
Priority date: 1983-01-18
Filing date: 1983-01-18
Publication date: 1984-08-01
Also published as: GB2133738B; GB8301265D0; DE3347652A1

Abstract

A disc record press suitable for moulding video discs comprises a pair of opposing mould blocks, (20,21) to which stamper plates (24,25) are affixed, assembled between flat and parallel rams (30,31). The mould block surfaces facing the stamper plates are normally slightly convex and the mould block surfaces facing the rams (26,27) are normally slightly concave such that when the press is retracted, the stamper plate pressing surfaces (28,29) are slightly convex. During disc pressing, the high pressing force causes the mould blocks and stamper plates to elasticably deform to have flat and parallel surfaces. As the press retracts, parting of the stamper plates from a moulded disc takes place gradually from the disc periphery towards its centre with an arcuate motion, thereby substantially avoiding damage to the disc modulations due to differential shrinkage and air influx. <IMAGE>

Description

SPECIFICATION Improvements in or relating to disc record preparation This invention relates to the pressing of thermoplastic disc records, such as video discs, which have a high areal density of recorded information. Other uses of the invention may include digital audio recording and mass storage for computers.

Techniques for moulding gramophone records and, latterly, video discs by compressing a "shot", otherwise known as a 'cake' or a, 'preform', of softened plastics material between a pair of heated stamper plates are well known. Typically, each stamper plate, otherwise known as a mould matrix, bears a negative replica of the modulations to be carried by the disc, and each stamper plate is mounted by means of clamps on a respective mould block between the rams of a record press.

While in a conventional gramophone record the information is presented by the modulation of a continuous groove of, typically, 50Wm to 1 00 > m width, the information in a video disc is commonly represented by a sequence of closely spaced pits of, typically, 0.5#im, to 1 Fm width and less than 1 Fm depth. In consequence, the mechanical precision required for reliably transferring such recorded information from a stamper to a video disc is far more stringent than for a conventional gramophone record.

In particular, it is vitally important that the information pits in a moulded video disc match closely in profile the protrusions on the stamper plate, and that these pits suffer little or no damage in the process of disc record manufacture.

It is an object of the present invention to provide a disc record press which gives to an improved correspondence between the profile of the information pits of a moulded disc record and the corresponding modulated profile of its respective stamper plate.

According to the invention there is provided a disc record press which includes at least one mould block assembled to lie between a ram and a stamper plate, said mould block having a concave surface facing said ram and a convex surface facing said stamper plate when said disc record press is in a retracted position, said convex and concave surfaces becoming approximately flat and parallel to one another when said record press is in the closed position and in process of moulding a disc record.

In order that the invention may be clearly understood and readily carried into effect, one embodiment thereof will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a sequence of operations involved in the pressing of a video disc using prior art mould blocks.

Figure 2 shows an equivalent sequence of operations involved in the pressing of a video disc using mould blocks in accordance with one embodiment of the invention.

The prior art mould blocks shown at 1 and 2 in Figure 1 a are typically formed of three parts, namely a base plate 3, an intermediate plate 4 and a front plate 5 as described, for example, in British Patent No. 1437175. Each mould block has internal channels and inlet and outlet connections for heating and cooling fluids which are circulated as appropriate during the pressing cycle. Parts 3, 4 and 5 are commonly of steel material, although inner and outer regions may be of a material of higher thermal conductivity such as berillium copper as described in British Patent No. 1509362. The component parts 3, 4 and 5 are rigidly assembled to form a sandwich construction and the upper and lower surfaces 6 and 7 of each mould block are precision ground to be flat and parallel to one another.The overall thickness of each mould block is typically 4cm, and its diameter is somewhat greater than the diameter of the disc record to be pressed, typically 35cms. The stamper plates are shown at 8 and 9 in Figure 1 (a). Each stamper plate is of a nickel or nickel alloy and is electroformed to have a profile at surface 10 which is a mirror image on the surface of the master disc to be replicated. Each stamper plate has flat and parallel surfaces ignoring the minute protrusions of the modulated surface and has a thickness of typically 250yam.

Chamfers at 11 and lands at 12 permit a widening of the disc record around its periphery, while chamfers at 13 permit each stamper plate to be clamped to its respective mould block by means of a chamfered clamping ring (not shown). When the stamper plates are clamped to their respective mould blocks each innerstamperface, ignoring the presence of the minute modulating protrusions, is flat and parallel to its respective outer mould block face 6 as shown in Figure 1(b).

The mould blocks are in operation acted upon by flat and parellel rams, partly shown, and 15, of a record press such that the modulated faces 10 of a stamper plates 8 and 9 are flat and parallel to one another, shown in Figure 2 (c) at the moulding state of the record pressing cycle. The mould blocks are heated to a temperature in the region of 170or which is sufficient to melt a preformed 'cake' of a vinyl based plastics material placed between the stamper plates, to form a moulded disc record at 16. At this stage the force at the rams is in order of 50 tonnes, 50000 kg. The mould blocks are then cooled and the plastic disc record solidifies.The disc record is then supported by grippers (not shown) which attach to the solidified plastic flash surrounding the disc periphery and the stamper plates are withdrawn from the modulated surfaces of the disc as the press retracts, as shown in Figure 1 (d). A much magnified view of a small element of the upper surface of a video disc is shown at 16A, with its corresponding element of the upper stamper plate at 17. During the cooling and solidifying of the moulded disc, the plastics material tends to shrink in the direction of the arrow A towards the centre of the disc, this shrinkage being considerably more marked than the shrinkage of the stamper plate due to thermal contraction.Before separating the stamper plates from the moulded disc an approximately radial shrinkage stress in the plastic disc acts against the outer vertical walls such as 18 of the minute protrusions of the stamper plate which represent the information to be replicated. This shrinkage stress can cause damage to the outer walls such as 19 of the moulded pits in the disc record surface as the stamper plate is vertically removed from the disc record on retracting the record press, indicated by arrow B. Further damage to the walls of the moulded pits may be caused by a sudden uncontrolled influx of air into the spaces between the moulded disc record and the stampler plates as the press is retracted. Damage to the walls of the moulded pits in a disc record can cause signal degradation in a video recording, or can give rise to errors in a digital recording.

The operation of a disc record press which includes improved mould blocks according to one example of the invention and which at least reduces the risk of causing such damage to the walls of the moulded pits is shown in Figure 2(a),(b),(c) and (d).

Mould blocks 20 and 21 are of generally similar construction to prior art mould blocks 1 and 1 of Figure 1 (a) but have their surfaces 22 and 23 which face respective stamper plates 24 and 25 slightly 'crowned' or tapered to have a convex profile which is symmetrical about the centre. Such profiling may be carried out by a grinding operation on an initially flat surface, and is typically spherical with a radius in the order of 40 metres. The opposite surface of each mould block, shown at 26 and 27, is slightly hollowed to have a concave profile. Such profiling may again be carried out by a grinding operation on an initially flat surface and this concave profile is again symmetrical about the centre and preferably parallel to the convex profile of surface 22 and 23.Stamper plates 24 and 25 are initially flat with parallel faces and generally similar to stamper plates 8 and 9 in Figure 1(a). On clamping to their respective mould blocks, the stamper plates are sufficiently flexible for their modulated surfaces 28 and 29 of Figure 2(b) to conform to the convex profiles 22 and 23 of their respective mould blocks.

When the mould block and stamper plate assemblies are positioned between the flat and parallel surfaces of a record press ram, partly shown at 30 and 31 of Figure 2(c), and on moulding a disc record, the force at the rams, which is in the order of 50 tonnes, is sufficient to elastically deform the mould blocks 20 and 21 such that their initially concave surfaces 26 and 27 conform to the flat surfaces of the rams 31 and 32, thereby flattening the mould blocks and their associated stamper plates. The modulated surfaces of the stamper plates 28 and 29 become flat and parallel to one another, to mould a disc having flat and parallel sides as shown at 32. On retracting the rams, the pressing force is released and the mould blocks and stamper plates revert to their original curved profiles, as shown in Figure 2(d).The parting of the stamper plates from the moulded disc record place gradually, separation first occurring at the periphery of the disc and progressing towards the centre. In consequence, the separation of individual protrusions on the stamper plates from their respective pits in the moulded disc record occurs with an arcuate motion, shown in exaggerated form by the arrow C of Figure 2(d), compared with the vertical motion shown by arrow B in the prior art pressing arrangement of Figure 1(d). This gradual parting of stamper protrusions from the disc, taking place progressively from the periphery to the centre, combined with the arcuate motion of the stamper protrusion, permits the parting to take place with reduced risk of damage to the outer walls of the pits in the disc despite the differential shrinkage of moulded disc and stamper plate.Additionally, the influx of air into the spaces between the moulded disc and the stamper plates which occurs when these items are parted takes place in a gradual and controlled manner thereby reducing the possibility of wall damage from this cause. The reduced wall damage resulting from the use of the invention lessens the risk of signal degradation in a video recording, or reduces the risk of unacceptably high error rate in a digital recording.

It will be appreciated that the above embodiment is given by way of example, and the invention may be applied to other methods of moulding disc records by applying pressure to a mouldable material.

CLAIMS (Filed on 21.11.83) 1. A disc record press which includes at least one mould block assembled to lie betwen a ram and a stamper plate, said mould block having a concave surface facing said ram and a convex surface facing said stamper plate when said disc record press is in a retracted position, said convex and concave surfaces becoming approximately flat and parallel to one another when said press is in the closed position and in process of moulding a disc record.

2. A disc record press according to Claim 1 which includes a pair of mould blocks assembled to face one another between a pair of rams, said rams having surfaces which are flat and parallel to one another, each mould block having a stamper plate affixed thereto, each mould block having a concave surface facing its associated ram and a convex surface facing its associated stamper plate when said disc record press is in a retracted position, said convex and concave surfaces becoming substantially flat and parallel to said ram surfaces and to one another when said record press is in the closed position and in process of moulding a disc record.

3. A disc record press according to either preceding claim in which each convex surface is tapered.

4. A disc record press according to either of Claims 1 or 2 in which each convex surface is crowned.

5. A disc record press according to Claim 4 in which each convex surface is spherical.

6. A disc record press according to Claim 5 in which each convex surface has a radius in the order of 40 metres.

7. A disc record press according to any preceding claim in which each concave surface lies substantially parallel to its associated convex surface.

8. A disc record press according to any preceding claim in which said convex and concave surfaces are formed by grinding operations on initially flat

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. outer vertical walls such as 18 of the minute protrusions of the stamper plate which represent the information to be replicated. This shrinkage stress can cause damage to the outer walls such as 19 of the moulded pits in the disc record surface as the stamper plate is vertically removed from the disc record on retracting the record press, indicated by arrow B. Further damage to the walls of the moulded pits may be caused by a sudden uncontrolled influx of air into the spaces between the moulded disc record and the stampler plates as the press is retracted. Damage to the walls of the moulded pits in a disc record can cause signal degradation in a video recording, or can give rise to errors in a digital recording. The operation of a disc record press which includes improved mould blocks according to one example of the invention and which at least reduces the risk of causing such damage to the walls of the moulded pits is shown in Figure 2(a),(b),(c) and (d). Mould blocks 20 and 21 are of generally similar construction to prior art mould blocks 1 and 1 of Figure 1 (a) but have their surfaces 22 and 23 which face respective stamper plates 24 and 25 slightly 'crowned' or tapered to have a convex profile which is symmetrical about the centre. Such profiling may be carried out by a grinding operation on an initially flat surface, and is typically spherical with a radius in the order of 40 metres. The opposite surface of each mould block, shown at 26 and 27, is slightly hollowed to have a concave profile. Such profiling may again be carried out by a grinding operation on an initially flat surface and this concave profile is again symmetrical about the centre and preferably parallel to the convex profile of surface 22 and 23.Stamper plates 24 and 25 are initially flat with parallel faces and generally similar to stamper plates 8 and 9 in Figure 1(a). On clamping to their respective mould blocks, the stamper plates are sufficiently flexible for their modulated surfaces 28 and 29 of Figure 2(b) to conform to the convex profiles 22 and 23 of their respective mould blocks. When the mould block and stamper plate assemblies are positioned between the flat and parallel surfaces of a record press ram, partly shown at 30 and 31 of Figure 2(c), and on moulding a disc record, the force at the rams, which is in the order of 50 tonnes, is sufficient to elastically deform the mould blocks 20 and 21 such that their initially concave surfaces 26 and 27 conform to the flat surfaces of the rams 31 and 32, thereby flattening the mould blocks and their associated stamper plates. The modulated surfaces of the stamper plates 28 and 29 become flat and parallel to one another, to mould a disc having flat and parallel sides as shown at 32. On retracting the rams, the pressing force is released and the mould blocks and stamper plates revert to their original curved profiles, as shown in Figure 2(d).The parting of the stamper plates from the moulded disc record place gradually, separation first occurring at the periphery of the disc and progressing towards the centre. In consequence, the separation of individual protrusions on the stamper plates from their respective pits in the moulded disc record occurs with an arcuate motion, shown in exaggerated form by the arrow C of Figure 2(d), compared with the vertical motion shown by arrow B in the prior art pressing arrangement of Figure 1(d). This gradual parting of stamper protrusions from the disc, taking place progressively from the periphery to the centre, combined with the arcuate motion of the stamper protrusion, permits the parting to take place with reduced risk of damage to the outer walls of the pits in the disc despite the differential shrinkage of moulded disc and stamper plate.Additionally, the influx of air into the spaces between the moulded disc and the stamper plates which occurs when these items are parted takes place in a gradual and controlled manner thereby reducing the possibility of wall damage from this cause. The reduced wall damage resulting from the use of the invention lessens the risk of signal degradation in a video recording, or reduces the risk of unacceptably high error rate in a digital recording. It will be appreciated that the above embodiment is given by way of example, and the invention may be applied to other methods of moulding disc records by applying pressure to a mouldable material. CLAIMS (Filed on 21.11.83)

1. A disc record press which includes at least one mould block assembled to lie betwen a ram and a stamper plate, said mould block having a concave surface facing said ram and a convex surface facing said stamper plate when said disc record press is in a retracted position, said convex and concave surfaces becoming approximately flat and parallel to one another when said press is in the closed position and in process of moulding a disc record.

surfaces.

9. A disc record press according to any preceding claim in which the pressing force at the rams is in the order of 50 tonnes.

10. A disc record press according to any preceding claim which is suitable for the moulding of video disc records.

11. A disc record press according to any preceding claim which is suitable for the moulding of discs for digital audio recording.

12. A disc record press according to any preceding claim which is suitable for the moulding of discs for computer mass storage.

13. A disc record press substantially as herein described and with reference to the accompanying drawings.