GB2087784A - Moulding optically smooth articles having large surface areas - Google Patents

Abstract

The method uses a mould surface 2, having a closely fitting liner 4, with a highly finished outer surface 6. The liner is formed at a temperature, preferably in the range 90 DEG to 105 DEG C, at which the liner material does not become plastic, thereby avoiding the disadvantages with regard to the surface finish associated with prior art techniques. In carrying out the method, the liner is removed from the base together with the moulded article and, being of flexible deformable or demountable form, is removed from inside the moulded article to leave a highly polished inner finish. It is possible to mould a male or female matrix by the same method. <IMAGE>

Description

SPECIFICATION Method for moulding articles having large surface areas This invention relates to a method for moulding shaped articles using a basic technique variously known as fabrication or replication.

Background of the invention It is well-known to mould shaped articles using a male form as base, coating the form with the material of which the article is to be made, for example, partially-cured resin and glass fibre, curing the resin, in the example given, to render the female article in the shape of the male form, and removing the article from the male form.

It is also known to prepare a female matrix in the manner described in the preceding paragraph, and then to prepare the final article, in the reproduced shape of the male form, using the female matrix as a mould.

In the case of either method, it is common practice to coat the mould with a parting agent to facilitate removal of the moulded article from its mould.

The present invention may be applied to either method described above and its object is to provide an improved moulding method.

Small moulded articles may fairly readily be stripped from male form, if the form is previously coated with a parting agent. However, large articles, for example, continuous articles of some 7 feet by 11 feet dimensions, present great difficulty in their removal from the form without damage. It is found that the total force holding the moulded article to the form may be 163,000 Ibs., in such an example.

Generally, the smoother the surface finish of the male form, and consequently of the moulded article and the greater the absolute refinishing of the male form and article, the greater the adhesive force per unit area. This force can be reduced only at the cost of degrading the surface finish obtained.

A very important reason why prior art techniques are unable to produce large area moulded articles of a finish suitable for optical purposes is that such techniques rely on heating the article to be moulded to a plastic state in the moulding operation. The temperature usually employed lies in the range of about 1400 to 1600 C. Reference is made, for example, to U.S. Patent No. 3,317,640 (Jones) which provides for the use of temperatures between 2900 to 4000F (about 143 to 2050 C). Moreover, the use of such temperatures is recommended in the manufacturer's handbook of Rohm and Hass, the principal manufacturer of the material known in the United States by the Trademark "Plexiglas" and in Europe as "Oroglas".The disadvantages of this approach in making large mirrors are that once the material becomes plastic it takes on any blemishes which are present in the mould, and further, cooling of the material is impossible to control, with the result that the sheet cools at an uneven rate thereby giving rise to physical distortions in the thickness and surface curvature of the sheet. These physical distortions cause unacceptable optical imperfections in the finished mirror and these disadvantages make the prior art moulding methods useless for the purposes of the present invention.

Summary of the invention In accordance with the present invention, a liner used in forming the moulded article is itself formed at a relatively low temperature so that the liner does not become plastic, thereby avoiding the problems discussed above. The temperature used is below 11 00C and preferably in the range of 900 to 1 050C. In this way the disadvantages of the prior art are overcome and the forming of articles such as very large, optically satisfactory mirrors becomes possible.

According to a further aspect of the invention, the method provided uses moulding apparatus including a male form base and a flexible or otherwise deformable or demountable liner shaped for closely fitting the male form.

The moulded article is formed over the liner and, when completed, is stripped from the male form together with the liner. The liner, being flexible, deformable or demountable, is then easily removed from inside the moulded article (whetherthis moulded article be the final article of the female mould for producing the final article).

Thus, the present invention is also distinguished from the prior art in providing three bodies, or layers; the base form, the liner and the moulded article, all of which are separable from one another instead of the two bodies previously provided.

It is known, according to the prior art, to coat the base form, for example with paint or varnish, to provide a highly polished surface or to glue or cement onto the base form a cover, which may then be processed to give it a highly polished finish. Such coating or cover then becomes a part of the base and is not removable therefrom in the process of moulding articles thereupon.

Preferably, according to the present invention, the base form is given a relatively rough finish, to permit of easy removal of the liner therefrom, and the liner is given a relatively fine finish, to impart such finish to the final article, or female matrix, moulded thereon.

Other features and advantages of the invention will be set forth in, or apparent from, the detailed description of the preferred embodiments which follows.

Brief description of the drawings In order that the invention may be clearly understood and readily carried into practice, an embodiment of the method of the invention will now be described in detail, by way of example, with reference to the accompanying drawing, the sole figure of which is an isometric diagram of part of a base form, liner and moulded article, showing the three layers thereof Description of the preferred embodiments The present invention is applied, in this example, to the construction of a plurality of partial forms which, when joined together by abutment of their edges, form a large, continuous body of partspherical, or part spheroidal, shape having a sufficiently accurate and highly finished surface texture which, when silvered or otherwise coated, provides a reflective surface suitable for optical purposes, or for the reflection of radio waves of wavelength near the infra-red region. Such a surface is referred to as a mirror smooth surface in this application. The following description relates to one only of the sections to be assembled and it will be understood that other sections may be manufactured in a similar manner.

Referring now to the accompanying drawing, a male base form 1, comprising in this example a metal weldment, is constructed of the required form, in this example one end section of a spherical form, and provided with a surface 2 finished to approxi mately 120 micro inch C.L.A.

The material selected for the base form construction is required to be rigid and not deformable with surface temperature cycling between ambient and 150 C. Remote-reading, surface temperature sensors, represented by a sole sensor 3 in the figure, are provided to monitor the instantaneous surface temperature during such cycling.

An intermediate liner 4 is shown in the figure removed from the base 1 and partially peeled off the underside of a moulded article 5.

The liner 4 is previously vacuum moulded from pre-cast methyl methacrylate sheet and is intentionally moulded to a smaller radius of curvature than that of the surface 2 of the base form 1. In initial assembly, the liner 4 is strapped, or otherwise secured edgewise upon the base form 1. The surface temperature of the liner base combination is raised so that the acrylic liner4 demoulds sufficiently to become an exact fit upon the base form. The temperatureitime regime required for such demoulding of liner 4, to become an exact fit on the base form 1 may either be calculated or determined empirically, with the temperature lying within the range of 90-105 C as mentioned above and described below.

To retain the liner 4 upon the base 1 so that the upper surface 6 of the liner 4 is both rigid and laterally static, a vacuum may be applied between the base 1 and liner 4, at the edge ofthe liner 4, by means not shown, but in accordance with known practice.

Indeed, it has been found that when the liner 4 is a close fit upon the base 1, the liner demoulding step described may be omitted and the vacuum applied to obtain exact and static fit of the liner 4 upon the base 1.

In either case, the vacuum is retained throughout the subsequent moulding stage for article 5.

A substrate is then fabricated upon the linercovered base using known technique to form an epoxy resin/glass fibre, honeycomb substrate 5 thereon. The epoxy resin is applied in uncured, or partially cured, state and is then finally cured to provide a rigid moulded article 5.

The finish of the surface 2 of the base permits of the moulded article 5, with the liner 4 attached thereto, to be readily removed from the base 1, since there is only a low adhesion force per unit area therebetween.

When the liner and article have been removed from the base 1 the liner may easily be stripped from the article 5 for two reasons. First, the liner 4 is flexible for removal and, second, there is no bond between the acrylic material of the liner and the epoxy resin material of the substrate.

The liner 4 is thus removed to leave the highlyfinished replicated surface 6 of the moulded article 5, without risk of damage.

It will be understood that the liner 4 is peeled progressively from the article 5 starting at one edge or corner. The liner 4 is thus parted along a shear line of a very small area. As the force of adhesion per unit area is itself small, the total stripping force required is small.

The surface 6 of liner 4 is similarly little liable to damage in the stripping operation, so that the liner 4 may be replaced upon the base 1 for repeated use.

As discussed above, an important aspect of the invention involves the use of relatively low temperatures in forming the sheet-like acrylic liner 4 so as to prevent the liner from becoming plastic. As mentioned, it has been discovered that if the liner sheet becomes plastic, it is useless for optical purposes because the article will then take on the blemishes of the mould, and because control of cooling of the material from the relatively high temperatures conventionally used is impossible as a practical matter, with result that there will be physical distortions with the product produced.The method of the present invention avoids these problems by forming the liner at tempertures below about 110 and preferably in the range of 900 to 1 050C. It is noted that the plastic material phase, i.e., the phase for temperatures above the critical temperature is known in the art as the "hot-break region", characterized by a break in the material exposing mirror-smooth edges, whereas, the phase below the critical temperature, i.e., the phase employed in accordance with the invention, is known as the "cold-break region", characterized by a break exposing irregular and jagged edges. Thus, this fact provides an empirical method for readily determining the exact critical temperature for any batch of plastic material.

Although the invention has been described in relation to exemplary embodiments thereof, it will be understood by those skilled in the art that variations and modifications can be effected in these exemplary embodiments without departing from the scope and spirit of the invention.

Claims (13)

1. A method for moulding an article of fabri- cated, synthetic resin and fibre construction having a surface area on the order of tens of square feet, and having a mirror smooth surface of a sufficiently accurate and highly-finished surface texture which, when appropriately coated, provides a reflective surface suitable for optical purposes, said method comprising the steps of: providing a mould having a surface of said accuracy and finish; forming a removable mould surface of said accur acy and finish having a said mirror smooth surface, said forming step including heating said removable mould surface to a temperature below the critical temperature at which said removable mould surface becomes plastic; and forming an article of a synthetic resin and fibre construction as a substrate on said removable mould surface.

2. A method as claimed in Claim 1, wherein said temperature is below 110 C.

3. A method as claimed in Claim 2, wherein said temperature is in the range of 900C to 1050C.

4. A method as claimed in Claim 1, wherein said article has a concave mirror smooth surface, a rigid convex base form being employed, said method comprising the steps of: i) providing a said base form with a relatively rough convex surface finish; ii) pre-forming a re-usable liner of thermo-plastic sheet to approximately the convex surface shape of the base form; iii) securing the liner upon the base form; iv) constraining the liner to become an exact fit upon the convex surface of the base form; v) fabricating a said article of synthetic resin and fibre construction as a substrate upon the convex surface of the liner; vi) releasing and removing the liner and the fabricated substrate together from the base form; and vii) stripping the liner from the fabricated substrate to provide the said article.

5. A method as claimed in Claim 4, in which the liner is pre-formed to a smaller radius of curvature than the convex surface of the base form.

6. A method as claimed in Claim 4, in which the pre-formed liner is constrained to become an exact fit upon the base form convex surface by raising the base form surface temperture for sufficient time to demould the pre-formed liner thereupon.

7. A method as claimed in Claim 4, in which the pre-formed liner is secured upon the base form by application of a vacuum between the liner and base mating surfaces.

8. A method as claimed in Claim 1, wherein said article has a convex mirror smooth surface and a rigid, convex base form is used, said method comprising the steps of: i) providing the bae form with a relative rough convex surface finish; ii) pre-forming a re-usable liner of thermoplastic sheet to approximately the convex shape of the base form; iii) securing the liner upon the base form; iv) constraining the liner to become an exact fit upon the convex surface of the base form; v) fabricating a concave article by synthetic resin and fibre construction as a substrate upon the liner convex surface; vi) releasing and removing the liner and the fabricated substrate together from the base form; vii) stripping the liner from the fabricated substrate to provide a mould form having a concave surface; and viii) moulding the said article having a convex mirror smooth surface employing said concave fabricated substrate as a mould.

9. A method as claimed in Claim 1, wherein: a) a plurality of component articles are moulded by the steps of: i) providing a base form with a relatively rough convex surface finish; ii) pre-forming a re-usable liner of thermo-plastic sheet to approximately the convex surface shape of the base form.

iii) securing the liner upon the base form; iv) constraining the liner to become an exact fit upon the convex surface of the base form; v) fabricating a said article by synthetic resin and fibre construction as a substrate upon the convex surface of the liner; vi) releasing and removing the liner and the fabricated substrate together from the base form; and vii) stripping the liner from the fabricated substrate to provide the said article; and b) the component articles so moulded are assembled in juxtaposition one to another so as to produce a composite article.

10. A method as claimed in Claim 1, wherein a plurality of component articles are moulded separately and the component articles so moulded are assembled in juxtaposition to one another so as to form a complete article.

11. A method as claimed in Claim 4, wherein the convex surface of the liner is polished to modify the surface characteristic thereof.

12. A method as claimed in Claim 8, wherein the convex surface of the liner is polished to modify the surface characteristic thereof.

13. A method as claimed in Claim 9, wherein the convex surface of the liner is polished to modify the surface characteristics thereof.