Diakon: A New Material for Coloured Telephones
The Author gives a brief description of the manufacture and properties of Diakon, a new moulding powder which is being used for coloured telephones.
The Manufacture of Diakon
Mould for Telephone No. 232
When Diakon granules, prepared as described above, are moulded, the resultant mouldings are completely transparent. For telephones, however, the powder is now mixed with carefully adjusted quantities of Ivory, Red, or Green colouring matter to give either of the three standard Post Office shades. At this stage the preparation of the "powder" is complete and it is purchased by the moulder for manufacturing into telephone parts, etc. It is estimated that over 60 tons of "powder" will be used in the preparation of coloured telephones for next year.
Unlike the Bakelite class of materials used for black telephones, Diakon is not a thermosetting material, i.e., heat and pressure do not harden it. In working Bakelite a steel mould heated to about 170' C. is filled with "powder" and closed under a pressure of the order of one or two tons per square inch; this condition is maintained for about three minutes during which time the material is "cured"; at the end of this period the mould is opened and the finished part removed hot and hard and an exact reverse of the mould. With Diakon, and incidentally with cellulose acetate, another thermal plastic from which telephone cradles are made, the process is briefly as follows:-
The mould is first closed under pressure and the Diakon is forced into the cold mould at a pressure of up to 15 tons/sq. in. from a cylinder in which it has When heated and to which the mould is connected. The Diakon must be at about 200' C. before injection may take place and at this temperature, although plastic, it is far from being fluid. Narrow vents are left to permit the exit of the air which originally fills the mould. When injection is completed the material remains for a short time under pressure until it has hardened sufficiently due to the falling temperature for the mould to be opened and the moulding removed. This whole process may be completed in about 90 seconds as against the three minutes necessary with Bakelite and ten minutes for the coloured Urea powders. An injection type press suitable for moulding telephone cradles is shown right.
Although the injection process is the economical method of producing Diakon mouldings
it is possible to compression-mould the material by water-cooling the mould before
removing the moulding, but only at the expense of speed of production. There is little
difference between the results of the two methods as regards the appearance or the
mechanical or electrical properties of the finished article. The appearance of coloured
telephones in Diakon is undoubtedly superior to those in any other moulding material at
present available but the Post Office is, at this stage, primarily interested in their
behaviour in service and, although it is as yet too early to predict complete
satisfaction, there is every indication that the new powder will obviate most of the
difficulties hitherto attendant upon the manufacture of coloured telephones and the
trouble which has previously been associated with their use.
There are several grades of Diakon available but their chief differences are in respect of their moulding properties. These differences do not result in any material change to the electrical properties of the finished moulding, and with the exception of the resistance and deformation when heated, only introduce minor alterations to the mechanical properties. The above figures include the whole range of values which will generally be encountered.
As already stated, Diakon, unlike Bakelite, softens under heat and is therefore unsuitable for use at raised temperatures, but it is not anticipated that this will give rise to difficulty in this country.
In conclusion the author wishes to tender his grateful acknowledgements to Messrs. Imperial Chemical Industries for their very material assistance in the preparation of the article and to Messrs. The Telephone Manufacturing Company Ltd., and Messrs. John Shaw Ltd., for the illustrations.
Comments from Andy Emmerson
No manufacturer would be able to make anything in red or green from Bakelite. It is effectively impossible to do so as the colouring pigments would not have sufficient 'oomph' to mask the colour of the wood flour that gave Bakelite its body. Only black or dark brown pigments can do this.
Coloured variants of the International Standard Electric Company (STC in Britain) 'Antwerp' telephone were made in Bakelite but these were made of black Bakelite that had been spray-painted.
What some people mistakenly call 'coloured Bakelite' was in fact Beetle/Beetleware/Betal (trade names), alias Urea Formaldehyde (technical name). The surface appearance is similar to Bakelite, although without the high-gloss finish of Bakelite and is often wrongly called Bakelite. Urea Formaldehyde can be mixed with white and other light colours (such as ivory, red and green) and has similar characteristics to Bakelite.
The chief fault of Urea Formaldehyde is that it cracks easily under stress, which is why you often see visible stress lines on telephone (1) on the main case around the places where brass inserts were placed (to take screws from the base) and (2) along the hand grip part of the handset.
Brown muck often accumulates in these cracks but can be removed with bleach or, better, hydrogen peroxide and an old toothbrush. The other fault appears on the ivory phones, when the ochre and white pigments separate out, looking like toffee ripple ice cream.
Urea Formaldehyde was a heavy plastic, which was a minor disadvantage for telephone parts but not a problem for the jewellery boxes and toothbrush mugs made in this material, where its weight was an advantage. Melamine Formaldehyde (trade names Melamine, Melmex, Beetle) was a later (post-war, I think) replacement for urea formaldehyde, very similar but with a slightly shinier finish. For a while Melamine was widely used for kitchenware and picnic plates.
Once the lighter and glossier acrylic plastics became available after the war, the GPO dropped the use of Urea Formadehyde. The transparent Polymethyl Methacrylate (trade names Perspex, Lucite) was used for clear telephone cases, whilst the coloured Diakon was used to mould the cases of telephones. Perspex and Diakon were widely used for domestic radio parts and all manner of other things.
Last revised: July 18, 2021