GLASS ENGINEERING FOR CATHODE RAY TUBE MANUFACTURE (ii)

Today we'll look at the CRT neck production and their joining to the cone to make a complete CRT bulb.  The neck was the only glass part of the CRT to be made at Mullard Blackburn as the site glass plant was well versed in dealing with tubing which was also used to manufacture valve envelopes as well as tube necks.  In the glass making machine, molten glass flows on the outer surface of a hollow refractory mandrel so that a tube of molten glass is forced vertically through an air blast venturi.  At the top of the mandrel was a series of air jets which cool the produced tube such that it solidified sufficiently to be grabbed and drawn upwards by pairs of friction wheels which lifted the tube two floors to a work station where it was cut to length.

Each length was gauged for external diameter and weighed to check both internal and outside diameter.  Any variation from mean specification was controlled by adjusting the speed at which the tubing was drawn.  The tubing required for CRT necks was cut to the required length and then fed into a glass lathe where it was heated with gas jets to soften the glass which was then formed into a conical bell which corresponded to the end of the CRT bulb.

The shaped neck was then transferred to another machine and held in a jig which held it approximately 40mm from the bulb aperture.  The jig moved around a carousel where gas jets were applied radially whilst turning the jig until the glass on both parts softened at which point the jig moved downwards until the plastic glass touched.  Continued heat and downward pressure ensured a perfect join.   The completed bulb continued to move around the machine with it's temperature being reduced at successive stations.  At one point of the rotation, an operator fused a hole in the side of the bulb into which an EHT lead-in connection was fused.   The EHT connection was a number of lengths of wire sealed into a glass bead ready for fusing into the bulb.  

Bulbs were then removed to an annealing furnace where they were slowly cooled to relax heat induced stresses, the most critical of these being at the bulb and tube joint area.  To ensure a no stress product with minimal risk of implosion, the join area was scrutinised under polarised light to show up any inherant weakness which had not been normalised by the annealing process.   The completed bulbs were at this time in the early 1950s transported by road to the Mitcham factory where the final processes to turn them into finished CRT were undertaken. 

We shall leave this story today with a nice photograph of the individual glass components of a Mullard Rectangular Tube: -