Editing Cathode-ray tube (section)

===Implosion===
[[File:CRT implosion.JPG|thumb|A CRT during an implosion]]
If the glass wall is damaged, atmospheric pressure can [[Implosion (mechanical process)|implode]] the vacuum tube into dangerous fragments which accelerate inward and then spray at high speed in all directions. Although modern cathode-ray tubes used in TVs and computer displays have [[epoxy]]-bonded face-plates or other measures to prevent shattering of the envelope, CRTs must be handled carefully to avoid injury.<ref name=Bali>{{cite book |last= Bali |first= S.P. |title= Colour Television: Theory and Practice |publisher= Tata McGraw–Hill |year= 1994 |page= 129 |url= https://books.google.com/books?id=53nnX4fnnNIC&pg=PA129 |isbn=978-0-07-460024-5}}</ref>

====Implosion protection====
[[File:Datapoint 1500 keyboard mounted in case.jpg|thumb|[[Datapoint]] 1500 terminal with exposed chassis, with its CRT suffering from a "cataract" due to aging PVA]]
Early CRTs had a glass plate over the screen that was bonded to it using glue,<ref name="auto63"/> creating a laminated glass screen: initially the glue was [[polyvinyl acetate]] (PVA),<ref name="auto41">{{Cite web|url=http://www.earlytelevision.org/Deksnis/doug_harland.htm|title=Illustrated Cataract Repair|website=www.earlytelevision.org}}</ref> while later versions such as the LG Flatron used a resin, perhaps a UV-curable resin.<ref>{{Cite web|url=https://patents.justia.com/patent/4841372|title=US Patent for Cathode ray tube contrast enhancement systems Patent (Patent # 4,841,372 issued June 20, 1989) - Justia Patents Search|website=patents.justia.com}}</ref><ref name="auto13"/> The PVA degrades over time creating a "cataract", a ring of degraded glue around the edges of the CRT that does not allow light from the screen to pass through.<ref name="auto41"/> Later CRTs instead use a tensioned metal rim band mounted around the perimeter that also provides mounting points for the CRT to be mounted to a housing.<ref name="braun"/> In a 19-inch CRT, the tensile stress in the rim band is 70&nbsp;kg/cm<sup>2</sup>.<ref>{{Cite web|url=https://www.freepatentsonline.com/4158419.html|title=Implosion protected CRT|accessdate=18 December 2022}}</ref>

Older CRTs were mounted to the TV set using a frame. The band is tensioned by heating it, then mounting it on the CRT; the band cools afterwards, shrinking in size and putting the glass under compression,<ref>{{Cite web|url=http://www.r-type.org/exhib/aar0024.htm|title=160AB22 @ The Valve Museum|website=www.r-type.org}}</ref><ref name="auto63"/><ref name="band">{{Cite web|url=https://patents.google.com/patent/US6757158B2/en?q=braun+tube&oq=braun+tube|title=Implosion resistant cathode ray tube with mounting lug having a compound bend|accessdate=18 December 2022}}</ref> which strengthens the glass and reduces the necessary thickness (and hence weight) of the glass. This makes the band an integral component that should never be removed from an intact CRT that still has a vacuum; attempting to remove it may cause the CRT to implode.<ref name="auto28"/>

The rim band prevents the CRT from imploding should the screen be broken. The rim band may be glued to the perimeter of the CRT using epoxy, preventing cracks from spreading beyond the screen and into the funnel.<ref>{{Cite web|url=https://patents.google.com/patent/US4930015A/en|title=Flat tension mask cathode ray tube implosion system|date=6 April 1988 |accessdate=18 December 2022 |last1=Dougherty |first1=Lawrence W. |last2=Lee |first2=Sae D. |last3=Rogers |first3=Melvin F. }}</ref><ref name="band"/>

Alternatively the compression caused by the rim band may be used to cause any cracks in the screen to propagate laterally at a high speed so that they reach the funnel and fully penetrate it before they fully penetrate the screen. This is possible because the funnel has walls that are thinner than the screen. Fully penetrating the funnel first allows air to enter the CRT from a short distance behind the screen, and prevent an implosion by ensuring the screen is fully penetrated by the cracks and breaks only when the CRT already has air.<ref name="auto63"/>