Implosion screens

[retroaudio]

I have recently obtained an ultra transportable 405 line TV. I dont know yet if there are any faults. Certainly at least one cap has disintegrated and one valve has lost its vacuum.

This isnt my main concern. The set has a perspex implosion screen which has began to discolour and has a nasty crack. It isnt flat it has a slight curve in order to follow the contour of the CR tube but is flat at the lip where it disappears under the plastic facia. Does any forum member have any experince of moulding such materials or know of people who do it. I am not completely stuck the depth of the chassis and tube are adjustable within the cabinet so at a push you could probably make up a flat implosion screen and it wouldnt foul the tube however I would like to make it as original as possible if I can. Any help would be appreciated.

Alex

[dominicbeesley]

You should be able to get hold of normal perspex (acrylix sheet) and make your own mould.

To make stuff at school we used to:

Heat perspex to ~100C in a normal oven

place on a form (two bits of wood or whatever that can be clamped to force the now bendy sheet into whatever shape you're after).

clamp

put back in oven at 100C for a while for it to settle

take back out and allow to cool slowly

seemed to work most times but sometimes you'd get wrinkes. Make sure you don't get it too hot or it will go too runny and stick to things - be very careful it does not stick to you as this gives nasty burns!

The difficult bit will be making the form - not sure how you'd do this nicely?!?

Dom

[Nickthedentist]

Originally Posted by dominicbeesley

The difficult bit will be making the form - not sure how you'd do this nicely?!?

Maybe you could pour plaster into the old one, then use the plaster as a mould? Pure speculation though

[chipp1968]

plaster woud give you the shape i suppose . it should be possible with some thought involved .
can we see a pic ?

[Heatercathodeshort]

Sounds like an Ultra VP17-72. Very difficult to fabricate this one. Good luck with it anyway. John.

[GMB]

If you are successful in heat forming perspex, be very careful about what you use to clean it.

Depending on how much residual stress remains on cooling, organic solvents which normally work fine (like white spirit or meths) have the strangest effect, causing total disintegration of the plastic by causing thousands of cracks to appear. The result is as if it was glass and you just dropped it.

Not a mistake I will be making a second time

[Hunts smoothing bomb]

Will perspex have the safety factor if the tube implodes?

I would suspect that it would just shatter and shards of glass would punch straight through it.

Cheers
Lee

[dominicbeesley]

The 1/4" stuff is pretty tough. I've used it as a replacement bus window before now and its very easy to scratch but pretty difficult to break

[geofy]

Originally Posted by Hunts smoothing bomb

Will perspex have the safety factor if the tube implodes?

I would suspect that it would just shatter and shards of glass would punch straight through it.

Could a sheet of toughened glass be used, or cut a piece from a car windscreen maybe, the next door neighbour might not notice

Geof

[Ian B]

You can't cut toughened glass, once the outer skin is penetrated it will shatter. So you have to cut ordinary glass to size, and then get it toughened.

Laminated safety glass can be cut, but not a task I'd try myself, too difficult!

Ian

[dominicbeesley]

And flat-laminated glass can't be bent into shape - the problem the OP was tackling.

In relation to this in general how strong does an implosion screen need to be? I would have though that 1/4" shatter resistant perspex could hold the small fragments of glass involved quite easily not sure about what sort of impact the gun assembly has if it decides to shoot forwards though...

Dom

[GMB]

I suspect that the main purpose of the strength of the implosion screen is to stop the user from punching through the tube rather than vice-versa.

I think the total energy involved is fairy tiny and when consider that it's an implosion so a lot of energy is lost by the bits colliding with each other.

It may even be that most of the strength is needed to withstand the momentary vacuum that it will be exposed to and is nothing to do with the fragments at all.

[chipp1968]

Alot of sets had perspex screens even going back to the 40s , not very thick stuff , so it should be fine . If the tube implodes ,even it it came through im sure it would be slowed down considerably.

[Heatercathodeshort]

The CRM1703 used in the Ultra portable is one the first 110 Degree tubes used in this country in 1959. It is almost a circular tube due to the extreme pressure exerted on its virtually flat bowl. The reason for the rounded corners was to reduce the stress on the CRT that amounts to over one ton on a 17" screen.
An even worse tube was the first semi square corner, unprotected 19" Mazda CME1901. The implosion violence when an example of these imploded had to be seen to be believed. The square corner FST tubes of the mid 80's would not have been possible without rimband protection.
Its highly unlikely that a CRT will self implode but implosion protection screens were highly tested items having to attain a high standard. Sorry to be such a party pooper but to be honest I think it unwise to mess with them. Regards, John.

[geofy]

Originally Posted by GMB

I suspect that the main purpose of the strength of the implosion screen is to stop the user from punching through the tube rather than vice-versa.

Better to just stop watching the buffoon politicians!

[Hunts smoothing bomb]

I ditched a Sobell portable 405 only set last year that was unsalvagable in every way, I disposed of it in my bin and took a hammer to the implosion screen once removed to make it smaller. I am quite strong and found it very difficult to break the toughened glass hitting it full pelt with a large claw hammer!

I too do not want to be a party pooper but I sometimes think that people underestimate the force an imploding screen can forwardly deliver.

Cheers
Lee

[chipp1968]

If the original is a plastic material then some reserch is need to find what it was .something thats brittle wont do.

[geofy]

Originally Posted by Ian B

You can't cut toughened glass, once the outer skin is penetrated it will shatter. So you have to cut ordinary glass to size, and then get it toughened.

This is worrying, I was surprised to find out that toughened glass is just ordinary glass which is heated and quenched similar to hardening steel. But I thought the toughening went right through, what would happen on these glass floors like at the Toronto Tower if someone punctured the skin of the glass, would it shatter? It's along way down

Geof

[FrankB]

As Nick says, a plaster mold of both sides would be the way to go.
Cut the Perspex a bit larger than the mold halves and heat form in the oven.
NOTE: UltraCal or HydraCal are better than plain plaster for this, as the plaster mold can shrink a larger percentage.
IF you make the mold, be sure it is THOROUGHLY dry before you put it in the oven.
Water in the plaster can make it explode.
This is the same way one can form polycarbonate sheet also, which would probably work too.
Wear welders gloves as that plastic & plaster can give you a nasty burn. Let cool slowly too. That will releave some of the heat stress.

Trim up with a craft tool- Dremel type- and you should be fine to go.

It takes a bit of time but is pretty easy to do.

[dseymo1]

Polycarbonate (Lexan) is probably the best material to use.
It's what riot shields and the like are made frm, and will withstand very high impact forces.