Acrylic domes

[tsalaf]

Does anyone have experience forming clear Acrylic domes by using air pressure to blow the sheet through a ring?

Any help is appreciated.

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[shogun]

first you need an electric woven to place the acrylic sheet into
the acryl must be molded, not extruded
next the acrylic square is placed on a table that has a hole into
over the table a metal ring must be pressed against the sheet
next step is pumping air in the hole in the table
see this : http://www.plexirom.ro/pages/fr/atelier/mari/7.htm
i hope this will help you
excuse my english...
best regards

[nitrousdoc]

HI there,
Unless you are able to 1) heat the plastic evenly and 2) ensure that the plastic is dimensionally stable (ie same thickness throughout) and 3) it is mechanically stable (ie will deform uniformly across the surface), I think you'll need a mold. Either expand into a negative mold or vacuform to a positive mold.
Hope this helps.
doug

[shogun]

i think my english is the blame
there are 2 kinds og acrylic sheets (we call it plexi )
onethat is extruded (wich is not recomended for thermoforming due to the problems above)
an other wich is poored into a mold to form the sheet
this kind of acryl is used for this kind of aplications
vacuumin' the dome might leave marks on the surface unless the mold is not perfectly smooth

[PLASTICMAN]

Heat the sheet slowly and evenly to about 450 F. Do not inject compressed air directly at the sheet, it will chill unevenly and cause an off-center dome. A baffle of some sort is required. Don't allow the sheet to touch anything that isn't preheated or the same off-center condition will occur.
Pressure required is a direct function of surface area and sheet thickness. The larger the sheet the less pressure needed. With thicker sheet you need more pressure.
A limit switch wired into the pressure valve and set to deactivate the valve when the bubble touches it is the best method for controlling bubble height. The bubble will "bounce" a bit as it touches and the shrinks back away from the switch.
Proper clamping of the sheet is critical since any slippage will once again cause problems with the shape.
Also the type of acrylic used makes a difference. Extruded stuff is sloppily manufactured and tends to make an inferior part. Cast or melt-calendered materials are a little harder to work with, but produce good looking parts.
My company has successfully blown 1.5" thick domes 48" diameter using this method.

[Andrew Whillans]

How big a dome?If it is small-ie. maximum base dia of 100mm then moulding can be accomplished in a pressure mouthguard moulder although beware-many moulders are only vacuum formers.Some,e.g. the "ERKOPRESS" type are fully fledged miniature blowmoulders using 6 bar air pressure to force the guard material onto or into a mould-for instance a hollow or raised spherical mould.The Erkodent company sells a range of materials from soft to quite hard in thickness range 1.0 to 6.0 mm.The harder materials are clear and white acrylic and harder,tougher materials such as a kind of polycarbonate known as Erkodur-this material is crystal clear and polishable.

[r1656]

If you call Acrylonics in San Jose,Ca (listed) Talk to Rick, he can tell you how it is done. You need a large bake oven and a jig. They come out nice but if it is just for one dome i'd have him make it for you

[Stuff-Builder]

For one of my previous jobs I had a thermoforming shop make a bunch of these. The mold really simple - he made a small box with a round hole in the top. Then he clamped the sheet into the thermoformer and brought it to temp, then just sucked the dome into the box using the vacuum.

Depending on the size you're wanting, you might be able to duplicate this using a heat gun and cheap vacuum generator (harbor freight has an "air conditioner evacuator" that does the trick). Just heat the plastic evenly until it sags into the box, then pull your vacuum to round out the dome.

Scott

[hammerheadted]

Like the guys are saying, only use Cast acrylic not Extruded.
As for heating, a stardard house oven will do with a flat plate
to heat on, but see if you can get a sheet of 1.0mm or thinner
Teflon sheet bigger than your Blank size, this allows you to heat
the acrylic without mark-off, available from most engineering plastics
shops.
As for the ring we've had success with MDF rings to blow through.
But steel is preferable and a heavy guage of 10 - 20mm is prefered
to stop leaking, and clamping is crutial, use over centre clamps, or
something that is fast at clamping.
And yes a diffused airflow from underneath, pressure regulated is
prefered, we use a peice of pool table baize as the diffuser, but
a clean rag glued down works.
And just setup a stick to blow to the right height.

[tsalaf]

Thanks to all for the usefull information.

[KIDMORGAN]

Hello, I Worked At A Company For 8 Yrs. That Made The Security Camera Domes For K-mart. We Hung The Plastic On Clamps That Were Made By Welding Steel Key Stock Between Two Vise Clamps (handle To Handle And Boby To Body, Making One Large Clamp). The Clamps Were Hung On Unistrut Rollers And Rolled Into An Oven With The Plastic Hanging Below. After Reaching The Required Temp For Forming, The Plastic Still Hanging From The Clamp Was Laid On A Felt Covered Table With A Hole On Center. Next A Pnuematic Ram Pushed A Fixture With A Ring Against The Plastic . Then The Filtered Air Entered Under The Plastic, Causing The Plastic To Rise. The Air Pressure Was Controled By A Normally Closed Photoswitch Looking Dead Center Of The Plastic. As The Plastic Rises To The Desired Height, The Air Shuts Off And The Plastic Begins To Drop, Causing The Air To Come Back On And So On, Until The Plastic Cools. This Was A Easy Way To Keep An 1/8" Tolerance. Our Domes And Hemi's Were Then Electoplated In A Vacuum, To Create Domed Mirrors

[nitin_insulink]

i am looking for thermoforming of the part as attached pictures , how can i do that .

what will be approx cost of tooling

Overall dia 260 mm
dia at bottom flanges : 200 mm
thickness 1.5 mm
acrylic material
daily qty. - 500 pcs


do i need some special machines for this process .

who can make tooling for this ?

what will be blank size ?

cycle time ?

regrds
nitin

[Spassbremse]

hi Nitin !

Have you thought about a injection molding machine ? A 500 pieces a day it could be the best way to keep it simple.

Mfg Spassbremse