clear acrylic moulding

[custy44]

Hoping someone can help me.

As strange as it may sound, i want to create a clear acrylic sheet (900 x 600mm) about 4 - 6mm thick, in the shape as bubble wrap (smooth on one side and with multiple bubbles on the other).

I have enquired at thermo / vacuum / injection moulders, but have had no luck.

What would be the best way? The clarity of the clear acrylic is important, and i dont want to spend a fortune.

Any advise or names of potential manufacturers would be greatly appreciated.

Thanks in advance

 

[patprimmer]

You have two options, neither of which I have ever seen done before with both control of the shape and perfect clarity at the same time.

You could use two sheets, thermoform both to the desired shapes then glue them together with solvent cement. Small jounts can be done with good clarity but a bit of a mark at the edges of each bubble.

Cast it from monomer with a blowing agent in the spots where you want the bubbles or use inert gas injection via fine needles and withdraw the needles just before the resin fully sets. It might not work and will lack precision but will have perfectly clear matrix and bubbles with no frothy bits at the edge of a weld.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[MacGyverS2000]

If you're willing to share, I'd love to know the application... it's just such an odd request.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Pud]

Do you mean "bubbles" in the sense of a hollow feature, or a solid lens array arrangement?

H

http://www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

 

[btrueblood]

I had the same question as Pud. If hollow, follow Pat's suggestions. If solid, injection molding will work.

 

[custy44]

Hi All,

Thanks for your replies. Great to see so many helpful people out there.

Pud / Btrueblood - I dont need the bubbles to be hollow. A solid sheet in the shape of bubble wrap is what i am after. Discussed with an injection moulder and he thought there would be a high rejection rate, because small bubbles may form in the corners of the mould due to the dimensions of the sheet, and the difficulty in injecting the resin to all corners / edges.

Patprimer, thanks for your input, will definately look into that.

Magyvers, not prepared to give away too much, but will be used in the display industry.

Any more advise would be greatly appreciated.

Regards

 

[btrueblood]

Can you live with gate witness marks in certain areas, like between each lens/bubble? Yes, the part would be tricky to mold, but with enough runners/gates and a certain amount of post-mold work, I'd think it do-able.

 

[patprimmer]

I was thinking hollow bubbles, not solid bumps.

You can cast that shape if you are prepared to go that route.

It should be mouldable depending on a how high the bumps are and how they are joined.

Say if you only have a few bumps that are very thick and there is a large distance between them, then you will have trouble filling and pacing them unless you can get a runner into each bump.

If you can connect each bump with a suitably thick section then no problem. You need to be able to ensure that material continues to flow in to replace lost volume as the material solidifies.

If the thick section is to thick relative to the thinner parts, then the material must be injected directly into the thick parts

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[MikeHalloran]

I'd start with a thick sheet of cast acrylic, and trepan around the 'bubbles' with a CNC mill.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Mikes advice is good up to a certain production number.

Smallish volume on a mill. Requires skills, but they are fairly readily available.

Middle volume by casting in a simple cheap mould but with a process that requires unique skills. These skills might be hard to find.

Large volume. Injection moulding if possible. Very high initial cost and risk analysis required to see if it's possible. It will be the cheapest and fastest for very large volume, that is if it can be done.


Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[MacGyverS2000]

Milling would be a PITA for bubbles... cylinders, fine, but the constant change in dome angle as Z axis increases would take a long time to create nicely rounded "bubbles".

Take a look at Evonik Cyro... they have a number of patterned acrylic sheets, and I think they already have one with small regular bumps in it (though not at the tightness of bubble wrap). I'm sure they could handle a job like that.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[btrueblood]

"Milling would be a PITA for bubbles"

No disagreement. But you could get the shape close quickly, I think, if you mill with a standard square edge cutter, and leave cylinders at the correct height, then finish the cylinders to dome shapes with a concave cutter (i.e. custom ground to leave the dome you want to finish with), it might be do-able with a bit less pain than multi-pass cutting with a ball end mill. Not sure about the surface finish, especially near the center, which is why I'd recommend starting with the center of the dome (cylinder) at the finish height.

If you want to spend some R+D time, I wonder if a calendaring method might work - i.e. preform a sheet, then stick it into a heated mold and clamp under pressure. On third thought, no, I can't see a way to vent the inevitable air from the dome areas without leaving witness marks.

 

[Pud]

Alloy tooling - cast acrylic.

Slow, but (reasonably) low capital cost. A decent CNC machining centre should be able to machine a mould to a near mirror finish, requiring minimal polishing.

H

http://www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

 

[custy44]

Thanks All, You have been a great help.

Will definately look into some of those options.

Macgyver, you mentioned that Evonik Cyro already offers patterned acrylic sheets. Do you know what methods they use to do this? (irrespective of the pattern itself)

Thanks

 

[patprimmer]

They either cast or calender.

If they are who I think they are, (Oroglass or Plexiglass) they are one of the original producers of cast acrylic sheet.

For a finished surface they probably extrude onto a series of rollers with the surface embossed into the rollers. That is certainly how they make prismatic sheet for flat diffusers in the lighting industry.

If they extrude and cross link in the extrusion process, they call that continuous cast. If they mix the monomers or pre polymers, and pour into a mould and react the resin to set it, they call that cell cast.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[MacGyverS2000]

Evonik Cyro's name brand is Acrylite:

http://www.acrylite.net/product/acrylite/en/Pages/default.aspx

I don't know their specific method, but you can get a sample kit for free.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

I just found it. It's Acrylite in the USA and Plexiglass in the rest of the world. It should be cell cast acrylic.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules