Is it a chemical fog or a physical fog...
E.G is it a physical change to the surface finish or a chemical one?
How does buffing effect it? Can the acrylic have a removeable membrane over the window part of it during molding (IT normally has a covering as delivered).
Del

hi Del the catt,

Yes, it is a physical change to the surface finish. There are white cloud like streaks on the transparent surface. It gets removed by buffing. This becomes an additional process and we wanted to know if there could be any solution which may be possible to eliminate this fog during the molding process itself.

The acrylic is already having a scratch proof membrane, but we are removing this just before the molding process. If not removed the thickness increases.

Regards,

Thank you Del the Cat!

A compliment like yours makes it worth while.

Hello 2tinker,

Thank you for the excellent and informative post. Your assumption is very correct and we are using a precut acrylic and them molding a GF nylon frame.

Here are our answers for your questions.

For 1. We are not preheating the acrylic.

For 2 & 3. The preload is very minimal. It is stimated to be about 0.1mm. We are not sure whether this could result in heat transfer issues.

For 5 & 6. Yes, the acrylic is presently in full contact with the faces of the mold. If the contact is reduced there are chances of the nylon to spread over the acrylic.

For 7. Molding is perfect and there is no shrinkage/warpage.

For 8. We need to try GF propylene.

For 9. There is no functional need of the entire cover to be transparent. Hence we decided to use acrylic window only in the area of the cover, where it is needed to have a visual see-through into the product.

Presently, we are getting the component with lesser fog when we use lesser shot weight. But this results in the failure of the cover during our weatherproofness testing at 0.5 bar wherein the water seeps through the joint. There is no consistency achieved with this.

Will polycarbonate in place of acrylic eliminate this problem?

Thanks and regards.

Hello again,

I am glad that, I was not all hot air, and that some of the ideas mentioned could be of practical benefit.

I would study 5&6. It may be the key to your success.

To try the cheapest possible way (by experiment) to see if in fact, having the faces of the mold touch the acrylic as you mold, is your problem.

1. Cut a shim out of of brass or stainless to increase the complete parting line of the mold/sans cavity area. Make the shim, say .008" or .2mm thick. Stick it to the parting line opposite side of vented surface so remember to keep your vents clear.
2. Carefully cut two more shims of .004" or .1mm thickness. Glue them at the pinch point of your metal to acrylic in a continuous frame fashion. You may stretch into the nylon overmold area to give the shim as much integrity as possible. This will hopefully provide an air/insulating space between the acrylic and the mold faces.
3. You can increase the shims to be sure you don't allow the acrylic to touch the faces of the mold. Glue the shim to both sides of the acrylic or mold steel to form an upstanding frame around the acrylic. This essentially suspends the acrylic between the mold faces. Clean the faces of the mold thoroughly so as no vapor from mold release agents or other attack the acrylic. Mold a few shots and see the results.
4. If it works , be careful to vent the airspace that lies between the mold face and acrylic face on the A and B side of the mold to atmosphere.
   
5. Good Luck "Slan Leat"
   

Hello there,

I forgot to answer the question on polycarbonate. I also addressed a reply to Dell's # 3 post back to you, the originator. How confusing?

1. I don't think polycarbonate is your best answer, it can lack the clarity of Acrylic and costs more.
2. You can try leaving the liner paper (brown paper not plastic) while you mold. Try it, you have little to lose.
3. If you flash out at the parting line use a Parting line shim to make up the thickness of 2 liner papers less .001-.002" squeeze. Don't block the vents.
4. Less shot weight is because you did not pack the part out and therefore it will sink as it cools leaving you with a non uniform surface. It will also reduce post molding shrinkage pressure on the acrylic You can not afford not to pack the part out so keep your hold times correctly to keep a flat sealing surface or you will end up molding an O-ring groove, more headache and expense.
5. Unfilled nylon can shrink .025 inches per inch where as glass filled can be as low as .008" per inch. Polypro shrinks .015"-.018" per inch and is more stable than nylon when GF'd.
6. 
   

Bye for now

Hi 2tinker,

Thank you for the valuable information. Your suggestion to reduce the heat at acrylic surface sounds very logical. We need to try out the experiment and will check for the improvement.

Thank you once again,

Regards,

Good vision.

Instead of using Acrylic sheet, how about using an Acrylic frame. This could be of the same size of the Nylon with a small projection, say 3 mm. After molding, a new, clear sheet of required size could be glued (using Chloroform or like solvent). Gluing inside might help to stand for greater pressure.

How about using, instead of Nylon, a relatively cold moldable compound like Epoxy or Phenol resin with suitable filler.

Acrylic has a tendency to crack and is somewhat like a glass and is used for a look thru window. It is often supported by a frame of other flexible non-cracking type material that can hold or retain it in position without transferring vibration of un-even force which may crack it or break it.

Acrylic fish ponds can be made in same way as glass fish ponds but they can hardly take force or vibrations.

Nylon is a reasonably good cheap material which has less transparency but is a goos golding material and can be easily molded or mechanically cut and shaped. I often get gears made from Nylon.

I prefer Polycarbonate holder and acrylic window material for LED displays. It is not easy to break Polycarbonate and it is little flexible also. I think PVC is not so good material as it can capture moisture. PVC is often used in place of Nylon as it is much cheaper.

The fogging is from gasses in the hot nylon depositing on the acrylic surface. The best thing to do is to remove as much of the hot gasses as possible from the flow front of the plastic. The first pace to look for improvement is to check the vents on the runner. They should be .05 - .10 mm deep, which may flash a little, but is of no consequence on the runner. The the exterior parting line of the part should have vents .02 - .04 mm deep. Vents should have a land of about 1 - 1.5 mm, then drop down into a relief channel of at least 1.5mm deep that runs to the mold's exterior.

The acrylic window should also have vent relief channels around the whole interior perimeter, but no actual vent, since the plastic pressure would compress the acrylic into the vents and cause flash. Since the perimeter of the window is surrounded by the molded nylon, you will need to drill a hole through the mold steel inside the window area to have the gasses reach the exterior. Polish the edges of the vent relief and/or make the transition from the shutoff to the relief channel gradual so as to not mark the acrylic surface.

This should help with the clouding of the window.

Hi Zoomer,

We also suspected the fogging may be due to the gases evolved during the molding. We have provided necessary air vents similar to your suggestions. However, we will try out additional air vents at the acrylic area as you had mentioned.

Thanks to you and all for the excellant ideas and suggestions. This way the forum enabled sharing of the experience & knowledge to the entire world instead of being localized.

All the best wishes, regards and thanks to all.

nameaspri

one thing to think about is the mold temp it might need to be warmer ive heard fogging issues with low temp of the mold ......i do agree with zoomer about the venting

Also there has to be an escape route for air bubbles some excessive material. Homogeneous mixing of fine glass powder (grains) is also essential. Shape of the glass powder grains and its quantity can change the optical properties drastically. Glass powder is generally added for hardening the otherwise soft frame material. If added in higher volume or of larger grain sizes then it inhibits flow, traps air bubbles, leaves gaps, causes uneven surfaces.

have you tried liquid nylon to form a bond? or use as a primer for a bond