air entrapment in poly-resin casts-[small rubber molds]

[einzats]

einzats
Hi, my problem is a old one that hasn't been effectively solved in an economically sound way. I am casting small resin parts {urathane,epoxy etc.] in small rubber molds. They are designed with efficient vent systems etc. I have used vibration casting, vacuum casting, pressure, rotational and centrifuge. I still get the occational pin-hole. I want to eliminate this problem completely--and efficiently ie. fast and cheap. Any suggestions?

 

[patprimmer]

It sounds to basic, but you didn't mention control of exotherm, are you sure you are not generating gasses from excess heat while cureing Regards
pat

 

[ir12]

Based on the idea that you are using correct process parameters on your TDS from mat'l supplier, you don't mention how you are degassing the material or how you are dispensing it into the molds. How are you meter, mix, & dispensing? Hand mixing? Eliminating the air in the material may help. Various types of dispensing,i.e. plural or one component equipment can used with a nitrogen blanket, w/heated tanks,etc. This would help eliminate pinholes and or porocity that you are getting. You also do not state if you are warming your tools prior to pouring. This might help with flow and wetout of the mold surface to allow the material to flow better.

 

[einzats]

To ir12, I am intrigued by these dispensing methods you discribe. Nitrogen blanket? The dispensing method I am using now is plastic syringe and pvc tubing [small gauge]and injecting from bottom. Sometimes I use vibration. Heating the molds is good, but the cure time of the material becomes excelerated meaning less time for air to escape. The material[2 part] is pre vacuumed and has extremely low viscosity. Gas or exotherm are not relavent in this application.I would like a process where the mold is pre-flooded with a type of gass that would be molecularily{real word?} attracted to the casting compound.
What do you think? So far I thick your on to something using a gass. Please return your opinion, it is appreciated---thanks--- einzats

 

[patprimmer]

Nitrogen gas will just make nitrogen bubles instead of air bubbles. Air is mainly nitrogen anyway.

The syringe methad sounds good, as you get some mixing with no gass present Regards
pat

 

[einzats]

Patprimer, thanks for your response. I did an experiment by drying a silica compound [heat] and coating the inside of the mold. After the cast was made I noticed the powder did enter all the cavities as expected [electro-static reaction I assume] and it absorbed into the material. Just one problem; the apex of some extremities of the cast had a slight powdery surface. Keep in mind I have made many perfect castings, I'm just trying to find a process which assures flawless castings everytime. This includes eliminating micro-scopic surface imperfections. I think a heavy gass in combination with a mechanical process of some kind will be the answer.
What do you think?
Thanks, einzats

 

[patprimmer]

Im sorry, I don't really understand your process, especially the purpose of the silica.

I still se no function from the presence of a gas other than air, unless you have one of the following issues

Moisture from the air creates problems.
Oxygen in the air causes oxidation of the parts.
The air is entrapped in the mould.
Exotherm is boiling a component in your resin, causing outgassing. Some of this gas is then trapped.

If moisture is the problem, simply dry the air

If oxidation is the problem, use nitrogen

If air entrapment is an issue, there are 4 possible solutions

Evacuate the mould
Fill slowly
Fill from the bottom
Use a gas that disolves in your resin. (I don't know anything about this in practice).

If exotherm is the problem, control the initial temp, and heat added or removed dureing the reaction. Also, try a partly reacted resin. This certainly works for acrylic.

If you are moulding powders, use a compression moulding technique rather than a casting technique Regards
pat

 

[patprimmer]

I forgot, if exotherm is boiling a component, the boiling point can be raised by pressurising, in an autoclave for instance Regards
pat

 

[einzats]

einzats responding. The crucial point in my process is; I am casting small parts with extreme detail. It is the miniscule protuburances where air becomes trapped. Think about a very detailed piece of jewellery being cast in wax. The "high-tech" way of producing "voidless" casts is by injecting the wax into a mold under a vacuum, with the wax under a high degree of atmospheric pressure. The machine is expensive, but the priciple works exceedingly well. I'm trying to find a low tech solution, that is effective and cheap. Autoclave does work, but with fexible molds, the down-side is increased flashing on the part.
Thanks for your input,
einzats