Casting Sand still going wrong

[BernieNUFCParticipant @bernienufc]

So further to my comment in my introduction reply i have had yet another failing of creating a casting mold so just wondering what now.

I used silica sand and mixed at 4% ratio with sodium silicate, talc'd the pre waxed former which is only 12" long by about 1/2" wide and cut from the easy composites modelling board just to try and get a good pattern.

I packed it in, not to hard but just like 'sv seeker' on you tube and then placed the lot in a bag with a small cup of vinegar and bake soda. Pulled it out some time later and its nice and hard and looks great.

On with the next part place the former over the other one and do exactly the same. This is where i might be going wrong, as i try to lift the flask off so i can put in the bag, but the former remains on top of the other one (not too worried by this tho i think) but leaves a fair amount of the sand stuck with it. For this piece (because of previous attempt) i used the release agent on it and it is very slippery but still the sand stuck, any pointers appreciated

Bernie

[BernieNUFCParticipant @bernienufc]

[Neil WyattModerator @neilwyatt]

No expert, have you tried 'rapping' the pattern with a hammer to get it to release cleanly?

[JasonBModerator @jasonb]

Are the pins locating your two pattern halves too tight and stopping the top half of the pattern lifting away cleanly with the box.

If not once you have packed the top half then turn the whole thing upside down and lift off the previously set half which should leave the unset half less disturbed and exposed so you can do as Neil suggests and lightly rap it before pulling out.

[BernieNUFCParticipant @bernienufc]

'No' to Neil's comment, would that not dislodge the sand in the flask though?

The pins could be too tight but i had hoped the sand would not have stuck still, but i do like the idea of turning them over and trying that, i will also make the fit a bit looser then.

thanks

[Bob StevensonParticipant @bobstevenson13909]

I don't think the sand mix is right…..

Most casting sand contains bentonite which has to coat every last grain of sand and acts like a binding agent.

[BernieNUFCParticipant @bernienufc]

Thanks bob, i have watched and googled this and i have come across 2-4% in weight of SS to Sand.

I was putting in 80ml for 2kg of sand, the way the first flask dried hard i figured i was in the right place.

[JasonBModerator @jasonb]

Bob, it sounds as if Bernie is using a chemical set rather than standard greensand which would contain bentonite. The Chemset is more often used for cores but can also be used for the main casting

[BernieNUFCParticipant @bernienufc]

I think Jason is right as i do not know what chemset means, but it is silica sand really very fine that is used in block paving and liquid sodium silicate added in the mix just before adding to the flask

I also have oil bonded sand but again that stuck to the former, so i will try 1 more go using each method tomorrow

thanks for replies

[JasonBModerator @jasonb]

What density board did you use and how have you prepared the surface? Talc should be adequate to release from oilbonded sand.

If too rough and not smooth enough just using wax won't allow it to release easily as the stand will tend to stick to any texture on the surface.

Can you post a photo of the pattern there may be something obvious that we can see like lack of draft angle or roughness

[BernieNUFCParticipant @bernienufc]

I am using PU600 High Density Polyurethane Model Board for the patterns.

I cut them on CNC router with new bits and are as good as i can get them. In prep for tomorrow i am putting on extra coats of release agent just to eliminate any non super smooth areas and make sure they are really smooth as you say.

I think after studying it i might not have enough draft as you indicate so will look at that as well.

For the sodium method i think that i did not get enough co2 down to the pattern so thats what am doing now and leaving it overnight to see and i also made some vent holes for it to get in easier.

cheers

[John ReeseParticipant @johnreese12848]

Could the surface finish on your pattern be a problem? It needs to be glossy slick to keep sand from sticking.

[BazyleParticipant @bazyle]

There are two techniques being mixed here. With sand plus sodium silicate the mix has zero repeat zero structural strength. Unless it is a very simple shape do not even attempt to remove the pattern before curing it with the CO2. No harm in leaving it with the first section when curing the second.

[BernieNUFCParticipant @bernienufc]

John, the pattern is as smooth a i can get it and i am getting it better ea coat of agent.

Thank you Bazyle, i think this is where i am going wrong with the silica method, so i am hoping i can get enough co2 to give the strength this time before trying part it.

Thanks all

[jason udallParticipant @jasonudall57142]

Just a thought..I see no mention of draught angle on the patterns..I believe ( been a long time abd that with oil bonded) that a draft angle is required only 3-5 degrees but needed none the less..
As I say been a while

[BernieNUFCParticipant @bernienufc]

Jason, a previous Jason did mention this and i will be looking to increase if needed, but you are right i will need to see what the degree should be thank you for reply

[BuzzaParticipant @buzza]

Have a look at 'Myfordboy' on Youtube. Lots of info. and videos about casting there.

[PatJParticipant @patj87806]

The trick with sodium silicate bound sand is to use the correct ratio (I use the lower end of the range, I think around 3%), and the worst thing you can do is over-gas the sand, which ruins its strength.

I generally gas for 5 seconds only.

I thread a screw into the pattern and use a small automotive slide hammer to tap it out.

I then insert the pattern back into the sand and mold the other half.

Leaving a pattern for too long in the sand can cause it to stick. I wax my patterns before each use.

For one piece patterns (most of what I do now), I don't fully remove the pattern from the first mold half, but try to at least break it loose before ramming the second mold half.

I use sodium silicate sand exclusively for both cores and molds. It works well if a fine and very dry sand is used, but generally the sand is not reusable.

If more than 3-4% of sodium silicate is used, it can increase the mold strength slightly (sometimes) but it makes molds and cores difficult to break up after casting, and I don't recommend it.

Edited By Pat Jorgensen on 23/07/2017 09:27:26

[BernieNUFCParticipant @bernienufc]

Thanks Pat, very informative and useful, i will apply this info later on today on another practise job

[Ady1Participant @ady1]

I recall colloidal graphite from an article I read, you paint it on the sand of the mould before the pour to help release the final product

Also makes fabby penetrating oil

Note: I would try to use an atomiser scoosher to apply it, not a brush, although it may be brushable

Edited By Ady1 on 23/07/2017 12:10:23

[Ady1Participant @ady1]

Searching for "diecasting and forging lubricants" may also help

[BernieNUFCParticipant @bernienufc]

Just finished my first cast then after the issues that i have had. I can easily say what i have learnt and its lots.

Although the end products are not what i want they are close in that i could not make a successful core after lots of attempts, but i now know how to fix that.

Todays cast ended up as the bottom flask being a good Silica Sand mold and the top one was a oil green sand mold to see which was best for me going forward.

Time to start right from the beginning and to note these points:
Put a marginally bigger draft on some of the patterns
Make sure Silica Sand is absolutely bone dry
That Silica sand does not give me a better end finish as i had expected compared to green sand
That preparation (release agent/talc/wax) is key
I need a better setup method for me to get the CO2 into the mold if i go this route with Sodium Silicate
I think i might need a vent at the top of the rims as there is a slight dip and these were the top of the mold in teh oil sand
This Forum is only a post away

In some respects i feel that i have lost 2 weekends with no product at the end, but with what i have learnt i am now looking forward to producing a better quality part which will be worth much more to me in the end run.

Thanks to all who posted, i am a happy man today.

Edited By BernieNUFC on 23/07/2017 16:13:23

Edited By BernieNUFC on 23/07/2017 16:38:40

[Author]

Posts