Help please. Cold-shut problems.

I have been making these castings for years, but recently I have changed a few things and run into problems.

So I used to;

• Pour metal straight down a sprue about 1" in diameter and then through a short gates to make a pair.
• Cut the gates with a trowel.
• This sometimes created wash.
• The surface finish was OK but owing to the pressure of the large sprue, showed sand detail.

So now I have;

• Made a pattern-plate and attached runners to it.
• Increased the pour to six castings in an effort to increase production.
• Made a pouring basin and used a 1/2" sprue.

The new castings have a prettier finish, but cold-shuts are happening too often on parts which were never difficult to make in the past.

I think that maybe I need to increase the diameter of the sprue. The metal is still at the hottest that I can make it – in other words nothing has changed with the temperature.

Is the arrangement of the patterns on the plate a sensible one?
Are the runners too long or not?

Your thoughts and suggestions please?

 

I'd be inclined to go back to a 1" sprue and increase the height of the sprue by 4" using a tin can packed with green sand boost to hydraulic pressure and also the volume of metal it can feed. The can just sits on top of the existing sprue and so long as both surfaces are reasonably flat there won't be any leakage.

 

Yes, I forgot to mention that I used to use the tin-can method. Old Foundryman doesn't like the method because it introduces too much pressure. Time to experiment; I will do one mould next time with the can reinstated.

Thanks Mark.

 

Consider, at least as a contributing factor, that there might be a break in the continuity of flow. The metal is initially splashing in and going in many directions but eventually will go toward its easiest direction, momentarily abandoning previous pathways. Once the easy path is filled it has to then re-establish flow but now has to remelt the previous front of (oxide covered) cold metal. This may well be happening at various points in your gating scheme simultaneously. Locating your sprue on one end so the metal needs to go only in one direction might help with no need for larger sprue or increased head.

Pete

 

Your gating is HUGE for aluminum, the rule of thumb is wide and shallow in your case I would go 3/8in deep, 7/8 in wide , your are just square ( Your gating like it is steel ) I totally agree with Kelly's advice on the arrangement,
I'll go to my pattern thread and add a pattern as an example

V/r HT1

P.s. your sprue is too big (diameter) ancient thread by Campbells Boy Poduckia(sp?) we could not find any reason for a hobbiest to use a sprue larger then 1/2 inch... Might have been lost on Alloy avenues

 

Thank you Pete, HT1 and Kelly.

I had forgotten the wide and flat rule and I was thinking about a stepped runner yesterday. Time to revise everything,

Cheers Charlie

 

A little late to the party.

I would be willing to bets that simply moving your sprue to one of the “y” intersections and tilting the mold down at the sprue would make the aluminum flow in a more organized way. As shown it can decide which of six directions to flow and flow that way for a while until another direction is lower resistance. One or two of the part cavities are bound to have some arrested flow resulting in slush formation and permanently stopping flow to it/them. Since you have your matchplate all made, I’d try it with that single mod.

Denis

 

Thanks Denis. I have destroyed that pattern-plate and have nearly finished the new one. I have also given myself a severe slap in the face because I didn't read my notes on good pattern-making-practice, that I wrote after reading the US Navy manual.

So the new plate has the runner reduced after the first gates and the parts reduced to four and arranged in two lines. This was to make it fit my smaller moulding boxes. I will experiment with set-up e.g. tilting a box; using a filter; sprue diameter, etc.

Cheers Charlie

 

I guess it may be just my quirk, but I can not think of mold I do not tilt. I used to have a couple that were cast level where I was feeding long straight edges from the middle of the their span. One of the reasons I switched was due to occasional cold shuts. Pouring my 36 and 48 inch camel back castings from one end with about an inch of tilt over their length is so much better. And I should add that cylinder-like molds cast with their long axis vertical I do cast level in the sense that the flasks are level, but the cavities are maximally elevated. ;-)

Denis

 

Looks like the metal is not filling as even as it should. Really a bad idea to pull an ingate of the spru basin.
I made a rough drawing of a layout. This is how I would have done this.

 

my hands are shaking some, hard to get a clear image.
The runner is 1x1 inch w/basin in the center of the runner. 8 impression,a 3/4 inch spru, 2 pop-offs on the runner. pour aluminum 1375-1400

 

another look at you images, I think if you took the two casting that are coming off your downgate off, it may clear things up for you.
I am thinking that job needs to be either a 4 impression or 8 impression pattern. 6 throws the flow off, and coming off the downgate. Just not good.

 

Sometimes I stick a tall water glass on the table and rest the phone on that, leaving the camera hang out over the paper or object I am 'shooting'.

 

Line them up three or four on each side with central runner either tapered or stepped down smaller at the end. The metal has inertia and will push it in one direction until force changes its direction, so the last castings will start to fill first and by making the runner and ingates bigger at the first castings they will take metal faster once the castings on the end start to fill which in theory will fill them all evenly. I myself like a bigger sprue as it makes it easier to fill while pouring...keeps more heat in the mold...will continue to feed the casting if I happen to sway out of the pour cup while filling...gives a bigger area to float impurities....will not get clogged up with impurities...last thing to solidify so I know the castings are solid once the sprue is solid. The down side to bigger sprues...more metal to melt....if you have a very tall mold can lead to velocity and erosion defects if not properly choked at the entry of the runner. This is not the case for your height. As for finish on a casting and temp...unless you know you are pouring too hot causing burn in and burn on type defects I would leave temp alone and focus on vents and sand properties first and then look at runner size and ingate size.

 

Pressurized Gating is tricky, the math for the metal flow has to be very good. And a good layout is difficult as the parts will be at different depths in the mold. The idea to fill the parts at the end of the runner first, metal backs up, fills the next and so forth, helps put more metal into each cavity quicker. Tends to squirt the metal through the ingate, is good for thin parts. Don't recommend pressurized gate for this housing.

 

Thanks Billy.

I have made a stepped-runner and a new pattern-plate for four housings. I will experiment with different sized sprues. Slow business here, as I have heaps of other jobs to get on with,

Cheers Charlie

 

Update time. I will do this in two posts to make it easier to understand.

The failed castings I showed at the start of this post have now worked really well. Process that was changed;

• I made a new pattern plate with a stepped-runner arranged longitudinally and two castings each side.
• I widened the in-gates and made them thinner.
• I tried a 1/2" sprue and a 5/8" sprue to see what difference that might make.
• I put a 5/8" spacer under the drag so that the metal had to flow uphill.

All the parts bar one came out really well with a nice surface finish and no wash. I use loose patterns on this job and the failed one moved off one of its dowels and therefore didn't align with the core. Sloppy pattern making.

Thank you to everyone who made suggestions of improvement.

The photo shows the results of two pours,

Cheers Charlie

 

In addition I did two other types of parts.

The flattish parts had cold-shuts last time as well, but the only thing I changed was to use Denis's suggestion of tilting the moulding boxes.

PS And use a stepped runner.