Metal Casting Zone

hello,
we cast plenty of parts in low pressure die casting and one of the common problems we face is inclusion/ oxide entrapment on the top face of the casting which is formed on the upper half of the die cavity. , we use mesh filters on the gate but problem is not eliminated.
advice remedies
thanks in advance
rags

im not an expert but it sounds like the mold hasnt been engineered correctly to allow gases to escape.

thanks xl,
The defects do not seem to be voids or air entrapment but visible only after machining and are hard aluminium oxide inclusions..i have attached some pics..your views.
thanks
rags

pics here

oops hope pics are uploaded now

Does it happen randomly or in batches mixes?
It sound likes you have to much turbulence in you runner system. And gasses are getting trapped in your mold Hydrogen dissolved in molten aluminum may generate cavities of its own ??

raghuvir wrote:
oops hope pics are uploaded now thanks for the pics.i know this this well as i get it also. this is not an answer as such but this is what ive found by experimenting. when i first started casting i was getting a lot of this problem. as i wanted much finer castings (2mm to 3mm) it really showed up. if i grab some aluminium scrap,melt, scalp and pour i can get this problem.if i use aluminium which ive previously melted ,scalped maybe two or three times i dont get this problem.im putting it down to impurities in the metal. in earlier times i used to stir and pour ! (bad practise!!) now i stir and let my metal settle ,scalp and pour. im not sure if this is where the problem starts. i tried different fluxes but the problem was still there. lately i havent had the problem as i melt my scrap,remove the dross and pour into ingots. for fine model work i use my sprues and old miss casts (which are becoming very rare!!!!!! as my "refined" metal stocks are getting used up i will be facing the same problem. keep us posted. my suggestion is to remelt your castings and see how much dross surfaces. pour the metal and examine the castings and see if the problem has diminished! if you do please post.

thanks xl and spw,
The problem is indeed random, there isnt much turbulence as we fill up quite slowly in low pressure die casting.

There are enough vents/ejector pins for the entrapped gasses, i would like to check the inclusion level in the molten metal , i understand that the K-MOLD test is a good check , can anyone provide a sketch for the k-mold which will help me to start at the root of the problem. Iam attaching a close up image of the defect again- mixture of al oxide and voids.

thanks again
rags

The round spherical voids indicate trapped gas porosity.
Two things to look at are the effectiveness of your metal de-gassing and the metal velocities. For Aluminum alloys, molten metal velocities should not exceed 0.5 to 1.0 m/s (per John Campbell's research).