My dear Douglas,
To prevent the casting flask blowout following suggestions should
1) Proper water powder ratio is very important if you use more water
then recommended then the investment will have more water in it and
while improper burnout the steam will crack the plaster from inside
and the result is blowout while casting.
2) The wax pattren should be minimum 18mm below from the top edge of
the flask, and 12 to 15mm from the wall of the flask.
3) Burnout very slowly. In your case of 3x3, 3x4 inch flask or 4x4
flask,if you put one each. Then put the flask in the oven after one
and a half hour of investing preheat the furnace(oven) at 100
degrees centigrade,load the furnace hold for one hour. Raise the temp
to 150 DC(Degree centigrade) hold it for one hour, third step is to
raise the temp to 200 DC hold it for one hour, fourth step is raise
the temp to 250 DC hold it for one hour, fifth step is to raise the
temp to 300 DC and hold the temp for an hour, sixth step is to raise
the temp to 720 DC hold it for 2 to 3 hours or over night if you are
going to cast in the following morning, then drop down the temp to
your required casting temp between 400 to 650 DC depending up on the
thick and thin castings.
Try this and you will be able to prevent the blowouts.
Best of Luck
May God bless us all with total health, Mental, Physical, Spiritual