Sand Casting Core Defects Series - Part 1

Part 1

In this two-part foundry series, we explore the most common shell sand core defects and provide probable causes to help eliminate them. If you run into these core defects, use this troubleshooting guide to check your system. Minimizing the amount of defects in the core room will boost overall production and save money on re-work costs.

Strongest:

Normally a 3/8″ to 1/2″ thick. For thick wall castings, 3/4″ or greater
Complete cure of the entire thickness. Golden brown in color
Cold Tensile – 350 minimum. Ideal would be 350 to 500 tensile
Good density – Blown tight

Weakest:

Wall thickness below 3/8″
Wall thickness under cured – yellow in color
Cold tensile below 350
Loose density – Not blown tight

When is a shell core at its strongest and weakest?

Core is over cured:

Box temperature above 550 degrees
Cure timers too long
Low melt point of sand (below 205 degrees)
Sand temperature too hot (above 90 degrees)

Core is under cured:

Box temperature is below 450 degrees
Cure timer is too short
Melt point of sand is above 218 degrees
Sand temperature is too cold (below 60 degrees)

Common Shell Core Curing Issues:

Core is showing uneven cure:

Not enough burners (heat) to cover the configuration of the box
Burners out (not working)
Heat controls or sensors not working (either side of the box)
Core in the box too long before removal
Loose pieces colder than the rest of the box (move heat closer to loose pieces and mount extra burners)
Mixture of low and high melt point sands (improperly blended)

Core surface is very dark but rest of core is under cured:

Box temperature too high (above 550 degrees)
Cure timers too short

Core Blowability Issues and Defects:

Core Has Poor Blowability:

Wet Sand
- Water in air lines
- Cold Sand – Below 60 degress. As the sand warms up, moisture is created
Low melt point – below 190 degrees. Sand sets up too quick, which slows the movement of the sand during blow cycle
Core box too hot – Above 550 degrees. The hotter the box gets, the quicker the sand sets up as it is flowing in, which slows up the movement of the sand
Blow valve malfunction – Proper air pressure not getting into sand box

Water in air lines
Cold Sand – Below 60 degress. As the sand warms up, moisture is created

Core Has Loose Blow or Poor Density:

Sand low in sand box
Vents plugged
Low blow pressure
Blow timer too short
Lack of venting opposite of blow end
Lack of vents at dead ends
Parting line not tight (sand blowing out)
Box too hot (above 550 degrees)
Lack of seal between blow plate and sand box
Blow valve blowing at wrong box position
Lumps in sand box (gets in front of blow holes)

Core Has Swirl Marks:

This core defect is usually found on cores that have a large area between smaller areas. The large area would be 4 inches or greater in diameter. This defect can be caused by:

Sand falling into the large area and beginning to set up before the remaining sand fills the area and the two sands do not knit together because of the different times the sand is in contact with the hot box. Filling the core box faster usually corrects the problem.
Air that is trapped in the large area that has not pushed to vents opposite of blow area. As the sand was filling the large area and moving around, the excess air blends in with the sand giving the loose blow, swirl appearance. Usually putting vents in the box where the defect is found corrects the problem.

Are you experiencing defects with your shell cores?

Contact our team today to discuss your defects. Backed by years of expertise, our team can run trials to find the best process within your application.