Hamilton Form, Ltd.
7009 Midway Road
Fort Worth, TX 76118

Phone: 817 590-2111
Fax: 817 595-1110
sales@hamiltonform.com

PCI NPCA CPCI

Frequently Asked Questions

"Mill Scale"- What is it and how to deal with it?

We have had several customers recently who have been concerned about "mill scale" and other minor surface defects on their new flat casting tables made with 3/8" x 12'-6" x 40'-0" skin material. To define mill scale, we must first describe how hot-rolled steel plates are produced using the rolling mill process. The process of rolling finished mild-carbon steel plate from ingots or slabs requires many passes of the metal through the rolls. Each pass approximates the final thickness and surface finish of the plate desired more closely than the last. The successive reduction in the thickness of the plate is accomplished by bringing the rolls slightly closer together between passes. The number of passes through the rolls and the type and amount of water cooling process used will determine the amount of mill scale and ultimate quality of the surface finish. Mill scale forms a protective coating on the surface of steel plates and normally should not be removed.

Your new form will typically arrive with a metal protective oil applied to all contact surfaces. If the form is not installed in a timely manner, and will be kept in storage for an extended period, we recommend the application of an additional metal preservative. Once the form is installed and ready for use, a simple wiping with a dry rag (combined with solvent if necessary) should remove the oil, dirt, and any loose mill scale. Some precast producers recommend using a petroleum-based form release agent for at least the first five or six casts. Once the table surface is "broken-in" they will then convert to a standard emulsion or water-based form release agent.

Some minor flaking of the mill scale may occur until the table is "broken-in" which should not be objectionable for most architectural and structural concrete products. However, for some as-cast architectural finishes, or if the mill scale is flaking excessively, we recommend any one of the following solutions:

  1. Lightly sand the affected areas with a 60-grit sanding disk. Warning: under no circumstances should a Hard-rock Disk Grinder be used on the skin surface.
  2. Mechanically clean the plate surface with a Hamilton Form Company Flat Deck Cleaning Machine equipped with 12" diameter brush filament made of tough, long-lasting nylon impregnated with a special abrasive.
  3. Clean the plate surface with a circular floor buffer equipped with a stiff-bristle brush.
  4. Lightly abrasive-blast the skin surface with a non-scarring medium removing only loose mill scale particles.
  5. Wipe the affected area with steel wool. Warning: Removing all of the mill scale on the skin surface will cause the steel plate to immediately oxidize in the presence of oxygen and water, and rust will form without a heavy coat of protective form oil between castings.

Another type of plate surface defect we have observed, especially in plates over 10 feet in width, is a formation of orange colored residue called "ferrous oxide." Ferrous oxide is normally not a problem for casting structural concrete products, however it may be a problem for casting as-cast architectural concrete products. To remove most of the ferrous oxide from a casting surface, we recommend any one of the following solutions:

  1. Mechanically clean the skin surface with a Hamilton Form Company Flat Deck Cleaning Machine as described above and flush the surface with water to remove the residue.
  2. Clean the plate surface with a circular floor buffer as described above and remove the residue with cloth rags and solvent.
  3. Clean the surface with a light solution of muriatic acid and water.