Avoiding Precast Staining and Bugholes – Proper Application of Release Agent
The application of release agent (commonly known as form oil) is a production step that sometimes is not given much thought or care during the production sequence. This step usually follows the stripping process and often the production teams are rushed to close the forms again in preparation for re-casting, particularly if double casting is required to meet the delivery schedule.As a result, the application technique is compromised in favor of speed, with the consequences (defects in the formed surface) revealed later once the product is cast and stripped.The main defects resulting from improper release agent application are:Staining
Staining is usually caused by excessive application of the release agent and dirt/residue which is transferred to the cast surface.
Excessive bugholes are usually caused by heavy application of the release agent, coupled with inadequate vibration resulting in air trapped at the cast surface.
Main methods of application are as follows:Spraying
- Clean the forms thoroughly before application, using the appropriate tools such as scrapers, brooms and compressed air.
- Ensure that the release agent does not get on the reinforcement when applied
- Less is better – aim to cover the entire formed surface with a light application – about the thickness of a human hair! If pooling or running occurs, it has been over-applied and is not only wasteful but detrimental to the finished product
- Ambient temperature and product type can determine the proper method of application
The use of a broad-nozzle/flat spray system has been found to be the most effective method for general application of release agent. It is important to keep the tip of the sprayer moving to ensure uniform cover, and to use the appropriate pressure and spray tip according to ambient temperature (higher pressure and smaller nozzle for lower temperatures and lower pressure and larger nozzle for higher temperatures. Only spray systems specifically designed for release agents should be used; other types may leave large droplets of agent on the forms which compromise the surface.Swabbing and Painting
Hand application with brushes is labor intensive and results in a thick layer of release agent. This method is usually for release agents that are too thick to be applied with the spray system. These types of heavy applications also attract more dirt.Wiping
Wiping is often favored for architectural products where a blemish-free surface is critical. Wiping on release agents with a sponge or rag will normally result in a thin coating, but is very labor intensive and can be wasteful if not done properly.An excellent guide to the basics of form release application can be found hereTypes of Release Agent:
Choosing the right type of release agent can definitely assist in producing a superior product. As such, it is important to recognize that there are 2 distinct categories of release agents:Barrier Types
Barrier release agent provides a physical barrier between the form and the concrete. These are usually blended petroleum distillates such as kerosene and diesel. When applied to the form, the lighter fraction of the oil evaporates, leaving the heavier oil behind, similar to grease on a cookie sheet to prevent the concrete from sticking to the form surface. However, this oil can migrate during vibration, causing sticking and staining.Reactive Types
Reactive release agents are typically fatty acids or other ingredients that react with the calcium hydroxide (lime) in fresh concrete to produce a metallic, waterproof soap interface. This prevents the concrete from bonding to the form, and results in a brilliant, uniform surface that is free of discoloration. These agents are also attracted to steel and wood, preventing migration.The Importance of Seasoning:
When starting up a production process with new forms, seasoning is a crucial first step. If skipped or not done properly, the results can be highly problematic and frustrating to deal with.
Seasoning allows the release agent to penetrate into the surface of the form and fill the open ”grain” in the surface. When the forms are new, this is basically done by removing any protective coating on the steel first (using solvents or other means), and then saturating the surface with release agent and allowing a period of time for the agent to penetrate the surface, usually at least 24 hours. After this, the excess release agent is wiped away and the form is ready for the first cast, although it may take a few casts and reapplication before the form surface is fully seasoned.
When seasoning, producers may encounter mill scale which is a grey flaky surface found on hot rolled steel consisting of mainly iron oxides. This is a by-product of the hot-rolling process during the manufacture of sheet plate. The presence of mill scale can be very detrimental to new molds, causing many problems such as severe sticking and pitting of the surface. It can be very difficult to remove using abrasives due to its hardness and scouring can be detrimental to forms as this can cause accelerated corrosion. The proper way to remove mill scale is with a diluted acid such as Muriatic acid that will dissolve mill scale, similar to how vinegar dissolves lime scale in a kettle. The acid remains on the form for a period of time to react with the mill scale and then washes away; leaving a smooth, scale free surface. Once removed, the form can be seasoned normally.
Click here for a video guide to removing mill scale