Many cell types will grow when attached to a rigid surface but not in suspension, a phenomenon termed „anchorage dependence”︁. Anchorage dependence can be studied by incorporating solid particles of varying size into gels. It has been found that colonies will form on glass fibrils 500 μ in length, but not in the presence of silica fragments smaller than the cells. This shows that the suspending medium is not itself inhibitory, and confirms the requirement for a rigid surface of adequate size.
The state of inhibited cells in suspension culture was examined by dispersing them in a methyl cellulose gel, in vessels lined with agar. In this system aggregation is prevented and the cells may be recovered quantitatively. Normal, as well as transformed, cells increase in size, and a proportion synthetize DNA during the first 24 hours in suspension culture. Growth and DNA synthesis in normal cells then virtually cease, while transformed cells continue to grow into colonies. The stationary normal cells remain competent for further growth for at least a week in suspension. When such cells are allowed to attach to a rigid surface in the presence of colchicine, DNA synthesis occurs and is followed by mitosis. These results indicate that suspended cells are blocked between mitosis and the end of the S phase of the cycle.
To anchor is to hold or resist the movement of an object; anchorage is the gaining of that hold. In orthodontics, terms such as “critical anchorage”, “noncritical anchorage”, or “burning anchorage” are often used to describe the degree of difficulty of space closure. Anchorage may be defined as the amount of movement of the posterior teeth (molars, premolars) to close the extraction space (Fig. 10-1A) in order to achieve selected treatment goals. Therefore, the barrier anchorage needs of an individual treatment plan could vary from absolutely no permitted mesial movement of the molars/premolars (or even distal movement of the molars required) to complete space closure by protraction of the posterior teeth.
When designing large structural components it’s critical to make an informed decision between castings and forgings. The following paper by Rexnord provides an in-depth examination.
Material selection is one of the most crucial decisions made in the design, manufacture, and application of large structural components. Material selection naturally influences the entire performance of the design, and thus it is critical that informed decisions are made during the design stage. Steel castings and steel forgings are two alternatives for large structural components. For many design engineers it is often assumed that a forging is a better product because it is formed or worked during the manufacturing process. It also assumed that castings are inferior because they may contain porosity. Nothing could be further from the truth. Each process has its advantages and disadvantages. It is just as possible to produce an inferior product whether it is a forging or a casting. This paper will present an honest evaluation of castings and mining forgings, so that those in the design community can make an informed choice.
Melt Shop Practice
One important distinction between wrought and cast steels is the de-oxidation practice that is used. Wrought steels are typically “aluminum killed,” which means that a small amount of aluminum is added during the melting process for the purpose of removing oxygen from the steel. While very effective at removing oxygen, the aluminum forms microscopic aluminum oxide particles, which are abrasive during the CNC machining process. Some steel casting shops de-oxidize with calcium, which also removes the oxygen but produces a softer, more machinable inclusion.
Large forgings are hammered or pressed into rough shapes, which then require extensive machining parts or welding to other components to produce a more complex shape. This adds to the cost of the overall product. Large forgings are limited as to the amount of mechanical working that can be done.
Most steel mining castings are produced in expendable sand molds. The mold is produced by forming sand around a pattern, which is a replica of the finished part. Molding sands are mixed with materials that will allow it to hold the desired shape after the pattern is removed. Holes or cavities are created by assembling sand cores in the mold. The pattern equipment also includes the gates and risers which are needed to produce a quality casting. The gating system is designed to allow the metal to flow into the mold in a controlled manner. Risers are reservoirs of molten metal which allow the casting to solidify without shrinkage porosity.