Materials of construction and finish

Screw feeders are normally of metallic construction because of the cost/strength requirements for industrial applications. Some small feeders are constructed of non-ferrous materials to allow metal detectors to be used around the casing, rather than on the free fall of the flow stream. There are also casings made of flexible materials, for vibrators or oscillating pads to deform the walls in order to stimulate flow. While these are usually successful in promoting flow, the regime of flow that is created is generally uncertain and in most cases definitely not of a mass flow nature. 

The specification of a surface finish for stainless steel equipment is a source of many potential problems and misunderstandings between purchasers and manufacturers. The implications for costs and like-for-hke comparisons between tenders, and the need to establish that the equipment condition satisfies all functional requirements of the application, dictate that the surface finish specified should be appropriately defined in all contractual documents. Descriptions such as crevice-free or pohshed 400 grit , may appear definitive, but all too often lead to ambiguities, disagreements of interpretation, or unfitness for purpose. 

The selection of surface finish is usually made on the basis of choosing the optimum economic method to meet the objectives related to suitability for the application. The choice may be compromised by global conditions, such as seeking machine or plant uniformity, or by narrow key functions of crucial importance, as with fatigue problems or stringent duty constraints. 

A useful starting point is to review the differing criteria on which the specification of surface finish is based. This illustrates the many attributes nature of the situation and how these may apply individually or in varied combinations, according to the application circumstances. The reason for the selection of a particular type of surface finish usually relates to one or more of four basic considerations, relating to  

The most common reasons for preparing a machine to a particular standard of appearance are given below  

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Materials of construction and finish must be selected to resist corrosion and environmental conditions. Pressures are generally low, unless there is a dust explosion danger, in which case the contaimnent depends upon the means used to counter the hazard. Total containment requites resistance to about 8... 

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