The surface treatment process is a complex and crucial part of manufacturing. It consists of several separate processes, including blast cleaning, painting and chemical washing. These processes differ from each other and the requirements for each process are different in different manufacturing plants. Blastman Robotics provide automatic solutions for blast cleaning, and our aim is to provide our customers with the best possible blast cleaned surface. We need know the requirements of other processes in the production line so that we can provide the best possible surface. Furthermore, we need to understand the characteristics of these processes. This blog will discuss the general requirements for different types of surface treatment processes.
The surface treatment process typically starts with heat treatment, washing or chemical treatment when producing new products. Repair facilities are different, because they typically skip this part of the process as it has already been performed in manufacturing. The parameters for these processes affect the surface condition. This in turn changes the initial surface condition for the next process, which is typically blast cleaning. Therefore, the blast cleaning supplier needs to know what happens to the surface so that they can take it into account when designing their process. For example, if washing is completely discarded and the workpiece is machined, there will be oil on the workpiece. Oil has an enormous impact on blasting and its consideration is crucial. Another example would be the duration and temperature of heat treatment. Mill scale is produced during heat treatment and the thickness of the layer affects blasting.
After the first phase of the process, the workpiece is typically blast cleaned. For this part, we need to know the initial condition of the workpiece and the requirements of the next process. There are two main parameters that need to be optimised: preparation grade and roughness. These two parameters define the adhesion and lifespan of the coating.
The preparation grade basically means how clean the workpiece is after blasting. It is defined by standards that are based on visual inspection. There are many standards used to evaluate a blast cleaned surface, but the two that are most commonly used are the Swedish Sa standard, used in Europe, and the SSPC standard that is used in North America. Preparation grade affects the lifespan and adhesion of the coating. It also affects the duration of the blasting process. Therefore, it is crucial that the desired preparation grade is well defined and monitored. This way, the process can be designed to be as efficient and cost-friendly as possible.
Table 1. Brief overview of blast cleaning standards
The roughness of the workpiece influences the adhesion of coating on the workpiece. Roughness can be easily and accurately measured with modern devices. There is no direct correlation between greater roughness and adhesion. This means that all coatings have specific limits for roughness and these limits depend on the properties of the coating and the thickness of the coating film. Incorrect roughness can drastically reduce the adhesion of the coating and reduce the lifespan of coating. The roughness limits are usually provided by the coating supplier.
After blast cleaning, a coating, usually paint, is applied to the workpiece. This is often the last phase of the surface treatment process and the first thing the customer sees. The coating is also a protective layer against corrosion for the workpiece. The surface created during the blast cleaning process needs to fulfil all the requirements of the coating, otherwise adhesion and lifespan will not be optimal. A surface with a low preparation grade has a lot of impurities, such as sulphites and chlorides, which cause blistering on the coating film. A surface with roughness below the specified limits does not provide sufficient area for the coating to adhere properly. A surface with roughness above the specified limits is not fully covered by the coating and the protective layer of the coating does not cover the entire surface. This can be fixed by applying a thicker film, but it usually reduces adhesion.
As we can see, the surface treatment process is complex and requires a great deal of accuracy. All quality defects in the process are immediately visible to the customer and can cause reclamation. Here at Blastman Robotics, we aim to understand every aspect of the surface treatment process. That way we can provide our customers with the best possible blast cleaned surface. In future blogs, I will be discussing the blast cleaning process and its optimisation in detail.
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Sakari Veijola, Process Specialist of Blastman Robotics Ltd