October 2015 Archives

(1) The principle of cleaning

As you notice when you wash dishes at home, some stains are easy to remove if you scrub with a drop of liquid soap on the sponge whereas there are some stubborn ones that stay on even after soaking them in water or hot water for a while. We will now take a look at the mechanism of how the cleaning works.

What matters the most here is to determine the ultimate goal of the cleaning task. It is necessary to select the best and most economical method of surface cleaning. When you determine how clean the product needs to be, it is important to assess the degree of permissible contamination without leaving harmful effects on the next process.

(2) Cleaning with detergents

This is the most common cleaning method, which is similar to washing dishes at home. The difference is as follows: heating up the detergents and accelerate the reaction in order to improve the cleaning efficiency and capacity, circulating the degreasing solution, shaking the workpieces, or mixing the degreasing solution using ultrasonic waves in order to save power by skipping the scrubbing process.
In general, the detergent is classified into the types described below. They can be used individually or used by the composite of these items.

[Table 1] is an example of the commonly-used degreasing bath called "boiling degreaser" or "alkaline degreaser". Heat up the solution to 50 - 60°C before use.

[Table 1] Example of degreasing bath composition (g/L)

* Note) g/l refers to grams dissolved per liter of degreasing solution.

(3) Solid material

The surface of metal products is contaminated with various types of grime listed here.The contaminants include organic/inorganic dirt, such as products formed by chemical reaction on the metal surfaces during processing, as well as dirt and dust in the air attached to the surface or onto the oil films, including rust-preventing/lubrication oils.

(4) The mixture of oil and solid refuse

Surface conditioning on the product surface may be performed by mechanical polishing before the surface treatment process.This is one of the decoration techniques achieved by buff polishing or sandblasting to add the aesthetic product value to metal surfaces, such as matting, satin finish, hairlines, and mirror gloss.
This section introduces details on the methods of buff polishing.The common buffing methods are as follows:

Emery polishing

(Emery polishing smooths out the product surface using a round buff with emery abrasive powder adhered to the glue in the circumferential direction. This buff is made of multi-layered fabric stitched up closely. Common types of abrasive powder for this buffing are #120 and #220 [coarser powder], and #320 and #400 [finer powder], which will be used from the lower number.To reduce friction and avoid grinding burn of the metal, oil-based abrasive paste will be used together.This paste is also called "grinding compound".)

Buffing

(This buffing adds mirror polishing or matte polishing to emery-polished products.The buff used here is called a "cloth buff", a round-shaped buff made of multi-layered fabric stitched up roughly. Apply oil-based abrasive paste onto the circumferential surface for polishing.)

A variety of buffing methods are available including the ones introduced here. Emery polishing may be enough for some products, but some of them require polishing on the glossy surface.

Grime attached to the metal surfaces is largely divided into the following two types: Oil-based grime, such as lubricants, and metal-based contaminant, such as oxide films formed during processing or heat treatment.To remove these contaminants, "degreasing" is performed on the former type whereas "acid washing" is performed on the latter type.

Let's change the topic here and look into material cleaning, which will be processed as part of the surface treatment of metal products.
In general, the surface treatment will be performed in the following steps:

Before performing surface treatment such as plating work, the pretreatment process is always required since plating work cannot be applied directly to the product surface.This step is called the pretreatment process of plating, which prepares the surface of the plating substrate for plating.In other words, this is a process for cleaning the substrate surface (including plastic and ceramic materials in addition to metal products in recent years.)The pretreatment process starts with the removal of various dirt and grime attached to the substrate surface in order to expose the surface.This task is referred to as "cleaning."
Now, let's see what kind of dirt and grime we will found on the substrate surface.

Grime on Metal Surfaces -1

Before moving onto the topic of metal cleaning, let's take a look at surface grime.In general, any the following types of dirt and grime can be attached to the surface of metal products to be processed for surface treatment:

(1) Antirust/lubrication oil

Storing finished metal products without any other processing will cause the products to develop rust or generate discoloration due to oxidation. These changes damage the product and interfere with the subsequent processing.To prevent such phenomena, it is common to apply an antirust coating before storing the products.As the antirust coating, apply petroleum-derived or animal/vegetable antirust oil onto the product surface.

(2) Lubrication oil for metal-forming process

Metal materials are formed into metal products after going through various metallic processes as listed below.During processing, many types of lubricants and coolants are used to facilitate the processing and to prevent wear by reducing friction between products and molds/tools while minimizing heat generation.