Metal aluminum alloy plate prepress processing

Screen printing on metal surfaces must be pre-treated. The metal surface pretreatment process is also referred to as prepress processing. There are many ways to pretreat metal surfaces, such as degreasing, polishing, drawing, oxidation, sand surface treatment, and spray painting. These processes can be used individually or in conjunction with the requirements of the product, depending on the conditions of the underlying materials, in order to achieve the desired results.

Aluminum alloy plate is the most contacted product in the screen printing industry. The following is representative of aluminum alloy surface treatment and briefly introduces the pretreatment process of metal screen printing. There are two main purposes of aluminum alloy plate pre-treatment: First, the decorative, that is to remove some of the defects of aluminum plate material to increase the aesthetic appearance of the surface; the second is the process, that is, with a number of pre-treatment process to change the surface finish of the material to Increases the adhesion of the print coating on it. There are many pre-treatment methods for aluminum alloy sheet, the introduction is as follows:

First, degreasing treatment

Degreasing is a process pretreatment. In order to make the surface of the aluminum plate have a certain affinity for the printing paint, it is required to remove the oil stain on the surface of the plate. The general encapsulant oil can be wiped off with fine sawdust. The oil in the machining process can be scrubbed with an organic solvent or degreasing with an alkaline solution. The commonly used alkaline degreasing solution formulation is described as follows:

â–² simple formula and process conditions sodium carbonate 50-60g
Water up to 1000ml
Solution temperature 60-80°C
Cleaning time 2-5min

â–² commonly used formula and process conditions sodium carbonate 30g
Phosphorus 50g
Sodium silicate 10g
Water up to 1000ml
Solution temperature 60-80°C
Cleaning time 2-5min

Second, polishing

Polishing is a common method to overcome the surface defects of metal materials and improve the surface finish. Polishing can be broadly divided into mechanical polishing, chemical polishing and electrochemical polishing. In mechanical polishing, the surface of the material is rough-ground by a cloth wheel polisher and a polishing paste, and fine polishing is performed with a fine cloth wheel to make the surface of the material achieve the desired smoothness and cleanness. It is mostly used for decorative treatment of anodic oxidation stained signboards and spiral patterns, and for folding aluminum plates used for cursor marks. Mechanically polished aluminum plates sometimes require chemical polishing and electrochemical polishing in order to obtain good brightness and good oxidation. Chemical polishing is mostly carried out in acidic solutions. Electrochemical polishing is a higher surface finishing method that can achieve a higher “bright surface” effect and reduce atmospheric oxidation of the aluminum surface.

â–² Aluminum chemical polishing solution formulation and process conditions are as follows (for reference)
Phosphoric acid <relative density 1.7; 90% (w)> 75% nickel sulfate 2g/1000ml
Sulfuric acid<relative density 1.84; 98%(w)>15% solution temperature 100-115°C
Nitric acid <relative density 1.42; 65%(w)>10% polishing time 1-15min

â–²Acid electrochemical polishing solution formula and process conditions are as follows (for reference)
Phosphoric acid Relative density 1.70 90%
Glycerine (glycerine) 1%
Solution temperature 55-65°C
Voltage 12-20V
Current density 10-30A/dm2
Polishing time 1-4min

â–²Alkaline electrochemical solution formulations and process conditions are as follows (for reference)
Water 1000ml Solution temperature 80-85°C
Anhydrous sodium carbonate 300g Polishing time 3-5min
Sodium phosphate 100g Voltage 12V
Sodium hydroxide 30 g Current density 3-6 A/dm2
The preparation should be heated while stirring, completely dissolved before use.

â–² After electrochemical polishing, the following solutions and process conditions can be used to remove water 1000ml
Phosphoric acid Relative density 1.70 90% 35g
Chromium (chromium trioxide) 20g
Solution temperature 90-100 °C Time: go to the end

Third, drawing and spiral processing

According to the needs of decoration, drawing can be made into several types, such as straight grain, random grain, thread ripple and spiral grain.
Straight-line drawing means that the surface of the aluminum plate is machined with a straight line by mechanical friction. It has the dual role of scratches on the surface of the brushed aluminum plate and the surface of the decorative aluminum plate. There are two kinds of continuous wire drawing and interrupted wire drawing. The continuous thread can be obtained by continuously horizontally rubbing the surface of the aluminum plate with a scouring pad or a stainless steel brush. Change the wire diameter of the stainless steel brush to obtain different thickness lines. Intermittent threads are generally processed on a brush machine. Production principle: using two sets of differential rotating wheels with the same direction, the upper group is a rapidly rotating roller, the next group is a slow transfer rubber roller, aluminum or aluminum alloy plate passes through the two sets of rollers, is delicate brushing Intermittent Z ruled.

Random pattern drawing is a kind of irregular pattern under the high-speed operation of copper wire brush, which makes the aluminum plate move without friction. This kind of processing requires higher surface area for aluminum or aluminum alloys.

Ripples are generally made on brush machines and scribing machines. Using the axial movement of the upper set of grinding rollers, the surface of the aluminum or aluminum alloy plate is brushed to obtain a wave pattern.

Swirl, also known as optical rotation, is a silk pattern obtained by rotating abrasive polishing on a surface of an aluminum or aluminum alloy plate by using cylindrical felt or grinding stone nylon wheel mounted on a drill press, using kerosene to mix and polish the ointment. It is mostly used for decorative processing of round signs and small decorative dials.

The thread is a small motor fitted with a round felt on the shaft, which is fixed to the table and forms an angle of about 60 degrees with the edge of the table. Do not make a carriage with a fixed aluminum platen. Apply a straight line of polyester film on the carriage to restrict the thread width. With the rotation of the felt and the linear movement of the carriage, a thread pattern with the same width is spirally wound on the surface of the aluminum plate.

Fourth, sandblasting

Sandblasting is to obtain a film light decoration or a subtle reflective surface to meet special design needs such as soft gloss. At the same time, the combination of printed materials and substrates can be made stronger during screen printing. Sandblasting is usually carried out in a special sandblasting machine. According to the requirements of the thickness of the sand surface, select the appropriate number of quartz sand, sprayed into a suitable sand surface. Uniform and moderate sandblasting can basically overcome the common defects of aluminum surfaces.

Fifth, oxidation dyeing

The use of aluminum as a substrate, the use of oxidation dyeing process to produce silk screen signage is a more common method of application. Among aluminum and aluminum alloy materials, high purity aluminum, industrial pure aluminum No. 1-4, special aluminum alloy LT66, and famous magnesium alloy are preferred materials. Due to the high copper content of the hard aluminum above No. 5, the wrought aluminum alloy is not suitable for oxidative dyeing due to its silicon content. Before the oxidation, the relevant chemical treatment or electrochemical treatment is required. Oxidation according to its processing methods are divided into chemical oxidation and electrochemical oxidation. The chemical oxide film is thin (about 0.5-2 UM), the film quality is soft, the corrosion resistance and friction resistance are worse than the electrochemical oxide film, and the effect after dyeing is also poor. Chemical oxidation is divided into alkaline solution oxidation, phosphate-chromate oxidation and chromic acid oxidation. Electrochemical oxidation can also be divided into sulfuric acid anodization, chromic acid anodization, oxalic acid anodization and so on.

The formulation and process conditions of various types of oxidation solutions (solution temperature, current density, oxidation time, impurity content, etc.) are all important factors affecting the quality of the film. In order to obtain an ideal and satisfactory oxide film, in addition to selecting the correct formulation, the correct operation method and the solution maintenance and adjustment techniques summarized from practical experience should be adopted to meet the needs of production. The following examples illustrate typical formulations and process conditions for the type of anodizing solution.

1. General Decorative Sulfuric Acid Anodizing Sulfuric Acid 150-200G/L
Anode Current Density 0.8-1A/DM Square Operating Temperature Requirements 15-25 Degrees Celsius
Time 40-60MIN
Voltage 18-24V
2. High wear resistant hard film parts of sulfuric acid anodizing sulfuric acid 700-100G/L
Anode Current Density 0.5A/DM Square Operating Temperature (0±5) Celsius Time 15MIN
Voltage 20V
3. Chromic Acid Anodized Chromium Anhydride 50-60G/L Anode Current Density 1.5-2.5A/DM Squared
Operating temperature &nb