1. Introduction

The finishing process is one of the most important processes in manufacturing timber engineered flooring. The complex painting not only determines the touch and visual profile of the finished flooring but also gives the flooring a solid and hard-wearing layer of protection. As the processing for different flooring is also different, one of the finishing projects is introduced in this article. This article will explain the function of the different processes and detail items in production. For the different flooring products or lacquers, the process design and procedure parameters will be slightly different.

2. Finishing Process and Equipment

There are 12 coating processes in the timber engineered flooring finishing phase, namely one water-based staining, one heavy putty, one light putty, three hardened primer lacquers, three stiffness primer lacquers, one abrasion-resistant primer lacquer, and two scratch-resistant top coatings.

The finishing line is divided into two sections: the front section includes water-based colouring and drying, UV putty filler and primer coating, followed by grooving and colour correction. The rear section consists of UV primer lacquer and top layer lacquer application. The separate processing design eliminates the embossed lacquer on the ends caused by the filler and reverse roller coating in the first section. The front and rear sections can work simultaneously without affecting production efficiency.

2.1. Grinding on the Unfinished Boards

As the surface of the flooring boards is still rough after thickness grinding, the subsequent water-based staining and putty filling may largely be affected by the small pores and burrs. The sanded flooring boards must be finely sanded before water-based staining to improve the surface fineness and enhance the coating adhesion.

2.2. Water-Based Staining

Water-based staining gives the flooring a uniform colour. Compared with solvent-based staining and padding, water-based staining has the advantages of good grain permeability, strong layering and being environmentally friendly. It can also enhance the adhesion of the coating film. For the rough top layer like white oak, red oak or brushed lamella, there are often all kinds of defects in the staining. The production line is equipped with a double-roller roller coater and a colour-evening brush machine to address these problems.

The water-based staining is applied at approximately 25 g/m2. The sponge roller in the double roller coater applies the colour agent to the board surface. The rubber roller flattens the colour blocks left by the sponge roller in the colouring, making it easier for the subsequent brushing to even out the colour.

The brush machine is equipped with two reverse rotating brush rollers with the bristles arranged at an angle to ensure that the brush filaments are in full contact with the board surface so that the colouring texture is naturally transparent.

The drying of the water-based colour agent occurs in the hot air drying channel. With the optimal parameters of working temperature and time, there is no moisture residue on the surface after drying, and a hand touching the surface is not stained. Otherwise, there will be a variety of defects, such as poor adhesion and sinking of the paint film.

After the water-based staining is dried, the burrs at the edges or surfaces that have not been removed by fine grinding will bulge again, affecting the subsequent painting. Therefore, the deburring process is usually arranged after the stain drying. The brush rollers of the deburring machine run at high speed to completely remove residual burrs.

2.3. Putty filling

To reduce the dosage of primer and top coating and prevent defects such as sagging and pinholes, putty is applied to the unfinished boards, divided into heavy putty (jelly-like) and light putty (fluid-like). Different types of putty are selected to match the requirements of different surface species and product categories. The amount and number of putty applied on the surface vary for different products.

Heavy putty, which has a high viscosity and solid content, is applied at 40-45g/m2 to fill in the pores and gaps by roller extrusion, providing a solid base for subsequent coatings. It is applied to the board surface by a rubber roller and then levelled with a chrome mirror-like roller.

The light putty is applied at a rate of 20-25g/m2 to fill in any remaining pores after sanding the heavy putty to level the board surface further. Also, the light putty is applied with a rubber roller and levelled with a chrome mirror-like roller.

2.4. Primer Coating Application

UV hardening primer lacquer

The UV hardening primer lacquer can increase the hardness of the coating effectively. In this process, three layers of UV hardening primer lacquer coats are equipped to improve the coating volume and the flatness of the paint surface.

The three coats are applied at 60g/m2, 20~25g/m2 and 30~35g/m2, respectively. Paint defect piling at the ends can be controlled by adjusting the distance between the rollers and the feed speed of the conveyor. To make the paint film evenly flow and spread, reducing the surface tension of the paint film and speeding up the solvent evaporation simultaneously, a set of IR heating ovens can be configured after the roller coater.

UV stiffness primer lacquer

The UV stiffness primer lacquer is often applied between the UV hardening primer lacquer and the UV abrasion resistant primer lacquer. This sandwich configuration can make the coating achieve optimal hardness and stiffness.

However, with the innovation of the lacquer formula, the stiffness of the primer layer is getting better and better without cracking. The process of stiffness primer lacquer has been gradually simplified.

UV abrasion resistant primer lacquer

The abrasion resistance of flooring has a direct impact on its service life. The abrasion resistance attributes to the functional coatings with wear-resistant particles evenly distributed in the primer or topcoat lacquer. The abrasion resistant primer applied is approximately 25 g/m2. An excessive amount of abrasion resistant primer can lower the transparency of the finish.

2.5. Top Coating Application

The UV top coating determines the appearance of the paint film and its physical properties. Using specific topcoat lacquers can improve the abrasion and scratch resistance of the product. There are two layers of topcoat in this process, the application of which are 7-8 g/m2 and 5-6 g/m2 separately. The coating is applied with a rubber roller and a quantitative chrome roller rotating inversely to improve the uniformity and fineness of the finish.

2.6. UV Curing

In the finishing process, the curing degree directly affects the adhesion, abrasion resistance hardness and the grinding performance of the finish. A photoinitiator in the UV coatings can promote the polymerisation of small organic molecules when irradiated with a certain wavelength of UV light to form a cured paint film.

UV curing is divided into semi-curing, surface curing, and full curing. The surface of the semi-cured paint film is slightly sticky and is usually followed by a roller coating. After surface curing, the surface of the paint film is not sticky, and nails can still scratch and leave traces like those usually followed by a grinding process. Full curing is mostly used after the top coat application process when all the properties of the flooring finish are fulfilled.

Depending on the curing requirements, the UV curing dryers must be equipped with the appropriate number of lamps to provide the illumination intensity.

2.7. Putty and Lacquer Grinding

After the putty has been applied, most of the paint is squeezed into the pores of the flooring lamella. It is essential to sand the boards after each putty application for better flatness and adhesion. The grinding depth is a very important process parameter, as poor grinding will affect the adhesion of the next coat, while too heavy a grinding will easily wear out and damage the staining layer.

The grinding of the primer should be performed in such a way as to ensure that the finished surface is as flat as possible while minimizing the grinding depth. The UV lacquer film itself is relatively hard. The optimal configuration of different grinding belts, including 320 and 400 mesh, and the feeding speed, can control the grinding performance on the surface.

3. Finish Properties

The finish properties are tested and evaluated based on the relevant standard. Generally, the abrasion weight loss should be no larger than 0.15 g/100r, with no wear-out on the abrasion ring. The hardness of the finish tested with a pencil hardness tester should be no less than 2H. The film adhesion is tested with a paint film scriber. There should be no large-scale peeling after being scratched and stripped by transparent tape.

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