Hot pressing process and appearance quality defects of impregnated paper-faced plywood and blockboard

Introduction

This paper introduces the raw material requirements, production process, and hot-pressing technology for the preparation of impregnated paper-faced plywood and blockboard in my country at the present stage, and analyzes the causes of product defects.

1. Requirements for raw materials used in impregnated paper-faced plywood and blockboard

2. Production process of impregnated paper-faced plywood and blockboard

3. Hot pressing process for impregnated paper-faced plywood and blockboard

Impregnated paper-faced plywood and blockboard veneer employ a hot-in, hot-out process. The hot-pressing parameters include platen temperature, unit pressure, and hot-pressing time, all of which are coordinated. Taking "one-time lamination" as an example, the hot-pressing process is as follows:

1) Pressing temperature. Temperature mainly catalyzes the chemical reaction of the impregnating resin, accelerating curing, shortening the hot pressing cycle, and improving efficiency. However, excessively high temperatures cause the resin to cure prematurely before it can spread evenly, resulting in micropores on the board surface; excessively low temperatures can easily cause wet spots on the board surface and low bonding strength. For impregnated paper-faced plywood and blockboard, the current process conditions with a temperature control of 110-130 ℃ are more suitable, and the heating system should use a heat-conducting oil system with a relatively uniform temperature.

2) Unit Pressure. Appropriate hot-pressing pressure ensures a good bond between the substrate and the adhesive paper. Insufficient pressure affects the bonding strength between the substrate and the adhesive paper, as well as the resin's flowability; excessive pressure can easily reflect substrate defects on the adhesive paper surface and also damage the substrate. Because the substrates of impregnated adhesive paper-faced plywood and blockboard are relatively soft, the unit pressure for hot pressing is generally controlled within the range of 0.8–1.2 MPa. Furthermore, to ensure more uniform hot-pressing pressure, most eco-board manufacturers now use 6-cylinder hot presses.

3) Hot-pressing time. The length of the hot-pressing time depends on the curing speed of the impregnating resin. If the time is too long, the resin will be over-cured, lose its elasticity, and easily produce surface cracks or internal stress, which will easily lead to cracks and warping in subsequent processing. If the hot-pressing time is too short, the resin will not cure sufficiently, which will easily cause the boards to stick together and affect the physical and chemical properties of the surface of the impregnated paper-faced plywood and blockboard, ultimately affecting the durability of the impregnated paper-faced plywood and blockboard. Generally, a hot-pressing time of 8 to 12 minutes is appropriate.

4. Appearance quality defects of impregnated paper-faced plywood and blockboard

Common appearance defects in impregnated paper-faced plywood and blockboard include dry spots, wet spots, delamination and bubbling, surface indentations, scratches, missing paper, edge defects, bleed-through, contamination, color mismatch, and paper tearing.

1) Dried spots on the board surface. Dried spots on the board surface, also known as white spots, refer to opaque white spots on the product surface. The following factors may cause dried spots: poor resin flowability or insufficient resin impregnation; uneven substrate surface; excessively high hot pressing temperature or low pressure; contamination of the pressing board, etc.

Dried flowers are one of the main defects of ecological boards. The main reason is that during the preparation of the base material, plywood or blockboard, there are defects such as gaps or knots inside. During the hot pressing process, the stress generated on the surface impregnated paper is inconsistent, which is reflected on the surface and forms defects.

2) Wet spots on the board surface. Wet spots, also known as water stains, are hazy marks on the product surface. The following factors may cause wet spots: high volatile content of the impregnated paper, low pre-curing degree; low hot-pressing temperature and short time; excessive moisture content of the substrate. For impregnated paper veneer plywood and blockboard, wet spot defects exist in the product, but are not common.

3) Delamination and blistering. Delamination and blistering may be caused by: excessively high hot-pressing temperature or time; excessively high moisture content in the substrate, resulting in significant steam pressure in the core layer during pressing; low substrate density, leading to poor compressive strength or excessive hot-pressing pressure. Therefore, defects such as delamination and blistering can be avoided by controlling the substrate moisture content and the hot-pressing process.

4) Surface ripples. Surface ripples are a unique surface defect of eco-friendly boards, appearing as wavy patterns when viewed from the surface. The main cause is gaps between the plywood and blockboard substrates during veneer splicing or between small wood strips within the blockboard. After hot pressing, these internal defects are reflected on the surface, forming the ripple. The solution is to improve the quality of the substrate as much as possible to reduce or eliminate internal defects.

5) Surface cracking. The main cause is the inconsistent expansion and contraction between the substrate and the impregnated paper, resulting in surface cracks. Since the substrate of impregnated paper-faced plywood and blockboard is plywood or blockboard, it expands or contracts under changes in external temperature and humidity. However, the expansion and contraction of the surface impregnated paper does not match that of the substrate, thus causing surface cracking. This defect is common in products, and in many cases, it takes months or even longer to become apparent, causing significant losses to manufacturers.

in conclusion

Developing a new technology or product is extremely challenging, requiring significant investment of funds and the wisdom and dedication of R&D personnel. Only when a new product's performance fully meets expectations and its quality stabilizes can it achieve longevity. Impregnated paper-faced plywood and blockboard, products created by Chinese manufacturers, are currently enjoying an increasingly large market share in the interior decoration and furniture industries due to their superior characteristics. However, the development of impregnated paper-faced plywood and blockboard is relatively recent, and some companies still have varying degrees of quality issues. It is crucial for impregnated paper-faced plywood and blockboard manufacturers and quality inspection agencies to strictly control quality and urgently implement technological innovation and improvement.