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

Introduction

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

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

2 Production process of impregnated paper-faced plywood and blockboard

3 Hot pressing process of impregnated paper veneer plywood and blockboard

The hot pressing process is used for the veneer of impregnated film paper veneer plywood and blockboard. The hot pressing process parameters include the pressing plate temperature, unit pressure, and hot pressing time. The three parameters are coordinated with each other. Taking "one-time lamination" as an example, the hot pressing process is as follows:

1) Platen temperature. Temperature primarily catalyzes the chemical reaction of the impregnating resin, accelerating curing, shortening the hot pressing cycle, and improving efficiency. However, excessively high temperatures prevent the resin from evenly distributing itself and causing premature curing, resulting in micropores on the board surface. Excessively low temperatures can easily cause wetting and weaken the bond strength. For impregnated adhesive-faced plywood and blockboard, a temperature range of 110-130°C is ideal under current process conditions. The heating system should utilize a thermal oil system for a more uniform temperature.

2) Unit pressure. A reasonable hot press pressure ensures a good bond between the substrate and the film. Too low a pressure can affect the bond strength between the substrate and the film and the resin's ability to spread. Too high a pressure can easily reflect substrate defects on the film's surface and damage the substrate. Because the substrates of impregnated film-faced plywood and blockboard are relatively soft, the unit pressure of the hot press is generally controlled within the range of 0.8 to 1.2 MPa. Furthermore, to achieve more uniform hot press pressure, eco-board manufacturers now generally use a six-cylinder hot press.

3) Hot-pressing time. The length of the hot-pressing time depends on the curing speed of the impregnating resin. If the hot-pressing time is too long, the resin will over-cure, lose its proper elasticity, and easily produce surface cracks or internal stress, which can easily cause cracks and warping during subsequent processing. If the hot-pressing time is too short, the resin will not cure fully, which can easily cause sticking and affect the physical and chemical properties of the surface of the impregnated film-faced plywood and blockboard, ultimately affecting the durability of the impregnated film-faced plywood and blockboard. Generally, the hot-pressing time should be 8 to 12 minutes.

4 Appearance quality defects of impregnated paper veneer plywood and blockboard

Common appearance quality defects of impregnated film paper veneer plywood and blockboard include dry flowers, wet flowers, delamination and bubbling on the board surface, surface indentations, scratches, paper missing, edge defects, bottom showing, contamination, color mismatch, paper tearing, etc.

1) Surface blemishes. Surface blemishes, also known as white spots, refer to opaque white spots on the product surface. Possible causes include: poor resin flowability or insufficient resin impregnation; uneven substrate surface; excessively high hot pressing temperature or low pressure; and contaminated press plates.

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

2) Surface blemishes. Surface blemishes, also known as water stains, are hazy marks on the product surface. The following reasons may cause blemishes: high volatile content in the impregnated paper, insufficient pre-curing, low hot pressing temperature and time, or excessive moisture content in the substrate. Surface blemishes can occur in impregnated paper-faced plywood and blockboard, but they are not common.

3) Delamination and blistering. Delamination and blistering may be caused by: excessively high hot pressing temperatures or prolonged hot pressing times; excessively high substrate moisture content, resulting in high steam pressure in the core layer of the sheet during lamination; low substrate density, poor sheet compression resistance, or excessive hot pressing pressure. Therefore, defects such as delamination and blistering can be avoided by properly controlling the substrate moisture content and hot pressing process.

4) Surface waviness. Surface waviness is a unique surface defect found in eco-friendly boards, creating a wavy texture when viewed from the surface. This defect is primarily caused by gaps between the veneers of plywood and blockboard, or between the small wood strips within blockboard. After hot pressing, these internal defects are reflected on the surface, creating this defect. The solution is to optimize the base material to minimize or eliminate these internal defects.

5) Surface cracking. This is primarily caused by a mismatch between the expansion and contraction of the substrate and the impregnated paper. Because the substrates of impregnated paper-faced plywood and blockboard are plywood and blockboard, they expand or contract when exposed to temperature and humidity fluctuations. However, the surface impregnated paper shrinks or expands inconsistently with the substrate, causing surface cracking. This defect is common in many products, often taking months or even longer to manifest, resulting in significant losses for manufacturers.

in conclusion

Developing a new technology or product is a challenging endeavor, requiring significant investment and the dedication and wisdom of R&D personnel. Furthermore, a product's longevity is only guaranteed when its performance fully meets expectations and its quality remains stable. Impregnated paper-faced plywood and blockboard, developed by Chinese manufacturers, are currently enjoying a growing market share in the interior decoration and furniture industries due to their superior properties. However, their development history is relatively short, and some manufacturers still experience quality issues. This requires strict quality control by manufacturers and quality inspection agencies, and urgent technological innovation and improvement are crucial.