Hot-pressing technology and appearance quality defects of impregnated laminated veneer plywood and blockboard

Hot-pressing technology and appearance quality defects of impregnated laminated veneer plywood and blockboard

Core Tips: Hot pressing technology and appearance quality defects of impregnated laminated paper veneer plywood and blockboard. This paper introduces the raw material requirements, production process and hot pressing technology of impregnated laminated paper veneer plywood and blockboard at present in China, and analyses the causes of product defects. 1. Requirements for raw materials for impregnated laminated paper veneer plywood and blockboard 2. Adhesion of impregnated laminated paper veneer

Introduction

The raw material requirements, production process and hot pressing technology of impregnated paper veneer plywood and blockboard in China at present are introduced, and the causes of product defects are analyzed.

1 Requirements for raw materials for impregnated laminated paper veneer plywood and blockboard

2. Production process of impregnated laminated paper veneer plywood and blockboard

Hot pressing process of 3 Impregnated laminated veneer plywood and blockboard

Impregnated paper veneer plywood and blockboard veneer adopt hot-in and hot-out process. The hot-pressing process parameters include pressing temperature, unit pressure and hot-pressing time. They coordinate with each other. Taking “one-time film coating” as an example, the hot-pressing process is as follows:

1) Plate temperature. Temperature plays a catalytic role in the chemical reaction of impregnated resin, accelerating curing, shortening hot pressing period and improving efficiency. However, the high temperature makes the resin harden ahead of time without uniform flow, resulting in micro-voids on the surface of the board; the low temperature is easy to produce wet flowers on the surface of the board, and the bonding strength is not high. For impregnated veneer plywood and blockboard, it is more appropriate to control the temperature of the existing process conditions at 110-130 C. The heating system should adopt a heat conducting oil system with more uniform temperature for heating.

2) Unit pressure. Reasonable hot pressing pressure can ensure a good combination between the substrate and the film paper. Low pressure affects the bonding strength and resin flow ability of the base material and the film paper, and high pressure can easily reflect the defects of the base material on the surface of the film paper and cause the damage of the base material. Because the base material of impregnated plywood and blockboard is soft, the unit pressure of hot pressing is generally controlled in the range of 0.8-1.2 MPa. In addition, in order to make the hot pressing pressure more uniform, now the six cylinder hot pressing machine is widely used in eco-board manufacturing enterprises.

3) Hot pressing time. The length of hot pressing time depends on the curing speed of impregnated resin, which will cause excessive curing of resin, loss of elasticity, surface cracks or internal stresses, and cracks and warps in the subsequent processing; too short hot pressing time, inadequate curing of resin, easy to produce stickers and affect impregnated adhesive. The physical and chemical properties of the surface of laminated veneer plywood and blockboard ultimately affect the durability of impregnated veneer plywood and blockboard. Generally, the hot pressing time is 8-12 minutes.

4. Appearance Quality Defects of Impregnated Adhesive Paper Veneer Plywood and Blockboard

Common appearance quality defects of impregnated paper veneer plywood and blockboard include dry flowers, wet flowers, stratified bubbling, surface indentation, scratches, paper shortage, edge defects, bottom penetration, pollution, color mismatch, paper tearing and so on.

1) Dry flowers on board. Dry flower on board, also known as white flower, refers to the opaque white spots on the surface of products. The following reasons may arise: poor fluidity of resin or insufficient impregnation of resin; uneven surface of base material; high hot pressing temperature and low pressure; pollution of pressure plate, etc.

Dry flower is one of the main defects of eco-board. The main reason is that there are cracks or scars in the interior of the base material Blockboard or plywood in the process of preparation. During hot pressing, the stress produced by impregnated paper on the surface is inconsistent, and defects are formed when it is reflected on the surface.

2) wet flowers on board. Wet flowers on the board, also known as watermarks, are foggy marks on the surface of products. The following reasons may cause wet flowers: high volatile content of impregnated paper, low degree of pre-curing; low hot pressing temperature, short time; high moisture content of base material. For impregnated paper veneer plywood and blockboard, wet flower defect exists in products, but it is not common.

3) stratification and bubbling. The causes of stratified bubbling may be: too high hot pressing temperature and too long hot pressing time; too high moisture content of the base material, resulting in a larger vapor pressure in pressing the core layer of the plate; low density of the base material, poor compressive performance of the plate or too large hot pressing pressure. Therefore, delamination or bubbling defects can be avoided under the condition of controlling the moisture content of the substrate and hot pressing process.

4) Surface ripple. Surface ripple is a special surface defect of ecological board, which is the existence of wavy texture from the surface of the board. The main reason is that there are gaps between the veneer splicing inside the plywood and the small strips inside the blockboard, and the internal defects are reflected on the surface after hot pressing. The solution is to make the base material as good as possible, reduce or eliminate internal defects.

5) Surface cracks. The main reason is the surface crack caused by the inconsistency between the expansion and contraction of the base material and the impregnated film paper. Because the base material of the impregnated film paper veneer plywood and the blockboard is plywood and blockboard, the base material will wet or dry shrink when the external temperature and humidity change, but the surface impregnated film paper will shrink or wet up. It is inconsistent with the base material, so surface cracking occurs. This defect is a ubiquitous defect of products. Many products can only appear after several months or a long time, causing great losses to the producers.

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