Research on Pulp Foaming Technology

Abstract : The paper studied the foaming technology of pulp slurry molding products, and put forward the method of low foaming of pulp products. A preliminary study was conducted on the properties of the foam tray and the structural features of the foam tray.

Keywords :pulp; paper tray; suction molding; foaming

1. Introduction of suction molding process

Pulp suction molding products are called paper trays. It is an internal packaging material that is researched and developed in recent years to replace the foamed plastic products in research and development of mechanical and electrical product packaging to solve the problem of “white pollution” and is an environmental protection product. The production process of the paper tray includes four major processes of pulping, suction molding, drying and hot pressing.

Pulping: Place waste paper such as waste newspapers and waste paper boxes into the pulper barrel, and add appropriate amount of clean water to immerse waste paper. Under the action of the tearing of the impeller rotating at a high speed in the pulper, the waste paper becomes pulp having a certain degree of fiber, and this kind of pulp is called a thick paste. The coarse pulp is removed by sand removal and screening to become seminal plasma. The seminal plasma is diluted with an appropriate amount of water, and additives such as binders, moisture-proofing agents, reinforcing agents, and colorants are added to prepare a working slurry.

Slurry molding: The working surface of the suction die is immersed in the working slurry, and a layer of pulp is adsorbed on the working surface of the die by vacuuming. Then, the mold was taken out of the working slurry, a portion of the water was drained, and the mold was released to make a wet blank of the paper tray.

Drying: The wet blank still contains about 60% of water, which is placed on a drying line and dehydrated by hot air to dry into a blank.

Hot-pressing and shaping: In the drying process, the blank is deformed due to baking and shrinkage, and the shape of the blank needs to be corrected by a hot-pressing shaping mold.

The paper tray is formed by the adsorption of a thin pulp on the working surface of a mold having a certain geometric shape. It has a certain strength, stiffness and cushioning properties and can therefore be used to support and protect packaged items. Due to the low stiffness and strength of the cardboard, although the geometrical concave-convex design can increase the stiffness and strength of the entire product, the increase is limited. Especially for strength, the increase is smaller. Therefore, a general paper tray is only suitable for use as an inner packaging material for articles of relatively small weight and volume.

2. Research on Pulp Foaming Technology

Can paper trays be used as inner packaging materials for articles with a large weight and volume?

Increasing the thickness of the paperboard can increase the strength of the paper tray, but at the same time its rigidity is greatly increased. With excessive stiffness, the cushioning properties of the trays are reduced, thereby also reducing the protection of the support. Pulp foaming, in the case of an increase in the thickness of the paperboard to improve the strength of the paper tray, so that the paper tray still has better flexibility and greater ability to compress deformation, which can provide reliable protection of the packaged items.

Due to the short fibers of the pulp, only low foaming can be used. Otherwise, the strength of the tray will be greatly reduced. Also, foaming should be done at a lower temperature (usually should not exceed 150°C), otherwise the paper will be scorched. Therefore, the pulp foam should be low temperature and low foaming.

Low foaming causes only slight changes in the shape and size of the blank, and the shape and size of the tray require less precision. Therefore, the foaming can be performed outside the cavity as long as the foaming pair is properly considered in the design. The influence of the shape and size of the tray can produce products that meet the requirements. Therefore, foaming can be performed during the drying process.

The cardboard thickness of the foamed paper tray is large. In order to improve the drying efficiency, in addition to adding additives such as binders, reinforcing agents, moisture-proofing agents, etc., an appropriate amount of heat-conducting agent should be added to make it faster and better. Dry and dry.

The low-temperature foaming agent is lighter than water, and the amount of waste paper adsorbed directly is not sufficient to achieve a good foaming effect. In order to evenly mix enough foaming agent in the working pulp, the foaming agent is first mixed with the dried fine wood chips, so that the foaming agent is adsorbed on the fine wood chips, and then put together in the working pulp to stir, so that good results can be obtained. Effect.

3 Experimental results and analysis

3.1 Tensile experiment

Cut 20mm wide and 100mm long specimens with a thickness of 10mm. Both ends of the specimen were clamped with clips and the breaking strength was measured on a tensile testing machine. The test results are shown in Figure 1.

Figure 1 Tensile test Figure 2 Drop test

The tensile breaking strength of the paperboard decreases as the blowing agent content increases. When a critical value is reached, the fracture strength begins to drop dramatically. The reason for this is that the degree of foaming increases and the bonding strength between the fibers decreases. In particular, when the degree of foaming exceeds a certain level, some of the fibers are detached from each other and the strength is drastically decreased.

3.2 Drop experiment

Put a 10mm thick foam board on the bottom of a 1000ml glass bottle filled with water for a free-fall drop test to test the maximum drop height.
The experimental results are shown in Figure 2.

When the degree of foaming is not too large, the maximum drop height increases as the degree of foaming increases. This means that the cushioning properties of the board increase as the degree of foaming increases.
However, when the foaming degree reaches a critical value, the maximum height of the drop decreases as the degree of foaming increases. This shows that the cushioning properties of the cardboard are reduced as the degree of foaming increases.

Because the small pores generated by the foaming increase the compressive deformation ability of the paperboard, at the same time, the elastic modulus of the paperboard is reduced and the flexibility is increased. Therefore, when the foaming degree is not great, the buffering performance of the paperboard increases with the increase of the foaming degree. However, when the foaming degree exceeds a certain critical value, the elastic deformation ability of the paperboard is significantly reduced due to the sharp drop in the bond strength between the fibers, and thus the buffering performance thereof decreases as the foaming degree increases.

4 Structural design of foamed paper tray

The structure of the foamed paper tray is between the structure of the foamed plastic product and the structure of the unfoamed paper tray. The strength of the paper tray is increased by appropriately increasing the thickness of the paperboard, and the stiffness, cushioning properties, and strength of the paper tray are improved by a simple geometric design. In order to uniform the drying speed of the various parts of the paper tray, the thickness of the paperboard should be uniform. At the same time, in order to facilitate demoulding, the geometry should be as simple as possible and the dimensions in the height direction should be as small as possible.

The thickness δ of the cardboard is generally 3 to 16 mm. If it is too thick, the difference between the drying speed of the surface layer and the inner layer is too great, and the drying speed is very slow.

The geometric shape is designed as a perforated plate structure, as shown in Fig. 3, or a flat bottomed plate structure with edges. Generally try to avoid the use of bubble-like structure, as shown in Figure 4.
If a bubble-like structure needs to be provided, the lateral dimension of the bubble should be appropriate and the height dimension should be as small as possible. In general, take the horizontal dimension L = (10 ~
15) δ, L ≥ 20mm; take the height dimension h = (3 ~ 5) δ, h ≤ 60mm. The side angle α should be larger for mold release. However, the slope is too large, which greatly reduces the bearing capacity of the bubble. Generally take the angle α = 20 ° ~ 30 °.

Fig.3 Porous plate structureFig.4 Bubble structure

5 Conclusion

The foamed paper tray can support packaged articles with a relatively large weight and volume, which greatly expands the application range of the paper tray.

Reprinted from: China Carton Network