Common problems and solutions in the production of pulp molded products

As environmental awareness grows and the range of pulp molded products becomes more comprehensive, their applications become more and more widespread. At UV Tableware, we produce compostable tableware made from bagasse pulp mainly, which is just one of the categories of pulp molded products. With the diversification of pulp molded products, we often see pulp molded products in high-grade gift packaging, industrial packaging, medical accessories, and other fields.

For those who are concerned about the pulp molded industry and the green aspects, they often discuss common problems in the production of pulp molded products, and for this topic, UV Tableware engineers participated in a forum in the industry, and after discussion and analysis, we made a brief overview from the following aspects.

Uneven thickness of pulp molded products

Common problems

1. The depth of the mold sinking into the pulp is not enough.
2. The opening rate of the molding art mold is not appropriate. The molding process of pulp molding products is a large three-dimensional papermaking process, which makes the molding die have the difference of top angle, edge, surface height, and tilt degree. If the opening rate is not appropriate, will cause the thickness of the product is not uniform.
3. Backflush water time is too short. If the backflush time is too short, the slurry in the grouting tank cannot be well stirred, and the uneven slurry leads to uneven fiber deposition on the surface of the molding mold, which reduces the product qualification rate.

Solutions

1. Adjust the depth of the mold sinking into the slurry (H), so that it meets the relationship H>=2h with the shape height of the product (h), so that the thickness of the product slurry distribution is consistent.
2. In order to make the slurry deposited on the mold evenly distributed, the value of the opening rate of each part of the dewatering hole on the mold is different. In general, the opening rate of the top corner is about 70%, the opening rate of the ribs is about 60%, the opening rate of the upper top surface is about 40%, and the opening rate on the bevel is more complicated, generally, the upper opening rate is high, between 35% and 50%, and the lower opening rate is low, about 20%, so as to make the slurry deposited on the mold more evenly distributed.
3. Determine the appropriate backflush water time.

Insufficient strength of pulp molded products

Common problems

1. The structure design of pulp molded products is not reasonable, the number of tendons on the bearing surface is not enough.
2. The ratio of the pulp material is not suitable.

Solutions

1. Increase the number of tendons, but do not pursue the number of increases, should ensure that the width of the tendons before the slope is not less than 12mm, so as to make the paper mold products have sufficient strength.
2. For industrial package products, the strength of corrugated pulp is better than that of other pulp products. Therefore, it is an effective way to improve the strength of paper mold products by appropriately adjusting the pulp ratio of paper mold products according to needs, i.e., appropriately increasing the proportion of corrugated paper. For example, under the same circumstances, the strength of a product made from a 7:3 ratio of corrugated paper to the newspaper is significantly higher than that of a product made from a 5:5 ratio of the other two kinds of waste pulp.
3. If the structure is reasonable and the pulp is fixed, try adding talcum powder to the pulp. The use of talc as filler can not only improve the strength of the product, but also achieve the purpose of high retention rate, but must pay attention to the choice of adding process, talc can not be directly added to the pulp pool, must be used to filler pretreatment system, so that the talc is fully dispersed after wetting added so that the role of chemical additives to make talc complexation with fibers. The specific process is: with 20% to 30% of the water suspension containing talcum powder seasoning in the mixer at high-speed water treatment for 6min, dilute with water and add chemical additives, and then processed for 5min, made of filler suspension, and then added to the pulp.

The water content of pulp molded products is large after forming

Common problems

1. Excessive beating leads to excessive fiber cutting.
2. The adsorption time is too short, resulting in high moisture content of the paper mold molding wet blank, the product is more difficult to demold.
3. The vacuum degree is too low. If the vacuum degree is too low, the pressure difference between the inner cavity of the molding mold and the suction hood outside the molding mold decreases, and there is not enough pressure difference to pump the molding mold, resulting in a large amount of water remaining in the molded wet blank at the end of adsorption.
4. The backflushing time is too long. If the backflushing time is too long, it will not only affect the adsorption effect, but also increase the water content of the formed wet billet, and even make the demolding of the subsequent process can not be carried out normally.

Solutions

1. When dealing with waste paper, try to maintain the fiber strength, on the basis of complete sparging, do not excessively increase the pulping degree, avoid excessive fiber cutting, in order to ensure the smooth filtration and demolding of the wet billet formed by the paper mold.
2. Moderately extend the adsorption time. Too long adsorption time will make the production efficiency lower and lead to a decrease in the overall vacuum. In addition, it will also waste a lot of energy, because after adsorption to a certain extent, the water content of the paper mold forming wet blank no longer decreases, and must rely on the later production process to further reduce the water content.
3. Increase the vacuum level appropriately. If the vacuum level is too high, the pressure difference between the inner cavity of the molding mold and the absorbing cover outside the molding mold becomes large, then there is enough pressure to suck the water in the molding mold, but the water content of the paper mold forming wet blank is too low and affects the hot pressing process, so that the quality of the product after hot pressing is light and poor in strength.
4. Appropriate to shorten the backflushing time. If the backflushing time is too short, the slurry in the grouting tank can not be stirred very well, and the uneven slurry will lead to uneven fiber deposition on the surface of the molding die and reduce the qualified rate of the product.

Serious deformation of pulp molded products

Common problems

1. Due to improper mold design, the local thickness of paper mold products is too large, causing uneven drying of paper mold products and deformation.
2. Paper mold products dry too quickly or unevenly heated. Under the condition of reasonable mold design, the paper mold products are often deformed, which is caused by too rapid drying or the surface of one party is heated at a higher temperature than other parties.

Solutions

1. Under the condition of not affecting the packaging function of paper mold products, improve the design of the mold to eliminate the phenomenon of over-thickness in the paper mold, and add some anti-deformation structures to improve the situation.
2. The temperature is relatively high in the early stage of drying, and the temperature should drop slowly near the midpoint, In the case of conditions allowed to configure a longer drying tunnel, as far as possible to reduce the drying temperature, extend the drying time to make the paper mold products uniform heating.
3. In order to reduce the deformation of paper mold products, can use more mechanical pulp, because its shrinkage rate is less than other viscous chemical pulp, and can reduce the paper mold products of the burr phenomenon.

Pulp molded products and the surface of the packaged product after the surface friction generated hairy debris   

Common problems

1. The raw material of industrial packaging products is generally waste paper, After drying in the production process, the fiber strength is lower than the original fiber, resulting in poor adhesion between the fiber products, once the friction, some fine fibers will fall off.
2. The tightness of the product is too low, causing the contact between the fibers is not close, affecting its hydrogen bonding during drying, resulting in poor bonding between the fibers, and after friction with the surface of the packaged product, the fine fibers are easy to fall off.

Solutions

1. Before forming, add 1%~2% of cationic starch to the slurry to increase the bonding strength between the fibers, mix well, and then send it to the forming machine for forming.
2. Add a set of cold pressing equipment, and the wet paper mold blanks after molding will first go through a cold-pressing process to increase the contact area between fibers.
3. Control the moisture of the paper mold blank before shaping at about 15%~20%, increase the pressure when shaping, and raise the shaping temperature to 140~150℃ if necessary.
4. Spray some chemical reagents on the surface of the product before hot pressing, which can make the surface of the product smooth and clean.
5. Coating film on the surface of the product can solve the problem of paper molded products falling off after friction, but the cost of the product will increase and is not conducive to recycling.

The above content is a summary of the problem points for the whole pulp molded category, for pulp molded tableware this sub-category, production of common problems and solutions, the follow-up will be a separate statement.

For more details, please contact UV Tableware for an in-depth discussion, thank you.