What is the principle and classification of roll forming? Roll forming is a kind of hollow forming process of thermoplastic. Its principle: the resin is added into the mold, the mold is closed, and the mold is rotated continuously by heating the mold and using the two right angle intersecting rotating shafts at the same time to make the mold rotate / roll in three dimensions. The resin evenly fills the mold cavity with its own gravity and gradually melts until the resin is completely melted and evenly adheres to the inner wall of the mold cavity, and then stops heating and turns to cooling After the products are cooled and consolidated, they are demoulded to obtain the required seamless hollow products.
The rotational speed of the mould is not high during the rolling process, and the products are all formed by the natural flow of resin, so the products are almost free of internal stress and are not prone to deformation, depression and other defects.
According to the different forms of resin used, roll molding can be divided into three categories.
1. The dry powder roll molding represented by polyethylene resin is the most widely used category in the roll molding process. Due to the forced extrusion of the resin from powder to melt without external force, the air bubbles formed between the powder particles in the melt can only grow up by their own aggregation, and then be removed from the melt. Whether the bubbles can be completely discharged directly affects the physical and mechanical properties of the products.
2. PVC paste resin roll molding industry is commonly known as plastic lining. Mainly produce soft PVC hollow products.
3. The roll molding resin monomer formed by monomer polymerization enters the mold cavity, and the polymerization and flow molding are completed under the appropriate temperature conditions and the effect of auxiliary agents. In comparison, the process is currently relatively less practical application.
The roll forming process has wide applicability. It can form seamless hollow products such as table tennis, yacht or submachine, or hundreds of cubic chemical tanks; the wall thickness of products can be adjusted from less than 2mm to several centimeters without changing the mold; it can complete the manufacturing of complex structural products in one step; it can add inserts or other assemblies in the products according to the design needs.
The rolling mould is simple in manufacture and low in cost, generally only 1 / 4 ~ 1 / 3 of the cost of other moulds of the same size; the rolling mould is especially suitable for the manufacture of large-scale products, which can realize the edge thickening to ensure the edge strength of large-diameter products; in addition, the rolling mould has no flash and less waste of materials. The disadvantages of rolling technology are: long production cycle, materials need to be ground in advance, more manual operation, high labor intensity, etc.
As an example, the whole process can be divided into four parts.
1. In the feeding process, clean the inner wall of the mold, measure the feeding materials according to the requirements, then close the mold, and prepare for the subsequent rolling operation.
2. In the forming process, the mold and rotation system enter the designated hot forming station, and turn on the vertical rotation to make the mold rotate and roll in three dimensions, and then heat it from the outside of the mold (hot air circulation, electric radiation, or open fire baking can be used) until the process setting temperature (260 ~ 320 ℃), keep the temperature for a certain time, to ensure that the melt adheres completely on the mold cavity wall, the thickness is uniform, and the bubbles are exhausted.
3. The cooling process maintains the three-dimensional rotation and rolling state of the mold, and turns to the cooling station with cold air. Spray and spray are used to cool the mould and products, so that the product is completely consolidated and finalized.
4. In the demoulding process, stop the mold rotation, turn to the loading / unloading station, open the mold and take out the products. Clean the inner wall of the mold and prepare for the next cycle.
The pigment products used in roll forming products should pay more attention to heat resistance, dispersibility, light resistance, weather resistance, safety and other characteristics.
1. Heat resistant roll forming is a non forced forming process. The melt is required to have very good flow performance;
The heat conduction form is a one-way natural transfer mode for resin, without friction and shear heat generation; the bubble contained in the melt needs enough time to accumulate and break the bubble naturally. Therefore, the processing temperature is much higher than other molding processes of the same kind of resin. Similarly, the operation time at this high temperature is much longer than that of other processes. If nitrogen is not used as protection in the mold cavity during rolling, the oxidation effect at high temperature must also be considered. Therefore, all the pigments must have excellent heat resistance, otherwise the quality of the products can not be guaranteed.
2. The dispersibility can be seen from the rolling process. If the resin system is not processed with the mixture in advance, it is only used for simple dry powder mixing (currently, quite a number of manufacturers use this method). Therefore, the pigment powder particles have not been effectively dispersed in the whole process. If the powder particles are coarse and hard to disperse, they will cause defects such as color spots and affect the product quality.
3. Light and weather resistant roll forming products are widely used outdoors, especially for yachts, piers and large toys. Therefore, the pigments used in these products must have excellent light and weather resistance.
4. A large number of civil products and toys in safety rolling products are directly in contact with human body, especially children's toys. Therefore, we must pay attention to the safety of pigment products.
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