Under certain conditions, the profile obtained by one-time molding of polymer materials (such as plates, sheets, rods, tubes) is re-molded and processed to obtain the final shape of the product.
Introduction
Secondary molding is one of the methods of plastic molding processing. A method that uses plastic profiles or parisons as raw materials to turn them into products of the desired shape through heating and external force.
Secondary molding is to heat plastic profiles such as sheets, tubes, and plates obtained by one-time molding to a rubber-like state, deform them through external force and shape them into various simple shapes, and then cool and shape them to obtain products. Secondary molding includes hollow blow molding, thermoforming, tenter film molding and other methods, and is only suitable for the molding of thermoplastic plastics. Secondary molding is a method of molding based on primary molding.
Secondary molding processing principle
For amorphous polymers whose Tg is much higher than room temperature: heat between Tg-Tf, then deform it into a certain shape; after the deformation is completed, place it at near room temperature to cool and set.
For partially crystalline polymers : heating and molding near the melting point, and mainly relying on crystallization after molding.
Secondary molding conditions
Secondary molding conditions: molding temperature, setting temperature, molding speed
Molding temperature: The temperature for secondary molding is the temperature at which the polymer can deform and has the maximum elongation. At this time, the temperature rises and transitions to a high elastic state. Since the chain segments begin to move and the viscosity of the system is very high, the frictional resistance to the movement of the chain segments is relatively large. The high elastic deformation lags significantly behind the stress change, and the internal friction is also large. Generally, the optimal molding temperature of amorphous thermoplastics is slightly higher than its Tg. For example, the optimal molding temperature of hard polyvinyl chloride (Tg=83℃) is 92~94℃, and the optimal molding temperature of polymethyl methacrylate (Tg=105℃) The temperature is 118℃.
Setting temperature: As the setting temperature decreases, the recoverable deformation decreases and the residual deformation (effective deformation) increases, so it is best for the setting temperature to be lower than Tg. Under the same setting temperature, the higher the molding temperature, the greater the residual shape variable obtained, and the better the dimensional stability of the product; however, the elongation of the product has a maximum value at this time. When the molding temperature is too high, the elongation will decrease. Unstable phenomena will occur. Under the action of high temperature and low speed, the product will soften and decompose due to heat, resulting in cracks and other phenomena in the product.
Molding speed: refers to the time required to complete a certain deformation or the amount of deformation completed within a certain period of time. It is related to the molding temperature, and the room temperature effect of the polymer must be considered. Below Tg, slow molding can obtain high elongation; above Tg, rapid molding can obtain higher elongation.
Methods
Mainly include the following molding methods:
Thermoforming
Thermoforming is a molding method in which thermoplastic plastic sheets are heated until softened and turned into products using appropriate molds or fixtures under gas pressure, liquid pressure or mechanical pressure. There are many methods for plastic thermoforming, which can generally be divided into:
Compression molding uses a single mold (male or female mold) or a pair of molds, and uses external mechanical pressure or self-weight to form sheets into various products. It is different from the compression molding of one-time processing. This method applies to all thermoplastics.
Differential pressure molding uses a single mold (male or female mold) or a pair of molds, or without a mold. Under the action of gas differential pressure, the heated to soft plastic sheet is made to adhere to the mold surface, and is then cooled to make various products. molding method. Differential pressure molding can be divided into vacuum molding and air pressure molding.
Thermoforming is particularly suitable for the manufacture of products with thin walls and large surface areas. Commonly used plastic varieties include various types of polystyrene, plexiglass, polyvinyl chloride, ABS, polyethylene, polypropylene, polyamide, polycarbonate and polyethylene terephthalate, etc.
Thermoforming equipment includes clamping systems, heating systems, vacuum and compressed air systems, and forming molds.
Biaxial stretching
In order to reorient the molecules of thermoplastic films or sheets, a biaxial stretching process is performed above the glass transition temperature. Stretching and orientation should be carried out between the glass transition temperature and melting point of the polymer. After directional stretching and rapid cooling to room temperature, the mechanical properties of the film or monofilament in the stretching direction are greatly improved.
Polymers suitable for directional stretching include: polyvinyl chloride, polyethylene terephthalate, polyvinylidene chloride, polymethyl methacrylate, polyethylene, polypropylene, polystyrene and some benzene. Ethylene copolymer.
Solid phase molding
Solid-state molding is a method in which thermoplastic profiles or blanks are shaped into products using a mold under pressure. The molding process is below the melting (softening) temperature of the plastic (at least 10-20°C below the melting point). All are solid phase molding. Among them, the processing of amorphous plastics in the high-elastic region above the glass transition temperature and below the melting point is often called thermoforming, while the processing below the glass transition temperature is called cold forming or room temperature molding, and is also often called plastic molding. Cold working method or normal temperature plastic processing.
This method has the following advantages: short production cycle; improved toughness and strength of products; simple equipment, capable of producing large and super-large products; and reduced costs. The disadvantages are: it is difficult to produce products with complex and precise shapes; the production process is difficult to control, and the products are easy to deform and crack.
Solid phase forming includes: sheet rolling, deep drawing or sheet stamping, hydroforming, extrusion, cold stamping, roller forming, etc.
