1. Plastic types
In order to facilitate research and use of plastics, it is necessary to classify plastics from different points of view. There are two generally accepted classification methods: first, according to performance of plastics after heating, plastics can be divided into two categories: thermoplastics and thermosetting plastics.
The molecular structure of polymers in thermoplastics is linear or branched, and they are often referred to as linear polymers. Amenable to molding when heated - a plastic part of a certain shape that retains a given shape after cooling. When reheated, it can be softened and melted, as well as reshaped into a plastic part of a certain shape, which can be repeated many times and is reversible. In molding process described above, there are usually no chemical changes, only physical changes.
Because thermoplastics are materials that can be repeatedly softened by heating and solidified by cooling, thermoplastics can be repeatedly solidified and formed by heating and melting, so waste thermoplastics can usually be recycled and reused, which is called "recycled material". .
(2) Thermosetting plastic
Thermosetting plastics also have a chain or dendritic structure at beginning of heating, are ductile and fusible, and can be molded into plastic parts of a certain shape. With continued heating, chemical bonds are formed between main chains of these chains or dendrimers, which gradually acquire a network structure (the so-called crosslinking reaction). When temperature rises to a certain value, crosslinking reaction proceeds further, and molecules finally turn into a solid structure, turning into a substance that does not melt or melt (the so-called solidification). When reheated, due to a chemical reaction between molecular chains, shape of plastic part is fixed and will not change. Plastic is no longer malleable until it chars at very high temperatures, which is irreversible. During molding process, both physical and chemical changes occur. Due to above characteristics of thermoset plastics, leftovers and waste products cannot be recycled.
It is clear that thermosets have better heat resistance than thermoplastics. Commonly used phenolic, melamine formaldehyde, unsaturated polyester, etc. are thermoset plastics. "
Because a hardened thermoset plastic cannot change its state even if it is heated further, it cannot go back into a molten state. Therefore, thermoset plastics cannot be reheated for molding, so replacement thermoset plastics are generally not recyclable.
Secondly, dependingSpecific uses and characteristics of plastics Plastics can generally be classified into three categories: general purpose plastics, engineering plastics, and specialty plastics.
(1) General Purpose Plastic
General purpose plastics generally belong to a class of plastics with high yield, wide application, relatively low performance and low price, such as: polyethylene, polypropylene, polyvinyl chloride, polystyrene, phenol plastics, aminoplasts, etc., they account for about 60% of total plastic production.
(2) Engineering plastics
Engineering plastics refer to plastics that can be used as structural materials. There is no clear boundary between them and general-purpose plastics. Engineering plastics have relatively good strength, impact resistance, heat resistance, hardness, and aging resistance. Some metal materials are used as engineering materials such as nylon, polycarbonate, polyoxymethylene, ABS, etc.
Refers to those plastics that have special functions and are suitable for special occasions, mainly including medical plastics, light-sensitive plastics, magnetically conductive plastics, superconductive plastics, radiation-resistant plastics, heat-resistant plastics, etc. Its main component is resin , some are specially synthesized resins, and some are made from aforementioned general purpose plastics and engineering plastics that have been specially treated or modified for special properties. This type of plastic has a small yield, excellent performance and a high price.
2. Forming characteristics of thermoplastics
The various properties of plastics related to molding process and molding quality are collectively referred to as processing properties of plastics. The degree of understanding and mastering of plastics process is directly related to whether plastics can be successfully molded and quality of plastic parts can be guaranteed. This also affects design requirements of mold. The following is main process of characterization and injection molding requirements of thermoplastics and thermosetting plastics.
In addition to thermodynamic properties, crystallinity, and orientation, properties of thermoplastics during molding also include shrinkage, flow, heat sensitivity, water sensitivity, hygroscopicity, and compatibility.
Plastic is usually formed by filling a mold cavity in a high temperature molten state. When a plastic part is removed from mold and cooled to room temperature, it will shrink from its original size in mold, a characteristic called shrinkage. It can be expressed as a percentage of plastic shrinkage per unit length, i.e. shrinkage (S).
Because thisShrinkage is not only caused by thermal expansion and contraction of plastic part itself, but also due to various molding process conditions and shape factors, shrinkage of plastic part after molding is called mold shrinkage. The change in size of a plastic part can be reduced or changed by adjusting process parameters or by changing mold structure.
Mold shrinkage can be divided into dimensional shrinkage and postshrinkage, both of which are directional.
① The size of plastic part is reduced. Due to thermal expansion and contraction of plastic parts, as well as physical and chemical changes inside plastic part, size of plastic part decreases after demoulding and cooling to room temperature, so this must be taken into account when designing forming parts of mold. ● Make a compensation to avoid exceeding size tolerance of plastic parts.
② Subsequent shrinkage of plastic parts. When plastic parts are formed, a series of stresses occur due to factors such as internal physical, chemical, and mechanical changes. After plastic parts are formed and solidified, residual stresses occur again. Shrinkage phenomenon. Generally, post-shrinkage of conventional plastic parts is relatively large within 10 hours of demoulding, and mold is mostly solidified after 24 hours, but it takes a long time to reach final shape. higher than thermoset plastics. The subsequent shrinkage of injection molded and injection molded plastic parts is greater than compression molded plastic parts.
In order to stabilize size of plastic parts after molding, sometimes according to performance and process requirements of plastics, plastic parts need to be heat treated after molding. After heat treatment, size of plastic parts also decreases, which is called post-treatment shrinkage. When designing molds for high-precision plastic parts, errors caused by subsequent shrinkage and post-processing shrinkage must be compensated for.
③Directional shrinkage of plastic parts. In plastic molding process, effect of orienting polymer along direction of flow will lead to anisotropy of plastic part, and shrinkage of plastic part will inevitably vary depending on direction: usually shrinkage occurs along direction of material flow. flow is large and strength is high, while vertical direction of material flow has little shrinkage and low strength. At same time, due to uneven distribution and density of additives in different parts of plastic part, shrinkage is also uneven, resulting in poor shrinkage of plastic part, which may cause warping, deformation, or even cracking of part. plastic part.