Serialization of Practical Guide to Rubber and Plastics Technology1.2 Fundamentals of Polymer Molding and Processing
For professionals involved in polymer materials, it is necessary to understand basics of forming and processing polymers. The following is a brief introduction to basics of polymer molding.1.2.1 Introduction to polymer structure
The structure of polymer molecular chain is shown in Figure 1.1.
(a) Linear macromolecules: Also known as linear polymers or thermoplastic polymers, they can be heated and cooled repeatedly.
(b) Branched-chain macromolecules: Also known as branched-chain polymers, they can usually be heated and cooled repeatedly.
(c) Bulky macromolecules: also known as bulky polymers or thermoset polymers. , insoluble and infusible.
Figure 1.1 Schematic diagram of polymer molecular chain structure
The physical state of polymer molecules is classified into glassy state (Tb-Tg), highly elastic state (Tg-Tf), viscous liquid state (Tf-Td), as shown in figure 1.2.
Figure 1.2 Schematic diagram of physical state of polymer molecules
The description is as follows: curve 1 is a crystalline polymer and curve 2 is an amorphous polymer.
Temperature embrittlement (Tb): this is lower temperature limit of polymer, because at this temperature it is easily destroyed by force.
Glass Condition (Tb-Tg): This is operating temperature of product. The higher Tg, higher ability of product to adapt to environment.
Highly elastic state (Tg-Tf): although physics is a solid state, under action of an external force, it can get a large high elastic deformation (does not correspond to "Hooke's theorem"), and plastic deformation will occur. when external force acts for a long time, for product packaging, blow molding, film (or fiber stretch) and other molding processes.
Viscous flow state (Tf - Td): commonly referred to as ductile strain (expressed by injection molding, extrusion, calendering, injection molding) used to measure efficiency of plastic injection molding. Low Tf promotes melting and less heat consumption during production; temperature range between Tf and Td is large, thermal stability of plastic melt is good, it can deform and flow in a relatively wide temperature range, and it is not easy to decompose, and it can also be said that temperature range of injection molding is very wide.
Glass transition temperature, viscous flow temperature, thermal decomposition temperature and melting point of commonly used plastics are shown in Table 1.2
Table 1.2, Glass Transition Temperature, Viscous Flow Temperature, Thermal Decomposition Temperature, Melting Temperature of Commonly Used Plastics1.2.2 Processing Properties of Polymers
A, total heat capacity, specific heat capacity, latent heat of fusion
(a) Total Heat Capacity: The heat required for a given mass of polymer (or a given polymer system) to rise from its initial state to unit temperature is called total heat capacity of polymer.
(b) Specific Heat Capacity: This is specific heat capacity of a polymer when heat required to heat a unit mass of a polymer to a unit temperature is converted into a polymer.
(c) Latent heat of fusion: refers to heat required to change a crystalline polymer from a solid state to a viscous liquid state.
When forming a material, heating characteristics of material are related to total amount of heat required by polymer and temperature of polymer and cylinder, while cooling characteristics of product at high temperature are related not only to total heat contained in polymer, but also related to product and related to mold temperature.
B, thermal conductivity, thermal diffusivity, heat transfer coefficient
Thermal conductivity. The rate at which heat passes through a polymer is called thermal conductivity.
Thermal diffusivity. The rate at which temperature is transferred in a polymer is called thermal diffusivity.
Heat transfer coefficient: The ratio of heat flux density (the ratio of heat passing through heat transfer surface to heat transfer surface per unit time) and heat transfer temperature difference.
Overall heat transfer coefficient: refers to heat transfer coefficient between two fluids separated by a solid wall.
Because thermal conductivity and thermal diffusivity of polymer is very small, it takes a long time to heat barrel and cool mold. Generally, built-in extruder shear unit and mixing ring can be installed in injection molding machine to increase shear. Cut out heat, improve heat transfer efficiency . Enlarge cooling circuit in mold to increase thermal diffusivity, and use new molds such as low thermal conductivity and adiabatic runner design that does not form condensate in runners.
C, density and specific volume
Density: The weight of polymer per unit volume.
Specific volume: volume per unit mass of polymer.
Usually, as density of polymer increases, tensile strength, stiffness, hardness and temperature of viscous flow of polymer increase, but compressibility, fluidity, etc. decrease. As temperature rises or pressure decreases, density of polymer decreases. Less specific volume increases, on contrary, density is increasingthe specific volume decreases.
D, expansion ratio and compression ratio
The change in specific volume of a polymer due to a change in temperature at constant pressure is called coefficient of expansion.
Compressibility. The change in specific volume of a polymer as a result of a change in pressure at a constant temperature is called compressibility.
The expansion ratio and compressibility are very useful for analyzing and controlling dimensional accuracy and performance of products.1.2.3 Processability of polymer
Polymers are divided into linear polymers and bulk polymers. However, bulk polymer is also a cross-linked three-dimensional network structure obtained by a chemical reaction between a linear polymer or some low molecular weight substances and a polymer with a lower molecular weight. Once structure is formed, it cannot be plasticized. These are what we often refer to as thermoset polymers. On other hand, linear polymer molecules have a long chain structure, and in their aggregates they are always intertwined with each other.
In polymers, due to strong attraction between molecules and molecules, polymer materials have mechanical strength and can be processed many times, which is why they are called thermoplastic polymers. The nature and processing behavior of polymer is related to long chain structure of polymer and mechanical state of entangled state of aggregation.
Therefore, it has unique processing properties such as good formability, extrudability, formability and plasticity. It is these technological properties that make polymeric materials suitable for various processing methods and, therefore, widely used.
Polymers are extruded during processing, for example, polymers are extruded in extruder and injection molding machine barrels, calenders and molds. Extrudability refers to ability of a polymer to deform, take shape, and retain shape when extruded.
Under normal conditions, polymers cannot be obtained by extrusion in solid state, and only when polymer is in a viscous-liquid state, macroscopic and useful deformations can be obtained by extrusion. Polymer melts are shear thinning liquids, which means that their viscosity decreases with increasing shear force or rate. The main indicator for measuring extrudability of polymeric materials is fluidity of materials, and level of fluidity determines size and complexity of extrudability of polymeric materials.
Figure 1.3 Cavity pressure-temperature curve
Fluency refers to ability of a material to deform and form into a shape under influence of temperature and pressure. Molding materials can be formed into molded articles of various shapes by molding methods such as injection molding, molding, and extrusion.
Formability mainly depends on rheology, thermal properties and other physical and mechanical properties of material. Too low a temperature will result in insufficient fluidity and molding difficulties. Although too high a temperature promotes molding, it will easily cause degradation of polymer. The area formed by four lines in Fig. 1.3 (the part with crossed lines) is best area for sculpting. Molding conditions not only affect moldability of polymers, but also affect mechanical properties, appearance, shrinkage, crystallization, and product orientation over a wide range. These processability properties are directly related to flowability of polymer.
C. Ability to rotate
Spinnability refers to ability of a polymer material to form continuous, hard fibers during processing. It mainly depends on rheological properties of material, melt viscosity, melt strength, heat resistance and chemical resistance of melt, etc.
Extensibility refers to ability of an amorphous or semi-crystalline solid polymer to deform when rolled or stretched in one or two directions. This property of material opens up possibility of producing products with a large aspect ratio (length to diameter, and sometimes length to thickness), using plasticity of polymers for production of films, sheets and fibers by calendering or stretching. However, stretching method is still most widely used in industrial production. The plasticity of linear polymers is due to long chain structure and flexibility of macromolecules. The plasticity of a polymer depends on ability of material to undergo plastic deformation and work hardening. Deformability is related to temperature to which solid polymer is subjected.
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