Molecular aggregation state in thermoplastic resin solid is sparse and dense. Dense part can be called crystalline part, and sparse part is called amorphous part.Most polymers will have a certain degree of crystalline part;

 

Therefore, we call content of crystalline part crystallinity. But in general, polymers with functional groups such as nylon and thermoplastic polyester, or polymers with more regular molecular arrangements such as polypropylene and polyethylene, have higher crystallinity, while copolymers or mixed polymers have lower crystallinity.
Actual crystallinity of general polymer is lower than its inherent crystallinity, therefore, its crystallinity can be improved by heat treatment or increasing mold temperature.
Polymers with high crystallinity increase in strength, decrease in elongation, and decrease in volume. The higher crystallinity of plastics, the higher its density, the higher melting temperature (melting point), the higher strength, the lower transparency, and the lower elongation.
It can be seen that there is a close relationship between crystallinity and physical properties. Changes in tensile properties of various resins are related to difference in crystallization of resin during molding process. The greater difference in crystallization, the greater change in tensile properties of polymer.
Polymers with good crystallinity will shrink in volume due to progress of their crystallization, which will affect dimensional stability of their products. Therefore, it is necessary to try to increase crystallinity as much as possible to inherent crystallinity during processing to prevent dimensional stability of product caused by post-shrinkage. In fact, in order to improve dimensional stability of product, some crystallization accelerators (nucleating agents) are often added to resin.

In thermoplastic resins, there is a phenomenon of glass transition temperature (glass transition point), that is: during process of increasing temperature, polymer will be in a temperature range that is neither solid nor viscous liquid before it melts. In rubbery state, we call starting temperature of rubber attitude as its glass transition temperature (Tg). In this temperature range, thermal expansion of polymer will suddenly become larger, deformation that occurs is different from rubber, which is an irreversible deformation.
Glass transition temperature characteristics are very important when using polymer products; for example, when product is placed at a temperature above glass transition temperature, it will cause unexpected deformation. On the contrary, if you want to change shape of product, you can perform it above glass transition temperature. In addition, when you want to increase crystallinity of product, you can also process it in this temperature range.

In nature, we divide aggregation state of matter at room temperature into three types: gas, liquid, and solid.

 

High molecular polymers represented by amorphous linear polymers, due to continuity of molecular structure and their huge molecular weight, their aggregation state is different from general low molecular compounds, but under different thermal conditions, they exist in three unique forms: vitreous state, high elastic state, and viscous flow state.
High-molecular polymers do not exist in gaseous state. Before being heated and possibly vaporized, molecular structure has been completely destroyed and become low-molecular vaporized substances or carbides.
Glassy state of high molecular polymers is actually a manifestation of solid state, which is characterized by properties generally possessed by solid substances within a certain temperature range, and is similar to ordinary glass in some mechanical properties.
Viscous flow state of high molecular polymer is a unique "liquid state", within a certain temperature range, it has mechanical properties that it can flow and is different from ordinary low molecular liquids.
Highly elastic state of high molecular polymers is a unique state between glass state and viscous flow temperature range. Macromolecular polymers, like other substances, have a relatively stable form under specific temperature and pressure conditions. For example, under ordinary conditions of use, plexiglass can be regarded as a representative of glass state, and liquid resin can be regarded as a representative of viscous flow state.
When external temperature and pressure change and reach a certain level, high molecular polymer will change original state and transform into another state. Task of injection molding factory is to provide these changing conditions. During processing, when plastic raw material (based on high polymer polymer) is subjected to temperature, pressure, and shear, its viscosity, physical structure, shape, etc. will change. Among them, temperature has the greatest influence, which is theoretical basis for plastic thermoforming.

Specific heat capacity is heat required when temperature of unit weight of material rises by 1 degree [J/kg.k].
Specific heat capacity of different polymers is different, and crystalline type is higher than non-opposite type. Because when heating polymer, supplementary heat energy must not only be consumed in temperature rise, but also in the change of polymer structure, crystal type must supplement amount of tears required for latent heat of melting to melt material.
During injection molding process, heating or cooling characteristics of plastic are determined by heat content and temperature difference of polymer. Heat transfer rate is proportional to temperature difference between heated material and heat source.
Generally, cooling is faster than melting, because generally temperature difference between barrel and material is small, temperature difference between melt and mold is large. Heating time depends on temperature difference between inner wall of barrel and material layer and thickness of material layer.

Thermal diffusivity refers to speed at which temperature is transferred in heated material. It is also called thermal conductivity. Its value is determined by amount of heat (specific heat capacity) required when temperature of material per unit mass rises by 1 degree and rate at which material absorbs heat (thermal conductivity).
Pressure has little effect on thermal diffusivity, while temperature has a greater effect on it.

Thermal conductivity reflects speed at which material spreads heat. The higher thermal conductivity, the faster heat transfer within material. Due to low thermal conductivity of polymer, it takes a certain amount of time whether it is heated in barrel or melt is cooled in mold. In order to improve heating and cooling efficiency, some technical measures need to be taken.
For example, heating barrel requires a certain thickness. This is not only to consider strength, but also to increase thermal inertia, to ensure that material can conduct heat well and stably. Sometimes low thermal conductivity of polymer is used, and hot runner mold is used. Thermal conductivity of polymers increases with increasing temperature. Thermal conductivity of crystalline plastics is more dependent on temperature than that of amorphous plastics.

 

Increase in density will reduce gas and solvent permeability of product, but it will increase product's tensile strength, elongation at break, rigidity and softening temperature; it will reduce compressibility, impact strength, fluidity, and creep resistance.
In injection molding process, polymer undergoes a repeated thermal process of cooling-heating-cooling. Temperature, gradient and polymer morphology change greatly, so density is constantly changing, which has an important impact on quality of injection molded products. .
Specific volume reflects volume occupied by a unit substance. This is a measure of space occupied by polymer structure under different process conditions, expansion and compression in various states, and size shrinkage of product are very important parameters.

Change in specific volume caused by temperature under constant pressure is coefficient of expansion. From high temperature to low temperature, polymer exhibits a shrinkage characteristic with a gradual decrease in specific volume. Specific volume of a polymer depends not only on temperature but also on pressure. Specific volume of polymer changes with pressure at different temperatures. As pressure increases, specific volume decreases and density increases. This property is of great significance for using pressure to control quality and dimensional accuracy of products.

Moisture permeability refers to permeability of water vapor to plastic film. Basic principle and definition are same as permeability. Ratio of luminous flux passing through object and luminous flux hitting object is called transmittance; ratio of scattered light in direction of incident light to all transmitted light is called haze or turbidity.
Haze is usually translucent and has a diffuse nature to incident light. Tensile strength refers to tensile load applied along longitudinal axis of sample under specified test temperature, humidity and tensile speed, maximum load when sample is broken is measured.

Compressive strength refers to strength at which a compressive load is applied to sample to rupture (for brittle materials) or yield (for non-brittle materials). Bending strength refers to strength of sample at two fulcrums when a concentrated load is applied to deform sample or until it ruptures.

Impact strength refers to joules consumed per unit area when sample is broken by impact. For some plastics with high impact strength, there is often a gap of a specified size in the middle of sample, which can reduce joules required when it breaks.
Different test pieces can use different test methods: falling ball impact test, high-speed tensile impact test. Coefficient of Friction refers to ratio of friction force to positive pressure. A positive pressure is applied to sample to determine ratio of dynamic and static when sample is rigidly moving.

Abrasion refers to mechanical destruction process in which particles are continuously separated from friction surface during friction process of plastic, which causes size of friction part to change continuously. It is also called abrasion. Plastic hardness refers to performance of plastics against other hard objects. There are two common types of Rockwell hardness and Shore hardness.
Shore hardness refers to depth that indenter of indenter is pressed under specified pressure and time. Shaw indenters can be divided into two categories, namely: A and D type. Applied load weight is 1.0, 5.0 kg, pressing time is 15 seconds. Type A is suitable for soft plastics, Type D is suitable for semi-rigid plastics; when A type is used to measure more than 95% of range, D-type should be used instead, when D-type measurement exceeds 95% of range, Rockwell indentation needs to be changed. Fatigue strength refers to strength at which plastics are destroyed in an environment with static destructive force and a small amount of alternating cycles; sources of fatigue load include tension and compression, bending, torsion, and impact.

Creep refers to characteristics of plastics that change over time under continuous action of a fixed external force under certain temperature and humidity conditions. Characteristics of this deformation increase with increasing load, decrease with decreasing load, its deformation gradually recovers. Sources of creep include tensile creep, compression creep, and bending creep. Durable strength refers to function of time when ability of plastics to withstand static loads for a long time decreases from high.

 

For example: strength of plastic before load is 1000 hours, but after load may only be between 50% and 70%. Coefficient of linear expansion refers to number of centimeters per centimeter of plastic stretch when temperature rises by 1 degree Celsius. Coefficient of linear expansion of plastic is generally about ten times that of steel.
Specific heat refers to unit of heat required when 1 gram of plastic is hard to rise by 1 degree Celsius. Thermal conductivity refers to unit of heat that a certain unit area and thickness of plastic can pass through. Thermal conductivity of plastic is very small, only about one percent of steel, so it is a good thermal insulation material. Heat resistance of plastics is a characteristic reflecting relationship between temperature of plastic parts and amount of deformation, heat resistance is more important for temperature-dependent plastic parts.

Decomposition temperature refers to temperature at which macromolecular chain of plastic breaks when heated, and it is also one of indicators for identifying heat resistance of plastic; when temperature of melt exceeds decomposition temperature, most of melt will show a yellowish color, strength of product will be greatly reduced.

Melt Index (MFI) MI refers to weight of melt flowing out of melt through small hole of tester within 10 minutes at a certain temperature and pressure, unit is expressed in grams/10 minutes.

Burning resistance of plastics is expressed by ratio of burning rate (burning length of sample during burning time) and burning weight loss rate (percentage of difference between weight before and after burning). It can be used as a reference number for combustion resistance.

Rapidly increase voltage to a certain limit value, stay for a period of time, plastic sample is broken down, and voltage at this time is said to be withstand voltage that sample can withstand.

Aging resistance of plastics refers to phenomenon that chemical structure is damaged due to external factors such as light, heat, oxygen, water, organisms, and stress during use, storage and processing, original excellent performance is reduced. Purpose of studying aging of plastics is to improve its stability and extend its service life.

Chemical resistance of plastics refers to whether plastic is corroded in a chemical medium. Evaluation is usually based on degree of change in weight, volume, strength, and color of plastic after a certain period of time in medium.

Molding shrinkage means that when thermoplastic is molded in mold, molded product that is released after cooling must have a shrinkage phenomenon, that is, molded product is smaller than size of mold cavity.