Ability of plastic in molten state after being heated to fill the entire cavity under pressure is called fluidity, which is generally tested with a professional fluidity test mold.
Fluidity of plastics of same type and different grades is different, and plastic parts of different structures have different requirements for fluidity of plastics. Fluidity of plastics is closely related to conditions such as molding temperature and pressure.
Usually, if fluidity is too large, on the one hand, molten material will not be filled tightly, resulting in loose products and affecting product quality; On the other hand, if fluidity is too large, edges of injection molded parts are prone to drape (burrs) sticking to mold and nozzle casting, resulting in blockage of nozzle. However, if fluidity of plastic is too small, for plastic parts with complex structure and long process, flow during injection molding will be difficult, and it is prone to lack of glue, shrinkage, etc., resulting in a large number of waste products; or it must be injected under high pressure conditions, which is easy to cause excessive internal stress of plastic parts. We must choose plastic molding with suitable fluidity according to product structure, size, and thickness.

Thermoplastics contain more or less moisture and volatiles, and an appropriate amount of moisture has a plasticizing effect.
If water and volatiles in plastic exceed a certain proportion, there will be many problems during injection molding (such as: degradation, fogging, strength reduction, etc.). In severe cases, bubbles (silver streaks) and rough surfaces may occur, light transmittance of transparent products will be destroyed (turbid). It is difficult to ensure accuracy of precision plastic parts.
However, absolutely dry plastic will reduce fluidity, increase brittleness, it is difficult to fill mold during molding, and it is also unusable. Special attention should be paid to this point. Some people think that the more fully dried plastic, the better, which is a wrong concept.
There are three main reasons for the high amount of moisture and volatiles in plastics;
A. Average molecular weight of plastic resin is low;
B. Plastic resin is not fully dried during production;
C. Plastics with high water absorption absorb the moisture in surrounding air due to improper storage. Different plastics have different drying temperature and drying time regulations.
Moisture content and drying temperature of commonly used plastics (example)

There are three main reasons for shrinkage:
A. Plastics have much larger thermal shrinkage than metals (one order of magnitude larger, about ten times larger).
B. Plastic product is not a rigid body after hardening, and plastic part has a certain elastic recovery after demolding.
C. When mold is just demolded, pressure begins to decrease, but plastic part is still attached to mold wall, and product undergoes plastic deformation.
Factors affecting shrinkage rate include nature of plastic, molding conditions, design of mold and product. Shrinkage rate of amorphous plastics is less than 1%, and shrinkage rate of crystalline plastics is more than 1%. Products of crystalline plastic injection molding have post-shrinkage phenomenon, size needs to be measured after cooling for 24 hours, and accuracy can reach 0.02mm.
Mold shrinkage of polymers

Processing temperature of plastic is temperature at which it reaches a viscous flow state. Processing temperature is not a point but a range (from melting point to decomposition temperature). When thermoforming plastics, appropriate processing temperature should be selected according to size, complexity, thickness, insert condition, temperature tolerance of colorant used, machine performance and other factors.
Processing temperature range of commonly used plastics is shown in the table below

Why is temperature reflected by thermometer often changed in injection molding production, and temperature set for same product (same mold) may be different when it is produced on different machines?
In fact, thermoforming temperature of plastic is relatively fixed, and above difference is only caused by temperature measurement method, temperature measurement point layout and performance difference of temperature sensor. Temperature displayed on temperature indicator controller is not actual temperature of melt in barrel, but an indirect, local temperature.

When setting injection molding process conditions, each injection molding worker needs to thoroughly understand relevant properties of plastics used in order to scientifically set process conditions and analyze problems in injection molding production process.
Types and grades of plastics (factors to consider when understanding composition and properties of plastics);
Density of plastic (a factor to consider when setting position of multi-segment injection);
Hygroscopicity of plastic and allowable water (factors considered when setting drying conditions);
Glass transition temperature, melting point and decomposition temperature of plastic (factors considered when setting barrel temperature);
Plastic melt index FMI (factors considered when setting injection pressure and back pressure);
Crystallinity of plastic (factors considered when setting mold temperature/material temperature);
Plastic allowable injection pressure range (factors considered when setting injection pressure);
Allowable residence time of plastic in barrel (factors to be considered when setting amount of residual material and shutting down);
Plastic molding shrinkage (factors considered when setting mold temperature/material temperature/pressure);
Mold temperature range during plastic molding (factors considered when setting mold temperature);
Other properties (such as chemical resistance, heat distortion temperature, etc.) are considered factors in post-processing of plastic parts.