Different forms of thermoplastics have different process properties, shrinkage properties, physical and mechanical properties, etc.
Generally speaking, for crystalline plastics, when processing temperature is higher than its melting point, its fluidity is better, cavity can be filled quickly, and its required injection pressure can also be smaller. Fluidity of amorphous plastics is poor, injection speed is slower, and injection pressure it needs is larger. Therefore, when designing mold, you can design a reasonable runner system size according to fluidity of plastic. On the one hand, it can avoid waste of material due to large size of runner system, at the same time extend injection molding cycle. On the other hand, it can avoid too small size of runner system, which causes filling and pressure holding difficulties. Of course, there are exceptions. For example, although polystyrene is an amorphous plastic, its fluidity is very good. Indicators that reflect fluidity usually have melt index (MFR) and apparent viscosity. MFR refers to mass of melt flowing out of standard capillary every 10min under a certain temperature and load in melt flow rate meter, its unit is g/10min. For high molecular polymers, under normal injection molding conditions, their flow behavior mostly does not obey Newton's law of flow and belongs to non-Newtonian fluids. Ratio of their flow shear stress to shear rate is called apparent viscosity. Apparent viscosity is not a constant at a certain temperature, but can change with shear stress, shear rate, and even some changes with time.

Thermoplastics undergo varying degrees of volume shrinkage from molten state to solid state. Crystalline plastics generally show greater shrinkage and shrinkage range than amorphous plastics, are more susceptible to influence of molding process. Shrinkage rate of crystalline plastic is generally 1.0%~3.0%, while shrinkage rate of amorphous plastic is 0.4%~0.8%. For crystalline plastics, subsequent shrinkage should also be considered, because after they are demolded at room temperature, they can crystallize and continue to shrink. Amount of post-shrinkage depends on thickness of product and ambient temperature. The thicker plastic, the greater shrinkage.
Attached Table 2-1: Mold shrinkage rate of common plastics

Note: Parameters with "*" are recommended by company.

Rheology of polymer refers to relationship between stress, deformation, deformation rate and viscosity during processing. This involves influence of temperature, pressure, time and molecular structure, molecular weight and distribution on these elements. According to rheology of plastics, plastics can be divided into shear-sensitive materials and heat-sensitive materials. The stronger dependence of viscosity on shear rate, the faster viscosity decreases with increase of shear rate. This kind of plastic is a shear-sensitive plastic. Common shear-sensitive plastics include ABS, PS, PE, PP, POM and so on. If melt viscosity is more dependent on temperature, viscosity will decrease as temperature rises. This kind of plastic is a heat-sensitive plastic. Common heat-sensitive plastics PC, PA, PMMA, etc. For high polymer polymers, shear rate has an effect on viscosity of above two materials. Increase of shear rate can reduce viscosity of melt to varying degrees, which can make melt "shear thinning" phenomenon. Therefore, when designing runner system, it is not that the larger runner size, the smaller pressure drop. A properly small runner size can increase shear rate of melt to reduce viscosity and further reduce pressure drop. Shear sensitive materials are more obvious. A smaller gate size can increase shear rate of melt and generate a large amount of frictional heat. Melt temperature will rise significantly, melt viscosity will decrease accordingly, thereby increasing fluidity. Therefore, use of small gates is often successful for shear-sensitive plastics. However, when wall thickness of product is thick, gate size should be appropriately increased in consideration of holding pressure to extend solidification time of gate.

Factor that affects performance of product is also orientation effect of plastic melt in flow process. Macromolecules of plastic melt are stretched under action of external force and arranged parallel to each other along flow direction. This arrangement is too late to be eliminated before plastic is cooled and solidified, freezes in solid product, forming an orientation effect. Orientation effect will weaken integrity of product, which is manifested in inconsistency of physical and mechanical properties in various directions, may also cause uneven shrinkage in various directions, which may lead to warping and deformation of product. Macromolecules in melt can be divided into "flow orientation" under action of shear stress and "stretch orientation" under action of stretching according to form of force and nature of action. Conditions for controlling orientation include following factors:
(1) Decrease of melt temperature and mold temperature will strengthen orientation effect;
(2) Increase of injection pressure can increase shear rate and shear stress, strengthen orientation effect;
(3) The thinner product thickness, the stronger orientation effect;
(4) A larger gate size will enhance orientation effect.
Sometimes some special measures are taken to enhance orientation effect, so that tensile strength and bending strength in orientation direction are improved. Such as stretch films, hinges, etc.