INJECTION MOLD DESIGNERS NEED TO TRULY UNDERSTAND THESE FOUR CONCEPTS

a. Fluidity
Different forms of thermoplastics have different process performance, shrinkage performance, physical and mechanical properties, etc.
Generally speaking, for crystalline plastics, when the processing temperature is higher than its melting point, its fluidity is better, it can quickly fill the cavity, and the injection pressure it requires can also be smaller. Amorphous plastics have poor fluidity, so the injection speed into the cavity is slower, and the injection pressure it requires is also larger. Therefore, when designing the mold, a reasonable runner system size can be designed according to the fluidity of the plastic. On the one hand, it can avoid the runner system size being too large and wasting materials, and at the same time extend the injection molding cycle. On the other hand, it can avoid the runner system size being too small and causing difficulties in filling and holding pressure. Of course, there are exceptions. For example, although polystyrene is an amorphous plastic, it has good fluidity. Indicators that reflect fluidity usually include melt index (MFR) and apparent viscosity. MFR refers to the mass of the melt flowing out of the standard capillary every 10 minutes under a certain temperature and load in the melt flow rate meter. Its unit is g/10min. For high molecular polymers, under normal injection molding conditions, their flow behavior does not obey Newton's flow law and belongs to non-Newtonian fluids. The ratio of their flow shear stress to shear rate is called apparent viscosity. Apparent viscosity is not a constant at a certain temperature, and can change with shear stress, shear rate, and even some with time.
b. Shrinkage
Thermoplastic plastics will undergo different degrees of volume shrinkage from molten state to solidified state. Crystalline plastics generally show a larger shrinkage rate and shrinkage range than amorphous plastics, and are more easily affected by the molding process. The shrinkage rate of crystalline plastics is generally 1.0%~3.0%, while the shrinkage rate of amorphous plastics is 0.4%~0.8%. For crystalline plastics, their post-shrinkage should also be considered, because they can continue to shrink after post-crystallization at room temperature after demolding. The amount of post-shrinkage depends on the thickness of the product and the ambient temperature. The thicker the product, the greater the post-shrinkage.
Appendix 2-1 :Molding shrinkage of common plastics

c. Rheology
The rheology of polymer refers to the relationship between stress, deformation, deformation rate and viscosity during processing. This involves the influence of temperature, pressure, time, molecular structure, molecular weight and its distribution on these factors. According to the rheology of plastics, plastics can be divided into shear-sensitive materials and heat-sensitive materials. The stronger the viscosity is dependent on the shear rate, the faster the viscosity decreases with the increase of shear rate. This plastic belongs to shear-sensitive plastic. Common shear-sensitive plastics include ABS, PS, PE, PP, POM, etc. If the melt viscosity is more dependent on temperature, the faster the viscosity decreases with the increase of temperature, this plastic belongs to thermosensitive plastic. Common thermosensitive plastics include PC, PA, PMMA, etc. For high molecular polymers, the shear rate has an effect on the viscosity of the above two materials. The increase of shear rate can reduce the viscosity of the melt to varying degrees, and can cause the melt to produce "shear thinning" phenomenon. Therefore, when designing the runner system, it is not the case that the larger the runner size, the smaller the pressure drop. A properly small runner size can increase the shear rate of the melt to reduce the viscosity and further reduce the pressure drop. This effect is more obvious for shear-sensitive materials. A smaller gate size can increase the shear rate of the melt, generate a large amount of frictional heat, significantly increase the melt temperature, and the melt viscosity decreases accordingly, increasing fluidity. Therefore, the use of a small gate is often successful for shear-sensitive plastics. However, when the wall thickness of the product is thicker, the gate size should be appropriately increased to extend the solidification time of the gate in consideration of pressure holding.
d. Orientation effect
Another factor affecting the performance of the product is the orientation effect of the plastic melt during the flow process. The macromolecules of the plastic melt are stretched under the action of external force and arranged parallel to each other along the flow direction. This arrangement is frozen in the solid product before the plastic cools and solidifies, forming an orientation effect. The orientation effect will weaken the integrity of the product, which is manifested as inconsistent physical and mechanical properties in various directions, and may also cause uneven shrinkage in various directions, which may cause warping and deformation of the product. According to the form of force on the macromolecules in the melt and the nature of the action, it can be divided into "flow orientation" under shear stress and "stretching orientation" under tensile action. The factors that control orientation are as follows:
(1) The decrease of melt temperature and mold temperature will strengthen the orientation effect;
(2) The increase of injection pressure can increase the shear rate and shear stress and strengthen the orientation effect;
(3) The thinner the product thickness, the stronger the orientation effect;
(4) The larger the gate size will strengthen the orientation effect.
Sometimes, some special measures are taken to enhance the orientation effect, so that the tensile strength and bending strength in the orientation direction are improved. Such as stretch film, hinge, etc.