In injection molded products, local stress state is different everywhere, and degree of deformation of product will be determined by stress distribution. If product is cooling. In presence of a temperature gradient, this type of stress will develop, so this type of stress is also called "forming stress".

 

There are two kinds of internal stress of injection molded products: one is molding stress of injection molded products, and the other is temperature stress. When melt enters mold with a lower temperature, melt near wall of cavity cools rapidly and solidifies, so molecular segments are "frozen". Due to solidified polymer layer, thermal conductivity is very poor, and a large temperature gradient is generated in thickness direction of product. Core of product solidifies very slowly, so that when gate is closed, melt unit in the center of product has not yet solidified, and injection molding machine cannot replenish cooling shrinkage at this time.
In this way, internal shrinkage of product is opposite to that of hard skin layer; core is in static tension and surface layer is in static compression.
When melt is filling mold, in addition to stress caused by volume shrinkage effect. There is also stress caused by expansion effect of runner and gate outlet; stress caused by former effect is related to flow direction of melt, and the latter will cause stress perpendicular to flow direction due to expansion effect of outlet.

Under rapid cooling conditions, orientation will lead to formation of internal stress in polymer. Due to high viscosity of polymer melt, internal stress cannot be relaxed quickly, which affects physical properties and dimensional stability of product.
Influence of each parameter on orientation stress:
Melt temperature is high, viscosity is low, shear stress decreases, and degree of orientation decreases; on the other hand, due to high melt temperature, stress relaxation is accelerated, and deorientation ability is enhanced.
However, without changing pressure of injection molding machine, cavity pressure will increase, and strong shear action will lead to an increase in orientation stress.
Prolonging holding time before nozzle is closed will lead to an increase in orientation stress.
Increasing injection pressure or holding pressure will increase orientation stress,
High mold temperature can ensure slow cooling of product and play a role in deorientation.
Increasing thickness of product reduces orientation stress, because thick-walled product cools slowly, viscosity increases slowly, and stress relaxation process takes a long time, so orientation stress is small.

As mentioned above, due to large temperature gradient between melt and mold wall during mold filling, outer melt that solidifies first will help prevent shrinkage of inner melt that solidifies later, resulting in compressive stress (shrinkage stress) in outer layer, inner layer produces tensile stress (orientation stress).
If mold is filled for a long time under action of holding pressure, polymer melt is added into mold cavity to increase pressure of mold cavity, which will change internal stress caused by uneven temperature. However, when holding time is short and cavity pressure is low, product will still maintain original stress state during cooling.
If mold cavity pressure is insufficient in the early stage of product cooling, outer layer of product will form a depression due to solidification and shrinkage; if mold cavity pressure is insufficient in the late stage when product has formed a chilled layer, inner layer of product will be separated due to shrinkage, or a cavity will be formed; if cavity pressure is maintained before gate is closed, it is beneficial to increase product density and eliminate cooling temperature stress, but a large stress concentration will be generated near gate.
From this point of view, when thermoplastic polymers are molded, the greater pressure in mold, the longer holding time will be, which will help reduce shrinkage stress caused by temperature, and on the contrary will increase compressive stress.

 

Existence of internal stress in product will seriously affect mechanical properties and performance of product; due to existence and uneven distribution of internal stress in product, cracks will occur in product during use. When used below glass transition temperature, irregular deformation or warping often occurs, it can also cause "whitening" and turbidity on the surface of product, and deterioration of optical properties.
Trying to reduce temperature at gate and increase slow cooling time will help improve uneven stress of product and make mechanical properties of product uniform.
Whether for crystalline polymers or amorphous polymers, tensile strength exhibits anisotropic characteristics. Tensile strength of amorphous polymers will vary depending on position of gate; when gate is consistent with mold filling direction, tensile strength decreases with increase of melt temperature; when gate is perpendicular to mold filling direction, tensile strength increases with increase of melt temperature.
Due to increase of melt temperature, deorientation effect is strengthened, and orientation effect is weakened to reduce tensile strength. Orientation of gate will affect orientation by affecting direction of material flow, and because anisotropy of amorphous polymer is stronger than that of crystalline polymer, tensile strength of the former in direction perpendicular to flow is greater than that of the latter big. Low temperature injection has greater mechanical anisotropy than high temperature injection. For example, when injection temperature is high, intensity ratio of vertical direction to flow direction is 1.7, and it is 2 when injection temperature is low.
From this point of view, increase in melt temperature will lead to a decrease in tensile strength for both crystalline polymers and amorphous polymers, but mechanism is different; the former is due to effect of reduction through orientation.