Picture shows a car lamp made of PC. PC polycarbonate is a material that is more prone to internal stress; among elastomers, wires of SEBS and PPO systems often crack due to internal stress; following explains what is internal stress.

Plastic internal stress refers to a kind of internal stress generated during plastic melt processing due to factors such as orientation of macromolecular chains and cooling shrinkage.
Essence of internal stress is unbalanced conformation formed during melt processing of macromolecular chain. This unbalanced conformation cannot immediately return to an equilibrium conformation compatible with environmental conditions when it is cooled and solidified. It is essentially a reversible high-elastic deformation, and frozen high-elastic deformation is stored in a plastic product in the form of energy. Under suitable conditions, this forced unstable conformation will transform into a free and stable conformation, potential energy will be converted into kinetic energy and released. When force and entanglement between macromolecular chains cannot withstand this kinetic energy, internal stress balance will be destroyed, plastic products will have stress cracking and warping deformation.

 

Almost all plastic products have internal stress to varying degrees, especially internal stress of plastic injection products is more obvious. Existence of internal stress not only causes stress cracking and warping deformation of plastic products during storage and use, but also affects mechanical properties, optical properties, electrical properties and appearance quality of plastic products.
For this reason, it is necessary to find out cause of internal stress and method to eliminate internal stress, reduce internal stress of plastic products to the greatest extent, make residual internal stress distribute as evenly as possible on plastic products, avoiding phenomenon of internal stress concentration, so as to improve mechanical and thermal properties of plastic products.

In injection molded products, local stress states are different everywhere, degree of product deformation will depend on stress distribution. If product has a temperature gradient during cooling, this type of stress will develop, so this type of stress is also called "forming stress".
There are two internal stress packages for injection molded products: one is molding stress of injection molded product, and the other is temperature stress. When melt enters mold with a lower temperature, melt near cavity wall quickly cools and solidifies, so molecular chain is "frozen."
Due to solidified polymer layer, thermal conductivity is very poor, resulting in a large temperature gradient in thickness direction of product. Core part of product solidifies quite slowly, so that when gate is closed, melt unit in the center of product has not solidified yet, and injection molding machine cannot supplement cooling shrinkage at this time. In this way, internal shrinkage of product is opposite to direction of hard skin layer; core is in static tension and surface layer is in static compression.
In mold filling and flow of melt, 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 the former effect is related to flow direction of melt, and the latter will cause stress in direction perpendicular to flow direction due to outlet expansion effect.

 

(1) Influence of directional stress. Under rapid cooling conditions, orientation will lead to formation of internal stress in polymer. Due to high viscosity of polymer melt, internal stress cannot relax quickly, which affects physical properties and dimensional stability of product.
Influence of various parameters on orientation stress:
Melt temperature is high, viscosity is low, shear stress is reduced, and degree of orientation is reduced; on the other hand, high melt temperature will accelerate stress relaxation and promote enhancement of de-orientation ability.
However, without changing pressure of injection molding machine, cavity pressure will increase, strong shearing action will lead to an increase in orientation stress.
Before nozzle is closed, extending holding time will cause orientation stress to increase.
Increasing injection pressure or holding pressure will increase orientation stress,
High mold temperature can ensure that product cools slowly and play a de-orientation effect.
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.
(2) Influence on temperature stress
As mentioned above, due to large temperature gradient between melt and mold wall during mold filling, outer layer of melt that solidifies first prevents shrinkage of inner layer of melt that solidifies later, resulting in compressive stress (shrinkage stress) in outer layer and tensile stress (orientation stress) in inner layer.
If holding pressure continues for a long time after mold is filled, polymer melt is added to mold cavity again to increase pressure of mold cavity, and this pressure will change internal stress due to temperature unevenness. However, when holding time is short and cavity pressure is low, inside of product will still maintain original stress state during cooling.
If mold cavity pressure is insufficient at the initial stage of cooling of product, outer layer of product will form depressions due to solidification and shrinkage; if mold cavity pressure is insufficient in later period when product has formed a chilled layer, inner layer of product will separate due to shrinkage or form a cavity; if cavity pressure is maintained before gate is closed, it is beneficial to increase density of product and eliminate cooling temperature stress, but a greater stress concentration will occur near gate.
It can be seen from this that when thermoplastic polymers are molded, the greater pressure in the mold, the longer holding time, which contributes to reduction of shrinkage stress generated by temperature, which will increase compression stress.

Existence of internal stress in the product will seriously affect mechanical properties and performance of product; due to existence and uneven distribution of internal stress of product, product will crack during use. When used below glass transition temperature, irregular deformation or warping often occurs, which can also cause surface of product to "whiten", turbid, and deteriorate optical properties.
Try to reduce temperature at gate and increase slow cooling time, which will help improve uneven stress of product and make mechanical properties of product uniform.
Regardless of crystalline polymer or amorphous polymer, tensile strength shows anisotropic characteristics. Tensile strength of amorphous polymers will vary depending on location of gate; when gate is consistent with filling direction, tensile strength decreases with increase of melt temperature; when gate is perpendicular to filling direction, tensile strength increases as melt temperature increases.
As melt temperature increases, de-orientation effect is strengthened, while weakening of orientation effect reduces tensile strength. Orientation of gate will affect orientation by influencing direction of material flow, and because amorphous polymers have stronger anisotropy than crystalline polymers, tensile strength perpendicular to flow direction of the former is greater than that of the latter. Low-temperature injection has greater mechanical anisotropy than high-temperature injection. For example, when injection temperature is high, strength ratio between vertical direction and flow direction is 1.7, and when injection temperature is low, it is 2.
From this point of view, increase in melt temperature will result in a decrease in tensile strength for both crystalline and amorphous polymers, but mechanism is different; the former is due to decrease in orientation.