Internal stress generation
In the injection molded products, the local stress state is different in each place, and the degree of product deformation will be determined by the residual stress distribution. If the product is cooled. If there is a temperature gradient, this kind of stress will develop, so this kind of stress is also called “molding stress”.
There are two types of internal stresses in injection molded parts: one is the molding stress of injection molded products, and the other is the temperature residual stresses. When the melt enters the mold at a lower temperature, the melt near the cavity wall cools rapidly and solidifies, so the molecular chain segments are “frozen”.
Due to the poor residual thermal stresses conductivity of the solidified polymer layer, a large temperature gradient is generated in the thickness direction of the product.
The heart of the product solidifies rather slowly, so that when the gate is closed, the melting unit in the center of the product has not yet solidified, and the injection molding machine is unable to compensate for the cooling shrinkage at this time.
Thus, the internal shrinkage of the product is in the opposite direction of the action of the hard skin layer; the core is in static tension while the surface layer is in static compression.
In the melt filling flow, in addition to the volume shrinkage effect caused by the stress. There are also stresses caused by the expansion effect of the runner and gate outlet; the former effect causes stresses related to the direction of melt flow, and the latter will cause stresses in the direction perpendicular to the flow due to the outlet expansion effect.
The process factors affecting the stress
In rapid cooling conditions, orientation will lead to the formation of stress in the polymer. Due to the high viscosity of the polymer melt, the internal stress cannot be relaxed quickly, affecting the physical properties and dimensional stability of the product.
Influence of each parameter on orientation stress
(1) Melt temperature, high melt temperature, low viscosity, and shear stress decrease orientation; on the other hand, because of the high melt temperature will make the stress relaxation accelerated, prompting the ability to strengthen the unorientation.
(2) In the case of not changing the pressure of the injection molding machine, the mold cavity pressure will increase, and the strong shear effect leads to an increase in orientation stress.
(3) Extending the holding time before the nozzle is closed will lead to an increase in orientation stress.
(4) Increasing the injection pressure or holding pressure will increase the orientation stress.
(5) High mold temperature can ensure that the product cools slowly and plays a role in unorientation.
(6) Increase the thickness of the product to reduce the orientation stress, because the thick-walled products cool slowly, viscosity increases, and the stress relaxation process of a long time, so the orientation stress is small.
Effect on temperature stress
(1) As mentioned above, due to the large temperature gradient between the melt and the wall when filling the injection mold, the outer layer of the melt that solidifies first has to help stop the shrinkage of the inner thin surface layer of the melt that solidifies later, resulting in compressive stress (shrinkage stress) in the outer layer and tensile stress (orientation stress) in the inner layer.
(2) If the mold is filled and continues for a long time under the action of holding pressure, the polymer melt is added to the mold cavity, so that the injection molding pressure in the mold cavity is increased, and this pressure will change the internal stress due to uneven temperature.
However, in the case of short holding time and low cavity pressure, the product will still maintain the original stress state when cooling.
(3) If the mold cavity pressure is insufficient at the early stage of product cooling, the outer layer of the product will form a depression due to solidification shrinkage; if the injection mold cavity pressure is insufficient at the later stage when the product has formed a cold hard layer, the inner layer of the product will separate due to shrinkage or form cavities.
(4) If the cavity pressure is maintained before the gate is closed, it is beneficial to improve the density of the product and eliminate the cooling temperature stress, but a large stress concentration will be generated near the gate.
(5) Thus, it seems that the greater the pressure in the mold the longer the holding time, which helps to reduce the shrinkage stress generated by the temperature and vice versa will increase the compressive stress.
The relationship between internal stress and product quality
(1) The existence of internal stress in the product will seriously affect the mechanical properties and performance of the product; due to the existence and uneven distribution of internal stress, cracks will occur in the process of using the product.
In the glass transition temperature below the use, often occur irregular deformation or warpage, but also cause the surface of the product “white”, cloudy, optical properties deterioration.
(2) Try to reduce the temperature at the gate, and increase the slow cooling time, which is conducive to improving the residual stress unevenness of the product, so that the mechanical properties of the product are uniform.
(3) No matter for crystalline polymer or non-crystalline polymer, the tensile strength shows anisotropic characteristics.
For non-crystalline polymers, the tensile strength varies depending on the location of the gate; when the gate is in the same direction as the mold filling, the tensile strength decreases as the melt temperature increases; when the gate is perpendicular to the injection mold filling direction, the tensile strength increases as the melt temperature increases.
(4) The increase of melt temperature leads to the strengthening of the reorientation effect, while the weakening of the orientation effect decreases the tensile strength.
The orientation of the gate affects the orientation by influencing the direction of the material flow, and since the anisotropy of non-crystalline polymers is stronger than that of crystalline polymers, the tensile strength in the direction perpendicular to the flow direction is greater for the former than for the latter.
The low-temperature injection molding has greater mechanical anisotropy than a high-temperature injection, e.g., the strength ratio of perpendicular to the flow direction is 1.7 at high injection temperature and 2 at low injection temperature.
(5) Thus, it appears that an increase in melt temperature leads to a decrease in tensile strength for both crystalline and non-crystalline polymers, but the mechanism is different; the former is due to the effect of reduction through orientation.