1. The generation of internal stress
In injection molded products, the local stress state is different everywhere, and the degree of deformation of the product will depend on the stress distribution. Such stresses can develop if the product has a temperature gradient while it is cooling. So this kind of stress is also called “forming stress”.
There are two kinds of internal stress packages for injection molded products: one is the molding stress and the other is the temperature stress.
When the melt enters a mold with a lower temperature, the melt near the cavity wall is rapidly cooled and solidified, so the molecular chain segment is “frozen”.
Due to the solidified polymer layer, the thermal conductivity is poor, resulting in a large temperature gradient in the thickness of the product.
The core of the product solidifies so slowly that when the gate is closed, the melt unit in the center of the product has not solidified.
At this time, the injection molding machine cannot replenish the cooling shrinkage.
In this way, the internal shrinkage of the product is opposite to that of the hard skin layer, the interior is under static stretching and the surface layer is under static compression.
In the melt-filling flow, in addition to the stress caused by the volume shrinkage effect, there is also the stress caused by the expansion effect of the runner and the gate outlet. The stress caused by the former effect is related to the flow direction of the melt, and the latter will cause a stress effect perpendicular to the flow direction due to the expansion effect of the outlet.
2. Process factors affecting stress
（1） Effect of directional stress
Under rapid cooling conditions, orientation causes the formation of internal stresses in the polymer.
Due to the high viscosity of the polymer melt, the internal stress cannot relax quickly, affecting the physical properties and dimensional stability of the product.
Effects of parameters on orientation stress:
The melt temperature
When the melt temperature is high, the viscosity is low, the shear stress is reduced, and the degree of orientation will be reduced.
On the other hand, due to the high melt temperature, stress relaxation will be accelerated, and the orientation release ability will be enhanced.
However, if the pressure of the injection molding machine is not changed, the cavity pressure will increase, and the strong shear effect will lead to an increase in the orientation stress.
Extending the holding time before the nozzle is closed will result in increased orientation stress.
Increasing the injection pressure or holding pressure will increase the orientation stress.
The high mold temperature can ensure that the product cools slowly and plays a role of deorientation.
Increasing the thickness of the product reduces the orientation stress. Because thick-walled products cool down slowly, their viscosity increases slowly, and the stress relaxation process takes a long time, so orientation stress is small.
(2) Influence on temperature stress
As mentioned above, due to the large temperature gradient between the melt and the mold wall during mold filling, the outer solidified first melt must prevent the inner solidified melt from shrinking, resulting in compressive stress (shrinkage stress) in the outer layer and tensile stress (orientation stress) in the inner layer.
If the mold is filled for a longer period of time under the effect of holding pressure, the polymer melt is refilled into the cavity, which will increase the cavity pressure and this kind of pressure will change the internal stress caused by uneven temperature.
However, under the condition that the holding time is short and the cavity pressure is low, the product will still maintain the original stress state during cooling.
If the cavity pressure is insufficient in the initial stage of product cooling, the outer layer of the product will form a depression due to solidification shrinkage.
If the cavity pressure is insufficient in the late stage when the product has formed a cold hard layer, the inner layer of the product may be separated due to shrinkage, or a cavity may be formed.
If the cavity pressure is maintained before the gate is closed, it will help increase the density of the product and eliminate the cooling temperature stress, but a large stress concentration will occur near the gate.
From this point of view, when the thermoplastic polymer is molded, the larger the pressure in the mold, the longer the holding time, which helps reduce the shrinkage stress caused by temperature, and conversely increases the compressive stress.
3. Relationship between internal stress and products quality
The existence of internal stress in the product will seriously affect the mechanical properties and serviceability of the product.
Due to the existence and uneven distribution of internal stress in the product, cracks may occur in the product during use.
When used below the glass transition temperature, irregular deformation or warping often occurs, and the surface of the product will be “whitened”, cloudy, and the optical properties will be deteriorated.
Trying to reduce the temperature at the gate and increase the slow cooling time will help improve the uneven stress of the product and make the mechanical properties of the product uniform.
Regardless of the crystalline polymer or the amorphous polymer, the tensile strength shows anisotropic characteristics.
Tensile strength to amorphous polymers will vary depending on the location of the gate.
When the gate is consistent with the filling direction, the tensile strength decreases as the melt temperature increases. When the gate is perpendicular to the filling direction, the tensile strength increases as the melt temperature increases.
As the melt temperature increases, the deorientation effect is strengthened, while the weakened orientation effect reduces the tensile strength.
The orientation of the gate can affect the orientation by affecting the direction of the flow.
Since the amorphous polymer exhibits stronger anisotropy than the crystalline polymer, the former has a higher tensile strength in the direction perpendicular to the flow direction than the latter.
Low temperature injection has greater mechanical anisotropy than high temperature injection. For example, when the injection temperature is high, the intensity ratio of the vertical direction to the flow direction is 1.7, and when the injection temperature is low, it is 2.
From this point of view, the increase in melt temperature will lead to a decrease in tensile strength for both crystalline and amorphous polymers, but the mechanism is different. The former is due to the effect of reduction by orientation.