Do you know all the factors that generate and influence internal stress in injection moulded products?
1. internal stress generation
In the injection moulded products, the local stress state is different in each place, and the degree of deformation of the products will be determined by the 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”.
Let’s take a look at the introduction of extrusion machine.
There are two types of internal stress packages for injection moulded products: one is the moulding stress and the other is the temperature stress. When the melt enters the mould at a lower temperature, the melt near the cavity wall cools and solidifies at a rapid rate, so that the molecular chain segments are “frozen”. Due to the poor thermal conductivity of the solidified polymer layer, a large temperature gradient is created in the direction of the thickness of the product. The heart of the product solidifies very slowly, so that when the gate is closed, the melt unit in the centre of the product has not yet solidified, and the injection moulding machine is then unable to compensate for the cooling shrinkage.
The internal shrinkage of the product is thus in the opposite direction to the action of the hard skin layer; the core is in static tension while the surface layer is in static compression.
When the melt is filling the mould and flowing, 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, the latter due to the expansion effect of the outlet will cause stresses in the direction perpendicular to the flow.
2.Process factors affecting stress
(1) to the effect of stress in the rapid cooling conditions, orientation will lead to the formation of internal stress in the polymer. Due to the high viscosity of the polymer melt, the internal stress can not be quickly relaxed, affecting the physical properties and dimensional stability of the product.
Influence of various parameters on orientation stresses.
Melt temperature, high melt temperature, low viscosity, lower shear stress and reduced orientation; on the other hand, high melt temperature will make the stress relaxation faster and promote the ability to strengthen the unorientation.
However, without changing the pressure of the injection moulding machine, the mould cavity pressure will increase and the strong shear effect will lead to an increase in orientation stress.
Prolonging the holding time until the nozzle is closed will lead to an increase in orientation stress.
Increasing the injection pressure or holding pressure will increase the orientation stress.
A high mould temperature ensures that the product cools slowly and acts as an unorientation.
Increase the thickness of the product to reduce the orientation stress, because thick-walled products cool slowly, viscosity increases slowly, the stress relaxation process of a long time, so the orientation stress is small.
(2) The effect on temperature stress
As mentioned above, due to the temperature gradient between the melt and the wall when filling the mould, the outer layer of the melt that solidifies first has to help stop the shrinkage of the inner 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.
If the mould is filled and then held under pressure for a longer period of time, the polymer melt is added to the mould cavity, increasing the cavity pressure, which changes the internal stresses due to uneven temperatures. However, if the holding time is short and the cavity pressure is low, the product will remain in the same state of stress as when it was cooled.
If the mould cavity pressure is not sufficient at the beginning of the cooling process, the outer layer of the product will shrink due to solidification and form a depression; if the mould cavity pressure is not sufficient 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 a cavity; if the mould cavity pressure is maintained before the gate is closed, it will help to increase the density of the product and eliminate the cooling temperature stress, but a large stress concentration will be generated near the gate.
Thus it seems that the greater the pressure in the mould the longer the holding time, which will help to reduce the shrinkage stress generated by the temperature and conversely increase the compressive stress.
3. The relationship between internal stress and product quality
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, the product will crack in the process of use. When used below the glass transition temperature, irregular deformation or warping often occurs, which can also cause the surface of the product to become “white” and cloudy, and the optical properties to deteriorate.
The lowering of the temperature at the gate and the increase of the cooling time will help to improve the unevenness of the stress in the product and make the mechanical properties of the product homogeneous.
For both crystalline and non-crystalline polymers, the tensile strength is anisotropic. 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 mould filling, the tensile strength decreases as the melt temperature increases; when the gate is perpendicular to the mould filling direction, the tensile strength increases as the melt temperature increases.
The increase in melt temperature leads to an increase in unorientation and a decrease in tensile strength due to a decrease in orientation. The orientation of the gate affects the orientation by influencing the direction of the flow, and because amorphous polymers are more anisotropic than crystalline polymers, the tensile strength in the direction perpendicular to the flow is greater for the former than for the latter. Low temperature injection has greater mechanical anisotropy than high temperature injection, e.g. the strength ratio in the vertical to flow direction is 1.7 at high injection temperatures and 2 at low injection temperatures .
It would appear that an increase in melt temperature, whether for crystalline or non-crystalline polymers, leads to a reduction in tensile strength, but by a different mechanism; the former is due to a reduced effect through orientation.