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Process Discussion: How is the internal stress formed?
Oct 23, 2017

1.the internal stress generated


In the injection molded products, the local stress state is different, the degree of deformation of the product will be determined by the stress distribution. If the product is in cooling. There is a temperature gradient, then such stress will develop, so this kind of stress is also called "forming stress".

   

Injection molded products, two kinds of internal stress package: one is injection molding products, the other is the temperature stress. When the melt enters the mold with a lower temperature, the melt near the cavity wall is cooled and solidified, and the molecular segments are "frozen". Due to the solidified polymer layer, the thermal conductivity is poor and a large temperature gradient is produced in the thickness direction of the product. The solidification of the product is rather slow, so that when the gate is closed, the melt unit of the product center has not yet solidified, and the injection molding machine can not feed the cooling shrinkage.


So that the internal contraction of the product and the direction of the hard cortex is the opposite direction; the heart is in the static stretch and the surface is in a static compression.

   

In the melt filling flow, in addition to the volume shrinkage effect caused by stress. As well as the stress caused by the expansion effect of the runner and gate outlet. The stress caused by the former effect is related to the direction of the melt flow, which will cause the stress in the direction perpendicular to the flow direction.


2. the impact of stress on the process factors


(1) Effect on stress In the case of rapid cooling, orientation leads to the formation of internal stress in the polymer. Due to the high viscosity of the polymer melt, the internal stress can not be loosened quickly, affecting the physical properties and dimensional stability of the product.


The influence of various parameters on the orientation stress:


A: The melt temperature is high, the melt temperature is high, the viscosity is low, and the degree of decrease of the shear stress is reduced. On the other hand, because the melt temperature is high, the stress relaxation is accelerated and the solution orientation ability is enhanced.


B: However, without changing the pressure of the injection molding machine, the mold cavity pressure will increase, strong shear and lead to increased stress.


C: Before the nozzle is closed, prolonging the holding time will result in an increase in the orientation stress.


D:Increase the injection pressure or packing pressure, will increase the orientation stress,


E: Mold temperature can ensure that the product is slow to cool, play the role of solution.


F: Increasing the thickness of the product reduces the orientation stress, because the thick-walled product cools slowly, the viscosity increases slowly, and the stress relaxation process takes a long time, so the orientation stress is small.


(2) the effect on temperature stress


As described above, since the temperature gradient between the melt and the ply is large at the time of filling, the solidified outer melt is required to assist in the shrinkage of the solidified inner layer melt, resulting in a compressive stress (shrinkage stress) , The inner layer produces tensile stress (orientation stress).

  

If the mold is maintained under the action of the holding pressure for a long time, the polymer melt is added to the mold cavity, so that the cavity pressure increases, the pressure will change due to uneven temperature and internal stress. But in the holding time is short, mold cavity pressure is low, the product will still maintain the original internal cooling stress state.

   

If the cavity of the product is insufficient in the initial stage of the cooling of the product, the outer layer of the product will form a depression due to the shrinkage of the product. If the cavity pressure of the product after the cold layer has been formed is insufficient, the inner layer of the product may be separated by shrinkage or The formation of holes; if the gate closed before the maintenance of mold cavity pressure, is conducive to improving the product density, eliminating the cooling temperature stress, but in the vicinity of the gate will produce a large stress concentration.

   

In view of this, the greater the pressure in the mold, the longer the pressure in the mold, the longer the shrinkage stress that contributes to the temperature will 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 of the product and the use of performance; due to the existence of products and uneven distribution of products, products in the course of the use of cracks. When used under the glass transition temperature, often irregular deformation or warping, but also cause the product surface "whitening", turbidity, optical properties deteriorate.

   

Trying to reduce the gate temperature, increase the slow cooling time, is conducive to improving the uneven stress products, so that the mechanical properties of products uniform.

   

Whether the crystalline polymer or non-crystalline polymer, tensile strength are showing the characteristics of the anisotropy. The tensile strength of the non-crystalline polymer will vary depending on the gate. When the gate and the filling direction are consistent, the tensile strength decreases with the increase of the melt temperature. When the gate is perpendicular to the filling direction, The tensile strength increases as the melt temperature increases.

   

As the melt temperature increases, the solution orientation is enhanced and the orientation effect is reduced to reduce the tensile strength. The orientation of the gate will affect the orientation by influencing the orientation of the stream, and because the anisotropic properties of the non-crystalline polymer are stronger than those of the crystalline polymer, the tensile strength in the direction perpendicular to the flow direction is higher than that of the latter Big. Low temperature injection has greater mechanical anisotropy than high temperature injection. If the injection temperature is high, the intensity ratio of vertical direction to flow direction is 1.7, and the injection temperature is low.

   

In view of this, the increase in melt temperature, both for crystalline and non-crystalline polymers will lead to lower tensile strength, but the mechanism is not the same; the former is due to the effect of reduced by the role of orientation.


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