How Is Internal Stress Formed?

Internal stress generation

In injection molded products, local stress states are different everywhere, and the degree of product deformation will depend on the stress distribution. If the product is cooling. If there is a temperature gradient, this kind of stress will develop, so this kind of stress is also called "forming stress".

There are two types of internal stress for injection molded products: one is the molding stress of the injection molded product, and the other is the temperature stress. When the melt enters the mold with a lower temperature, the melt close to the cavity wall quickly cools and solidifies, so the molecular chain is "frozen." Due to the solidified polymer layer, the thermal conductivity is very poor, resulting in a large temperature gradient in the thickness direction of the product. The core part of the product solidifies quite slowly, so that when the gate is closed, the melt unit in the center of the product has not solidified yet, and the injection molding machine cannot supplement the cooling shrinkage.

In this way, the internal shrinkage of the product is opposite to the direction of the hard skin layer; the core is in static tension and the surface layer is in static compression.

In addition to the stress caused by the volume shrinkage effect during the melt filling and flow. There is also the stress caused by the expansion effect of the runner and gate outlet; the stress caused by the former effect is related to the flow direction of the melt, and the latter will cause stress in the direction perpendicular to the flow direction due to the outlet expansion effect.

2. Process factors affecting stress

(1) The influence of directional stress Under 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 cannot be relaxed quickly, which affects the physical properties and dimensional stability of the product.

The influence of various parameters on orientation stress:

a Melt temperature, the melt temperature is high, the viscosity is low, the shear stress is reduced, and the degree of orientation decreases; on the other hand, the high melt temperature will accelerate the stress relaxation and promote the enhancement of de-orientation ability.

b However, without changing the pressure of the injection molding machine, the cavity pressure will increase, and the strong shearing action will lead to an increase in the orientation stress.

c. Extending the holding time before the nozzle is closed will cause the orientation stress to increase.

d Increasing the injection pressure or holding pressure will increase the orientation stress,

e The high mold temperature can ensure that the product cools slowly and play a de-orientation effect.

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) 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 layer of melt that solidifies first helps to stop the shrinkage of the inner layer of melt that solidifies later, resulting in compressive stress (shrinkage stress) in the outer layer. , The inner layer produces tensile stress (orientation stress).

If the holding pressure continues for a long time after the mold is filled, the polymer melt is filled into the mold cavity again to increase the pressure of the cavity, and this pressure will change the internal stress caused by uneven temperature. However, when the holding time is short and the cavity pressure is low, the inside of the product will still maintain the original stress state during cooling.

If the mold cavity pressure is insufficient at the initial stage of cooling of the product, the outer layer of the product will form depressions due to solidification and shrinkage; if the mold cavity pressure is insufficient in the later stage when the product has formed a chilled layer, the inner layer of the product will separate due to shrinkage, or Cavities are 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 greater stress concentration will occur near the gate.

It can be seen from this that when thermoplastic polymers are molded, the greater the pressure in the mold, the longer the holding time, which helps to reduce the shrinkage stress generated by the temperature, which will increase the compression 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 the internal stress of the product, the product will crack during use. When used below the glass transition temperature, irregular deformation or warping often occurs, which can also cause "whitening", turbidity, and deterioration of optical properties on the surface of the product.

Try to reduce the temperature at the gate and increase the slow cooling time, which 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 non-crystalline polymer, the tensile strength shows anisotropic characteristics. The tensile strength of amorphous polymers will vary depending on the placement of the gate; when the gate is the same as the filling direction, the tensile strength decreases with the increase of 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 de-orientation effect is strengthened, while the weakened orientation effect reduces the tensile strength. The orientation of the gate will affect the orientation by influencing the direction of the material flow, and because amorphous polymers have stronger anisotropy than crystalline polymers, the tensile strength perpendicular to the flow direction of the former is higher than that of the latter. Big. Low temperature injection has greater mechanical anisotropy than high temperature injection. For example, when the injection temperature is high, the strength ratio between the vertical direction and 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 result in a decrease in tensile strength for both crystalline and amorphous polymers, but the mechanism is different; the former is due to the reduction of orientation.