Relationship between plastic internal stress and injection molding parts
Relationship between plastic internal stress and injection molding parts
In the injection molding products, the local stress state is different, and the deformation degree of the product is determined by the stress distribution. If there is a temperature gradient when the product is cooled, this kind of stress will develop, so this kind of stress is called "forming stress".
There are two kinds of internal stress in injection molded parts: one is molding stress, the other is temperature stress. When the melt enters the mold with lower temperature, the melt near the die cavity wall rapidly cools and solidifies, so the molecular chain segment is "frozen".Due to the poor thermal conductivity of the solidified polymer layer, there will be a large temperature gradient in the thickness direction of the product, but the core of the product will solidify very slowly, so that when the gate is closed, the melt unit has not solidified. If the injection molding machine stops feeding the cooling shrinkage and shrinkage, because the internal shrinkage of the product is opposite to the action direction of the hard skin layer, the core will be in the position Under static tension, the surface layer is in static compression.
In the process of melt filling, in addition to the stress caused by the volume shrinkage effect, there are also the stresses caused by the expansion effect of the runner and gate outlet; the stress caused by the former effect is related to the melt flow direction, and the latter will cause the stress action perpendicular to the flow direction due to the outlet expansion effect.
For semi crystalline polymers, another effect should be paid attention to, that is, when the glass transition temperature is exceeded, some molecular segments of the amorphous phase retained between the crystalline units will start to move, but will be limited by the crystalline phase to prevent the return of the tensile chain, thus forming internal stress. For crystalline polymer, there is also a kind of deformation induced stress; when the stress applied to the crystalline polymer melt exceeds the elastic deformation limit, the lattice will flow along the sliding surface, resulting in the displacement of plastic deformation, instead of a part of elastic deformation.
Under the condition of stress relaxation with constant total deformation, the stress gradually decreases to a certain minimum value which is not equal to zero, which is called "deformation induced".
It can also be assumed that there is a crystallization model for crystalline polymers. The stacking displacement is formed during the crystallization process, which makes it difficult for the lattice to accumulate further on the sliding surface. Therefore, the reaction force is generated, which is equal to the stress required to maintain the lattice displacement structure. Moreover, the lattice displacement structure is formed in the non-equilibrium state without stress. This is the explanation of the "deformation induced internal stress" displacement mechanism, but it is not applicable to amorphous polymers.
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, and when used below the glass transition temperature, irregular deformation or warping will often occur, and the surface of the product will be "white", turbid, and the optical performance will be deteriorated.
The internal stress reduces the resistance of the products to light, heat and corrosive media. Under the action of environment, stress cracking or "cracking" occurs. Therefore, it is of great significance to reduce or homogenize the internal stress of products. However, the internal stress can also be used. For example, the anisotropic mechanical characteristics of the oriented internal stress can be used to produce higher strength in the direction of stress. Products such as tensile film and braided tape can be used selectively in application. But for injection molded parts, it is expected that the internal stress is small and evenly distributed.
Reducing the temperature at the gate and increasing the slow cooling time can improve the uneven stress in the products and make the mechanical properties uniform. For crystalline polymers, the tensile strength is anisotropic.
With the increase of melt temperature, the tensile strength of both crystalline and amorphous polymers decreases, but the mechanism is different: the former is affected by the decrease of crystallinity; the latter is affected by orientation.
