Internal stress
(1) Internal stress generation
In an injection molded article, the local stress states are different, and the degree of deformation of the article will be determined by the stress distribution. This type of stress develops if the product has a temperature gradient when it is cooled, so this type of force is called "forming stress."
The internal stress of the injection molded article includes two types: one is the molding stress of the injection molded product, and the other is the temperature stress. When the melt enters the lower temperature mold, the melt that passes into the wall of the mold cavity cools rapidly to solidify, and the molecular segments are "frozen".
Due to the solidified polymer layer, the thermal conductivity is very poor, a large temperature gradient will occur in the thickness direction of the product, and the core of the product solidifies so slowly that the melt unit has not solidified when the gate is closed. If the injection molding machine stops feeding the cooling contraction, since the internal shrinkage of the product is opposite to the direction of the hard skin layer, the core will be in static stretching and the surface layer will be statically compressed.
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 direction of the melt flow, The effect of the stress perpendicular to the flow direction will be caused by the effect of the outlet expansion.
Another effect should be noted for semi-crystalline polymers, that is, when the glass transition temperature is exceeded, the molecular segments of some amorphous phases remaining between the crystallization units will start to move, but they are limited by the crystalline phase. The return of the stretch chain is prevented, thus forming an internal stress. For crystalline polymers, there is also a 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, causing a plastic deformation displacement, replacing a part. Elastic deformation.
Under the stress relaxation condition with constant deformation, the stress gradually drops to a certain minimum value that is not equal to zero. This reserved value is “deformation-inductionâ€. For the explanation of this case, it is also conceivable that the crystalline polymer has a crystallization model, and a stacked displacement is formed during the crystallization process, which makes it difficult to further accumulate the crystal lattice on the sliding surface, thus generating a reaction force whose size is equal to the retention lattice. The stress required to displace the structure, and this lattice displacement structure is formed in a non-equilibrium state without stress. This is an explanation of the "deformation-induced internal stress" displacement mechanism, but it does not apply to amorphous polymers.
(2) 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 stress in the product, the product will crack during use, and when used below the glass transition temperature, irregularities often occur. Deformation or warpage can also cause the surface of the product to be "whitened", turbid, and deteriorated in optical properties.
The internal stress reduces the resistance of the product to light, heat and corrosive media. Under the action of the environment, stress cracking or "cracking" occurs. Therefore, it is important to reduce or homogenize the internal stress of the product. However, internal stresses also have an available side. For example, the intrinsic stress can be used to produce anisotropic mechanical properties, resulting in higher strength in the direction of force, and selective use of articles in applications, such as the production of stretched films and Braided belt, etc. However, it is desirable for the injection molded article to have a small and uniform internal stress.
Reducing the temperature at the gate and increasing the slow cooling time are beneficial to improve the stress unevenness in the product and make the mechanical properties uniform. For crystalline polymers, the tensile strength is characterized by anisotropy.
The increase in melt temperature, whether for crystalline or amorphous polymers, leads to a decrease in tensile strength, but the mechanism is different: the former is affected by the decrease in crystallinity; the latter is affected by the orientation. .
2. Impact strength
The impact strength of the injection molded article exhibits more outstanding anisotropy. In addition to the molecular structure of the polymer and the conditions of the injection molding process, the impact strength is also related to the structural shape, gate and position, number, distribution and arrangement of the product. This is because the impact strength is mainly determined by the internal stress (orientation stress, temperature stress, deformation-induced stress) formed by the polymer processing.
3. Product shrinkage
(1) Shrinkage process
The shrinkage of the injection molded article during the molding process can be divided into three stages.
The first stage is the pressure holding stage before the gate solidifies. The shrinkage of the product is highly dependent on the degree of compensation of the melt. Due to the low mold temperature, the melt temperature is continuously decreasing, and the melt density and viscosity are continuously increasing. Therefore, the compensating ability of the melt at this time mainly depends on the magnitude of the holding pressure and the time for maintaining the transfer into the mold.
The second stage is the cooling phase from the start of the gate solidification to the demolding. At this stage, no melt enters the cavity, and the weight of the product does not change, but the density or specific volume of the product will change.
The third stage is the shrinkage from the start of demolding to the stage of use. This is a free contraction.
(2) Control of shrinkage rate
1 injection molding process
a. The mold temperature should not be too high. For example, for POM products. When the mold temperature was 80 ° C and 40 ° C, the shrinkage was 5%.
b. The barrel temperature should not be too high. For example, a polyoxymethylene product has a shrinkage of 2.5% when the melt temperature is 190 ° C and 10 ° C.
c. The injection pressure can be appropriately increased. For example, for a polyoxymethylene product, when the injection pressure is 78 MPa and 9.8 MPa, the shrinkage is 5%.
d. Properly increase the injection rate.
e. The holding time should be set longer.
f. Increase the cooling time as appropriate.
g. Control the cooling rate of the mold.
2 materials
a. Select a material with uniform particles, so that the particles are evenly heated, and the temperature is uniform everywhere, so that the cooling rate is uniform.
b. It is necessary to select a material with a suitable molecular weight and a melt index and a uniform molecular weight distribution, so that the process conditions are easy to control, the flow is stable, and the shrinkage is reduced.
c. Providing conditions for reducing crystallinity and stabilizing crystallinity for crystalline polymers, and creating factors for reducing the orientation of the non-crystalline polymers.
d. Select a polymer with low hygroscopicity, and reduce shrinkage by drying and reducing moisture.
e. Select a polymer with good fluidity and low melt index.
f. Use a composite with a reinforcing filler to reduce shrinkage.
3 in terms of mold
a. According to the shrinkage rate of the mold, the mold cavity tolerance is designed reasonably, and the mold material with small expansion coefficient is selected.
b. The gate cross-sectional area is appropriately increased to help reduce shrinkage.
c. Shorten the inner flow channel and reduce the flow length ratio, which is conducive to feeding.
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