I. Internal Stress

·  Internal Stress Generation

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

The internal stresses in injection molded products include two kinds: one is the molding stress of injection molded products and the other is the temperature stress. When the melt enters the mold at a lower temperature, the melt that is leaning into the wall of the mold cavity cools rapidly and solidifies, so the molecular chain segments are "frozen".

Because of the solidification of the polymer layer, thermal conductivity is very poor, in the direction of the thickness of the product will produce a large temperature gradient, while the heart of the product is solidified quite slowly, so that when the gate is closed, the melt unit is not solidified, if the injection molding machine and stop the cooling shrinkage to make up for the time, due to the contraction of products inside the role of the hard skin layer with the role of the direction of the opposite direction, the heart of the static tensile and the surface layer is in the static Compression.

In the melt mold flow, in addition to volume contraction effect caused by the stress, there are due to the runner, gate outlet expansion effect caused by the stress; the former effect caused by the stress and melt flow direction, the latter due to the outlet expansion effect will cause perpendicular to the flow direction of the stress effect.

For semi-crystalline polymers should also pay attention to another effect, that is, when more than the glass transition temperature, the crystalline units retained between some of the non-crystalline phase of the molecular chain will begin to move, but was limited by the crystalline phase, preventing the stretching of the chain to return to the formation of internal stresses. 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, generating a displacement of the plastic deformation and replacing part of the elastic deformation.

Under conditions of stress relaxation with constant total deformation, the stress gradually decreases to a minimum value not equal to zero, and this retained value is "deformation-induced".

For the explanation of this situation, it can also be assumed that the crystalline polymer has a crystallization model, during the crystallization process, the formation of stacking displacement, so that the lattice on the sliding surface of the further accumulation of difficulties, so that the generation of the reaction force, the magnitude of which is equal to the amount of stress required to maintain the structure of the lattice displacement, and the structure of such a lattice displacement in the absence of stresses in the formation of the non-equilibrium state. 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 products will seriously affect the mechanical properties of the products and the use of performance; due to the existence and uneven distribution of internal stress in the products, the products in the process of cracking, in the glass transition temperature below the use of the use of the use of irregular deformation or warping often occurs, but also cause the surface of the products, "whiteness", turbidity, optical properties deteriorate.

Internal stress reduces the resistance of products to light, heat and corrosive media, in the environment, stress cracking or "cracking", therefore, reduce or equalize the internal stress of products is of great significance. However, internal stress can also be utilized, for example, the mechanical characteristics of anisotropy produced by oriented internal stress can be utilized to produce higher strength in the direction of force, and the products can be used selectively in applications such as the production of stretch films and woven belts, etc. However, for injection molded products, it is hoped that internal stress will be reduced or homogenized. However, for injection molded products, it is hoped that the internal stress is small and evenly distributed.

Reduce the temperature at the gate, increase the slow cooling time, is conducive to improving the uneven stress in the product, so that the mechanical properties are uniform. For crystalline polymers, tensile strength are characterized by anisotropy.

Increase in melt temperature, whether crystalline or non-crystalline polymers will lead to a reduction in tensile strength, but the two mechanisms are not the same: the former is due to a decrease in the degree of crystallinity; the latter is through the effect of orientation and influence.

Second, impact strength

The impact strength of injection molded products show more prominent anisotropy. In addition to the impact strength of the molecular structure of polymers and injection molding process conditions, but also with the shape of the product structure, gate and location, number, distribution and arrangement of forms.

This is because the impact strength, mainly formed by the polymer processing process of internal stress (orientation stress, temperature stress, deformation - induced stress) is determined.

Third, product shrinkage

·  Shrinkage process

The shrinkage of injection molded products in the molding process can be divided into 3 stages.

The first stage is the holding pressure stage before the gate solidifies. The shrinkage of the product depends largely on the degree of compensation of the melt. Due to the low temperature of the mold, the temperature of the melt is decreasing, and the density and viscosity of the melt are increasing. Therefore, the compensation ability of the melt at this time depends mainly on the size of the holding pressure and the time to maintain the transfer to the mold.

The second stage is the cooling stage from the beginning of the solidification at the gate to the demolding. At this stage, no more melt enters the mold cavity, and the weight of the product will 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 use stage. This is free shrinkage.

·  Shrinkage control

Injection molding process aspects:

. The mold temperature should not be too high. For example, for polyformaldehyde products. When the mold temperature 80 ℃ 40 ℃, then the shrinkage is 5%.

. The barrel temperature should not be too high. For example, for paraformaldehyde products, when the melt temperature is 190°C 10°C, the shrinkage rate is 2.5%.

. The injection pressure can be increased appropriately. For example, for paraformaldehyde products, when the injection pressure is 78Mpa 9.8Mpa, the shrinkage rate is 5%.

. Appropriately increase the injection rate.

. The holding pressure time should be set longer.

. Increase the cooling time appropriately.

. Control the cooling speed of the mold.

Material:

. Select materials with uniform particles so that the particles are heated uniformly and the temperature is consistent everywhere, so that the cooling rate is uniform.

. To choose the material with suitable molecular weight size and melt index, and uniform molecular weight distribution, so that the process conditions are easy to control, and the filling flow is stable, which is conducive to reducing shrinkage.

. For crystalline polymers provide conditions to reduce crystallinity and stabilize crystallinity, and for non-crystalline polymers create factors to reduce deorientation.

. Selection of polymers with low hygroscopicity can reduce shrinkage by drying and reducing moisture.

. Select polymers with good fluidity and low melt index.

. Selection of composites with reinforcing fillers can reduce shrinkage.

In terms of molds:

. According to the mold shrinkage rate, design the mold cavity size tolerance reasonably, choose the mold material with small expansion coefficient.

. Appropriately large gate cross-sectional area can help reduce shrinkage.

. Shorten the inner runner and reduce the flow length ratio, which is helpful to make up the shrinkage.