Three factors to control the internal quality of injection molding products

Three factors to control the internal quality of injection molding products

Internal stress generation

In the injection molding products, the local stress states are different, and the deformation degree of the products will be 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 force is called "forming stress".

There are two kinds of internal stress in injection molding products: one is molding stress, the other is temperature stress. When the melt enters the mold with lower temperature, the melt entering the mold cavity wall cools rapidly 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, while 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, because the internal shrinkage of the product is opposite to the action direction of the hard skin layer, the center will be In static tension, the surface layer is in static compression.

In the process of melt filling flow, in addition to the stress caused by volume shrinkage effect, there are also the stress caused by the expansion effect of runner and gate outlet; the stress caused by the former effect is related to the direction of melt flow, and the stress caused by the expansion effect of outlet will be perpendicular to the direction of flow.

For semi crystalline polymers, another effect should be paid attention to, that is, when the glass transition temperature is exceeded, the molecular segments of some non crystalline phases retained between the crystallization units will start to move, but they are limited by the crystalline phase, preventing the return of the tensile chain, thus forming the 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, 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 minimum value which is not equal to zero, and this retention value is called "deformation induced".

For the explanation of this situation, it can also be assumed that there is a crystal model of crystalline polymer. During the crystallization process, stacking displacement is formed, which makes it difficult for the lattice to further pile up on the sliding surface, so a reaction force is generated, which is equal to the stress required to maintain the lattice displacement structure, and the lattice displacement structure is formed in the non-equilibrium state without stress. This is the explanation of the displacement mechanism of "deformation induced internal stress", but it is not suitable for amorphous polymers.

Relationship between internal stress and product quality

The existence of internal stress in products will seriously affect the mechanical properties and service performance of products; due to the existence and uneven distribution of internal stress in products, cracks occur in the use process of products, and irregular deformation or warpage often occur when products are used below the glass transition temperature, which will also cause "whitening", turbidity and deterioration of optical properties of products.

 Internal stress reduces the resistance of products to light, heat and corrosive media. Under the action of environment, stress cracking or "crazing" 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 mechanical characteristics of anisotropic orientation can be used to produce higher strength in the direction of stress. In the application, products can be selectively used, such as the production of tensile films and woven tapes. However, it is expected that the internal stress of injection molding products is small and evenly distributed.

Reducing the temperature of the gate and increasing the slow cooling time will help to 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 will decrease, but the mechanism is different: the former is affected by the decrease of crystallinity; the latter by the effect of orientation.

impact strength

The impact strength of injection molding products shows more prominent anisotropy. The impact strength is not only related to the molecular structure of the polymer and the injection process conditions, but also related to the product structure shape, gate and location, number, distribution and arrangement.

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

Product shrinkage

Shrinkage process

The shrinkage of injection molding products can be divided into three stages.

The first stage is the pressure maintaining stage before the gate solidification. The shrinkage of the product depends largely on the compensation of the melt. Because of the low temperature of the die, the melt temperature is decreasing and the melt density and viscosity are increasing. Therefore, the compensation ability of melt mainly depends on the magnitude of holding pressure and the time to maintain the transfer to the mold.

The second stage is the cooling stage from gate solidification to demoulding. At this stage, no melt will enter the cavity, the weight of the product will not be changed, but the density or specific volume of the product will change.

The third stage is the shrinkage from the beginning of demoulding to the use stage. This is a free contraction.

Control of shrinkage

Injection molding process:

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

The barrel temperature should not be too high. For example, when the melt temperature of POM products is 190 ℃ 10 ℃, the shrinkage is 2.5%.

The injection pressure can be increased properly. For example, for POM products, when the injection pressure is 78mpa 9.8MPa, the shrinkage is 5%.

Increase the injection rate properly.

The pressure holding time should be set longer.

Increase the cooling time appropriately.

Material:

Select the material with uniform particles to make the particles evenly heated, with uniform temperature and uniform cooling speed.

The materials with proper molecular weight and melt index and uniform molecular weight distribution should be selected so that the process conditions are easy to control, the filling flow is stable and the shrinkage is reduced.

For crystalline polymers, the conditions of reducing crystallinity and stabilizing crystallinity should be provided, and for non crystalline polymers, the factors of reducing solution orientation should be created.

The shrinkage can be reduced by drying and reducing water content.

Select the polymer with good fluidity and low melt index.

The composite material with reinforced filler can reduce the shrinkage.

On the mold side:

According to the shrinkage rate of the die, the tolerance design of the die cavity is reasonable, and the die material with small expansion coefficient is selected.

A proper increase in gate cross-sectional area will help to reduce shrinkage.

Shortening the inner flow passage and reducing the flow length ratio is conducive to feeding.