All that stuff about injection molding

In 2020, the domestic chemical market ushered in a wave of price increases. Taking the main products of the plasticizer industry chain as an example, the price basically showed a straight rise. Time passed the end of the year, the injection molding machine market in 2021 seems to have been infected. Recently, many injection molding machine manufacturers issued price hike letters intensively, saying that due to the impact of the epidemic, the price of raw materials rose sharply, so they made the decision to adjust the price.

Many insiders believe that with the rise of new downstream applications, such as the plastic back of 5G mobile phones and lightweight structural parts of automobiles, injection molding products are expected to usher in a new round of business cycle, and the industry is expected to continue to rise in 2020-2021. So today we will learn about those things about injection products!

What are injection molded products

Injection products refers to the product molding machine is used to heat the plastic to plasticize and melt, and then injected into the molding mold cavity molding, cooling, solidification after stripping, injection molding machine Injection molding products.

1. Liquidity

Different thermoplastics have different technological properties, shrinkage properties and physical properties. For crystalline plastics, when the processing temperature is above the melting point, the fluidity is better, and the cavity can be filled quickly, requiring less injection pressure. For amorphous plastics, their fluidity is poor, so the injection speed is slow and the pressure is high.

Therefore, in the design of the mold, the fluidity of the plastic can be used as the basis for calculating the size of the flow channel system. This can not only avoid the waste of materials, prolong the injection molding cycle, but also avoid the problems of filling difficulties and pressure preservation difficulties due to the small size of the flow channel system.

2. Contractility

When thermoplastics change from melting state to solidification state, their volume will shrink in different degrees. In general, the shrinkage range of crystallized plastics is 1.0% - 3.0%, and that of amorphous plastics is 0.4% - 0.8%. Therefore, the shrinkage rate and shrinkage range of crystallized plastics are larger than that of amorphous plastics, and they are more susceptible to the influence of molding process. After demoulding, crystalline plastic can also crystallize at room temperature and then continue to shrink. Its shrinkage depends on the thickness and temperature of the product. The thicker the product, the greater the shrinkage.

3. Rheology

Rheology refers to the relationship between stress, deformation, deformation rate and viscosity during machining. According to the rheological properties of plastics, plastics can be divided into shear sensitive plastics and heat sensitive plastics. Common shear sensitive plastics are ABS, PS, PE, PP, POM, etc. The viscosity of this plastic will decrease rapidly with the increase of shear rate. Common heatsensitive plastics include PC, PA, PMMA and so on. This kind of plastic has a strong dependence on temperature. The higher the temperature, the faster its viscosity decreases.

From the perspective of polymer, the shear rate has an impact on the viscosity of the above two kinds of plastics. The increase of shear rate can reduce the viscosity of melt to different degrees, so that melt produces "shear thinning" phenomenon, and the rheological properties of injection products will also change.

4. Orientation effect

Orientation effect will be produced during the flow of plastic melt, which can affect the properties of injection products. The macromolecules of plastic melt are stretched under the action of external forces, and then line up parallel to each other along the direction of flow. This arrangement cannot be eliminated before the plastic cools and solidifies, and will freeze in the solid product, forming the orientation effect. Orientation effect will weaken the integrity of the product, resulting in uneven shrinkage and warping deformation of injection products.

Cause and treatment of deformation of injection products

Causes of deformation

1. Residual stress caused by the shape of the product, especially the relationship between the forming shrinkage and the thickness of the product.

2. Residual stress due to forming conditions

3. Residual stress generated during demoulding

4. Deformation caused by insufficient cooling time

Solutions

1. Instant: fully cooling and curing in the mold (prolong the cooling time timing

To increase the cylinder temperature and reduce the injection pressure.

2. Short-term: make the mold cool evenly.

3. Long-term: Avoid the difference of product thickness, in the place of product thickness

Set the gate, because the straight line is easy to cause warping, make a large R curve, product reversible bending mold, increase the number of ejecting rod, increase the release slope.

4. Mold design phase through mold flow analysis of product deformation trend, design optimization, easy to adjust the process.

Treatment of insufficient brightness of injection products

1. If the mold cavity processing is not excellent enough, such as scars, microholes, wear, rough and other defects, it will certainly affect plastic parts, resulting in a large degree of adverse consequences, resulting in the gloss of plastic parts is not good enough. Therefore, the mold should be carefully processed, so that the roughness on the surface of the cavity can be greatly reduced, and, when necessary, can also choose to polish and chrome plating, to achieve the best results.

2. If there are some untreated oil stains or water stains on the surface of the mold cavity, or, too much release agent will make the surface of the plastic parts appear dull and dull. In this case, those oil and water stains should be removed in time, and limit the amount of release agent.

3. If the demoulding inclination of plastic parts is too small, it is difficult to demoulding, or demoulding by too much force, resulting in the smooth surface of plastic parts is too small, it should be increased demoulding inclination, so as to ensure the smoothness of plastic parts.

4. If the exhaust performance of the mold is not very good, too much gas will remain in the model, which will also lead to the poor luster of the final product after forming. Therefore, it should be checked and corrected to ensure that the die exhaust system has sufficient exhaust performance to avoid the existence of trapped gas.

5. If the cross sectional area of the gate or runner is too small, or the change fluctuation is large, the shear force acting on the melt during flow will be too large, resulting in the turbulent dynamic flow of the solution, which will also lead to the poor glossiness of the final product. Therefore, the cross-sectional area of the runner and gate should be appropriately increased to reduce the shear force to avoid turbulence and improve the gloss of the product.

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