3 Things to Consider When Choosing Plastic Injection Molding Materials

The shrinkage rate

The factors affecting thermoplastic shrinkage are as follows:

1.1 Thermoplastics In the molding manufacturing process, due to the volume change from crystallization, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation, and other factors.

Thermoplastics have a larger shrinkage rate, wide shrinkage range, and obvious directivity compared with thermosetting plastics.

In addition, the shrinkage rate after forming, annealing, or humidification treatment is generally larger than thermosetting plastics.

1.2 Characteristics of plastic parts during molding, the molten material contacts the surface of the cavity, and the outer layer is cooled immediately to form a low-density solid shell.

Due to the poor thermal conductivity of plastic, the inner layer of plastic parts is cooled slowly and formed a high-density solid layer with large shrinkage.

So the wall thickness, slow cooling, and high-density layer thick shrinkage. In addition, the presence or absence of inserts, the layout, and the number of inserts directly affect the direction of soft materials flow, the density distribution, and the size of shrinkage resistance, so the characteristics of plastic parts have a greater impact on the size and direction of shrinkage.

1.3 Feed port form, size, and distribution of these factors directly affect the direction of the feed flow, density distribution, pressure holding, shrinkage, and forming time.

The direct feed port and the large section of the feed port (especially the thick section) will be narrowed but the direction is large, and the wide and short lengths of the feed port will be small. Close to the feed port or parallel to the direction of the feed flow, the shrinkage is large.

1.4 Injection molding conditions mold temperature is high, melt material cooling slow, high density, large shrinkage, especially for crystallizing material properties because of high crystallinity, and volume change, so the shrinkage is greater.

The mold temperature distribution is also related to the internal and external cooling of plastic parts and the uniformity of density, which directly affects the shrinkage and directivity of each part.

In addition, the holding pressure and time also have a great impact on the contraction, the pressure is large, and the long time will shrink but the direction is large.

The injection pressure is high, the melt material viscosity difference is small, the interlayer shear stress is small, the elastic bounce after demoulding is large, so the shrinkage can also be reduced appropriately, the material temperature is high, the shrinkage is large, but the directivity is small.

Therefore, adjusting the mold temperature, pressure, injection speed and cooling time and other factors can also be appropriate to change the shrinkage of plastic parts.

Mold design according to the shrinkage range of various plastics, plastic parts wall thickness, shape, feed port form size, and distribution, according to experience to determine the shrinkage rate of each part of the plastic parts, and then calculate the cavity size.

For high-precision plastic parts and difficulty to master the shrinkage rate, the following methods are generally suitable for mold design:

(1) Take a smaller shrinkage rate of the outer diameter of the plastic parts and a larger shrinkage rate of the inner diameter, to leave room for correction after the mold test.

(2) Mold test to determine the form, size, and forming conditions of the pouring system.

(3) To post-treatment plastic parts after the post-treatment to determine the size of the change (measurement must be in the demoulding 24 hours after).

(4) Correct the mold according to the actual shrinkage situation.

(5) Re-test the mold and can appropriately change the production plastic molding process conditions and slightly modify the shrinkage value to meet the requirements of plastic parts.

Liquidity

a) The fluidity of thermoplastics can be generally analyzed from a series of indexes such as molecular weight, melting index, Archimedes helix flow length, performance viscosity, and flow ratio (process length/plastic part wall thickness).

Small molecular weight, wide molecular weight distribution, the molecular structure is poor, melting index is high, screw flow length is long, viscosity is small, flow ratio is large, the flow is good, the same name of the plastic must check its instructions to determine whether the flow is suitable for injection molding.

According to mold design requirements, the fluidity of commonly used transparent plastics can be roughly divided into three categories:

Good fluidity PA, PE, PS, PP, CA, poly (4) methyl shicene;

Medium fluidity polystyrene series plastic resins (such AS ABS, AS), PMMA, POM, polyphenyl ether;

Poor liquidity PC, hard PVC, polyphenyl ether, polysulfone, polyaromatic sulfone, fluorine plastic.

b) The fluidity of various plastics also varies due to various forming factors. The main influencing factors are as follows:

When the material temperature is high, the fluidity increases, but there are also differences among different plastics.

The fluidity of PS(especially impact-resistant type and high MFR value), PP, PA, PMMA, modified polystyrene (such AS ABS, AS), PC, CA, and other plastics varies greatly with the temperature.

For PE and POM, temperature increase or decrease has little influence on their liquidity. So the former is in the molding time to adjust the temperature to control the flow.

When the pressure of injection molding increases, the molten material is subjected to large shear action, and the liquidity also increases, especially PE and POM are more sensitive, so the molding should adjust the injection pressure to control the liquidity.

Mold structure gating system form, size, layout, cooling system design, molten material flow chemical resistance (such as surface finish, feed passage section thickness, cavity shape, exhaust system), and other factors directly affect the actual flow of molten material。

In the cavity, where the molten material reduces the temperature, increasing the flow resistance will reduce the flow. Mold design should be based on the fluidity of the plastic used, choose a reasonable structure.

Molding can also control the material temperature, mold temperature and injection pressure, injection speed, and other factors to properly adjust the filling situation to meet the molding needs.

Crystallinity

Thermoplastics can be divided into two categories: crystallized plastics and non-crystallized (also known as amorphous) plastics according to the fact that no crystallization occurs when they condense.

The so-called crystallization phenomenon is the plastic from a melting point state to condensation, the molecules from independent movement, completely in an unordered state, into molecules that stop free movement.

According to a slightly fixed position, there is a tendency to make the molecular arrangement into a regular model of a phenomenon.

As the standard to distinguish the appearance of these two types of plastics, the transparency of the thick wall plastic parts is determined by the general crystalline material is opaque or translucent (such as POM, etc.), the amorphous material is transparent (such as PMMA, etc.).

However, there are some exceptions, such as poly (4) methyl thiophene is a crystalline plastic but has high transparency, and ABS is amorphous material but not transparent.

In the mold design and material selection of an injection molding machine, we should pay attention to the following requirements and precautions for crystallized plastics:

The injection molding plastic materials temperature rises to the molding temperature required more heat resistance, to use the plasticizing capacity of large equipment.