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What is plastic shrinkage, fluidity and crystallinity?

What is plastic shrinkage, fluidity and crystallinity?

Date of issue:2019-04-12 Authors:admin Click:

First, the shrinkage rate

The factors that affect the shrinkage of thermoplastics are as follows:

1. Plastics During the molding process of thermoplastics, due to the volume change of crystallization, the internal stress is strong, the residual stress in the plastic parts is large, and the molecular orientation is strong, so the shrinkage rate is compared with that of thermosetting plastics. Larger, wide range of shrinkage, obvious directionality, and shrinkage after shrinkage, annealing or humidity control after molding is generally larger than thermosetting plastics.

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2. When the plastic part is formed, the molten material contacts the surface of the cavity and the outer layer is immediately cooled to form a low-density solid outer casing. Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part is slowly cooled to form a high-density solid layer with a large shrinkage. Therefore, the wall thickness, the slow cooling, and the high density layer thickness are large.

In addition, the presence or absence of inserts and insert layouts and quantities directly affect the flow direction, density distribution and shrinkage resistance, so the characteristics of the plastic parts have a great influence on the shrinkage size and directionality.

3. The form, size and distribution of the feed port directly affect the flow direction, density distribution, pressure-preserving and shrinking action and molding time. The direct feed port and the feed port have a large cross section (especially thicker cross section), but the shrinkage is small but the directivity is large, and the feed port width and the short length are small. The shrinkage is large near the feed port or parallel to the flow direction.

4. Molding conditions The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large. Especially for the crystallized material, the crystallinity is high and the volume changes greatly, so the shrinkage is larger. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic parts, which directly affects the amount of shrinkage and directionality of each part.

In addition, maintaining pressure and time also has a large effect on shrinkage. When the pressure is large and the time is long, the shrinkage is small but the directivity is large. The injection pressure is high, the viscosity difference of the melt is small, the interlaminar shear stress is small, and the elastic rebound is large after demolding, so the shrinkage can also be reduced appropriately, the material temperature is high, the shrinkage is large, but the directivity is small. Therefore, factors such as adjustment of mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of the plastic part.

5. When  designing the mold , according to the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the size and distribution of the feed port, the shrinkage rate of each part of the plastic part is determined empirically, and then the cavity size is calculated.

For high-precision plastic parts and difficult to grasp the shrinkage rate, it is generally advisable to design the mold by the following method:

The test mold determines the form, size and molding conditions of the casting system. The plastic parts to be treated are post-treated to determine the dimensional change (measured must be 24 hours after demolding). 
Correct the mold according to the actual shrinkage. Re-test the mold and adjust the process conditions slightly to correct the shrinkage value to meet the requirements of the plastic parts.

Second, liquidity

The fluidity of thermoplastics can generally be analyzed from a series of indices such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (flow length / plastic wall thickness).

Low molecular weight, wide molecular weight distribution, poor molecular structure, high melt index, long snail flow length, low viscosity, and good flow ratio. For injection molding.

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

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

Medium-sized polystyrene resin (such as ABS, AS), PMMA, POM, polyphenylene ether;

Poor flowability of PC, hard PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastic. 
The fluidity of various plastics also varies with each molding factor. The main factors affecting are as follows:

1. When the temperature of the material is high, the fluidity increases, but different plastics also have different characteristics. PS (especially high impact resistance and MFR), PP, PA, PMMA, modified polystyrene (such as ABS, AS) The fluidity of plastics such as PC and CA varies greatly with temperature. For PE and POM, the temperature increase and decrease has little effect on the fluidity. Therefore, the former should adjust the temperature to control the fluidity during molding.

2. When the pressure injection pressure is increased, the molten material is subjected to large shearing action and fluidity is also increased. Especially PE and POM are sensitive, so the injection pressure should be adjusted to control the fluidity during molding.

3. The form, size, arrangement, cooling system design, melt flow resistance (such as profile finish, channel section thickness, cavity shape, exhaust system) of the mold structure casting system directly affect the melt type. The actual fluidity in the cavity, the flowability is reduced when the melt is lowered in temperature and the fluid resistance is increased. The mold should be designed according to the fluidity of the plastic used, and a reasonable structure should be selected.

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

Third, crystallinity

1. Thermoplastics can be divided into two types: crystalline plastics and amorphous (also known as amorphous) plastics.

The so-called crystallization phenomenon is that when the plastic is from the molten state to the condensation, the molecules move independently, completely in a disordered state, and the molecules stop moving freely, at a slightly fixed position, and have a tendency to make the molecules arranged into a regular model. a phenomenon.

2. As the standard for judging the appearance of these two types of plastics, the transparency of thick plastic parts of visible plastics is generally opaque or translucent (such as POM), and amorphous materials are transparent (such as PMMA). . However, there are exceptions. For example, polytetramethyl decene is a crystalline plastic with high transparency, and ABS is amorphous but not transparent.

3. When designing the mold and selecting the injection molding machine, it should be noted that when the crystal type plastic is used, the heat required for the material temperature to rise to the molding temperature is large, and the equipment with large plasticizing ability is used.

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