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The difference between crystalline and amorphous plastics

Author:Yancheng GWELL Equipment Co., Ltd. Click:341  Release Date:2022/09/14

1、 What is crystalline plastic?

Crystalline plastics have a clear melting point, and when solid, the molecules are arranged in a regular manner. The regular arrangement area is called the crystal area, the disordered arrangement area is called the amorphous area, and the percentage of the crystal area is called the crystallinity. Typically, polymers with a crystallinity of over 80% are called crystalline plastics. Common crystalline plastics include polyethylene PE, polypropylene PP, polyoxymethylene POM, polyamide PA6, polyamide PA66, PET, PBT, etc.

2、 The Effect of Crystallization on the Properties of Plastics

1) Mechanical properties

Crystallization makes plastics brittle (with a decrease in impact strength), with strong toughness and poor ductility.

2) Optical performance

Crystallization makes plastics opaque, as light scattering occurs at the interface between the crystalline and amorphous regions. Reducing the size of the spherulites to a certain degree not only improves the strength of the plastic (reduces intergranular defects) but also improves transparency (no scattering occurs when the size of the spherulites is less than the wavelength of light).

3) Thermal performance

Crystalline plastics do not exhibit a high elastic state when the temperature rises, but exhibit a viscous flow state when the temperature rises to the melting temperature TM. Therefore, the usage temperature of crystalline plastics has advanced from Tg (glass transition temperature) to TM (melting temperature).

4) Improvements have been made in solvent resistance, permeability, and other properties due to the tighter arrangement of crystalline components.

3、 What are the factors that affect crystallization?

1) Polymer chain structure, with good symmetry, unbranched or few branched chains, or small side group volume, and high intermolecular forces, is easy to lean against each other and cause crystallization.

2) Temperature, the movement of polymers from disordered coils to the surface of growing crystals, and higher mold temperatures improve the activity of polymers, thereby accelerating crystallization.

3) Pressure, if there is external force during the cooling process, can also promote the crystallization of polymers. Therefore, high injection pressure and holding pressure can be adjusted during production to control the crystallinity of crystalline plastics.

4) Nucleating agents are added to the molding material to eliminate this contradiction, as low temperatures are beneficial for rapid nucleation but slow down grain growth. This allows the plastic to crystallize rapidly at high mold temperatures. Common nucleating agents include talc powder, etc.

4、 What are the requirements for injection molding machines, extruders, and molds for crystalline plastics

1) When crystalline plastics melt, they require more energy to destroy the lattice, so converting a solid into a molten melt requires a greater amount of heat input. Therefore, injection molding machines and extruders need to have larger plasticizing capabilities, as well as corresponding improvements in injection volume and yield.

2) The melting point range of crystalline plastics is narrow. In order to prevent the rubber material from crystallizing and blocking the nozzle when the nozzle temperature decreases, the nozzle aperture should be appropriately increased and a heating ring that can independently control the nozzle temperature should be installed.

3) Due to the significant impact of injection mold temperature on crystallinity, injection mold waterways should be as much as possible to ensure uniform mold temperature during molding.

4) Crystallinity undergoes significant volume shrinkage during the crystallization process, resulting in a significant molding shrinkage rate. Therefore, it is important to carefully consider its molding shrinkage rate in mold design. For the extrusion of PE pipes, it is necessary to fully consider the shrinkage rate of the material.

5) Due to the obvious anisotropy and high internal stress, attention should be paid to the position and size of the gate, reinforcing ribs, and the position and size in mold design, otherwise warping and deformation can easily occur. It is quite difficult to improve it by relying on the forming process.

6) The crystallinity is related to the wall thickness of plastic parts. The cooling of the wall thickness is slow, the crystallinity is high, the shrinkage is large, and shrinkage and porosity are prone to occur. Therefore, in mold design, attention should be paid to controlling the wall thickness of plastic parts.

5、 The Forming Process of Crystalline Plastics

1) The heat released during cooling is large, so it should be fully cooled. When forming at high mold temperature, pay attention to the control of cooling time.

2) The difference in specific gravity between the molten state and the solid state is large, resulting in large molding shrinkage and easy occurrence of shrinkage and porosity. It is important to pay attention to the setting of the pressure maintaining pressure

3) When the mold temperature is low, cooling is fast, crystallinity is low, shrinkage is small, and transparency is high. Crystallinity is related to the wall thickness of plastic parts. When the wall thickness of plastic parts is large, cooling is slow, crystallinity is high, shrinkage is large, and physical properties are good. Therefore, crystalline plastics must control the mold temperature according to requirements.

4) The anisotropy is obvious, the internal stress is high, and after demolding, there is a tendency for the crystallized molecules to continue to crystallize. They are in an energy imbalance state and are prone to deformation and warping. Therefore, it is necessary to improve the material temperature, mold temperature, moderate injection pressure, and injection speed.

There are many types of plastics in the market, but people who make plastics generally only know that they are divided into two categories: engineering plastics and daily plastics. In essence, plastics can be divided into crystalline plastics and amorphous plastics.

Crystalline plastics: nylon, propylene, ethylene, polyformaldehyde, etc;

Non crystalline plastics: polycarbonate, ABS, styrene, vinyl chloride, etc.

Why are they divided into the above two categories? Due to different molecular structures.

(1) The surface of crystalline plastic is slippery and cannot be brushed or chrome plated, making it difficult to decorate the surface. Currently, the color of the coating cannot last long and easily falls off. Spraying, printing, and coloring of packaging bags are generally achieved by using electronic high current bristles to print colors, but they cannot last long and the adhesive is difficult to find.

(2) The surface of amorphous plastic can absorb other molecules, such as ink, chrome plating, spray coating, etc. Therefore, general product casings, watch cases, TV cases, etc. are made of amorphous plastic, which is easy to decorate and not easy to fall off.

However, the above two types of plastics should not be mixed together. If mixed together, they will be scrapped, causing bubbles and mercury lines on the surface, and serious peeling. Therefore, attention should be paid to mixing the two types of plastics, otherwise the consequences will be significant. Therefore, especially for waste plastics, it is necessary to distinguish which type is what material, and separate them before using them. However, mixing similar plastics does not have a significant impact, such as mixing propylene and ethylene. After mixing ABS and benzene, there is no significant impact.


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