What are crystalline plastics: 5 types of plastics and key points of injection molding

• What are crystalline plastics: 5 types of plastics and key points of injection molding
  • What is crystallization?
  • What are crystalline plastics?
  • The difference between crystalline and amorphous polymers
  • The Influence of Crystallization on Material Properties
  • What factors influence the crystallization of plastics ?
  • How to control plastic crystallization?
  • Molding process of crystalline plastics
  • Applications of crystallization in plastics
  • The difference between crystallization and recrystallization
  • Crystalline plastic materials commonly used in injection molding
    • polyethylene
    • Polyamide (PA)
    • PBT resin
    • Polypropylene (PP)
    • Polyoxymethylene (POM)
  • Core Injection Molding Technology and Key Points for Crystalline Plastic Materials
  • in conclusion

Plastics are the materials commonly used in the mass production of plastic products ; these plastics are often referred to as polymers. They contain components that contribute to the production of high-quality products. These components can be further improved in several ways. This leads to crystallization technology , which transforms a liquid into a solid. Crystallization is a process that converts a liquid into a solid. This process involves polymers or plastics. It is a separation technique that materials research companies often use to obtain solid crystals from impure mixtures. Therefore, it is sometimes considered a purification technique.

But for plastics, it’s far more than just a purification method. It’s far more than separating a solid from a liquid. For plastics, it’s a reinforcing agent, an improving agent. This article will introduce the basic concepts of crystallization technology , as well as the types of plastic crystallization , technology comparisons, and common materials . Please continue reading to learn more about plastic crystallization. First, let’s understand the meaning of crystallization.

What is crystallization?

Crystallization is a unique manufacturing process widely used by leading companies worldwide. It is the process of transforming a liquid into a solid. Simply put, crystallization is an extraction or transformation process that extracts solid crystals from an impure mixture. The main steps involve dissolving the target substance in a solvent; heating and cooling the mixture ; and finally, extracting the solid crystals. Therefore, crystallization is not a single process but a combination of multiple steps.

Crystallization is an effective separation technique , and it is generally considered more efficient than evaporation. So, how can this separation method be applied to plastics?

What are crystalline plastics?

Now we understand this separation technique. Remember, we’re talking about plastics. What does crystallization have to do with plastics? Can we extract crystals from melted plastics? Or, what are its uses for plastics?

Crystallization, also known as polymer crystallization, refers to the process of ionic chain alignment. Ionic chains in plastics align through crystallization. Therefore, crystallization is the process of ion formation in plastics. Because the atomic chains in plastics are usually irregular and deform after the melt cools, crystallization can correct this deformation, forming a more regular structure. Crystallization is easier for linear plastics. That is, plastics with linear ions crystallize more easily.

Therefore, crystalline plastics have a distinct melting point, and their molecules are arranged in a regular pattern when solid. The regularly arranged regions are called crystalline regions, and the disordered regions are called amorphous regions (or “non-morphic regions”). The percentage of crystalline regions is called crystallinity. Polymers with a crystallinity of 80% or higher are generally called crystalline plastics.

The difference between crystalline and amorphous polymers

The essential difference between crystalline and amorphous polymers lies in the orderliness and disorderliness of their molecular structures. As the range of molecular motion gradually decreases at lower temperatures, molecules move closer together and tend to arrange themselves in a close and regular manner under the influence of intermolecular forces, forming a “close-packed” structure.

This is a common phenomenon for small molecules because they occupy a small volume and can easily form a close-packed structure. However, for polymers, because individual molecules are very long and occupy a large volume, their molecular structure needs to meet specific requirements to form a close-packed structure. Otherwise, when cooled, the molecules will become disordered and form a chaotic amorphous structure.

The Influence of Crystallization on Material Properties

In terms of mechanical properties, the modulus, rigidity, and hardness of plastics all increase with increasing crystallinity. Creep resistance and stress relaxation also improve with increasing crystallinity. Tensile strength exhibits a trough with increasing crystallinity; that is, before a certain level of crystallinity, tensile strength decreases with increasing crystallinity, and then increases with further increases in crystallinity. However, the impact strength of plastics decreases with increasing crystallinity.

In terms of optical properties, excessive crystallinity can affect the transparency of plastics because the refractive index differs significantly between crystalline and amorphous regions. Furthermore, in terms of thermal properties, crystallinity in plastics can increase their heat distortion temperature, thereby raising their service temperature. When plastics are non-crystalline or have low crystallinity, their service temperature is the glass transition temperature; when the crystallinity reaches 40% or more, the maximum service temperature can reach near the plastic’s crystalline melting point. Therefore, controlling the crystallinity of plastics allows for significant control over their properties through modification.

What factors influence the crystallization of plastics ?

• Polymer chain structure: Polymers with good symmetry, few or no branches, small side group volume, and strong intermolecular forces tend to aggregate and crystallize easily.
• Temperature: As polymers move from disordered clumps to the surface of growing crystals, higher mold temperatures increase polymer mobility and accelerate crystallization.
• Pressure: External forces during cooling can also promote polymer crystallization. Therefore, injection and holding pressures can be increased during production to control the crystallinity of crystalline plastics.
• Nucleating agents: While low temperatures promote rapid nucleation, they slow down crystal growth. To eliminate this contradiction, nucleating agents are added to the molding material. This allows the plastic to crystallize rapidly at high mold temperatures, and the addition of nucleating agents usually refines the crystal structure. This explanation clarifies the true role of nucleating agents.

How to control plastic crystallization?

Controlling plastic crystallization mainly involves controlling the degree of crystallinity and the quality of crystallization, both of which have a significant impact on the properties of plastics. The higher the degree of crystallinity of the polymer, the higher the melting temperature and heat resistance, and the higher the elastic modulus, hardness, tensile strength, flexural strength, etc., but the lower the toughness.

For the same plastic material at the same degree of crystallinity, if the product contains large spherulites, its light transmittance is poor and its appearance lacks aesthetic appeal. Obvious interfaces between spherulites can easily lead to stress concentration, resulting in poor toughness but beneficial rigidity and hardness. An increase in the number of microcrystals and a decrease in the number of spherulites, along with finer crystal sizes, improves its physical properties, enhances gloss, and increases transparency. Furthermore, if the material contains small β-type spherulites, it exhibits good toughness, improving impact strength and yield strength. The striations formed during stretching can further improve the toughness of the product and significantly enhance tensile strength, gloss, hardness, and barrier properties.

Molding process of crystalline plastics

• The heat released during cooling is large, so it must be cooled thoroughly. When molding at high mold temperatures, pay attention to controlling the cooling time.

• The difference in specific gravity between the molten and solid states is large, resulting in significant molding shrinkage and a tendency to develop shrinkage cavities and air bubbles. Therefore, it is important to pay attention to the setting of the holding pressure.

• At low mold temperatures, cooling is rapid, resulting in low crystallinity, minimal shrinkage, and high transparency. Crystallinity is related to the wall thickness of the plastic part; thicker parts cool more slowly, have higher crystallinity, greater shrinkage, and better physical properties. Therefore, the mold temperature for crystalline plastics must be controlled as required.

• Significant anisotropy and high internal stress; after demolding, uncrystallized molecules tend to continue to crystallize, resulting in an energy imbalance state that makes them prone to deformation and warping. Therefore, the material temperature and mold temperature should be appropriately increased, along with moderate injection pressure and injection speed.

Applications of crystallization in plastics

After crystallization, the properties of plastics change. Or more precisely, crystalline plastics acquire certain properties. They become harder and stronger. Solvent penetration has less of an effect on them. Crystallization also improves their heat resistance and chemical resistance.

This means that the plastic’s chemical and heat resistance is enhanced, but its impact resistance decreases. Therefore, crystallization has both advantages and disadvantages. Increased heat resistance, but decreased impact resistance; increased strength, but increased shrinkage; increased flexibility, but a narrower processing window. It all depends on the desired product. Therefore, you need to assess your own needs. Furthermore, the effects of crystallization vary for many plastics. This is due to the many unique properties of plastics. Remember, ionic form is key to promoting crystallization. Small ions form a three-dimensional lattice, thus forming large crystals. Crystallization is a separation technique that transforms a liquid into a solid.

The difference between crystallization and recrystallization

Besides crystallization, there is also recrystallization. But what are the differences between them? What is their essence? First, we need to clarify that crystallization is a separation technique. These solids are usually the result of chemical reactions occurring in solutions. Therefore, through crystallization, crystals can be obtained from impure mixtures. Companies utilize this process for the production and purification of crystals.

Recrystallization is the process of purifying crystals. Therefore, the former involves obtaining crystals from a liquid, while the latter involves purifying them. Although crystals obtained through crystallization are usually pure, impurities may still remain. Therefore, we use recrystallization to purify these crystals and remove these impurities. These two processes differ. The first step extracts the crystals, and the second step purifies them. They are two closely related steps in the crystal production and purification process.

Crystalline plastic materials commonly used in injection molding

There are many crystallization methods, each depending on the properties of the plastic. The following are crystalline plastic materials commonly used in injection molding processes.

polyethylene

First, let’s talk about polyethylene. It’s not a new type of plastic, but one of the most commonly used plastics today. You can find it in many products, including electronics, household goods, toys, and insulation materials. Electrical wires and bubble wrap are also made from this plastic.

Polyethylene exhibits good flowability during molding, thus eliminating the need for high-temperature curing. Its stable ionic structure makes it easily deformable, facilitating the production of various shapes. Some polyethylene plastics have high densities, making them highly sensitive to temperature. For objects with thick walls, rapid injection pressure and speed are required. Temperature control must also be extremely sensitive.

Polyamide (PA)

This is a type of plastic. Polyamide (PA) has unique crystallization characteristics and is very sensitive to temperature changes. Unlike other plastics, PA has a melting point and melts into a liquid. Therefore, PA requires a higher molding temperature than other plastics. It’s important to note that drying PA above 90°C will cause it to fade. This is the second type of plastic that crystallizes easily.

PBT resin

There are two types of PBT resin: PBT resin and PET resin. Each has its unique characteristics and similarities. PBT resin has good molding properties and low melt viscosity, making it easy to crystallize. These resins contain glass fibers, which improve their properties. Their molding temperatures are typically between 40-90°C, but can sometimes be molded at lower temperatures. Furthermore, these resins cure very quickly, so injection speeds must be very fast. It is best to pre-dry the resin before processing to prevent it from absorbing water during melting and causing decomposition. Attention should also be paid to the injection pressure, which should be controlled at around 50-130 MPa.

Polypropylene (PP)

It shares similar properties with the first plastic. Its liquefaction rate is related to the barrel temperature, typically set around 280°C, with an optimal control temperature of 270°C. Polypropylene is well-suited for separating solids and searching for crystals. Its strong ionization properties aid this process. However, these ionization properties are prone to deformation and warping at low temperatures. Therefore, temperature control and maintaining temperature equilibrium are crucial when processing polypropylene. This is essential for the success of any process using polypropylene.

Polyoxymethylene (POM)

Polyoxymethylene (POM) comes in two forms: homopolymer and copolymer. Both are resins with poor flowability and are prone to thermal decomposition, therefore, heating temperatures must be strictly controlled. Copolymers generally have superior performance compared to homopolymers, and thus require higher processing temperatures. However, it is crucial to ensure that the cooling time is not excessive, otherwise the finished product will turn yellow.

Core Injection Molding Technology and Key Points for Crystalline Plastic Materials

Precise temperature control	Preheating or melt temperature: It needs to be high enough to ensure melt fluidity, but not too high to damage material properties.Mold temperature (critical): The properties of crystalline plastics largely depend on their crystallinity, and mold temperature directly affects the crystallization rate and morphology. Generally, the higher the mold temperature, the more regular the crystals (higher crystallinity), and the harder and more transparent the product (depending on the material), but the shrinkage rate increases.
Pressure control	Injection pressure: Used to quickly fill the mold cavity. The crystalline material has relatively good fluidity, so the pressure should not be too high to prevent burrs.Holding pressure and time: Provides a “feeding” effect, compensates for cooling shrinkage, and improves density and dimensional stability. The holding time of crystalline plastics is usually longer to allow for full crystallization.
Cooling and crystallization (shrinkage control)	Runners and gates: Optimized design to reduce shear heat and pressure loss.Exhaust system: Effectively removes air and water vapor, preventing condensation spots and affecting crystallization.Cooling water system: Precise control of mold temperature is crucial, especially for thick-walled products, where uniform cooling is essential.
Advanced injection molding technology	Thin-wall injection molding: suitable for materials with good flowability such as PP or PE, for rapid prototyping of thin-walled long-process products.Gas-assisted injection molding (GAIM): Gas is used to fill the molten material in the mold cavity to form a hollow structure, reducing material usage and stress. It is suitable for crystalline plastics such as PP.Vacuum injection molding: Removes gas from the mold cavity, improves surface finish, and reduces air bubbles. It is suitable for crystalline plastics that require high transparency and high cleanliness (such as medical applications).Two-color injection molding: Combining materials with different melt indices or different crystallinities to achieve multi-color or multi-material integrated molding.

in conclusion

In summary, some plastics crystallize more easily due to their ionic form, while others crystallize more slowly. We know that crystallization is the process of separating impurities to obtain solid crystals. Its advantages include increasing the density of the plastic, enhancing its strength, improving its heat and chemical resistance, and thus increasing its flexibility. Its disadvantages include reducing the plastic’s impact resistance and causing shrinkage and deformation. However, its advantages and disadvantages depend on the type of plastic.

In addition, we are familiar with five different crystalline plastic materials and the key points of crystalline material injection molding technology. If you need to manufacture large quantities of custom plastic products using various crystalline plastic materials through injection molding, please contact Elimold. We provide professional injection molds and a one-stop service for mass production of plastic products. Contact us now for a project quote.