Telling about the injection molding process of crystalline plastics

First, the characteristics of crystalline plastics

1. Mechanical properties: The crystal structure enhances the intermolecular force and enhances the toughness, but at the same time reduces the ductility and impact resistance;

2, optical properties: due to the light scattering between the molecular crystal surface and the amorphous surface of the crystalline plastic, its transparency is not high, often opaque;

3, thermal performance: has a stable melting point, does not appear in the high-elastic state during the temperature rise, when the temperature rises to the melting temperature TM, it is in a viscous flow state;

4, solvent resistance: crystal tightly arranged to make it better impermeability and higher solubility.

Second, the injection molding points of crystalline plastics

1. Factors affecting crystallization

A. A polymer structure with good symmetry, little branching or small side group volume, because of the large inter-matrix forces, it is easier to abut each other and a crystallization reaction occurs;

B. The higher temperature increases the molecular mobility of the crystalline plastic, and the active molecular activity during cooling is more likely to cause crystallization tendency;

C. Pressure is also conducive to promoting crystallization. The molecular polymerization volume under external force is reduced, and the power of polymerization is generated. Therefore, high injection pressure and pressure holding pressure are commonly used to increase the crystallinity of the plastic;

D. The nucleating agent is an additive for promoting rapid crystallization of the crystalline plastic at a high mold temperature, and is used to improve the contradiction of slowing down the proportion of crystal growth during rapid prototyping at low temperature;

2, the requirements of injection molding equipment

A. Higher temperature: Since crystalline plastics require more energy to destroy their crystal structure during melting, there is a higher requirement for the plasticizing temperature and nozzle temperature of the barrel, so higher temperatures are required. system;

B. Increase the diameter of the nozzle: The melting point of the crystalline plastic is relatively narrow. In order to prevent the influence of temperature change on the injection molding effect, the temperature control of the nozzle is generally increased by increasing the diameter of the injection nozzle and installing the nozzle heating ring;

C, uniform mold temperature: in order to ensure a uniform crystal distribution requires uniform mold temperature control, so the mold should have a good multi-channel temperature control system;

D. More reasonable gate design: Since the anisotropy of crystallization is significant and the internal stress is larger, the position, size and shape of the gate should be better designed during the design of the mold, especially at the rib position. Should avoid the occurrence of warpage and deformation;

E. Control of product wall thickness: Since the crystallinity has a great relationship with the wall thickness of the product, the product with excessive wall thickness has a slow cooling rate, a higher crystallinity, and a larger shrinkage ratio due to crystallization, so that it is more likely to occur. Shrinkage holes, pores, because the product wall thickness should be optimized.

3, the main points of the injection molding process

A, reasonable shrinkage is expected: due to the smaller molecular volume of crystalline plastic after crystallization, thereby increasing its shrinkage, because the size problem caused by shrinkage should be pre-designed during injection molding production;

B. Optimization of cooling system: Since the melting temperature is high, the heat released during cooling is also more. In order to increase the cooling rate, the cooling system should be used reasonably;

C, mold temperature control: by using low mold temperature for injection molding, due to rapid cooling, melt crystallinity decreases, shrinkage decreases and transparency increases, reasonable mold temperature control can be carried out according to demand to achieve reasonable product output;

D. In order to solve the problem of orientation and stress release caused by crystallinity, by appropriately setting the gate, it is also possible to reduce deformation and warpage of the product by increasing the temperature and mold temperature, adjusting the injection pressure and the injection speed. The problem.