Chapter 2 Forming Process of Plastic Packaging Materials (1)

1. What are the molding methods of plastic packaging products?
A: Plastic packaging products are divided into soft plastic packaging products and hard plastic packaging products according to the different properties of the products. The so-called floolible package refers to various film packaging products. Soft plastic packaging products include extrusion blown film process, extrusion casting process, calendering process, coextrusion compounding process, dry compounding process, hot melt compounding process, wet compounding process, solvent casting process, coating process Etc. The principle is that hard plastic packaging products include hollow containers, turnover boxes, calcium plastic boxes, various trays, cups, dishes, and buffer packaging. The molding process includes extrusion, co-extrusion, extrusion blowing, extrusion blowing, injection molding, injection blowing, and injection blowing. It can be said that due to the diversity of plastic packaging, its molding process covers almost all the processes of plastics processing.
The secondary processing of plastic packaging products also includes various processing fields such as various machining, welding, bonding, printing, vacuum evaporation, bronzing, transfer printing, and electroplating.

2. What is the "tri-state" of plastics? What role does it play in plastic molding?
A: Figure 2-1 shows the temperature-deformation curve of a polymer under a certain load. (slightly)
As can be seen in Figure 2-1, for a typical amorphous polymer, there are several distinct characteristic temperatures:
1 Embrittlement temperature TB Polymers below the brittle temperature, brittle like glass, plastics have no use value. Recently developed a method for crushing some plastics and elastomers with high flexibility below the TB temperature that are difficult to be crushed at room temperature by general pulverization methods;
2 Glass transition temperature Tg Plastics are called glassy plastics at the TB-Tg temperature, which is the extreme service temperature range for plastics. Polymers in the glassy state can be machined: car, planer, grinding, nailing, bonding, painting, and printing;
3 The melting temperature Tf The plastic between Tf-Tg is called plastic under high elasticity state. The plastic can perform molecular crystallization, stretching orientation, shrinkage, vacuum thermoforming and heat setting under high elastic state. In other words, a polymer molecule in a highly elastic state can have a certain degree of freedom at a fixed position due to the force of molecular heat movement; a polymer having a highly regular molecular structure can be crystallized in a highly elastic state; Under the action of external force, the molecules are oriented in the direction of the direction of the external force; they can also be vacuum thermoformed or pressed in the vicinity of the softening temperature. A class of engineering plastics such as nylon can be annealed or quenched in high elasticity to improve various polymer properties.
4 Decomposition temperature TD reaches TD temperature, and the polymer molecules undergo thermal decomposition, and the polymer decomposes violently beyond this temperature. For this reason, the melt molding temperature of the polymer can only be performed in a molten state between Tf and TD.
Glassy, ​​highly elastic, and molten states are called polymer tri-states. Some polymers Tg and Tf see Table 2-1

Table 2-1 Temperature and melting temperature of several commonly used polymers

3. What pretreatments need to be done before plastic melt molding?
A: In China, PVC resin is generally supplied to the market with resin powder. The resin powder is divided into seven grades only by the relative molecular mass of suspended PVC, which does not contain any additives. For this reason, PVC products must be formulated, mixed and granulated before they can be melt-molded.
For other plastics, such as PE, PP, PVC, ABS, PS, etc., according to the requirements of the molded products, the granular resin grades can be selected from the grades produced by petrochemical plants and can be directly applied to molding. Petrochemical plants have been granulated by adding various additives according to the performance characteristics of each grade. Some grades lacking, need to be self-formulated in the molding factory, then the added materials should be dried, mixed and agitated, solid additive grinding, granulation and shaping.
Plastics or plastics with high hygroscopicity, which have strict moisture control requirements, should be pre-dried. For example: polyamides should be dried at 100°C for 2 hours; PCs should be dried at 120°C for 3 hours; and polyester resins should be at 100-130. °C dry 4h. Drying of resins and fillers, such as CaCO3, consists of fixed bed ovens and fluidized bed dryers. The advantages and disadvantages of various dryers are listed in Table 2-2.

Table 2-2 Advantages and disadvantages of various dryers

The hygroscopicity of several common plastics is shown in Table 2-3. Dry conditions for several plastics are shown in Table 2-4.

Table 2-3 Hygroscopicity of several common plastics