The main factors affecting the qualitative and product quality of hollow core processing are raw materials, temperature, warp ratio, skin ratio, head size, blow molding pressure, mold cooling, and molding shrinkage rate. They are now described as follows.
- Selection of raw materials
Taking polyethylene glycol products as an example, the selection of raw material grades must be based on the intended use of the product. The main parameters are density and sleeve index. Softening temperature and rigidity are functions of density. The higher the density, the higher the softening temperature and rigidity. In order to maintain the structural shape of the product and develop thin-walled products, it is required to use resin varieties with high density and low melt index.
Secondly, containers containing detergents and other chemical reagents, as well as containers with other material inserts, have higher requirements for the product’s resistance to cyclic stress cracking. And stress cracking is related to density, molecular weight, and molecular weight distribution. The melt index is a characterization of molecular weight, so the environmental stress cracking resistance of polyethylene is related to the melt index. When the melt index decreases, the environmental stress cracking resistance increases.
In addition, observing the phenomenon of self sagging of the mold, it can be found that when the mold reaches a certain length, the descent speed of the mold increases, and the length of the mold at this time is called the critical length. The critical length can be used as a rough standard for the sagging of the billet. The sag of the billet is closely related to the melt index. The higher the melt index, the faster the sagging speed of the billet, that is, the shorter the critical length.
In order to improve the rigidity, impact strength, environmental stress cracking resistance, and molding stability of hollow products, it is advisable to use resins with low melt index and high density. Commonly used high-density polyethylene often uses ethylene/butene or ethylene/ethylene copolymers. The melt index is 0.2~0.4. The melt index of low-density polyethylene blended with it is generally between 0.3 and 1. When the blending ratio of low-density polyethylene is small, the melt index should not exceed 2.
- Molding temperature
Each grade of resin has a certain melting temperature, which should be strictly controlled during processing. Whether the temperature is low or high, it will result in a rough surface of the product. In order to improve the appearance quality, the temperature at the mold lip should be slightly higher than the material temperature (about 5C higher), and the extrusion temperature also directly affects the degree of billet sagging, expansion ratio, cooling and setting time, etc. Therefore, the melting temperature is an important parameter for controlling product quality and determining production efficiency.
- Expansion ratio
Shadow expansion ratio refers to the phenomenon where the extruded molten billet is larger than the die size. The transfer of energy compared to the extraction of polyethylene end of plastic soil products results in poor elasticity of the cultivation disk. Understanding the limit expansion ratio is of great significance for controlling the shape or wall process, improving product quality, determining die design dimensions, and selecting dies.
