Polyethylene for hollow blow molding is the topic in this post.
(1) Polyethylene type for hollow blow molding
The types of polyethylene used for hollow molding include low-density polyethylene (LDPE), high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), high molecular weight polyethylene (HMWHDPE), etc.
(2) Selection of polyethylene for hollow blow molding
In the hollow molding process, the selection of polyethylene is mainly based on two indicators: density and melt flow rate (MFR). The industry generally uses density as a measure of molecular structure, and melt flow rate (MFR) is an index to measure its average molecular weight.
The so-called plastic melt flow rate (MFR) refers to the mass of the melt through the standard die every 10 min under a certain temperature and load, and the unit is g/10 min.
Generally, the MFR increases, the molecular weight of PE decreases, the melt viscosity is small, the fluidity is good, the molding processing temperature is low, and the molding is easy, but the mechanical properties of the product are poor. On the contrary, the smaller the MFR, the larger the molecular weight of PE, the higher the melt viscosity, the poor fluidity, the higher the molding processing temperature, and the better the mechanical properties of Zhilin.
Therefore, the early selection of PE is to consider not only the molding processability but also the use of the performance of the product.
LDPE has excellent properties such as lightweight, good transparency, cold resistance, flexibility, excellent high-frequency insulation, and easy molding and processing. It is one of the main plastics used in hollow molding. It can be used to hollow form various daily and medical packaging bottles, as well as flexible packaging and can linings, and can also be used to blow various films.
When used for blow molding heavy packaging films, LDPE with an MFR of 0.3-2g/10min is usually used.
When used for blow molding light packaging films, LDPE with MFR of 2-7g/10min should be used.
It is used for blow molding hollow molding various bottles and other packaging containers. Generally, the MFR range of LDPE is 0.3-4g/10min.
HDPE has a high average molecular weight, short and few branches, so it has high density and high crystallinity.
The tensile strength, stiffness, and hardness of HDPE are better than LDPE, which is conducive to the thinning and lightweight of products. At the same time, HDPE has better heat resistance, gas barrier properties, and chemical stability than LDPE.
At room temperature, HDPE has a small breaking length rate and poor ductility, but it has a large tensile strength under appropriate temperature conditions. This can be used to obtain highly oriented products. After orientation, the mechanical properties of the product can be greatly improved.
Better than HDPE with good comprehensive performance, can be used for various packaging films, moisture-proof. Hollow products such as oil-resistant packaging containers and car seats.
When used for blow molding heavy packaging film, HDPE with MFR below 0.5g/10min is usually used.
When used for blow molding light packaging film, HDPE with MFR below 2g/10min should be selected.
When hollow molding various bottles and other hollow products, the MFR range of HDPE is generally 0.2-1.5g/10min.
LLDPE has a linear structure, with many short branches on the macromolecular chain and almost no long branches. LLDP has a relatively large molecular weight and a narrow distribution.
LLDPE has higher tensile strength and impact strength than LDPE, greater hardness and rigidity, excellent heat resistance and environmental stress cracking properties, and balanced vertical and horizontal shrinkage, and is not easy to warp. It can be used in various packaging films and moisture-proof packaging containers.
When used for blow molding heavy packaging films, LLDPE with an MFR of 0.3-1.6g/10min should be used.
When used for blow molding light packaging film, LLDPE with MFR of 0.3-3.3g/10min should be used.
When hollow molding various bottles and other packaging containers, the general MFR range of LLDPE is 0.3-1.0g/10min.
Due to the high molecular weight of HMWHDPE, it has high tensile strength and rigidity, excellent wear resistance, chemical resistance stability, good impact resistance, and environmental stress cracking resistance. It can be used for the hollow molding of various large-scale and complex geometrical hollow products. Such as automobile fuel tanks, sprayers, and container barrels, and industrial storage tanks.
Metallocene polyethylene has a narrow molecular weight distribution, and the composition and structure of macromolecules are very uniform. The molecular structure has high regularity and high crystallinity. And the crystal size formed is uniform. It has high transparency, high impact strength, and puncture resistance, especially excellent low-temperature toughness.
Due to the narrow molecular weight distribution, the molding processability is poor. At present, a certain amount of LDPE or HDPE is often added to metallocene polyethylene for blending modification.