With the rapid development of plastic molding technology, plastic products have taken on many new styles and uses, providing convenience for people. Due to their numerous applications and various excellent properties, plastic products have become indispensable in people's clothing, food, housing, transportation, and daily necessities.
Today, we'll introduce you to a molding process called blow molding, and discuss the requirements for masterbatches used in products made using this process.
About blow molding
Plastic blow molding is a common molding method for thermoplastic plastics. It involves placing a semi-molten preform obtained from extrusion or injection molding into a mold of various shapes. Compressed air is then used within the preform, causing it to expand and adhere tightly to the mold cavity wall, thus imparting a specific shape and size. The finished product is then obtained after cooling and demolding. Plastic blow molding is the third largest plastic processing technology after extrusion and injection molding. The process is simple, energy-efficient, and highly productive, allowing for mass production and a wide variety of products. This molding technology can produce hollow containers such as bottles, barrels, jugs, and cans of different shapes and capacities, as well as everyday consumer goods, transportation facilities, and children's toys. It can also produce various industrial hollow products for different purposes, and is widely used in industries such as chemical, food, pharmaceutical, automotive, transportation, fisheries, sports, water facilities, military, and home appliances.
Blow molding classification
There are two main types of blow molding processes: extrusion blow molding and stretch blow molding.
1. Extrusion blow molding
Extrusion blow molding involves heating and melting thermoplastic resin through a screw extruder, then extruding it into a tubular preform of a certain length under pressure. After processing, the preform is placed in a pre-prepared sealed mold, and compressed air is used to blow-form it into the finished product. During extrusion blow molding, products of different sizes, such as bottles and barrels, are produced depending on the size of the extruder. It can also be used to produce bottled and barrelled containers, finding wide application in many fields. However, in practice, the extruded preform has an open end, and weld lines are applied after processing. Therefore, the process must be performed according to a specific mold; otherwise, the final product will not meet the required specifications.
Disadvantages of extrusion blow molding : The end of the extruded parison needs to be open, the bottom of the product made from the parison needs to have a weld line, and improper operation during molding will affect the strength of the entire part.
2. Stretch blow molding
Stretch blow molding utilizes high-molecular polymers, under the manipulation of highly elastic molecules, to create transparent, high-strength packaging containers according to specific needs. The stretch blow molding process generally involves two steps: the first step is to create a small tubular preform; the second step is to heat-treat the preform, which imparts elasticity to it.
Hollow blow molding designs a wide variety of products, making it difficult to establish a unified quality standard during implementation. In summary, hollow blow molding processes require good compressive and impact resistance, and the wall thickness must remain uniform when loading certain products, avoiding any uneven thickness. Product quality defects can also occur during hollow blow molding operations, affecting production. Effective control measures must be implemented by relevant personnel during operation.
Performance requirements for blow molding masterbatch products
Almost all thermoplastic resins can be used to manufacture blow-molded products, with the most important resins including polyethylene, polyester, polypropylene, polyvinyl chloride, polyamide, and polycarbonate. For masterbatches used in blow molding, many properties need to be considered, with particular attention to: chemical resistance, light/weather resistance, heat resistance, and safety.
01
Chemical resistance
Plastic containers used for packaging raw materials and products, especially chemical preparations, in industrial and agricultural production must ensure that the substances contained in the container itself do not chemically react with the substances being packaged. Therefore, special attention should be paid to the resistance properties of pigments, such as acid/alkali resistance, solvent resistance, water resistance, and oil resistance, depending on the contents being packaged. Even higher requirements apply to packaging containers for strong oxidizing chemicals.
02
Security
Numerous plastic containers are used for packaging food, beverages, and other cosmetics or kitchen cleaning products that come into contact with the human body. All the raw materials used in the production of these plastic containers, including pigments, must comply with relevant chemical safety regulations and usage permits.
03
Heat resistance
Under the process temperature conditions of hollow blow molding of different resins, pigments must be able to maintain color stability and the required performance.
04
Light/weather resistance
Hollow blow molded products are often used outdoors, so attention should be paid to the lightfastness of the pigments. In particular, some oils and cosmetics are stored in outdoor shop windows, and there are special lightfastness requirements for the purpose of brand promotion.
05
Migration resistance
Hollow blow-molded products are mainly used as liquid packaging containers. If the pigments used in them have migration resistance, the migrated color will migrate into the liquid that comes into contact with it, causing contamination.
PE blow molding masterbatch formulation
Knowledge
Since we use polyethylene as a raw material for many blow-molded products in our daily lives, this episode of Yudi's Little Classroom will specifically introduce the formulation knowledge of polyethylene (PE) blow molding masterbatch.
Hollow blow molding masterbatches based on polyethylene are mainly used in oil drums, cosmetics, etc. The formulation design considerations are largely the same as those for blown film-grade masterbatches, such as dispersibility, heat resistance, migration resistance, and safety. The coloring requirements are: smooth product surface, good dispersibility, good migration resistance, and good solvent resistance. Of course, PE blow molding also has its own specific requirements and varieties; we have compiled a few for your reference.
01
Coverage requirements
Blow-molded products are mostly used for packaging lubricating oil boxes and some cleaning agents. In order to ensure the safe storage of the contents, the product's opacity is required. Therefore, a large amount of titanium dioxide needs to be added to the masterbatch formula. Attention should be paid to the selection of titanium dioxide type and the impact of adding titanium dioxide on the performance of organic pigments.
02
High performance requirements
Because some oil packaging is from multinational brands, strict requirements are placed on the performance of the product's outer packaging for brand promotion purposes. Therefore, the selection of pigments tends to favor high-performance ones.
03
Special raw material requirements
Many lubricant packaging materials use aluminum as raw material. The so-called aluminum pigment is actually flake-shaped aluminum silver powder, which is made by melting aluminum ingots into molten aluminum, then atomizing it into micron-sized spherical aluminum powder under high pressure with nitrogen, and finally grinding the micron-sized spherical aluminum powder into flake-shaped aluminum silver paste. Therefore, when selecting aluminum raw materials for lubricant packaging, it is necessary to consider both brightness and opacity, and choose an appropriate particle size to meet the requirements.
04
Additive requirements
The content of polyethylene wax in the blow-molded polyethylene wax masterbatch formula should not be too high, because wax is a low molecular weight substance. Too much wax will affect the crackability of the oil drum and is one of the reasons why blow-molded drums break in drop tests.
05
Selection of resin raw materials
Blow-molded products extensively use medium to high molecular weight HDPE resin, giving the products excellent impact resistance. Therefore, LDPE and LLDPE are selected as carriers for PE blow-molding grade masterbatches, which have good fluidity and dispersibility.
