Blow Molding Technology: Products · Molds · Process

Author: Norman C. Lee (USA), translated by Chai Chengzhi et al.
Translator: Chai Chengzhi, Li Shu
Country:
Publisher:
Publish Date: 2003-01-01
Features: All blow-molded products are formed by blowing compressed air into the billet internally, causing the billet to expand and fill the mold surface. Therefore, the mold determines the outer surface dimensions of the product. The blow-up ratio of the product is the ratio of the finished product diameter to the billet diameter. If the billet is blown into a cylinder, this definition of the blow-up ratio is correct. When the shape is irregular, the blow-up ratio is determined by the cross-sectional area of these irregular shapes to determine the additional blow-up ratio of these areas. In other words, compared to the total blow-up ratio, the irregularly shaped areas have their own independent blow-up ratio areas. While ensuring product quality and providing the best product value, the packaging industry, like other industries, must also strive to improve production efficiency. One effective way to reduce packaging costs without losing market demand is to use blow molding processing methods to produce packaging containers, because the amount of resin used is the most important factor in the cost. In this way, reducing the weight of the container material reduces the cost. To meet packaging needs while achieving manufacturing efficiency, the quality of the container and the production efficiency of the processing machine must be comprehensively considered. To produce a functional, low-cost, and efficient product, factors such as design, aesthetics, consumer transportation, processing processes, and container performance must be fully considered. This software can also display the distribution of the area-to-stretch ratio, i.e., how much the area of the billet is stretched compared to its original surface. The area at the corner is significantly stretched more than other parts of the product, and this stretching data can be displayed on the graph and returned to the original billet [Figure 6.2(c)], which accurately shows how to improve the design of a 1.52 mm thick billet to achieve the best uniform wall thickness. The stretching data can be displayed on the graph and returned to the area-to-stretch ratio, i.e., the billet can be designed to be thicker in selected areas by the program. For example: If the product characteristics require the corners to be thicker, the programmed design will make the area of the billet blown into the corner thicker, rather than making the entire billet thicker. Although this program does not include the analysis of this situation, with the help of the previous graphical area-to-stretch data, the design of billet sagging and thinning can be improved, and the best balance can be maintained between material savings and product quality. Blow-molded molds can be composed of various modules and inserts, but they are usually made up of two halves. When the mold is closed, one or both of the two halves form the cavity for blowing one or more billets. For bottle containers, the two halves of the mold are similar in shape. However, for industrial products, the shape may be more complex, and there may even be slides and inserts. Generally, there is no distinction between concave and convex dies, but the exception is the double-wall container mold. The billet cutting port is usually set at both ends of the two halves of the bottle mold. The air inlet rod can play an additional forming role and can form the inner surface of the bottle neck. Both halves of the mold must have internal cooling water channels. The guide pins and guide bushings installed on both sides of the templates of the two halves of the mold must ensure accurate alignment of the cavity to allow the mold to close. The precise guiding devices on the two halves of the mold reduce assembly time. Figure 10.1 shows the two halves of the blow-molded mold for small bottles. Figure 10.2 shows the position of the cooling water channels. In some blow molding processes, achieving mold closure requires two steps. The first step is to quickly close the mold with low pressure, with an opening distance of 6.2 to 13 mm. The second step is to close the mold slowly with high pressure to protect the mold from damage and reinforce the billet cutting port joint, as shown in Figure 10.9. The mold does not require vertical positioning; they can occasionally tilt with the billet, which will help with the uneven distribution of resin. For example, for some irregular products, such as large kettles with handles, appropriate tilting can save the length of the billet.
12.2 Polymers
Polymers or resins (naturally produced products from trees and plants) are obtained through a chemical process called polymerization. Here, it refers to the combination of one or more simple molecules into larger, more complex macromolecules. The most important thing is to selectively produce these molecules or molecular chains to make them polymers or plastic materials with various hardness, strength, color, weather resistance, or other properties to meet the needs of various applications. As mentioned earlier, monomers are the basic repeating units in polymer molecular chains, containing the elements of the final plastic. The normal state of monomers is gas or liquid, and under appropriate conditions, monomers themselves or other monomers chemically combine to form the required polymer.
12.2.1 Homopolymers, Copolymers, and Terpolymers
Polymers composed entirely of one monomer are called homopolymers, and polyethylene and polypropylene are two commonly used homopolymers. From polyethylene and polypropylene monomers, people found a way to easily mix ethylene and propylene, thus obtaining a series of completely different polymers. This type of polymer composed of two monomers is generally referred to as a copolymer. Its properties are different from any of the homopolymers it contains, and this important synthetic method opens up an important polymerization path for the required polymer. Another example is monomers such as acrylonitrile, butadiene, and styrene. These three different monomers combine to form terpolymers, resulting in three types of copolymers and two more complex copolymer structures. For example, the two materials mentioned later, ABS plastic and nitrile rubber, are both widely used terpolymers today.
12.2.2 Thermoplastics and Thermosets
As mentioned earlier, polymers can be divided into two major categories based on physical properties: thermoplastics and thermosets. Thermoplastics can soften, melt, and be repeatedly molded under heat and pressure. They allow the product to be crushed and reused and can be repeatedly molded many times. Thermosets undergo chemical crosslinking during the molding process, forming a network-like three-dimensional structure. This type of polymer will decompose at high temperatures. This material has excellent high-temperature resistance and can be used for parts and handles on cookware, automotive engine parts, circuit boards for installing electronic components, and components used on aircraft, etc.
12.2.3 Amorphous and Crystalline
Thermoplastics can be divided into two categories based on morphology: amorphous and crystalline polymers. In amorphous polymers, molecules exist in a disordered state, and these long chains are intertwined, forming a glassy state. Generally, amorphous resins have a small shrinkage rate when cooled, and amorphous plastics are easier to process because they have a wide melting temperature range, which helps reduce the molding stress in the product. Products made from these resins are rigid and have low to medium impact strength (such as polystyrene and polymethyl methacrylate) or excellent impact strength and transparency (such as polycarbonate). In crystalline polymers, molecules are arranged in a specific direction in an ordered manner, and the polymer molecular chains are also basically arranged in an ordered manner. Crystalline polymers are usually more flexible and softer, and they have a large shrinkage rate when cooled. The melting range of these materials is usually very narrow. Typical crystalline resins are polyethylene and nylon, while the stereoisomer shape of polypropylene can show its crystalline and amorphous morphology.
12.5 Plastic Material Coloring
Generally, thermoplastics can be molded in a wide range of colors. Coloring can be provided by pre-colored base materials or by adding solid or liquid masterbatches before material plasticization. A masterbatch is a highly concentrated material in which pigments are highly dispersed in a carrier resin. Masterbatch suppliers make them by blending according to the required color, and then they are formulated according to a concentrated recipe, with pigments accounting for 20% to 60% of the carrier resin. Additives such as antioxidants, stabilizers, and anti-stick agents can be mixed together. It is recommended to mix a certain concentration of masterbatch with the basic resin to obtain the desired color. There is a lower limit ratio, for example, if 454 kg of resin is mixed with 454 g of masterbatch, the lower limit ratio is 100:1. Generally, the lower the ratio [(25:1) to (30:1)], the more accurate the color required. Other factors such as screw length-to-diameter ratio and screw rotation speed can also be adjusted appropriately based on the mixing percentage. The reason is that the mixture is mixed using a screw, so the time the resin spends in the extruder is short. The carrier resin is best compatible with the resin being produced, and the melt flowability of the carrier resin must be high to mix the pigments well and extrude the resin evenly. An important quality test of masterbatch is color matching, and the hue must be consistent when comparing the formed product with color chips or standard color chips. It is important to accurately determine the light source used for color matching, and ultraviolet, fluorescent, and sunlight are commonly used light sources. Colors can only be matched under one light source, which is what is known as conditional color matching. When the masterbatch is sensitive to heat, the retention time needs to be reduced appropriately. Generally, red has the best thermal stability, while yellow and orange have medium thermal stability. The darker the color, the lower the processing temperature required, and masterbatches of hygroscopic resins must be mixed before drying.
12.6 Recycled Material in Extrusion Blow Molding
Extrusion blow molding generally produces a certain amount of scrap, and in some products, the scrap can account for up to 100% of the total product weight. The economical approach is to recycle and reuse the scrap. The recycled material must be kept clean, as impurities can damage the surface condition of the product and reduce the performance of the product or resin. Impurities can also stick to the machine head, causing other problems, so all materials and all material handling equipment (grinders, storage bins, feeders) must be kept clean. The amount of recycled material used in specific products is determined by several factors, and the performance for different uses is shown in Figure 12.5(a) to (g).
12.6.1 Recycled Material Usage
The amount of recycled material must be determined through multiple physical property tests to find the maximum or optimal amount of recycled material required for the final product. Repeatedly processed raw materials can lead to deterioration in the physical properties of the material.
12.6.2 Processing Characteristics
For some materials, adding about 50% recycled material can affect the hanging strength and mold exit expansion, and poor consistency of the billet width is caused by these issues.
12.6.3 Physical Properties
When a large amount of recycled material is added, some important physical properties of certain resins will be lost. Most resin suppliers recommend that the amount of recycled material should not exceed 3 times heating. Products that must withstand impact and repeated pressure should have the recycled material content precisely controlled. For co-extruded products, recycled material should be used in the intermediate layer. Reducing scrap can be achieved by making the distance between the mold and the machine head as close as possible and using the appropriate size of the machine head. High-fine powder or fine-grained recycled material requires a temperature reduction of 5.5 to 11.1°C in the feeding section, which helps prevent the plastic from melting too early. The amount of recycled material can change the amount of masterbatch added and the processing ability of the processing machine for special color masterbatches, because many colors are heat-sensitive materials and will fade during repeated processing. Recycled material is different from pure granules because the newly formed particles are irregular in shape and have a rubber-like material that tends to stick together.
12.7 Used and Industrially Reused Materials
The factors to consider when processing recycled high-density polyethylene (HDPE) and pure material are mostly the same. Since most other materials cannot be used for blow molding, people look for suitable occasions to use recycled material. Various grades of high-density polyethylene are the most widely used plastic materials, and the main considerations for using recycled material are that fragments or sheets are more difficult to slide than granules, which can cause bridging at the feeding point of the extruder. A large number of fine sheets increase the likelihood of this problem. To prevent the possibility of this problem, it is important to determine the amount of fine particles. Under given application conditions, the optimal ratio of recycled material to pure material is focused on the long-term or short-term performance requirements of the product. By exposing HDPE strips to accelerated or outdoor ultraviolet radiation to verify the material's long-term toughness and compare it with the initial value, the selected material ratio can be ensured to meet the long-term and short-term performance requirements of the product. The same test method also applies to the recycled material of other plastic varieties. When recycled material is normally used in pure material, the percentage of the mixture is usually determined by mixing pure material and recycled material to meet the technical requirements of the processing. The melt flow of HDPE feed material and other plastics has different characteristics, for example, high-density polyethylene used to produce milk bottles has a low melt index. Another example is the colored detergent bottles, with a melt index of 5, and general bottles are also around 5. This melt index plastic is suitable for blow-molded products but not for injection molding, which requires good fluidity (melt index between 20 and 30). Although films (garbage bags, padding packaging, etc.) cannot be formed by injection molding, they are suitable for extrusion or blow molding. To understand the flow characteristics of a given recycled material, experiments must be conducted, ensuring that qualified products are produced using the appropriate recycled material.