Blow Molding Technology -- Products. Molds. Process

Author: (USA) Lee (N.C.) (Author), Chai Chengzhi, Li Shu (Translator)
Publisher:
Publication Date: 2003-01-01
Features: All blow-molded products are formed by blowing compressed air into the billet, causing it to expand and fill the mold cavity. Therefore, the mold determines the external surface dimensions of the product. The blow-up ratio of the product is the ratio of the diameter of the finished product to the diameter of the billet. If the billet is blown into a cylindrical shape, 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, irregular-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 resin usage is the most important factor in cost. Thus, reducing the weight of container materials reduces costs. To meet packaging needs while achieving manufacturing efficiency, the container quality and 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, that is, how much the stretched area of the billet is 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)], where it is accurately shown 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, that is, the billet can be designed to be thicker in selected areas by the program. For example, if the product characteristics require thicker corners, 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 mold kits can be composed of various modules and inserts, but they are usually composed of two halves. When clamped, one or both of these two halves serve as the mold cavity for blowing one or more billets. For bottle containers, the two halves have similar shapes. For industrial products, the shape may be more complex, and it may have some slides and inserts. There is usually no distinction between concave and convex dies, but exceptions are for wall-walled containers. The cut-off mouth is generally set at both ends of the two halves of the bottle. The air inlet rod can play an additional shaping role and can shape the inner surface of the bottle neck. Both mold halves must have inner cooling water channels. The guide pins and bushings installed on both mold halves' template surfaces must accurately align the mold cavity to ensure proper closure. The precise guide mechanism on the two mold halves reduces assembly time. Figure 10.1 shows the two halves of the blow mold for small bottles. Figure 10.2 shows the location of the cooling water channels. In some blow molding processes, achieving mold closure requires two steps. The first step is to rapidly bring the mold to a 6.2 to 13 mm open distance with low pressure. Then, the second step is to slowly close the mold with high pressure to protect it from damage and reinforce the cut-off mouth joint, as shown in Figure 10.9. The molds do not require vertical positioning; they can occasionally lean with the billet, which will help with the uneven distribution of the resin. For example, for some irregular products, such as large pots 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 and more complex macromolecules. The most important thing is to selectively produce these molecules or molecular chains to become 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 and contain the elements of the final plastic. In their normal state, monomers are gases or liquids, and under appropriate conditions, they can chemically combine with themselves or other monomers to form the required polymers.
12.2.1 Homopolymers, Copolymers, and Terpolymers
Polymers composed entirely of one monomer are called homopolymers. Polyethylene and polypropylene are two common homopolymers. From polyethylene and polypropylene monomers, people found a way to easily mix ethylene and propylene, thus obtaining a series of completely different polymers. These polymers composed of two monomers are generally referred to as copolymers. Their properties are different from those of any of the homopolymers they contain. This important synthetic method opens up an important polymerization pathway for the required polymers. Another example is monomers such as acrylonitrile, butadiene, and styrene. These three different monomers combine to form terpolymers, producing three types of copolymers and two more complex copolymer structures. For example, the ABS plastic and nitrile rubber mentioned later 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 ground and reused and can be molded many times. Thermosets undergo chemical crosslinking during the molding process, forming a network-like three-dimensional structure. This structure of polymers will decompose at high temperatures. This material has excellent high-temperature resistance and can be used to make parts and handles for cookware, automotive engine parts, circuit boards for installing electronic components, and components used in aircraft, among others.
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 an unordered state, with long chains intertwined and capable of forming a glassy state. Generally, amorphous resins have a small shrinkage rate when cooled, and processing amorphous plastics is relatively easy because of the wide melting temperature range, which helps reduce the mold shrinkage stress in the final product. Products made from these resins are rigid, with low to medium impact strength (such as polystyrene and PMMA) or excellent impact strength and transparency (such as polycarbonate). In crystalline polymers, molecules are arranged in an ordered manner in a specific direction, and the polymer molecular chains are also basically arranged in an ordered manner. Crystalline polymers are usually tougher and softer, with a large shrinkage rate when cooled. These materials typically have a very narrow melting range. Typical crystalline resins include polyethylene and nylon. Depending on the stereoisomer shape of polypropylene, it can show both crystalline and amorphous states.
12.5 Plastic Material Coloring
Generally, thermoplastics can be molded in a wide range of colors. Color can be provided by pre-colored base material or by adding solid or liquid masterbatches before material plasticization. A masterbatch is a material that contains highly dispersed high-concentration pigments in a carrier resin. Masterbatch suppliers make them by blending according to the required color, then preparing them according to a concentrated formula, where pigments account for 20%–60% of the carrier resin. Additives such as antioxidants, stabilizers, and anti-stick agents can be mixed simultaneously. It is recommended to mix a certain concentration of masterbatch with the base 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 ratio is 100:1. Generally, the lower the ratio [(25:1)–(30:1)], the higher the accuracy of the required pigments. Other factors, such as screw length-to-diameter ratio and screw rotation speed, can also be adjusted appropriately based on the mixing percentage. This is because the mixing is done using a screw, and the resin spends less time in the extruder. The carrier resin should be highly compatible with the resin being produced, and the melt flowability of the carrier resin must be high to ensure good mixing of the pigments and uniform extrusion of the resin. An important quality inspection of masterbatches is color matching, and the hue must match when comparing the formed product with color chips or standard color chips. Precise measurement of the light source used for color matching is important. UV light, fluorescent light, and sunlight are commonly used light sources. Colors can only be matched under one light source, which is known as conditional color matching. When masterbatches are thermally sensitive plastics, the retention time should be appropriately reduced. Generally, red has the best thermal stability, while yellow and orange have medium thermal stability. The deeper the color, the lower the processing temperature required. Masterbatches of hygroscopic resins must be mixed before drying.
12.6 Reclaimed Material in Extrusion Blow Molding
Extrusion blow molding generally produces a certain amount of flash, and on some products, the flash can account for up to 100% of the total product weight. The economical approach is to recycle and reuse the flash. The reclaimed 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 (crushers, storage bins, feeders) must be kept clean. The specific amount of reclaimed material used in a particular product is determined by several factors, and the different usage properties are shown in Figure 12.5(a)–(g).
12.6.1 Reclaimed Material Usage
The maximum or optimal amount of reclaimed material must be determined through multiple physical property tests to meet the final product requirements. Repeatedly processed raw materials can degrade the physical properties of the material.
12.6.2 Processing Characteristics
For some materials, adding reclaimed material to about 50% can affect hanging strength and mold exit expansion, and poor billet width consistency can result from these issues.
12.6.3 Physical Properties
When a large amount of reclaimed material is added, some important physical properties of certain resins may be lost. Most resin suppliers recommend that the amount of reclaimed material not exceed 3 heating cycles. Products that must withstand impact and repeated pressure should have the reclaimed material content precisely controlled. For co-extruded products, reclaimed material should be used in the intermediate layer. Reducing flash can be done by making the distance between the mold and the machine head as close as possible and using appropriately sized nozzles. High-fine-grained powder or granular reclaimed material requires a temperature reduction of 5.5–11.1°C in the feed section, which helps prevent the plastic from melting too early. The content of reclaimed material can change the amount of masterbatch added and the processing machine's ability to handle special color masterbatches, as many colors are thermally sensitive materials and will fade during repeated processing. Reclaimed material differs from pure granules because the reformed particles are irregularly shaped and feel rubbery, making them prone to sticking together.
12.7 Used and Industrially Reused Materials
The factors to consider in the forming process of high-density polyethylene (HDPE) reclaimed material and pure material are mostly the same. Since most other materials cannot be used in blow molding, people look for suitable opportunities to use reclaimed material. High-density polyethylene of various grades is one of the most widely used plastic materials, and the main consideration when using reclaimed material is that fragments or sheets slide more poorly than granules, which can lead to bridging at the feeder of the extruder. A large number of fine sheets increase the likelihood of this problem. To prevent the possibility of this issue, it is important to determine the amount of fine particles. Under specific application conditions, the most suitable ratio of reclaimed 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 light to verify the material's long-term toughness and comparing it with the initial value, it can be ensured that the selected material ratio meets both the long-term and short-term performance requirements of the product. The same test method also applies to the reclaimed material of other plastic grades. When reclaimed material is normally used in pure material, the percentage of the mixture is determined by mixing pure material and reclaimed material to meet the technical requirements of the processing. The melt flow characteristics of HDPE feeding are different from those of other plastics. For example, high-density polyethylene used to produce milk bottles has a low melt index. Another example is detergent-colored bottles, which have a melt index of 5, and general bottles are around 5. This melt index plastic is suitable for blow-molded products but not for injection molding, which requires good fluidity (melt index between 20–30). Films (garbage bags, padding packaging, etc.) cannot be formed by injection molding but are suitable for extrusion or blow molding. To understand the flow characteristics of a given reclaimed material, experiments must be conducted to ensure that qualified products are produced using the appropriate reclaimed material.