Alexei Volkov
Injection molding is the most common method used in plastics processing, and it is considered the most versatile and accurate. It is used to create a variety of parts, ranging in both size and shape, from small bottle caps to large automotive parts. The injection molding process involves injecting melted plastic into a mold made of steel, which then fills the mold's cavities to form the desired shape. The mold's accuracy and surface roughness requirements depend on the type of plastic and product being created. To ensure the accuracy of the mold, it should be made of materials with high heat resistance and good wear resistance, such as steel or aluminum.
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What You'll Learn
Injection molding for accuracy
Injection molding is a highly accurate manufacturing process for producing parts by injecting molten material into a mould. It is the most common method used in plastics processing and is applicable to all thermoplastics and some thermosetting plastics. Injection molding is also used with other materials such as metals, glasses, elastomers, and confections.
The accuracy of injection molding depends on three main aspects: injection molds, injection materials, and injection machines. The mold is typically made of steel or aluminium, and precision-machined to form the features of the desired part. The mold should have high heat resistance to avoid deformation and ensure accuracy. The choice of material for the mold depends on cost considerations and the product life cycle. The dimensional tolerance of precision injection molds should be controlled below 1/3 of the product's dimensional tolerance.
The injection system and the part-forming system are in direct contact with the plastic and are the most complex and varied parts of the mold. The part-forming process involves injecting molten plastic into the mold cavity, where it cools and solidifies to the shape of the cavity. The accuracy of the final product depends on controlling the injection molding tolerances, which can be optimized through design for manufacturing (DFM) materials selection, tool design, and process control.
To achieve high accuracy, the mold should utilize steel or aluminum for their superior hardness, quick heat transfer, and durability. The number of cavities in the mold should not be too many, and the base plate, support plate, and cavity wall should be thicker to avoid deformation under high temperature and pressure. The materials for making cavities and runners should undergo strict heat treatment and have high hardness, good wear resistance, and strong corrosion resistance.
Overall, injection molding is a highly accurate process that can be controlled to within 0.005 inches of accuracy, making it suitable for producing precise and high-quality parts at an affordable cost.
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Rotational molding for large hollow products
Injection molding is the most common method used in plastics processing. However, rotational molding is a great option for large hollow products. This process is also known as roto molding, roto casting, or roto mold. It is a high-temperature, low-pressure plastic-forming process that uses heat and biaxial rotation to produce hollow, one-piece parts.
Rotational molding is ideal for creating large, hollow parts such as oil tanks, water tanks, kayaks, and canoes. It is also used to make highly specialized products, including UN-approved containers for the transportation of nuclear fissile materials. The process is quite slow compared to other high-speed, high-pressure processes, with only one or two cycles per hour. However, it is a remarkably effective method for creating hollow products, and it is much cheaper than other types of molds.
The rotational molding process involves heating and rotating a mold filled with plastic resin powder or polymer so that it melts and uniformly coats the interior. The mold is heated in an oven while it rotates, and the hollow part is rotated through two or more axes, rotating at different speeds to avoid the accumulation of polymer powder. The length of time the mold spends in the oven is critical: too long and the polymer will degrade, and too little time will result in large bubbles in the polymer, impairing the mechanical properties of the finished product.
There are some design considerations to keep in mind when using rotational molding. For example, when creating large flat surfaces, steps or recesses should be added to prevent the material from distorting. Parallel walls should also be evenly spaced to prevent them from getting too thin. Rotational molding is not suitable for creating extremely sharp corners and edges, as the material can get caught up in these points, leading to uneven coatings.
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Blow molding for uniform thickness
Injection molding is the most common method used in plastics processing. However, blow molding is a highly developed process with many variants, and it is the primary method for producing hollow plastic objects.
Blow molding differs from other molding processes in that the mold defines only the external shape of the object, while the inner shape is defined by fluid pressure, usually compressed air. This means that the inner form is free of constraints, but it also means that high precision and independent internal features are impossible.
Blow molding can be performed through extrusion blow molding, injection blow molding, or stretch blow molding. In extrusion blow molding, a cylinder of semimolten plastic, called a parison, is extruded downward between two open mold halves. Once the parison is long enough, the mold is closed, and a blow pin pressurizes the inside of the parison. The air pressure forces the parison to inflate until it touches the entire surface of the mold cavity.
The two issues affecting the parison tube thickness are gravity-induced flow or "sagging" and die swell. Gravity tends to make the bottom of the tube thicker at the beginning of blowing. To prevent this problem, a special die that produces a variable wall thickness, starting thinner and ending thicker, can be used. Such programmed parison thicknesses can also be used to create a uniform final thickness in complex molds. Additionally, the blowing time should be kept as short as possible to minimize gravity-induced flow.
In injection blow molding, the parison is made by injection molding, which produces a parison with a well-controlled wall thickness and more robust screw caps compared to extrusion blow molding. Injection blow molding offers much better control over wall thicknesses and screw caps, and with the addition of stretching, better mechanical and barrier properties. Therefore, it is used for more demanding container applications, such as carbonated beverage bottles.
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Mold wear resistance
Injection molding is the most common method used in plastics processing. It is applicable to all thermoplastics and some thermosetting plastics, and it is used to create a high volume of precise and high-quality parts at an affordable cost. Injection molds can be made of steel or aluminum, with steel molds offering secure and accurate foundations.
The mold wear resistance of plastic injection molds depends on the chemical composition of steel and heat treatment hardness. The most common tool steels vary in wear resistance based on their hardness. A strong enhancement of wear resistance can be achieved by introducing a second harder phase, such as a harder grade of steel. The carbide content, carbide size and shape, carbide distribution, and the hardness of the matrix are all factors that influence wear resistance.
The wear resistance of plastic injection molds can be improved by increasing the hardness of the mold, which can be achieved through heat treatment. A study found that the wear resistance of steel increased with higher heat treatment temperatures. However, the study also found that only at a higher frequency of 8 Hz was there a relationship between hardness and wear resistance. At a lower sliding frequency of 1 Hz, no significant differences in wear resistance were observed between the different materials.
The surface finish of plastic injection molds can also influence their wear resistance. A smoother surface reduces friction between moving parts, leading to less wear. Machining, polishing, and applying protective coatings can improve the surface finish and enhance the durability of the mold over time. Additionally, the design of the mold can impact wear resistance. Adding ribbing and reinforcement features can distribute stress more evenly, reducing wear in high-stress areas. Designing parts with precise tolerances helps minimize unnecessary friction and wear, especially in high-load or moving applications.
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Mold steel heat resistance
Injection molding is the most common method used in plastics processing. It is applicable to all thermoplastics and some thermosetting plastics. Injection molds come in various structures, depending on the application and design requirements. The mold bases offer secure and accurate foundations for molds. The mold rests upon this rigid structure, typically constructed out of soft steel such as S50C, P20, or, more expensively, 718, 1.2344 steel.
To ensure the accuracy of the mold in use and to minimize deformation, mold steel should have high heat resistance performance. The steel's chemical makeup and how it responds to heat treatment determine its wear resistance. Heat-treated mold materials can provide enough surface hardness to ensure the mold has sufficient rigidity. The mold needs to withstand greater pressure and friction during injection molding.
Different types of steel are used depending on the type of plastic being used. For example, some plastics require steel that is highly polished and resistant to corrosion. Common materials include P20, H13, and S7 steels, with H13 being favored for high-temperature applications due to its toughness and resistance to wear. H13 steel offers excellent heat resistance and wear resistance, making it suitable for high-heat molding applications, such as automotive parts. P20 steel is a medium carbon Cr-Mo plastic mold steel introduced from the United States. It is good for making molds for plastic molds and die-casting low melting point metals. This steel has good machinability and mirror polishing performance. 420 MQ plastic mold steel is another option that provides high hardness and resistance to corrosive vapors produced during molding.
To improve the heat resistance of mold steel, annealing and tempering processes can be performed. Annealing should be done after hot working and before re-hardening. The steel is heated at a specific rate and held at temperature for a certain amount of time before being cooled slowly. Tempering is done immediately after quenching and within a specific temperature range to achieve the desired hardness.
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Frequently asked questions
Injection moulding is the most accurate plastic moulding technique. It is also the most versatile and commonly used method.
Injection moulding involves injecting molten plastic into a steel mould with cavities. Once the plastic cools, the parts are ejected. This process is used to create a variety of parts, ranging in size and shape.
Injection moulding is used to create large automotive parts, small bottle caps, and small, intricate parts used in surgical equipment.
