You will now receive a comprehensive guide to the injection production process, one of the most widely used methods for manufacturing plastic parts and products. In this guide, you’ll learn about the history, principles, equipment, materials, and advantages and disadvantages of injection molding. Additionally, we will show you some applications and examples of injection molding in different industries.
First, we’ll review the history of injection molding and understand how it evolved from its simple beginnings to the complex technology it is today. We’ll explore key milestones and groundbreaking developments along the way, and their impact on the industry as a whole.
Next, we’ll dive into the principles of injection molding. We’ll explain how the process works, which involves heating plastic pellets, injecting them into a mold, then cooling and removing the finished product. We will also discuss some of the key parameters in this process, such as temperature, pressure, and time, and how they affect the quality of the final product.
After understanding the principles of injection molding, we will introduce the equipment used for this process. We will describe in detail the various types of injection molding machines, including fully electric, hydraulic, and hybrid types. We will also discuss how to select the right injection molding machine for a specific application and how to maintain and repair this equipment.
Then, we’ll look at the materials used in injection molding. We’ll cover the various types of plastics, including thermoplastics and thermosets, as well as their properties and applications. We’ll also discuss how to choose the plastic material that’s best for your specific product.
Next, we will discuss the advantages and disadvantages of injection molding. We’ll explain why this approach is so popular, including its advantages of being efficient, flexible and economical. We will also discuss its disadvantages, such as the need for a large initial investment and the potential impact on the environment.
Finally, we will show applications and examples of injection molding in different industries. We’ll discuss its applications in industries such as automotive, electronics, medical, and consumer goods, as well as some successful case studies.
What is Injection Molding?
Injection molding is a process in which a thermoplastic polymer is heated above its melting point, resulting in the conversion of the solid polymer to a molten fluid with a reasonably low viscosity. This melt is mechanically forced, i.e., injected, into a mold in the shape of the desired final object. The low viscosity of the molten polymer allows complete filling of the mold where the article resides until it is cooled below the freezing point of the polymer. In the case of semicrystalline polymers, the crystallinity of the object (which governs its mechanical and appearance properties) is usually controlled by in-mold cooling of the object at a defined cooling rate. In the last step, the mold is opened and the part is ejected and recovered .
Injection molding is one of the most important processes for mass production of objects from thermoplastics, usually without additional finishing being required. Today, most injection molding machines are the universal types, which can accept all types of molds, within limits. The economics of this process are excellent for articles with complex geometry, giving this process an advantage over other techniques. Cost per molding improves with scale, despite the sizable initial cost of injection molding machines.
The essential mechanisms of injection molding are heat transfer and pressure flow. The essential equipment is an injection molding machine, sometimes known as a press, and a mold which may also be referred to as a tool or sometimes a die. The product of the process is a molding, which confusingly and inaccurately is sometimes called a mold.
History of Injection Molding
Molding is the process of shaping a liquid or flexible raw material using a rigid frame called a mold. This process, which is very popular today in many industries, took off at the height of the Second World War when the need for mass-produced products increased.
The Industrial Rise of Molding: Mass Production
In 1872, the American inventor John Wesley Hyatt and his brother Isaiah created the first injection molding machine. It was a simple machine that used a piston to push plastic through a hot cylinder into a mold. The engine was primarily employed for mass production of items such as buttons and combs.
The German chemist Arthur Eichengrün developed the injection molding press in 1919. In 1939, he filed a patent application for the injection molding of plasticized cellulose acetate, which was much less flammable than the solutions utilized at the time.
When the Second World War broke out, the need for cheap, mass-produced goods increased. For example, the war in Asia and attacks at sea disrupted rubber production. Tanks and other war applications generated a massive demand for metal. Plastics were used to fill these gaps, providing an affordable substitute.
James Watson Hendry then created the first screw injection molding tool, which quickly became popular because of its precision and control, and therefore better production quality. Throughout the post-war period, plastics remained popular. When business leaders recognized the enormous cost advantages over competing materials, they re-evaluated global supply chains. Plastics became firmly entrenched in the economy and manufacturing processes of the mid-twentieth century.
The Invention of Injection Molding Up to the Present Day
Hendry then developed the first gas-assisted injection molding system in the 1970s. The technique made it possible to manufacture complex parts, which could be cooled quickly. This greatly improved the flexibility and strength of the manufactured objects while reducing production time and cost.
In 1979, plastic production surpassed steel, and in 1990, injection molding made extensive use of aluminum molds.
Today, screw injection machines represent the vast majority of injection machines. Other widely used molding methods include blow molding, compression molding, or vacuum molding (thermoforming).
Now, the injection molding market has reached $300 billion. This process produces more than 5 million tons of plastic parts yearly worldwide. Almost all manufacturing sectors use it: electronics, automotive, household goods and appliances, etc. Plastic injection molding remains an affordable and efficient method of producing high-quality parts and products.
Injection Molding Equipment
The essential equipment for injection molding is an injection molding machine and a mold. The injection molding machine consists of two main components: the injection unit and the clamping unit.
The injection unit is responsible for melting the plastic material and injecting it into the mold under high pressure. The injection unit consists of a hopper, a screw, a barrel, a nozzle, and a heater. The hopper is where the plastic material is fed into the machine. The screw is a rotating device that pushes the material forward and mixes it homogeneously. The barrel is a hollow cylinder that surrounds the screw and contains the heater. The heater is used to heat up the material to its melting point. The nozzle is a tapered opening at the end of the barrel that directs the molten material into the mold.
The clamping unit is responsible for holding the mold closed during the injection and cooling phases. The clamping unit consists of a fixed platen, a movable platen, a toggle mechanism, and a hydraulic cylinder. The fixed platen is attached to the injection unit and supports the mold. The movable platen is attached to the hydraulic cylinder and moves back and forth to open and close the mold. The toggle mechanism is used to apply a high clamping force to keep the mold closed under high injection pressure. The hydraulic cylinder is used to power the movement of the movable platen.
The mold is a hollow metal block that contains one or more cavities in the shape of the desired part. The mold also contains channels for cooling water, ejector pins, sprue, runner, and gate. The cooling water channels are used to circulate cold water around the mold to cool down the part and solidify it. The ejector pins are used to push the part out of the mold when it is opened. The sprue is the main channel that connects the nozzle to the runner. The runner is a network of channels that distributes the material from the sprue to the gate. The gate is a small opening that connects the runner to the cavity.
Injection Molding Materials
The most common materials used for injection molding are thermoplastic polymers, which can be melted and reshaped repeatedly without losing their properties. Some examples of thermoplastic polymers are polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), nylon, acrylonitrile butadiene styrene (ABS), polycarbonate (PC), etc.
Some thermoplastic polymers can be blended with other materials such as additives, fillers, reinforcements, or colorants to modify their properties or appearance. Some examples of additives are stabilizers, lubricants, antioxidants, flame retardants, etc. Some examples of fillers are calcium carbonate, talc, glass beads, etc. Some examples of reinforcements are glass fibers, carbon fibers, etc. Some examples of colorants are pigments, dyes, etc.
Some thermosetting polymers can also be used for injection molding, but they require a different process called reaction injection molding (RIM). Thermosetting polymers are polymers that undergo an irreversible chemical reaction when heated and cured in a mold. They cannot be melted or reshaped once they are cured. Some examples of thermosetting polymers are epoxy, polyester, polyurethane, etc.
Injection Molding Process Steps
Injection molding is a manufacturing process that produces plastic parts by injecting molten material into a mold. The process consists of the following steps:
- Clamping: The two halves of the mold are closed and clamped together by a hydraulic or mechanical force. The clamping force must be sufficient to withstand the pressure of the injected material and prevent the mold from opening or cracking.
- Injection: A screw or plunger pushes the molten plastic into the mold cavity through a nozzle. The injection speed, pressure and temperature must be controlled to ensure the quality and consistency of the parts.
- Cooling: The molten plastic inside the mold begins to cool and solidify, taking the shape of the mold cavity. The cooling time depends on the material, wall thickness and mold design. During this stage, the plastic shrinks slightly and may form internal stresses.
- Ejection: After the cooling time has elapsed, the mold is opened and the part is ejected by a mechanism such as ejector pins or air blast. The ejection force must be enough to remove the part without damaging it or the mold.
- Post-processing: The ejected part may undergo additional processing such as trimming, drilling, painting or assembly to meet the specifications and requirements of the final product.
Advantages and Disadvantages of Injection Molding
Injection molding is a process of producing plastic parts by injecting molten plastic into a mold and then cooling and ejecting the final part. It has many advantages and disadvantages that should be considered before choosing it for a project. Some of the advantages are:
- It allows for complex geometries with tight tolerances. Injection molding can produce parts with intricate shapes and details, as well as high accuracy and consistency. The typical tolerance range for injection molded parts is ± 0.500 mm (0.020’’), but it can be as low as ± 0.125 mm (0.005’’) for some applications.
- It is compatible with a wide range of materials and colors. There are over 25,000 engineered materials that can be used for injection molding, including thermoplastics, thermosets, resins, and silicones. These materials have different physical, mechanical, and chemical properties that can suit various needs and requirements. Injection molding also allows for color customization by using masterbatches, pre-colored resins, liquid colorants, or salt and pepper blends.
- It is very efficient. Injection molding has a fast cycle time, usually between 10 to 60 seconds, which means it can produce a large number of parts per hour at a low cost per part. Injection molding can also use multi-cavity or family molds to produce several parts in one cycle, further increasing the production rate and efficiency.
- It offers high repeatability and reliability. Injection molding can produce identical parts over and over again with minimal variation and defects. This is ideal for applications that require high quality and consistency across large volumes of parts.
Some of the disadvantages are:
- It has a high initial cost and lead time. Injection molding requires designing and manufacturing a mold, which can be expensive and time-consuming depending on the complexity and size of the part. The mold also needs to be tested and refined before mass production, which adds to the overall cost and lead time. Injection molding is not suitable for low-volume or prototype production, as the mold cost may not be justified by the number of parts produced.
- It has limitations on part design and material selection. Injection molding requires the part to have certain features that facilitate the molding process, such as draft angles, uniform wall thickness, smooth transitions, adequate venting and gating, etc. These features may limit the design flexibility and creativity of the part. Injection molding also requires the material to have certain characteristics that allow it to flow and fill the mold cavity, such as melt viscosity, thermal stability, shrinkage rate, etc. These characteristics may limit the material options and performance of the part.
- It produces waste and environmental impact. Injection molding generates some waste material during the process, such as runners, sprues, flash, etc. These waste materials need to be recycled or disposed of properly to reduce the environmental impact. Injection molding also consumes a lot of energy and water to heat up and cool down the plastic material during the process. These resources need to be managed efficiently to reduce the carbon footprint of injection molding.
