What is Injection Molding?
Injection molding is a popular manufacturing process that facilitates producing a large volume of high-quality, uniform and cost-effective plastic parts. During the injection molding process, a plastic or resin material is heated to its melting point and then injected into a custom mold. The part forms inside the mold and is released once it has properly cooled. Injection molding is automated and can be repeated thousands and sometimes millions of times, creating exact copies of a custom part.
The first step of getting a plastic part injection molded is to have a computer-aided design (CAD) model of the part produced by a design engineer. The three-dimensional (3D) CAD model then goes to an injection molding company where a plastic mold maker (or toolmaker) will make the mold (tool) that will be fitted into an injection molding machine to make the parts.
Molds are precision-machined usually from steel or aluminum and can become quite complex depending on the design of the part. Plastic materials shrink at different rates when they cool, so the mold has to be constructed with consideration for the shrinkage rate of the material being used for the parts. In other words, a formula is applied in the construction of the mold to slightly increase the size so that when the plastic shrinkage occurs, the part will be to the dimensional specifications of the CAD model.
ICOMold has been working with customers all over the world and in many different industries. We are a low-cost leader producing high-quality injection-molded plastics with a quick turnaround. ICOMold is proud to serve both first-time inventors and Fortune 500 companies. We are an ISO 9001:2008 certified injection molding company. Ready to begin your injection molding project? We can help you select the best material for your project.
Injection Molding Basics
The actual process of plastic molding is just an expansion of this basic process. The plastic goes into a barrel or chamber by gravity or is force-fed. As it moves down, the increasing temperature melts the plastic. Then, the molten plastic is forcibly injected into the mold under the barrel with an appropriate shape. As the plastic cools, it solidifies. The plastic molded like this has a reverse shape of that of the mold. A variety of shapes can be produced by the process. The process of plastic molding is cheap due to the simplicity involved and the quality of the plastic material is modifiable by changing the factors involved in the custom injection molding basics process:
- The pressure of injection can be varied to change the hardness of the final product. Injection pressure causes the material to flow. Pressure increases as mold filling becomes more complex. There is a direct relationship between injection pressure and injection line pressure.
- The thickness of the mold affects the quality of the part produced. On average, the minimum wall thickness of an injection molded part ranges from 2mm to 4mm (.080 inch to .160 inch). Parts with uniform walls thickness allow the mold cavity to fill more precisely since the molten plastic does not have to be forced through varying restrictions as it fills.
- The temperature for melting and cooling determine the quality of the plastic formed. In plastic injection molding, the temperature of the melt in the cavity is generally between 200 and 300 degrees, the melt is formed in the cavity, cooled and solidified into a product profile.
Insert Injection Molding
Insert molding is a process that starts with placing metal, ceramic, or plastic parts into the plastic injection mold. After those parts have been properly placed, the mold is filled with molten plastic in the regular injection molding process.
Gas Assist Injection Molding
Gas assist injection molding is a process that utilizes an inert gas to create one or more hollow channels within the injection-molded plastic parts. At the end of the filling stage, the gas is injected into the still liquid core of the molding. From there, the gas follows the path of least resistance and replaces the thick molten ejections with gas-filled channels.
What Materials are Used in Injection Molding?
There are many plastic or resin materials available in the manufacturing industry. Selecting the right match for your custom injection molding project is essential to producing a high-quality part. Each material will have its own set of unique characteristics and chemical makeup. When selecting the best material for your injection molding project, consider the durability, performance, texture, flexibility, density and color.
Here are some of the materials ICOMold offers for injection molding manufacturing.:
Sometimes called by its trademarked names Lexan, Makrolon, Makroclear, arcoPlus®, polycarbonate is a commercially available thermoplastic that can withstand strong impacts, is transparent and amorphous. It can be pliable at room temperature and may be reformed without the application of heat. Polycarbonate can be liquified at its melting point, making it easy to use for injection molding applications. Some common applications of polycarbonate include windshields, phone cases, pens, vehicle headlights and more. Its versatility is just one advantage of this common thermoplastic.
Polypropylene (PP) is the second most widely-produced plastic globally, after polyethylene (PE). Its behavioral characteristics are similar to PE, but it is slightly harder and more heat-resistant. It is a commodity–grade polymer popular in the packaging and labeling industries. PP is tough, fatigue- and chemical-resistant, but vulnerable to UV radiation and is flammable. It is a versatile plastic, as it is easily customized with additives. It is naturally white. Due to the elasticity of PP, it makes a good material for living hinges. It is also used in many common products such as buckets, packaging, bottle caps, toys and many other items.
Polyvinyl Chloride (PVC) is a popular polymer that is naturally white and brittle. It is available in two forms, flexible and rigid. The first form is a rigid or unplasticized polymer (RPVC or uPVC) commonly used for construction products such as vinyl siding or pipes. The second form, flexible, plasticized PVC, is made bendable by the addition of phthalates. It is used for flooring, insulation, electrical wires and more.
Acrylic (polymethyl methacrylate PMMA), sometimes called plexiglass, is a thermoplastic homopolymer that is often used as an impact–resistant replacement for glass. The light transmittance of acrylic is 92%, which is similar to optical quality glass. Acrylic is UV and light resistant and will not fade. It is ten times more impact resistant than glass. There are numerous benefits to using acrylic for injection molding manufacturing. Common acrylic applications include lenses, medical devices, screens, furniture, paint, beauty products and more.
Injection molding resins (or thermoplastic resins) are plastics that are easy to manipulate or mold once heated to a certain temperature and become solid once they are cooled. Resins are ideal for the injection molding process because they can be liquified, injected into a mold using pressure and then cooled to form a custom product. Some common resins used for injection molding include crystalline plastics, amorphous plastics, imidized plastics, polyethylene, polypropylene, acrylonitrile butadiene styrene, polyoxymethylene, polystyrene and more.
Nylon (PA) or Nylon Polyamide is a common synthetic thermoplastic polymer that is popular due to its high melting temperature, tensile strength, low friction and resistance to chemicals. It is both durable and flexible. Nylon can be combined with other thermoplastics to achieve increased strength. Nylon can be combined with many additives to create different variants and material properties. Its overall versatility is just one of its many advantages.
Glass-filled Nylon is made by adding glass in powder form to Nylon or by extruding the Nylon resin with glass fibers. The result is a Nylon product that has increased stiffness and strength, amongst other enhanced properties. There are additional benefits that make glass-filled Nylon an ideal material for specific projects.
Acrylonitrile Butadiene Styrene or ABS is a thermoplastic polymer widely used for injection molding. It is durable and can stand up to scratching, is budget-friendly and is easy to machine and fabricate. ABS can be used in conjunction with a metal coating as many metals can adhere strongly to ABS plastic. ABS is perfect for projects that demand strength, stiffness and impact resistance. It can be painted, glued and is easy to recycle. ABS plastic properties are not highly sensitive to temperature or humidity. Molding ABS at a high temperature produces resistance to heat and a nice gloss, while molding it at a low temperature will result in strength and impact resistance. Therefore, its final properties and strength are influenced by the conditions used during the manufacturing process.
Injection molding thermoplastic rubbers are plastics that are easy to manipulate or mold once they have been heated to a certain temperature and then become solid at room temperature or through a cooling process. Thermoplastic rubbers are ideal for injection molding manufacturing because they can be liquified, injected into a mold using pressure and then cooled to form a custom product. There are many advantages and applications of thermoplastic rubber.
Polystyrene (PS) is a transparent thermoplastic most often used to create single-use products. It is odorless and non-toxic. Polystyrene is available in either solid or foamed. As a solid, it is used to make plastic cutlery, electronics, toys and auto parts. As a foam (Styrofoam) it can be extruded to make packing peanuts, disposable drinking cups and more.
Low–Density Polyethylene (LDPE) is a form of polyethylene (PE) that is made using high pressure and high temperature. LDPE is a flexible type of polyethylene used mainly for packaging. It has superior moisture resistance but is not suitable for products that require weatherability or that must withstand high temperatures. LDPE has good impact strength and chemical resistance. LDPE can be used to make six-pack rings, toys and computer components. In recycled form, polyethylene can be used to make trash cans, tiles, paneling and furniture. Polyethylene products account for 34% of total plastics on the market.
Fiber–reinforced plastics (FRP) is a cost-effective material that results in lighter weight products with increased durability. They are a composite material consisting of a resin and select fibers such as glass or carbon. By adding fibers to resin, the material receives additional support. Fiber–reinforced plastic for injection molding lowers material and labor costs, reduces production time and creates minimal waste. The result is a product that is durable and resistant to warping and cracking.
High impact polystyrene (HIPS) is polystyrene that has rubber additives to improve its flexibility and impact strength. While polystyrene is normally transparent, HIPS is opaque and can be painted and labeled. High impact polystyrene is a cost-effective material that is easy to customize, recycle and manufacture. HIPS is ideal for smaller parts and products that require higher levels of detail and variable thickness.
Polyoxymethylene (POM, acetal, polyacetal, polyformaldehyde) is a combination of plastic material used to create parts with good dimensional stability, low friction and high rigidity. It is opaque white and is available in many colors. Similar to other thermoplastics, POM becomes a liquid at its melting point, making it an easy material to use in the injection molding process. POM is commonly used for eyeglass frames, fasteners, guns, knife handles, lock systems and other engineering and automotive parts.
History of Injection Molding
The injection molding process is generally dated back to 1868, when John Wesley Hyatt of billiard ball maker Phelan and Collander was searching for a suitable replacement material for the ivory in billiard balls. Hyatt invented a way to inject celluloid into a mould that processed it into a finished form. In 1872 John and his brother Isaiah patented the first injection molding machine. This machine was relatively simple compared to the complex machines used by today’s injection molding companies. It consisted of a basic plunger to inject the plastic into a mold through a heated cylinder. The industry was slow to adopt the injection molding process, eventually beginning to produce plastic items such as collar stays, buttons and hair combs. Not until the 1940s did the concept of injection molding really grow in popularity because World War II created a huge demand for inexpensive, mass-produced products.
The industry was revolutionized in 1946, when James Hendry built the first screw injection molding machine with an auger design, replacing Hyatt’s plunger. The auger mixes the injection molded material in a cylinder and pushes the material forward, injecting it into the mould. This allowed colored plastic, or recycled plastic, to be mixed in with the virgin material before getting injected into the mould.
Today, screw-type injection molding machines account for 95% of all injection machines. The industry has evolved immensely over the years due to technological advancements and machine automation. It has come from producing combs and buttons, to a multitude of custom injection molded products for virtually every industry including automotive, medical, construction, consumer, packaging, aerospace and toys.
Advantages of Injection Molding
The major advantage of injection molding is that it is very cost effective and fast. Unlike a “cutting” manufacturing process, injection molding reduces the possibility of undesired sharp edges. This process produces smooth and finished products that require no further finishing. Not only is the injection molding process simpler and more reliable than others methods, but it is also extremely efficient. Here are some of the top advantages of the injection molding process:
Fast and efficient: The injection molding process is extremely fast compared to other methods and the high production output rate makes this process even more efficient and cost-effective. Once the injection molds have been designed to specifications the actual molding process is very fast compared with other methods of molding, allowing for more parts to be manufactured from a single mold. Manufacturing speed is dependent on the complexity of a mold design, but on average only about 15 to 30 seconds pass between cycle times.
Complexity of part design: Injection molding can handle extremely complex parts and uniformity, as well as the ability to make millions of virtually identical parts. Since the plastic injection molds are subjected to extremely high pressure it is possible to add a large amount of detail to the part’s design.
Waste reduction: A high-volume injection molding company shares a commitment to quality, sustainability and optimal safety. The benefit to the environment is by only using required amount of plastic to build parts. Recycling excess plastic benefits environment and minimizes waste.
Lower labor costs: Much of the injection molding process is automated and controlled by a single operator or mold technician. This keeps labor costs at a minimum. The molding equipment typically runs with a self-gating, automatic tool to keep operations streamlined and production ongoing, requiring minimal supervision.
Strength: While it is possible to use fillers in the injection molds, these fillers reduce the density of the plastic while being molded and help add to its finished strength after the molding process. Finding the proper balance of design considerations determine the part’s need for strength and stability. The strength and durability of plastics has increased over the years. Modern lightweight thermoplastics can withstand even the most rugged environments.
Versatility in materials and color: Choosing the right material and color for a project are two of the essential factors in creating plastic parts. One mold can produce may variations of your product. Color and material type are relatively easy to change in injection molding machines.
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