You have most likely seen something that was made with plastic injection molding. Remote controls, scale model kits, numerous children’s toys, appliances, and many parts of a vehicle interior. Have you ever wondered how they were made? Have you wondered why so many products and parts are made with plastic injection molding instead of another manufacturing avenue? It’s very popular, and it may be the right choice for your next product also. If you’ve been thinking about launching a new product, but were unsure of the background into plastic injection molding, read on as we’ll pull back the curtain on a process that can create your future:
What is Injection Molding?
Injection molding is a process that utilizes a molten material that is forced into a mold to form a part. The molten material could be metal, glass, rubber, or plastic. A mold is filled under pressure with the molten material, then cooled to form a solid part before being removed from the mold.
What is Plastic Injection Molding?
Injection molding that utilizes plastic resin as the molten material is specifically called ‘Plastic Injection Molding’. It uses one of thousands of resins available to form solid parts from an engineered resin.
Injection molding has been around since the early 1870s when the first plastic injection molding machine was developed to produce billiard balls. For the next six decades, plastic injection molding was used to develop new products with newly formed plastic resins. In the 1930s plastic injection molding took off with great options for products using thermoplastics, such as polyolefins, polystyrene, and polyvinyl chloride (PVC).
What Products Are Made by Injection Molding?
With the technology and engineered resins currently available, it may be a fair question to ask what isn’t made by injection molding. Injection molding can utilize metal, plastic, rubber, and glass as the molten material, which means almost anything can be made by injection molding At SEA-LECT Plastics we utilize plastic injection molding to make drink tumblers, outdoor adventure gear, military and automotive products, and even musical instruments.
What are the Steps in Injection Molding?
The injection molding process has four main steps:
Step 1: Melt the material to a molten state
Step 2: Injection the molten material into the mold
Step 3: Allow the molten material to cool to a solid state inside the mold
Step 4: Remove the cooled part from the mold
What Does the Injection Molding Machine Look Like?
The injection molding machine has four main components:
Component 1: The Injection Unit, which includes:
- The Hopper
- The Screw
- The Barrel
- The Heater
- The Nozzle
Component 2: The Clamping Unit, which includes:
- The Mold Cavity
- The Mold Core
- The Moveable Platen
Component 3: The Controls
Component 4: The Base
How Does an Injection Molding Machine Work?
To start the injection molding process, the machine draws in the material that will be melted into a molten state. Many materials are in a pellet form when they are fed into the machine. The pellets or raw materials are drawn into the machine via the Hopper, which in many cases uses gravity to feed the material in.
Once the material is inside the machine, an internal screw (also called an auger) turns to move the materials through the barrel to a heater. Inside the heater, the material is changed from a solid state to a molten state. Once in a molten state, the material is injected through the nozzle under pressure.
The nozzle is directed into the mold. When the cycle starts, the two halves of the injection mold close. That forms the space that will be filled with molten material as the auger forces molten material forward under high pressure.
The molten material fills the injection molding cavity, and is then cooled to harden the material. The injection mold is typically cooled with a liquid coolant, such as water, and it flows through the mold to draw away heat from the mold. When the part is cooled enough for removal from the mold, the halves open and the part is ejected out. The complete process, from injecting the molten material to a cooled part, takes one minute or less.
What are the Most Common Plastics Used for Injection Molding?
In plastic injection molding, thermoplastics are one of the most often chosen materials. Thermoplastics melt under high heat, and can be reheated repeatedly to form new parts over and over before degradation occurs. Thermoplastics can also be colored with color additives, glitter, and their properties can be altered with specialized additives. These are five thermoplastics that are used most:
Acrylonitrile Butadiene Styrene (ABS)
ABS is a common resin that you can find in many applications needing a tough exterior and thermal stability. It is a thermoplastic that is a lower cost option available. It’s easily machined and shaped to specific designs while also able to be colored to match specific applications for many different industries. It is a strong material offering minimal chemical and electrical resistance in many environments.
Nylon has been around for 80 years, and has numerous applications and variations. It’s one of the resins frequently used in fabric applications like carpet and flooring, food packaging, and automotive parts. One of the first uses after development was ladies nylons, which is where the name of the product stems from. Nylon has many uses from toothbrushes, women’s stockings, to parachutes. Beyond these areas, food packaging is prevalent with clear nylon film. For clothing, nylon is mixed with other fabrics like cotton, polyester, and spandex to make a material that is durable, strong, and has the ability to wick away moisture. It is highly recyclable, so that is an added benefit to its use.
Polypropylene is on the list of the most common used and manufactured plastics in the world. It has many properties, such as chemical resistance, flexibility, and insulation. While it can be flammable, and degrade in UV exposure, it is still a popular choice in many applications.
HIPS (High-Impact Polystyrene)
HIPS offers good impact resistance, dimensional tolerance stability, a hard surface that is good for machining, and a good surface for applications needing aesthetic properties. HIPS offers the ability to be printed on, which makes it a good choice for signs. It can be glued or bonded together, and other materials can be added for more opportunities to change the decoration of the end product. It is low in cost, which also makes it a favorite choice for injection molding.
Polycarbonate is another resin that offers strength and a lower cost. It is a high-grade thermoplastic that also is naturally transparent. That makes it a great option for safety wear, bulletproof glass, and medical uses including test tubes, beakers, and pipettes.
What is the Biggest Advantage of Using Injection Molding?
Injection molding has many advantages when compared to other manufacturing processes to create parts or components for a product. It is most often used for mass-production processes where the same part is reproduced over and over. Some parts may be produced in quantities of a thousand, while some may be replicated a million times over the course of the injection mold’s life. One of the major advantages of choosing injection molding as your primary manufacturing process is that you can produce a large quantity of parts for a low cost. The tooling will be the largest investment of the project, but you have the ability to scale the project as needed, whether that is more or less parts in a short amount of time.
Are There Additional Benefits to Injection Molding?
Injection molding has other benefits other than being easily scalable. Some of those include:
- Injection molding produces a very low amount of waste. At initial start-up of the machine, you will have some shots through the molding die to purge air and contaminants out. Those will most likely not be useable in the future and will need to be discarded. From there, there will be minimal amount of waste from the manufacturing process in the form of a sprue, the runners, the gates, and flash from the edges of the parts, but all may be able to be ground back into pellet form and reused.
- There are thousands of resins available that have amazing strength, corrosion resistance, and durability that can rival metals, alloys, and aerospace materials. Choosing the functional and visual criteria that the parts need to demonstrate may be harder than finding a perfect resin to match your ideal product.
- Injection Molding is very repeatable from day to day and year to year. The third part can be the same as the hundredth, the thousandth, or the one millionth. If you’re producing a large quanitity of parts over the course of a few years, it will be beneficial to have consistency and repeatability in your products.
- Injection molds can make multiple parts at one time. If you need three of one part, five of another, and then just one of the last part, one mold may be able produce all pieces in one molding shot. In other production processes, that may take three different dies or three different processes altogether.
- The colors available for your products are limitless. Beyond colors, you could choose inlays and overmolds to create unique finishes and designs that aren’t an option with other materials and manufacturing processes.
- You have the ability to design a large amount of assembly and human interaction out with automation. Human interaction typically adds cost to the process and final product. Eliminating as much as possible will reduce cost to individual parts, the overall product, and additional assembly or manufacturing.
- With sustainability trending as a hot topic, plastics are now being developed to increase biodegradation in many environments.
Are there Drawbacks to Injection Molding?
As with every process, there are downsides. These are some things that make injection molding less appealing than other processes available:
- The initial cost of an injection-molding die may be more than your budget allows. Your tooling designer and mold maker should be able to advise you in ways to reduce costs for your molds without sacrificing quality or time. If you need a low quantity of parts, it may be beneficial to use a softer die material to reduce the cost and lead-time of the finished die.
- Mold makers and tooling designers are specialists and experts in their craft, so finding the right partner may be more effort than searching the internet. Computer software is available to make an educated guess at what the final tool will look like, but an expert tool design will be able to refine the tooling beyond what a computer model will predict.
- Injection molding dies will have a longer lead-time than some other processes. A machining program can be generated overnight, and within a day or two you may be able to have a sample part for demonstration. In order to be competitive in that timeframe, you may need to find a rapid prototype operation that can 3D print a part in plastic instead of a part machined from a metal or hard plastic.
- Design revisions may not be easy to make in an injection-molding die. Some dies can be made modular so that pieces can be changed out, but certain details in your finished part may make that impossible. A large enough design revision may require a new mold to be created, and that can add a significant cost to the project. Your designer should be able to reduce the amount of changes needed, and a rapid prototype can help discover areas of opportunity for reduction or revisions to optimize the injection mold.
Hopefully this information gives you insight into injection molding overall, and a deeper understanding on plastic injection molding. SEA-LECT Plastics excels at injection mold design, tooling creation, manufacturing, assembly, and global logistics. We are a turnkey manufacturer that can organize a new product launch from concept to consumer. We have a complete turn-key option for your product. When you’re ready to discuss your current or next project, or want to know more about plastic injection molding, contact us at email@example.com or give us a call at (425) 339-0288.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.