Plastic injection molding significantly contributes to modern manufacturing, producing a diverse range of components used in nearly all products on the market. From kitchen knife handles and phone cases to dashboards on cars and drone turbine blades, you’ll find a plastic injection molded part there.
To help businesses better understand the intricacies of this production method, we’ve prepared this thorough injection molding guide. This includes what it is, how it works, and some common applications. We’ll also examine the advantages of plastic injection molding, as well as what to expect.
What Is Injection Molding Meaning?
Injection molding is a manufacturing method used to create rubber or plastic parts in a fast and efficient manner. It is a popular manufacturing procedure that sees great use in various industries, becoming one of the go-to processes for its durability, flexibility, and relative affordability.
The process involves injecting a heated material into a mold, which is then cooled to solidify the material within. Once enough time is given, the mold is opened to reveal the molded product, then cleaned and processed to achieve the desired outcome.
Origins of Injection Molding
The injection molding process was invented in 1872 by brothers John and Isaiah Hyatt. The initial patent for an injection molding machine featured a simple plunger that injects the molten material into the premade mold via a heated cylinder. The birth of the first injection molding machine gave way to an entirely new industry, the plastic manufacturing market, mass-producing common items like plastic buttons and collar stays.
Over time, certain improvements were added to the injection molding process, gradually reflecting what we see in the modern day. One of these advancements is the development of new materials, including soluble cellulose acetate and thermoplastics like PVC. These materials were easier to use and less combustible, making injection molding safer while improving production capabilities.
Injection Molding Process Step by Step
The injection moulding process contains several steps from beginning to end, with each requiring talent and skill for optimized output. In this section, we will look at how does injection molding work and include injection molding best practices to ensure the best results.
Material Preparation
Every plastic injection molding process starts with the selection of materials for the project. The plastic materials are carefully chosen based on the project’s specific requirements, like density, durability, and temperature resistance. For some projects, manufacturers may combine different plastic types to achieve the desired outcome, perfectly balancing the best features of each material.
After selecting the plastic material, the mold is inserted into the injection molding machine. A clamp secures the two sides tightly, creating a pressurized cavity for the molten material.
Melting & Injection
The plastic pellets are slowly fed into a barrel through a mechanical hopper, gradually filling the container. The barrel’s rotating screw moves the pellets on the barrel, melting them in the process with the help of high-temperature heating bands. This continues until the plastic turns from solid pellets to molten liquid plastic, which is then injected into the mold.
Mold Filling
The molten plastic fills the mold, making sure every nook and cranny of the cavity is filled with the plastic material. The plastic injection machine uses high pressure to rapidly push the plastic into the mold, as any delay could cause the plastic to harden and clog the funnel.
Cooling & Solidification
Screws attached to the mold continue to add pressure, preventing the molten plastic from escaping. It also compensates for potential shrinkage that occurs during the cooling process.
Speaking of cooling, a circulating system helps lower the temperature of the mold, typically using either water or oil. As the plastic cools, it solidifies and takes the shape within the mold, which could take some time to complete.
Ejection
Once the plastic is properly cooled and hardened, the mold is slowly opened using precision tools to prevent any damage to the part. Ejector pins or mechanisms on the machine then push the plastic part away from the mold, which falls into a bin and is moved for the next step. The mold is then cleaned and prepared for another round of injection molding.
Secondary Operations
Secondary processing refers to additional steps done to complete the injection molded part before shipping. This includes the removal of excess plastic through trimming or sanding, as well as polishing and finishing to enhance the aesthetics and performance of the completed part. The secondary operations used on the plastic part depend on the specific project requirements, so each project may undergo different secondary processes.
Quality Inspection
The last step in the plastic injection molded parts manufacturing is a meticulous quality inspection and testing. While most injection molding manufacturers conduct routine testing after each step of the process, a final round of inspections guarantees the overall quality of the product.
Inspectors follow established parameters when performing the final inspection, checking every aspect of the part per industry and facility benchmarks. The parts also undergo several tests using calibrated testing tools, making sure they pass with flying colors. Only those that meet industry standards are sent for packaging and shipment, recording the results for future reference.
What are the Main Sections of an Injection Mold?
An injection mold plays a key role in creating injection molded parts and components. Composed of several parts, a mold houses the molten plastic material that would form into the component once cooled to the right temperature. Let’s break down the individual parts and see how they function in the overall mold.
Clamp Plates
These parts ensure that each half of the mold is securely attached to the injection molding machine. While most clamp plates utilize bolts to hold the metal mold in place, some use strong magnets to attach the mold to the platen.
Nozzle/Sprue Bushing
Comprised of two pieces, namely the nozzle and sprue bushing, this is how the molten plastic enters the mold. The nozzle is where the liquid plastic material is pushed through, while the sprue bushing and locating ring center the nozzle and ensure it’s properly aligned. Once both parts come in contact, that’s when the plastic is extruded into the cavity.
Feed System
The feed system facilitates the flow of liquid plastic into the mold cavities, acting as guides as it goes to the individual runners and the gates. Once cooled and extruded, the sprue and runners could be removed from the plastic part and reused.
Feeder systems are also called cold runner systems, as they use unheated channels that cool along with the plastic. A hot runner system is also available for high-volume production, though it has a higher initial cost.
Cavities
Cavities are crevices within the mold that feature the intricate designs and the final destination of the molten plastic material. For optimal production, molds will feature an even number of cavities to maintain their balance.
Cooling System
Injection molding machines have several cooling channels connected to the mold that facilitate the temperature reduction and solidification of the molten plastic. While water is the typical medium that flows through the cooling channels, high-temperature applications may use oil depending on project requirements.
Guide Pillars/Bushings
To ensure proper alignment of the cavity and core mold halves when closed, injection molding machines use guide pillars and bushings. These are found on either end of the mold and guide the mold during the closing procedure.
Ejector System
The ejector system uses an assortment of pins or bars to remove the formed plastic part from the core. The system ensures safe and hassle-free removal while preventing potential damage to the completed component.
Ejector Plate System
A part of the ejector system, the ejector plate holds the pins together and ensures equal force is used during the extraction process.
What are the Key Principles of Injection Mold Design?
When it comes to creating a design for an injection mold, style and aesthetics by themselves are not enough. The final design must strike a balance between form and function to guarantee that the completed part will function as intended. Here are injection mold design basics that must be considered when creating a functional mold.
Thickness
A key component in effective injection mold design is achieving uniform thickness throughout all the parts. Remember that the molten plastic shrinks during the cooling process, which must be calculated in the design. Uniform part thickness reduces the risk of warping, sinking, and distortion of the part once it’s cooled.
One great practice is checking the design’s overall thickness using the CAD software’s analysis tool. Thick walls may offer better durability and be less prone to warping, though they take more time to fully solidify. Coring out sections can also help reduce cooling time while maintaining their thickness.
Draft angle
Draft angles refer to the slopes found on the mold’s vertical faces that make part ejection easier and safer. Without these, either the part will stick to the mold or will warp once removed.
Industry standards dictate at least one degree of draft for all injection molded parts, though those with extra texture may need an additional degree. Moreover, draft angles should always be directed away from the parting line for easier release.
Corners and radii
Here’s a reality in injection molded parts: sharp corners equal potential breaking, as they can make releasing the part more difficult and create stress concentrations. So, it’s important to add corners to prevent this from happening.
For best results, interior corners should have a radius of .5 times the part’s thickness, while 1.5 times for exterior corners. Additionally, fillets for bosses and ribs must have a 0.15 thickness ratio to avoid sinking, as it seamlessly balances visual appeal and integrity.
Ribs
Adding ribs to your injection mold design may elevate the strength and integrity of the completed part, though it can affect the overall aesthetics if not properly planned. To circumvent this issue, it’s best to make the rib’s thickness 50% of the wall’s thickness, especially at the base. A radius equal to 1/8 of the wall’s thickness and a draft of 0.5 to 1 degree will make it stronger and shrink-proof.
Bosses
In injection mold design, bosses act as attachment points and are an integral feature in most final parts. As such, knowing how to properly add them to the final design will be important to maximize the feature.
The rule of thumb is that the boss’s outer diameter should be double that of what would be inserted to accommodate shrinking. Its wall thickness should be half of the part and extend to the base wall level, with the height not going over five times its outer diameter. Also, merging the boss to the sidewalls may cause sinking, so use ribs to support it instead.
Undercuts
Undercuts are typically used when a mold must intersect with the part without pulling away from it. While there are no limits to using undercuts, it can increase mold cost and production times, which translates to more money spent.
Some ways to circumvent undercuts are to reposition the parting lines, eliminating the need for an undercut. Another is adding holes below the undercut, allowing the part to pull straight from the mold.
Mechanical Strength
Knowing the strengths and weaknesses of the material you plan to use will play a role in determining how to improve the final part’s mechanical strength. These include the following:
- Increasing part thickness, though this will boost material cost
- Add curvature or corrugation
- Choosing a more durable resin
- Add radii to reduce stress concentrations
- Consider the load and environmental conditions
What Kinds of Materials are Used for Injection Molding?
Now that we have a good grasp of the injection molding process and the best practices in creating an injection mold design, it’s time to look at the materials for injection molding.
Why is it important to know the injection molding materials, you might be wondering. The plastic materials, typically in the form of pellets, will determine the overall toughness and capabilities of the completed injection molded part. Some considerations to look at when choosing the proper material for your project include:
- Mechanical properties – structural integrity and toughness
- Environmental sustainability – resistance to heat, moisture, and chemicals
- Cost – the price of the material
- End use – will the product be in contact with food, electricity, heat, or others
We’ve also listed some of the most common materials used in plastic injection molding. While this is not an exhaustive list, these are frequently sought for their unique properties.
Acrylic (PMMA)
Also known as polymethyl methacrylate or PMMA, acrylic is a plastic type with a clear color and is highly resistant to weather and scratches. This material is typically used as an alternative to glass.
Acrylonitrile butadiene styrene (ABS)
Despite its lightweight structure, ABS is a highly preferred thermoplastic for its durability, thermal and corrosion resistance, and great impact resistance. Due to its resilience, this plastic is widely used as the main material for toy bricks, automotive parts, and protective headgear.
Nylon (polyamide, PA)
Preferred for its excellent flow, nylon is a great thermoplastic for making diverse injection molded parts. Different types of nylon material have specific qualities that can match your project needs, like Nylon 6, which has higher strength, and Nylon 46, which has high resistance to heat.
Polycarbonate (PC)
Strength, durability, and a clear color place this material at the top of the list for various industrial applications. Polycarbonate is available in several grades for different functions, including food-grade and medical-grade options. They can also be incorporated with fillers to enhance their capabilities.
Polyethylene (PE)
Polyethylene is a soft and malleable plastic that has a low melting point, which makes it easy to process into different injection molded parts. The material is also highly versatile, seeing great use as the key component in consumer goods, packaging, and automotive parts.
Polyoxymethylene (POM)
Also known as acetal, this engineering thermoplastic has great dimensional stability, low friction, and enhanced chemical resistance. It’s typically used in making injection molded precision parts and is the go-to material for parts that need tight tolerances.
Polypropylene (PP)
While sharing some components with PE, polypropylene has a harder and stiffer structure that sets it apart. Alongside its strength, toughness, and chemical resistance, a key feature of PP is its high fatigue resistance, where it can tolerate repeated flexing without breaking or deteriorating.
Polystyrene (PS)
One of the more accessible and affordable plastic materials available in the market, polystyrene is mainly used to create lightweight products with resistance to moisture and radiation. It’s also used in making delicate packaging products that are seen in multiple industries.
Thermoplastic elastomer (TPE)
Thermoplastic elastomer shares similar qualities to rubber but skips the long curing process. Highly versatile and flexible, the material blends thermoplastic and elastomer to produce soft yet rigid injection molded parts.
Thermoplastic polyurethane (TPU)
A result of combining polyols, diisocyanates, and short-chain diols, injection molded parts made from thermoplastic polyurethane or TPU have extreme flexibility and elemental resistance. The material is used in a wide range of products, including consumer goods, industrial parts, and phone cases, to name a few.
Plastic Injection Molding Advantages & Disadvantages
After going through the basics of this injection molding guide, it’s time to dive deeper into the strengths and weaknesses of the process. We have to acknowledge this part of the production method to properly prepare and have the right expectation, should we choose to move forward with injection molding.
Pros of Plastic Injection Molding
Low-Cost High-Volume Production – a typical injection molding cycle takes 10 seconds from plastic melting to ejection, allowing for thousands of parts to be completed per hour. Production costs are also relatively lower compared to other methods, as molds can be used for long periods before replacing them.
Consistent Quality Output – not only does injection molding handle mass production of parts, but it also guarantees consistency in each component made. As it uses a specific mold during the process, you can expect the same quality for every batch produced.
Broad Range of Material Options – There’s no shortage of available materials that one can choose from when opting for injection molding. Whatever your production requirements are, there’s a plastic material that fits the task perfectly.
Lower Workforce Expenses – in addition to lower manufacturing costs due to the mold and materials, plastic injection molding also helps reduce overall expenses. Plastic injection molding machines have automation features that greatly reduce the need for manual labor.
Ability to Create Complex Designs – molds can be created to have complex designs that the molten plastic will copy upon injection. The completed parts will typically have minimal deviation from the design, ensuring each detail is replicated to the last curve.
Cons of Plastic Injection Molding
High Initial Mold and Setup Costs – a significant part of the plastic injection molding is the creation of the actual mold, which may require a large investment. Setting up the machine may also take some time and money, especially when specific equipment is needed to complete the project.
High Cost of Redesign or Adjustments – along with the initial amount for the mold, any adjustment made may also cost some time and money to complete. This is especially true if the mold needs a complete redesign, since a new one will be made from scratch.
Plastic Injection Molding Applications
Since its inception, plastic injection molding has been intended to address the diverse needs of various industries. As of modern times, numerous markets utilize injection molded parts as vital components of their products. Here are some industries that use injection molded plastic parts, along with their typical applications:
Aerospace
The durability, longevity, and inherent flexibility of injection molded plastic parts make them integral to the automotive industry. Aerospace injection molding parts can be seen in various ways, especially in the manufacturing of sensitive components and drones.
- Circuit and component enclosures
- Chassis and turbine blades for drones and model planes
- Pilot tubes, bezels, and battery housings for aircraft
- Component for radomes
Electronics
Plastic injection molding and electronics go hand in hand, as the manufacturing process matches the high demand the industry has. Electronics injection molding allows for creativity and flexibility, ensuring there will always be a part for whatever the next concept the market has.
- Connectors, fittings, and housing for electrical circuits
- Buttons and keypads for electrical devices
- Wire spools and cable ties for wire management
- Outlet covers, switch plates, and dashboard knobs
Medical
The medical and pharmaceutical fields have some of the strictest regulations for manufacturing and production, since any contamination may have serious consequences. As such, they require plastic injection molding medical parts that are not only medical grade but also adhere to FDA and ISO standards.
- Manufacturing of medical products like syringes, IVs, and surgical instruments
- Parts and components for processing pharmaceutical products
- Parts and components for medical devices
Food and Beverage
With the strict health and safety requirements of the food and beverage industry, plastic injection molded parts are relied on to fulfill product processing and packaging. These parts and components must be BPA-free, non-toxic, and meet FDA and GMA certification standards.
- Parts for food and beverage processing
- Filtering and dispensing components
- Food and beverage containers
- Overcaps and enclosures for food and beverage products
Household Goods
Almost anything seen in the modern home has a plastic component to it, from the kitchen to the living room. Thanks to their durability, cost-effectiveness, and diverse design capabilities, many manufacturers in this industry opt to use injection molded plastic parts for their products.
- Key components for kitchenware and storage containers
- Parts for furniture and household appliances
- Main material for children’s toys
- Parts for personal care items like toothbrush handles
Product Packaging
Due to being lightweight and durable, injection molded plastic has been the preferred method for making various packaging options for different products. These products can also be easily modified to fit the diverse demands of their target markets.
- Bottles and jars for food, cosmetics, and household products
- Containers for general-purpose or specific markets
Fox Mold – Your Trusted High Quality Injection Molding Manufacturer in China
If you’re looking for a reliable injection molding manufacturer to handle your next project, look no further than Fox Mold. For more than a decade, we’ve been leading the industry in custom plastic injection molding that meets the needs of any industry. We offer a range of tailored services, including prototyping, design, mold making, mass production, and after-sales, with a specialization in plastic injection molding, overmolding, insert molding, and LSR molding.
Backing our commitment to excellent order fulfillment is our in-house manufacturing facility strategically located in Guangdong, China, that houses modern workshops and the latest machinery. With our solid production lines and 15 expert engineers handling each project with keen attention to detail, we ensure 99% accuracy and short lead times at the most competitive rates.
Moreover, we complement our strong production capabilities with the strictest quality control standards, adhering to global regulations like the FDA and IATF. Our support team is also ready to assist you with every concern, with dedicated project managers taking care of your project from start to finish.
Wrapping Up
Plastic injection molding is a simple yet intricate process that many industries rely on for valuable parts and components. Knowing how it works, its components, and viable applications will help businesses better understand its value and know how to integrate it into their product offerings.
Start your next plastic injection molding project with Fox Mold. We’re experts in injection molding manufacturing with over 13 years of experience. Talk to our consultant today and see how we can quickly and efficiently bring your ideas to market.
