Plastic products are widely used in several industries, ranging from home appliances to medical devices, automotive parts, and more. For this reason, plastic injection molding is a popular manufacturing process used across various sectors. If you’re seeking answers to the question, “What is injection molding?” Then you’re in the right place.
In this article, we’ll cover the essential details of the injection molding process. While the process is reliable and highly efficient, it must be adequately controlled to maintain the quality of final products. Therefore, we’ll take you through the step-by-step process while covering practical tips to get the best from injection molding.
What Is Plastic Injection Molding?
Injection molding is a manufacturing process where plastic polymers are melted and then solidified under pressure in pre-machined molds to form specific shapes. It allows for the efficient production of plastic molded components with a wide range of shapes, including complex designs, all at a cost-effective rate.
Importance of Injection Molding in the Manufacturing Industry
The popularity of plastic injection molding in the manufacturing industry today is no surprise. Plastics are now famous in many industries because of their affordability, durability, and extensive capabilities. High-end plastics offer exceptional strength, excellent durability & flexibility, and pleasant aesthetics.
Moreover, the availability of many recyclable plastic materials ensures environmental sustainability. Plastic molding techniques occur in a closed industrial loop which helps minimize waste production. Beyond being eco-friendly, the process is cost-effective, highly flexible, and precise. It ensures product manufacture within a short cycle time.
Brief History of Injection Molding
Injection molding traces its theoretical foundation to the 19th century when the first injection molding machine was patented. Early production was limited to simple items like buttons and combs, and manufacturing occurred on a very small scale. By the 1930s, the development of popular thermoplastics, such as polystyrene, polyvinyl chloride (PVC), and polyolefins, laid the groundwork for modern production.
As the demand for affordable alternatives to metals grew, modern injection molding systems were developed. James Hendry’s creation of the first extrusion screw machines marked a significant transformation in the industry. Over time, fundamental molding principles continued to evolve, along with continuous advancements in injection molding machines.
Today, the technique has matured significantly, enabling the production of components that excel in a wide range of applications. Modern processes are also compatible with recyclable and more cost-effective plastics, making the method safer and more efficient.
Step-by-Step: Plastic Injection Molding Process
Now that you know the evolution of injection molding and why it is so valuable in the manufacturing industry, it’s essential to understand the step-by-step process to help you get the best from your project.
1. Design the Mold
Creating the mold is a critical step in the process. Machinists utilize CAD designs and CNC machining to determine the mold’s specifications, including dimensions, geometry, and surface texture.
The tooling mold is meticulously crafted with features like cavities, cores, sprues, gates, runners, ejector systems, and moving parts. The cavity is the fixed section where molten plastic is injected, while the core, a moving part, shapes the final component.
Manufacturers typically use high-quality steel or aluminum to produce durable molds capable of enduring multiple production runs. For short-run projects, 3D printing can help reduce production costs. This stage demands a high level of skill and precision, with rigorous testing and simulations to ensure the mold meets specific requirements.
The design, fabrication, and approval of the mold are often the most time-consuming phases of the injection molding process, and the initial costs can be quite high. The common types of mold tooling include:
Single Cavity Mold
This is the most basic type of mold involving the molding of only one part per injection cycle. It is suitable for small to medium production runs. It also ensures better control of the process, allowing for continuous running of the injection molding. Moreover, the tooling costs for a single cavity mold are relatively lower. Thus, they are excellent choices in applications with lower budgets.
Multi Cavity Mold
Multi-cavity molds can produce multiple identical parts simultaneously, making them ideal for high-volume productions. They produce numerous components in a single injection cycle, shortening production time and lowering costs. However, these types of injection molds are more difficult to manufacture and require larger tonnage machines. Thus, the initial mold costs are higher when compared to single-cavity molds.
While multi-cavity molds have a higher initial cost, they help save costs in the long run. As a result, they are an excellent choice for projects requiring thousands to hundreds of thousands of units.
Stack Mold
A stacked mold comes with multiple mold plates arranged in parallel. They often have up to four levels to ensure increased efficiency. Building a stack mold takes longer than a traditional mold, resulting in higher initial costs.
However, manufacturers can reduce the clamp tonnage required per cycle to ensure lower operational costs. Furthermore, manufacturers can design stack molds to accommodate multiple materials, making it even more efficient.
2K Injection Mold
2K injection mold helps manufacturers combine two or more different materials or colors to make one plastic part. This technology allows for the creation of custom components that meet specific functional and aesthetic requirements. The chemical bonding process joining the materials helps give the finished product a superior aesthetic and performance. Overall, 2K injection molding is an advanced and versatile manufacturing process, enabling unique design and material combinations.
2. Select Plastic Material
Before starting the molding process, it’s crucial to choose the most suitable plastic resin for the desired component. This selection must consider how the material interacts with the mold tool, as some options may not be compatible with specific mold designs.
Testing the chosen material with the tool is essential to ensure the final product meets the required properties. Different plastics have unique molecular structures, resulting in varying mechanical and thermal characteristics. These properties will determine the material’s suitability for certain molds and products.
The most common injection molding plastics include the following:
Plastic Type | Advantages | Specific Industrial Applications |
Acrylonitrile-Butadiene-Styrene (ABS) | High-temperature resistance, excellent mechanical properties, good chemical resistance | Automotive dashboards, casings for electronic devices, protective headgear |
Nylons (PA) | High toughness, wear resistance, good chemical resistance, high melting point | Gears, bushings, cable ties, machine housings |
Polycarbonate (PC) | High impact strength, transparency, good dimensional stability, heat resistance | Safety glasses, automotive lighting lenses, medical instrument housings |
Polypropylene (PP) | Excellent chemical resistance, high fatigue strength, lightweight, resistant to moisture | Automotive bumpers, battery cases, living hinges, laboratory containers |
Teflon (PTFE) | Outstanding chemical resistance, very low friction, excellent electrical insulating properties | Seals, gaskets, non-stick coatings for cookware, wire insulation |
Acrylic (PMMA) | High transparency, weather resistance, good surface hardness, lightweight | Aircraft windows, light fixtures, display cases, optical lenses |
Polyvinyl Chloride (PVC) | Good chemical and weather resistance, flame retardant, durable, versatile | Pipe fittings, electrical conduit, window frames, blood bags |
PEEK | High temperature resistance, excellent mechanical properties, good chemical resistance | Aerospace components (e.g., turbine blades), medical implants, pump gears |
High-Density Polyethylene (HDPE) | High impact resistance, good chemical resistance, lightweight, low moisture absorption | Fuel tanks, chemical containers, industrial piping, playground equipment |
Polystyrene (PS) | Good clarity (in transparent grades), easy to process, lightweight, cost-effective | Disposable laboratory ware, insulation panels, product packaging, appliance housings |
Choosing the right option requires a solid understanding of its structure and characteristics. Conducting field tests can further assess how it performs in real-world applications. Once the appropriate selection is made, production can begin.
3. Setup Machine
A critical step in the molding process is the setup of an injection molding machine. Mold installation necessitates meticulous attention to detail to ensure the correct alignment and securing of the mold to the machine platens. Before installing the mold, it is critical to check the machine’s specifications to ensure it can accommodate its size and weight.
The machine platens should be clean and free of debris or contamination that could interfere with the molding process or damage the mold. When the machine is ready, the mold is carefully placed onto the platens and aligned with the machine’s nozzle. Proper alignment is required to ensure the molten plastic resin flows evenly into the mold cavity, producing high-quality parts with consistent dimensions.
Once in place and secured to the machine platens with bolts, clamps, or other fastening mechanisms, running a test is crucial to ensure it works properly. The test involves passing some molten resin pellets through the molding machine to check for blockages, leaks, or other issues.
Setting up an injection molding machine requires skill and expertise. Proper machine setup ensures that the finished parts are high quality and consistent, reducing the risk of mold or machine damage.
4. Injection Process
There are different types of injection molding, including:
Plastic Injection Molding Process
It is a continuous cycle that includes several steps. After closing the mold, the plastic resins are heated and fed into the machine. Upon applying the appropriate pressure to the tool and screw, the gate opens, and the screw moves forward. Molten plastic is then injected into the mold, filling it.
When the melted plastic reaches the end of the barrel, the machinist closes the gate that controls the injection of the plastic. The screw then moves back and draws through a predetermined amount of plastic, creating pressure in the screw for injection.
Simultaneously, the clamp pressure helps to hold the two parts of the mold tool close together. It is critical to balance the injection pressure and clamp pressure to ensure that the part correctly forms and that no plastic escapes the tool during the injection.
Overmolding Process
Overmolding is a specialized injection molding technique that combines two or more different materials to produce a single finished part. Molding a base or substrate part, placing it into a mold cavity, and overmolding it with a second material, results in a finished product with improved functionality, durability, and aesthetics. Rigid plastic is often the base material, while the second is softer.
The overmolding process is typically divided into two stages:
- Molding the substrate or base part: The machinist molds the first material, usually a rigid plastic, into the desired shape and size at this stage. Then, the insertion of the base part into a second cavity begins the overmolding process.
- Overmolding in a different material: A second material, usually a softer and more flexible plastic or elastomer, is molded over the base part at this stage. The second material bonds with the first, resulting in a strong, long-lasting finished part.
Insert Molding Process
Insert molding uses pre-fabricated inserts placed into a mold cavity and molded over plastic resins to create a finished part. The following steps are typically involved in the insert molding process:
- Creating the inserts: The inserts, made of metal, ceramic, or plastic, are created before molding. Their designs allow them to fit into the cavity and their adhesive coatings aid their bonding with the plastic resin.
- Putting the inserts in the cavity: The insert molding design involves specialized tools, jigs, and fixtures. They help place the inserts into the cavity. Machinists place them precisely to ensure that they remain securely in place during molding.
- Molding the plastic resin: This step involves injecting the plastic resin into the cavity and molding over the inserts.
5. Cooling and Solidification
After filling the mold with plastic, including its sprues, runners, gates, and other components, it is kept at a specific temperature to allow for uniform solidification of the material into the desired part shape. Maintaining a holding pressure during this time helps prevent backflow into the barrel and to reduce shrinking effects.
The holding pressure is critical to ensuring that the plastic packs out and forms correctly, resulting in a high-quality final product. The part’s size and complexity, the type of plastic used, and some other factors determine the holding time.
The screw retracts after the holding phase, releasing pressure and allowing the newly formed part to cool in the mold. The part remains in the mold during this phase, and the mold temperature remains constant to ensure uniform cooling throughout the part.
The cooling time is critical to ensuring the plastic part sets appropriately and retains its shape ejection from the mold. If the part is removed too soon, it may warp or deform, resulting in injection molding defects. Depending on the size and complexity of the part, the time can range from a few seconds to several minutes.
6. Ejection of the Part
The mold opens after the plastic part cools, and ejector pins or plates push the finished piece out of the tool. The injection molded part is placed in a compartment or conveyor belt to await finishing processes. The finishes can be polishing, painting, or removing sprues. Polishing can help smoothen rough surfaces or defects on the part, whereas painting can add color to the part.
7. Inspection and Quality Control
After finishing, an inspection of the parts follows to ensure they meet the required specifications. If there are any flaws discovered, the parts may have to be reworked or scrapped. After successful inspection, packaging, and distribution of the molded component to suppliers and end users can begin.
General Rules of Thumb for Injection Molding Process
Before starting the molding process, it is crucial to consider the correct parameters. Then you find the right balance between performance, speed, and cost.
Ensure Part Design with Uniform Wall Thickness
Ensuring uniform wall thickness is an essential principle in injection molding design. Lack of consistency in the thickness can lead to numerous issues. These include mechanical faults such as stress concentration and micro-cracks spread over time.
Additionally, the variability in the wall thickness can cause shrinkage inconsistencies, resulting in further complications. Ideally, limiting the variability to a maximum of 60% when it cannot be avoided will help you achieve optimal results.
Select Materials that Fit Your Specific Requirements
The materials used will significantly influence the durability of injection mold tooling due to inevitable wear and tear. For instance, lightweight materials such as polypropylene have minimal effects on the mold’s lifespan. In contrast, abrasive materials like nylon or glass tend to shorten the lifespan of the mold.
Aluminum or steel mold tooling is more suitable for producing between 8000 to 12,000 pieces. However, if the number of pieces to be produced is lower than this, then other types of molds may be more appropriate.
High Production Volume Is Ideal for Injection Molding
The decisions you make on production volume can either decrease or increase injection molding costs. Generally, mass production reduces the overhead cost while increasing production volume. Therefore, it is a generally good idea to increase the production volume for cost benefits. Ensure that the mold tooling can withstand longer production cycles to reduce the overall molding costs.
Consider the Expected Lead Time
It may take 5 to 10 weeks to manufacture steel or aluminum mold used in the molding process. Part manufacturing may also take up to 3 weeks. This lead time may sometimes be too long for your project. In that case, you should consider choosing alternative processes requiring less time while offering similar results.
Tooling Costs
Injection molds are often complex and require experienced engineers for their design and building. A simple single-cavity mold may cost up to $2,000. On the other hand, a complex multi-cavity mold created from hardened steel can cost over $100,000. While it is possible to reuse these molds, it is essential to consider how much they cost upfront. A reliable injection molding company will help refine your mold design to maximize your budget.
Why Using Injection Molding?
Plastic injection molding is incredibly versatile, playing critical roles in developing products for different industries.
It offers several advantages, including the following:
Cost-effective
Injection molding is a cost-efficient process that minimizes waste by using recyclable materials. Plastics are generally more affordable than alternatives like metals and rubber, making them a preferred choice in many applications. Additionally, recycling excess plastic for future production further reduces costs.
Once the initial costs of mold design and production are out of the way, the labor costs are often low. Being a process with a high production rate, we can say that the mold pays for itself over time. Thus, it is an excellent choice for producing high-quality, high-volume plastic parts.
High Production Efficiency
Plastic molding process is famous for its fast production rate and efficiency. This high production output provides cost-saving advantages and makes the process ideal for mass production. The technique also focuses on repeatability, with low scrap rates compared to conventional processes.
A single mold tooling can produce hundreds and thousands of parts before requiring replacements. Many aluminum molds last up to 10,000 cycles, while production steel molds can last over 100,000 cycles. This ensures a repeatable, high-output process that meets tolerance and aesthetic requirements. You must note that the production speed often depends on the size and complexity of the intended injection molded part.
High Precision and Accuracy
Modern high-speed injection molding equipment enables the consistent production of precision-molded plastic parts. This process excels in delivering components with uniform quality, tight tolerances, and high dimensional stability. For applications requiring extreme accuracy, such as gears and connectors, plastic injection molding is often the preferred method, achieving tolerances as precise as +/-0.0002”.
In addition, manufacturers often equip molding machines with advanced control systems. These automated systems monitor and adjust the process parameters in real time. As a result, plastic parts are molded to exact specifications.
Versatility in Design
Injection molding ensures the production of components with intricate and complex geometrical features. Manufacturers often optimize the process parameters to achieve design versatility. Optimizing parameters like injection pressure, temperature, and speed will ensure the uniform injection of the plastic material into the cavity.
Additionally, machinists can enhance the process with advanced technologies like simulation software, automation, and 3D printing. These technologies help design and test mold tooling, process parameters, and molded parts. However, the design versatility of injection molding depends on the plastic material, part complexity, and mold design.
Wide Range of Materials
As mentioned earlier, injection molding is compatible with an extensive range of materials and colors. Over 25,000 engineering materials are available, including thermoplastics, thermosetting plastics, silicones, resins, etc. These options allow you to strike the perfect balance between materials’ mechanical, chemical, and physical properties.
You can easily combine materials to improve the product’s stiffness, strength, or impact resistance. For example, the overmolding design allows you to mold plastic materials over a metal substrate.
Is Injection Molding Right for Your Project?
Despite its many advantages, this technique has a few drawbacks to note before you begin your project. They include the following:
High Initial Investment
Custom mold tooling is necessary for every molded component. The creation of these molds often increases the initial costs of injection molding. It also makes it less economical for low-volume production runs. Mold tooling for simple designs and small batches can cost up to $5,000. In contrast, complex molds for large-scale production can cost a lot more.
Limited Flexibility
In contrast to CNC machining, which allows the production of parts from a computer design file that a software engineer can modify at any time, significant alterations to a plastic mold after tooling can be complex. There are two primary methods for changing a plastic mold.
One method is to increase the size of the mold cavity by cutting sections of the metal mold. This can result in the addition of plastic material to the part. Removing plastic material from the part, on the other hand, can be more difficult. Welding metal into the cavity may be possible in some cases but may not always be possible. Therefore, there may be a need to create a new cavity or an entirely new mold.
Long Lead Times
CNC machining can deliver parts within 5 to 10 days, while industrial 3D printing typically takes 3 to 5 days. In contrast, molding processes have a longer lead time, often requiring 5 to 7 weeks to manufacture tooling, followed by 2 to 4 weeks for production and shipping.
A high-quality plastic injection mold is designed to last, incorporating complex runner and water-cooling systems to improve material flow and cooling efficiency. Due to the intricacy of the design, prototyping, testing, and tooling phases, the entire process can take several months, making it one of the more time-intensive methods in plastic manufacturing.
However, once the planning and tooling phases are complete, a single mold can produce millions of parts over its lifespan. The large-scale production capability often justifies the initial investment and extended lead time associated with this process.
Material Waste
Some factors contribute to material waste during injection molding. They include:
- Mold design: Improper design and manufacture of mold may result in incorrect filling, packing, or filling of parts. This increases the waste produced.
- Overpacking: Injecting more plastic resin than necessary into the cavity will require trimming excess materials off the component. Thus, more waste is produced during the process.
- Sprue and runner waste: The design of these channels makes them larger in diameter than the desired final component. When eventually removed after molding, it results in material waste.
- Material handling: Poor handling of raw plastic pellets may lead to contamination, making them unusable.
Environmental Concerns
Like many other industrial processes, injection molding has some associated environmental concerns. Molding machines use a lot of energy for heating and cooling materials, operating machinery, and powering auxiliary equipment. This high amount of energy contributes to air pollution through gas emissions. The scrap materials from the process may also end up in landfills, contributing to environmental pollution.
Key Uses of Injection Molded Parts
Plastic injection molding is a highly versatile manufacturing technique that is essential in developing a wide range of products, including appliances, automotive parts, medical devices, and more.
Let’s look at some of the applications of the process:
Automotive Industry
Plastic components enhance the aerodynamic profile of the body and various parts of a vehicle. With the evolving demands of the sector, injection molding for automotive applications is the go-to option for manufacturers.
It produces components with excellent strength-to-weight ratios to save energy and minimize emissions. These parts also have better dimensional stability properties and are resistant to weather and wear. Examples of injected molded parts in the automotive industry are dashboards, bumpers, and minor elements like mirror housings, cup holders, and many others seen in cars.
Packaging Industry
The packaging industry uses components that are effective at developing creative packaging designs that are appealing to customers. Aside from an appealing finish and aesthetics, the products must adhere to stringent rules for safety and sanitization. Injection molding can increase the overall quality of products without causing any significant problems.
Medical Industry
Custom plastic parts are used in many medical components, offering remarkable mechanical qualities and the highest precision. The medical sector is at the forefront regarding accuracy and sustainability. Medical plastic part manufacture is subject to very strict regulations. To be durable, medical parts also need the right chemical compounds and structural qualities that can withstand greater temperatures.
Injection molding is an excellent option because it offers outstanding durability, flexibility, strength, and easy modification. These qualities are essential for any operation in the healthcare sector. Common medical parts like dentures, surgical equipment, surgical room preparation equipment, and support for fractures use the plastic injection molding technique.
Consumer Goods Industry
The industry that manufactures household items relies heavily on plastic molding techniques. The primary benefit of this method for consumer goods is its ability to provide the desired finishing and aesthetics.
This technology produces components that are both highly durable and of exceptional quality. Household items have utilized this process since plastic products became prevalent, demonstrating its widespread application. Examples of consumer goods produced with these techniques include window holders, cutlery holders, handles, racks, and more.
Aerospace Industry
The aerospace industry has stringent rules regarding structural strength and precision. Plastic injection molding is used to create the aerodynamic components of most aircraft today. The aerospace industry is changing patterns to adopt this method for part manufacturing.
Many products in the industry, ranging from small parts to complex components, use injection molding. Examples of such components are turbine blades and housings, panels, chassis components, panels, enclosures, etc.
Choose Your Injection Molding Services
Plastic injection molding is a reliable, high-efficiency, and relatively cost-effective method of creating plastic components for various applications. It offers several advantages to the manufacturing industry and end-use consumers. However, it is not enough to know what is injection molding. While the technique may sound straightforward, many things can go wrong, leaving you with defective products that may affect your business revenue.
Therefore, you must work with the best injection molding company to stay above your competitors continuously. RapidDirect is your best option for on-demand injection molding services. With decades of experience, global exposure, and extensive technical expertise, you can be sure of getting excellent results. The team of experts at RapidDirect will guide you through the process, from material choice to mold design and part creation.
We are ISO 13485 and ISO 9001 certified, ensuring that we uphold stringent standards for unmatched quality, performance, and consistency of injection molded parts. Our quotation platform enables you to check DFM analysis and confirm the mold design before the production stage. RapidDirect provides outstanding solutions to meet your unique demands regardless of the production volume. Contact us today, and let’s start molding!