Injection Molding 101: Process, Types, Advantages and Disadvantages
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Injection Molding 101: Process, Types, Advantages and Disadvantages
Plastic parts play an integral part of our life with almost every gadget, appliance, or device requiring them. They play roles such as protecting us from inside electrical parts, mounting the electrical parts tightly into position, or creating functional parts. Additionally, some plastics act as stand-alone tools that we use in our daily lives.
This explains why plastic is amongst the most abundant man-made objects on earth. Back in the 1950s, the world barely produced 2 million tonnes per year. However, currently, that number is in the ceiling as the world produces more than 450 tonnes per year.
Despite relying on them to carry out our daily tasks, most people don’t know how the process of creating plastic parts works. Injection molding is one of the most popular methods for creating plastics. In this article, you will learn how injection molding works, the machine components, and the services associated with it.
What is Injection Molding?
Injection molding refers to a manufacturing process where molten plastic materials or metal, are injected into a mold at high pressure.
The process is widely used to mass-produce complex parts with consistent quality and precision.
It is important to create or select the right plastic injection molds as they can affect the final output. Furthermore, the mold also determines how details of complex parts are well captured. Every injection molding project requires a unique mold depending on size and shape.
How Does the Injection Molding Process work?
The plastic injection molding process is a meticulous sequence of steps, each playing a crucial role in the creation of high-quality plastic parts. Let’s delve into the intricacies of each stage:
Clamping
The process initiates with the clamping stage, where the mold is securely closed to prevent any leakage thermal expansion of molten plastic. The clamping unit applies pressure to hold the mold halves together, ensuring a tight seal. This phase sets the foundation for the subsequent stages, providing stability for the injection and cooling processes.
Injection
The injection phase begins with the molten plastic being injected into the mold cavity at high pressure. Molten plastic refers to the plastic that has been melted to the melting point. This step requires precise control over the injection speed, pressure, and temperature to ensure the material fills the cavity completely and uniformly.
Typically, a reciprocating screw within the injection molding machine pushes the molten material forward into the mold under controlled conditions. When the injection pressure is applied molten plastic flows through the runner system and into the mold cavity, where it takes on the shape of the desired part.
Dwelling
Following the injection of melted plastic into the mold cavity, there is a brief pause known as the dwelling phase.
During this stage, the material remains stationary within the metal mold, allowing it to settle and pack evenly. Proper dwelling time is essential to ensure that the material is distributed uniformly and fills any complex features or details of the part. This phase helps minimize potential defects such as air pockets or voids and promotes the formation of a dense and uniform structure within the part.
Cooling
After the dwelling phase, the mold undergoes the cooling stage, where the melted plastic inside the mold cavity solidifies. Cooling is achieved through various methods, such as circulation of coolant through channels within the mold or by allowing the mold to cool naturally in ambient air.
Proper cooling is crucial for achieving the desired part properties, dimensional accuracy, and minimizing warpage or distortion. The cooling rate and uniformity are carefully controlled to prevent internal stresses within the part and ensure consistent cooling across the entire mold.
Mold Opening
Once the plastic solidifies, the mold undergoes the mold opening stage, where the two halves of the mold are separated to reveal the newly formed part.
Mold opening is typically achieved using hydraulic or mechanical systems that apply force to the mold’s clamping mechanism, causing it to release and open. Precision and consistency are crucial during this phase to ensure that the part is ejected smoothly without damage. Proper plastic injection mold opening also helps prevent any deformation or distortion of the part as it is released from the cavity.
Ejection( Removal of products)
After the mold opens, the final plastic parts are ejected from the mold cavity, marking the final stage of the injection molding process. The ejected parts may be manually removed by an operator or automatically ejected using ejector pins or plates integrated into the mold design.
Care must be taken during the part removal process to avoid any damage or deformation to the parts. Any excess material, known as flash, may also be trimmed or removed at this stage to achieve the desired final appearance and dimensions of the parts.
Main Injection Molding Parameters
The injection molding cycle consists of many parameters that need to be in check in order to produce high-quality plastic components. These parameters include:
Injection pressure: The force applied to inject molten plastic into the mold cavity. It’s crucial for achieving proper filling of the mold and ensuring part quality.
Mold temperature: This refers to the temperature at which the mold is maintained during the injection molding process. It affects material flow, part cooling, and overall part quality.
Nozzle pressure: The pressure exerted at the injection nozzle to force molten material into the mold. It’s part of the overall injection pressure but specifically refers to the pressure at the entry point of the mold
Recovery rate: Refers to the rate at which the material rebounds or recovers after being compressed during the injection process.
Shrinkage rate: This refers to the percentage decrease in dimensions of the molded part from its molten state to its solid state.
Clamping force: The force applied to hold the mold halves together during injection and cooling.
Screw speed: It’s the speed of the screw within the injection molding machine barrel. Scre speed determines the rate at which plastic is fed into the mold cavity and affects both filling and cooling times.
Injection Molding Machine Components
Hopper: It is a container located at the top of the injection molding machine where raw material, typically plastic pellets or resin, is stored. It feeds the material into the barrel of the moulding machine.
Barrel: It refers to a long, cylindrical chamber within the injection molding machine where the raw material is melted and mixed. It houses the reciprocating screw and heaters.
Reciprocating screw: An important component that moves back and forth within the barrel. Its primary function is to melt and homogenize the raw material while also injecting it into the cavity under pressure.
Heaters: They are used to heat the barrel of the injection molding machine, maintaining the temperature required to melt the raw material. They ensure consistent material flow and proper molding conditions.
Movable Platen: This refers to one of the two plates that hold the halves together during the injection molding process. It can move horizontally to open and close the mold, allowing for part ejection and mold changeovers.
Nozzle: It’s the point where the molten plastic material exits the barrel and enters the mold cavity. It ensures a precise and controlled flow of material into the mold.
Mold: A precision tool typically made of steel or aluminum that forms the desired shape of the plastic part. It consists of two halves (core and cavity) that come together to create a void where the molten material is injected and solidified.
Mold Cavity: This is the void or impression within the mold where the molten plastic is injected to form the final part. It is shaped precisely to match the desired geometry of the finished product.
Types of Injection Molding Services
Conventional Injection Molding: A standard process where the molten plastic material is injected into a mold cavity, cooled, and ejected as a solid part.
Insert molding: Involves placing pre-formed parts (inserts) into the cavity before injection, resulting in a single integrated part.
Overmolding: Process where a substrate is molded over with a second material, commonly used for adding soft-touch surfaces or multi-material aesthetic.
Gas-Assisted Injection Molding: Utilizes nitrogen gas to displace molten plastic material in the cavity, creating hollow sections for reducing material usage and improving part strength.
Co-injection Molding: Simultaneously injects two or more different materials into the cavity to produce multi-material parts.
Two-shot or multi-shot injection molding: Injects different materials or colors sequentially into the same mold cavity, enabling the production of multi-material or multi-color parts in one step.
Micro-injection molding: A specialized process for producing extremely small plastic parts with tight tolerances, commonly used in electronics and medical devices.
Liquid Injection Molding (LIM): Utilizes liquid silicone rubber (LSR) or similar elastomers injected into a cavity under high pressure, ideal for producing flexible parts with complex geometries.
Common Materials used for injection molding
The choice of material is not arbitrary; it’s a strategic decision based on the intended application. Whether it’s the transparency of polycarbonate in optical components or the wear resistance of nylon in mechanical parts, each material plays a vital role in shaping the success of the injection molding process.
Polypropylene (PP)
Polypropylene, commonly known as PP, is a versatile thermoplastic with a wide range of applications in injection molding. Its low density, high chemical resistance, and excellent fatigue resistance make it a preferred choice for producing items such as packaging, containers, and automotive components.
ABS
Acrylonitrile Butadiene Styrene, or ABS, is renowned for its strength, impact resistance, and dimensional stability. These properties, coupled with its ability to be easily molded and colored, make ABS a top choice for manufacturing consumer goods, automotive parts, and electronic housings.
Polyethylene (PE)
Polyethylene, identified as PE, is a lightweight plastic known for its flexibility and low cost. With variations such as HDPE and LDPE, it finds applications in diverse industries, from packaging and containers to agricultural products and toys.
Polystyrene (PS)
Polystyrene, or PS, boasts clarity, rigidity, and affordability. Commonly used for disposable items like packaging, food containers, and disposable cutlery, PS is favored for its ease of molding and cost-effectiveness.
Nylon (PA 6)
Nylon, specifically Nylon 6 or PA 6, is recognized for its strength, toughness, and resistance to abrasion. Ideal for producing mechanical parts, nylon is widely used in applications like gears, bearings, and other components requiring high volumes, durability and precision.
Polycarbonate (PC)
Polycarbonate, known as PC, is prized for its transparency, impact resistance, and high-temperature tolerance. These characteristics make it suitable for manufacturing optical lenses, electronic components, and durable transparent parts in various industries.
POM (Acetal/Delrin)
Polyoxymethylene, or POM, commonly referred to as Acetal or Delrin, is a high-strength engineering plastic with excellent dimensional stability. Its low friction and wear and good chemical resistance make it ideal for producing gears, bushings, and other precision components.
The properties of common materials used in injection molding, as summarized in Table 1 below, offer a concise guide for material selection in the manufacturing process. From the physical characteristics to chemical resistance and typical applications, this overview provides valuable insights for informed decision-making in the dynamic realm of injection molding.
PP
ABS
PE
PE
Semi-rigid, tough
thermoplastic polymer
Crystalline thermoplastic
Lightweight
flexible, low cost
Clarity, rigidity, affordability
Made from the monomer
Combination of acrylonitrile, butadiene, styrene
Variations include HDPE and LDPE
Ideal for disposable items like packaging
Lightweight,durable, and strong
Strong,impact resistant, rigid structure
Versatile applications in packaging. toys, etc.
Easy molding; cost-effective
0.89-0.91g/cm³
1.05-1.07 g/cm³
0.91-0.97 g/cm³
1.04-1.07 g/cm³
Opaque
Opaque
Opaque
Opaque
320 °F(160°C)
176-203°F(80-95℃)
176-203°F(80-95℃)
158°F(70℃)
Excellent resistance to chemicals
Very good
Excellent resistance to chemicals
Limited resistance to some chemicals
30-38MPa
40-50MPa
10-45MPa
40-50MPa
PC
POM
Strength, toughness, resistance to abrasion
Transparency, impact resistance.high temp
High-strength engineering plastic
Ideal for mechanical parts like gears, bearings
Suitable for optical lenses, electronic components
Excellent dimensional stability
Durable and precise
Durable and transparent
Low friction, wear resistance
1.12 g/cm³
1.20-1.22 g/cm³
1.4l g/cm³
Opaque
Transparent
Opaque
185 °F(85°C)
302°F(150°C)
212°F(100°C)
Excellent resistance to chemicals
Good resistance to chemicals
Good resistance to chemicals
50-80MP
55-75MPa
60-70MPa
Advantages and Disadvantages of Injection Molding
The realm of injection molding is a landscape of unparalleled efficiency and precision, yet it is not without its challenges. In this section, we will explore both the benefits that make injection molding a cornerstone of manufacturing and the limitations and challenges that inject nuances into its mastery.
Pros
High Accuracy and Precision
Injection molding allows for the creation of parts with extremely tight tolerances and consistent dimensions. The process enables manufacturers to produce intricate geometries and complex shapes with remarkable accuracy, ensuring that each part meets exact specifications.
Complex Details
One of the standout features of injection molding is its ability to produce parts with intricate details and complex features. This includes thin walls, intricate patterns, undercuts, threads, and other intricate details that may be challenging or impossible to achieve with alternative manufacturing methods.
Durability
Injection-molded parts are renowned for their durability and strength. The process ensures that materials are evenly distributed, resulting in parts that exhibit excellent structural integrity and resistance to wear, impact, and environmental factors. This durability makes injection molded parts suitable for a wide range of applications across various industries.
Automation
Injection molding is highly automated, leading to increased production efficiency and reduced labor costs. Automated injection molding machines can run continuously with minimal human intervention, ensuring consistent part quality and high throughput rates. This automation also allows for rapid production cycles, enabling manufacturers to meet tight deadlines and scale production as needed.
Cost-Effective for Mass Production
While the initial tooling costs for injection molds can be significant, injection molding becomes highly cost-effective for large-scale production runs. Once the molds are created, the per-unit cost of manufacturing decreases significantly, making injection molding an economical choice for mass production of identical or similar parts.
Wide Range of Materials
Injection molding supports a vast array of materials, including thermoplastics, thermosets, elastomers, and composites. This versatility in material selection allows manufacturers to choose the most suitable material for their specific application requirements, whether it be for its mechanical properties, chemical resistance, or aesthetic appeal.
Reduced Post-Processing
Injection molded parts often require minimal post-processing, as they typically have smooth finishes and accurate dimensions straight out of the mold with minimal waste material. This reduces the need for additional machining, sanding, or finishing operations, saving time and resources in the manufacturing process. Additionally, the consistency and precision of injection molding minimize the occurrence of defects, further reducing the need for rework or corrections.
Cons
High Initial Tooling Cost
One of the significant drawbacks of injection molding is the high cost associated with creating the molds. Designing and fabricating molds tailored to specific part geometries can be expensive, particularly for complex or intricate designs. This initial investment can be prohibitive for smaller production runs or for companies with limited budgets.
Takes Longer to Get Started
Compared to some rapid prototyping methods like 3D printing, injection molding typically requires more time to set up and get started. The process involves designing and fabricating molds, conducting tooling trials, and fine-tuning process parameters before full-scale production can commence. As a result, there can be a longer lead time from concept to finished parts compared to faster prototyping methods.
Size Limitations
Injection molding may have size limitations, particularly for larger parts. The size of the injection molding machine and the capacity of the cavity can constrain the maximum dimensions of the parts that can be produced. Manufacturing oversized parts may require specialized equipment or multiple mold cavities, which can further increase costs and complexity.
Design limitations
While injection molding offers great versatility in creating complex shapes and details, there are still design limitations to consider. Certain part geometries, such as sharp corners, thin walls, or deep recesses, may pose challenges in mold filling, cooling, or ejection. Designing parts for injection molding often requires careful consideration of draft angles, wall thicknesses, and other design guidelines to ensure successful moldability and part quality. Additionally, features like undercuts or complex internal geometries may require additional mold features or secondary operations, adding complexity and cost to the manufacturing process.
Common injection molding defects
Injection molding is a finely tuned process that demands precision at every stage. However, even with meticulous attention, certain defects can emerge, affecting the quality and functionality of the final product. Understanding and addressing these common injection molding defects is crucial for achieving consistently high-quality results.
In this section, we will explore and dissect seven prevalent defects that can occur during the injection molding process. From the causes behind warping to the remedies for short shots, our journey will navigate through the intricacies of defect identification and mitigation. Let’s embark on this exploration, unraveling the challenges and solutions that contribute to the refinement of the injection molding craft.
Warping
Warping in injection molding refers to the unintended twists or bends caused by uneven internal shrinkage during the cooling process. This defect is often the result of non-uniform or inconsistent mold cooling, creating stresses within the material.
Preventing warping involves ensuring uniform wall thickness in mold design and allowing parts sufficient time to cool gradually. Materials with semi-crystalline structures are particularly prone to warping.
Surface Delamination
Surface delamination is a condition where a part’s surface separates into thin layers, resembling peelable coatings. This issue arises from contaminants in the material or excessive reliance on mold release agents.
Delamination can compromise the material’s structural integrity and appearance. Preventive measures include adjusting mold temperatures, reducing dependence on release agents, and ensuring proper pre-drying of the plastic before molding.
Sink Marks
Sink marks, characterized by depressions or dents on the surface of a molded part, can be a result of uneven cooling or insufficient packing of the already molten material or plastic. Our exploration will uncover the intricacies of sink marks, pinpointing their causes, and proposing remedies to achieve smooth and flawless surfaces.
Knit or Weld Lines
Knit or weld lines, are marks where two flows of molten resin unite during their journey through the mold. These lines typically form around holes in the geometry. When the plastic flows around a hole, the meeting point of two flows can result in a visible line.
Weld lines reduce the overall strength and durability of the component. Factors such as resin temperature, injection speed, and pressure influence the formation of weld lines. Strategic modifications to mold design, like removing partitions, can help eliminate weld lines.
Drag Marks
Drag marks, also known as streaks or scuffing, can mar the appearance of an otherwise perfect part. We will scrutinize the factors leading to drag marks, from mold temperature to injection speed, and discuss practical approaches to eliminate this defect and enhance the visual appeal of molded parts and products.
Flow Lines
Flow lines are intricate patterns, often off-color, streaks, or variations on a part’s surface. These markings are a visual manifestation of molten plastic’s journey through the injection mold. When plastic moves at varying speeds, it solidifies at different rates, resulting in these lines.
The appearance of flow lines is indicative of potential issues with injection speed or pressure. Ensuring consistent wall thickness and strategic gate placement can minimize this defect.
Short Shots
Short shots refer to instances where the resin fails to entirely fill the mold cavity, leading to incomplete and unusable parts. Factors such as restricted flow within the mold, narrow gates, blocked gates, trapped air pockets, and inadequate injection pressure contribute to short shots.
Understanding these issues is crucial for optimizing the injection molding process and ensuring complete, consistent part production.
Short shots refer to instances where the resin fails to entirely fill the mold cavity, leading to incomplete and unusable parts. Factors such as restricted flow within the mold, narrow gates, blocked gates, trapped air pockets, and inadequate injection pressure contribute to short shots.
Understanding these issues is crucial for optimizing the injection molding process and ensuring complete, consistent part production.
History of Injection molding
Injection molding, originating in the late 19th century, began with John Wesley Hyatt’s patent for a machine using a plunger to mold celluloid. The 1940s saw the introduction of screw injection machines, bringing consistency to the process, while the 1950s integrated hydraulic systems and expanded material options.
In the 1980s, computer numerical control (CNC) machining revolutionized precision and automation, influencing design possibilities significantly.
Today, injection molding dominates various industries due to its efficiency in producing intricate, high-quality components at scale. It’s widely used in automotive, electronics, medical, and consumer goods sectors. This method not only streamlines production but also enables the creation of complex shapes and features, pushing the boundaries of design possibilities.
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.
