11.2 Injection molding process and mold design
4 min read•july 23, 2024
is a key manufacturing process for creating plastic parts. It involves melting polymer pellets, injecting the molten material into a mold, and cooling it to form the final product. The process requires precise control of temperature, pressure, and timing.
Mold design is crucial for successful injection molding. It encompasses the cavity shape, cooling channels, ejection system, and . Proper mold design ensures high-quality parts, efficient production, and minimizes defects like shrinkage and warpage.
Injection Molding Process
Steps of injection molding process
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- Plasticizing involves heating and melting polymer pellets in the barrel of the injection molding machine
- Reciprocating screw rotates, melting and mixing the polymer (polypropylene, polyethylene)
- Molten polymer accumulates at the front of the screw, forming a homogeneous melt
- Injection step involves injecting molten polymer into the under high pressure (50-200 MPa)
- Screw acts as a plunger, pushing the molten polymer through the nozzle and into the mold
- High injection speed ensures rapid filling of the mold cavity (0.1-1 s)
- Packing and holding step involves injecting additional polymer to compensate for shrinkage during cooling
- Pressure is maintained (50-100 MPa) to ensure complete filling of the mold cavity
- Holding time allows the gate to freeze off, preventing backflow of the melt
- Cooling step involves solidification of the molded part inside the mold
- depends on part thickness (1-5 mm), polymer properties (thermal conductivity), and mold temperature (20-100 ℃)
- Sufficient cooling is essential to maintain part dimensions and prevent warpage
- Ejection step involves removing the solidified part from the mold
- Ejector pins or plates push the part out of the mold cavity
- Mold opens, and the part is ejected automatically or manually
Components of injection molding machines
- Hopper stores and feeds polymer pellets (granules) into the barrel
- Barrel heats and melts the polymer pellets using electric heaters or oil/water circulation
- Contains heating elements and temperature sensors for precise temperature control (150-400 ℃)
- Reciprocating screw plasticizes, mixes, and injects the molten polymer
- Screw design (compression ratio, L/D ratio) affects mixing, homogeneity, and injection performance
- Nozzle connects the barrel to the mold and directs the molten polymer into the mold cavity
- Nozzle type (open, shut-off) and size influence flow and prevent drooling
- opens and closes the mold, and provides clamping force during injection
- Sufficient clamping force (100-10,000 kN) is required to keep the mold closed during high-pressure injection
- Mold contains the cavity that shapes the molded part
- Consists of fixed and moving platens, core and cavity inserts, and cooling channels
Mold Design
Elements of mold design
- Mold cavity determines the shape and dimensions of the molded part
- Cavity design must account for shrinkage (0.5-5%) and draft angles (0.5-2°) for easy part ejection
- Parting line separates the fixed and moving halves of the mold
- Location and design affect part appearance (witness lines) and ease of ejection
- Cooling channels control the temperature of the mold and the cooling rate of the molded part
- Proper cooling channel design (diameter, spacing, layout) ensures uniform cooling and reduces
- Ejection system removes the solidified part from the mold cavity
- Ejector pins, plates, or sleeves must be designed to minimize marks on the part surface
- Venting allows air and gases to escape from the mold cavity during injection
- Inadequate venting can lead to incomplete filling, burn marks, and surface defects (silver streaks)
Gate and runner systems
- Gate is the point at which the molten polymer enters the mold cavity
- Gate location, size, and type affect flow patterns, part appearance, and cycle time
- Common gate types include pin, edge, fan, and submarine gates
- Runner channels distribute the molten polymer from the sprue to the gates
- Runner design influences flow balance, pressure drop, and material usage
- Cold runners are solidified in the mold and ejected with the part
- Hot runners maintain the polymer in a molten state, reducing waste and cycle time
Shrinkage in injection molding
- Shrinkage is the reduction in volume and dimensions of a molded part as it cools and solidifies
- Occurs due to thermal contraction and polymer crystallization
- Shrinkage rate depends on polymer type (amorphous, semi-crystalline), part geometry, and processing conditions
- Crystalline polymers (polyethylene, polyamide) generally exhibit higher shrinkage than amorphous polymers (polystyrene, ABS)
- Mold design must account for shrinkage to achieve the desired final part dimensions
- Mold cavity is made slightly larger than the desired part size to compensate for shrinkage
- Differential shrinkage can lead to warpage and dimensional instability
- Caused by variations in wall thickness, cooling rates, and flow patterns
- Shrinkage control can be improved by optimizing processing parameters and mold design
- Uniform cooling, proper gate placement, and consistent wall thickness help minimize differential shrinkage
Key Terms to Review (18)
ASTM Standards: ASTM Standards are a set of guidelines and criteria established by ASTM International to ensure quality, safety, and performance in materials and products. These standards play a crucial role in the testing and evaluation of polymers, providing a basis for comparing different materials, processes, and applications across various industries.
Clamping Unit: A clamping unit is a crucial component of an injection molding machine that securely holds the mold in place during the injection process. It ensures that the mold remains closed against the pressure exerted by the molten polymer being injected, which is vital for producing high-quality molded parts. The clamping unit plays a key role in maintaining the integrity of the mold and preventing any material leakage, ultimately influencing the efficiency and accuracy of the entire injection molding process.
Cooling Time: Cooling time is the duration required for a molten polymer to solidify within a mold after it has been injected during the molding process. This time is crucial as it influences the final part's dimensional accuracy, mechanical properties, and overall production efficiency. Proper management of cooling time ensures optimal cycle times and can significantly affect the quality of the molded product.
Cycle time: Cycle time is the total time required to complete one full cycle of the injection molding process, from the start of injection to the ejection of the molded part. This term is critical in assessing the efficiency and productivity of the injection molding operation, influencing factors such as production rate, equipment utilization, and overall manufacturing costs.
Design for manufacturability: Design for manufacturability (DFM) is a design approach aimed at improving the manufacturing process by considering how a product will be produced during its design phase. This approach focuses on simplifying the design, reducing production costs, and enhancing product quality by addressing manufacturing capabilities and constraints early in the development process. By integrating manufacturing considerations into the design process, DFM helps ensure that the final product is not only functional but also economical to produce.
Draft angle: Draft angle is the slight taper or incline designed into the walls of a mold to facilitate the easy removal of the molded part from the mold. This feature is crucial in ensuring that the molded items can be efficiently ejected without causing damage to the part or the mold itself. The appropriate draft angle helps minimize friction during ejection and can influence the quality of the final product.
Gate design: Gate design refers to the specific configuration and placement of the entry point in an injection mold, which is crucial for the efficient flow of molten polymer into the cavity. It plays a significant role in determining the quality of the molded part, as well as its manufacturing efficiency and production cycle time. A well-thought-out gate design minimizes defects, ensures uniform filling, and helps manage cooling and solidification processes.
Injection Molding: Injection molding is a manufacturing process used to create parts by injecting molten material into a mold. This method is crucial for producing complex shapes and high volumes of polymer products, making it a key player in various industries.
Injection Unit: An injection unit is a crucial component of the injection molding machine that is responsible for melting and injecting plastic material into the mold. This unit typically consists of a hopper for feeding raw plastic, a barrel where the material is heated and melted, and a screw that helps to mix and transport the molten plastic into the mold cavity. The efficiency and design of the injection unit directly impact the quality of the final molded product.
ISO 9001: ISO 9001 is an international standard that specifies requirements for a quality management system (QMS). It helps organizations ensure they meet customer and regulatory requirements while continuously improving their processes. By focusing on quality and customer satisfaction, ISO 9001 plays a crucial role in enhancing the efficiency and reliability of manufacturing processes, including injection molding and mold design.
Mold cavity: A mold cavity is the hollow space within a mold where the molten material is injected to create a specific shape or form during the injection molding process. This cavity is designed to replicate the desired final product, allowing for precise and intricate designs. The quality of the mold cavity directly influences the finished product's dimensions, surface finish, and overall integrity.
Multi-cavity mold: A multi-cavity mold is a type of injection mold that contains multiple cavities for producing more than one part in a single cycle. This design increases production efficiency by allowing the simultaneous creation of several identical parts, reducing cycle time and overall costs. Multi-cavity molds are crucial in high-volume manufacturing, ensuring consistency and maximizing the use of resources.
Process optimization: Process optimization refers to the methodical approach of improving a manufacturing process to achieve the best possible outcome in terms of efficiency, quality, and cost-effectiveness. In the context of designing molds and injection molding processes, it involves adjusting various parameters like temperature, pressure, and cycle time to minimize waste and enhance production speed while ensuring that the final product meets required specifications.
Short shot: A short shot refers to a manufacturing defect in the injection molding process where the mold does not fill completely, resulting in an incomplete part. This issue can lead to weak points, aesthetic defects, and functional failures in the final product, making it a critical aspect of quality control in mold design and production.
Single cavity mold: A single cavity mold is a type of mold used in the injection molding process that produces one part per cycle. This design is often employed for simpler, low-volume production runs, ensuring efficiency and reduced costs for manufacturing individual components. The design of a single cavity mold directly influences the overall cycle time, quality of the part produced, and the economics of the injection molding process.
Sink mark: A sink mark is a surface defect that appears on a molded part, typically as a small depression or dimple, resulting from uneven cooling and solidification during the injection molding process. This defect occurs when thicker sections of a part cool more slowly than thinner areas, causing the material to shrink and create an indentation at the surface. Understanding sink marks is crucial for optimizing mold design and processing parameters to improve the overall quality of molded products.
Thermoplastics: Thermoplastics are a type of polymer that become soft and moldable upon heating and solidify upon cooling. This unique property allows them to be reshaped and recycled, making them incredibly versatile in various applications across different industries.
Thermosetting polymers: Thermosetting polymers are a type of polymer that, once cured or hardened, cannot be remelted or reshaped. This process involves irreversible chemical reactions, which give thermosetting polymers their strength and heat resistance. Their rigid structure and excellent mechanical properties make them ideal for applications requiring durability and stability under heat, particularly in manufacturing processes like injection molding.