Injection molding is a molding method that combines injection and molding. The general injection molding process involves adding polymer-containing granules or powder into the barrel (also called the barrel) of an injection molding machine.
Through heating, compression, shearing, mixing, and conveying, the material is homogenized and melted (this process is also called plasticizing). Then, pressure is applied to the molten material using a plunger or screw, and the high-temperature melt is injected through the nozzle in front of the barrel and the mold's gating system into a pre-closed low-temperature mold cavity. After cooling and solidification, the mold can be opened, and the product ejected, resulting in a plastic product with a specific geometric shape and precision.
This method is suitable for the mass production of complex-shaped plastic parts and is one of the important plastic processing methods.
Injection Molding Steps
Injection molding generally consists of six steps: mold clamping, injection, pressure holding, cooling, mold opening, and product ejection, as shown in Figure 1-1. Each molding step represents a different stage of injection molding. Under normal production conditions, the injection molding machine can automatically complete these steps by setting the injection molding machine parameters. The following is a brief explanation of each injection process.
The mold opening and closing of an injection molding machine is accomplished by the mold clamping system. For hydraulic-mechanical (linkage) injection molding machines, see Figure 1-2. The mold opening and closing are mainly achieved through mechanical movement. When clamping the mold, a clamping force is applied to the mold to overcome the pressure of the cavity during injection molding. The mold opening action is mainly to remove the product and enter the next production cycle.
Injection, Holding Pressure, and Plasticizing
The injection, holding pressure, and plasticizing processes are primarily accomplished by the injection system of the injection molding machine: In one cycle of the injection molding machine, a certain amount of plastic is heated and melted within a specified time. Then, under specific pressure and speed, the molten plastic is injected into the mold cavity through a screw. After injection, a certain pressure is maintained on the molten material injected into the mold cavity. See Figure 1-3.
Cooling
The product cooling stage involves the high-temperature molten metal being injected through the nozzles in front of the barrel and the mold's gating system into a pre-closed low-temperature mold cavity, where it cools and solidifies. This cooling and solidification process significantly impacts the production cycle and requires setting a reasonable cooling time based on the product's manufacturing process requirements.
Product Ejection
Product ejection is accomplished by the ejection system on the injection molding machine. After the injection molding machine opens the mold, the product ejection system moves forward, ejects the product, and then retracts (Figure 1-4). The product ejection method can be set to hold, retraction, or intermediate ejection modes, depending on the part removal requirements.
Main injection molding process
The injection molding process is mainly completed on an injection molding machine, including plasticizing and metering, injection filling, and cooling and shaping. The purpose of studying the injection molding process is to adjust the injection molding process parameters according to the plastic and the product to control the quality of the injection molded products.
Plasticizing Measurement
Plasticizing refers to the entire process of heating plastic in the barrel to a fluid state with good plasticity. The plastic raw material is uniformly melted at a high temperature by heat generated from friction with the rotating injection molding machine screw, or by heat supplied by a heater outside the injection molding machine barrel, preparing it for injection into the mold. Plasticizing can be considered the preparation process for injection molding. The molten plastic should reach the specified molding temperature upon entering the mold cavity and provide a sufficient quantity of molten plastic within a specified time. The temperature of the molten plastic should be uniform at all points, with little or no thermal decomposition to ensure continuous production.
Plasticizing can be divided into plunger-type plasticizing and screw-type plasticizing. Screw-type plasticizing involves not only rotational motion but also backward linear motion; the screw rotates and retracts simultaneously. The backward linear motion is the result of the reaction force between the material in the screw channel and the molten material at the screw head on the screw during rotation. The polymer undergoes three states from the rear to the front in the barrel: glassy state, elastic state, and viscous flow state. The corresponding screw is divided into three sections: the rear solid conveying section (feeding section), the intermediate melting section (compression section), and the front homogenization section (metering section). The screw groove depth of the general-purpose screw gradually becomes shallower from the feeding section to the metering section, as shown in Figure 1-5.
Injection Molding
Injection molding is the process of injecting the plasticized molten metal from the metering chamber into the mold cavity. This is a complex and crucial stage, involving the flow of high-temperature melt into the relatively cooler mold cavity. It determines polymer orientation and crystallization, directly impacting product quality. Injection molding consists of two stages: the injection stage and the holding pressure stage. The injection stage begins with the screw propelling the melt until the melt fills the mold cavity. The holding pressure stage begins with the melt filling the mold cavity and ends with the gate "freezing." The holding pressure stage can be further divided into the holding pressure compensation flow stage and the holding pressure switching reverse flow stage.
During the holding pressure stage, under the holding pressure, the melt in the mold cavity undergoes cooling, compensation, and further compression and densification.
The holding pressure compensation flow stage refers to the situation where the nozzle pressure (injection pressure) reaches its maximum value, but the mold cavity pressure has not yet reached its maximum value. In other words, the peak pressure in the mold cavity lags behind the injection pressure by a certain period. It requires a period of compaction pressure holding, meaning the melt must fill all parts of the mold cavity within a very short time, and the melt itself must be compressed. The pressure during this flow process is very high, also known as secondary injection pressure. Both the holding pressure flow and the pressure flow during mold filling are compacted flows of the melt under high pressure. The characteristic of this flow is that the volumetric velocity is very low and does not play a dominant role, while pressure is the main factor affecting the process. During the holding pressure stage, the mold pressure and melt temperature gradually change. The reason for the holding pressure flow is that the melt near the mold cavity wall contracts after cooling, causing a change in melt volume. Thus, before the gate "freezes," the melt continues to replenish the mold cavity under the holding pressure, producing a compensating holding pressure flow.
From the holding pressure stage to the backflow stage. The gate "freezes," the holding pressure ends, screw pre-plasticizing begins, and the nozzle pressure drops to zero. At this point, although the gate is "frozen," the melt inside the mold has not yet completely solidified. Under the reaction of the mold cavity pressure, the melt inside the mold will flow back to the gate system, causing the mold cavity pressure to decrease. The mold cavity pressure at this time is called the sealing pressure. The backflow time and sealing pressure depend on factors such as polymer properties, barrel and mold temperature, and gate size.
Cooling and Shaping
The cooling and shaping process begins when the gate "freezes" and continues until the part is demolded. The main characteristic of this process is temperature. Generally, from the time the gate "freezes" until demolding, the part needs to continue cooling in the mold cavity for a period of time to ensure sufficient rigidity and prevent distortion upon demolding. During this process, the temperature of the melt inside the mold cavity gradually decreases. The change in cavity pressure is related to the holding time. The longer the holding time, the greater the residual stress in the cavity. Ideally, the residual stress at demolding should be zero. If the residual stress is greater than zero, demolding becomes difficult; if the residual stress is less than zero, surface depressions or internal vacuum bubbles are likely to appear on the part. The change in plastic volume is essentially the change in plastic density. That is, the longer the holding time, the higher the plastic temperature at the time the gate "freezes," the higher the cavity pressure, and the greater the density of the part. With a fixed holding time, the higher the demolding temperature and the higher the cavity pressure, the lower the density of the part, and the greater the post-molding shrinkage, resulting in greater internal stress within the part.
Plastic parts can be demolded once they have cooled sufficiently within the mold. The demolding temperature should not be too high, and is generally controlled between the heat distortion temperature and the mold temperature.
Applications of Injection Molding
Injection molding is used in a wide range of industries to produce a diverse array of products, from simple household items to complex medical devices.
Injection molding is widely used in the automotive industry for producing parts such as dashboards, bumpers, interior components, and engine parts.
Lightweight components for improved fuel efficiency
Complex geometries with high precision
High-strength materials for safety-critical parts
The medical industry relies on injection molding for producing sterile, precision components such as syringes, IV connectors, surgical instruments, and implantable devices.
Biocompatible materials for patient safety
Sterilizable components for medical applications
Tight tolerances for critical medical functions
Injection molding is used to produce a vast array of consumer products, including household items, toys, electronics, packaging, and personal care products.
High-volume production of affordable products
Wide range of colors and finishes
Customizable designs for brand differentiation
The electronics industry uses injection molding for producing casings, connectors, switches, and other components that require precision and electrical insulation.
Precision components for delicate electronics
Materials with high electrical insulation properties
Heat-resistant materials for electronic components
Injection molding is widely used in the packaging industry for producing containers, caps, closures, and other packaging components with tight seals and precise dimensions.
Lightweight and durable packaging solutions
Customizable shapes and sizes
Barrier properties for food and pharmaceutical packaging
The aerospace industry uses injection molding for producing lightweight, high-strength components such as interior panels, brackets, and connectors.
Lightweight materials for fuel efficiency
High-strength components for critical applications
Materials that meet strict aerospace certifications
CNC Machining in Injection Molding
Computer Numerical Control (CNC) machining plays a crucial role in the injection molding process, from mold making to part production.
The Role of CNC Machining in Injection Molding
CNC machining is a manufacturing process that uses computerized controls to operate machine tools such as mills, lathes, routers, and grinders. In the context of injection molding, CNC machining is primarily used for:
Mold Making
CNC machining is used to create the molds used in injection molding. This process allows for high precision and accuracy, ensuring that the mold produces parts that meet exact specifications.
Prototyping
CNC machining is often used to produce prototypes of injection molded parts. This allows designers to test the form, fit, and function of the part before committing to expensive mold tooling.
Low-Volume Production
For low-volume production runs, CNC machining can be a cost-effective alternative to injection molding. It allows for the production of parts without the need for expensive molds.
Mold Repair and Modification
CNC machining is used to repair and modify existing molds, extending their lifespan and ensuring consistent part quality over time.
CNC Machining vs. Injection Molding
| Factor | CNC Machining | Injection Molding |
|---|---|---|
|
Production Volume |
Best for low to medium volumes (1-1,000 parts) |
Best for high volumes (1,000+ parts) |
|
Initial Cost |
Low (no need for expensive molds) |
High (due to mold tooling costs) |
|
Per-Unit Cost |
High (labor and machine time) |
Low (economical for large volumes) |
|
Material Options |
Wide range of metals, plastics, and composites |
Wide range of plastics and some metals |
|
Lead Time |
Short (days to weeks) |
Long (weeks to months due to mold making) |
|
Part Complexity |
Limited (difficult to produce complex geometries) |
High (can produce very complex shapes) |
|
Surface Finish |
Good, but may require additional finishing |
Excellent (mold finish is transferred to part) |
Injection Molding vs. Other Manufacturing Processes
Injection molding is just one of many manufacturing processes available. Understanding how it compares to other methods can help in selecting the most appropriate process for a given application.
3D Printing
Initial Cost
Production Speed
Part Complexity
Material Options
Surface Finish
Scalability
Best For:
Prototyping, low-volume production, complex geometries, and custom parts.
When to Choose 3D Printing Over Injection Molding:
Low production volumes (1-100 parts)
Complex geometries that are difficult to mold
Quick turnaround times
Prototyping and design validation
CNC Machining
Initial Cost
Production Speed
Part Complexity
Material Options
Surface Finish
Scalability
Best For:
Prototyping, low to medium volume production, precision parts, and parts requiring tight tolerances.
When to Choose CNC Machining Over Injection Molding:
Low to medium production volumes (1-1,000 parts)
Simple to moderately complex geometries
High precision and tight tolerances
Use of exotic or specialized materials
Vacuum Casting
Initial Cost
Production Speed
Part Complexity
Material Options
Surface Finish
Scalability
Best For:
Prototyping, small batch production, and parts requiring high detail and smooth surfaces.
When to Choose Vacuum Casting Over Injection Molding:
Small batch production (1-50 parts)
High-detail parts with complex geometries
Short lead times
Low-cost tooling for temporary needs
Process Selection Guide
The choice between injection molding and other manufacturing processes depends on several factors, including production volume, part complexity, material requirements, and budget. Use this guide to determine the most suitable process for your project:
Choose Injection Molding When:
You need high-volume production (1,000+ parts)
You require complex geometries with tight tolerances
You need consistent part quality and precision
You want to use a wide range of materials
You need efficient production with minimal waste
You require high surface finish quality
Consider Other Processes When:
Your production volume is low (1-1,000 parts)
You need a quick turnaround for prototyping
Your budget is limited for tooling costs
You need to test multiple design iterations
You require highly customized or unique parts
You need to use materials not suitable for injection molding
Case Studies
Explore real-world examples of how injection molding has been used to solve complex manufacturing challenges across various industries.
Automotive Dashboard Component
A leading automotive manufacturer needed to produce a complex dashboard component with integrated air vents, button housings, and decorative elements.
Challenge:
Complex geometry with multiple undercuts, tight tolerances, and aesthetic requirements.
Solution:
Multi-cavity mold with side actions and hot runner system to ensure consistent quality across all cavities.
Results:
• 40% reduction in production time
• 99.8% first-pass quality rate
• Annual production of 500,000 units
Medical Syringe Component
A medical device company required precision-molded syringe barrels with exceptional dimensional accuracy and biocompatibility.
Challenge:
Ultra-tight tolerances (±0.02mm), medical-grade materials, and zero-defect requirements.
Solution:
Clean room manufacturing with all-electric injection molding machines and advanced process monitoring.
Results:
• 99.99% quality compliance
• FDA approval achieved
• 25% cost reduction vs. alternatives
Smartphone Housing
A consumer electronics manufacturer needed lightweight, durable housings for their latest smartphone model with integrated antenna components.
Challenge:
Thin-wall design, electromagnetic compatibility, and premium surface finish requirements.
Solution:
Advanced polymer blend with metallic coating, precision temperature control, and specialized ejection system.
Results:
• 30% weight reduction achieved
• Premium surface finish quality
• 2 million units produced annually
Industry Success Metrics
Average Quality Rate
Average Time Reduction
Average Cost Savings
Parts Produced Annually
Development Trends of Injection Molding
In accordance with the key development directions for China's plastics industry, enterprises are shifting from the past "three lows and two highs" (low added value, low quality, low value and high pollution, high energy consumption) to a growth model focused on high quality and high added value. Green and environmentally friendly products conducive to low-carbon economic development will receive policy incentives and market favor. The pursuit of original innovation and high added value in new products and processes will become the overall trend for the future development of plastics processing enterprises. To achieve rapid, stable, and healthy development in the injection molding industry, injection molding enterprises must emphasize development and innovation while focusing on technology, operation, and management. Development and innovation are the overall, long-term, and fundamental development strategies for the injection molding industry.
Development and innovation are necessary to adapt to environmental changes. We live in an era of great change; the breadth, depth, and speed of environmental changes in China and its surrounding areas are unprecedented. China's injection molding industry has developed rapidly recently. China has become one of the largest production, sales, and export bases for plastic products such as automobiles, motorcycles, computer casings, other IT products, hardware tools, electrical appliances, and small household appliances. Moreover, the market size of China's injection molded products has been increasing year by year in recent years, showing enormous development potential. China's injection molding industry serves as a bridge connecting the international market, possessing vast and promising development potential. China boasts a robust financial and trade service system, along with advanced logistics, information flow, and commercial flow, providing the injection molding industry with ample opportunities for innovation and development.
As mold making enters an era of personalization, the plastics manufacturing industry must also define its market position, concentrating resources to target specific market segments. Based on the principle of market segmentation, focusing on a particular area is crucial to establishing core competitive advantages. Therefore, we should not engage in simple price wars with competitors, but rather address the issue of a market with a prevalence of general-purpose plastic parts and a scarcity of high-tech, high-value-added, and distinctive plastic products, guiding injection molding companies to diverge from their competitors.
The injection molding market is vast, brimming with opportunities. The strong create opportunities, seize opportunities, and the mediocre lose them. Winning by producing ordinary plastic parts is an external and temporary advantage. We should strive to win through the unique value of injection molding. By offering reliable quality, stable performance, exquisite appearance, and reasonable prices, we aim to gain the recognition of customers across the manufacturing industry chain, thereby enhancing the reputation of the injection molding industry within the plastics sector.
To achieve development and innovation in the injection molding industry, it is necessary to re-analyze market demands and trends, the challenges and opportunities, one's own strengths and weaknesses, and the key links and steps. For a long time, we have only focused on simple plastic parts processing, and most injection molding companies' business has been limited to injection molding, without further processing after the initial molding. Over time, the profit margins of injection molding companies have shrunk, making it difficult to improve performance. To achieve development and innovation in the injection molding industry, it is necessary to formulate entirely new business content, methods, personnel frameworks, management systems, and business strategies. Product processing must also move from simple injection molding parts processing to product design and development, and then to establishing a complete integrated production operation model.
With the transformation and upgrading of injection molding companies and the progress and development of the injection molding industry, injection molding has entered an era of full automation, unmanned operation, and precision. This new development provides new opportunities for injection molding companies, while also placing higher demands on the capabilities of injection molding technicians. Injection molding companies and their employees need to keep pace with the times in terms of professional competence.