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Exploring Injection Molds: The Precision Foundation of Industrial Manufacturing

• 2025-07-11

Exploring Injection Molds: The Precision Foundation of Industrial Manufacturing

    In the vast landscape of modern industrial manufacturing, injection molds hold a pivotal position and can be regarded as the mother of industry. From the various plastic products that are readily available in daily life to key components in high-end fields such as automobiles, electronics, and medical care, injection molds, with their unique forming processes, endow plastic materials with ever-changing shapes, meeting the diverse needs of human society. This article will delve into the world of injection molds, providing a comprehensive analysis of their design, manufacturing, application, and cutting-edge developments.

Auto Mould_Taizhou Jiefeng Mould Co.,Ltd. (jfmoulds.com)

The basic framework and working principle of injection molds

    Injection molds are precision tools used for plastic molding, mainly composed of two major parts: the moving mold and the fixed mold. They achieve the production of plastic products through the coordinated operation of multiple systems. The basic working principle is to use the pressure of the injection molding machine to inject molten plastic into the mold cavity. After cooling and solidification, plastic products of the required shape are formed. Finally, the products are removed through the mold opening action.

A comprehensive analysis of the workflow

    1.Mold closing: The mold closing device of the injection molding machine drives the moving mold to move towards the fixed mold, causing the moving mold and the fixed mold to close tightly, forming a closed cavity and gating system. During the mold closing process, the guiding system plays a crucial role in ensuring that the moving mold and the fixed mold are accurately aligned. The mold closing accuracy is usually required to reach ±0.03mm or even higher.

    2. Injection molding: Plastic pellets are heated to a molten state in the barrel of an injection molding machine and then, driven by a screw, are injected into the mold cavity through the injection system at a speed of 100-500mm /s. Injection pressure is determined by the shape, size of the plastic product and the characteristics of the plastic material, generally ranging from 50 to 300MPa. During the injection molding process, it is necessary to precisely control the injection speed and pressure to ensure that the plastic melt can fill the cavity evenly and quickly, avoiding defects such as short shot and trapped air.

    3. Holding pressure: After the cavity is filled with the plastic melt, a certain pressure (holding pressure) is continuously applied to compensate for the shrinkage of the plastic during the cooling process and ensure the dimensional accuracy and surface quality of the plastic product. The holding pressure is usually slightly lower than the injection pressure. The holding time varies depending on the thickness and complexity of the plastic product, generally ranging from a few seconds to tens of seconds.

    4. Cooling: While maintaining pressure, the cooling system starts to work. The coolant circulates in the cooling water channels, taking away the heat from the mold and enabling the plastic products to cool and solidify rapidly. The cooling rate is generally controlled at 5-20 ℃/s. The cooling time is determined by the thickness of the plastic product and the thermal properties of the plastic material, usually ranging from tens of seconds to several minutes. The uniformity of the cooling process is crucial to the quality of plastic products. If the cooling is not uniform, it may lead to defects such as deformation and warping in plastic products.

    5. Mold opening and ejection: After the plastic product cools and solidifies, the mold opening device of the injection molding machine drives the moving mold to separate from the fixed mold, and then the ejection system works to ejection the plastic product out of the mold cavity. During the ejection process, attention should be paid to the distribution of the ejection force and the ejection speed to avoid causing damage such as whitening, deformation or cracking to the plastic products. The plastic products after ejection can become finished products after subsequent trimming, processing and other procedures.

Motorcycle Mould_Taizhou Jiefeng Mould Co.,Ltd. (jfmoulds.com)

Design criteria and key points of injection molds

    The design of injection molds is a complex and meticulous task, which requires a comprehensive consideration of multiple factors such as the shape, size, precision requirements, production batch size of plastic products, the characteristics of plastic materials, and the performance of injection molding machines. An excellent injection mold design not only ensures the quality of plastic products but also enhances production efficiency and reduces production costs.

Processability analysis of plastic products

    Before designing injection molds, a comprehensive processability analysis of plastic products should be conducted first to assess whether they are suitable for injection molding processes. This includes the analysis of structural features of plastic products such as shape, wall thickness, draft Angle, reinforcing ribs, fillets, holes, and threads.

    Shape complexity: The more complex the shape of plastic products is, the greater the difficulty of mold design and manufacturing will be. For plastic products with complex shapes, special mold structures such as lateral core-pulling mechanisms and inclined top mechanisms may be required to achieve the molding of the products. For instance, for plastic products with side holes or side depressions, a lateral core-pulling mechanism needs to be designed to pull out the lateral core after injection molding to facilitate smooth demolding.

    Uniformity of wall thickness: The wall thickness of plastic products should be as uniform as possible to avoid sudden changes in wall thickness. Uneven wall thickness can cause inconsistent shrinkage of plastic products during the cooling process, thereby resulting in defects such as deformation and warping. Generally speaking, the wall thickness of plastic products should be between 1 and 4mm. For large plastic products, the wall thickness can be appropriately increased, but it is also necessary to pay attention to controlling the difference in wall thickness.

    Demolding slope: To facilitate the smooth demolding of plastic products from the mold cavity after cooling and curing, a certain demolding slope needs to be set on the inner and outer surfaces of the products. The size of the draft Angle depends on factors such as the shape and size of the plastic product, the characteristics of the plastic material, and the surface roughness of the mold. Generally speaking, a release slope of 0.5° to 2° is more appropriate. For plastic products with complex shapes or high precision requirements, the release slope can be appropriately reduced, but the minimum should not be less than 0.2°.

    Reinforcing ribs and fillets: Reinforcing ribs can enhance the strength and rigidity of plastic products and reduce their deformation. When designing reinforcing ribs, it is important to ensure that their thickness is less than the wall thickness of the product to avoid shrinkage marks. At the same time, rounded corners should be designed at the corners of plastic products to avoid stress concentration and enhance the strength and appearance quality of the products. The radius of the fillet is generally no less than 0.5mm.

    Holes and threads: For holes and threads on plastic products, their forming methods and precision requirements should be taken into consideration. For small holes and deep holes, special mold structures or processing techniques may be required to ensure their precision. For threads, the appropriate forming method should be selected based on the specifications and precision requirements of the threads, such as direct forming, secondary processing forming, etc.

Optimal design of mold structure

    After completing the processability analysis of plastic products, the next step is to optimize the design of the mold structure. This includes aspects such as the selection of mold types, the determination of the number of cavities, the design of parting surfaces, the design of gating systems, the design of ejection systems, the design of cooling systems, and the selection of mold materials.

    Mold type selection: Common injection mold types include single parting face molds, double parting face molds, and hot runner molds, etc. The single parting surface mold has a simple structure and is suitable for the molding of most plastic products. The double parting surface mold is suitable for plastic products with point gate feeding and can conveniently remove the solidified material from the gating system. Hot runner molds are suitable for plastic products with high requirements for raw material utilization and can reduce the generation of waste. When choosing the type of mold, a comprehensive consideration should be made based on the characteristics of plastic products and production requirements.

    The determination of the number of cavities mainly depends on factors such as the production batch of plastic products, the clamping force and injection volume of the injection molding machine, and the cost of the mold. For plastic products with large production batches, multi-cavity molds can be adopted to enhance production efficiency. For plastic products with smaller production batches, single-cavity molds can be adopted to reduce mold costs. When determining the number of cavities, it is also necessary to consider whether the clamping force and injection volume of the injection molding machine can meet the requirements to avoid problems such as insufficient injection or flash.

    Parting surface design: The parting surface is the interface between the moving mold and the fixed mold in a mold. Its design directly affects the demolding of plastic products, the structure of the mold, and the manufacturing difficulty. When designing the parting surface, the following principles should be followed: To ensure that the plastic product can be demolded smoothly, the parting surface should be selected as much as possible at the maximum cross-section of the plastic product. For the convenience of mold processing and manufacturing, the shape of the parting surface should be as simple as possible. It is conducive to ensuring the dimensional accuracy and appearance quality of plastic products, and avoiding defects such as flash and burrs at the parting surface. Common forms of parting surfaces include planar parting surfaces, inclined surface parting surfaces, curved surface parting surfaces, etc.

    Gating system design: The design of the gating system should be comprehensively considered based on factors such as the shape, size, wall thickness of the plastic product, the characteristics of the plastic material, and the number of cavities. The size of the main runner should be determined based on the size of the injection molding machine nozzle to ensure that the plastic melt can smoothly enter the runner. The shape and size of the runner should be designed based on the number of cavities and the distribution of plastic products to ensure that the plastic melt can be evenly distributed to each cavity. The type and size of the gate should be selected based on factors such as the appearance requirements, dimensional accuracy and molding process of the plastic product, in order to control the flow rate and volume of the plastic melt and ensure the quality of the plastic product. In addition, the cold material cavity and exhaust groove should be reasonably designed to ensure the smooth progress of the injection molding process.

    Ejection system design: The design of the ejection system should select the appropriate ejection method and ejection position based on the shape, structure and size of the plastic product. The ejection method should ensure that the plastic products are not damaged during the ejection process. The ejection positions should be evenly distributed to avoid problems such as whitening and deformation. At the same time, the reset issue of the ejection system should also be considered to ensure that the ejection system can be smoothly reset after the ejection action is completed, so as to facilitate the next injection molding cycle.

    Cooling system design: The design of the cooling system should be comprehensively considered based on factors such as the shape of the plastic product, wall thickness, thermal properties of the plastic material, and injection molding cycle. The layout of the cooling water channels should be reasonable to ensure that the mold can be cooled evenly and avoid local overheating or overcooling. The diameter and length of the cooling water channels should be determined based on the flow rate and velocity of the coolant to ensure the cooling effect. In addition, attention should be paid to the distance between the cooling water channels and other components of the mold to avoid mutual interference.

    Mold material selection: The choice of mold material directly affects the service life, processing performance and cost of the mold. Commonly used mold materials include steel, aluminum alloy, copper alloy, etc., among which steel is the most frequently used mold material. When choosing mold materials, a comprehensive consideration should be made based on factors such as the batch size of plastic products, precision requirements, characteristics of plastic materials, and the structure of the mold. For molds with large production batches and high precision requirements, steel with high strength, good wear resistance and small deformation during heat treatment should be selected. For molds with smaller production batches and lower precision requirements, steel or other materials with lower prices can be selected.

Manufacturing process and quality control of injection molds

    The manufacturing of injection molds is a crucial step in transforming designs into actual products. The quality and performance of the molds are directly determined by the quality of the manufacturing process. Modern injection mold manufacturing typically employs a variety of advanced processing techniques and equipment to ensure the high precision and quality of the molds.

Precision machining technology

    Machining: Machining is the fundamental process in the manufacturing of injection molds, including turning, milling, drilling, boring, grinding, etc. Through mechanical processing, the basic shape and size processing of mold parts can be completed. During the mechanical processing, it is necessary to strictly control the processing accuracy and surface roughness to meet the requirements of mold design. For instance, for key components such as the cavity and core of a mold, their dimensional accuracy is typically required to be controlled within ±0.01mm, and the surface roughness should reach Ra0.8 - Ra0.4μm.

    Electrical discharge Machining (EDM) : EDM is a processing technique that utilizes the principle of electrical discharge corrosion. It is suitable for machining mold parts with complex shapes that are difficult to complete by mechanical processing methods. During the electrical discharge machining process, a certain discharge gap is maintained between the tool electrode (usually copper or graphite) and the workpiece. The high-frequency pulse current generated by the pulse power supply instantly ionizes and breaks down the working fluid in the discharge gap, forming a discharge channel. This generates high temperature and high pressure, causing local melting and vaporization of the workpiece material, thereby achieving the purpose of material removal. Electrical discharge machining can produce high-precision and complex-shaped mold parts, such as those with irregular holes, narrow grooves, patterns, etc.

    Wire cutting processing: Wire cutting processing is a special form of electrical discharge machining. It uses a moving fine metal wire (usually molybdenum wire or copper wire) as the tool electrode and cuts the workpiece through the high-frequency pulse current generated by the pulse power supply. Wire cutting processing is mainly used for processing the contour shapes of parts such as the core, cavity and insert of molds, as well as various irregular holes and narrow slits. Wire cutting processing has the advantages of high processing accuracy, good surface quality, and the ability to process hard materials, which can meet the requirements of high precision and complex shapes in injection mold manufacturing.

    Numerical Control Machining (CNC) : CNC machining is a processing method that uses digital information to control mechanical movement and processing procedures. It converts the design data of mold parts into numerical control codes through computer programming and inputs them into numerical control machine tools. The numerical control machine tools automatically complete the processing procedures according to the preset programs. CNC machining has the advantages of high processing accuracy, high production efficiency and stable processing quality, and can achieve automated and high-precision processing of mold parts. In injection mold manufacturing, numerical control machining is widely applied in various mechanical processing and electrical discharge machining processes, such as numerical control milling and numerical control electrical discharge machining.

Quality inspection and control

    To ensure the quality of injection molds, strict quality inspection and control are required during the manufacturing process. Quality inspection includes the inspection of the dimensional accuracy, shape accuracy, surface roughness, hardness and other aspects of mold parts, as well as the inspection of the assembly quality and performance of molds.

    Dimensional accuracy inspection: Dimensional accuracy is one of the important indicators of injection mold quality, and it directly affects the dimensional accuracy of plastic products. During the processing of mold parts, high-precision measuring instruments such as three-coordinate measuring machines and optical projectors should be used to conduct real-time detection of the parts' dimensions to ensure that they meet the design requirements. For the key dimensions of the mold, such as the dimensions of the cavity and core, and the flatness of the parting surface, strict control is required. The tolerance range is usually controlled within ±0.01mm

    Shape accuracy inspection: Shape accuracy is also one of the important indicators of injection mold quality. It includes the straightness, flatness, roundness, cylindricity, etc. of mold parts. During the processing of mold parts, corresponding measuring tools and methods should be adopted to inspect the shape accuracy of the parts, ensuring that the shape of the parts meets the design requirements. For instance, for key components such as the cavity and core of molds, the shape accuracy requirements are relatively high. Usually, optical measuring instruments, profile measuring instruments, etc. are used for inspection.

    Surface roughness detection: Surface roughness has a significant impact on the demolding performance of injection molds and the surface quality of plastic products. During the processing of mold parts, tools such as surface roughness measuring instruments should be used to detect the surface roughness of the parts to ensure that the surface roughness of the parts meets the design requirements. For key parts such as the cavity and core of the mold, the surface roughness requirement is usually between Ra0.8 and Ra0.4μm. For some plastic products with higher requirements, the surface roughness requirement can be reduced to below Ra0.2μm.

    Hardness testing: Hardness is one of the important indicators for evaluating the performance of mold materials. It directly affects the service life and wear resistance of the mold. After the processing of mold parts is completed, tools such as hardness testers should be used to test the hardness of the parts to ensure that the hardness of the parts meets the design requirements. Different mold materials and heat treatment processes have different hardness requirements. For instance, the hardness of commonly used mold steel P20 is generally between HRC30 and 35, while that of S136 is typically between HRC48 and 52.

    Mold assembly quality inspection: Mold assembly is the process of assembling various mold parts into a complete mold. The quality of assembly directly affects the performance and service life of the mold. During the mold assembly process, it is necessary to strictly follow