For previous reading, please refer to Stamping die.
A mold that allows molten plastic raw materials to fill cavity under pressure and solidify into products and parts, including injection molds, compression molds, extrusion molds, blow molding molds, thermoforming molds, foaming molds, etc.

 

‌Overview of principle of plastic molds

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Plastic molds are important tools in plastic processing industry. They are used in conjunction with plastic molding machines to give plastic products a complete configuration and precise size. Its working principle is mainly based on molding process of plastics. Following are key points:

‌Basic composition‌: Plastic molds usually consist of a movable mold and a fixed mold. Movable mold is installed on movable mold plate of injection molding machine, and fixed mold is installed on fixed mold plate.

‌Working principle‌: During injection molding process, movable mold and fixed mold are closed to form a pouring system and a cavity. Heated and melted plastic raw materials are pushed into mold cavity by screw of injection molding machine under high pressure, molded product is obtained after cooling and solidification. When mold is opened, movable mold and fixed mold are separated to remove plastic product.

 

‌Widely used‌: Due to diversity of plastic varieties and processing methods, types and structures of plastic molds are also diverse, and are widely used in many fields such as automobiles, home appliances, electronic communications, and daily necessities. In summary, plastic molds provide efficient and accurate solutions for production of plastic products through their precise structure and molding process. Plastic molds are tools that are used in plastic processing industry to match plastic molding machines, give plastic products complete configurations and precise dimensions. Due to wide variety of plastic varieties and processing methods, structures of plastic molding machines and plastic products are different, types and structures of plastic molds are also diverse. A combined plastic mold for compression molding, extrusion molding, injection molding, blow molding and low foaming molding, which mainly includes a concave mold with a variable cavity composed of a concave mold combination base plate, a concave mold assembly and a concave mold combination card plate, and a convex mold with a variable core composed of a convex mold combination base plate, a convex mold assembly, a convex mold combination card plate, a cavity truncation assembly and a side sectional combination plate. Coordinated changes in mold convex, concave mold and auxiliary molding system. A series of plastic parts of different shapes and sizes can be processed. With rapid development of plastics industry, continuous improvement of strength of general and engineering plastics, application scope of plastic products is also expanding, and amount of plastic products is also increasing. Plastic mold is a tool for producing plastic products. It is composed of several groups of parts, and there is a molding cavity in this combination. During injection molding, mold is clamped on injection molding machine, molten plastic is injected into molding cavity, cooled and shaped in cavity, then upper and lower molds are separated, and product is ejected from mold cavity by ejection system. Finally, mold is closed for next injection molding, and the entire injection molding process is carried out in a cycle. Generally, a plastic mold consists of a movable mold and a fixed mold. Movable mold is installed on movable mold plate of injection molding machine, and fixed mold is installed on fixed mold plate of injection molding machine. During injection molding, movable mold and fixed mold are closed to form a pouring system and a cavity. When mold is opened, movable mold and fixed mold are separated to remove plastic product. Although structure of mold may vary due to different types and properties of plastics, shape and structure of plastic products, and type of injection molding machine, basic structure is same. Mold is mainly composed of a pouring system, a temperature control system, molding parts, and structural parts. Among them, pouring system and molding parts are parts that are in direct contact with plastic and change with plastic and product. They are the most complex, most variable, require the highest processing finish and precision in mold. Pouring system refers to flow channel part before plastic enters cavity from nozzle, including main channel, cold material hole, diverter channel and gate, etc. Molding parts refer to various parts that constitute shape of product, including movable mold, fixed mold and cavity, core, molding rod and exhaust port, etc. Driven by high-tech drive and application needs of pillar industries, my country's plastic molds have formed a huge industrial chain. From upstream raw and auxiliary materials industry and processing and testing equipment to downstream machinery, automobiles, motorcycles, home appliances, electronic communications, construction materials and other major application industries, plastic molds are developing vigorously.

 

Following aspects should be considered when choosing a suitable plastic mold: ‌ Understand product requirements‌: Clarify purpose, working environment and physical performance requirements of plastic products, such as non-toxicity, corrosion resistance, high strength, etc., which will directly affect selection of mold materials.
‌Material properties‌: Consider physical properties of mold material such as hardness, strength, toughness, wear resistance, thermal conductivity, and chemical properties such as corrosion resistance and oxidation resistance to ensure that mold can work stably under high temperature and high pressure‌.
‌Processing and cost‌: Select materials that are easy to process, polishable, weldable, have good heat treatment properties to shorten manufacturing cycle and reduce processing costs. At the same time, balance between performance and cost should be considered comprehensively‌.
‌Mold structure‌: According to shape, size and complexity of plastic products, select a suitable mold structure, such as a large sprue, a fine sprue or a hot runner mold to meet production needs‌.
In summary, selection of a suitable plastic mold requires a comprehensive consideration of product requirements, material properties, processing and cost, and mold structure.
Common plastic mold materials‌
Plastic molds play an important role in plastic processing industry, choice of their materials has a vital impact on performance and service life of mold. Following are some common plastic mold materials:

 

‌Aluminum alloy material‌: Good thermal conductivity, corrosion resistance, easy processing, high economic benefits, suitable for small batch production or rapid manufacturing.
‌Ordinary steel materials‌: economical and affordable, suitable for manufacturing simple, low-pressure parts, such as 45 steel, 50 steel, etc.
‌Bearing steel materials‌: good toughness and wear resistance, suitable for manufacturing large molds with medium and high pressures, such as GCr15, SUJ2, etc.
‌Stainless steel materials‌: good oxidation resistance and corrosion resistance, suitable for manufacturing food packaging machinery, medical equipment and high-demand plastic product parts, such as SUS304, SUS420J2, etc.
‌Engineering plastic materials‌: high strength, strong casting performance, such as nylon (PA), polyimide (PI), aramid (PPS), etc., suitable for manufacture of high-quality plastic molds.
These materials have their own characteristics, and selection needs to be weighed according to specific needs and conditions.
‌Stainless steel mold materials are of various types. Following are some common stainless steel mold materials:
‌Martensitic stainless steel‌: such as 4Cr13MoV, 3Cr17, 5Cr13, etc., have good corrosion resistance and mechanical properties, and are suitable for manufacturing molds with higher requirements. ‌Precipitation hardening stainless steel‌: such as 17-4PH, AISI630, etc., have excellent corrosion resistance and weldability, as well as high strength and hardness, are suitable for manufacturing complex and precision molds.

 

‌High heat-resistant and high-hardness stainless steel‌: such as 8566, W360, etc., have excellent heat resistance, hardness and toughness, and are particularly suitable for hot forging molds.
‌Other stainless steel materials‌: such as 420 medium carbon high chromium corrosion-resistant plastic mold steel, 9Cr18 high hardness and high wear-resistant stainless steel, etc., are also commonly used to manufacture different types of molds. These stainless steel mold materials have their own characteristics, and when choosing, they should be weighed according to specific needs and conditions to ensure performance and service life of mold.
‌Overview of plastic mold shrinkage ratio‌
‌Definition‌: Plastic mold shrinkage ratio, or plastic shrinkage rate, refers to percentage of difference between size of plastic part at molding temperature and size after it is taken out of mold and cooled to room temperature. It reflects degree of size reduction of plastic part after cooling.
‌Influencing factors‌: Plastic shrinkage is affected by many factors, including plastic type, molding conditions, mold structure, shape of plastic parts, complexity of internal structure and whether there are inserts.
‌Common plastic shrinkage ratio‌: Different plastics have different shrinkage rates, such as PP shrinkage is 1.0-2.5%, PMMA is 0.1-0.4%, PC is 0.5-0.7%, etc. In practical applications, if manufacturer does not specify, middle value is usually taken as a reference.
‌Importance‌: In plastic mold design, accurate grasp of shrinkage of plastics is crucial to determine size of each part of mold cavity to ensure production of qualified plastic parts. In summary, shrinkage ratio of plastic molds is an important parameter in plastic processing. It is affected by many factors and plays a key role in mold design.

 

‌Plastic mold manufacturing process can be summarized into following key steps: Plastic mold manufacturing process can be summarized into following key steps:

1. Accepting tasks and design preparation: ‌Accept task book and clarify requirements of plastic parts. Collect, analyze and digest original data, including part design, molding process, etc.

‌2. Mold design:‌ Detailed analysis and design based on product drawings and samples. Determine mold type, structure, material and molding method, etc. Complete design documents such as engineering drawings and assembly drawings.

3. Mold manufacturing:‌ Mold manufacturing and processing according to design documents. Including forging mold blanks, machining, heat treatment, grinding, polishing and other processes.

4. Mold inspection and mold trial:‌ Inspect completed mold. Conduct mold trials to verify performance of mold and quality of product.

5‌, Mold modification, mold repair and maintenance:‌ Make necessary modifications and adjustments based on results of mold trial. Maintain mold to ensure long-term performance.
This process is based on product drawings and samples provided by customer, and is combined with actual production needs.
Ranking and introduction of world's top ten injection molding machine manufacturing companies

1. Haitian HT: Haitian Group was founded in 1966 and has now developed into a large multinational company with multiple manufacturing companies and subsidiaries.

2. YIZUMI: Guangdong YIZUMI Precision Machinery Co., Ltd. focuses on manufacturing of compression molding equipment, integrating design, research and development, and production.

3. LK: Guangdong LK Plastic Machinery Intelligent Manufacturing Co., Ltd. is mainly engaged in research and development, manufacturing and sales of precision injection molding machines.

4. Demag: Founded in Germany, it enjoys a high reputation worldwide. It was acquired by Sumitomo in 2008 and is famous for its excellent injection molding technology.

5. KraussMaffei: Founded in Germany in 1838, it is a large-scale manufacturer of plastic and rubber processing machinery and equipment under China National Chemical Corporation.

6. Other brands: Husky, ENGEL, BORCHE, NISSEI, TEDERIC are all well-known injection molding machine manufacturers in the industry, providing high-quality injection molding machine products. These companies have significant influence and market position in the field of injection molding machine manufacturing.