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Exploring Injection Molds: Unlocking the Core Code of Plastic Molding

• 2025-07-10

Exploring Injection Molds: Unlocking the Core Code of Plastic Molding

    In the vast kingdom of plastic products, injection molds can be regarded as the magical craftsmen behind the scenes. They silently determine the "birth appearance" of plastic products and profoundly influence product quality and production efficiency. They are undoubtedly the key players in the field of plastic molding.

Automobile radiator crossbeam accessories Manufacturer in China (jfmoulds.com)

I. The exquisite Structure of Injection Molds

    The structure of an injection mold is like a precise industrial castle, with all parts working in coordination and none can be missing.

1. Forming System: The "Engraver" that endows Products with soul

    Composed of a cavity and a core, the cavity acts like a concave mold, meticulously shaping the outer surface of the product. The core is like a punch, precisely shaping the inner surface of the product. Sometimes inserts are also embedded to facilitate processing, replacement or maintenance. For instance, when manufacturing plastic cups, the cavity determines the outline of the cup, while the core defines the space inside the cup.

    High-quality die steels such as P20, H13, and S136 are often used, and they undergo heat treatment to enhance wear resistance. The surface is polished or electroplated, like chrome plating, to reduce resistance during demolding and make the product surface smooth and beautiful.

2. Gating System: The "high-speed Channel" for Plastic melt

    The main runner connects the injection molding machine nozzle and the runner, and its conical shape facilitates demolding. The distribution channel distributes plastic to various cavities in different shapes, such as circular, trapezoidal and U-shaped. The gate is a narrow passage for molten material to enter the cavity. Different types of gates have different functions. Side gates are suitable for most products. Point gates are often used in three-plate molds and can automatically cut off the gates. The latent gate is hidden inside the product. Fan-shaped gates are suitable for wide and thin parts, reducing internal stress. The cold material well is responsible for collecting the cold material at the front end to prevent the gate from being blocked.

    Hot runner systems have significant advantages in mass production. They keep the runner in a molten state all the time, without generating cold material, saving materials and shortening the molding cycle. However, the mold cost is high and precise temperature control is required during maintenance.

3. Ejection System: The facilitator for the "birth" of products

    The ejection pin is the most commonly used ejection method and is suitable for most products. The push plate is used for deep cavities or thin-walled parts to prevent the product from being punctured. Pipe jacking is specifically used for the ejection of tubular products. The air top uses compressed air to assist demolding and is suitable for soft plastics. For instance, when manufacturing plastic toys, a thimble might be used to push them out. For the production of thin-walled plastic containers, a push plate needs to be pushed out.

    The ejection positions should be evenly distributed to prevent product deformation. The ejection stroke should be reasonable to avoid insufficient or excessive ejection.

4. Cooling System: The "Temperature Regulation Master" of Molds

    The cooling water channels are arranged around the cavity and core, and are usually processed by drilling. In complex areas, water barriers will be set up to enhance the cooling effect, and jet pipes will be used for deep cavity cooling. For instance, when manufacturing large plastic casings, the rationally distributed cooling water channels can enable the products to cool down quickly and evenly.

    The distribution of cooling water channels should be uniform to avoid local overheating, and they should be as close as possible to the surface of the cavity, but not too close to affect the strength of the mold.

5. Guidance and Positioning System: "Precise Mating Device" for molds

    The guide pins and guide sleeves provide guiding functions to ensure the precise alignment of the moving mold and the fixed mold. High-precision molds will also adopt conical surface positioning to prevent lateral offset and ensure product accuracy. For molds used in the production of precision electronic components, the guiding and positioning system is of vital importance.

6. Exhaust System: The "Scavenger" for Eliminating trapped air

    Exhaust on the parting surface is achieved by opening shallow grooves (0.02-0.05mm) on the parting surface. The ejector pin exhaust is discharged through the gap between the ejector pins. Special exhaust inserts, such as exhaust steel (porous metal), will also be set up at the parts prone to trapped gas to prevent the product from having trapped gas defects such as charring and bubbles.

7. Lateral core-pulling System: The "Magician" Solving Complex Structures

    The slider is driven by inclined guide pins to achieve horizontal core-pulling. The inclined top is achieved by the movement of the ejection plate to complete the inclined core-pulling. Hydraulic/cylinder core-pulling is suitable for long-distance or complex core-pulling. For instance, when manufacturing plastic pipe fittings with side holes, a side core-pulling system is required to form the side holes.

8. Mold Base: The "solid foundation" of molds

    The fixed mold fixed plate is connected to the fixed template of the injection molding machine, the fixed mold fixed plate is connected to the mobile template of the injection molding machine, the support plate increases the rigidity of the mold, and the ejection plate installs ejection mechanisms such as ejector pins to support and fix each component of the mold, making the mold a stable whole.

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

Ii. Diverse Classifications of Injection Molds

    Injection molds are like a large family and can be classified into various types based on different standards.

1. Classification by mold structure

    Two-plate mold: The simplest structure, consisting of A fixed mold (plate A) and a moving mold (Plate B), the gating system and the product are demolded on the same parting surface. It is suitable for simple-structured products such as plastic LIDS and ordinary boxes, and is appropriate for single-cavity or symmetrical multi-cavity layouts. The advantages are low cost, short manufacturing cycle and convenient maintenance. The disadvantage is that the gate needs to be trimmed manually and there is a lot of waste in the runner.

    Three-plate mold: It has an additional release plate compared to the two-plate mold, forming a three-layer structure of fixed mold → release plate → moving mold. The gating system and the product are demolded at different parting surfaces, and the gate can be automatically cut off (such as point gates). It is suitable for products with multiple gates or complex injection methods, such as precision electronic components and multi-cavity molds. The advantages are that the gate automatically separates, reducing manual pruning and being suitable for complex glue injection. The disadvantages are complex structure, high cost and long mold opening stroke.

    Hot runner molds: The runner of the gating system remains in a molten state all the time, with no cold material produced, and is directly injected into the cavity. They are divided into fully hot runners and semi-hot runners. It is suitable for mass production, such as bottle caps, medical supplies, auto parts, etc. The advantages are no runner waste, material saving, shortened molding cycle and improved degree of automation. The drawback is that the mold cost is high and maintenance is complex (precise temperature control is required).

2. Classification by the number of cavities

    Single-cavity mold: Only one product is produced in one injection, suitable for large-sized products or the trial mold stage, such as molds for large plastic sculptures.

Multi-cavity molds: Produce multiple identical products in one injection (such as 2-cavity, 4-cavity, 8-cavity, etc.), suitable for large-scale production of small parts, such as buttons and small plastic ornaments.

    Family molds: Different shapes of products (such as matching upper and lower covers) are produced within the same mold, but the filling and cooling of each cavity need to be balanced.

3. Classification by the gating system

    Cold runner mold: The runner cools together with the product, and runner waste needs to be trimmed. It has a low cost and is suitable for simple injection molds and low-cost production processes.

    Hot runner molds: The runner remains molten, with no waste, making them suitable for mass production and high-quality product requirements.

    Warm runner mold: It lies between cold and hot runners, with controllable temperature, and is suitable for injection molding of special materials (such as PVC).

4. Classification by core-pulling method

    Side-free core-pulling mold: The product has no side holes or upside down, and can be directly ejected from the mold, such as simple flat plastic products.

    Slider core-pulling die: The slider is driven by inclined guide pins or hydraulic cylinders to form side holes/grooves, such as plastic housing dies with side grooves.

    Inclined top core-pulling mold: It achieves inclined core-pulling through the movement of the ejection plate and is suitable for internal inverted structures, such as some plastic container molds with internal clamps.

Iii. Material Selection for Injection Molds

    The material of injection molds is a key factor in determining the performance of the molds, and factors such as the type of plastic, production batch, and product complexity need to be comprehensively considered.

1. Die steel (mainstream material）

    Pre-hardened steel: It has been pre-hardened before leaving the factory, with a hardness ranging from HRC28 to 42. It has good processing performance and is often used in the production of molds with small batch sizes and general precision requirements, such as ordinary plastic daily-use molds.

    Quenched and tempered steel: It requires heat treatment to enhance hardness and wear resistance. Its hardness can reach HRC50-60. It is suitable for molds with large production batches and high requirements for precision and wear resistance, such as molds for automotive plastic parts.

    Corrosion-resistant steel: Containing elements such as chromium and nickel, it has excellent corrosion resistance and is used for molding corrosive plastics (such as PVC) or molds with high appearance requirements and rust prevention, such as molds for medical plastic products.

2. Aluminum alloy: 

    Low density, light weight, good processing performance, excellent thermal conductivity, and short forming cycle. However, it has low hardness and poor wear resistance, and is suitable for the production of small-batch, large-sized or complex-shaped molds with low requirements for dimensional accuracy, such as the rapid trial production of large plastic toy molds.

3. Beryllium copper alloy: 

    It has excellent thermal conductivity, several times that of die steel, which can significantly shorten the forming cycle and improve production efficiency. It has good strength and hardness, and its wear resistance is superior to that of aluminum alloy. It is often used in molds with high requirements for cooling speed, precision and surface quality, such as precision electronic product shell molds.

    Injection molds, the core force in the field of plastic molding, play an irreplaceable role in industrial production with their exquisite structure, diverse classification and meticulous material selection, and continuously drive the plastic products industry.