Structural features of EPP molds

Cavity and core: The cavity determines the outer contour of the EPP product, and the core is used to shape the internal shape of the product, including holes, depressions and other structures.

The high-precision matching of the two is the basis for ensuring the dimensional accuracy and surface quality of EPP products.

For example, in the EPP mold for manufacturing car seats, the cavity and core need to accurately simulate the curve and structure of the seat to ensure that the EPP seat parts after molding are perfectly matched with the car interior and provide a comfortable driving experience.

Parting surface: The parting surface is the interface where the mold is separated during the opening and closing process. Its selection is directly related to whether the EPP product can be demolded smoothly, the complexity of mold manufacturing and the quality of the product.

A reasonable parting surface is usually selected at the maximum contour of the product shape. At the same time, it is necessary to consider keeping the molded product on the side of the moving mold as much as possible to facilitate subsequent demolding operations.

Taking the production of large EPP packaging box molds as an example, the parting surface is often set in the middle of the box body, which can not only ensure the complete demolding of the box body after molding, but also simplify the mold structure and reduce manufacturing costs.​

Gating system: The gating system is responsible for introducing the heated and molten EPP raw materials into the mold cavity. It consists of main runners, branch runners and gates.

The main runner connects the injection molding equipment and the mold, the branch runner evenly distributes the melt from the main runner to each cavity, and the gate is the entrance for the melt to enter the cavity. Its size, shape and position have a significant impact on the flow rate of the melt, filling uniformity and product quality.

For some small and simple EPP product molds, direct gates may be used. The melt enters the cavity directly from the main runner. The structure is simple but may leave gate marks on the surface of the product; for EPP products with high appearance requirements or large and complex, such as EPP packaging molds for high-end electronic products, point gates or latent gates are often used to reduce the adverse effects of gates on the appearance of products. ​

Demolding system: Since EPP products shrink and adhere closely to the mold cavity and core surface during cooling, an effective demolding system is required to help the products leave the mold. Common demoulding methods include ejector demoulding, push plate demoulding and pneumatic demoulding.

Ejector demoulding is achieved by placing ejectors on the surface of the core or cavity, and the ejectors eject the product when the mold is opened; push plate demoulding uses a push plate to push the entire product out of the core, which is suitable for products with complex shapes that are difficult to demould with ejectors; pneumatic demoulding uses compressed air to form an air cushion between the product and the mold to achieve smooth demoulding of the product.

When producing EPP toy models, ejectors and pneumatic demoulding may be used in combination. First, the product is initially lifted from the core with an ejector, and then pneumatic demoulding is used to completely remove the product from the mold to avoid damage to the fine structure of the toy model. ​

Material selection for EPP molds​

Steel: For EPP molds with large production batches and extremely high requirements for mold strength and wear resistance, high-quality steel is often used, such as pre-hardened steels such as P20, 718, and NAK80.

These steels have good cutting performance and heat treatment characteristics. After proper heat treatment, the hardness can reach HRC30-40, which can meet the needs of long-term high-intensity production.

In the automotive industry, when manufacturing EPP car bumper molds, the use of P20 steel can ensure that the molds always maintain high precision and good surface quality during mass production, reduce mold wear and maintenance frequency, and improve production efficiency and product quality stability. ​

Aluminum alloy: Aluminum alloy materials have the advantages of low density, excellent thermal conductivity, and good processing performance. They are suitable for manufacturing EPP molds with requirements on weight, relatively small production batches, or high requirements on mold heat dissipation performance, such as 6061 and 7075 aluminum alloys. Aluminum alloy molds have a short manufacturing cycle and low cost, and good thermal conductivity helps to speed up the cooling speed of EPP products and improve production efficiency.

In some small hand-made custom EPP molds or rapid prototyping molds, aluminum alloys are widely used. For example, when making small EPP molds for architectural decoration, aluminum alloy materials are used for easy processing and flexible modification, which can quickly meet the diverse needs of different customers for shapes. ​

Other materials: In some special cases, other materials are used to make EPP molds.

For molds with low precision requirements and temporary use, wood can be used as a low-cost option, but its strength and durability are poor and it is only suitable for small-scale production or experimental purposes.

In addition, in situations where mold weight and cost restrictions are extremely strict and complex shapes need to be made, plastic or composite materials may be used, but these materials are relatively weak in strength and heat resistance and have limited application range. ​

EPP mold manufacturing process​

Design stage: With the help of computer-aided design (CAD) software, carefully construct a three-dimensional model of the mold according to the shape, size and performance requirements of EPP products.

In the design process, it is necessary to comprehensively consider the structural rationality of the mold, the optimal choice of the parting surface, the scientific design of the pouring system and the demoulding system, and other factors. At the same time, the mold strength and stiffness analysis is carried out to ensure that the mold can withstand high temperature, high pressure and other working conditions during the production process.

For example, when designing an EPP packaging mold for a high-end electronic product, the flow process of EPP raw materials in the mold cavity is simulated by CAD software, the pouring system design is optimized, and problems such as insufficient filling, excessive filling or weld marks are avoided, while ensuring that the mold structure strength meets the requirements of long-term use. ​

Processing link​

Mechanical processing: According to the designed mold drawings, the mold materials are finely processed using advanced equipment such as CNC machining centers, electrical discharge machining (EDM), and wire cutting machining (WEDM).

CNC machining can accurately manufacture complex shapes such as mold cavities, cores, and parting surfaces to ensure the dimensional accuracy and surface quality of the mold; EDM machining is suitable for processing complex shapes and fine structures that are difficult to complete by mechanical machining, such as narrow grooves and small holes in the mold; WEDM machining is mainly used to cut the outer contours of the mold and special shape parts. When manufacturing the core of the EPP mold, for the core with complex curved surfaces, the general shape is first processed by CNC, and then the details are accurately processed by EDM to ensure that the shape accuracy and surface roughness of the core meet strict requirements.​

Surface treatment: In order to improve the wear resistance, corrosion resistance and demolding performance of the mold, the mold needs to be surface treated after processing.

Common treatment methods include nitriding, hard chrome plating, polishing, etc. Nitriding can form a nitrided layer with high hardness and good wear resistance on the mold surface, significantly extending the service life of the mold; hard chrome plating makes the mold surface smoother, enhances demolding performance, and has a certain anti-corrosion ability; polishing is mainly used to improve the finish of the mold cavity and core surface, and reduce surface defects in the EPP product molding process.

For molds for producing high-end EPP packaging products, after polishing, the mold surface roughness can reach Ra0.01-0.05μm, making the surface of the molded EPP product smooth as a mirror, meeting the packaging requirements of high-end products. ​

Assembly and debugging: After cleaning and deburring the processed mold parts, assemble them strictly in accordance with the design requirements.

During the assembly process, the matching accuracy between the parts must be strictly controlled to ensure smooth opening and closing of the mold and accurate positioning.

After assembly, the mold is fully debugged, and the molding effect of the mold is tested through mold trials, including the dimensional accuracy, surface quality, and demolding of EPP products.

According to the results of the mold trials, the mold is adjusted and optimized in a targeted manner, such as adjusting the size of the pouring system, modifying the parameters of the demolding mechanism, etc., until the mold can stably produce EPP products that meet quality standards.

When debugging a new EPP foam board mold, if adhesion is found when the foam board is demolded, it is determined that the demolding slope is insufficient. By appropriately increasing the demolding slope, the demolding is smooth after another mold trial, and the quality of the foam board meets the standard. ​

Key points for the use and maintenance of EPP molds

Precautions for use

Temperature control: The molding of EPP raw materials needs to be carried out within a specific temperature range, so the heating and cooling temperatures of the mold must be strictly controlled when using the mold.

Generally speaking, the heating temperature of the EPP mold is between 120-180℃, and the cooling temperature is between 30-60℃.

Too high a temperature may cause EPP raw materials to decompose and products to deform, while too low a temperature will affect the molding speed and product quality. When producing EPP insulation materials, if the mold heating temperature is too high, the surface of the insulation material will turn yellow and bubble, seriously damaging the product quality; if the cooling temperature is too low, the insulation material will cool down for too long, reducing production efficiency. ​

Pressure regulation: During the injection of EPP raw materials into the mold cavity, the injection pressure must be precisely controlled.

Excessive pressure may cause mold deformation, flash on the product, and other problems, while too low a pressure will cause insufficient filling of the product.

According to the shape, size and mold structure of the EPP product, the injection pressure should be reasonably adjusted. For EPP products with complex shapes and thin walls, the injection pressure should be appropriately increased to ensure that the raw materials can smoothly fill all parts of the mold; for large products with thick walls, the injection pressure should be reduced to prevent the mold from being subjected to excessive pressure. ​

Demolding operation specifications: Follow the correct operation method when demolding to avoid damaging the mold and EPP products.

According to the predetermined demoulding sequence, start the demoulding system first, push the product out of the mold cavity for a certain distance, and then use manual or mechanical assistance to completely remove the product.

For fragile EPP products, such as thin-walled structures or products with fine surfaces, be extra careful when demoulding. If necessary, use a demoulding agent to assist demoulding, but pay attention to the amount of demoulding agent and the uniformity of application to avoid affecting the quality of the product.

When producing EPP crafts, due to the fine patterns and shapes on the surface of the crafts, special demoulding tools must be used for demoulding, and the demoulding agent must be evenly applied on the mold surface to ensure that the crafts are completely and intactly removed from the mold. ​

Key points for maintenance​

Daily cleaning: After each production, the EPP raw materials, demoulding agents and other impurities remaining on the mold surface and inside the cavity should be cleaned in time.

Compressed air, brushes and other tools can be used for cleaning. For stubborn dirt, special mold cleaning agents can be used for cleaning. Keeping the mold clean can prevent impurities from accumulating on the mold surface, affecting the quality of the product and the service life of the mold.

In the workshop producing EPP packaging materials, after get off work every day, a special person is arranged to use compressed air to blow away the debris on the surface of the mold cavity and core, and then wipe the mold surface with a brush dipped in cleaning agent to ensure that the mold can be used normally next time. ​

Regular inspection: Regularly conduct a comprehensive inspection of the mold, covering the mold appearance, dimensional accuracy, connection status of each component, working status of the pouring system and demoulding system, etc.

Check whether there are defects such as wear, cracks, deformation on the mold surface, measure whether the dimensions of the key parts of the mold are within the tolerance range, and ensure that the performance indicators of the mold meet the standards. If problems are found, they should be repaired and handled in time.

Check the EPP mold every other week, measure the mold cavity size with calipers and other measuring tools, check whether the ejector pin can move flexibly, and replace the ejector pin with a new one in time if the ejector pin is severely worn to ensure the normal operation of the mold. ​

Lubrication and maintenance: Regularly lubricate and maintain the movable parts of the mold, such as guide pins, guide sleeves, ejector pins, etc.

Choose suitable lubricants, such as lithium-based grease, lubricating oil, etc., and apply them on the surface of moving parts to reduce friction and wear, ensure smooth opening and closing of the mold, and normal operation of the demolding system.

Usually, the moving parts of the mold are lubricated and maintained at least once a week. Applying lithium-based grease evenly on the surface of the guide pin and guide sleeve of the EPP mold can effectively reduce the friction of mold opening and closing, extend the service life of the guide pin and guide sleeve, and improve the opening and closing accuracy of the mold. ​

Rust prevention: For EPP molds that have been idle for a long time, anti-rust measures should be taken. First clean the surface of the mold, then apply anti-rust oil or anti-rust agent, then wrap it with plastic film and store it in a dry and ventilated environment.

This can prevent the mold from rusting and avoid affecting the mold accuracy and service life due to rust. In the winter production off-season, after the EPP mold is thoroughly cleaned and rust-proofed, it is wrapped with plastic film and stored in the warehouse. When it is used in the peak season next year, the mold can still maintain good performance. ​