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1. Overview of EPP materials
1.1 Principle of EPP Foam Molding
The forming principle of EPP is to use polypropylene (PP) particles as the main material, catalyst, foam stabilizer, foaming agent and other auxiliary materials, put these raw materials into the pressure tank, after a certain period of pressure holding treatment (the beads are filled with air of a certain pressure) to make them expand evenly, and then use a spray gun to inject compressed air into the mold of the EPP molding machine, heat the mold, then foam molding, and then cool down through cooling water to demould, and finally dry and set. In addition, it is also possible to perform secondary die-cutting of the formed EPP material (only simple cavity die-cutting, not curved surface die-cutting). Finished EPP products are widely used to make cushioning and acoustic insulation layers that do not require high precision.
1.2 Epp Foam Products Production Processes
The EPP processing process includes 6 main operation steps: preheating and clamping, feeding, steam entering the mold, cooling, demoulding, drying and shaping, as shown in the figure.
(1) Preheat the mold and close the mold. The mold is preheated first, so that the surface temperature of the mold reaches the melting point of PP particles. The PP particles are molded into a fixed shape, which can be designed and customized according to the different characteristics of the product.
(2) Feeding. First, the mold is brought to the temperature required by the process, and then the EPP particles are blown in from the mold feed port with high-speed air. At the same time, ensure that the air outlet volume is greater than the air inlet volume, so that the air outlet is unobstructed, so that the raw materials fill all parts of the mold.
(3) Steam is passed into the mold. The process includes steam washing, transverse steam, pressure holding. Steam scouring means that the steam is washed from top to bottom, and the air is discharged from the steam chamber and the condensate flows out of the steam chamber; Transverse steam means that steam sweeps from one side of the steam chamber to the material, penetrates the material and reaches the other side; Pressure holding refers to the need for pressure holding or double-sided evapotranspiration after transverse steaming.
(4) Cooling. After the introduction of steam, the internal temperature of the mold generally reaches 140 °C, and in order to facilitate the release of the product, cooling water needs to be introduced to reduce the temperature in the film to 70 °C.
(5) Demoulding. The pressure inside the mold is relieved, and the temperature drops to a temperature suitable for mold release, and the mold removal operation can be carried out.
(6) Drying and shaping. Set the temperature of the drying chamber to 60~80 °C to keep the air dry and well ventilated.
2. EPP material properties
2.1 Color
Generally speaking, there are 3 main colors of EPP: white, pink, and black. White is an ordinary material, mostly used for ordinary packaging; Pink is a functional material, with antistatic function, mostly used in electronic product packaging; Black is a flame retardant and is mostly used for auto parts and logistics pallets. In addition, there are other colors that are not commonly used, such as gray, yellow, blue, etc., which need to be imported and need to meet a certain order quantity, and are generally less used.
2.2 Foaming rate and density
The foaming rate of PP refers to the multiple of the volume increase before and after PP foaming, the foaming rate is generally 5~45 times, and the density is 20.0~180.0 kg/m3.
The conversion relationship between the foaming rate and density of EPP: foaming ratio = EPP atomic weight/density, density = EPP atomic weight/foaming rate, where the EPP atomic weight is 900.
2.3 Foaming mold size limit
The size of EPP is generally limited by the equipment and mold frame, and the size (length× width) of the largest EPP product that can be produced in China is 2,600 mm× 1,600 mm.
3. Application in the packaging of auto parts
With the packaging industry’s increasing requirements for lightweight and green packaging materials, EPP materials with good energy absorption and lightweight have become the first choice for some high-value, complex-shaped electrical parts packaging materials. The application of EPP material can not only develop personalized positioning devices according to the shape of the parts to ensure the quality of the parts, but also:
It is possible to reduce the use of disposable protective materials for parts. At present, according to the application characteristics and requirements of auto parts production in packaging appliances, EPP materials are used to carry out special packaging design for some auto parts products, and PP hollow boards, injection molding boxes and other materials that have been used will be gradually replaced.
The application of EPP material in the packaging of auto parts has three advantages: first, the packaging container and lining made of high-strength EPP material have excellent elasticity, impact resistance and pressure resistance, which can reduce or reduce the impact force generated by the outside world, and play an excellent role in protecting the goods; Second, the EPP material has a certain hardness and anti-friction performance, and the separator material made of it can be in direct contact with the product, which can not only prevent friction, but also improve the packing efficiency; Thirdly, the addition of electrostatic protection materials can achieve the effect of electrostatic protection, so as to effectively protect electronic components. It can be seen that EPP materials can be used in the packaging design of a variety of automotive parts, including small instrument products and large roof racks and sunroofs.
