Thin-wall injection molding : What you need to know

Most food packaging uses thin-wall injection molding technology. Thin-wall injection molding products have attracted much attention due to their advantages such as low part weight, small production scale, low material expenditure and short molding cycle.

The molding processes of plastic packaging boxes mainly include injection molding, extrusion molding, blow molding, etc. Among them, injection molding has a simple process, convenient operation, and strong repeatability of molded products. It is the most commonly used molding process for plastic packaging boxes.

Injection molding, also known as injection molding or injection molding. This is an important polymer material molding processing method. Injection molding products are widely used, and their total amount accounts for about 80% of engineering plastic products and 30% of the total plastic products.

In a simple view, when the wall thickness is less than 1mm, it is called thin wall. More comprehensively, the definition of thin wall is related to the process/wall thickness ratio, the viscosity of the plastic and the heat transfer coefficient.

The process L from the main channel of the mold to the farthest point of the finished product, divided by the wall thickness t of the finished product, is called the process/wall thickness ratio. When L/t>150, it is called thin wall. If the thickness of the process is inconsistent, it can be calculated in sections as shown in Figure 1.

Thin-wall injection molding technology is also called thin-wall plastic injection molding technology. The definition of thin wall is related to the process/wall thickness ratio, the viscosity of the plastic and the heat transfer coefficient. There are currently three main definitions of this technology:

1. The ratio of flow length to thickness L/T, that is, the ratio of the flow length L from the melt entering the mold to the farthest point of the cavity that the melt must fill to the corresponding average wall thickness T is 100 or 150 or more for thin-wall injection molding;

2. The thickness of the molded plastic part is less than 1 mm, and the projected area of ​​the plastic part is more than 50 c㎡;

3. The wall thickness of the molded plastic part is less than 1 mm (or 1.5 mm), or t/d (plastic part thickness t, plastic part diameter d, for disc-shaped plastic parts) is less than 0.05. Injection molding is defined as thin-wall injection molding.

From this, we can see that the critical value of the definition of thin-wall injection molding will also change, and it should be a relative concept.

The mold wall is cold. When the molten metal fills the mold cavity, the mold wall will form a solidified layer, thereby reducing the thickness of the flow channel. This situation is more serious when the wall thickness is thinner. 1mm wall thickness has a 0.2mm thick solidified layer, and the flow channel is 0.6mm thick. 0.5mm wall thickness has a 0.2mm thick solidified layer, and the flow channel is 0.1mm thick. When the filling is not completed, if the flow channel disappears due to the excessive thickness of the solidified layer, the finished product will not be fully filled.

Injection molding machine is the basic mechanical equipment for injection molding, including injection molding system, clamping system, hydraulic transmission system, electrical control system, etc. As for the injection molding system, it should ensure that the plastic raw materials can be evenly injected into the mold under pressurized conditions; the clamping system is used to realize the locking of the movable mold and the fixed mold of the injection mold, and it should ensure that there is a large enough clamping machine to prevent overflow; the hydraulic transmission and electrical control system should ensure that the injection molding machine has sufficient power to achieve normal operation, and the sensitive response program controls each action of the injection molding machine.

Compared with traditional injection molding machines, thin-wall injection molding machines have higher performance requirements. The injection pressure should be higher, above 200MPa; the injection speed should be faster, above 200mm/s; the clamping force should be greater, and the clamping force provided by the clamping mechanism on the projected area of ​​the product per square inch is 5-8 tons.

The injection mold includes the pouring system, cooling system, temperature control system and demolding and ejection system. When designing the injection mold of the thin-walled plastic packaging box, it is necessary to combine the purpose and appearance of the packaging box for reasonable design. The mold performance requires high strength, good impact resistance, high cooling performance, and good exhaust performance.

High-fluidity plastics should be used. The melt index (MI) of PP plastics is as high as 60 (g/10min), such as Basell's Moplen RP1086. The reason why many finished products use PS/ABS is that the toughness of PC and the fluidity of ABS are required, which is also suitable for thin-wall injection molding.

At present, the plastic raw materials that are more widely used in thin-wall injection molding are polystyrene (PS), polypropylene (PP), polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), PC/ABS blends and PA6. These plastic raw materials usually have good fluidity, large flow length, high impact strength, high heat deformation temperature, high thermal stability, low directionality and good dimensional stability.

Taking polypropylene (PP) as an example, polypropylene has good chemical stability to most acids, alkalis, solvents, oils and other chemicals except oxygen, and the chemical stability increases with the increase of crystallinity. For tableware, because they are used to hold high-temperature food, they have higher requirements for the properties of materials under high temperature conditions. The melting point of polypropylene can reach 175°C, and it can withstand hot water cooking and disinfection. Even when not subjected to external force, its products will not deform at 150°C.

For thin-walled injection-molded plastic packaging products, compared with ordinary injection-molded products, on the one hand, they have the difficulties of injection molding of thin-walled injection molded parts, such as warping, material shortage, pores, etc.; on the other hand, they also have the difficulties of injection molding of packaging box products, such as high appearance requirements and certain products requiring higher precision. The production and processing of packaging plastic products are developing in the direction of high speed, high precision and high automation, and there is still a lot to explore in the future.

Injection molded cups or lunch boxes are adding nucleating agents, which generally require rigidity enhancement, rigid-toughness balance, transparency enhancement or both. Here is a recommended nucleating agent for thin-walled injection-molded lunch boxes-Longpu 9086, which has independent intellectual property rights, a doctoral team leading product innovation, continuous iteration of product upgrades, rich technical reserves, and stable use by well-known listed companies.

Thin-wall injection molding is mainly used in:

According to research, thin-wall injection molding is widely used in food containers, medicine boxes, cosmetics, stationery and even multimedia packaging. For example: disposable packaging boxes, cosmetic bottles and other daily necessities.