Plastic material selection principles

The successful application of plastic materials in products cannot be achieved without the following 4 major aspects.

1) The choice of plastic material.

2) Design of plastic parts.

3) Design of plastic molds.

4) Control of injection molding process.

Among them, the selection of plastic materials and the design of plastic parts are the responsibilities of product design engineers; the design of plastic molds and the control of injection molding processes are the responsibilities of mold engineers and injection molding operators. Product design engineers do not need to be proficient in every detail of injection molds and injection processes, but at least need to understand and be familiar with them, especially the knowledge related to the quality and cost of plastic parts.

It is very important to note that these four aspects do not exist in isolation, but need to be organically integrated into a whole, product design engineers, mold engineers, injection mold process personnel and plastic material supplier technical engineers to form a team to communicate and cooperate fully and effectively to ensure the successful application of plastic materials in the product. Among them, the role of plastic material suppliers technical engineers are often overlooked, they are experts in plastic materials, plastic material properties, applications and advantages and disadvantages of the very understanding, they can plastic material selection and plastic parts design to make useful suggestions. Therefore, in product development, the earlier the plastic material supplier technical engineers involved the better.

There are many plastic materials with different properties, and the selection of suitable plastic materials becomes very difficult. In general, the choice of plastic materials is usually used in the past products used materials (not to have merit, but to seek no fault), or recommended by plastic material suppliers, mold manufacturers, but this choice is often not the best choice. The selection of plastic materials requires product design engineers not only a comprehensive understanding of the performance parameters of plastic materials, but also need to consider the product load conditions, application environment, function, appearance, assembly and cost factors.

The selection of plastic materials must be determined at the early stage of product design, because the difference in the performance of plastic materials may cause the design of plastic parts completely different, including the size of the wall thickness, reinforcement thickness, screw fit hole size and assembly process selection, etc.; in addition, the different shrinkage of plastic materials also makes the same set of injection molds to process different plastic size differences, once the injection mold begins processing Once the injection mold starts processing, the possibility of changing the plastic material becomes very small.

1. Load condition of plastic parts

When selecting a plastic material, the product design engineer should carefully analyze the load on the part in the actual application, including the size, type and duration of the load, and then compare the physical properties of the plastic material to select the appropriate material. Five typical load conditions and the engineer should consider the plastic material properties are shown in Table 1.

Table 1 Typical load conditions and plastic material properties to be considered

2. Product use environment

Product use environment including ambient temperature and contact medium. Plastic materials can only be in a certain temperature range to preserve performance, below or above the temperature range, plastic parts in the mechanical stress or chemical attack is very vulnerable to become fragile and failure, and different plastic materials working temperature range is not the same.

As shown in Figure 1, the tensile strength of a brand of POM decreases significantly at an ambient temperature of 80°C, compared to an ambient temperature of 23°C. Therefore, it is necessary to specify the temperature of the product's operating environment, including the maximum and minimum operating temperatures, as well as the long-term operating 20 temperature. When considering the temperature, it is also necessary to consider the ambient temperature of the product during assembly, post-processing, and transportation.

The selection of plastic materials should also consider the various chemical media that the product may be exposed to in the manufacturing and use environment, such as mold release agents, stamping oils, degreasers, lubricants, cleaning agents, lubricants, printing dyes, adhesives, etc. In addition, the impact of water, rain, ultraviolet light, sand on plastic materials should be considered. Reasonable choice of plastic materials to ensure compatibility with the environment.

3. Price

In product design, cost is always a very important factor, the selection of plastic materials need to take into account the price of the material, in the conditions of meeting the product application requirements, try to choose the cheaper plastic materials. Figure 2 shows the price comparison of common plastic materials. It should be noted that, due to the different densities of plastic materials, in comparing the cost of plastic parts is not only to compare the price of raw materials, but also need to compare the density.

4. Assembly requirements

The selection of plastic materials needs to take into account the way in which the parts are assembled into the product, and choose the right material according to the assembly requirements of the product. Some plastic materials are suitable for gluing connection, some are suitable for ultrasonic welding, while the snap connection requires plastic materials with sufficient strength, flexibility and dimensional stability. For example, the upper and lower cover of a product requires ultrasonic welding assembly, the upper cover of the plastic material has been selected for ABS, then the lower cover of the plastic material should have a better ultrasonic welding fusion performance with ABS, can be ABS or PMMA, etc..

5. Dimensional stability

In the selection of plastic materials, suitable plastic materials should be selected according to the dimensional stability requirements of the parts and products.

6. Appearance

Whether the parts have transparency requirements, whether they need nibbling, whether they need electric locking, whether there are requirements for surface roughness, etc., these appearance requirements will affect the choice of plastic materials.

7. Safety specifications

When choosing plastic materials, consider whether the product needs to meet certain safety norms or certain certification requirements, such as 3C certification, FDA, USDA, UL, etc.; also consider whether the product needs to meet the requirements of UL94 flame retardant grade, flame retardant grade is divided into 5VA, 5VB, V-0, V-1, V-2, HB. different plastic materials have different flame retardant grades, and even the same Different plastic materials have different flame retardant grades, and even the same plastic material has different flame retardant grades because of different material grades.