If the CPU is regarded as the core of the computer, the water meter movement is equivalent to the "CPU" of the water meter. The quality of the water meter movement is directly related to whether the flow rate of the water meter meets the standard, the length of service life of the water meter, and the quality of the water meter Bad and customer's reputation, in today's water meter industry, where hundreds of contention and fierce competition, customers' requirements for water meters are getting higher and higher, which puts higher requirements on the quality of water meter movements.
As we all know, the water meter movement is composed of two core components: the counting mechanism and the measurement mechanism, and more than 95% of the components of the movement are from injection molding.
The selection of injection materials is of great significance for injection molding. Different materials must correspond to different process conditions. In order to better improve the consistency of injection molded products, we need to further understand the injection molding process conditions and the interrelation and influence of various factors, analyze the various factors that affect the injection molding process, and propose control solutions.
1. Shrinkage
1.1 Plastic types
ABS, PP, PC and other thermoplastics with heat softening and cooling hardening characteristics will change their volume during injection molding, their internal stress will be strengthened, and the molecular orientation between plastic molecules will become stronger and violent Molecular movement will cause thermoplastics to have a stronger shrinkage rate than thermosetting plastics. The direct result is that plastic products will have more obvious shrinkage. Therefore, according to the structure of plastic parts, anti-shrinkage holes should be reasonably designed. For plastic precision parts, the If the shrinkage deformation is more demanding, the part structure should be modified in time to meet the requirements.
1.2 Plastic parts characteristics
Compared with metal parts, the thermal conductivity of plastics is relatively poor. During the injection molding cooling process, the plastic that contacts the surface of the mold cavity will be cooled faster, and the internal cooling of the plastic parts will be slower. A high-density solid layer will be formed inside the plastic part, which is also one of the roots of plastic parts prone to shrinkage deformation. Therefore, the temperature difference between the inside and outside of the plastic part is reduced during the cooling process, and the time for the plastic part to cool completely is shortened, which can effectively suppress the shrinkage deformation. For example: Put the molded plastic parts into water for rapid cooling.
1.3 Molding conditions
Factors such as equipment performance, mold temperature, melt characteristics, pressure size, and speed will affect the shrinkage rate of injection molding materials. Different factors will produce different shrinkage results. For example: when the mold temperature is high, the melt material The cooling process will be relatively long, which will eventually lead to large changes in the shrinkage and volume of plastic products. In the hot summer, turning on the ice water machine to cool the mold is a way to reduce the shrinkage of the material.
2. Liquidity
Generally speaking, the greater the molecular weight of the material, the better the fluidity. Before injection molding, the plastic composition and characteristics must be fully understood. The factors that affect the fluidity of plastics are as follows:
The first point of temperature, in general, the fluidity of the material is proportional to the temperature, the higher the temperature, the better the fluidity. However, there are exceptions. For example, the fluidity of PE and POM materials is not so affected by temperature, so for most materials, the fluidity of the material can be controlled by controlling the temperature of each section of the barrel.
The second point of pressure, when the injection pressure is increased, the shear force received by the material in the barrel will increase, which will increase the fluidity of the material.
The third point of the mold structure is whether it is an ordinary mold or a hot runner mold. In addition, the casting form of the mold and the size of the pouring port will have a greater impact on the fluidity of the material.
3. Crystallinity
Whether the thermoplastic material crystallizes in a condensed state can be divided into two categories: crystalline materials and non-crystalline materials. These two types of materials can be distinguished by visual inspection. Generally speaking, crystalline Materials are opaque or translucent, and amorphous materials are transparent, but there are exceptions. Generally speaking, the crystallization phenomenon will increase the brittleness and ductility of the material. The main factors that affect the crystallization of the material are temperature and pressure. The increase in temperature and pressure can increase the activity of the material molecules, thereby speeding up the formation of crystals.
Special attention should be paid to the temperature control of crystalline materials and non-crystalline materials in the injection molding process, otherwise injection molding defects are prone to occur.
4. Screw characteristics
The use of different screws for different plastics should also have clear requirements. The general situation is: transparent products (such as PC materials) must use chrome-plated screws. If nitride screws are used, the surface of the products is prone to black spots and yellowing; flame retardant grade Products must use chrome-plated screws. Chrome-plated screws are not suitable for production containing glass fiber products, because chrome plating is not wear-resistant; nitrided screws are not suitable for production of products containing flame retardant grades, because nitriding is not resistant to corrosion.
5. Material thermal performance and cooling rate
Different materials have different thermal conductivity properties. Plastics with high thermal conductivity require a shorter cooling time when they are finally cooled. Such materials will generally be demolded earlier. based on the material properties of thermal expansion and contraction, they need to be done after demolding. It is necessary to prevent obvious cooling deformation of the material. At this time, the cooling rate should be controlled within an appropriate range. In addition, the cooling rate of each plastic itself is also very different, so the cooling rate should be treated differently.