Explanation of terms related to injection molding (3)

1. What is the dimensional relationship between the nozzle of the injection machine and the sprue of the injection mold?

In order to ensure that there is no overflow between the main channel and the nozzle of the injection machine during injection molding, which will affect the demoulding. When designing the mold, the spherical surface at the beginning of the sprue must be slightly larger than the spherical radius of the nozzle head of the injection machine, as shown in Figure 4.10, that is, R is 1 ~ 2mm larger than R. The diameter of the small end of the main flow channel is slightly larger than that of the nozzle, that is, D is 0.5 ~ 1mm larger than D.

2. How many installation forms of injection mold and injection machine?

The movable mold and fixed mold fixing plate of the injection mold shall be installed on the movable mold and fixed mold respectively. There are two methods for installing the mold on the injection machine: one is to fix it directly with screws; The screw holes on the mold fixing plate and the injection machine template should be completely consistent. For large molds with large weight, it is safer to directly fix them with screws; The other is fixed with screws. As long as there is a screw hole near the outside of the pressing plate where the die fixing plate needs to be placed, the pressing plate can be fixed. Therefore, the fixing of the pressing plate has great flexibility.

3. How to check the injection machine according to the maximum injection volume?

The maximum injection volume refers to the maximum volume of plastic injected by the injection machine at one time. When designing the mold, ensure that the total injection volume required for molding plastic parts is less than the maximum injection volume of the selected injection machine, that is: (4.3) where

——The maximum injection volume allowed by the injection machine, g or cm.

——The utilization coefficient of the maximum injection volume of the injection machine is generally taken as 0.8;

——The mass or volume of plastic required by the gating system, g or cm;

——The mass or volume of a single plastic part, g or cm;

——Number of cavities.

4. How to check the injection pressure of the injection machine?

The injection pressure required for plastic molding is determined by the variety of plastic, the type of injection machine, the form of nozzle, the shape of plastic parts and the pressure loss of gating system. For plastics with high viscosity and plastic parts with thin shape and long process, the injection pressure should be higher. As the pressure loss of plunger injection machine is greater than that of screw type, the injection pressure should also be greater. The verification of injection pressure is to verify whether the rated injection pressure of the injection machine is greater than the injection pressure required for molding.

5. When selecting the injection machine, which installation parts should be checked for relevant dimensions?

In order to make the injection mold can be smoothly installed on the injection machine and produce qualified plastic parts, the dimensions related to the installation of the injection machine and the mold must be checked when designing the mold. Generally, the parts that should be checked when designing the mold include the size of nozzle, the size of locating ring, the maximum and minimum thickness of the mold and the size of mounting screw hole on the template

6. What is plastic?

Plastic is composed of high molecular synthetic resin as the basic raw material and a certain amount of additives. It is plastic under a certain temperature and pressure to make it into a material with a certain structural shape and can maintain its shape at room temperature.

7. What are the ingredients of plastics?

Plastics are composed of resin and additives (or additives). Resin is the main component, which determines the type of plastic (thermoplastic or thermosetting) and basic properties (such as thermal properties, physical properties, chemical properties, mechanical properties, etc.). The function of additives is to improve the performance of molding process, improve the performance of plastic parts and reduce costs. Additives include fillers, plasticizers, colorants, lubricants, stabilizers, curing agents, etc

8. According to the molecular structure and thermal properties of resins in plastics, which kinds of plastics are divided and what are their characteristics?

According to the molecular structure and thermal properties of resins in plastics, plastics are divided into two categories: Thermoplastics and thermosetting plastics.

 (1) Characteristics of thermoplastics: 1) the molecular structure of the resin is linear or branched chain structure. 2) it softens and melts when heated to become a flowable viscous liquid. In this state, it can be molded into a plastic part with a certain shape, and maintain the shaped shape after cooling. If heated again, it can soften and melt, and can be molded into plastic parts of a certain shape again, which can be repeated many times. 3) In the above process, there are only physical changes but no chemical changes.

 (2) Characteristics of thermosetting plastics: 1) the molecular structure of the resin is finally the body structure. 2) At the beginning of heating, its molecules are linear structure, plastic and soluble, and can be molded into plastic parts of a certain shape. When heating continues, the main chains of linear polymer molecules form chemical bond bonding (i.e. cross-linking), and the molecules are in a network structure. When the temperature reaches a certain value, the cross-linking reaction further develops, and the molecules finally change into body structure. The resin becomes neither molten nor dissolved, and the shape of the plastic part is fixed and will not change. This process is called curing. If reheated, it will no longer soften and plastic. 3) In the above forming process, there are both physical and chemical changes.

9. What are the main properties of plastics?

Plastic has many excellent properties, which makes it widely used in various fields. Its main performance includes:

 (1) density is small: Plastic density is generally between 0.83 and 2.2g/cm3, only steel 1/8 to 1/4, the density of foamed plastics is smaller, and its density is generally less than 0.01g/cm3. The density of plastic is small, which is of great significance for reducing the weight of mechanical equipment and energy saving, especially for vehicles, ships, aircraft and spacecraft.

 (2) High specific strength and stiffness: the absolute strength of plastic is not as high as that of metal, but the density of plastic is small, so the specific strength is low（ σ b/ ρ）、 Specific stiffness (E)/ ρ） Quite high. In particular, reinforced plastics made of various high-strength fibrous, flake and powdery metal or non-metal fillers have higher specific strength and stiffness than metals.

 (3) Good chemical stability: most plastics have good resistance to acid, alkali, salt, water and gas. Under normal conditions, they do not react with these substances.

 (4) Good electrical insulation, heat insulation and sound insulation.

 (5) Good wear resistance and self lubrication: plastic has small friction coefficient, good wear resistance and good self lubrication. In addition, it has high specific strength and low transmission noise. It can work effectively under liquid medium, semi dry or even dry friction conditions. It can be made into machine parts such as bearings, gears, cams and pulleys. It is very suitable for occasions with low speed and low load.

 (6) Strong adhesion.

 (7) Good molding and coloring properties.

10. What is the orientation behavior in plastic molding?

The orientation behavior of plastics is the phenomenon that polymer molecular chains tend to be arranged in parallel along the stress direction under the action of stress. Orientation can be divided into two cases:

 (1) Flow orientation of solid filler in injection and pressure injection molded plastic parts; (2) Flow orientation of polymer molecules in injection and pressure injection molded plastic parts.