Injection molding, also known as injection molding, is an injection molding method. The advantages of the injection molding method are fast production speed, high efficiency, automatic operation, a wide variety of patterns, shapes from simple to complex, and sizes from large to small. Moreover, the product size is accurate, the product is easy to be updated, and it can form parts with complex shapes. Injection molding is applicable to mass production, products with complex shapes and other molding processing fields. 
At a certain temperature, the fully melted plastic material is stirred by a screw, and then injected into the mold cavity with high pressure. After cooling and curing, the molded product is obtained. This method is suitable for the mass production of parts with complex shape and is one of the important processing methods.
Basic information editing
The injection molding process can be roughly divided into the following six stages:
Mold closing, glue injection, pressure maintaining, cooling, mold opening, and product removal.
If the above process is repeated, products can be produced in batches and periodically. The molding of thermosetting plastics and rubber also includes the same process, but the barrel temperature is lower than that of thermoplastics, but the injection pressure is higher. The mold is heated. After the injection of materials, the mold needs to be cured or vulcanized, and then the film is removed while hot.
Nowadays, the trend of processing technology is developing towards high-tech. These technologies include micro injection molding, high filling composite injection molding, water assisted injection molding, mixed use of various special injection molding processes, foam injection molding, mold technology, simulation technology, etc.This process is also used for commonly used for toy injection molds.
In 1868, Hayat developed a plastic material, which he named Sailu. Cylulu was invented by Alexander Parks in 1851. HYATT improves it so that it can be processed into finished shape. Hayat and his brother Isaiah registered the patent right of the first plunger injection machine in 1872. This machine is relatively simpler than those used in the 20th century. It basically works like a giant hypodermic syringe needle. This huge needle (diffuser) injects plastic into the mold through a heated cylinder.
In the 1940s, World War II created a huge demand for cheap, mass produced products., Low price, mass produced products.
In 1946, American inventor James Watson Hendry built the first injection molding machine, which allows more accurate control of the injection speed and quality of the products produced. The machine also allows the color or recycled plastics to be thoroughly mixed and injected into the raw biomass before the materials are mixed and injected. In 1951, the United States developed the first screw injection machine, which has not applied for a patent, and this device is still in use.
In the 1970s, Hendry went on to develop the first gas assisted injection molding process, which allowed the production of complex, hollow products and rapid cooling. This greatly improves design flexibility as well as power and end of line manufactured components, while reducing production time, cost, weight, and waste.
Injection molding process
Injection molding equipment and mold
1. Barrel temperature: The temperature to be controlled during injection molding includes barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of plastics, while the second temperature mainly affects the flow and cooling of plastics. Each kind of plastic has different flow temperature. The flow temperature and decomposition temperature of the same kind of plastic are different due to different sources or brands. This is due to different average molecular weight and molecular weight distribution. The plasticizing process of plastic in different types of injection machines is also different, so the selection of barrel temperature is also different.
2. Nozzle temperature: The nozzle temperature is usually slightly lower than the maximum temperature of the barrel, which is to prevent the “salivation” of molten materials that may occur in the straight through nozzle. The nozzle temperature shall not be too low, otherwise, the nozzle will be blocked due to the early solidification of the melted material, or the performance of the product will be affected due to the early solidification of the melted material injected into the mold cavity.
3. Mold temperature: Mold temperature has a great impact on the internal performance and apparent quality of products. The mold temperature depends on the crystallinity of the plastic, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed, injection pressure, molding cycle, etc.).
The pressure in the injection molding process includes plasticizing pressure and injection pressure, and directly affects the plasticization of plastics and the quality of products.
1. Plasticizing pressure: (back pressure) When the screw injection machine is used, the pressure on the molten material at the top of the screw when the screw rotates backward is called plasticizing pressure, also called back pressure. The pressure can be adjusted through the relief valve in the hydraulic system. During injection, the size of plasticizing pressure needs to change with the screw design, product quality requirements and different types of plastics. If these conditions and the screw speed are unchanged, increasing the plasticizing pressure will strengthen the shearing effect, that is, increase the melt temperature, but will reduce the plasticizing efficiency, increase the countercurrent and leakage flow, and increase the driving power.
Pressure curve in injection molding
In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the color mixture uniform and the gas in the melt discharged. In general operation, the plasticizing pressure shall be determined as low as possible on the premise of ensuring good product quality. The specific value varies with the type of plastic used, but it rarely exceeds 20kg/cm2.
2. Injection pressure: In current production, the injection pressure of almost all injection machines is based on the plunger or screw top
The applied pressure (converted from the oil pressure) shall prevail. The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, give the molten material the speed of mold filling and compact the molten material.
1. Injection pressure
The injection pressure is provided by the hydraulic system of the injection system. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine. Driven by the pressure, the plastic melt enters the vertical runner (also the main runner for some molds), the main runner and the branch runner of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate. This process is called the injection molding process, or the filling process. The existence of pressure is to overcome the resistance in the melt flow process, or conversely, the resistance in the flow process needs to be offset by the pressure of the injection molding machine to ensure the smooth filling process.
During the injection molding process, the pressure at the nozzle of the injection molding machine is the highest to overcome the flow resistance of the melt in the whole process. Then, the pressure gradually decreases along the flow length to the front wave front of the melt. If the internal air exhaust in the mold cavity is good, the last pressure in the front of the melt is atmospheric pressure.
There are many factors affecting the melt filling pressure, which can be summarized into three categories: ⑴ material factors, such as the type and viscosity of plastics; ⑵ Structural factors, such as the type, number and position of the gating system, the shape of the mold cavity and the thickness of the product; ⑶ Molding process elements.
2. Injection time
The injection time here refers to the time required for the plastic melt to fill the mold cavity, excluding auxiliary time such as mold opening and closing. Although the injection time is very short and has little impact on the molding cycle, the adjustment of injection time has a great effect on the pressure control of the gate, runner and cavity. Reasonable injection time is helpful to the ideal filling of melt, and it is very important to improve the surface quality of products and reduce dimensional tolerance.
The injection time is far less than the cooling time, which is about 1/10~1/15 of the cooling time. This rule can be used as the basis for predicting the total molding time of plastic parts. In the process of mold flow analysis, the injection time in the analysis results is equal to the injection time set in the process conditions only when the melt is completely driven to fill the cavity by the rotation of the screw. If the pressure maintaining switch of the screw occurs before the cavity is filled, the analysis result will be greater than the setting of the process conditions.
3. Injection temperature
Injection temperature is an important factor affecting injection pressure. The barrel of the injection molding machine has 5-6 heating sections, and each raw material has its proper processing temperature (see the data provided by the material supplier for detailed processing temperature). The injection temperature must be controlled within a certain range. Too low temperature leads to poor plasticization of melted materials, which affects the quality of molded parts and increases the process difficulty; The temperature is too high, and the raw materials are easy to decompose. In the actual injection molding process, the injection temperature is often higher than the barrel temperature, and the higher value is related to the injection rate and material properties, up to 30 ℃. This is due to the high heat generated by the shearing of the molten material when it passes through the injection port. This difference can be compensated in two ways during the mold flow analysis: one is to measure the temperature of the molten material during the empty injection molding, and the other is to include the nozzle in the modeling.
4. Holding pressure and time
At the end of the injection molding process, the screw stops rotating and just moves forward. At this time, the injection molding enters the pressure holding stage. During the pressure maintaining process, the nozzle of the injection molding machine continuously feeds the cavity to fill the volume vacated due to the shrinkage of the part. If the cavity is filled without pressure maintaining, the part will shrink by about 25%, especially the rib area due to excessive shrinkage. The pressure maintaining pressure is generally about 85% of the maximum filling pressure, which should be determined according to the actual situation.
5. Back pressure
Back pressure refers to the pressure that needs to be overcome when the screw reverses and reverses for material storage. The use of high back pressure is beneficial to the dispersion of pigments and the melting of plastics, but it also extends the screw retraction time, reduces the length of plastic fibers, and increases the pressure of the injection molding machine. Therefore, the back pressure should be lower, generally not more than 20% of the injection molding pressure. When injecting foamed plastics, the back pressure should be higher than the pressure formed by the gas, otherwise the screw will be pushed out of the barrel. Some injection molding machines can program the back pressure to compensate for the reduction of the screw length during melting, which will reduce the input heat and reduce the temperature. However, due to the difficulty in estimating the results of such changes, it is not easy to make corresponding adjustments to the machine.
Defect resolution editing
The injection molding process is a complex processing process involving mold design, mold manufacturing, raw material characteristics and raw material pretreatment methods, molding process, injection molding machine operation and other factors, and is closely related to the processing environment conditions, product cooling time, post-treatment process. Therefore, the quality of products is not only determined by the injection molding accuracy and metering accuracy of the injection molding machine, or by the quality of mold design and the precision level of mold processing. Generally, it is also affected and restricted by other factors mentioned above.
Under the constraints of so many complex factors, the defects of injection molded products are inevitable. Therefore, it is particularly important to find the internal mechanism of defect generation and predict the location and type of possible defects of products, and to guide mold design and improvement, summarize the rules of defect generation, and formulate more reasonable process operation conditions. We will explain the mechanism and solutions of injection molding defects from three main factors that affect the plastic material characteristics, mold structure, injection molding process and injection molding equipment in the injection molding process.
Injection molding (1 piece)
Classification of Common Defects in Injection Molding Products
The plastic raw materials used in the injection molding process are diverse, and the types and forms of mold design are also varied. In addition, the operators’ familiarity with specific injection molding machines, operating skills and practical experience are also different. At the same time, the objective environment (such as ambient temperature, humidity, air cleanliness) will also vary with the seasons, These objective and subjective conditions together determine the occurrence of defects in injection molded products.
Generally speaking, there are three main aspects to evaluate the performance of plastic products:
First, appearance quality, including integrity, color and luster;
Second, the accuracy between size and relative position, that is, size accuracy and position accuracy;
Third, the mechanical properties, chemical properties, electrical properties, etc. corresponding to the use, namely functionality
Therefore, if any of the above three aspects has problems, it will lead to the generation and expansion of product defects.
Classification of Common Defects in Injection Molding Products
Appearance defects: silver grain discoloration, weld line
Process problems: flash, shrinkage, lack of glue
Performance problems: warping and embrittlement