What is injection molding?

 Injection molding, also known as injection molding, is a molding method that combines injection and molding. The advantages of injection molding are fast production speed, high efficiency, automated operation, a wide variety of colors, shapes from simple to complex, sizes from large to small, and precise product size. The product is easy to update and can be made into parts with complex shapes. Injection molding is suitable for mass production and molding processing fields such as complex-shaped products.

At a certain temperature, the completely molten plastic material is stirred by a screw, injected into the mold cavity with high pressure, and cooled and solidified to obtain a molded product. This method is suitable for mass production of parts with complex shapes and is one of the important processing methods.

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History of Injection Molding

 In 1868, Hyatt developed a plastic material he named celluloid. Celluloid had been invented in 1851 by Alexander Parks. Hyatt improves it so that it can be processed into finished shapes. Hyatt and his brother Isaiah registered the patent for the first plunger injection machine in 1872. This machine is relatively simpler than those used in the 20th century. It operates basically like a giant hypodermic needle. This giant needle (diffusion barrel) 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-priced, mass-produced products.

In 1946, American inventor James Watson Hendry built the first injection molding machine, which allowed for more precise control of injection speed and quality of produced items. This machine also enables material mixing before injection, so that colored or recycled plastics can be thoroughly mixed into virgin matter. In 1951, the United States developed the first screw injection machine. It did not apply 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 and allowed the production of complex, hollow products that cooled rapidly. This greatly increases design flexibility as well as the strength and endpoints of manufactured parts while reducing production time, cost, weight and waste.

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Injection molding temperature control

 ⒈ Barrel temperature: The temperatures that need to be controlled during the injection molding process include barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of plastics, while the latter temperature mainly affects the flow and cooling of plastics. Each type of plastic has a different flow temperature. The same type of plastic has different flow temperatures and decomposition temperatures due to different sources or brands. This is due to different average molecular weights and molecular weight distributions. The plasticization process of plastics in different types of injection machines is also different, so the barrel temperature is also different.

⒉ Nozzle temperature: The nozzle temperature is usually slightly lower than the maximum barrel temperature. This is to prevent the "drooling phenomenon" that may occur in the straight-through nozzle. The nozzle temperature cannot be too low, otherwise it will cause premature coagulation of the melt and block the nozzle, or the premature coagulation of the melt will be injected into the mold cavity, affecting the performance of the product.

⒊ Mold temperature: The mold temperature has a great influence on the intrinsic performance and apparent quality of the product. The mold temperature is determined by the presence or absence of plastic crystallinity, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.).

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Injection molding pressure control

 The pressure in the injection molding process includes plasticizing pressure and injection pressure, which directly affects the plasticization of plastics and the quality of products.

⒈ Plasticizing pressure: (back pressure) When using a screw injection machine, the pressure on the top of the screw when the screw rotates backward is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted by the overflow valve in the hydraulic system. In injection, the size of the plasticizing pressure needs to be changed according to the design of the screw, the requirements of product quality and the type of plastic. If these conditions and the speed of the screw remain unchanged, increasing the plasticizing pressure will strengthen the shearing effect, that is, it will increase the temperature of the melt, but it will reduce the efficiency of plasticization, increase backflow and leakage, and increase the driving power.

In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the colorant mixes evenly and discharges the gas in the melt. In general operation, the plasticizing pressure should be determined as low as possible while ensuring the quality of the product. Its specific value varies with the type of plastic used, but it usually rarely exceeds 20 kg/cm2.

⒉ Injection pressure: In current production, the injection pressure of almost all injection machines is based on the pressure applied by the plunger or screw top to the plastic (converted from the oil circuit pressure). The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, increase the speed of molten material filling the mold, and compact the molten material.

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Injection molding cycle

 The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. It actually includes the following parts: Molding cycle: The molding cycle directly affects labor productivity and equipment utilization.

Therefore, in the production process, the relevant time in the molding cycle should be shortened as much as possible under the premise of ensuring quality. In the entire molding cycle, the injection time and cooling time are the most important, and they have a decisive influence on the quality of the product.

The filling time in the injection time is directly inversely proportional to the filling rate. The filling time in production is generally about 3-5 seconds. The holding time in the injection time is the pressure time of the plastic in the mold cavity, which accounts for a large proportion of the entire injection time, generally about 20-120 seconds (special thick parts can be up to 5~10 minutes). Before the molten material is frozen at the gate, the amount of holding time has an impact on the dimensional accuracy of the product. If it is later, there is no effect.

The holding time also has a most favorable value. It is known that it depends on the material temperature, mold temperature, and the size of the main channel and gate. If the size of the main channel and gate and the process conditions are normal, the pressure value with the smallest fluctuation range of the product shrinkage rate is usually used as the standard. The cooling time is mainly determined by the thickness of the product, the thermal and crystal properties of the plastic, and the mold temperature.

The end point of the cooling time should be based on the principle of ensuring that the product does not cause changes when demolding. The cooling time is generally between 30 and 120 seconds. It is unnecessary to have a cooling time that is too long, which will not only reduce production efficiency, but also cause demolding difficulties for complex parts, and even produce demolding stress when forced demolding.

The other time in the molding cycle is related to whether the production process is continuous and automated, as well as the degree of continuity and automation.

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Injection molding parameters

 ⒈Injection pressure

Injection pressure is provided by the hydraulic system of the injection molding system. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine. Under the pressure, the plastic melt enters the vertical flow channel (also the main flow channel for some molds), the main flow channel, the branch flow channel of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate. This process is the injection molding process, or the filling process. The existence of pressure is to overcome the resistance in the flow process of the melt, 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 throughout the whole process. Afterwards, the pressure gradually decreases along the flow length to the front end of the melt wave front. If the exhaust inside the mold cavity is good, the final pressure at the front end of the melt is atmospheric pressure. There are many factors that affect the melt filling pressure, which can be summarized into three categories: ⑴Material factors, such as the type and viscosity of plastic; ⑵Structural factors, such as the type, number and position of the gating system, the cavity shape of the mold and the thickness of the product; ⑶Process elements of molding.

⒉ Injection time The injection time here refers to the time required for the plastic melt to fill the 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 the injection time has a great effect on the pressure control of the gate, runner and cavity. Reasonable injection time helps the melt to fill ideally, and is of great significance for improving the surface quality of the product and reducing the dimensional tolerance. The injection time should be much lower than the cooling time, which is about 1/10 to 1/15 of the cooling time. This rule can be used as a basis for predicting the total molding time of plastic parts. When performing mold flow analysis, the injection time in the analysis result is equal to the injection time set in the process conditions only when the melt is completely pushed by the screw to fill the cavity. If the screw pressure holding switch occurs before the cavity is filled, the analysis result will be greater than the setting of the process conditions.

⒊ Injection temperature The injection temperature is an important factor affecting the injection pressure. The barrel of the injection molding machine has 5 to 6 heating sections, and each raw material has its appropriate processing temperature (for detailed processing temperatures, please refer to the data provided by the material supplier). The injection molding temperature must be controlled within a certain range. If the temperature is too low, the molten material will not be plasticized well, which will affect the quality of the molded parts and increase the difficulty of the process; if the temperature is too high, the raw materials will easily decompose. In the actual injection molding process, the injection molding temperature is often higher than the barrel temperature. The higher value is related to the injection molding rate and the performance of the material, and can be up to 30°C. This is caused by the high heat generated by the shearing of the molten material when passing through the injection port. There are two ways to compensate for this difference when performing mold flow analysis. One is to try to measure the temperature of the molten material when injecting into the air, and the other is to include the nozzle when modeling.

⒋ Holding pressure and time When the injection molding process is about to end, the screw stops rotating and only moves forward. At this time, the injection molding enters the holding stage. During the holding process, the nozzle of the injection molding machine continuously adds material to the cavity to fill the volume vacated due to the shrinkage of the part. If the cavity is filled and the pressure is not maintained, the part will shrink by about 25%, especially the ribs will shrink too much and form shrinkage marks. The holding pressure is generally about 85% of the maximum filling pressure, which should be determined according to the actual situation.

⒌ Back pressure Back pressure refers to the pressure that needs to be overcome when the screw reverses and retracts to store materials. Using high back pressure is conducive to the dispersion of colorants and the melting of plastics, but it also prolongs 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 exceeding 20% ​​of the injection pressure. When injecting foam 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 in screw length during melting, which will reduce the input heat and reduce the temperature. However, since the results of this change are difficult to estimate, it is not easy to make corresponding adjustments to the machine.

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Injection molding defect solution

 The injection molding process is a complex 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 processing environment conditions, product cooling time, and post-processing process. Therefore, the quality of the product is not only determined by the injection molding accuracy and metering accuracy of the injection molding machine, or simply by the quality of the mold design and the precision level of mold processing. Usually, it is also affected and restricted by the other factors mentioned above.

Under the constraints of so many complex factors, the appearance of defects in injection molded products is inevitable. Therefore, it is particularly important to seek the internal mechanism of defect generation and predict the location and type of defects that may occur in the product, and use it to guide mold design and improvement, summarize the laws of defect generation, and formulate more reasonable process operation conditions. We will explain the mechanism and solution of injection molding defects from the three main factors that affect the injection molding process: plastic material characteristics, mold structure, injection molding process and injection molding equipment.

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Defects of injection molded products

 There are many kinds of plastic raw materials used in the injection molding process, and the types and forms of mold design are also varied. In addition, the operator's familiarity with a specific injection molding machine, the operating skills and practical experience of the workers are also different. At the same time, the objective environment (such as ambient temperature, humidity, and air cleanliness) will also vary with the seasons. These objective and subjective conditions jointly determine the defects of injection molded products.

Generally speaking, there are three main aspects for evaluating the performance of plastic products:

First, appearance quality, including integrity, color, and gloss;

Second, the accuracy between size and relative position, that is, dimensional accuracy and position accuracy;

Third, mechanical properties, chemical properties, electrical properties, etc. corresponding to the purpose, that is, functionality

Therefore, if there is a problem in any of the above three aspects, it will lead to the generation and expansion of product defects.

Classification of common defects of injection molded products Appearance defects: silver streaks, discoloration, weld marks Process problems: flash, shrinkage, lack of glue Performance problems: warping, embrittlement

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