Injection molding, also known as injection molding, is a molding method that combines injection and molding. The advantages of injection molding method are fast production speed, high efficiency, operation can be automated, variety of designs and colors, shapes can be from simple to complex, sizes can be from large to small, and the product size is accurate, the product is easy to update, and it can be made into complex shapes. Parts and injection molding are suitable for mass production and molding processing fields such as products with complex shapes.
At a certain temperature, the completely melted 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.
The injection molding process can be roughly divided into the following 6 stages:
Mold clamping, injection, pressure keeping, cooling, mold opening, and product removal.
By repeating the above process, products can be produced periodically in batches. The molding of thermosetting plastics and rubber also includes the same process, but the barrel temperature is lower than that of thermoplastics, and the injection pressure is higher. The mold is heated. After the material is injected, it needs to undergo a curing or vulcanization process in the mold, and then release the film while it is hot .
Nowadays, the trend of processing technology is developing in the direction of high and new technology. These technologies include: micro injection molding, high filling compound injection molding, water-assisted injection molding, mixed use of various special injection molding processes, foam injection molding, mold technology, simulation technology, etc.
The History Of Injection Molding
- In 1868, Hayate developed a plastic material, which he named celluloid. Celluloid was invented by Alexander Parkes in 1851. Hayate improved it so that it can be processed into a finished shape. Hayate and his brother Isaiah registered the patent for the first plunger injection machine in 1872. This machine is relatively simpler than the machines used in the 20th century. It basically operates like a giant hypodermic needle. This huge needle (diffusion cylinder) 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 more precise control of the injection speed and quality of the products produced. This machine also mixes materials before injection, so that colored or recycled plastics can be thoroughly mixed and injected into virgin substances. In 1951, the United States developed the first screw injection machine, which 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 that allowed the production of complex, hollow products that were quickly cooled. This greatly improves design flexibility and strength and end-of-end manufacturing of parts, while reducing production time, cost, weight, and waste.
Injection Molding Process
- Cylinder temperature: The temperature that needs to be controlled during the injection molding process includes the temperature of the cylinder, the temperature of the nozzle and the temperature of the mold. The first two temperatures mainly affect the plasticization and flow of the plastic, while the latter temperature mainly affects the flow and cooling of the plastic. Each plastic has a different flow temperature. For the same plastic, due to different sources or grades, its flow temperature and decomposition temperature are different. This is due to the difference in average molecular weight and molecular weight distribution. Plastics in different types of injection The plasticization process in the machine is also different, so the temperature of the barrel is also different.
- Nozzle temperature: The nozzle temperature is usually slightly lower than the maximum temperature of the barrel. This is to prevent the “salivation phenomenon” that may occur in the straight-through nozzle of the molten material. The temperature of the nozzle should not be too low, otherwise it will cause the premature solidification of the melt and block the nozzle, or the performance of the product will be affected due to the premature solidification of the material injected into the cavity.
- Mold temperature: The mold temperature has a great influence on the internal performance and apparent quality of the product. The temperature of the mold depends on the crystallinity of the plastic, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed and 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 product quality.
- Plasticizing pressure: (back pressure) When using a screw injection machine, the pressure on the top of the screw when the screw rotates and retreats 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 with the design of the screw, the requirements of the product quality, and the type of plastic. If these conditions and the speed of the screw are the same, increasing the plasticizing pressure will strengthen the shear Function, that is, it will increase the temperature of the melt, but it will reduce the efficiency of plasticization, increase the reverse flow and leakage, and increase the driving power.
In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the mixing of the pigments uniform, and the gas in the melt can be discharged. In general operation, the decision of plasticizing pressure should be as low as possible under the premise of ensuring good product quality. The specific value varies with the types of plastics used, but usually rarely exceeds 20 kg/cm².
- Injection pressure: In the current production, the injection pressure of almost all injection machines is based on the pressure applied by the plunger or the top of the screw to the plastic (converted by the oil pressure) as the standard. The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the barrel to the cavity, to give the molten material the rate of filling the mold and to compact the molten material.
The time required to complete an injection molding process is called the molding cycle, also called the molding cycle.
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, they all 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, and the filling time in production is generally about 3-5 seconds. The holding pressure time in the injection time is the pressure time for the plastic in the cavity, and it accounts for a large proportion in the entire injection time, generally about 20-120 seconds (extra-thick parts can be as high as 5-10 minutes).
Before the melted material at the gate is sealed, the holding time has an impact on the accuracy of the product size, if it is later, it will have no impact. The holding time also has the most favorable value, which is known to depend on the material temperature, mold temperature, and the size of the main runner and gate. If the dimensions of the sprue and the gate and the process conditions are normal, usually the pressure value that obtains the smallest fluctuation range of the product shrinkage rate shall prevail. The cooling time is mainly determined by the thickness of the product, the thermal and crystalline properties of the plastic, and the mold temperature.
The end of the cooling time should be based on the principle of ensuring that no changes are caused when the product is demolded. The cooling time is generally between 30 and 120 seconds. Too long cooling time is not necessary. It will not only reduce production efficiency, but also affect complex parts. It is difficult to demold, and demoulding stress may even occur when demolding forcefully. The other time in the molding cycle is related to whether the production process is continuous and automated, and the degree of continuity and automation.
The Parameter Of Injection Molding
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. The plastic melt is pushed by the pressure and enters the vertical flow channel of the mold (or the main flow channel for some molds), the main flow channel, and the split flow through the nozzle of the injection molding machine. And enter 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 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 progress of the filling process.
In 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 process. After that, the pressure gradually decreases along the flow length to the wavefront of the front end of the melt. If the inside of the cavity is well vented, the final pressure at the front end of the melt is atmospheric pressure.
There are many factors affecting the melt filling pressure, which can be summarized into three categories:
- Material factors, such as the type and viscosity of plastics, etc.;
- Structural factors, such as the type, number and position of the gating system, the cavity shape of the mold, and the size of the product Thickness, etc.;
- process elements of molding.
The injection time mentioned 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 short and the impact on the molding cycle is small, the adjustment of the injection time has a great effect on the pressure control of the gate, runner and cavity. Reasonable injection time is helpful for the ideal filling of the melt, and it is very important for improving the surface quality of the product and reducing the dimensional tolerance.
The injection time is much lower than the cooling time, 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. In the mold flow analysis, only when the melt is completely driven by the screw rotation to fill the cavity, the injection time in the analysis result is equal to the injection time set in the process conditions. If the holding pressure switch of the screw occurs before the cavity is full, the analysis result will be greater than the setting of the process conditions.
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 appropriate processing temperature (for detailed processing temperature, please refer to the data provided by the material supplier). The injection temperature must be controlled within a certain range. If the temperature is too low, the melt will be poorly plasticized, 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 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 the performance of the material, up to 30°C. This is due to the high heat generated by the shear when the melt passes through the injection port. There are two ways to compensate for this difference in mold flow analysis. One is to try to measure the temperature of the melt during air injection, and the other is to include the nozzle when modeling.
4.Holding pressure and time
At the end of the injection molding process, the screw stops rotating and only advances forward. At this time, the injection molding enters the pressure holding stage. During the pressure holding process, the nozzle of the injection molding machine continuously feeds the cavity to fill the volume vacated by the shrinkage of the part. If the pressure is not maintained after the cavity is filled, the part will shrink by about 25%, especially the shrinkage marks are formed at the ribs due to excessive shrinkage. The holding pressure is generally about 85% of the maximum filling pressure, of course, it should be determined according to the actual situation.
Back pressure refers to the pressure that the screw needs to overcome when reversing and retreating the material. The use of high back pressure is conducive to the dispersion of the color material and the melting of the plastic, but it also prolongs the screw retraction time, reduces the length of the plastic fiber, and increases the pressure of the injection molding machine. Therefore, the back pressure should be lower, generally not more than injection molding. 20% of stress. When injecting foam, 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 reduces the heat input and lowers the temperature. However, as the result of this change is difficult to estimate, it is not easy to adjust the machine accordingly.
The Defect Resolution Of Injection Molding
The injection molding process is a complex process that involves many factors such as mold design, mold manufacturing, raw material characteristics and raw material pretreatment methods, molding process, injection molding machine operation, etc., and is closely related to processing environmental conditions, product cooling time, and post-processing technology. Manufacturing processes. Therefore, the quality of the product is not only determined by the injection accuracy and measurement accuracy of the injection molding machine, or is only determined by the quality of the mold design and the accuracy level of the mold processing. Generally, it will also be affected by the other factors mentioned above. Constraints.
Under the constraints of so many complex factors, the appearance of defects in injection molded products is inevitable. Therefore, seek the internal mechanism of defects and predict the location and types of defects that may occur in the products, and use them to guide mold design and improvement, and to summarize defects. The generated rules and the formulation of more reasonable process operating conditions are particularly important. 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 equipment during the injection molding process.
Classification of common defects in injection molded products
The plastic raw materials used in the injection molding process are diverse, and the types and forms of mold design are also diverse. In addition, the operators’ familiarity with the specific injection molding machine and 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 seasons. These objective and subjective conditions together determine the occurrence of defects in injection molded products.
Generally speaking, there are three main aspects in evaluating the performance of plastic products:
- First, the appearance quality, including completeness, color, gloss;
- 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 purpose, that is, functional
Therefore, if there is a problem in any of the above three aspects, it will lead to the production and expansion of product defects.
Classification of common defects in injection molded products
- Appearance defects: silver streak discoloration, weld marks
- Process problems: flash, shrinkage, lack of glue
- Performance issues: warpage, embrittlement
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