Summary of knowledge of injection molding

【Summary】:

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, various designs, shapes from simple to complex, sizes from large to small, precise product dimensions, easy product replacement, and complex shapes. Parts, injection molding are suitable for mass production and complex shapes and other molding processing fields. At a certain temperature, a completely melted plastic material is stirred by a screw, injected into a mold cavity with high pressure, and cooled and solidified to obtain a molded product. This method is suitable for the mass production of complex shapes.

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, various designs, shapes from simple to complex, sizes from large to small, precise product dimensions, easy product replacement, and complex shapes. Parts, injection molding are suitable for mass production and complex shapes and other molding processing fields.

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

Injection process

temperature control

Injection molding equipment and molds

⒈Barrel 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 plastic has a different flow temperature. The same plastic has different flow and decomposition temperatures due to different sources or grades. This is due to the difference in average molecular weight and molecular weight distribution. Plastics are injected in different types. The plasticizing process in the machine is also different, so the barrel temperature is also different.

⒉Nozzle temperature:

The nozzle temperature is usually slightly lower than the maximum temperature of the barrel. This is to prevent the "salting" of the melt in the straight-through nozzle. The temperature of the nozzle should not be too low, otherwise it will cause the early setting of the melt and block the nozzle, or affect the performance of the product because the early setting material is injected into the mold cavity.

⒊Mould temperature:

The mold temperature has a great influence on the intrinsic properties 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.).

Pressure control

The pressure in the injection molding process includes two kinds of plasticizing pressure and injection pressure, which directly affect the plasticization of plastics and product quality.

⒈Plasticizing pressure:

(Back pressure) When a screw injection machine is used, the pressure that the melt on the top of the screw receives when the screw rotates and retracts is called the plasticizing pressure, also known as the back pressure. This pressure can be adjusted by the relief valve in the hydraulic system.

In the injection, the size of the plasticizing pressure needs to be changed with the design of the screw, the quality of the product and the type of plastic. If these conditions and the speed of the screw are unchanged, increasing the plasticizing pressure will strengthen the shear The effect is to increase the temperature of the melt, but it will reduce the efficiency of plasticization, increase the countercurrent and leakage, and increase the driving power.

In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, mix the color material uniformly, and discharge the gas in the melt. general

Pressure curve in injection molding

In operation, the plasticizing pressure should be determined as low as possible under the premise of ensuring good product quality. The specific value varies with the type of plastic used, but it rarely exceeds 20 kg / cm2.

⒉ Injection pressure:

In current production, the injection pressure of almost all injection machines

The applied pressure (converted from the oil circuit pressure) shall prevail. 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 melt filling rate, and to compact the melt.

Injection molding

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 sections:

Injection 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 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 dwell time in the injection time is the pressure time on the plastic in the cavity, which accounts for a large proportion of the entire injection time, usually about 20-120 seconds (for thick parts, up to 5-10 minutes).

Before the melt is frozen at the gate, the amount of holding time will have an impact on the accuracy of the product size. If it is later, it will have no effect. The dwell time also has the best value. It is known that it depends on the material temperature, the mold temperature, and the size of the main runner and gate. If the dimensions of the main flow gate and the gate and the process conditions are normal, the pressure value that results in the smallest fluctuation range of the shrinkage of the product is usually used.

The cooling time is mainly determined by the thickness of the product, the thermal and crystallization properties of the plastic, and the mold temperature. The end of the cooling time should be based on the principle of ensuring that the product does not cause changes during demolding. The cooling time is generally between 30 and 120 seconds. Excessively long cooling time is not necessary, which not only reduces production efficiency, but also affects complex parts. It causes difficulty in demolding, and it may even produce demolding stress when forcibly demolding. Other times in the molding cycle are related to whether the production process is continuous and automated and the degree of continuous and automated.

parameter

⒈Injection 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. Under the pressure of the plastic melt, the plastic melt enters the vertical flow channel of the mold (also the mainstream channel for some molds), the mainstream channel, and the shunt through the nozzle of the injection machine. And enter the mold cavity through the gate. This process is the injection process, or the filling process. The existence of pressure is to overcome the resistance in the flow of the melt, or conversely, the resistance in the flow 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 throughout the melt. After that, the pressure gradually decreases along the flow length toward the wavefront of the foremost end of the melt. If the cavity is well vented, the last pressure at the front 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 the plastic;

⑵ Structural factors, such as the type, number and location of the pouring system, the shape of the cavity of the mold, and the thickness of the product;

(3) Molding process elements.

⒉Injection time

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. A reasonable injection time is helpful for the ideal filling of the melt, and it is of great significance for improving the surface quality of products and reducing dimensional tolerances.

The injection time is 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 entire molding time of the plastic part. In the 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 filled by the screw to fill the cavity. If the holding pressure of the screw is switched before the cavity is full, the analysis result will be greater than the process conditions.

⒊Injection temperature

Injection temperature is an important factor affecting injection pressure. The injection molding machine barrel has 5 to 6 heating sections, and each raw material has its own suitable 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 not be plasticized, which will affect the quality of the molded parts and increase the difficulty of the process. In the actual injection molding process, the injection temperature is often higher than the barrel temperature. 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 when the melt is sheared 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 molding, and the other is to include the nozzle when modeling.

⒋Holding pressure and time

Near the end of the injection molding process, the screw stopped rotating and only advanced forward, at this time the injection molding entered the pressure holding stage. During the dwelling 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 cavity is not filled after the pressure is maintained, the part will shrink by about 25%, especially the ribs will shrink due to excessive shrinkage. The holding pressure is generally about 85% of the maximum filling pressure, of course, it must be determined according to the actual situation.

Back pressure

Back pressure refers to the pressure to be overcome when the screw is reversed and backed up for storage. The use of high back pressure is beneficial to the dispersion of the pigment and the melting of the plastic, but at the same time it prolongs the screw retraction time, reduces the length of the plastic fiber, and increases the pressure of the injection molding machine, so the back pressure should be lower, generally not more than injection molding 20% of pressure. When injecting foam plastic, 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 be programmed with back pressure to compensate for the reduction in screw length during melting, which reduces input heat and reduces temperature. However, because the results of such changes are difficult to estimate, it is not easy to make corresponding adjustments to the machine.