What Is Injection Molding?

After the read, you will learn about:

What Is Injection Molding?

Injection process

  1. Temperature control
  2. Mold temperature
  3. Molding cycle
  4. Process elements of molding
  5. Back pressure

Injection molding also known as injection moulding, 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 the mass production of parts with complex shapes and is one of the important processing methods.

Injection process

1. Temperature control

1.1. Barrel temperature

The temperature that needs to be controlled in the injection molding process includes the barrel temperature, nozzle temperature and mold temperature. 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.

1.2. Nozzle temperature

The nozzle temperature is usually slightly lower than the maximum temperature of the barrel. This is to prevent the possible salivation of the melt in the straight-through nozzle. 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.

2. 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.).

2.1. Pressure control

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

2.2. Plasticizing pressure

(Back pressure) When using a screw injection machine, the pressure on the top of the screw when the screw is rotated and retreated is called the plasticizing pressure, also known as the 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 variety of plastics used, but usually rarely exceeds 20 kg/cm².

2.3. Injection pressure

In the current production, the injection pressure of almost all injection machines is based on the plunger or the top of the screw to the plastic

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 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.

3. 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.

3.1. 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 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 pressure holding time in the injection time is the pressure time for the plastic in the cavity, which takes up 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. It is not necessary for the cooling time to be too long. 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.

3.2. 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. 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 that affect the filling pressure of the melt, which can be summarized into 3 categories, material factors, such as the type of plastic, viscosity, etc.;

Structural factors, such as the type, number and location of the gating system, the shape of the mold cavity and the thickness of the product, etc.;

4. Process elements of molding

4.1. Injection 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.

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.

4.2. 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 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.

4.3. 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.

5. Back pressure

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 20% injection molding 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.