Plastic Injection Molding
The plastic injection moulding process refers to a plastic moulding technology that melts the plastic raw materials and then injects them into the mould through the injection end to form a predetermined shape after cooling. The injection moulding equipment includes the primary parts such as the feeding end, the melting end, the injection end, the mould and the mould clamping device. Through different combinations of various components, injection moulding can be performed on materials with different properties.
The injection moulding process generally includes six stages:
Material melting, moulding, filling, pressure preserving, cooling, and de-moulding
1. The melting phase of plastic injection moulding
Different materials have different melting temperatures. Below this temperature, the material is not transparent. Above this temperature, problems such as black spots will be caused by coking. Therefore, temperature control during the melting stage is crucial. Different melting temperatures are selected according to different materials. The range of temperature changes can also be used as a treatment for product failures;
2. Mold clamping device of plastic injection moulding
Mould clamping refers to closely affixing the surfaces of moulds A and B. The principle of mould clamping is to maintain the stability of the moulds and to maintain a close fit under the impact of injection pressure without gaps and displacements. Phenomenon: The more precise the product is, the higher the requirement for mould clamping is, the more accurate the mould-closing technology can be injected with high-quality products, and the mould-clamping technology is also one of the indicators of continuous improvement of injection moulding technology;
3. The filling stage of plastic injection moulding
The filling is the first step in the injection moulding cycle. When the nozzle injects the raw material in the molten state into the mould, the raw material travels along with the gap in the mould and fills up the space. After the filling rate reaches 95%, it begins to enter the hold pressure stage; The shorter the time, the higher the moulding efficiency, but the actual moulding process is also subject to many conditions, such as the mould structure, runners, gates, cold material wells, etc.;
4. Hold pressure stage of plastic injection molding
Holding pressure applies continuous pressure to the injection channel when the filling is completed. This process compacts the melt increases the plastic’s density, and compensates for the plastic’s shrinkage; the injection screw is still moving slowly during the holding pressure. The plastic is under the cooling and solidification process of the die wall is accelerated, the melt viscosity rises, the die resistance rises, the material density continues to increase, and the plastic parts are moulded. At this time, the pressure holding pressure reaches a peak; the closing force of the clamping force in the holding pressure process is the most stringent. A little looseness will produce bad products, so the performance of the mould clamping device is shown here;
5. Cooling stage of plastic injection moulding
After the molten material enters the mould, it begins to solidify due to the temperature difference. During the curing process, many changes will occur. The molten material can only be cured to a certain degree of rigidity to be successfully moulded into a qualified product. The cooling and solidification period is about 70% to 80% of the entire moulding cycle. Therefore, shortening the cooling time is the primary method to improve the injection efficiency; the main factors affecting the cooling time are: mould design, mould materials, cooling water pipe configuration, coolant flow, coolant properties, injection materials, etc., and reasonably control these aspects Has a significant impact on improving injection moulding efficiency;
6. Demoulding stage of plastic injection moulding
Demoulding is the final stage of plastic injection moulding. The principle is that the mould release force is uniformly applied. Uniform mould release can avoid defects such as deformation, damage, and top marks caused by uneven force distribution; Two kinds of mould release and stripper stripping, reasonable use of the stripping method is an important means to improve the qualified rate of injection moulding, especially in the injection of transparent products, the optimization of stripping mode to achieve the maximum requirements;
Technical parameters of plastic injection moulding process:
1. Injection pressure of plastic injection moulding
The required injection pressure in the entire filling process is the injection pressure, which is provided by the hydraulic system of the injection moulding equipment; the injection pressure overcomes the resistance during the melt flow, and the frit enters the vertical flow path and the mainstream of the mould via the nozzle, channels, runners, through the gate into the mould cavity.
Several factors affect the injection pressure: material flow properties, mould structure and product thickness, mould exhaust system, moulding process elements;
2. The injection time of plastic injection moulding
The injection time is from the beginning of the filling to the beginning of the pressure maintenance. Generally, the injection time is concise, accounting for about 10%-15% of the entire injection moulding process. Although the injection time has little effect on the moulding cycle, it has a significant influence on pressure control of the gate, runners and cavities. A reasonable filling time is beneficial to improve the product surface quality and reduce the product dimensional tolerance; the injection time can be used to reverse the cooling time range, which can be used as an auxiliary factor to optimize the injection efficiency;
3. The injection temperature of plastic injection moulding
Injection temperature is the main factor affecting injection pressure. The raw material will go through 5-6 heating stages after entering the barrel from the drum. The temperature of each stage is different. According to the melting temperature range of different materials, proper control of temperature is the key to the success of injection moulding; if the temperature is too low, the material is melted and melted. The poor formation will affect the quality of moulding and increase the difficulty of the process. If the temperature is too high, it will cause coking decomposition, causing black spots and bubbles and other undesirable phenomena; in the actual injection moulding process, the injection temperature is often much higher than the temperature of the barrel, according to the difference in materials, the maximum temperature difference can reach about 30 °C, in the mould flow analysis, the temperature difference must also be taken into account;
4. Pressure preservation and pressure holding time of plastic injection moulding
Near the end of the injection moulding process, the screw stops spinning and only advances. At this point, the injection enters the hold pressure phase. During the pressurization process, the nozzle of the injection moulding machine continuously feeds the cavity to fill the vacated volume due to the workpiece’s contraction. If the pressure is not maintained after the cavity is filled, the parts will shrink by about 25%, especially when the ribs contract due to excessive shrinkage. The holding pressure is generally about 85% of the maximum filling pressure. The holding time must be determined according to the volume of the product and the length of the cooling moulding time. Generally, the melting time of the melt at the injection port is better, and the proper pressure holding pressure and holding time is an essential means to control the defective rate of products;
5. Back pressure of plastic injection moulding
Back pressure refers to the pressure the screw needs to overcome when reversing and retreating. The use of high back pressure is beneficial to the dispersion of pigments and the melting of plastics, but it also prolongs the screw retraction time, reduces the length of plastic fibres, and increases the pressure of the injection moulding machine. Therefore, the back pressure should be lower and generally not exceed the injection moulding 20% of pressure. When the foam is injected, the back pressure should be higher than the pressure formed by the gas, otherwise, the screw will be pushed out of the cartridge.
Plastic Injection moulding processing optimization measures
The plastic injection moulding process is not static and logical. Due to different products, different materials, different injection moulding machines, material pretreatment and other conditions, the injection moulding process will produce many uncertainties. Therefore, as an injection moulding worker, the specific circumstances of the processing process are to optimize the adjustment to reflect the highest expression of injection moulding processing technology.
Plastic Injection molding optimization can be specifically implemented in the following ways:
1. Material pretreatment
According to the different water absorption of different materials, some materials need to be dried before processing to remove the moisture in the material to reduce the proportion of defective products in the injection moulding process;
2. Maintain the state of injection moulding equipment
Injection moulding machines are high-precision equipment in which the various systems have a high degree of division and cooperation. Maintaining good working conditions can operate more efficiently and reduce the probability of failure in the production process.
Injection moulding machine maintenance includes dust treatment, lubrication treatment, replacement of worn parts, resetting of fastening screws, calibration of pressure system, etc.;
3. Mold design optimization
In the development process, the mould needs to be developed according to the basic properties of the applied raw materials. Different shrinkage rates, water absorption, and moulding flow properties need to be considered. The flow direction of the melt in the mould cavity is designed, and the pressure is uniform. Gas discharge and other conditions are critical components for reducing the difficulty of injection moulding and improving production efficiency. Efficient moulding tools are essential.
4. Mold maintenance
High-efficiency moulds are necessary, but it is also essential to maintain the efficient operation of the moulds. Maintaining good moulds mainly includes: keeping the moulds clean, avoiding wear, timely replacement of worn parts, maintaining lubrication, controlling the influence of temperature difference, and timely adjusting the state etc.,
How to effectively improve the efficiency of injection moulding processing, in addition to the excellent operation of the above injection moulding equipment, good performance of the mould and material properties also requires continuous research and experimentation by operators in the long-term production to be more efficient and high quality. The processing of a good product. Yutung Industrial’s many years of experience in plastics service has resulted in efficient and high-quality injection moulding processing services, bringing a different product experience to our customers.
The following are the pictures we made through our plastic injection moulding process:
