Thermoplastic Injection Molding
The thermoplastic injection molding process only requires the strength of the frame to support the weight of materials, molds, and the frame itself, making the processing of injection molded products fully convenient, with short cycles and low costs, and more widely used.
Injection Molding: 5 Elements to Control in the Manufacturing Process
Injection molding process has the advantage of strong adaptability to plastic varieties and can form plastic parts with complex shapes, precise dimensions, and metal or non-metal inserts in one go. It has high production efficiency and is easy to realize automation. It is an important method for thermoplastic injection molding.
To achieve precise mold control and quality manufacturing, injection molding companies need to be highly sensitive to the manufacturing process (especially in the LSR injection molding process) , including temperature control, film opening, and closing speed, pressure during the injection molding process, and the time of each link. , location, etc.
Temperature Control in Injection Molding
In the injection molding process, temperature control is essential. It mainly depends on the oil temperature and barrel temperature during machine operation. In addition, there are nozzle temperature and mold temperature.
1.Oil Temperature
When the hydraulic press is working, the machine needs to continuously operate the hydraulic oil to move and friction, thereby generating heat energy. Its temperature is controlled by cooling water. The mold manufacturing factory must confirm and control the oil when starting up. The temperature is around 45℃. If the oil temperature is too high or too low, it will affect the pressure transmission.
2.Barrel Temperature
Generally, materials need to be placed in the barrel, and the temperature here is mainly the heating temperature of the barrel surface. Injection molding factories need to control the temperature of the barrel, which can convert solid granular plastic raw materials into One with good overall liquidity. Through the operation of the machine, auxiliary materials such as color and pink masterbatch can be fully fused into a semi-liquid state to meet the needs of filling the mold.
The temperatures that need to be controlled in 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 temperatures and decomposition temperatures due to different sources or grades. This is due to different average molecular weights and molecular weight distributions. Plastics in different types of injection molding have different flow temperatures and decomposition temperatures. The plasticizing process in the machine is also different, so the temperature of the barrel is also different.
Under normal circumstances, the barrel heating needs to be carried out in three stages, from the hopper to the nozzle, from low to high, so that the plastic can gradually melt and plasticize, it can reduce errors in the process of manufacturing the mold. When the factory is manufacturing molds, the temperature of the solid conveying section near the hopper should be lower. To prevent the temperature of the hopper from being too high, the cooling water cooling system is also an indispensable part. When the injection molding process enters the compression section, the material is in a compressed state and gradually melts. This is the second heating of the barrel. The material is in a fully molten state. After the pre-plasticization is terminated, a metering chamber is formed to store the plasticized material to ensure that it is in a molten state. This is the third stage of heating of the barrel.
3.Nozzle Temperature
The nozzle has the functions of accelerating melt flow, adjusting melt temperature, and homogenizing materials. The nozzle temperature is usually slightly lower than the maximum temperature of the barrel. This is to prevent the melt from leaking in the straight-through nozzle. The temperature at the nozzle position is very important. It cannot be too high, but it cannot be too low, otherwise, the melt will easily solidify prematurely and the nozzle will be blocked.
4.Mold Temperature
Mold temperature refers to the surface temperature of the mold cavity in contact with the product, gate, and runner. Mold temperature has a great influence on the intrinsic performance and apparent quality of products. For example, the appearance of the product, airlines, pinch lines, size, and intrinsic performance have a great impact on the mold temperature. The mold temperature depends on the crystallinity of the plastic, the size and structure of the product, performance requirements, and other process conditions, such as melt temperature, injection speed, injection pressure, molding cycle, etc.
Usually, mold manufacturers will increase the mold temperature, which can improve the fluidity of the melt, increase the shrinkage of the product, and release and reduce the internal stress of the product. The mold temperature is usually controlled by adjusting the temperature of the mold temperature controller, but the mold temperature controller is an auxiliary equipment. It is not part of the injection molding machine. It has an independent heating and cooling system and a device for circulating the media.
5.Hot Runner Temperature
It is divided into insulation type and heating type. The characteristic of the hot runner system is to keep the melt of the main channel and the branch channel from solidifying during each injection molding cycle so that no material and cold glue are produced. It can also reduce plastic raw material consumption and energy consumption during molding, and shorten the molding cycle. Its temperature setting is based on the processing temperature of the materials used. Generally, the temperature is higher than that of the nozzle, and it maintains its molten state without causing drooling.
Injection Molding Speed Setting
- Setting the Speed of Mold Opening and Closing: The setting of mold opening and closing is generally based on the principle of “slow-fast-slow”. This setting mainly considers the machine, mold, and cycle.
- Ejection Setting: It can be set according to the structure of the product. For complex structures, it is best to eject some at a slow speed and then use quick demoulding to shorten the cycle.
- Shooting Speed: Set according to the size and structure of the product. If the structure is complex and the wall is thin, it can be fast. If the structure is simple and the wall is thick, it can be slow. It should also be set from slow to fast according to the performance of the material.
Pressure Adjustment for Injection Molding
The pressure during the injection molding process directly affects the plasticization of the plastic and the quality of the product.
- Injection Pressure
The injection melt flows from the nozzle into the main gate of the mold through the runner gate and into the mold cavity through injection pressure. The injection pressure at the screw head must maintain a certain pressure after overcoming the resistance loss encountered by the melt after entering the mold cavity. Fill the mold cavity.
In current production, the injection pressure of almost all injection machines is based on the pressure exerted by the plunger or the top of the screw on the plastic (converted from the oil line pressure). The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the barrel to the cavity, give the molten material a filling rate, and compact the molten material.
- Holding Pressure
When the high-temperature solution fills the mold cavity, it enters the pressure-holding and shrinkage stage, which lasts until the gate is frozen. To compensate for the volume shrinkage of the solution caused by the drop in temperature. Therefore, the setting of the holding pressure will have an important impact on the shaping process of the melt and the quality of the product.
- Sol Back Pressure
Sol back pressure is an auxiliary pressure to control the density of the solution in the material tube. Adjusting the back pressure can effectively discharge the gas of the raw materials in the metering section of the discharge pipe, make the color of the colored raw materials evenly mixed, and also directly affect the shrinkage and dimensional accuracy of the product.
(Backpressure) When using a screw injection machine, the pressure on the melt at the top of the screw when the screw rotates and retreats are called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted through the relief valve in the hydraulic system. In injection, the size of the plasticizing pressure remains unchanged with the screw speed. Increasing the plasticizing pressure will increase the temperature of the melt, but will reduce the plasticizing speed. In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the color materials can be mixed evenly, and the gas in the melt can be discharged. In general operations, the plasticizing pressure should be determined as low as possible while ensuring excellent product quality. The specific value varies with the type of plastic used, but it usually rarely exceeds 20 kg/cm2.
- Clamping Pressure
The clamping force refers to the maximum clamping force that the mold clamping mechanism of the injection molding machine can exert on the mold, also called the mold clamping force. When the injection molding machine is injected, the molten material enters the mold cavity after passing through the material tube, nozzle, and gate. Part of the injection pressure is lost in the nozzle and gate, and the rest is the melt pressure in the mold cavity, usually called the mold cavity pressure. The clamping pressure must be set to be greater than the mold cavity pressure during mold filling to prevent the mold from being stretched open and causing tipping. Excessive mold clamping will reduce the accuracy and life of the mold.
Injection Molding Time
- Injection Time
It refers to the time during which the screw advances and the molten material fills the mold cavity. The length of the injection time can be determined based on the size, thickness, and direct observation of the appearance of the product. The injection time setting device of the injection molding machine can only be set one time whether it is multi-stage or one-stage injection.
- Pressure Holding Time
After mold filling, melt feeding should be continued to maintain the feeding flow stage until the gate is frozen to obtain dense products. The pressure holding time of the injection molding machine is a time that must be set for each section.
- Cooling Time
It is the cooling stage from the solidification of the gate to the demoulding, which can make the product just formed in the mold have a certain degree of rigidity and strength, preventing ejection deformation during demoulding. At this time, the melt can no longer enter the mold cavity, and the weight of the product Can’t change any more. The cooling time and melting (accumulation) are mostly carried out simultaneously and are generally longer than the melting time.
Position Control in Injection Molding
- Mold Opening and Closing Position
It can be set according to the speed of opening and closing the mold. The key is to set the starting position of low-pressure protection. That is, the starting position of low pressure should be the point that is most likely to protect the mold without affecting the cycle. The end position should be at the slow closing speed. When molding, the position where the front and rear molds of the mold come into contact.
- Ejection Position
This position can satisfy the complete demoulding of the product. First, set it in increments from small to large. Note that the retraction position must be set to “0” when loading the mold, otherwise, the mold will be easily damaged.
- Glue Location
Calculate the material amount based on the product size and screw size, and then set the corresponding position.
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