First, dry plastic according to material requirements. For example, ABS drying temperature is 80℃ and drying time is 1-2 hours, or engineering materials must be dehumidified with a dehumidifier to achieve effect of no moisture or moisture. Then start pre-molding process: externally heat melting cylinder, and achieve required temperature inside melting cylinder through thermal balance. Then add material in solid particle state to hopper, and send particles to melting solution through screw return. Due to high temperature conduction, granular plastic is plasticized by thermal energy, transformed from particle state into a viscous liquid state, is sheared and compressed by screw thread to make density and viscosity uniform. This is the first process of completing pre-molding.

This process is to inject pre-molded liquid glue into mold cavity from measuring chamber, that is, barrel, through driving force of screw. This process is divided into two stages: the first stage, liquid material enters mold cavity from screw through mold nozzle until mold cavity is filled; the second stage is pressure maintaining stage. After the first stage injection is completed, injection pressure must be continued to maintain liquid flow so that liquid in mold cavity can further replenish pressure and increase density of product before it is completely cooled. This is role of pressure holding. The whole process is called injection molding.

When all holding pressure is removed, liquid in mold cavity continues to be transferred with temperature of mold, causing cooling, and shaping. As time goes by, temperature decreases until product can withstand allowed ejection after mold opening. Degree of shaping and deformation are OK products, which is called cooling shaping. After above three major parts of process, injection molding process is completed.

Main functions are transportation, melting, mixing, compression, and metering. Therefore, it plays an important role in quality of plasticization. It can be said to be heart component of injection molding machine and is closely related to quality of molded product.
The entire screw is divided into three sections: metering section, compression section, and feeding section.

 

Solid particles are fed into screw groove from barrel. Due to rotation of screw, a shear effect is formed, and temperature conduction of melt barrel begins to melt. That is to say, through this section, granular rubber is plasticized into a liquid flow.

Main function is to continue melting, mixing, shearing, compressing, pressurizing, and exhausting liquid plastic. Plastic will completely dissolve in this section and volume will shrink, so design of compression degree of screw (compression ratio) is very important.

Main functions are mixing, melting, and conveying. It also provides sufficient pressure to maintain a uniform temperature of melt and stabilize flow of molten plastic. If metering section is long, mixing effect will be good, but if it is too long, melt will stay in melting barrel for too long will cause thermal decomposition, while if it is too short, temperature will be uneven, plasticizing and melting effect will be poor.

① screw length, ② screw thread depth.
Screw aspect ratio: = screw working length (mm) ÷ screw diameter (mm).
Generally used ratio is 1:20 times. If screw is too long, plasticizing effect will be good and mixing effect will be better and more uniform. For example, (PA, PE, PP, POM) require a longer screw.
Screw compression ratio: = Feeding section tooth depth (MM) ÷ Metering section tooth depth (MM).
Compression ratio has a direct impact on plastics. If compression ratio is appropriate, it can increase density of plastic and make molecules more tightly bonded, which helps to reduce intrusion of air and reduce temperature rise caused by pressure, which affects difference in output. If compression ratio is inappropriate, it will destroy characteristics of plastic. The higher compression ratio, the higher temperature rise will be for plastic during plasticization process in barrel, and relative output will be greatly reduced. When teeth of feed section are deeper, the feeding volume is larger, but torque required by screw is also greater, and compression ratio is larger. If depth of feed section is too shallow, compression ratio will be smaller, it will easily burn and turn yellow if it is too large.
Appropriate compression ratio: PE 1:1.7~2.2, ABS 1:2~2.5, PP 1:2.2~3, Ronny 1:1.8~2.5.
Therefore, reason why transparent PC often turns yellow and scorched when producing transparent PC is that PC stays in barrel for too long, screw compression ratio is too large, scorch occurs due to shearing and heating of threads during return process.

Check whether barrel temperature of injection molding machine is normal and meets required temperature of material. Check whether plastic is dried, check mold temperature, whether ejector pin is back in place, whether ejector pin and slider cross and collide. If above problems are all OK, then make adjustments.
Set holding position to O, holding pressure to O, injection pressure to middle number, injection speed to middle number, tail section injection speed to 20% or less, and dosage position is based on size of product. Injection end position below 180T can be between 10MM, and above 180T can be between 20MM. Why should it be controlled within 10MM and 20MM? Main purpose is to avoid loss of injection pressure at the front end of screw and product instability.

If injection end position of the first beer exceeds 20MM and product is not full yet, then pressure must be increased, and pressure increase should be within 7% each time until product is full. This sets optimal dosage position. On the contrary, if product is full, but end position is still more than 20MM, then amount of melt should be reduced until product is full and position is within 20MM. Phase reflection position has returned to O, which proves that glue in barrel has been shot out and product is not yet full. Melting amount should be increased until above requirements are met, and then pressure switching position is set.

No matter what injection mode is used, correct setting of pressure transfer position is the most important. If you want to obtain the best production status and high-quality products, you must ensure that 98% of sol content of part has been injected into mold cavity before transfer. If pressure transfer is too early and solution in mold cavity is less than 95%, holding pressure needs to be used to push solution into mold cavity again. In this case, it is easy to cause insufficient filling, and at the same time, stability of product is also very poor, causing shrinkage and uneven running. On the contrary, if pressure transfer is too late, molten material in mold cavity will be more than 98% or even 100%. In this way, product is compressed too much, causing large peaks and internal stress in many parts. This makes it difficult to demould, may even cause damage to mold, because pressures of holding pressure and injection pressure are different.
Calculation method of pressure holding and cutting position:
Correct position of transfer pressure = material amount - [(material amount - end position) × 98%].
For example: 95-[(95-8)×98%]=9.74mm.

How melt back pressure works
According to speed and pressure set by computer, melt motor is filled with oil, so that sol motor starts to rotate. During rotation, screw sends dissolved material in melt barrel to head of screw, so that material in screw head gets pressure. After overcoming resistance of screw retreat system, screw can retreat. In addition to resistance of this system, there is also oil return resistance of injection cylinder, which is back pressure. We usually adjust back pressure, that is, adjust oil return pressure. The more back pressure screw is loosened, the lower oil return pressure will be and the lower back pressure will be. Correct adjustment method is based on fact that glue in nozzle does not overflow quickly.
Back pressure control
Effect of screw back pressure on melt temperature is closely related to properties of polymer. When screw is pre-molded, it will retreat under action of material pressure, allowing return oil from injection cavity to flow back to oil tank through back pressure valve. In this way, oil drain pressure of back pressure valve at screw position is used to establish melt pressure at screw head. After adjustment, shear force of melt is affected, mixing effect and compactness are controlled, and back pressure is increased, which helps to compact material the screw groove and drive away gas in material.
In short, increase in back pressure will increase resistance of return system. If screw retreat speed slows down, it will prolong thermal history of material in screw and change plasticization effect. Therefore, back pressure plays an important role in the entire molding process.

Complete molding process, in order, should include: preparation before molding, injection process, post-processing of plastic parts, etc.
1. Preparation before molding includes: inspection of raw material appearance and measurement of process performance. Raw materials should be fully preheated and dried; barrel should be cleaned before replacing raw materials; product inserts with inserts should be preheated first; if demoulding is difficult, a release agent should be used; due to different types and shapes of injection raw materials, structure of plastic parts, presence or absence of inserts, and use requirements, preparation work before molding of various plastic parts is not exactly same.
2. Injection process: Injection process generally includes several steps: feeding, plasticizing, injection, cooling and demoulding.
(1) Feeding: Since injection molding is an intermittent process, quantitative feeding is required to ensure stable operation, uniform plasticization of plastics, and ultimately obtain good plastic parts.
(2) Plasticization: Added plastic is heated in barrel and process of converting solid particles into a viscous flow state and having good plasticity is called plasticization. Main factors that determine quality of plastic plasticization are heating conditions and shearing effects of material. Heating of material through barrel causes polymer molecules to relax. A transition from solid to liquid occurs. These requirements are closely related to characteristics of plastics, control of process conditions and structure of plasticizing device of injection machine.
3. Injection: Injection process can be divided into several stages: mold filling, pressure maintaining, reverse flow, cooling after the gate freezes, and demoulding.
(1) Mold filling: Plasticized melt is pushed to the front end of barrel by plunger or screw, enters and fills cavity through the nozzle and mold pouring system. This stage is called mold filling.
(2) Pressure holding: When mold melt cools and shrinks, plunger or screw that continues to maintain pressure forces melt near gate to continuously fill into mold, so that plastic in cavity can be formed into a complete and dense shape. For plastic parts, this stage is called pressure holding.
(3) Backflow: After pressure holding is completed, pressure in cavity is released. At this time, melt pressure in cavity will be higher than that in front of the gate. If gate has not frozen, backflow will occur, causing defects such as shrinkage, deformation, and loose texture of plastic parts.
(4) Cooling after gate is frozen: When plastic in pouring system has frozen, remove pressure of plastic in barrel and add new material. At the same time, cooling media such as cooling water, oil or air are introduced to further cool mold. This stage is called cooling after gate freezing.
(5) Demolding: Mold can be opened after plastic part is cooled to a certain temperature, and plastic part is pushed out of mold under action of pushing mechanism.

 

4. Post-processing of plastic parts: After injection molded plastic parts are demoulded or mechanically processed, annealing and humidity control are often required.
(1) Annealing treatment: It is a process in which injection molded parts are placed in a fixed-temperature heated liquid medium or a hot air circulation oven for a period of time, and then slowly cooled.
(2) Humidity control treatment: Humidity control treatment is a post-processing method that places newly demolded plastic parts in hot water to isolate air, prevent oxidation of plastic parts, and accelerate moisture absorption balance. Purpose is to stabilize color, performance and size of plastic parts.