The core problem of injection molding is to use all methods to obtain a plasticized plastic solution, inject it into the mold cavity, and cool it under controlled conditions to make the products reach the required quality. In the injection molding of modified engineering plastics, the most important process conditions are temperature, pressure and corresponding effects. The effects of these process conditions are reviewed separately.

I. temperature is a very important parameter in injection molding of temperature modified engineering plastics. The temperature to be controlled mainly includes the temperature of modified engineering plastics solution and mold temperature. The temperature of plastic solution mainly affects the plasticization and activity of plastics, and the mold temperature mainly affects the activity and cooling of plastics.

1. Selection of solution temperature. The temperature of the modified engineering plastics solution is controlled by the barrel, so the concave convex temperature of the barrel is related to the plasticizing quality of the modified engineering plastics. The selection of the barrel temperature is related to the characteristics of the modified engineering plastics used. The selection of the barrel temperature mainly focuses on ensuring that the plastic plasticization is outstanding, and the injection can be completed smoothly without causing local degradation of the solution. The selection of the barrel temperature is also related to the type of injection molding machine. The plastic in the plunger injection molding machine only relies on the barrel wall and the surface of the split shuttle for heat transfer. The heat transfer rate is small, so a higher barrel temperature is required. In the screw injection molding machine, due to the rotation of the screw, more conflict heat can be obtained to accelerate the heat transfer. Therefore, the temperature of the selected barrel can be lower. The temperature of material barrel shall also be selected in combination with the structural characteristics of the product in the mold. For thin-walled plastics, due to the narrow mold cavity, large resistance to solution injection and fast cooling, the temperature of material barrel shall be higher for smooth stamping; When thick wall plastic parts are injected, the barrel temperature can be lower. For products with complex shapes or inserts, or when the flow channel is tortuous and long, the temperature of the barrel should also be higher. The melt temperature has a great influence on the molding process, the molding properties of plastics, the molding conditions and the physical and mechanical functions of products. Generally, with the increase of temperature, the viscosity of the solution decreases, the pressure drop in the barrel, nozzle and mold pouring system decreases, and the active length of the solution in the mold increases, thus improving the forming function. The injection rate increases, the dissolution time and filling time are reduced, the injection cycle is shortened, and the surface finish of the product is improved. However, when the temperature is too high, the plastic is prone to thermal degradation, which will degrade the function.

2. Selection of mold temperature. The temperature of the mold affects the behavior of the plastic melt when it overflows, and affects the function of the plastic products. The temperature of the mold actually determines the cooling rate of the solution. The faster the cooling speed is, the faster the solution temperature drops, which increases the viscosity, makes it difficult to move, causes the injection pressure loss to increase, reduces the effective mold filling pressure, and even causes insufficient mold filling. Generally, the confirmation of mold temperature should be comprehensively considered according to the function of the processed plastic, the functional requirements of the product, the shape and size of the product, and the process conditions of the molding process.

II. Injection pressure. Injection pressure is an important factor for solution filling and forming. It pushes the solution to move towards the front end of the barrel and forces the solution to fill the mold cavity and form.

1. Effect of injection pressure

(1) push the melt in the barrel forward to mix and plasticize the plastic together.

(2) it shall be able to overcome the active resistance of the pouring system and the mold cavity to the melt, so that the melt can achieve the satisfied mold filling speed and active length.

(3) it shall be able to compact the melt in the mold cavity and supplement the shortening of the melt due to cooling.

2. Injection pressure in the barrel for screw injection molding machine, materials are pushed forward together with melting through the rotation of screw. During injection, the resistance in the barrel mainly comes from the conflict resistance of the melt and the resistance at the nozzle. Due to the high temperature at the front end of the barrel, the conflict factor between the melt and the barrel is small, and the pressure drop is also small. However, the plunger injection molding machine is different. The plunger not only pushes the melt forward, but also pushes the unmelted and semi molten materials forward. The pressure drop caused by unmelted solid particles can reach 80% of the total pressure of the barrel. Therefore, the pressure loss in the barrel of the plunger type injection molding machine is much greater than that of the screw type.

3. Pressure in the mold. The injection pressure used by the injection molding machine is the actual molding pressure when it reaches the mold cavity after passing through the barrel, nozzle and mold pouring system. The size of forming pressure has a great impact on the appearance and connotation quality of products. In order to ensure the quality of products, there are often certain requirements for the injection rate, and the injection pressure is closely related to the injection rate. In terms of mechanical strength and shortening rate of products, each product has its own optimal injection rate, which changes within a certain scale and is usually confirmed by experiments. But the first effect is the product wall thickness. After the cavity is filled, the injection pressure will compact the melt. This pressure can be equal to or lower than the injection pressure. If the pressure of injection and compaction is equal, the shortening rate of the product can be reduced and the scale can be stabilized, but the residual stress and forming cycle of demoulding are larger. However, for crystalline plastics, since the melting point of plastics can be improved when the compaction pressure is high, the demoulding can be advanced and the forming cycle can be shortened. In the injection process, with the increase of injection pressure, the filling speed of melt accelerates, the active length increases, and the density also increases, resulting in the increase of product weight.

III. time (molding cycle) the time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. It mainly includes the following parts.

1. Injection time. Injection time includes mold filling time and pressure maintaining time. It is the most important time segment in the molding cycle and has a decisive impact on the quality of products.

(1) the filling time is the time when the screw (or plunger) moves forward. The filling time is directly inversely proportional to the filling rate. In production, the filling time is generally 3-5s.

(2) the pressure maintaining time refers to the compaction time of the melt in the mold cavity, which accounts for a large proportion in the whole injection time, generally about 20-120s (up to 5-10min for extra thick products). The length of holding time is related to the material temperature, mold temperature, the size of sprue and gate, and the pressure value with the smallest fluctuation of product shortening rate is usually the optimal pressure.

2. Cooling time cooling time also has a decisive impact on the quality of products. The length of the cooling time is mainly determined by the thickness of the product, the thermal and crystallization functions of the plastic and the mold temperature. The end of the cooling time shall be based on the principle of ensuring no deformation during demoulding. The cooling time is generally 30-120s The cooling time is too short, and the product is easy to deform; If the cooling time is too long, it will not only reduce the production efficiency, but also form demoulding difficulties for complex products. Forced demoulding may even lead to demoulding stress.

3. Other times except injection molding time and cooling time are other times, including mold opening, mold release agent coating, insert placement, mold closing, mold release, etc. Their selection is related to whether the production process is connected and automated.

This article is from Jiangmen Engineering Plastics:http://www.wywantong.com/