Control of injection molding process has a direct impact on final part quality and economics of process. Control parameters of machining process must be thoroughly studied in order to obtain maximum benefit and the best part quality. With continuous updating of home appliance products, complexity of product design continues to increase, internal and external quality requirements for injection molded parts continue to increase, which puts forward higher requirements for mold manufacturing and process adjustment control of injection molding enterprises.

 

With continuous update and improvement of mold processing methods and precision, complex product design can be realized; with continuous update and improvement of electrical control design of injection molding machine, complex structure mold can produce products with a better quality level. With advanced equipment and good molds, there must be a good level of process control, so that machine, mold and products can be perfectly combined.
The most important process conditions in injection molding process are temperature, pressure, speed and corresponding action time that affect plasticizing flow and cooling. These factors influence and restrict each other, such as increasing melt and mold temperature, injection pressure and speed can be reduced, and vice versa, injection pressure and speed need to be increased. Core of influence in various process conditions is viscosity change of plastic, which is very important for rational selection of parameters in injection process and their mutual influence.
With deepening of research on flow and deformation of plastic melt in injection process, importance of parameter selection of injection speed to improve product quality is increasingly recognized.

Using high-speed injection during injection process has following advantages:
(1) Reduce injection time and shorten molding cycle;
(2) Improve process of plastic, which is conducive to molding of thin-walled parts;
(3) Improve surface gloss of product;
⑷ It can improve strength of welding line, so that welding lines are not obvious;
⑸ Prevent cooling deformation, etc.
Using low-speed injection during injection process has following advantages:
(1) Prevent flashing of molded products;
(2) Prevent jet pattern and flow pattern from being produced;
(3) Prevent burning marks;
⑷ Prevent phenomenon of air inclusion in plastic melt;
⑸ Prevent molecular orientation deformation, etc.
Advantages of high-speed injection are also disadvantages of low-speed injection, and vice versa. Therefore, using a combination of high speed and low speed in injection process can make full use of their respective advantages and avoid their respective disadvantages, thereby ensuring quality of product and economy of process. This is what we usually refer to as multi-stage injection technology, which is commonly used on modern injection molding machines.
At present, most of injection molding machines above medium size have five to six levels of injection pressure, speed changes, and three to four levels of holding pressure changes (due to pressure holding stage, melt has filled cavity, and melt flows into mold after holding pressure. Feeding material of cavity is already limited, so influence of pressure holding speed is not large).

Due to complex shape of plastic products, flow and deformation of plastic melt flowing through main runner, runner, gate and various sections in cavity during injection process are extremely complex. In recent years, through research of rheology and combination of CAE technology, it is concluded that the most important condition for quality of product and small internal stress is to make flow field of melt reasonably uniform, that is, speed of melt flow front. It is uniform and stable when it flows through different sections at different times during injection molding process, that is, linear speed must be kept constant, and V=constant.
Products produced in this way are of good quality, and are not prone to defects such as shrinkage and insufficient filling. At the same time, flow field of melt in cavity is uniform, which ensures reasonable orientation of polymer molecules and obtains better product surface quality.
Due to complex geometry of injection molded product, area of plastic melt flowing through each section of mold cavity is large or small, and resistance is also large or small, so flow rate Q=V×S (S is cross-sectional area) is also a variable, which causes flow rate to be a function of time, and injection pressure is also a function of time. Taking into account economy of process, that is, shortening injection time and molding cycle, it needs to be realized by multi-stage injection, so as to ensure quality and economy of parts.

 

According to flow of melt during injection process, we generally divide injection speed into five parts:
The first part is main runner and branch runner. In principle, high-speed filling is used, which can shorten molding cycle, but it must be considered that this high-speed injection should not cause poor surface quality;
Second part is gate and part of product near gate. This stage makes corresponding changes for different raw materials and gate forms. Usually, a lower injection speed is used, especially for high-viscosity resins such as PC, PMMA, ABS, etc., as well as side gates (straight gates, such as cabinet transparent sheets, window machine panel door covers and longitudinal blades, etc.), to prevent jet patterns and gate fog spots. If gate adopts point gate or lap gate and raw material is low-viscosity resin such as PP, PA, PBT, etc. and surface quality near gate is not required, high-speed injection can be used;
Third part is main part of molded part, that is, about 70% to 80% of weight of molded part after products near gate are filled. This part adopts principle of high-speed injection, in order to shorten molding cycle and reduce viscosity of melt in metal mold cavity changes, and at the same time, it can improve gloss of surface of product, and seek to reduce deformation caused by reduction of plastic pressure during final filling, improve strength of weld seam, and improve appearance of weld line;
Fourth part is close to about 85% to 90% of cavity. This part uses medium-speed injection. Purpose is to transition to next stage of low-speed injection, while preventing molded product from being too thick and unbalanced due to gate produce flash;
Fifth part is final cavity filled part, where low-speed injection should be used to prevent flashing and burning marks, improve stability of size and weight of product, control and reduce clamping pressure.

Knowing selection principle and reasonable distribution of injection speed, how to accurately set injection speed curve in actual operation process has become focus of technicians.
Given injection speed: craftsman should grasp two points when setting injection speed curve, ⑴ is size of injection speed, ⑵ is given injection position. Usual practice is to use "0 injection method" to give screw injection position, that is, set injection pressure and speed of second stage to 0, adjust injection position to observe size and shape of actual injection molded product, compare it with position to be determined, then give secondary injection pressure, speed, and position, set tertiary injection pressure and speed to 0, also compare whether input position is reasonable, and so on. Then adjust speed, generally from low to high, under premise of not affecting surface quality of product, try to use high-speed filling to shorten molding cycle.
Correction of injection speed: When correcting injection speed curve, it needs to be determined in reverse, that is, first determine adjustment of injection speed, then check conversion position of injection speed change, because injection position basically meets our expectations when injection curve is given. However, due to change of injection speed, filling results of same injection speed switching position will deviate.
When performing multi-stage injection, injection pressure and injection speed are also interrelated and affect each other. When filling time is short, flow rate of melt (injection speed) is also higher due to longer process, therefore required injection pressure is also higher.
In addition, increasing filling time (that is, reducing injection speed), polymer is cooled for a long time, so that frozen layer generated by polymer encountering cold mold is also thicker, resulting in thinning of intermediate melt flow layer, and melting of melt. As viscosity increases, resistance increases, so higher injection pressures are also required to fill cavity.
Therefore, when adjusting injection pressure, injection pressure should be adjusted slightly higher, so that flash, material shortage and other defects of product can be accurately controlled by adjusting injection speed of each section. This is also idea of pressure and speed control design of injection molding machine: injection pressure is one stage, injection speed is six stages, and defects of product can be overcome by adjusting injection speed.
Causes and solutions: Gate flow marks are caused by flow lines caused by melt injection; injection angle of gate is aligned with core on mold to avoid direct injection, and injection speed is slowed down when melt flows through gate due to process adjustment. Since primary injection speed is slowed down, melt solidifies when it encounters cold mold and solidified layer is thicker, required injection pressure is large. Therefore, secondary injection speed is increased, filling time is reduced, injection pressure is reduced as much as possible, and at the same time, product is prevented from shrinking. When cavity is about to be filled, injection speed is slowed down to prevent flash.

 

Causes and solutions: Fusion flow marks are caused by rise of shear friction temperature when melt passes through five point gates, then rapidly cooled after encountering cold mold wall through side-feeding. After two streams meet, they advance together. Frictional shearing of each other leads to melt fracture and fusion flow marks.
On the mold, side gate is changed into a fan-shaped gate, and fusion angle is increased, so that two streams begin to fuse at a place close to gate, try to avoid temperature difference and temperature drop of two streams.
In addition, process adjustment will amplify first-level speed. When melt flows through gate, injection speed will be slowed down to reduce shear rate. When product is filled to 50%, injection speed will be increased to ensure complete filling and maintain a low injection pressure. Weld lines, slow down when filling cavity to reduce flash and reduce uneven gloss of weld lines.
Through above theoretical discussion and case analysis, following conclusions can be drawn:
① By adjusting injection speed, injection defect can be overcome, quality of product can be improved, and economy of process can be improved;
② For different product structure designs, material types and pouring systems, selection and adjustment of injection speed curve should be carried out separately;
③ In actual production operation process, "0 injection method" can effectively carry out given injection speed curve;
④ There are many related factors affecting injection speed, which need to be correctly understood and adjusted.