Control of the injection molding process directly impacts the quality of the final part and the economy of the process. The control parameters of the injection molding process must be studied in depth in order to facilitate maximum efficiency and best part quality.
The most essential process conditions in the injection molding process are the injection temperature, injection pressure, injection speed, and the corresponding individual action times that affect the plasticizing flow and cooling.
What is injection speed in injection molding?
Injection speed refers to the speed at which the screw or plunger moves during injection. As it goes faster, it takes a shorter time to complete one injection stoke, and allows greater yield.
Why is injection speed important?
The injection speed has a great impact on the degree of molecular orientation in the component and therefore influences the shrinkage and the development of a joint line. The strong molecular orientation results from fast cooling near the wall and insert.
What is the unit of injection speed?
The performance of an injecting unit is expressed by the injection rate (the volume of molten plastic that is injected in one second, cm3/s) or the injection speed (the speed of forward movement of the plunger, mm/s).
How do you calculate injection speed?
How To Calculate The Injection/Plasticising Rate?
Screw diameter = 35mm.
The material being processed = Polycarbonate (PC)
Maximum available injection velocity = 220mm/sec.
Maximum screw stroke = 240mm.
Maximum shot volume = 231cm3.
Injection speed selected = 80mm/sec.
What are the characteristics of high and low injection speeds?
The following advantages exist for the use of high injection speed in the injection process.
(1) Reduction of injection time and shortening of the molding cycle.
(2) Improving the flow of plastic and facilitating the molding of thin-walled parts.
(3) Improving the surface gloss of the product.
(4) It can improve the strength of the fusion line and make the fusion pattern less obvious.
(5) Prevents the generation of cooling deformation, etc.
The advantages of using a low-speed injection in the injection process are as follows.
(1) Preventing the generation of flying edges in molded products.
(2) Preventing the generation of jet lines and flow lines.
(3) Preventing the production of burn marks.
(4) Prevention of air entrapment in the plastic melt.
(5) Prevention of molecular orientation deformation, etc.
The advantages of high-speed injection are also the disadvantages of low-speed injection and vice versa. Therefore, the combination of high speed and low speed in the injection process can make full use of their respective advantages and avoid their respective disadvantages, thus ensuring the quality of the product and the economy of the process. This is what we usually refer to as multi-stage injection technology, which has been commonly used in modern injection molding machines.
At present, most medium-sized injection molding machines have five to six injection pressure, speed changes, and three to four pressure-holding pressure changes (because in the pressure-holding stage, the melt has been filled with the cavity, the melt through the pressure-holding pressure into the cavity of the complementary shrinkage material has been limited, so the impact of pressure-holding speed is not significant).
What are the principles of injection speed selection?
Due to the complex shape of plastic products, the flow and deformation of the plastic melt in the injection process through the main flow channel, manifold, gate and various sections of the mold cavity is extremely complex. In recent years, through the study of rheology and combined with CAE technology.
It is concluded that the most important condition to make the products with good quality and low internal stress is to make the flow field of the melt reasonably uniform, that is, the melt flow front velocity remains uniform and stable when flowing through different sections at different moments of the injection process, that is, the linear velocity must remain constant, V=constant.
This produces products with good quality, not easy to appear shrinkage and underfilling and other defects, while the uniform flow field of the melt in the cavity ensures a reasonable molecular orientation of the polymer, which can obtain a better surface quality of the product.
Due to the complex geometry of injection molded products, the area of plastic melt flowing through each section of the mold cavity is large or small, and the resistance is also large or small, so the flow rate:
(S is the cross-sectional area) is also a variable, which results in the flow rate as a function of time, while the injection pressure is also a function of time, taking into account the economics of the process, that is, to shorten the injection time and molding cycle, so it is necessary to use multi-stage injection to achieve, so as to ensure the quality and economy of the parts.
How do choose the injection speed reasonably?
The first injection speed selection part is the main flow channel and manifold part
In principle, the high-speed filling is used, which can shorten the molding cycle, but it must be considered that this high-speed injection should not make the surface appear poor quality.
The second injection speed selection part is the gate and the part of the product near the gate
The second injection speed selection part is the part of the product near the gate and the gate. This injection speed selection part of the injection process varies according to the raw material and the form of the gate.
For example, PC, PMMA, ABS, etc., and side gates (straight gates, such as cabinet transparent sheet, window panel cover and longitudinal blade, etc.), to prevent the phenomenon of spray pattern and gate fogging.
If the gate is a pointed gate or lap gate and the raw material is low viscosity resin such as PP, PA, PBT, etc., and the surface quality near the gate is not high, then high-speed injection can be used.
The third injection speed selection part is the main part of the molded part
The third injection speed selection part is the main part of the molded part, that is after the product near the gate is filled to about 70% to 80% of the weight of the molded part.
At the same time, it can also improve the gloss of the product surface, seek to reduce the deformation caused by the reduction of plastic pressure during the final filling of the mold, improve the strength of the fusion joint, and improve the appearance of the fusion line.
The fourth injection speed selection part is close to filling the cavity 85% to 90%
This injection speed selection part uses medium-speed injection, the purpose is to make the transition to the next stage of low-speed injection, and to prevent the molded products from thickening and from flying edges due to uneven gating.
The fifth injection speed selection part is the final cavity-filling part
Where the low-speed injection should be used to prevent fluttering and burning marks, improve the stability of the size and weight of the products, and also to control the reduction of clamping pressure.
Giving and correction of the injection speed curve
Knowing the selection principle and reasonable distribution of injection speed, how to accurately set the injection speed curve in the actual operation process becomes the key point for the technician to grasp.
Giving of injection speed
When setting the injection speed curve, the process personnel should grasp two points: (1) the size of the injection speed and (2) the given injection position.
The usual practice is to use the “0 injection method” to determine the injection position of the screw, that is, the second injection pressure, the speed at 0, adjust the injection position to observe the actual injection molding product size and shape, and the required to determine the position for comparison.
Then the second injection pressure, speed, and position are given, and the third injection pressure and speed are placed at 0. Again, compare whether the input position is reasonable, and so on to determine the amount of material to be fed.
Then adjust the speed, generally from low to high, and try to use high-speed filling to shorten the molding cycle without affecting the surface quality of the product.
Correction of injection speed
When correcting the injection speed curve needs to be determined in reverse, i.e. first determine the adjustment of injection speed and then carry out a review of the conversion position of the change in injection speed, because the injection position basically meets our expectation when the injection curve is given, however, the change in injection speed will cause deviation in the filling result of the conversion position of the same injection speed.
Injection pressure and injection speed are also interrelated and affect each other when the multi-stage injection is performed. When the filling time is very short, the melt flow rate (injection speed) is also higher due to the long process, and thus higher injection pressure is required.
In addition, when the filling time is increased (i.e., the injection speed is reduced), the polymer is cooled for a longer period of time, which results in a thicker frozen layer when the polymer meets the cold mold, resulting in a thinner molten flow layer, increased viscosity of the melt, and increased resistance, thus requiring a higher injection pressure to fill the cavity.
Therefore, when adjusting the injection pressure, the injection pressure should be adjusted to a slightly higher level, which can make it possible to accurately control the flying edges and missing material and other defects of the product by adjusting the injection speed of each section.
This is the idea of pressure and speed control design of injection molding machine: injection pressure for one section, injection speed for six sections, through the adjustment of injection speed to grasp to overcome the defects of the product.
Injection speed in injection molding troubleshoot
Causes and solutions 1:
The gate flow marks are due to the melt injection to produce flow lines; the mold will be the gate injection angle aligned with the core to avoid direct injection, and the other process adjustment will slow down the injection speed when the melt flows through the gate.
Due to the slowing down of the primary injection speed, the melt solidifies when it meets the cold mold and the solidification layer is thicker, so the required injection pressure is larger. Therefore, the secondary injection speed is increased to reduce the filling time and minimize the injection pressure, and prevent the shrinkage of the product.
Causes and solutions 2:
The fusion flow marks are caused by the melt passing through the five-point gates when the shear friction temperature rises, after the side gate meets the cold mold wall and cools rapidly, the two streams of material merge and advance together when the mutual friction shear causes the melt to break and produce fusion flow marks.
In the mold, the side gate is changed into a fan gate, and the angle of fusion is increased so that the two streams start to fuse at a very close distance from the gate, and the temperature difference between the two streams and the temperature drop is avoided as much as possible.
In addition, the process is adjusted to increase the speed of the first stage, and the injection speed is slowed down when the melt flows through the gate to reduce the shear rate.
Through the above theoretical discussion and example analysis, the following conclusions can be drawn.
① By adjusting the injection speed, injection defects can be overcome, the quality of products can be improved, and the economy of the process can be improved.
② For different product structure designs, material types, and pouring systems, the injection speed curve should be selected and adjusted separately.
③ The “0 injection method” can be used to effectively determine the injection speed curve in the actual production process.
④ There are many factors that affect the injection speed, so it is necessary to adjust them after understanding them correctly.
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