Multi-stage injection and segmented injection plastic flow principle of injection molding machine

Time:2021-04-01 11:40:27 / Popularity: / Source:

Multi-stage injection plastic flow principle 

1. Multi-stage injection plastic flow principle:

1. First shot glue is fed at a low speed, move nozzle to cold head, then increase second shot speed to fill cavity to shorten time for plastic at gate to flow to end, so that viscosity of plastic in filling is maintained to a minimum solidification. However, it is difficult to control correct holding pressure switching point for high-speed injection, so multi-stage deceleration must be used to effectively control holding pressure switching point.
2. Operation principle of segmented injection:
Use electro-optical control to direct flow proportional valve in hydraulic system to instantly obtain injection speed at one point to achieve segmented shooting speed.

2. Relationship between shooting speed and pressure of segmented injection:

1. In segmented injection molding process, no matter how many stages of filling process are changed, injection pressure has only one pressure, that is, one pressure, while pressure holding stage can only maintain one speed regardless of number of stages of holding pressure.
2. Examples of segmented injection:
⑴ Same finished product can be molded with a smaller clamping force, which can extend life of machine and mold;
⑵Segmented deceleration and mastering correct switching point of holding pressure can effectively ensure stability of quality;
⑶ When molding plastic flow is too good, to prevent burrs from producing, adopt low-speed injection glue, but do not allow raw material to cool and solidify as principle. After molten resin passes through thick part, increase shooting speed to quickly fill mold cavity, and flow mark (striped pattern of molten grease gradually centering on gate) is caused by boundary formed between resin that flows into mold cavity and resin that flows into cavity after cooling too fast.
(4) In the case of thick molding at plastic inlet (ie gate), too fast shooting speed will cause turbulence, cold material is easy to remain in channel and form flow marks. Therefore, cold material head should be pushed open at a slow speed and low pressure to allow plastic behind to enter smoothly.
⑸ In injection molding process, nozzle part is in contact with mold. Because mold cooling water cools mold at a lower temperature than nozzle, part of heat is taken away by mold, and nozzle is prone to produce cold stubs. These cold stubs are injected into mold, they will be blocked at gate and cause flow lines or silver bar-like marks. Probing segmented injection can improve this defect.
⑹ Precision and small parts, fine gate size, most mold cavity gate balance production is extremely difficult, it can be overcome by opening gate to same size and using multi-stage injection technology.
⑺ Change of switching position between first-stage low-speed injection and second-stage high-speed injection can correct local displacement of weld line, such as displacement of obvious part to a less obvious position (such as labeling place).
⑻ Phenomenon of depression and poor fusion is opposite to each other during molding, and this method can be used to improve at the same time; shooting rate of recessed parts of product drops sharply. After filling surface layer to cold solidification, shooting speed is increased quickly to fill mold cavity. Rapid injection of glue should be used at place where weld line is generated to prevent poor fusion (generally, surface depression of molded product occurs in thick part, which is caused by volume shrinkage of molten resin when it is cooled and solidified).
⑼ In process of holding pressure, stepwise reduction can reduce residual stress of molded product.
⑽ Molding conditions of thin molded products and long flow distance require high pressure to be successfully completed, but high-speed and high-pressure injection can easily cause residual stress at gate, which affects quality. Therefore, high-speed injection, medium-speed filling, and low-speed holding pressure are adopted to eliminate residual stress and prevent deformation of finished product.
⑾ Improve mold cooling: if movable mold temperature is low, fixed mold temperature is high, molded product will not warp inwardly, and if temperature of movable mold is high, molded product may be warped outwardly.
⑿ Scorching (phenomenon caused by compression and combustion of gas in mold cavity) is most likely to occur at parting line or welding place. Surface of resin shows black carbonization traces, air gas can be smoothly discharged from mold cavity, and rate of shot must be reduced.

Deformation molding process--special use of multi-stage pressure holding

Multi-stage injection and multi-stage pressure holding are part of molding process. Control of multi-stage injection can effectively improve appearance of product, while multi-stage pressure holding can effectively improve other requirements such as burrs, size, and deformation. Multi-stage injection has been widely used.
Of course, this also depends on product structure and mold structure, and it cannot be generalized.
As shown in figure below, 8 o’clock inversion sequence, 1.8mm flesh thickness, because G1 is filled first, ground pressure will be greater, and ground side deformation will be very large. Under normal conditions, deformation of sky side is about 1.5, and ground side is about 2.5. Deformation assembly causes assembly problems, and needs to be adjusted to within 2.0.
This product has effectively reduced degree of deformation through adjustment of time sequence and multi-stage holding pressure, and the best is reduced to 0.25. Please see following for specific parameters:
Multi-stage injection 
multi-stage holding pressure 
Filling is still filled normally as before, about 2.3 seconds to complete whole, and follow-up will rely on 3 stages of pressure to control degree of deformation
Packing pressure 1: 45bar. Packing speed 1: 8mm/s. Packing time 1: 1.5s. This section is to ensure that surrounding shrinkage is OK, and deformation degree exceeds 2.0 at this time.
Packing pressure 2: 10bar. Packing speed 2: 1mm/s. Packing time 2: 1s. This section is to make material close to stagnation, to ensure that remaining valve needles are closed and no glue is injected.
Packing pressure 3: 75bar. Packing speed 3: 10mm/s. Packing time 3: 2s. This section is for G1 to continue to maintain a large pressure holding gate near gate to pull deformation back. The smaller pressure and time, the greater deformation, and vice versa, the better deformation. When pressure is 85 and time is 3.5s, deformation is only 0.25.
Of course, this adjustment method will be larger due to pressure at G1, where stress mark will be slightly larger, and it needs magic cotton to wipe it. Two hooks are broken due to sticking to female mold. At this time, add a little undercut on side wall of male mold.
PS: In fact, similar products do not have this problem when thickness exceeds 2.5mm. This phenomenon occurs when thickness is below 2.0mm. Later, many products have been improved by this method, and molding process can be regarded as advancing with times.

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