1. Principle of multi-stage injection plastic flow:
1. The first shot is injected at a low speed, nozzle is moved away from cold material head, and then second shot speed is increased to fill mold cavity to shorten time for plastic in gate to flow to the end, so that viscosity of plastic in filling is maintained at minimum solidification, but it is difficult to control correct pressure holding switching point for high-speed injection, so it is necessary to use multi-stage deceleration to effectively control pressure holding switching point.
2. Principle of segmented injection:
Flow proportional valve in hydraulic system is controlled by electro-optical control to instantly obtain injection speed at a point position to achieve segmented injection speed.
2. Relationship between injection speed and pressure of segmented injection:
1. In segmented injection molding process, no matter how many stages of filling process there are, injection pressure has only one pressure, that is, one pressure, and no matter how many stages of pressure holding are there, speed can be kept at one.
2. Segmented injection examples:
⑴ Same finished product can be molded with a smaller clamping force, which can extend life of machine and mold;
⑵ Grasping correct pressure holding switching point by segmented deceleration can effectively ensure stability of quality;
⑶ When molding plastic flows too well, in order to prevent generation of burrs, low-speed injection is adopted, but principle of not cooling and solidifying raw materials is to increase injection speed to quickly fill mold cavity after molten resin passes through thick part. Flow mark (stripe-like appearance of molten fat gradually with gate as center) is caused by resin that first flows into mold cavity cooling too fast and junction formed between resin that flows in second.
⑷ If molding is thicker at injection port (i.e. gate), too fast an injection speed will cause turbulence, cold material will easily remain in channel and form flow marks. Therefore, injection should be slow and low-pressure to push away the cold material head so that plastic behind can enter smoothly.
⑸ In injection molding process, nozzle part contacts mold because mold temperature is lower than nozzle due to cooling water of mold. Part of heat is taken away by mold, and nozzle is prone to produce cold material heads. These cold material heads are injected into mold and will be blocked at gate, causing flow lines or silver strip-like marks. Segmented injection can improve this defect.
⑹ For precision and small parts, gate size is fine, and it is extremely difficult to make gate balance of most mold cavities. This can be overcome by opening gate of same size and then using multi-stage injection technology.
⑺ Change of switching position at junction of the first low-speed injection and the second high-speed injection can correct local displacement of weld line, such as moving obvious part in appearance to a less obvious position (such as labeling position).
⑻ Phenomenon of depression and poor fusion is mutually opposed in molding, and this method can improve both at the same time; injection speed of depressed part of product drops sharply, injection speed is quickly increased to fill mold cavity after filling surface until it cools and solidifies. Rapid injection should be used to prevent poor fusion at weld line (generally, surface sinking of molded product occurs at thick part, which is caused by volume shrinkage of molten resin when it cools and solidifies).
⑼ Segmented reduction during pressure holding process can reduce residual stress of molded product.
⑽ Molding conditions of thin molded products with long flow distance require high pressure to complete smoothly, but high-speed and high-pressure injection can easily cause residual stress in gate, thereby affecting quality. Therefore, high-speed injection, medium-speed filling, and low-speed pressure holding are adopted to eliminate residual stress, thereby preventing deformation of finished product.
⑾ Improvement in mold cooling: If moveable mold temperature is low and fixed mold temperature is high, molded product will not warp inward. If moveable mold temperature is high, molded product may warp outward.
⑿ Burning (a phenomenon caused by compression and combustion of gas in mold cavity) is most likely to occur at parting line or weld. Surface of resin shows black carbonized marks. In order to allow air gas to be discharged from mold cavity smoothly, injection speed must be reduced.
Basic parameters of injection molding process use tips
1. Injection speed
01 What is injection speed
Usually injection speed we set refers to speed at which screw advances. But what is really important is speed at which melt advances in cavity, which is related to size of cross-sectional area in flow direction.
02 How to determine injection speed
As a principle, injection speed should be as fast as possible.
Its determination depends on cooling rate and melt viscosity of melt: a high injection speed is used for a melt with a fast cooling rate or high viscosity.
Note: Speed of cooling depends on performance of material itself, wall thickness and temperature of mold.
03 Injection speed is too fast/too slow
Injection speed is too fast: it is easy to have burnt spots, flash, internal bubbles or melt spray
Injection speed is too slow: it is easy to have flow marks, welding marks, and cause rough and dull surface
2. Turning point
01 How to determine turning point
Generally speaking, turning point refers to switching point from injection to holding pressure when product is injected to 95% full under zero holding pressure. For thin-walled products (such as cable ties): generally 98% of product is injected
For unbalanced flow channels: generally 70%-80%, depending on specific situation. It is recommended to use slow-fast-slow multi-stage injection.
02 Transition point is too high/too low
Transition point is too high: insufficient mold filling, weld marks, depressions, small size, etc.
Transition point is too low: flash, difficult demolding, large size, etc.
3. Holding pressure
01 How to determine holding pressure
Optimized holding pressure is generally middle value between minimum holding pressure and maximum holding pressure.
Minimum holding pressure: Based on accurate transition point, a certain holding pressure is given, when product just has insufficient mold filling
Maximum holding pressure: Based on accurate transition point, a certain holding pressure is given, when product just has burrs.
(From minimum and maximum holding pressures, we can see safe range of product process fluctuations)
02 Generally speaking:
PA holding pressure = 50% injection pressure
POM holding pressure = 80% injection pressure;
For products with high dimensional requirements, 100% injection pressure can be achieved
PP/PE holding pressure = 30-50% injection pressure
4. Holding time
01 How to determine holding time
Holding time is determined based on gate condensation. It is determined by product weighing.
02 Holding time is too long/too short
Holding time is too long: affecting cycle
Holding time is too short: insufficient weight, internal voids in the product, and small size
Note: Holding pressure will affect length of holding time. The greater holding pressure, the longer holding time.
5. Screw speed
Goal of pre-plasticization is to obtain a uniform and stable melt (i.e., uniform plasticization, no cold material, no degradation, and no excessive gas)
01 How to determine screw speed
As a principle, screw speed must be determined so that sum of screw pre-plasticization time, retraction time, and retraction time of shooting platform is slightly shorter than cooling time.
02 Screw speed is too fast/too slow
Screw speed is too fast: uneven plasticization (causing cold material, insufficient mold filling, and breakage of product), material decomposition (causing scorch spots, color difference, breakage, etc.)
Screw speed is too slow: affects cycle
6. Cooling time
As a principle, cooling time should be set as short as possible, with basic requirements that product does not deform, does not stick to mold, and has no deep ejection marks.
And: Screw speed:
PA<1.0m/s;
POM < 0.7 m/s;
PP/PE/PS < 1.3m/s;
ABS/PC/PMMA < 0.6m/s
7. Back pressure
01 What is back pressure?
Back pressure refers to force of hydraulic cylinder to prevent screw from retreating during pre-molding, which is equal to reaction force of melt at the front end of screw on screw.
02 How to determine back pressure
Determination of back pressure depends on performance of different materials and is usually provided by material supplier.
Generally speaking: PA: 20-80 Bar; POM: 50-100 Bar; PP/PE: 50-200 Bar
03 How to determine back pressure
Back pressure is too high: material decomposition; drooling; longer pre-plasticization time is required
Back pressure is too low: uneven plasticization (especially for masterbatch containing color), unreal plasticization (thus causing product bubbles, scorch spots, etc.)
8. Back-sucking amount
01 How to determine back-sucking amount
Determination of back-sucking amount (combined with determination of back pressure) is based on principle of no drooling
02 Too much/too little back-sucking amount
Too much back-sucking amount: bubbles, scorch spots, unstable material pads
Too little back-sucking amount: drooling, unstable cushion (due to check valve not being able to close)
9. Clamping force
01 Determination of clamping force
Size of clamping force depends on projected area of cavity and size of injection pressure
02 Clamping force is too large/too small
Clamping force is too large: poor exhaust (scorch spot, insufficient filling), mold deformation
Clamping force is too small: flash
10. Melt temperature
01 How to determine melt temperature
Usually, determination of melt temperature depends on performance of different materials and is provided by material supplier. (Melt temperature and mold temperature of materials used are shown in attached table)
02 Setting barrel temperature
03 Melt temperature is too high/too low
Melt temperature is too high: material decomposition (causing bubbles, color difference, burnt spots, fractures, etc. in product)
Melt temperature is too low: uneven plasticization of material, cold material in melt (causing insufficient mold filling, cold material, product fractures, etc.)
11. Mold temperature
01 Why mold temperature is needed
No matter how high or low mold temperature is, its function is always to keep mold at a certain temperature during stable production and play a cooling role.
Mold temperature that really matters refers to temperature of mold cavity, not temperature displayed on mold temperature machine. Usually, during stable production, cavity temperature will reach a stable dynamic balance and be about 10 degrees higher than displayed temperature
(For large molds, mold must be fully heated before production, especially for thin-walled and large-length-ratio product molds)
02 What does mold temperature affect?
It will affect fluidity and cooling rate of melt.
Because it affects fluidity, it affects product appearance (surface quality, burrs) and injection pressure;
Because it affects cooling speed, it affects product crystallinity, which in turn affects product shrinkage and mechanical strength properties.
03 Mold temperature is too high/too low
High mold temperature: good fluidity; high crystallinity; large shrinkage (resulting in smaller size); deformation; longer cooling time required
Low mold temperature: poor fluidity (resulting in flow lines, weld marks); low crystallinity; small shrinkage (resulting in larger size)
Gud Mould 