Injection cycle

The injection cycle of an oil pressure driven injection molding machine refers to the period from the closing to the next closing. The die is generally divided into four sections: quick closing, slow closing, low pressure protection and high-pressure locking.

Injection starts after the high pressure locking is completed, and it is also divided into several sections. The injection process is filled with molten plastic. When the cavity is filled and the pressure rises suddenly, the end of the injection is also called the packing phase. When the control is not suitable, the finished product will produce rough edges.

The pressure holding starts after the injection is completed. In fact, cooling starts after the cavity is filled, that is, from the pressure maintaining. When the mold is cooled, the finished product is cooled and contracted. The function of pressure maintaining is to fill the depression formed by shrinkage by passing through the cold channel which has not yet solidified, so as to make the finished product full (without Dent) when demoulding.

When the cold runner solidifies, it is no longer meaningful to maintain pressure again, and the pressure retaining can be terminated. The pressure retaining can be divided into several sections, and the pressure holding pressure of each section is different (generally, it is decreasing by stages), and it is divided by time. The total holding time is determined by the weight of the finished product or the absence of dents from the finished product. Adjust from a short holding time, and increase the holding time a little at a time for each injection until the weight of the finished product no longer increases or the dent is acceptable, the holding time will not be increased.

The "cooling time" parameter on the injection molding machine is from the completion of pressure maintaining to the opening of the mold, but the cooling has started as early as the cavity is filled with plastic“ The purpose of cooling time is to make the finished product continue to cool and solidify, and it will not deform due to jacking out. It should be said that "cooling time" is obtained from the test.

Mold closing

Recently, elbow joint injection molding machines have regenerative die sealing oil to achieve higher die closing speed. It is suitable to use the mould without high impact.

High pressure locking die

The minimum and can make the finished product not produce the edge of the locking force, can shorten the high-pressure mold locking period required time. Moreover, the life of the die, the tie rod, elbow joint and the formwork of the injection molding machine will be extended due to the low locking force. If a mold uses 50t of locking force, it is not necessary to use 51T locking force, although your injection molding machine has higher locking force.

injection

The highest injection rate can be used when the finished product does not produce bubbles or black spots due to burning plastic. Especially for thick wall injection, there is a large amount of space for air storage in the mold cavity filled by molten plastic. Too high injection speed will make the air in the mold cavity not able to discharge the outside of the mold, and cause bubbles.

The minimum injection pressure can reduce the required mold locking force (expanding force) correspondingly, while the lowest barrel temperature can shorten the "cooling time".

Pressure maintaining

The shortest holding time can be determined from the weight of the finished product or acceptable dent. There are many thin-walled products that do not need to keep pressure, because the inner layer of the finished product is basically solidified immediately after injection.

Cooling time

Once said: the mold is a heat exchanger at all. Yes, the mold will continue to take away the heat from melting through cold water channel, and the appropriate design of the mold can improve the efficiency of heat exchange. However, ice water cooling can reduce "cooling time" when allowed. If ice water cooling is made into mold condensation, dry fan and sealed mold locking device can reduce dew point and prevent condensation.

If the plasticizing capacity is not enough to make bottleneck, the following treatment can be done in screw design and parameter adjustment:

1. barrier screw can increase plasticizing capacity.

2. the large diameter (c) screw can increase the plasticizing capacity.

3. the plasticizing capacity can be increased by increasing the groove depth of the screw.

4. the plasticizing capacity can be increased by increasing the rotating speed of screw (this method cannot be used for some plastic sensitive to shear, such as PVC and PET).

5. reduce back pressure as much as possible, otherwise, the plasticizing speed will be increased.

6. the oil pressure sealing nozzle is used to make the opening and closing die plasticized.

7. the design of the pre plasticizer can make the screw plasticized in the cycle, except for injection and pressure holding time.

8. the pressure maintaining device is adopted to make the screw plasticized in the pressure retaining section.

Die opening

In the case of no tearing of finished products and no large opening noise, the highest speed of mold opening is used. Some injection molding machines have pressure reducing equipment before opening, and even high-speed mold opening will not produce noise. In order to realize the precise position of the stop mode under the high speed open mode, the brake valve or closed-loop control can be used.

Eject

In small injection molding machines with small top output, pneumatic jacking can be used, which is higher than that of oil pressure. The electric ejector is faster than the pneumatic one.

The mold can be designed to drive the ejector by the opening action, instead of the ejector on the injection molding machine. Only once can it be pushed out. This is the simplest method to open the mold while pushing out.

The independent oil, gas or circuit control can realize the function of multiple jacking.

Equipped with video and computer equipment, it can quickly analyze whether all finished products fall off after one jacking. The second ejection is only carried out when not all of them fall, so 99% of the cycles in the previous example only pop up once, saving the average cycle time.

Top back

Some finished products can be ejected by the vibration of injection molding machine. The thimble does not need to be fully withdrawn at a time to shorten the time of multiple ejector.

The last jacking can start at the same time as the die closing. Because the stroke of thimble is shorter than that of template, the thimble will always be fully back before locking the mold.

Shortest cycle time

The shortest cycle time consists of the time required for mold closing, injection, pressure maintaining, cooling and mold opening. The feeding is carried out simultaneously in "cooling time" and opening and closing die, even when maintaining pressure. Multiple jacking is carried out simultaneously when the die is opened, and the last jacking is carried out simultaneously when the die is closed. In this case, there are up to three actions at the same time, each with independent drive. It may be that three are oil circuits (such as three oil pumps), three are circuits (electric injection molding machines) or combination of oil, gas and circuits.

Electric injection molding machine generally has four servo motors, which drive injection molding, feeding, opening, closing and demoulding respectively. The advantage of this method is that parallel action can shorten the cycle. In fact, the oil injection molding machine can achieve this goal when using three independent oil paths. Therefore, this advantage is not a patent of electric injection molding machine.

Because the injection molding can not be carried out when the mold is opened, the four servo motors can not operate simultaneously. The analysis in Figure 10 shows that three parallel actions have reached the shortest period.

Empty run time

Many injection molding machines have been marked with empty operation time, but generally only calculated theoretical time, ignoring the acceleration and deceleration of the template, of course, the quality of the mobile mold is not calculated, which is shorter than the actual opening and closing die time. According to EUROMAP, the time of empty operation is the time required for the template opening and closing, while the opening and closing travel is 0.7 times of the four column space. In the shortest period, only the opening and closing time (empty running time) and injection time are related to the design of injection molding machine.

Thin wall injection molding

Thin wall injection is defined as a injection molding with a wall thickness of 0.5mm or less, or a process / wall thickness ratio of more than 300. In order to avoid the solidification of melt plastics when the cavity is not filled, thin-walled injection molding is high speed (and high acceleration deceleration) injection. The so-called high-speed injection molding refers to the forward speed of the screw is more than 300 mm / s. Under high speed injection, the injection time is generally within 0.5s. High speed injection is assisted by accumulator. The oil pump can fill the accumulator in "cooling time", and it can also be filled by small oil pump in time other than injection and pressure preservation. The stored high pressure oil is released during injection, which can generally increase the injection speed by 3 times.

Blow air demoulding

If the product can be demoulded by blowing air, it is easy to blow while opening the mold. Generally, after the mold is opened, the air blowing is delayed, and the release time is hidden in the opening time.

Pneumatic demoulding

The pneumatic jacking out is realized on a small injection molding machine with little jacking force, which makes the opening and closing die parallel with the top out and back, saving about 1 s of the needle jacking operation time, which is considerable on the small injection molding machine, and finally reaches the cycle time of 2S.

Second best

The cost of injection molding machines with 3 independent oil paths is high. Two injection molding machines with parallel action generally use one oil circuit for the mold locking device and the other for injection device. This is based on the general division of injection molding machines

The consideration of the oil circuit board of the mold locking device and the injection device oil circuit board.

The hybrid oil injection molding machine combines two driving advantages, which is skillfully matched. The general design is the other action of electric feeding and oil pressure, such as Hesky's hyectoric injection molding machine.

Double pump injection

The double pump is designed to use only one pump for injection. The oil path can be changed slightly so that the double pumps are even during injection, thus increasing the injection speed nearly double.

Hot runner mold

The heat flow path can shorten the cycle in many ways.

1. if the diameter of the cold runner is larger than the thickness of the finished product, the cooling time should wait for the runner to cool to a certain extent before the mold can be opened, but the finished product has already cooled, causing waste. The runner of hot runner die does not need cooling, and the cooling time is determined by finished products.

2. the plastic quantity of cold runner is a percentage of the finished product plastic quantity, even heavier than the finished product weight, so the injection and feeding time will be prolonged. The injection and feeding time of the runner plastic are saved by using the hot runner mold.

3. The cold runner has a water inlet, and the opening stroke should be increased.

4. When injection molding with multiple cavities, the cold runner does not guarantee whether the finished product falls or not. The cycle will be slowed down by using the mechanical arm (fully automatic operation) or manual removal (semi-automatic operation).