How to shorten the cycle time during injection molding ?

In this increasingly competitive business world, improving production efficiency is a matter of great concern. We would like to share with you various ways to improve the injection molding cycle by using existing or newly designed injection molding machine or molds.

A hydraulic injection molding machine, its injection cycle refers to the period from mold closing to the next mold closing. Generally, mold closing is divided into four stages: fast mold closing, slow mold closing, low-pressure mold protection and high-pressure clamping.

After high-pressure mold clamping finished, it will start injection and be divided into multiple stages. During injection, the molten plastic fills the cavity. When the cavity is filled, the pressure rises suddenly, so the end of the injection is also called the packing phase. When the control is not appropriate, the finished product will have burrs.

Pressure holding starts after injection is completed. In fact, cooling starts after the cavity is filled, that is, from pressure holding. When the mold is cooling, the finished product shrinks due to the cold.

The function of pressure holding is to fill the depression formed by shrinkage, through the cold runner that has not yet solidified, suing a lower holding pressure generally than the injection pressure. Therefore, The finished product is full without dents when demolding.

When the cold runner solidifies, the pressure holding can be terminated, because it is meaningless to hold pressure again. The holding pressure can be divided into multiple stages. Each different holding pressure is divided by time. The total holding time is determined by the weight of the finished product or the absence of dents on the finished product. Start with a short holding time, increase a little holding time for each injection until the weight of the finished product no longer increases or the dents are acceptable, then the holding time does not need to be increased.

The parameter of "cooling time", set on the injection molding machine, is the period from the completion of holding pressure to the opening of the mold. However, the cooling has already started after the cavity is filled with plastic. The purpose of the "cooling time" is to allow the finished product continue to cool and solidify, so that it will not be deformed when ejected. It should be said that the "cooling time" is derived from the test.

Recently, toggle injection molding machine designed have a regenerative mold closing oil circuit, in order to achieve a higher mold closing speed. It is suitable for the use under the premise that the mold is not subject to high impact.

Using the lowest clamping force that does not produce burrs on the finished product, which can shorten the time required for the high-pressure clamping stage. Moreover, using low clamping force can extend the life of the mold, the tie rod, the toggle and the template of the injection molding machine. If a mold is sufficient with a clamping force of 50 tons, there is no need to use a clamping force of 51 tons, even if your injection molding machine has a higher clamping force.

The highest injection speed can be used when the finished product does not produce bubbles or black spots due to burnt plastic. Especially for thick-wall injection molding, there is a large amount of air storage space in the cavity filled with molten plastic. Higher injection speed will prevent the air being discharged from the mold in time, resulting in bubble formation.

Using the lowest injection pressure can reduce the required clamping force. Accordingly, it can shorten the "cooling time" by using the lowest barrel temperature.

The shortest time of holding pressure can be determined from the weight of the finished product or the acceptable sink mark. There are many thin-walled products without using packing pressure, due to the inner layer of the finished product is basically solidified immediately after injection.

There is a saying: the mold is essentially a heat exchanger. That is right, the mold continuously takes away the heat of the melt through the cold water channel.
A mold designed properly can improve the efficiency of heat exchange. However, if allowed, ice water cooling can shorten the "cooling time". If using ice water cool to make mold condensation, the dry fan and mould clamping device can lower the dew point and prevent condensation.

If the plasticizing capacity is insufficient and becomes a bottleneck, during screw design and parameter adjustment, the following treatments can be made:
1. Barrier screws can increase plasticizing capacity.
2. Large diameter (C) screws can increase plasticizing capacity.
3. Increasing the groove depth of the screw can increase plasticizing capacity.
4. Increasing the screw speed can increase plasticizing capacity (some shear-sensitive plastics such as PVC, PET, etc. cannot use this method).
5. Reduce the back pressure as much as possible, otherwise it will increase the plasticizing speed.
6. Use oil seal can be able to plasticize when opening and closing the mold.
7. Adopt a pre-plasticizer design allows the screw to plasticize during the cycle except for the injection and hold pressure time.

8. Use pressurizer can make the screw to plasticize during the packing stage.

Without tearing the finished product or making a loud mold opening noise, it need to use the highest speed to open the mold. Some sophisticated injection molding machine have a decompression device before mold opening, which will not make noise even at high-speed mold opening. In order to achieve accurate mould clamping position at high-speed mold opening, it can use a brake valve or closed-loop control.

On a small injection molding machine with low ejection force, pneumatic ejection can be used, which is faster than hydraulic ejection. Moreover, electric ejection is faster than pneumatic ejection.

The mold can be designed to be ejected by the mold opening action instead of using the ejection device on the injection molding machine. However, it can only eject once by this method. This is the simplest method of ejecting while opening the mold.

Using independent oil circuits, air circuits or electric circuit control, it can realize the function of ejecting while opening the mold for multiple ejections.

Equipped with video and computer equipment, it can quickly analyze whether the finished product has fallen off after one ejection. The second ejection is performed only when it has not fallen off. Therefore, 99% of the cycles in the above example are ejected only once, which can save the average cycle time.

Some finished products can be multiple ejection by the vibration ejection in the injection molding machine. The ejector pin does not need to be fully retracted each time to shorten the time of multiple ejections.

The last ejector back can be started at the same time as the spotting. Since the ejector pin stroke is shorter than the mold plate, the ejector pin will always be fully retracted before clamping.

The shortest cycle time consists of the time required for clamping, injection, pressure holding, cooling and mold opening. Feeding in the cooling time, at the same time, mold opening and closing, and even pressure holding are in process. Multiple ejections are carried out simultaneously during mold opening, and the last ejector back is carried out simultaneously during mold closing. In this case, there are up to three actions at the same time, and each action has an independent drive. These three actions may be oil circuits such as three oil pumps, or may be circuit such as electric injection molding machine, or may combine with oil circuits, air circuits and circuits.

Generally, electric injection molding machine have four servo motors, which is drive injection, feeding, opening and closing demolding respectively. The advantage is that parallel actions can shorten the cycle. In fact, hydraulic injection molding machine can also achieve this goal when using 3 independent oil circuits. Therefore, this advantage is not exclusive to electric injection molding machine.

Since injection cannot be performed when the mold is open, respectively the four servo motors cannot operate simultaneously.

The specifications of many injection molding machine indicate non-loading running time. However, the time is generally calculated theoretically, ignoring the acceleration and deceleration of the mold plate. It is also not calculating the quality of the moving mold. Therefore, this time is shorter than the actual mold opening and closing time. According to Euromap standards, non-loading running time is the time required for the mold plate to open and close, and the opening and closing stroke is set at 0.7 times in the four-column space. In the shortest cycle, only the mold opening and closing time (Non-loading running time) , and the injection time are related to the design of the injection molding machine.

Thin-wall injection molding is defined as injection molding with a wall thickness of 0.5 mm or less, or a process or wall thickness ratio of more than 300. In order to prevent the molten plastic from solidifying before filling the cavity, thin-wall injection molding adopts all high-speed way. The high-speed injection molding refers to the forward speed of the screw being more than 300 mm/s. Under high-speed injection molding, the injection time is generally within 0.5 second. High-speed injection is assisted by an accumulator. The oil pump fills the accumulator during the cooling time, and also can use a small oil pump to fill it except the time of injection and pressure holding. The stored high-pressure oil is released during injection, which can generally increase the injection speed by 3 times.

If the product can be demoulded by air blowing, it is easy to do it while opening the mold. Generally, the air blowing is delayed after the mold is opened, which makes the demoulding time is in the mold opening time.

The pneumatic ejection is realized on a small injection molding machine which requires little ejection force. It can make the opening and closing molds, ejection and ejector back run simultaneously. In this case, it can save ejector pin action time of one second, which is considerable in this machine. Therefore, a cycle time of two seconds can be achieved finally.

The cost of an injection molding machine with three independent oil circuits is relatively high. Injection molding machines with two parallel-action, one is generally a clamping device using one oil circuit, and the other is an injection device using another oil circuit. This is based on the consideration that the injection molding machine is generally divided into the oil circuit board of a clamping device and an injection device.

The hybrid injection molding machine combines the advantages of two drives, and can cooperate with them skillfully. The general design is electric feeding combined with other actions of oil pressure, such as Husky's hyelectric injection molding machine.

Double pump design uses only one oil pump during injection. The oil circuit can be slightly modified so that the injection is double pumped during injection, thereby increasing the injection speed by nearly double.

Hot runners 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 mold can be opened by waiting for the "cooling time" that the runner to cool in a certain degree. However, the finished product has already cooled, resulting in waste. Therefore, the runner of the hot runner mold does not need to be cooled, and the cooling time is determined by the finished product.

2. The amount of plastic in the cold runner is a percentage of the amount of plastic in the finished product. Some are even heavier than the weight of the finished product. Therefore, the injection and feeding time will be longer. Using hot runner molds can save the injection and feeding time required for the runner plastic.

3. The cold runner has a gate, and the mold opening stroke needs to be increased.

4. During multi-cavity injection molding, the cold runner does not guarantee whether the finished product falls or not. It needs to be taken out by a robot arm (fully automatic operation) or manual operation (semi-automatic operation), resulting in slowing down the cycle.