How to shorten the injection cycle?

Injection cycle

The injection cycle of a hydraulically driven injection molding machine is from the start of clamping to the next clamping. The clamping is generally divided into four sections: rapid clamping, slow clamping, low pressure clamping and high pressure clamping.

The injection begins after the high-pressure clamping is completed and is also divided into multiple sections. The molten plastic fills the cavity during injection. When the cavity is filled, the pressure rises, so the end of the injection is also called the extrusion section. When the control is not suitable, the finished product will produce burrs.

The pressure holding begins after the injection is completed. In fact, the cooling starts from the filling of the cavity, that is, from the pressure holding. When the mold cools, the finished product shrinks due to cold. The function of holding pressure is to fill the shrinkage formed by shrinkage through the cold-flow passage which has not been solidified, and the holding pressure is generally lower than the injection pressure, so that the finished product is full (no dent) when demolding.

When the cold runner is solidified, it is meaningless to maintain the pressure again, and the pressure can be terminated. The holding pressure can be divided into multiple sections, and the holding pressure of each section is different (generally decreasing step by step), which is divided by time. The total dwell time is determined by the weight of the finished product or from the finished product without dents. Starting from a short dwell time, a little dwell time is added for each injection, until the weight of the finished product no longer increases or the dent is acceptable, the dwell time does not need to be increased.

The "cooling time" parameter placed on the injection molding machine is a period of time from the completion of the pressure holding to the opening of the mold, but the cooling has begun as soon as the cavity is filled with plastic. The purpose of "cooling time" is to continue to cool and solidify the finished product, and it will not be deformed by ejection when it is ejected. It should be said that the "cooling time" is derived from the test.

Molding

Recently designed toggle-type injection molding machines have regenerative clamping lines for higher clamping speeds. It is suitable for use under the premise that the mold is not subject to high impact.

High pressure mode locking

With the lowest clamping force that allows the finished product to produce no burrs, the time required for the high pressure clamping section can be shortened. Moreover, the molds, the drawbars of the injection molding machine, the toggles and the formwork will also prolong their life due to the low clamping force. If a mold is sufficient to use a 50t clamping force, then a 51t clamping force is not required, even though your injection molding machine has a higher clamping force.

injection

The highest injection speed can be used in the case where the finished product does not generate bubbles or does not generate black spots due to charred plastic. Especially for thick-wall injection molding, a large amount of air in the cavity is filled with molten plastic. The injection speed is too high, so that the air in the cavity can not be discharged outside the mold, resulting in bubbles.

Using the lowest injection pressure can correspondingly reduce the required clamping force (expansion force), while using the lowest barrel temperature can reduce the "cooling time".

Holding pressure

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

Cooling time

To put it another way: the mold is a heat exchanger at all. It is true that the mold continuously removes the molten heat through the cold water channel, and the well-designed mold can improve the efficiency of heat exchange. However, ice water cooling can reduce the "cooling time" when allowed. If the ice water is cooled to form a mold condensation, the dry blower and the sealed mold clamping device can reduce the dew point and prevent condensation.

If the plasticizing capacity is insufficient to make a bottleneck, the following processing can be performed during screw design and parameter adjustment:

1. The barrier screw increases plasticizing ability.

2. Large diameter (C) screws increase plasticizing power.

3. Increasing the groove depth of the screw increases the plasticizing ability.

4. Increasing the speed of the screw increases the plasticizing ability (some shear-sensitive plastics such as PVC, PET, etc. cannot be used).

5. Reduce back pressure as much as possible, otherwise it will increase the plasticizing speed.

6. The oil pressure seal is used to plasticize when the mold is opened and closed.

7. The pre-plasticizer design enables the screw to be plasticized in the cycle except for the injection and holding time.

8. The pressure maintaining device is used to make the screw plasticized in the pressure maintaining section.

Mold

Open the mold at the highest speed without tearing the finished product and without producing a large mold opening sound. Some exquisite injection molding machines have decompression equipment before opening the mold, and even high-speed mold opening will not produce a sound. In order to achieve accurate position of the mold at high speed mold opening, brake valve or closed loop control is available.

Eject

In a small injection molding machine with little ejection force, pneumatic ejection can be adopted, which is higher than the speed of oil pressure ejection. Electric ejector is faster than pneumatic ejector.

The mold can be designed to be ejected by the mold opening action, instead of using the ejection device on the injection molding machine, but this method can only be ejected once, which is the simplest method of ejecting the side of the mold. With independent oil circuit, gas path or circuit control, it can realize the function of ejector edge opening and opening.

With video and computer equipment, you can quickly analyze whether the finished product has been dropped after being ejected. The second ejection is performed when not all of the drops, so the 99% cycle of the above example is only topped out, saving the average cycle time.

Retreat

The multiple ejection of some finished products can be triggered by the vibration of the injection molding machine. The thimble does not need to be fully retracted each time to shorten the time for multiple ejections. The last retreat can start at the same time as closing the mold. Since the stroke of the thimble is shorter than the template, the thimble will always be fully retracted before the mold is locked.

Shortest cycle time

The shortest cycle time consists of the time required for clamping, injection, holding, cooling and mold opening. The addition is carried out at the "cooling time" and the opening and closing of the mold, even when the pressure is maintained. Multiple ejections are performed simultaneously at the time of mold opening, and the last retraction is simultaneously performed at the time of mold clamping. In this case, there are up to three actions at the same time, each with an independent driver. It is possible that all three are oil circuits (such as three oil pumps), and three are circuits (electric injection molding machines) or a combination of oil circuits, gas circuits and circuits.

Electric injection molding machines generally have four servo motors that drive injection molding, feeding, opening, clamping and demoulding. The advantage is that parallel actions can shorten the cycle. In fact, the hydraulic injection molding machine can achieve this goal when using three independent oil circuits. Therefore, this advantage is not a patent for electric injection molding machines. Since the injection molding is not possible, the four servo motors cannot be operated at the same time.

Dry running time

Many injection molding machine specifications indicate the idle running time, but generally only the calculated theoretical time, ignoring the acceleration and deceleration of the template, of course, the quality of the moving mold is not calculated, which is shorter than the actual opening and closing time. . According to Euromap's standard, the idle time is the time required for the template to open and close, while the opening and closing stroke is set to 0.7 times the four-column space. In the shortest cycle, only the opening and closing time (empty running time) and the injection time are related to the design of the injection molding machine.

Thin wall injection

Thin wall injection molding is defined as injection molding with a wall thickness of 0.5 mm or less, or a process/wall thickness ratio of 300 or more. In order to prevent the molten plastic from solidifying when it is not filled with the mold cavity, thin-wall injection molding is high-speed (and high acceleration deceleration) injection molding. The so-called high-speed injection molding means that the forward speed of the screw is above 300 mm/s. In high-speed injection molding, the injection time is generally within 0.5s. High-speed injections are all aided by accumulators. The oil pump fills the accumulator at the "cooling time", and can also be filled with a small oil pump at a time other than injection and holding pressure. The stored high-pressure oil is released at the time of injection, generally increasing the injection speed by three times.

Blowing off the mold

If the product can be blown off with a blow, it is easy to blow while opening the mold. Generally, the air is blown after the mold is opened, and the demolding time is hidden in the mold opening time.

Pneumatic demoulding

The pneumatic ejection is realized on a small injection molding machine with a small top mold force, which makes the opening and closing mold parallel to the top ejection, saving the thimble action time of about 1 s, which is considerable on the small injection molding machine. Finally, the cycle time of 2s was reached.

Retreat next

The cost of an injection molding machine with three independent oil circuits is higher. Two parallel-action injection molding machines generally use one oil circuit for the clamping device and another oil circuit for the injection device. This is based on the consideration of the oil-filled plate of the injection molding machine and the oil circuit board of the injection device. The hybrid oil-electric injection molding machine combines the strengths of the two drives and is cleverly matched. The general design is other actions of electric feeding with oil pressure, such as Husky's Hyelectric injection molding machine.

Double pump injection

The dual pump design uses only one oil pump for injection. The oil circuit can be changed slightly so that the pump is double pumped, which increases the injection speed by nearly double.

Hot runner mold

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 "cooling time" will wait for the runner to cool to a certain extent to open the mold, but the finished product has already cooled and caused waste. The flow path of the hot runner mold is not cooled, and the finished product determines the cooling time.

2. The amount of plastic in the cold runner is a percentage of the finished plastic. Even if it is heavier than the weight of the finished product, the injection and feeding time will be lengthened. The use of hot runner molds eliminates the injection and feeding time required for runner plastics.

3. The cold runner has a nozzle and the mold opening stroke is increased.

4. When multi-cavity injection molding, the cold runner does not guarantee that the finished product will fall or not. It must be removed by mechanical arm (automatic operation) or manually removed (semi-automatic operation), and the cycle will be slowed down.