In this increasingly competitive business society, improving production efficiency is an issue worthy of attention. This article describes a variety of ways to improve injection cycle by using existing or newly designed injection molding machines or molds.

 

Injection cycle of a hydraulically driven injection molding machine refers to start of mold clamping to next mold clamping. Mold clamping is generally divided into four stages: fast mold clamping, slow mold clamping, low pressure mold protection and high pressure mold clamping.
Injection starts after high-pressure clamping is completed, and is also divided into multiple sections. Molten plastic fills cavity during injection. When cavity is filled, pressure rises sharply, so end of injection is also called packing phase. When control is not appropriate, finished product will have burrs.
Holding pressure starts after injection is completed. In fact, cooling starts after cavity is filled, that is, it starts from pressure holding. When mold cools, finished product shrinks under cold. Function of holding pressure is to pass through cold runner that has not yet solidified, fill recesses formed by shrinkage at a holding pressure generally lower than injection pressure, so that finished product is full (without dents) when it is demolded.
When cold runner is solidified, it is meaningless to hold pressure again, pressure can be terminated. Holding pressure can be divided into multiple sections, holding pressure of each section is different (usually decreasing step by step), which is divided by time. Total holding time is determined by weight of finished product or absence of dents on finished product. Adjust from a short holding time, add a little holding time for each injection, until weight of finished product no longer increases or dents are acceptable, holding time does not need to be increased.
"Cooling time" parameter set on injection molding machine is a period of time from completion of pressure holding to opening of mold, but cooling begins as early as mold cavity is filled with plastic. Purpose of "cooling time" is to make finished product continue to cool and solidify so that it will not be deformed by ejection when it is ejected. It should be said that "cooling time" is derived from experiment.

Recently designed toggle-type injection molding machines have regenerated mold clamping oil circuits in order to strive for higher mold clamping speed. It is suitable for use under premise that mold is not subject to high impact.

Use of the lowest clamping force that can prevent finished product from producing burrs, can shorten time required for high-pressure clamping section. Moreover, tie rods, toggles, mold plate and injection molding machines will also extend their life due to use of low clamping force. If a mold with a clamping force of 50t is sufficient, there is no need to use a clamping force of 51t, even though your injection molding machine has a higher clamping force.

The highest injection speed can be used when finished product does not produce bubbles or black spots due to scorched plastic. Especially for thick-walled injection molding, a large amount of air-storing space in mold cavity is filled with molten plastic. Too high injection speed prevents air in cavity from being discharged out of mold, resulting in bubbles.
Using the lowest injection pressure can correspondingly reduce required clamping force (expansion force), while using the lowest barrel temperature can shorten "cooling time".

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

There is a saying: mold is basically a heat exchanger. Yes, mold continuously takes away heat of melting throughcold water channel, and a properly designed mold can improve efficiency of heat exchange. However, if allowed, ice-water cooling can shorten "cooling time". If ice water is cooled to make mold condensation, dry air blower and sealed mold clamping device can reduce degree of dew point and prevent condensation.
If plasticizing capacity is not enough and it becomes a bottleneck, following treatments can be done during screw design and parameter adjustment:
1. Barrier screw can increase plasticizing capacity.
2. Large diameter (C) screw can increase plasticizing capacity.
3. Increasing groove depth of screw can increase plasticizing capacity.
4. Increasing speed of screw can increase plasticizing capacity (some plastics that are sensitive to shear, such as PVC, PET, etc., cannot use this method).
5. Reduce back pressure as much as possible, otherwise it will increase plasticizing speed.
6. Hydraulic sealing nozzle is adopted, so that it can be plasticized when opening and closing mold.
7. Use of pre-plasticizer design enables screw to be plasticized during cycle except for injection and holding time.
8. Adopt pressure-holding device, so that screw can be plasticized in pressure-holding section.

Open mold at the highest speed without tearing finished product and producing a loud mold opening noise. Some sophisticated injection molding machines have decompression equipment before mold opening, even high-speed mold opening will not produce noise. In order to achieve accurate mold stop position under high-speed mold opening, brake valve or closed-loop control can be used.

On small injection molding machines with low ejection force, pneumatic ejection can be used, which is higher than hydraulic ejection speed. Electric ejection is faster than pneumatic ejection.
Mold can be designed to be ejected by mold opening action instead of ejection device on injection molding machine. This method can only be ejected once. This is the simplest method of ejecting while opening mold.
Using independent oil circuit, gas circuit or circuit control, it can realize function of multiple ejection while opening mold while ejecting.
Equipped with video and computer equipment, it can quickly analyze whether finished products are all dropped after one ejection. Second ejection is performed when not all of them are dropped, so 99% of cycles in above example are ejected only once, which saves average cycle time.

Multiple ejection of some finished products can be ejected by vibration of injection molding machine. Thimble does not need to be fully retracted each time to shorten time of multiple ejections.
Last ejection can be started at the same time as mold clamping. Since stroke of thimble is shorter than that of mold plate, thimble will always be fully retracted before mold is locked.

The shortest cycle time consists of time required for mold closing, injection, pressure holding, cooling and mold opening. Feeding is carried out at the same time during "cooling time", mold opening and closing, and even while maintaining pressure. Multiple ejections are performed at the same time when mold is opened, and last ejection is performed at the same time when mold is closed. In this case, up to three actions can be performed at the same time, each action has an independent drive. It may be that all three are oil circuits (such as three oil pumps), all three are circuits (electric injection molding machine) or a combination of oil circuits, air circuits and circuits.
Electric injection molding machines generally have 4 servo motors to drive injection molding, feeding, opening, closing and demolding respectively. Advantage is that parallel operation can shorten cycle. In fact, hydraulic injection molding machine can also achieve this goal when using 3 independent oil circuits. Therefore, this advantage is not a patent for electric injection molding machines.
Since injection cannot be made when mold is opened, four servo motors cannot operate at the same time. Analysis in Figure 10 shows that the shortest period of three parallel actions has been reached.

Many injection molding machine specifications indicate dry running time, but generally it is only calculated theoretical time, ignoring acceleration and deceleration of mold plate, of course quality of moving mold is not calculated, which is shorter than actual mold opening and closing time. According to Euromap's standard, idle running time is time required for opening and closing of mold plate, opening and closing stroke is set at 0.7 times four-column space. In the shortest cycle, only mold opening, closing time (idle running time) and injection time are related to design of 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/wall thickness ratio of 300 or more. In order to prevent molten plastic from solidifying before filling cavity, thin-wall injection molding is high-speed (high acceleration and deceleration) injection molding. So-called high-speed injection molding means that forward speed of screw is above 300 mm/s. Under high-speed injection molding, injection time is generally within 0.5s. High-speed injection is assisted by accumulators. Oil pump fills accumulator during "cooling time", or a small oil pump can be used to fill accumulator during times other than injection and pressure holding. Stored high-pressure oil is released during injection, which can generally increase injection speed by 3 times.

If product can be demolded by blowing, it is easy to blow while opening mold. Generally, air blowing is delayed after mold is opened to hide the demolding time in mold opening time.    

Pneumatic ejection is realized on a small injection molding machine that requires little ejection force, so that mold opening and closing are parallel to ejection, which saves about 1s of ejector action time, which is considerable on a small injection molding machine, finally reached a cycle time of 2s.

Cost of an injection molding machine with three independent oil circuits is relatively high. Two parallel-action injection molding machines generally use one oil circuit for clamping device and another oil circuit for injection device. This is based on consideration that injection molding machine generally divides clamping device oil circuit board and injection device oil circuit board.
Hybrid oil-electric injection molding machine integrates advantages of two drives, and is ingeniously matched. General design is electric feeding and other actions of hydraulic pressure, such as Husky's Hyelectric injection molding machine.

Dual pump design uses only one oil pump during injection. Oil circuit can be slightly changed so that two pumps are used for injection, thereby increasing injection speed to nearly double.      
Hot runner mold
Hot runner can shorten cycle in many ways.
1. If diameter of cold runner is larger than thickness of finished product, "cooling time" will have to wait for runner to cool to a certain extent before mold can be opened, but finished product has already cooled, causing waste. Runner of hot runner mold does not need to be cooled, and finished product determines cooling time.     
2. Amount of plastic in cold runner is a percentage of amount of finished product, even if it is heavier than finished product, injection and feeding time will be lengthened. Use of hot runner molds saves injection and feeding time required for runner plastics.
3. There is a nozzle in cold runner, mold opening stroke should be increased.
4. In multi-cavity injection molding, cold runner does not guarantee whether finished product will fall or not. It needs to be taken out with a robotic arm (full automatic operation) or manually (semi-automatic operation), cycle will be slowed down.