The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. It actually includes the following parts:

Molding cycle: The molding cycle directly affects labor productivity and equipment utilization. Therefore, in the production process, the relevant time in the molding cycle should be shortened as much as possible under the premise of ensuring quality. In the entire molding cycle, injection time and cooling time are the most important, and they all have a decisive influence on the quality of the product. The filling time in the injection time is directly inversely proportional to the filling rate, and the filling time in production is generally about 3-5 seconds.

The holding pressure time in the injection time is the pressure time for the plastic in the cavity, and it accounts for a large proportion of the entire injection time, generally about 20-120 seconds (extra-thick parts can be as high as 5-10 minutes). Before the melted material at the gate is frozen, the holding time has an impact on the accuracy of the product size. If it is later, it has no effect. The holding time also has the most favorable value, which is known to depend on the material temperature, mold temperature, and the size of the main runner and gate. If the dimensions of the sprue and the gate and the process conditions are normal, usually the pressure value that obtains the smallest fluctuation range of the product shrinkage rate shall prevail. The cooling time is mainly determined by the thickness of the product, the thermal and crystalline properties of the plastic, and the mold temperature. The end of the cooling time should be based on the principle of ensuring that no changes are caused when the product is demolded. The cooling time is generally between 30 and 120 seconds. It is not necessary for the cooling time to be too long. It will not only reduce production efficiency, but also affect complex parts. It is difficult to demould, and demoulding stress may even occur when demolding forcefully. The other time in the molding cycle is related to whether the production process is continuous and automated, and the degree of continuity and automation.

General injection molding machines can be adjusted according to the following procedures:

According to the temperature range provided by the material supplier, adjust the barrel temperature to the middle of the range and adjust the mold temperature.

Estimate the required injection volume and adjust the injection molding machine to two-thirds of the estimated maximum injection volume. Adjust the inverted cable (pumping glue) stroke. Estimate and adjust the secondary injection time, and adjust the secondary injection pressure to zero.

Preliminarily adjust the first-level injection pressure to half (50%) of the limit of the injection molding machine; adjust the injection speed to the highest. Estimate and adjust the required cooling time. Adjust the back pressure to 3.5 bar. Remove the degraded resin in the barrel. Adopt semi-automatic injection molding mode; start the injection molding process and observe the action of the screw.

If necessary, adjust the injection speed and pressure appropriately. If you want to shorten the filling time, you can increase the injection pressure. As mentioned earlier, since there is a process before full mold filling, the final mold filling pressure can be adjusted to 100% of the first-level injection pressure. The pressure must eventually be adjusted high enough so that the maximum speed that can be reached is not limited by the set pressure. If there is flash, the speed can be reduced.

After observing a cycle, adjust the injection volume and the switching point. Set the program so that the filling of 95-98% based on the injection weight can be obtained in the first level of injection molding.

When the injection volume, switching point, injection speed and pressure of the first-level injection molding are adjusted properly, the second-level holding pressure adjustment procedure can be carried out.

Adjust the holding pressure appropriately as needed, but do not overfill the cavity.

Adjust the screw speed to ensure that the melt is completed just before the cycle is completed, and the injection cycle is not restricted.

Shorten cycle time to increase productivity

For most injection molding manufacturers, the injection cycle can directly affect the following two main purposes:

1. Get more parts from machinery every day;

2. The parts meet the requirements of the customers.

The injection cycle consists of the following:

Cycle start-the screw begins to advance, injection;

Part gate cooling

The screw begins to rotate-the plasticizing stroke begins

Screw return is completed-screw rotation stops

Glue pumping occurs if necessary

Mold opening (may include pulling out the mold core)

The part can be ejected after being fully cooled

Eject

The mold is closed (may include the return of the mold core)

The mold closes-the cycle restarts.

The automatic injection cycle is in the same continuous sequence, and the same thing is repeated again and again. There are three main parts of the cycle:

Mold opening time, filling time, mold closing time, pressure holding time

The goal of improving productivity is to complete all necessary actions in a very short time, complete the ejection, and ensure that the mold is protected (including pulling out and retracting the slider and the side mold core). Therefore, any problem with the mold or injection molding machine that delays the mold opening time must be repaired. In addition, if the mold opening time is different for each injection, the parts will also be different.

Injection molding fills the cavity, in terms of materials with better fluidity (such as Delrin? Polyoxymethylene, Zytel? and Zytel? ST nylon, and Crastin?, Rynite? Polyester), the filling time should account for 1/10 to the entire cycle 1/8 of the time, the most important thing when filling is the fast and stable screw advancing time, and the lowest and stable injection pressure.

When the screw advances forward, the melt will be injected from the material pipe through the nozzle, vertical flow channel, horizontal flow, gate, and then into the mold cavity, where resistance will be encountered. This resistance is affected by nozzle diameter, runner size, gate size, product thickness, and mold exhaust design.

Flow resistance should be improved and reduced in the mold to achieve filling balance and stability. Otherwise, due to uneven filling, the size of the products injected into different cavities will be different, the strength will be insufficient, or the appearance will be poor.

Holding pressure time, when injecting crystalline materials, holding pressure time is the most important part of the process. This period of time is from the melt filling 99% of the cavity to the end of the gate solidification. The strength and toughness of the mold part are determined by whether the pressure is maintained on the melt after injection molding until the part/gate solidifies. When maintaining the pressure, it is also necessary to reserve a small section of the glue position in front of the screw. This pressure holding period is the key to prevent the cavity after the melt solidification and shrinkage, or the weakness of the gate position, which causes insufficient strength of the module.

Cooling time, when the melt enters the mold cavity and hits the metal surface, the melt cooling step has already begun. Since the solidification temperature of semi-crystalline materials such as Delrin? POM, Zytel? and Zytel? ST nylon, and Crastin? and Rynite? polyester is very high, the time required for cooling is very small. For general modules, after the melt is completed, the modules should have enough cooling time. If a problem is found when the module is ejected, the cooling time can be extended slowly until the problem is solved.

Mold opening time, the mold opening time is an important part of the entire cycle, especially for molds with embedded parts. Even in relatively standard molds, the mold opening time is often higher than 20% of the entire cycle.