Injection time is a crucial parameter in the injection molding process. It refers to the time it takes for the melt to fill the mold cavity from the start of injection, directly affecting the quality of the plastic part, production efficiency, and raw material consumption. Properly setting the injection time ensures uniform filling of the melt within the mold cavity, reducing defects such as bubbles, material shortages, and weld marks. Conversely, injection times that are too short or too long can adversely affect part quality. In actual production, determining the injection time requires comprehensive consideration of the plastic part’s structural dimensions, material properties, mold design, and other process parameters, requiring repeated testing and optimization.
The injection time is closely related to the size and complexity of the part. Large, complex parts with thin walls and deep cavities require a longer injection time to ensure the melt fully fills every corner of the cavity. For example, large parts like car bumpers, with their large cavities and numerous complex structural details, typically require an injection time of 30 to 60 seconds. Small, simple parts like mobile phone buttons, on the other hand, may only require 2 to 5 seconds. If the injection time is insufficient, the melt is stopped before it has completely filled the cavity, resulting in problems such as missing material and incomplete shapes. However, if the injection time is too long, the melt remains in the cavity for an extended period, potentially generating significant internal stresses due to excessive cooling. This can lead to warping and cracking of the part after demolding.
Material fluidity is another key factor influencing injection time. Different types of plastics have varying melt flow properties. Plastics with good fluidity, such as polyethylene ( PE) and polypropylene (PP), can quickly fill the mold cavity at lower injection pressures, resulting in relatively short injection times. Plastics with poor fluidity, such as polycarbonate (PC) and polyoxymethylene (POM), require higher injection pressures and longer injection times to ensure complete cavity filling. For example, for a plastic part of the same size, the injection time might be 5 seconds with PE, while it might take over 10 seconds with PC. Furthermore, the material’s melt flow rate (MFR) also influences injection time. A higher MFR value indicates better material fluidity and a shorter injection time; a lower MFR value indicates a longer injection time.
There’s a close correlation between injection speed and injection time. Within a certain range, increasing injection speed can shorten injection time. A fast injection speed increases the melt’s flow rate within the mold cavity, allowing it to fill the cavity in a shorter time. This is particularly suitable for molding thin-walled parts, as it avoids material shortages caused by rapid melt cooling during the filling process. However, excessively fast injection speeds can also have drawbacks. For example, turbulence can easily occur as the melt flows within the mold cavity, entraining air bubbles. It can also cause material degradation due to excessive shear rates, impacting part performance. Therefore, when adjusting injection time, it’s important to balance the injection speed and the injection time to find the optimal balance. For example, for complex parts, a staged injection method can be used: using a lower injection speed in the initial filling phase to prevent melt impact with the cavity walls and vortexes. Increasing the injection speed in the later stages of the filling phase can shorten the overall injection time and ensure complete cavity filling.
The injection time setting also needs to consider the mold’s venting performance and cooling system. If the mold’s venting performance is poor, a too-short injection time will prevent the air in the cavity from being expelled in time, resulting in defects such as bubbles or burns. In this case, the injection time should be appropriately extended to allow the air more time to escape through the venting grooves. However, if the mold venting is good, an excessively long injection time will result in unnecessary time waste and reduced production efficiency. Furthermore, the mold’s cooling system will also affect the injection time. Molds with good cooling performance will cool the melt quickly within the cavity, and appropriately shortening the injection time can reduce the generation of internal stress. Molds with poor cooling performance, on the other hand, require adjusting the injection time to coordinate the cooling process and avoid deformation of the plastic part due to insufficient cooling. In actual production, the optimal injection time is usually determined through mold trials. An initial injection time is first set based on the part’s volume and material properties. Then, by observing the part’s appearance quality and dimensional accuracy, the injection time is gradually adjusted until satisfactory part quality is achieved.
