Injection molding machine adjustment skills: slow first then fast injection method and its application
During the injection molding process, speed control during the injection phase directly impacts part quality and production efficiency. The “slow-first, fast-later” injection method, a classic machine tuning technique, demonstrates significant advantages for complex parts and defect-prone applications. This method essentially divides the injection process into two or more stages: a lower injection speed is used initially. Once the front end of the melt has steadily filled a certain proportion of the cavity, a higher injection speed is used to complete the remaining filling. For example, when producing parts with fine patterns or precision inserts, a slow initial injection speed prevents high-speed melt impact on the mold, which can blur the pattern or shift the insert. Subsequent rapid injection reduces cooling losses during melt flow and ensures complete cavity filling. Practice has proven that the proper application of the slow-first, fast-later injection method can increase the surface quality rate of plastic parts by over 30% while shortening the molding cycle by 5%-10%.
The principle behind the slow-then-fast injection method stems from the flow characteristics of the melt within the mold cavity. When the melt first enters the cavity, its temperature is high and its fluidity is excellent. However, excessive injection speeds can easily lead to a series of problems: First, the violent impact at the front of the melt creates turbulence, entraining air and forming bubbles. Second, localized pressure increases near the mold gate, potentially leading to flash or internal stress concentrations within the part. Third, for structures with alternating thin and thick walls, high-speed melt flow can lead to unstable flow fronts at locations with sudden changes in wall thickness, resulting in weld marks. Slow injection speeds in the initial stages ensure smooth laminar flow, gradually expelling air from the cavity and reducing shear stress damage to the melt’s molecular chains. When the cavity is 70%-80% filled, the injection method switches to rapid speed. At this point, the melt has established a stable flow path. High-speed injection effectively prevents underfilling due to rapid cooling and uneven dimensional shrinkage caused by prolonged flow times.
Implementing the slow-first-faster injection method requires precise division of the injection stages based on the part’s structure and material properties. For small and medium-sized parts, this method typically involves two stages: the first stage, controlled at a speed of 10-30 mm/s, fills the cavity 60%-70% of the way; the second stage, increased to 50-100 mm/s, completes the remaining fill. For large, complex parts, a multi-stage injection control approach is required. For example, the injection speed may be further reduced when encountering structures like ribs and corners to ensure that the melt evenly coats every detail. For plastics with poor flow properties, such as PC and PMMA, the initial injection speed can be increased to 20-40 mm/s to prevent premature cooling of the melt. For materials with high flow properties, such as PE and PP, the initial injection speed should be reduced to 5-20 mm/s to prevent flash. Furthermore, injection pressure must be coordinated with injection speed. During the slow injection stage, pressure should be sufficient to overcome melt flow resistance, typically 80-120 MPa. During the fast injection stage, pressure can be reduced by 5-10% to reduce the risk of overfilling the cavity.
The slow-first-fast-then-fast injection method has specific application strategies for the production of different types of plastic parts. When producing parts with deep cavities, an initial slow injection speed allows the melt to slowly climb along the cavity walls, preventing “air entrapment” caused by high-speed flow. Once the melt reaches the bottom of the deep cavity, rapid injection uses inertia to push the melt toward the cavity top, reducing density unevenness caused by gravity. For parts containing glass fiber reinforcement, the slow phase reduces glass fiber orientation deviation and ensures its uniform distribution within the cavity. The rapid phase prevents glass fiber sedimentation due to slow flow, ensuring stable mechanical properties of the part. In the production of transparent plastic parts, this method effectively reduces shear heating during melt flow and prevents haze caused by material degradation. For example, in the production of PC lampshades, an initial injection speed of 20 mm/s to the gate and then a final injection speed of 80 mm/s can increase light transmittance by 2%-3%.
When applying the slow-first-fast-later injection method, it’s important to avoid common pitfalls to maximize its advantages. Some operators, driven by efficiency, excessively increase the speed in the second stage, causing the melt to violently impact the cavity end, leading to localized overheating or internal stress. In this case, mold trials should be conducted to determine the maximum safe speed, typically based on the absence of silver streaks and bubbles on the part surface. Furthermore, the stage switching point must be precisely set. If the switch is made too early, insufficient filling in the slow stage will lead to unstable melt flow in the fast stage. Switching too late will prolong molding time and reduce production efficiency. Pressure sensors installed on the mold can monitor the position of the melt front in real time and enable automatic switching. For beginners, a “gradual debugging method” can be used: starting with a fixed initial speed, gradually increasing the second stage speed, observing changes in part defects, and then reversing the initial speed to ultimately find the optimal parameter combination. Scientific application of this technique can not only improve part quality, but also extend mold life and reduce production costs.
Controlling the color of injection molded parts during production by adjusting the temperature
Color consistency of injection molded parts is an important indicator of product quality, and temperature, as a key parameter in injection molding, has a decisive influence on the coloring effect of the plastic melt. Temperature directly affects the final color of the plastic part by changing the dispersion of the pigment, the crystallinity of the plastic, and the fluidity of the melt. For example, when the temperature is too low, the pigment particles cannot be fully dispersed in the melt, resulting in color spots or streaks on the surface of the plastic part; excessively high temperatures may cause organic pigments to decompose and discolor. For example, red pigments tend to fade to brown at temperatures above 250°C. In actual production, even a temperature fluctuation of ±5°C can result in significant color differences in the same batch of plastic parts. Therefore, controlling color through precise temperature adjustment has become one of the core technologies in injection molding production.
The barrel temperature has the most direct impact on the color of injection molded parts, and the temperature settings in different areas need to match the characteristics of the pigment and plastic. The temperature of the barrel feed section should be slightly higher than the glass transition temperature of the plastic to ensure smooth transportation and initial melting of the raw materials. Too high a temperature at this stage will cause the raw materials to melt and agglomerate prematurely, affecting the uniform mixing of the pigment. The temperature of the compression section needs to reach the melting temperature of the plastic so that the pigment particles are fully dispersed under the action of shear force. For example, when processing ABS plastic, the temperature of the compression section is controlled at 200-220°C, which can increase the dispersion of inorganic pigments such as titanium dioxide by 40%. The temperature of the homogenization section needs to be adjusted according to the heat resistance of the pigment. For azo pigments with poor heat resistance, the temperature should be controlled below 230°C; for high-temperature resistant phthalocyanine pigments, it can be increased to 260-280°C to ensure uniform melt temperature.
