When it comes to injection molding manufacturing process, most friends think of injection molding technology. In fact, injection molding technology is an important part of injection molding manufacturing process, but we cannot forget what is purpose of injection molding technology - is to obtain more long-term stable profits for enterprise. Leaving this purpose, what technology is no good. So what are factors that affect adjustment of process parameters?

Injection pressure is provided by hydraulic system of injection molding manufacturing process system. Pressure of hydraulic cylinder is transmitted to plastic melt through screw of injection molding machine. Under pressure push, plastic melt enters vertical flow path of mold (also mainstream channel for some molds), main flow channel, and branch flow path through nozzle of injection molding machine, enters mold cavity through gate. This process is an injection molding manufacturing process, or a filling process. Pressure is present to overcome resistance during melt flow, or conversely, resistance present during flow process needs to be offset by pressure of injection molding machine to ensure a smooth filling process.
During injection molding manufacturing process, pressure at nozzle of injection molding machine is highest to overcome flow resistance throughout melt. Thereafter, pressure gradually decreases toward front end of melt along flow length. If internal exhaust of cavity is good, final pressure at the front end of melt is atmospheric pressure.
There are many factors affecting melt filling pressure. There are three categories in general: (1) material factors such as type and viscosity of plastic, and (2) structural factors such as type, number and location of casting system, and cavity of mold. Shape and thickness of product, etc.; (3) Process elements of molding.

 

Injection time mentioned here refers to time required for plastic melt to fill cavity, and does not include auxiliary time such as opening and closing of mold. Although injection time is short and effect on molding cycle is small, adjustment of injection time has a great effect on pressure control of gate, runner and cavity. Proper injection time helps ideal filling of melt and is very important for improving surface quality of product and reducing dimensional tolerances.
Injection time is much lower than cooling time, which is about 1/10~1/15 of cooling time. This rule can be used as basis for predicting molding time of plastic parts. In the case of mold flow analysis, injection time in analysis results is equal to injection time set in process conditions only when melt is completely driven by screw rotation to fill cavity. If holding pressure of screw occurs before cavity is filled, analysis result will be greater than setting of process conditions.

Injection temperature is an important factor affecting injection pressure. Injection molding machine barrel has 5 to 6 heating sections, and each material has its proper processing temperature. Injection temperature must be controlled within a certain range. If temperature is too low, melt plasticization is poor, affecting quality of molded parts, increasing difficulty of process; if temperature is too high, and raw materials are easily decomposed. In actual injection molding manufacturing process, injection temperature is often higher than barrel temperature, higher values are related to injection rate and material properties, up to 30℃. This is caused by high heat generated by shearing of melt as it passes through injection port. In mold flow analysis, difference can be compensated in two ways, one is to try to measure temperature of melt-to-air injection molding manufacturing process, the other is to include nozzle in molding.

At the end of injection molding manufacturing process, screw stops rotating and only advances, at which point injection molding manufacturing process enters packing phase. During pressure keeping process, nozzle of injection molding machine continuously feeds cavity to fill volume vacated due to shrinkage of workpiece. If cavity is filled without holding pressure, part will shrink by about 25%, especially if tendon is too large to form a shrinkage mark. Holding pressure is generally about 85% of maximum filling pressure, of course, it should be determined according to actual situation.

 

Back pressure refers to pressure that screw needs to overcome when reversing backstock. Use of high back pressure facilitates dispersion of colorant and melting of plastic, but at the same time prolongs screw retracting time, reduces length of plastic fiber, and increases pressure of injection molding machine, so back pressure should be lower, generally not exceeding 20% of injection pressure. When molding foam, back pressure should be higher than pressure created by gas, otherwise screw will be pushed out of barrel. Some injection molding machines can program back pressure to compensate for reduction in screw length during melting, which reduces input heat and reduces temperature. However, since result of this change is difficult to estimate, it is not easy to adjust machine accordingly.