Multi-stage injection molding is a molding method in which screw has different injection speeds and injection pressures at different positions in an injection process according to program setting. This method is suitable for injection molded products with complex shapes. Multi-stage injection molding process should include: multi-stage melt, multi-stage back pressure, multi-stage injection pressure, multi-stage injection speed and multi-stage pressure holding, etc. It does not only refer to multi-stage injection process. Here we will focus on multi-stage injection process and briefly introduce other processes.

Development of injection molding technology has also experienced a development process from simple to complex. It is result of invention of injection molding materials, advancement of injection molding machine technology and product demand. Early injection molding machines did not have multi-stage injection molding capabilities.
Injection molding machine was invented in the late 19th century, and the first molding machine was patented in 1872 by two brothers, John and Isaiah Hyatt. Although simple by today's standards, equipment soon contributed to development of emerging plastic manufacturing industry, but it could only produce combs, buttons and other simple plastic products.
It has been more than 50 years since screw plastic injection molding machine was newly introduced in the 1950s. At present, 80% of engineering plastics industry uses injection molding. In recent years, increasing demand for injection molded products in automotive, construction, home appliances, food, pharmaceutical industries has promoted development and improvement of injection molding technology. Modern injection molding machines all have function of multi-level injection molding control (such as: multi-level melt, multi-level back pressure, multi-level injection pressure, multi-level injection speed and multi-level holding pressure, etc.).
In modern times, injection molded products have been widely used in various fields. Shapes of products are very complex, and properties of polymers used are also very different. Even for products of same material, due to different runner systems and geometric shapes of various parts, different parts have requirements for flow (speed, pressure) of mold filling melt, otherwise it will affect rheological properties of melt in this part or crystallization orientation of polymer, as well as apparent quality of product.

In actual injection molding process setting, many injection molding workers do not know how to use multi-stage injection molding control program correctly, especially how to find location of multi-stage injection molding, which is very blind, takes a long time to adjust machine, has many defective products, wastes a lot of raw materials, and has high production costs. Injection molding machine with multi-stage injection function cannot play its advantages.
At present, most of injection molding machines are multi-stage controlled injection speeds. Usually, the entire injection process can be divided into 3 or 4 areas as shown in Figure a, and each area can be set to its own appropriate injection speed. As shown in Figure b, method of using low speed at the beginning of injection, high speed when filling mold cavity, and low speed injection when filling is close to the end is adopted. By controlling and adjusting injection speed, appearance of product, such as burrs, spray marks, silver bars or burn marks, shrinkage, and warping, can be prevented and improved, so that size of each molded injection molded part is uniform.

 

Multi-stage injection control program can reasonably set multi-stage injection pressure, injection speed, holding pressure, and melting method according to structure of runner, form of gate, and structure of injection molded part, which is conducive to improving plasticization effect, improving product quality, reducing defect rate, extending life of mold and machine.

It is more scientific to set at least three or four stages when general injection molding parts are injected. Water inlet runner is the first stage, gate is second stage, product is injected to about 90% of glue is third stage, and remaining part is fourth stage (also called last stage).
For injection molding of products with simple structure and low appearance quality requirements, a three-stage injection program can be used. However, for injection molding of products with more complex structure, many appearance defects, and high quality requirements, more than four stages of injection control programs are required.
When setting several injection programs, it is necessary to conduct scientific analysis and reasonable settings based on factors such as structure of runner, form/position/quantity/size of gate, structure of injection molded part, quality of product, and exhaust effect of mold.