Injection molding cycle, also known as molding cycle, refers to time required to complete an injection molding process. It includes all the time in injection molding process, and time composition of injection molding cycle is shown in Table 1.
Table 1 Time composition of injection molding cycle

Molding cycle directly affects production efficiency and equipment utilization. Therefore, during injection molding, on premise of ensuring quality of product, relevant time of each stage in molding cycle should be shortened as much as possible.
In the whole molding cycle, injection time and cooling time are the most important, they are not only main components of molding cycle, but also play a decisive role in quality of product.

Filling time in injection time refers to time from start of injection to filling mold cavity. Its size is inversely proportional to filling rate; filling rate is determined by injection rate. Filling time is shortened, filling speed is increased, orientation is decreased, shear rate is increased, and apparent viscosity is decreased. In order to ensure product quality, filling rate should be properly controlled. For products with high melt viscosity, high glass transition temperature, fast cooling rate, glass fiber reinforced products and low foaming products, rapid injection (ie high pressure injection) should be used to reduce melt filling time. In actual production, filling time of small products is generally 3 to 5s, and time of large products is tens of seconds.

 

Dwell time is time it takes to compact plastic in cavity. Length of holding time is determined by structural size of product, material temperature, size of main channel and gate. Usually, after holding pressure and injection temperature are determined, it is determined by testing according to use requirements of product. Specific method is: first select a shorter holding time to form product, weigh product quality after demoulding, then gradually increase holding time, repeat molding and weighing process, when product quality no longer increases with extension of holding time, critical pressure holding time is set as optimal holding pressure time. Under normal process conditions, reasonable sprue and gate size, optimal pressure holding time is usually time when fluctuation range of product shrinkage is the smallest.
Pressure holding time accounts for a large proportion of the entire injection time, generally about 20 to 120s, and products with complex shapes or extra thick products can be as high as 5 to 10 minutes; for products with simple shapes and small external dimensions, pressure holding time can also be very short, such as a few seconds.
Before gate melt freezes, length of dwell time has a great influence on quality of product. If dwell time is too short, product will not be compact, dents will easily occur, and dimensions will be unstable; if dwell time is too long, product will be unstable. Internal stress increases, resulting in deformation, cracking, and difficulty in demolding.

Cooling time is mainly determined by thickness of product, thermal and crystalline properties of plastic, and mold temperature. Length of cooling time should be based on fact that product does not deform during demoulding. The shorter time, the better, generally between 30 and 120s. If cooling time is too long, it not only prolongs production cycle, reduces production efficiency, but also causes difficulty in demoulding for complex products; if cooling time is too short, products are prone to defects such as deformation.

The other time in molding cycle refers to time of opening mold, demoulding, applying mold release agent, placing insert and closing mold, which is related to degree of continuity and automation of production process.
For determination of injection time and molding cycle, please refer to Table 2 and Table 3.
Table 2 Injection times for some plastics

Table 3 Empirical methods for determining molding cycle time