Fundamentals of injection molding

Fundamentals of injection molding

Injection molding is an engineering technology that transforms plastics into useful products that retain their original properties. Important conditions for injection molding process are temperature, pressure and corresponding exposure time, which affect course of plasticization and cooling.

1. Temperature control

1. Cylinder temperature: The temperature that needs to be controlled during injection molding process includes cylinder temperature, nozzle temperature, and mold temperature. The first two temperatures mainly affect plasticization and fluidity of plastics, while last temperature mainly affects fluidity and cooling of plastics. Each plastic has a different pour point The same plastic has a different pour point and decomposition temperature due to different sources or grades This is due to difference in average molecular weight and molecular weight distribution Plastics in different types of injection The plasticizing process in machine is also different, so choice of barrel temperature is also different.

Fundamentals of injection molding

Injection workshop

2. Nozzle temperature: The nozzle temperature is usually slightly below maximum barrel temperature, which prevents "salivation phenomenon" that can occur in a straight-through nozzle. The temperature of nozzle must not be too low, otherwise it will cause molten material to solidify prematurely and block nozzle, or affect product performance due to injection of prematurely solidified material into cavity

3. Mold temperature: Mold temperature has a great influence on internal performance and appearance quality of product. The mold temperature level determines presence or absence of plastic crystallinity, product size and structure, performance requirements, and other process conditions (melt temperature, injection speed and pressure, molding cycle, etc.).

2. Pressure control: The pressure in injection molding process includes plasticizing pressure and injection pressure, and directly affects plasticizing of plastics and product quality.

1. Plasticizing pressure: (back pressure) When an injection molding machine is used, pressure on molten material at top of screw as screw rotates and recedes is called plasticizing pressure, also known as back pressure. The value of this pressure can be adjusted by means of a pressure relief valve in hydraulic system. In injection, amount of plasticizing pressure is constant with speed of screw. As plasticizing pressure increases, melt temperature will increase, but plasticizing speed will decrease.

In addition, increasing plasticizing pressure can often lead to a uniform melt temperature, uniform mixing of color material, and degassing of melt. In general, plasticizing pressure should be kept as low as possible while ensuring good product quality. The specific value depends on type of plastic used, but usually rarely exceeds 20 kg/cm2.

2. Injection pressure: In current production, injection pressure of almost all injection molding machines is based on pressure exerted on plastic by top of plunger or screw (converted from oil circuit pressure). The role of injection pressure in injection molding is to overcome resistance to flow of plastic from cylinder into cavity, ensure filling speed of molten material, and seal the molten material.

Fundamentals of injection molding

Injection molded trays

3. Forming cycle

The time it takes to complete an injection molding process is called an injection molding cycle, also known as an injection molding cycle. In fact, it includes following aspects:

Moulding cycle. The molding cycle directly affects labor productivity and equipment utilization. Therefore, in production process, corresponding molding cycle time should be as short as possible in order to ensure quality. In entire molding cycle, injection time and cooling time are most important and they have a decisive influence on quality of product. The filling time during injection is directly inversely proportional to filling speed, and filling time in production is usually around 3-5 seconds.

Pressure holding time during injection is time of pressure on plastic in cavity, which accounts for a relatively large part of total injection time, usually around 20-120 seconds (for very thick parts, this can be up to 5-10 minutes) ) . Before molten material at gate freezes, holding time will affect dimensional accuracy of product, and if later, it will have no effect. The holding time also has an optimal value, which, as you know, depends on temperature of material, temperature of mold, dimensions of gate and gate.

If gate and gate dimensions and process conditions are normal, pressure value with smallest range of product shrinkage rate fluctuation will usually prevail. The cooling time is mainly determined by thickness of product, thermal and crystallization properties of plastic, and mold temperature.

The end of cooling time should be based on principle of ensuring that product does not change when demoulded. The cooling time is typically 30 to 120 seconds. If cooling time is too long, it is not necessary, which not only reduces production efficiency, but also affects difficult production. Parts also cause difficulties in demoulding, and even if forced demoulding, demoulding stress will occur. Other times in molding cycle are related to whether manufacturing process is continuous and automatic, and degree of continuity and automation.