Injection molding is a molding method of injection and molding. Advantages of injection molding method are fast production speed, high efficiency, automated operation, a wide variety of colors, shapes from simple to complex, sizes from large to small, and precise product size. Product is easy to update and can be made into parts with complex shapes. Injection molding is suitable for mass production and molding processing fields such as complex-shaped products.
At a certain temperature, completely molten plastic material is stirred by a screw, injected into mold cavity with high pressure, cooled and solidified to obtain a molded product. This method is suitable for mass production of parts with complex shapes and is one of important processing methods.

 

Injection molding equipment and mold

Temperatures that need to be controlled in injection molding process include barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect plasticization and flow of plastics, while the latter temperature mainly affects flow and cooling of plastics. Each type of plastic has a different flow temperature. Same type of plastic has different flow temperatures and decomposition temperatures due to different sources or brands. This is due to different average molecular weights and molecular weight distributions. Plasticization process of plastics in different types of injection machines is also different, so barrel temperature is also different.

Nozzle temperature is usually slightly lower than maximum barrel temperature. This is to prevent "drooling phenomenon" that may occur in straight-through nozzle. Nozzle temperature cannot be too low, otherwise it will cause premature coagulation of melt and block nozzle, or premature coagulation will be injected into mold cavity and affect performance of product.

Mold temperature has a great influence on intrinsic performance and apparent quality of product. Mold temperature is determined by presence or absence of plastic crystallinity, size and structure of product, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.).

Pressure in injection molding process includes plasticizing pressure and injection pressure, which directly affects plasticization of plastics and quality of products.

(Back pressure) When using a screw injection machine, pressure on the top of screw when screw rotates backward is called plasticizing pressure, also known as back pressure. Size of this pressure can be adjusted by overflow valve in hydraulic system.
In injection, size of plasticizing pressure needs to be changed according to design of screw, requirements of product quality and type of plastic. If these conditions and speed of screw remain unchanged, increasing plasticizing pressure will strengthen shearing effect, that is, it will increase temperature of melt, but it will reduce efficiency of plasticization, increase backflow and leakage, and increase driving power.
In addition, increasing plasticizing pressure can often make temperature of melt uniform, colorant mixes evenly and gas in melt is discharged. In general operation, plasticizing pressure should be as low as possible while ensuring quality of product. Specific value varies with type of plastic used, but it rarely exceeds 20 kg/cm2.

 

Pressure curve in injection molding

In current production, injection pressure of almost all injection machines is based on pressure applied by plunger or screw top to plastic (converted from oil circuit pressure). Role of injection pressure in injection molding is to overcome flow resistance of plastic from barrel to cavity, give molten material filling rate and compact molten material.
Injection molding cycle
Time required to complete an injection molding process is called molding cycle, also known as molding cycle. It actually includes following parts:

 

Molding cycle: Molding cycle directly affects labor productivity and equipment utilization. Therefore, in production process, relevant time in molding cycle should be shortened as much as possible while ensuring quality. In the entire molding cycle, injection time and cooling time are the most important, they have a decisive influence on quality of product.
Filling time in injection time is directly inversely proportional to filling rate. Filling time in production is generally about 3-5 seconds. Holding time in injection time is pressure time of plastic in cavity, which accounts for a large proportion of the entire injection time, generally about 20-120 seconds (extremely thick parts can be up to 5-10 minutes).
Before molten material freezes at gate, amount of holding time affects dimensional accuracy of product. If it is later, it has no effect. Holding time also has a most favorable value, which is known to depend on material temperature, mold temperature, size of main channel and gate. If size of main channel and gate and process conditions are normal, pressure value that results in the smallest fluctuation range of product shrinkage rate is usually used as standard.
Cooling time is mainly determined by thickness of product, thermal and crystallization properties of plastic, and mold temperature. End point of cooling time should be based on principle of ensuring that product does not cause changes when demolding. Cooling time is generally between 30 and 120 seconds. It is unnecessary to have a long cooling time, which will not only reduce production efficiency, but also cause demolding difficulties for complex parts, and even produce demolding stress when forced demolding. The other time in molding cycle is related to whether production process is continuous and automated, as well as degree of continuity and automation.

Injection pressure is provided by hydraulic system of injection molding system. Pressure of hydraulic cylinder is transmitted to plastic melt through injection molding machine screw. Under pressure, plastic melt enters vertical flow channel (also main flow channel for some molds), main flow channel, and branch flow channel of mold through nozzle of injection molding machine, and enters mold cavity through gate. This process is injection molding process, or filling process.
Existence of pressure is to overcome resistance in flow process of melt, or conversely, resistance in flow process needs to be offset by pressure of injection molding machine to ensure smooth progress of filling process.
During injection molding process, pressure at nozzle of injection molding machine is the highest to overcome flow resistance of melt throughout process. Afterwards, pressure gradually decreases along flow length to the front wave front of melt. If exhaust inside mold cavity is good, final pressure at the front end of melt is atmospheric pressure.
There are many factors that affect melt filling pressure, which can be summarized into three categories:
⑴ Material factors, such as type and viscosity of plastic;
⑵ Structural factors, such as type, number and position of gating system, cavity shape of mold and thickness of product;
⑶ Process elements of molding.

Injection time mentioned here refers to time required for plastic melt to fill cavity, excluding auxiliary time such as mold opening and closing. Although injection time is very short and has little effect on molding cycle, adjustment of injection time has a great effect on pressure control of gate, runner and cavity. Reasonable injection time helps melt to fill ideally, it is of great significance to improve surface quality of product and reduce dimensional tolerance.
Injection time is much lower than cooling time, which is about 1/10 to 1/15 of cooling time. This rule can be used as a basis for predicting the total molding time of plastic parts. When performing mold flow analysis, injection time in analysis result is equal to injection time set in process conditions only when melt is completely pushed by screw to fill cavity. If screw pressure holding switch occurs before cavity is filled, analysis result will be greater than setting of process conditions.

Injection temperature is an important factor affecting injection pressure. Barrel of injection molding machine has 5 to 6 heating sections, and each raw material has its appropriate processing temperature (for detailed processing temperatures, please refer to data provided by material supplier). Injection temperature must be controlled within a certain range.
If temperature is too low, melt is not plasticized well, which affects quality of molded part and increases difficulty of process; if temperature is too high, raw material is easy to decompose. In actual injection molding process, injection temperature is often higher than barrel temperature. Higher value is related to injection rate and performance of material, and can reach up to 30℃.
This is caused by high heat generated by shearing of molten material when it passes through injection port. There are two ways to compensate for this difference when doing mold flow analysis. One is to try to measure temperature of molten material when injecting into air, and the other is to include nozzle in modeling.

When injection molding process is about to end, screw stops rotating and only moves forward. At this time, injection molding enters holding stage. During holding process, nozzle of injection molding machine continuously replenishes cavity to fill volume vacated due to shrinkage of part.
If cavity is filled without holding pressure, part will shrink by about 25%, especially ribs will form shrinkage marks due to excessive shrinkage. 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 needs to be overcome when screw reverses and retreats to store material. High back pressure is beneficial to dispersion of colorants and melting of plastics, but it also prolongs screw retraction time, reduces length of plastic fibers, and increases pressure of injection molding machine. Therefore, back pressure should be lower, generally not exceeding 20% of injection pressure. When injecting foam plastics, back pressure should be higher than pressure formed 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 will reduce input heat and reduce the temperature. However, since results of this change are difficult to estimate, it is not easy to make corresponding adjustments to machine.