What is Plastic Injection Molding And How Does it Work?

Injection molding process

The injection molding process mainly consists of 6 stages, including mold closing - filling - holding pressure - cooling - mold opening --Demolding. These six stages directly determine the molding quality of the products, and these six stages are a complete and continuous process.

Injection Molding Process - Filling Stage

Filling is the first step in the whole injection molding cycle, and the time is counted from the start of injection molding when the mold is closed until the mold cavity is filled to about 95%. Theoretically, the shorter the filling time, the higher the molding efficiency; however, in actual production, the molding time is subject to many conditions.

High-speed filling. High speed filling with high shear rate, plastic due to shear thinning effect and the presence of viscosity decline, so that the overall flow resistance to reduce; local viscous heating effect will also make the thickness of the curing layer thinner. Therefore, in the flow control phase, the filling behavior often depends on the volume size to be filled. That is, in the flow control phase, the shear thinning effect of the melt is often large due to high speed filling, while the cooling effect of thin walls is not obvious, so the utility of the rate prevails.

Low-rate filling. Heat transfer controlled low speed filling has a lower shear rate, higher local viscosity and higher flow resistance. Due to the slower rate of thermoplastic replenishment, the flow is slower, so that the heat transfer effect is more pronounced, and heat is quickly taken away for the cold mold wall. Together with a smaller amount of viscous heating phenomenon, the thickness of the curing layer is thicker, and further increase the flow resistance at the thinner part of the wall.

Due to the fountain flow, in front of the flow wave of plastic polymer chain row to almost parallel to the front of the flow wave. Therefore, when the two molten plastics intersect, the polymer chains at the contact surface are parallel to each other; together with the different nature of the two molten plastics, resulting in a microscopically poor structural strength of the melt intersection area. When the part is placed at a proper angle under light and observed with the naked eye, it can be found that there are obvious joint lines, which is the formation mechanism of melt marks. The fusion marks not only affect the appearance of the plastic part, but also have a loose microstructure, which can easily cause stress concentration, thus reducing the strength of the part and making it fracture.

Generally speaking, the strength of the fusion marks is better when the fusion is made in the high temperature area. In addition, the temperature of the two melt strands in the high temperature region is close to each other, and the thermal properties of the melt are almost the same, which increases the strength of the fusion area; on the contrary, in the low temperature region, the fusion strength is poor.

Injection Molding Process - Holding Stage

The role of the holding stage is to continuously apply pressure to compact the melt and increase the density of the plastic to compensate for the shrinkage behavior of the plastic. During the holding pressure process, the back pressure is higher because the mold cavity is already filled with plastic. In the process of holding pressure compaction, the injection molding machine screw can only slowly move forward for a small movement, and the flow rate of plastic is also slower, which is called holding pressure flow. As the plastic is cooled and cured by the mold wall, the viscosity of the melt increases quickly, so the resistance in the mold cavity is great. In the later stage of holding pressure, the material density continues to increase, and the molded part is gradually formed. The holding pressure phase should continue until the gate is cured and sealed, at which time the cavity pressure in the holding pressure phase reaches the highest value.

In the holding phase, the plastic is partially compressible because the pressure is quite high. In the higher pressure area, the plastic is denser and the density is higher; in the lower pressure area, the plastic is looser and the density is lower, thus causing the density distribution to change with position and time. The plastic flow rate is very low during the holding process, and the flow no longer plays a dominant role; the pressure is the main factor affecting the holding process. During the holding process, the plastic has been filled with the mold cavity, and the gradually cured melt is used as the medium to transfer pressure. The pressure in the mold cavity is transferred to the surface of the mold wall with the help of plastic, which has the tendency to open the mold and therefore requires proper clamping force for mold locking.

In the new injection molding environment, we need to consider some new injection molding processes, such as gas-assisted molding, water-assisted molding, foam injection molding, etc.

Injection molding process - cooling stage

In injection molding, the design of cooling system is very important. This is because only when the molded plastic products are cooled and cured to a certain rigidity, the plastic products can be released from the mold to avoid deformation due to external forces. Since cooling time accounts for about 70% to 80% of the whole molding cycle, a well-designed cooling system can significantly shorten the molding time, improve injection molding productivity and reduce costs. Improperly designed cooling system will make the molding time longer and increase the cost; uneven cooling will further cause warping and deformation of plastic products.

According to experiments, the heat entering the mold from the melt is emitted in two parts, a part of 5% is transferred to the atmosphere by radiation and convection, and the remaining 95% is conducted from the melt to the mold. Plastic products in the mold due to the role of cooling water pipe, heat from the plastic in the mold cavity through heat conduction through the mold frame to the cooling water pipe, and then through thermal convection by the coolant away. The small amount of heat that is not carried away by the cooling water continues to be conducted in the mold until it is dissipated in the air after contacting the outside world.

The molding cycle of injection molding consists of mold closing time, filling time, holding time, cooling time and demolding time. Among them, cooling time accounts for the largest proportion, which is about 70% to 80%. Therefore, the cooling time will directly affect the length of the molding cycle and the yield of plastic products. The temperature of plastic products in the demolding stage should be cooled to a temperature lower than the heat deformation temperature of plastic products to prevent the relaxation of plastic products due to residual stress or warpage and deformation caused by external forces of demolding.

Factors that affect the cooling rate of the product are:

Plastic product design aspects. Mainly the wall thickness of plastic products. The greater the thickness of the product, the longer the cooling time. Generally speaking, the cooling time is about proportional to the square of the thickness of the plastic product, or proportional to the 1.6 times of the maximum runner diameter. That is, doubling the thickness of the plastic product increases the cooling time by 4 times.

Mold material and its cooling method. Mold material, including mold core, cavity material and mold frame material, has a great influence on cooling rate. The higher the heat conduction coefficient of mold material, the better the effect of heat transfer from plastic in unit time, and the shorter the cooling time.

The way of cooling water pipe configuration. The closer the cooling water pipe is to the mold cavity, the larger the diameter of the pipe and the more the number, the better the cooling effect and the shorter the cooling time.

Coolant flow rate. The larger the flow of cooling water, the better the effect of cooling water to take away heat by thermal convection.

The nature of the coolant. The viscosity and heat transfer coefficient of the coolant will also affect the heat transfer effect of the mold. The lower the viscosity of the coolant, the higher the heat transfer coefficient, the lower the temperature, the better the cooling effect.

Plastic selection. The plastic's is a measure of how quickly the plastic conducts heat from a hot place to a cold place. The higher the thermal conductivity of the plastic, the better the thermal conductivity, or the lower the specific heat of the plastic, the easier the temperature change, so the heat can easily escape, the better the thermal conductivity, and the shorter the cooling time required.

Processing parameters setting. The higher the material temperature, the higher the mold temperature, the lower the ejection temperature, the longer the cooling time required.

Design rules of cooling system:

The cooling channel should be designed in such a way that the cooling effect is uniform and rapid.

The purpose of the cooling system is to maintain proper and efficient cooling of the mold. Cooling holes should be of standard size to facilitate processing and assembly.

When designing a cooling system, the mold designer must determine the following design parameters based on the wall thickness and volume of the molded part - the location and size of the cooling holes, the length of the holes, the type of holes, the configuration and connection of the holes, and the flow rate and heat transfer properties of the coolant.

Injection Molding Process - Demolding Phase

Demolding is the last part of an injection molding cycle. Although the product has been cold-set, demolding still has an important impact on the quality of the product. Improper demolding may lead to uneven force during demolding and deformation of the product during ejection. There are two main ways of demoulding: top bar demoulding and stripping plate demoulding. When designing the mold, we should choose the suitable demoulding method according to the structural characteristics of the product to ensure the product quality.

For molds with top bar, the top bar should be set as evenly as possible, and the position should be chosen at the place with the greatest release resistance and the greatest strength and stiffness of the plastic part to avoid deformation and damage to the plastic part.

The stripping plate is generally used for the demolding of deep-cavity thin-walled containers and transparent products that do not allow traces of push rod. The characteristics of this mechanism are large and uniform demolding force, smooth movement and no obvious traces left behind.