The process of EVA mainly includes four periods: filling, pressure maintaining, cooling and demoulding. These four periods directly determine the molding quality of the products, and these four periods are a continuous process without defects.

1. Filling period

Filling is the first step in the process of all injection cycles, starting from the beginning of mold closing to the end of mold cavity filling to about 95%. Theoretically, the shorter the filling time is, the higher the molding power is. But in practice, the molding time may be restricted by many conditions.

High speed fill. Because of the effect of shear thinning, the viscosity of the plastic decreases, which reduces the overall resistance to movement. Some viscous heating effects also make the thickness of the solidified layer thin. Therefore, in the period of activity control, the filling method generally depends on the size of the volume to be filled. That is to say, in the active control period, because of high-speed filling, the shear thinning effect of the melt is generally great, while the cooling effect of the thin wall is not obvious, so the effect of the rate is dominant. L

Low speed filling. As shown in Figure 1-3, when the heat conduction is controlled to fill at low speed, the shear rate is low, some viscosity is high, and the movement resistance is large. Due to the slow filling rate and activity of thermoplastics, the heat conduction effect is obvious, and the sensitive heat is taken away by the cold mold wall. With a few viscous heating phenomena, the thickness of the solidified layer is thicker, which further increases the resistance of movement at the thinner part of the wall.

Because of the fountain activity, the plastic polymer chain in front of the active wave is almost parallel to the active wave front. Therefore, when two strands of plastic melts meet, the polymer chains on the contact surface are parallel to each other; in addition, the properties of two strands of melts are different (the residence time in the mold cavity is different, and the temperature and pressure are also different), so the microstructure and strength of the junction area of the melts are poor. When the parts are placed at the right angle under the light, it can be found that there is obvious joint line attack, which is the formation mechanism of the weld mark. The weld lines not only affect the appearance of the plastic parts, but also cause stress convergence due to the slackness of the microstructure, and then make the strength of some of them reduce and crack.

Generally speaking, the strength of the weld line is better in the high temperature region. Because of the high temperature phenomenon, the polymer chain has better activity and can penetrate and coil each other. In addition, the temperature of the two melts in the high temperature region is relatively close, and the thermal properties of the melts are almost the same, which increases the strength of the weld area. On the contrary, in the low temperature region, the weld strength is poor.

2. Pressure maintaining period

The effect of EVA processing is to continue to apply pressure, compact the melt, increase the density of the plastic (densification), to compensate for the shortening of the plastic. In the process of pressure maintaining, since the cavity is now filled with plastic, the back pressure is higher. In the process of pressure maintaining compaction, the screw rod of injection molding machine can only move forward slowly, and the movement speed of plastic is also relatively slow. At this time, the activity is called pressure maintaining activity. During the pressure maintaining period, the solidification of plastic is accelerated by the cooling of the mold wall, and the viscosity of the melt increases rapidly, so the resistance in the mold cavity is very large. In the later period of pressure maintaining, the density of material continues to increase, and the plastic parts gradually form. During the pressure maintaining period, the mold cavity pressure reaches the highest value until the gate solidifies and seals.

During the pressure maintaining period, due to the appropriate high pressure, some compressible properties of plastics appear. In the area with high pressure, the plastic is relatively dense and has high density; in the area with low pressure, the plastic is relatively loose and has low density, so the composition density distribution changes with the direction and time. In the process of pressure maintaining, the flow rate of plastics is very low, and the main effect is not revived. In the process of pressure maintaining, the plastic is full of the mold cavity. At this time, the gradually solidified melt is used as the medium to transfer the pressure. The pressure in the cavity depends on the plastic to transfer to the surface of the mold wall, which has the trend of opening the mold. Therefore, appropriate mold locking force is required to lock the mold. Under normal condition, die expanding force will quietly open the die, which can help exhaust the die. However, if the die expanding force is too large, it is easy to form the burr, overflow or even open the die. Therefore, when selecting the injection molding machine, it is necessary to select the injection molding machine with enough mold locking force to prevent the phenomenon of mold expansion and to maintain the pressure.

3. Cooling period

In the process of EVA injection mold, the plan of cooling system is very important. This is because the molded plastic products only need to be cooled and solidified to a certain rigidity, and only after demoulding can the plastic products be prevented from deforming due to external forces. As the cooling time accounts for about 70% - 80% of the total molding cycle, the excellent cooling system can greatly shorten the molding time, improve the injection productivity and reduce the cost. If the cooling system is not properly planned, the forming time will be prolonged and the cost will be increased; if the cooling system is not uniform, the warpage and deformation of plastic products will be further formed.

According to the experiment, the heat from the melt into the mold is roughly divided into two parts: one is 5% transferred to the atmosphere by radiation and convection, and the other 95% transferred to the mold from the melt. Due to the effect of cooling water pipe in the mold, the heat of plastic products is transferred from the plastic in the mold cavity to the cooling water pipe through heat conduction through the mold base, and then taken away by the coolant through heat convection. A small number of heat that is not taken away by the cooling water will continue to conduct in the mold, and will be scattered in the air after contacting with the outside world.

The molding cycle of EVA injection molding consists of closing time, filling time, holding time, cooling time and demoulding time. The cooling time accounts for the largest proportion, about 70% - 80%. Therefore, the cooling time will directly affect the molding cycle and production of plastic products. During the demoulding period, the temperature of the plastic products shall be cooled to a temperature lower than the thermal deformation temperature of the plastic products, so as to prevent the plastic products from being slack due to the residual stress or warping and deformation caused by the external force of demoulding.

The factors that affect the cooling rate of products are:

Plastic products planning. The first is the wall thickness of plastic products. The greater the thickness of the product, the longer the cooling time. Generally speaking, the cooling time is approximately proportional to the square of the thickness of the plastic product, or to the 1.6-th power of the maximum flow channel diameter. That is to say, the thickness of plastic products is doubled, and the cooling time is increased by 4 times.

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

Cooling water pipe allocation method. The closer the cooling water pipe is to the mold cavity, the larger the pipe diameter, the more the number, the better the cooling effect and the shorter the cooling time.

Coolant flow. The larger the flow rate of cooling water is, the better the effect is.

The nature of the coolant. The viscosity and thermal conductivity of the coolant also affect the heat conduction effect of the mold. The lower the viscosity of the coolant, the higher the thermal conductivity and the lower the temperature, the better the cooling effect.

Plastic selection. Plastic refers to the measurement of the heat transfer rate of plastic from the hot place to the cold place. The higher the heat transfer coefficient of plastic, the better the heat transfer effect, or the lower the specific heat of plastic, the temperature changes slightly, so the heat dissipation is simple, the heat transfer effect is better, and the cooling time is short.

EVA machining parameter setting. The higher the material temperature, the higher the mold temperature, the lower the ejection temperature and the longer the cooling time.

Planning rules for cooling system:

The planned cooling channel shall ensure uniform and sensitive cooling effect.

The purpose of the planned cooling system is to maintain proper and powerful cooling of the mold. Standard standards shall be applied for cooling holes to facilitate processing and assembly.

When planning the cooling system, the die planner must choose the following planning parameters according to the wall thickness and volume of the plastic part: the orientation and standard of the cooling hole, the length of the hole, the type of hole, the arrangement and connection of the hole, the activity rate and heat transfer property of the coolant.

4. Demoulding period

Demoulding is the final step in the injection molding cycle of EVA. Although the product has been cold solid forming, demoulding still has a very important impact on the quality of the product. Improper demoulding method may cause uneven stress of the product during demoulding and deformation of the product during ejection. There are two methods of demoulding: top bar demoulding and stripper plate demoulding. In order to ensure the quality of the product, the proper demoulding method should be selected according to the structural characteristics of the product.

For the mould with ejector rod, the ejector rod should be set evenly as far as possible, and the orientation should be selected at the place with the maximum resistance to demoulding and the maximum strength and rigidity of the plastic part, so as to prevent the deformation and damage of the plastic part.

The stripper plate is generally used for demoulding of deep cavity and thin-walled containers and transparent products without pusher marks. This arrangement is characterized by large and uniform demoulding force, stable movement and no obvious traces left.