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What is the plastic injection molding process?

The injection moulding process mainly consists of 6 stages: mold closing – filling – holding pressure – cooling – mold opening –Demolding, and other 6 stages. These 6 stages directly determine the moulding quality of the product, and these six stages are a complete nonstop process. also we concentrate on the 4 stages filling, holding pressure, cooling and demolding.

Filling stage

The filling is the first step in the whole injection moulding cycle, and the time is counted from when the mold is closed to start injection until the mold cavity is filled to about 95%. Theoretically, the shorter the stuffing time, the advanced the molding effectiveness.  However, molding time (or injection speed) is subject to many conditions in actual production.

High-speed filling

The shear rate is higher when filling at high speed, and there is a decrease in the plastic viscosity due to the effect of shear thinning, which reduces the overall flow resistance. Localized viscous heating effects also result in a thinner cured layer thickness. Therefore, in the flow control phase, the filling behaviour often depends on the volume size to be filled. In the flow control phase, the shear-thinning effect of the melt tends to be significant due to high-speed filling, while the cooling effect of the thin wall is not significant, 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 pace of thermoplastic replenishment, the flow is slower, so the heat transfer effect is more pronounced, and heat is quickly removed from the cold mould wall. Together with a minor viscous heating phenomenon, the thickness of the curing layer is thicker and further increases the flow resistance at the thinner part of the wall.

Because of the fountain flow, in front of the flow wave of plastic polymer chain row to almost parallel to the flow wave before. Thus, when the two beaches of plastic melt match, the polymer chains at the contact face are resemblant to each other; together with the different nature of the two strands of melt (additional retention time in the mould cavity, temperature and pressure are also extra), resulting in the poor structural strength of the melt intersection area on a microscopic level. When the part is placed at an appropriate angle in the light and observed with the naked eye, it can be found that there are clear 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 making the part’s strength decrease and fracture.

Generally speaking, the strength of the fusion marks is better when the fusion is made in a high-temperature area because the polymer chains are relatively more active at high temperatures and can penetrate and twist around each other. In addition, the temperature of the two melts in the high-temperature region is closer to each other, and the thermal properties of the melts are almost the same, which increases the strength of the fusion area. On the contrary, the fusion strength is poor in the low-temperature region.

Holding phase

The role of the holding phase is to continuously apply pressure to compact the melt and increase the plastic density (densification) to compensate for the shrinkage behaviour of the plastic. The back pressure is higher during the holding pressure process because the mold cavity is already filled with plastic. 

In the process of holding pressure compaction, the injection moulding machine screw can only slowly move forward for a slight 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 excellent. 

In the later holding pressure stage, the material density increases, and the moulded 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.

The plastic is partially compressible in the holding phase because the pressure is relatively 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 thickness is lower,  therefore causing the viscosity distribution to change with position and time.

The plastic flow rate is meagre during the holding process, and the flow no longer plays a dominant role. During the holding process, the plastic fills the mold cavity, and the gradually solidifying 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 tends to prop up the mold, so it needs proper clamping force to lock it. 

Under normal circumstances, the clamping power will slightly open the decay, which is helpful for the exhaust of the mould. However, if the clamping force is too large, it will easily cause the molded products to have burrs, overflow, and even open the mold. 

Therefore, when choosing the injection molding machine, you should choose the injection molding machine with a large enough clamping force to prevent the mold-up phenomenon and can effectively maintain pressure.

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

Cooling stage

Cooling systems are important in the field of injection molding, because molded plastic parts can only cool and solidify to a certain extent. After de-moulding, external forces can prevent plastic products from being deformed. Since cooling time accounts for about 70% to 80% of the entire moulding cycle, a well-designed cooling system can significantly reduce molding time, increase injection molding productivity and reduce costs. A poorly designed cooling system can increase molding time and cost.  Uneven cooling will further beget groundwork and distortion 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 rest is conducted from the melt to the mold. Plastic products in the mold due to the 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 the heat convection by the coolant away. The small amount of heat 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 injection moulding cycle consists of mold closing time, filling time, holding time, cooling time and demolding time. Among them, cooling time accounts for the most significant proportion, about 70% to 80%. Thus, the cooling moment will directly affect the distance of the molding cycle and the yield of plastic produces. 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 the warping 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 width of the product, the longer the cooling moment. 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 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, significantly influences the cooling rate. The advanced the heat conduction measure of mold material, the better the effect of heat transfer from plastic in unit time and the shorter the cooling moment.

The way of cooling water pipe configuration.

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

Coolant flow rate.

The greater the flow of cooling water (generally, achieving turbulent flow is better), the better the effect of cooling water to heat convection to take away heat.

The nature of the coolant.

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

Plastic selection.

The higher the thermal conductivity of the plastic, the better the thermal conduction effect, or the lower the specific heat of the plastic, the easier the temperature is to change, so the heat is easy to dissipate, the thermal conduction effect is better, and the required cooling time is shorter.

Processing parameters are set.

The higher the material temperature, the higher the mould temperature, the lower the ejection temperature, and the longer the cooling time required.

Demolding Phase

Demolding is the last stage in an injection molding cycle. Although the product has been cold-set, the demolding still impacts the quality of the product. Improper demolding may lead to uneven force during demolding and cause deformation of the product when ejecting.

There are two main ways of demolding.

  1. Top bar demoulding
  2. Stripping plate stripping.

When designing the mold, we have to choose a suitable demoulding method according to the product’s structural characteristics to ensure the product’s quality.

For the mold with the top bar, the top bar should be set as evenly as possible, and the position should be chosen in the place with the most significant release resistance and the biggest strength and stiffness of the plastic part to avoid deformation and damage of the plastic part.

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

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