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  • The four stages of filling, holding pressure, cooling and demolding tell you - what is the plastic injection molding process

The four stages of filling, holding pressure, cooling and demolding tell you - what is the plastic injection molding process

The four stages of filling, holding pressure, cooling and demolding tell you - what is the plastic injection molding process

The plastic injection molding process mainly includes four stages: filling, pressure-holding, cooling, and demolding, which directly determine the molding quality of the products, and these four stages are a complete and continuous process.

I. Filling stage

The filling is the first step in the whole injection molding cycle, and the time is counted from the time the mold is closed to the time the mold cavity is filled to about 95%. Theoretically, the shorter the filling time, the higher the molding efficiency, but in practice, the molding time or injection speed is subject to many
However, in practice, the molding time or molding speed is subject to many conditions.

The shorter the filling time, the higher the molding efficiency, but in practice, the molding time or injection speed is subject to many conditions.

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

Low-speed filling, heat transfer control at low-speed filling, the shear rate is lower, the local viscosity is higher, and the resistance to flow is greater. Due to the slower rate of thermoplastic replenishment, the flow is slower, so the heat transfer effect is more pronounced, and heat is quickly taken away for the cold film wall. Together with a relatively small amount of viscous
In the stagnant heating phenomenon, the curing layer thickness is thicker, and further increases the flow resistance at the thinner part of the wall.

Due to the fountain flow, the plastic polymer chain in front of the flow wave is lined up almost parallel to the front of the flow wave. Therefore, the two plastic melt in the intersection, the contact surface of the polymer chain parallel to each other; coupled with the two melt properties are different (in the film cavity retention time is different, temperature, pressure is also different), resulting in melt adhesive.
This, together with the different properties of the two melts (different retention times in the film cavity, different temperatures, and pressures), results in a microscopically poor structural strength in the melt intersection area. When the parts are placed at an appropriate angle in the light and observed with the naked eye, obvious bonding lines can be found, which is the formation mechanism of melt marks. The fusion marks not only affect the appearance of the plastic part but also
Due to the loose microstructure, it is easy to cause stress concentration, which makes the strength of the part reduce and fracture.

Generally speaking, the strength of the fusion marks produced in the high-temperature area is excellent, so the polymer chains are more active and can penetrate and twist each other in the high-temperature area.
On the contrary, in the low-temperature region, the fusion strength is poor.

II. Pressure-holding stage

The role of the holding phase 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 backpressure 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 make a small movement forward, 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 rapidly during the holding pressure stage.
The resistance in the mold cavity is very high.

In the later stage of holding pressure, the density of material continues to increase, and the plastic is gradually formed.

During the holding phase, the plastic exhibits partial compressibility due to the high pressure. In the area of higher pressure, the plastic is denser and has higher density; in the area of lower pressure, the plastic is looser and has a lower density, thus causing the density distribution to change with time and until.
The density distribution changes with time.

During the holding process, the flow rate of plastic is low, 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 filled the mold cavity, and the gradually cured melt is used as the medium for pressure transfer.

The pressure in the mold cavity with the help of plastic transfer to the surface of the film wall, there is a tendency to open the mold, so the need for appropriate mold force for mold, mold force in normal circumstances will slightly open the mold, for the mold exhaust has a helpful role;

However, if the mold force is too large, it is easy to cause the molded products to burr, overflow material, and even open the mold. Therefore, in the selection of an injection molding machine, you should choose a large enough to prevent the mold-up phenomenon and can effectively maintain pressure.

III. Cooling stage

The design of the cooling system is very important in the injection molding mold. This is because only when the molded plastic products are cooled and cured to a certain rigidity, the deformation of plastic products due to external forces can be avoided after demolding.

Since cooling time accounts for about 70-80% of the whole molding cycle, a well-designed cooling system can significantly shorten the molding time, improve injection molding productivity, and reduce costs. The improperly designed cooling system will make the molding time longer and increase the cost; uneven cold cutting will further, cause warping and deformation of plastic products.

According to the experiment, the heat entering the mold from the melt is roughly emitted in two parts, part of which 5% is transferred to the atmosphere by radiation and convection, 95% of which 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 the
The heat is transferred to the cooling water pipe by heat conduction through the mold frame and then taken away by the cooling liquid through heat convection. The small amount of heat that is not carried away by the cooling water continues to be transferred in the mold until it comes into contact with the outside world and then is dispersed in the air.

The first phase 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, about 70-80%. Therefore, the cooling time will directly affect the length of the molding cycle and the output of plastic products.

The temperature of plastic products in the demolding stage should be cooled down to lower than the heat deformation temperature of plastic products to prevent the relaxation of plastic products caused by residual stress or the warping and deformation caused by the external force of demolding.

IV. Demolding stage

Demolding is the last part of the injection molding cycle. Although the products have been cooled down, the demolding still has a very important impact on the quality of the products.
The improper way of demoulding may lead to uneven force during demolding and deformation of the product when ejecting.

There are two main ways of mold release: top bar release and stripper plate release. When designing the mold, we have to choose the appropriate release method according to the structural characteristics of the product to ensure quality of the product.

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 greatest resistance to release 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 rods. This structure is characterized by large and uniform demolding, smooth movement, and no obvious traces left behind.
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