injection moulding process

injection moulding process

Injection Moulding Process

The definition of injection moulding process

The general injection moulding process is to feed polymer pellets or powder into the injection moulding machine via barrel, After heated, compression, shear, mixing and transmit, it gets homogeneous and melting (The process is called the plasticizing) there. Then with pressure applied by the plunger or screw, the high temperature melt polymer would go through the nozzle via the runner system into pre- closed low temperature plastic mould cavity, cool and get solidified there, and finally it is ejected out of the plastic mould after mould open. A Plastic product with a certain geometry and accuracy will be achieved and this method can be applied for the complex shape plastic production, also it is one of the most important methods for the mass production.

injection moulding process

injection moulding

Plastic injection moulding process is generally divided into 4 stages:

  1. Filling,
  2. Packing,
  3. Cooling and
  4. Ejection,

and These 4 stages is a complete and continuous process and directly determine the quality of the products:

1, Filling Stage; filling is the first step in the process of a whole injection moulding process cycle, the time starts from the plastic mould close until the mould cavity filled about 95%. In theory, the shorter the filling time is, the higher the production efficiency will be, but in practice, the moulding time or injection speed should be restricted by many conditions.

  • High speed filling: the shear rate will be higher when the injection speed is high, and the viscosity is reduced by shear, which leads to lower overall flow resistance, and the local viscous heating can also cause the solidified layer thickness smaller. Therefore, in the flow control stage, the filling behavior is often determined by the size of the volume. Due to the high speed filling, the shear thinning effect become larger and the cooling effect of the thin wall is not obvious.
  • Low speed filling. The heat transfer control is filled with low speed, and the shear rate is lower. And the flow resistance is larger for the local viscosity is higher. Due to the slow heat transferring, the heat transfer effect is more obvious, and the heat quickly is taken away by the cold mould wall. With a small amount of viscous heating, the thickness of the solidified layer is thicker, so the flow resistance of the wall is further increased.

Due to fountain flow, the plastic polymer chain flow front moves almost parallel. Therefore contact surface polymer chains are parallel to each other when two strands of plastic melt meet and the heterogeneous nature of the two strands of the melt due to different time, pressure and temperature, results in meet area poor strength. In the light of the appropriate angle, naked eye can find a clear joint line, which is the forming mechanism of the weld line.

Weld line not only affect the appearance of plastic parts, but also make the stress concentration because of the loose structure, which leads to the joint part broken easily. Generally speaking, under the high temperature, the strength of the weld line is better, because the high temperature makes the polymer chain more active and can penetrate each other. In addition, near high temperature region, the thermal properties of the melt are almost same.so the strength is very good there.

2, Packing(holding) Stage; packing(holding) is to continue to exert pressure to the melt to increase the density of plastic, in order to compensate the shrinkage of plastic. In the process of holding pressure, due to the mould cavity has been filled with plastic, back pressure is high. In the holding process, the injection moulding machine can only move slowly forward, and the plastic flow velocity is also relatively slow, which process is called the pressure holding flow.

During the holding period, the melt plastic accelerates to get cooled due to the mould wall, and the melt viscosity increases quickly, so the flow resistance in the mould cavity is very big. In the late period of the holding pressure, the density of the material increases continuously, and the plastic parts are gradually formed. The holding pressure is kept until the gate is sealed.

In the holding period, due to the very high pressure, the melt plastic can be compressed. In the area of high pressure, the plastic is more compact, the density is higher; while the pressure is lower on other place, the plastic is loose and its density is lower, so the density distribution changes with different position and time. In the holding process, plastic flow rate is very low so the flow is no longer a leading role, and the pressure now is the main factor affecting the packing process.

In the process of holding pressure, the plastic fill the mould cavity, which is gradually solidified at this time. In the cavity pressure is transfer to the mould wall surface by means of the solidifying plastic, which pressure trends to open the mould. Therefore, it is necessary to apply appropriate clamping force to the mould. In normal circumstances, the pressure will slightly open the mould a little, which is helpful for mould venting; but if this force is too large, it easily cause moulding flash, overflow and even open the mould. Therefore, when buy the injection moulding machine, we should choose machine with large enough clamping force, in order to prevent the mould open and can effectively keep holding pressure.

3 Cooling Stage; for the injection mould, cooling system design is very important. This is because after released from the mould the moulded plastic products only cool to certain rigidity to avoid deformation due to external force. Since the cooling time accounts for about 70% ~ 80% of the whole moulding cycle, a good cooling system can greatly shorten the moulding time, improve the injection production and reduce the cost.

Improper design will make the moulding time longer and increase the cost. And the poor cooling even will cause the plastic products warping deformation . According to the experiment, the heat transferred from the melt into the mould is generally divided into two parts, 5% by radiation and convection transferred to the atmosphere, and the remaining 95% moves from the melt to the mould wall directly.Heat in the melt plastic is transferred to the water cooling pipe via mould base where heat is carried away by the coolant. Other heat that is not cooled by cooling water continues to be scattered into the air.

The injection moulding process cycle consists of the close time, filling time, packing time, cooling time and ejection time. Cooling time takes the largest proportion of cycle time, about 70% ~ 80%. So the cooling time will directly affect the plastic product manufacture cycle time and production.

After ejection out of the mould, the temperature of the plastic products should be cooled lower than that of the deformation, in order to prevent the distortion caused by the residual stress and the external force. Factors mainly affect the cooling quality lists following:

  • The main  factor is plastic product wall thickness. The greater the injection part wall thickness is, the longer the cooling time will be. Generally speaking, the cooling time is approximately proportional to the square of the thickness of the plastic products, or it is proportional to the 1.6 time of the maximum flow channel diameter. That is, if the thickness of the plastic product doubles, the cooling time will increase by 4 times.
  • Plastic mould steel and its cooling mode. The mould steel here mainly includes the mould core, cavity and mould base. The higher the thermal transfer coefficient of the mould steel is, the better effect of the heat transfer in the unit time will be and the cooling time will be shorter. The layout of cooling water pipe is very important. The closer the cooling water pipe is to the mould surface, the bigger the diameter is and the more the number are, the better the cooling effect is and the shorter the cooling time is.
  • Coolant flow. The quicker the cooling water flow is, the more heat the cooling water takes. Properties of coolant, the viscosity and heat transfer coefficient of the cooling liquid also affect the heat transfer effect of the mould. The lower the viscosity of the cooling liquid is, the higher the heat transfer coefficient will be. The lower the temperature is, the better the cooling effect will be.
  • Plastic choice, Plastic Thermal Conductivity is a measure of a plastic that transfer the heat from the hot to the cold. High Plastic Thermal Conductivity coefficient means the better thermal transfer effect or the low specific heat of plastic (temperature is easy to change).the better the heat transfer effect is, the shorter the cooling time will be.
  • Injection moulding process parameter. The higher the material temperature and mould temperature are, the longer the cooling time will be.
  • Cooling system design : the cooling pipes are designed to ensure the uniform and rapid cooling effect. The purpose of the cooling system is to maintain the proper and efficient cooling. Cooling pipes shall be machined in standard size for easy processing and assembly. In the design of cooling system, the mould design must be based on the wall thickness, size of the plastic parts, the location and size of the cooling pipes, the length of the pipes, the pipes type, layouts of the pipes, the flow rate and the heat transfer characteristics of the cooling material.

4 Ejection Stage; it is the last stage of the plastic injection moulding process  cycle. Although the products have been solidified, the ejection is very important for the product quality, the improper ejection may lead to the deformation and other defects when the force is uneven.

There are two main ejection ways: ejection pins and stripper plates. Mould ejection system should be designed according to the structural of the product in order to ensure product quality. For the ejection pins mould, ejection pins layout should be as uniform as possible and the large ejection resistance and high strength and stiffness area should be chosen for ejection to avoid plastic deformation and damage. And stripper plates are generally used for the deep and thin-wall container which does not allow the ejection traces, especially for the transparent products. The advantages of the stripper plate are the large and uniform ejection force, smooth movement, no obvious ejection traces.