Injection Moulding Process
The injection moulding process begins with the material mix flowing from the hopper of the feeder into the barrel of the extruder, where the mixture is heated and given shear by the rotation of the screw, thus melting the material mix. The molten material is injected at high-pressure into a closed mould, filling all the cavities in the mould. As the mould is relatively cool, the molten material will solidify or start to crystallize, when the polymer has semi crystalline characteristics. The solidified material will then fill completely all the mould cavities. After achieving its modulus, the well formed finished articles are ejected from the mould. This process is repeated in hundreds or even thousands of injection cycles until the compound material is finished.
A cycle of the injection moulding process consists ofthe following stages:
The machine with the mould closed , injects the molten materialinto the mould by operating the injection screw. The pressure and flow rate of the material play a very important role.
In this stage the pressure inside the mould is maintained while the crystalization process occur. The duration of the holding process depends on how fast the material can completely crystallize, for example Polypropylene requires a longer holding time due to its slow crystallization rate.
During the holding stage, the screw rotates and goes backwards, while generating a mixing effect and the barrel is refilled with material from the hopper. This process also causes a plasticizing effect on the material.
During this cooling process the pressure decreases and the material solidifies. This process takes the longest time and determines the cycle time.
Next the mould opens and by operating the ejection mechanism the moulded articles are ejected.
The resin supplier provides injection grade resins of various polymers. This article discusses the usage and the processing practice of Polypropylene (PP) as the material, considering its large demand in the injection moulding application. Practically PP does not need a drying process, unless the polymer is filled with minerals. Two important parameters in the injection moulding process are the Pressure and Temperature of the process. Best results can be achieved when the temperature range is around 200 to 290 oC, while the temperature at the feeding section is 20-25oC lower than at the injection section. If better mixing is required, the temperature profile should be like a bell, where the highest temperature is located in the middle. In case the temperature is too high, flashing and shrinkage occur that cause sink marks, warpage, and shrinkage. The moulded material can also be more fragile. On the other hand, when the temperature is too low, it is possible to cause flow marks, weld lines, surface clarity problems, lamination, and even short shots.
The injection pressure depends on the size and the level of complexity of the mould. Generally, the injection pressure is around 1000 to 1500 psi. It must be high enough in order to get an excellent filling process inside the cavities and avoid shrinkage, hollow space, sink marks, and poor dispersion problems. Excessive pressure can cause the material to stick to the mould and result in flashing. In case there is a shrinkage problem, the injection time should be increased.
Back pressure of around 50-100 psi is generally enough. It is should be kept at a minimum unless greater shear is required due to pigment dispersing problems.
When the screw recovery is too slow, resulting in an unacceptable throughput level, increasing the screw rpm and decreasing the back pressure are good steps to overcome the problem. Another solution is to speed up the melting process by increasing the temperature in the zone after the feeding section. Any additive, that decreases the friction between the polymer and barrel in the feeding section, also slows down the screw recovery due to the low material conveying ability.
The mould temperature is set around 20 - 60 oC. It should be high enough for PP article surfaces to be free from flow marks, weld lines, lamination, hollow sections, short shots, and becoming fragile. However, if the temperatture is too high problems, such as shrinkage, warpage, sinking, and even material sticking to the mould surface, can happen. Uniform mould cooling is the general rule, but in case shrinkage and warpage appear, differential cooling is needed.
In order to direct the molten material in the mould to flow from the bushing sprue to the gate, which is the entry port to the mould cavity, the first consideration is to take the shortest way, and avoid curved paths, especially turns. The second consideration is that the runner track should be balanced so that the material can be fill all cavity(ies) uniformly and simultaneously.
There are two runner systems in injection moulding, i.e. cold runner and hot runner. The cold runner system lets the melted material cool down starting from the sprue and becomes colder when it reaches the runner. The outer part of the runner will solidify earlier, and when the mould cools,the entire cross section of the runner will solidify. When the moulded material is ejected from the mould, the runner will come off together with the articles. A cold runner is designed with a high ratio of volume/surface area in order to minimalize heat loss, premature solidification in the runner, and pressure drop.
In the hot runner system, a heating mechanism is involved in the process so that the material is in a molten condition when passing through the runner as well as when it fills the cavities for the solidification process. When the articles are ejected, the runner will stay in the mould, resulting in no extra work to seperate the runner solids from the articles.