Plastic injection moulding has been used since the late 1800's and is today the most common manufacturing process to convert raw plastic material to the required form. This process is mostly used for thermoplastic materials which may in turn be melted, reshaped and cooled, but it can also be used for thermosetting plastics. A wide variety of high quality products can be manufactured using injection moulding. These can significantly vary in complexity, size as well as in application, on a fully automated basis at high speed. [1,4]
(The theory of)Plastic injection moulding can be divided into the following four individual steps:
Plasticizing - this consists of the melting of the solid plastic granules are to the required liquid consistency for injection at its correct melt temperature.
Injection - is the injection of the/this melt into the mould to fill the cavity or cavities.
Chilling - is the cooling of the moulded part to return it to its solid state. As it is cooled it also shrinks.
Ejection - is the removal of the cooled, moulded part from the mould cavity. [6]
Plastic Injection Moulding
In typical plastic injection moulding process, clean plastic granules are fed from the hopper into the barrel of the injection moulding machine. The plastic granules are melted by using the combined effect of heat, friction and pressure. While moving them forward, the reciprocating screw is also pressing them to the heated barrel. As the screw plunger moves forward, the melted plastic is injected into the mould cavity, through a nozzle and sprue, with the use of gates and runners. As the plastic component cools, it solidifies into the required shape and ejected from the mould. The stages in this process are described in detail in the next section.[1,2,groover]
Injection Moulding Cycle
The process cycle for injection moulding is typically between 2 seconds and 2 minutes. It consists of the following stages: clamping, injection, cooling and ejection.
Clamping
Prior to the injection of the material into the mould, the two halves of the mould must first be securely connected by the clamping unit. This is done by attaching the two halves of the mould to the platen. The two halves of the mould are called the mould cavity and the mould core. The mould cavity is mounted to a stationary platen and aligned with the nozzle of the injection unit. The mould core is mounted to a movable platen, which slides along the tie rods. While the material is being injected, a sufficient force is needed to keep the mould securely closed. This force can be created in two ways; either by the use of toggle lock or by the use of hydraulic lock. In the case of the toggle lock, the movable platen is moved by a mechanical toggle device which is actuated by a hydraulic cylinder. On the other hand, when the hydraulic lock is used the movable platen is moved by a hydraulic piston. The greater the machine and clamping force needed, the longer it will take to close and clamp the mould. Today's machines range from 15 to 4,000 metric tonnes available clamping force.[1,4, groover, ktieb irqiq]
Injection
The process of heating and injecting the plastic is done by the injection unit of the injection moulding machine. The raw plastic pellets are fed into the barrel of the injection machine from the hopper. The material is moved forward by a ram injector or a reciprocating screw. Today, the most commonly used is the reciprocating screw, which moves the material forward by both sliding axially and rotating. As the screw rotates, the plastic pellets are moved forward towards the mould. While moving forward, the plastic pellets are subjected to shear that is generated by the screw, which creates most of the heat that melts the plastic. This shear is created due to a non-uniform minor diameter of the screw?. The barrel wall is also fitted with ceramic resistance heaters to compensate for any heat losses through the barrel wall, and these heaters also help to obtain the required processing temperature. In the case of the ram injector, instead of the reciprocating screw, a ram or a plunger is used in order to push the material through a heated section. (The advantage of heating by internal shear is that it is self-regulating in that as polymer temperature rises, the viscosity falls. Thus, less heat is generated for the same amount of shear.)
The molten plastic is injected into the mould by the buildup pressure that is generated inside the barrel and by the forward movement of the screw. As the screw moves forward, the non-return valve is opened and the plastic is injected into the mould through the nozzle. ? The molten plastic passes through the sprue and runners in order to fill the cavity inside the mould. The plastic enters the cavity through a gate which directs the flow?. As the material starts to cool, it begins to shrink. In order to counter for this, the screw is held in the forward position for some time. This helps to keep a holding pressure on the solidifying material and also allows compensating material to enter the mould. When the material solidifies inside the mould, the screw retracts and is filled with more material for the next shot. [1,4, groover, ktieb iehor]
Cooling
As soon as the molten plastic makes contact with the surface of the mould, it begins to cool and solidify in the required shape. To reduce the residual stresses on the mould, the hold pressure is reduced to zero when the material has achieved a certain solidification state. This is done by moving the reciprocating screw backwards a few millimetres and thus closing the non-return valve. After the hold pressure is removed, the material is held into the mould for a certain cooling time until the temperature of the part is below the rubbery transition temperature. This allows the part to be removed from the mould without excessive distortion or shrinkage. The cooling is done by passing pressurized water through cooling channels built inside the mould in order to control the mould temperature. The maximum thickness of the wall and the thermodynamic properties of the material that are used affect the time the part takes to cool. [1,4, groover]
Ejection
After the cooling of the moulded part, the mould is opened and the part is ejected. The mould is usually designed so that the part sticks to the mould core when the mould is opened. ?After that the mould is opened, the extra plastic of the runner system is cut from the required moulded part by the mould?. Sometimes this is not done by the machine but by an operator. Ejector pins, coupled to an ejector bar, are used to push the moulded part out of the mould. A hydraulic actuator or air operated ejector valve are used to move the ejector bar. The collection of the part is either done by an automatic robot or it is felt to fall freely into a collecting box by the use of conveyor belts. When the part is clear from the mould tool, the complete moulding cycle can be repeated.[1,4, groover]
Post Processing
When the moulded part is collected, some post processing is needed. If the extra material of the runners is not cut by the mould, the part must be trimmed by the use of cutters. After cutting and separating the extra plastic of the runners from the required part, this plastic is grinded by regrind machines and mixed with the raw material in the hopper so that it can be reused. The major advantages of this process are: that this plastic does not need to be treated again in order to remove humidity from it together with the fact that there is no waste of material since the plastic that is cut is being reused.[4]
Defects
Shrinkage
Shrinkage occurs as the plastic cools in the mould. The amount of shrinkage depends on the molecular structure of the plastic that is used. Sink marks may occur in plastic parts in areas of thicker cross section such as junctions between side walls and base where plastic takes a longer period to cool. Higher temperatures increase the shrinkage of the plastic part due to higher molecular energy and subsequent ability to recoil. Shrinkage can be compensated by using higher packing pressure.
Weld lines
Weld lines occur when melt flow has been divided around an obstacle in the tool and rejoins on the other side. This can be seen as a hairline feature on the moulded part. Weld lines can affect the mechanical properties of the moulded part and due to this, special care should be taken when designing the mould so that the weld lines are minimized and placed in areas of least effect. The situation can be improved by increasing the melt temperature, mould temperature and the injection speed but this can create other problems.
Splash marks
If moisture in the material is present at the point where the material enters the cavity gate, there is a sudden decompression of the material which leads to a situation where the moisture volatilises off. This causes the splash effect. To counter for this, the plastic pallets are treated in order to remove moisture from them before putting them into the hopper.
Distortion and Moulded in stress
This defect occurs due to the molecular chain orientation of the plastic used. As the polymer is forced through small channels, the molecular chains become aligned. As the cooling is very fast, the polymer molecular chains do not have enough time to relax to their preferred coiled state. Due to this, they sort of freeze in their uncoiled state. After moulding, the part may distort as the molecular chains will try to recoil.
Other defects are:
burned parts due to high melt temperature;
ejector marks due to short cooling time and a high ejection force;
wrapage of parts due to (non-uniform cooling rate) uneven surface temperatures of the mould and non-uniform wall thickness of mould design;
surface imperfections due to high melt temperature;
incomplete cavity filling due to small injection stroke or injection speed that is too slow, and the latter causes freezing before mould is filled.[1,2,4]
Advantages and Disadvantages
Advantages
Versatile process - used to manufacture a wide range of products;
High level of production;
Able to replicate high tolerance level in the products being produced;
Lower costs for labour as most of the work is done by the machine;
Less scrap cost due to the precise design of the mould;
Ability to use a wide range of materials;
Little need to finish parts after moulding.
Disadvantages
Expensive equipment;
High running costs;
Need to design mouldable parts.[3,5]
Applications and uses
Injection moulding can be used for a wide range of products, from domestic to automotive use. Some typical applications of plastic injection moulding are packaging, bottle caps, automotive dashboards, automotive switches, pocket combs, wire spools, inside fridge doors and many other plastic products.[5]
Links:
1: http://www.azom.com/Details.asp?ArticleID=265#_The_Injection_Moulding
2: http://www.cheresources.com/injectionzz.shtml
3: http://www.global-plastic-injection-molding.com/injection-molding-how-plastic-is-molded.html
4: http://www.custompartnet.com/wu/InjectionMolding
5: http://en.wikipedia.org/wiki/Injection_molding
6: http://www.morganindustriesinc.com/injection-molding-process.htm
7:groover
8:ktieb irqiq
9: ktieb iehor
