Injection Moulding

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CERAMIC INJECTION MOULDING Ceramic injection moulding (CIM) is an innovative process of manufacturing wide range of products including the products which are complex in nature and fragile. This is because Ceramics have significant advantages characterised by their outstanding physical, chemical, mechanical, thermal and electrical properties. It is principally an adoption of the polymer injection moulding process that has been the main stay of polymer processing for over a century. Ceramic injection molding is ideal for high-volume production of complex, tight-tolerance components. Ceramic injection molding offers significant advantages over conventional forming methods, as given below: • Cost-effective technique for complex designs • Ability to produce net or near-net shape parts • Very tight tolerance control • Low-cost, high-volume manufacturing MATERIALS USED: The materials used in the mixture for injection moulding process mainly consist of ceramic powder, binder and plasticizer. BINDER: A binder is an organic ingredient used to bind together two or more other materials in mixtures. Its two principal properties are adhesion and cohesion. PROPERTIES OF BINDER: A binder should:       Have good flow property Be stable under mixing and moulding conditions Have good compatibility with other components of the mix Impart adequate strength to the moulded parts after binder removal Have a long life Be low at cost

PLASTICIZER: A substance added to a material mixture to produce or promote plasticity and flexibility and to reduce brittleness PARTS OF CERAMIC INJECTION MOLDING MACHINE

These machines are similar in construction and operation to that of plastic injection moulding machines. However, certain differences exist as detailed below: 1.HOPPER: The ceramic material used in the ceramic injection moulding process is supplied in the form of small pellets. These pellets are loaded into the hopper and then gravity fed into the barrel to pass through the plunger assembly. 2. BARREL: This is a chamber in which the plunger is located. The barrel is heated by heater bands. The heating cylinder or barrel is attached at the feed end to a metal stock which is usually called the hopper block. This block is water cooled so as to prevent material from fusing together before it can fall from hopper into barrel. The injection unit can slide usually on bushes set into the hopper block, along polished steel rods so that adjustment is possible between the nozzle and the mould such movement is useful for cleaning the unit for achieving sprue break. 3. BAND HEATERS: They are clamped to the outside of the barrel and are electrically connected so as to form number of zones. Each zone contains one or more bands and temperature of each zone is capable of controlling, so as to improve accuracy and efficiency. 4. NOZZLE: It is the connection between the mould and barrel through which the material flows during its passage to the mould cavities. Nozzles are divided into two types: Open nozzles: It minimizes polymer breakage.  Sealing nozzles: They are used for materials of sharp melting point e.g. nylon-88. Sealing nozzles are further divided into three types:    Thermal: They require sharp melting points. Mechanical: They are very efficient. Spring loaded: They are used to obtain required seal.[9]

CERAMIC INJECTION PROCESS:

There are essentially five steps in CIM process, as discussed below: 1. Tool Design and Manufacture: A key element in the success of the process is the ability to design tooling that is appreciable to both the requirements of the finished part and the complications of the subsequent manufacturing steps. Tool design must also consider the flow characteristics of the specific powder/binder feed stock. As with plastic injection moulding, all tools parting lines, gates and ejector pins. With ceramics, mould construction must also account for the shrinkage rate of the material. 2. Mixing: Preparation of mixture depends upon:  Flow characteristics  Quality of the final parts METHOD OF PREPARATION OF MIXER: SELECTION OF THE CERAMIC POWDER: In selecting the powder, two main issues that need to be addressed are (a) the composition of the powder and (b) powder characteristics such as the size distribution, shape and shape distribution, surface area etc. PROPERTY REQUIREMENTS ON THE CERAMIC CORE:        Good thermal shock resistance Good refractoriness Acceptable strength at metal poring temperature Low co-efficient of thermal expansion No metal-core reaction Good handling strength Low process shrinkage

PURPOSE OF BINDERS: The ceramic powders do not flow like fluids and so it is not possible to mould them into required shapes by any flow dependent shaping techniques. Binders which are viscous fluids in molten condition are therefore mixed with these powders so as to facilitate the powder particles to flow into the die cavity, fill homogeneously. The binder also helps hold the particulate structure in the desired shape until it is removed completely during the debinding stage. MIXING OF POWDERS AND BINDERS: The feed stock for injection moulding is made by high shear mixing of the powder with the binder systems. During mixing, the binder and the powder are combined to form homogenous mass. In order to ensure that the powder particles are completely surrounded by the binder at this stage, powder content must be broken up by the shear forces. It is therefore necessary that medium to high shear mixers are used for the purpose. Also, the binders systems used are normally solids at room temperature and therefore the mixing is to be carried out at temperatures where they form viscous fluids. The mixture of ceramic powder and binder is heat treated up to 250°C. (Refer Fig. ) HIGH SHEAR SIGMA MIXER Sub-micron size ceramic powders are mixed thermomechanically with a proprietary multi-component blend of binder constituents. The end result is a homogeneous blend of materials that is cooled and granulated for use as a moulding feedstock. 3. Moulding: Components are moulded in injection-moulding machines. The feedstock is gravity fed into the barrel of the moulding machine where it is heated and moved up to the nozzle by a plunger. Then plunger moves forward and rams the material through the nozzle into the cavity configured to the required shape. When the material in the cavity cools sufficiently to hold its shape, the machine opens the tool along the parting line and ejector pins are activated to push the part out of the cavity. The machine then closes the tool and the process is repeated. At this stage of the process, the component is referred to as a green part. It is common for these components to be produced in tools with multiple cavities thereby increasing productivity and reducing unit component cost.

Fig4. Preparation of Feed Stock

4. De binding: In this step the binder is removed from the moulded part. This can be done by several methods. Applicability of each method is defined by the chosen binder system. The strength of the debinded part is generally decreased after any of the debinding processes. Therefore the green component has to be handled very carefully. Thermal debinding: During thermal debinding the binder is removed through an applied heat. Debinding is carried out in a debinding reactor, often with a forced atmosphere circulation, at gently elevated temperatures. It can be formed depending on the selected powder. The green component obtained is treated in the electric hot furnace by placing the component in the block made up of Aluminium and aluminium powder is filled by spraying it on the component. The furnace contains molybdenum coils situated on the sides of the inner furnace. The component is maintained up to a temperature 650°C about 6 to 8 hours and then cooled. At this temperature the binder gets debinded from the green component. 5. Sintering: The green part is then placed in one of several high temperature furnaces. The firing atmosphere required by the particular ceramic powder dictates the specific type of furnace utilized. The temperature maintained will be around 1250°C for several hours. During sintering, the part will experience shrinkage ranging from 15% to 30%. This is determined by the binder system and ceramic powder being utilized. The fully sintered parts retain the complex shape of the moulded part and close dimensional tolerances can be achieved. MOULD DESIGN: A mould is a hollow cavity of a die in which molten material is injected and cooled to produce the desired component. A mould can produce several parts with in less amount of time. Mould design is extremely important as it affects the dimensions, appearance and properties of the injection moulded components. Some of the characteristics of the mould are given below: o The mould consists of two components, the injection mould and the ejector mould. Ceramic mixture enters the mould through a sprue in the injection mould. The sprue bushing seals tightly against the nozzle of the injection barrel of the moulding machine and allow molten ceramic to flow from the barrel into the mould, also known as the cavity. The sprue bushing directs the molten ceramic to the cavity images through channels that are machined into the faces of these plates, called runners. The molten ceramic flows through the runner and enters one or more specialized gates and into the cavity geometry to form the desired part. The amount of resin required to fill the sprue, runner and cavities of a mould is called a blast. Trapped air in the mould can escape through air vents that are ground into the parting line of the mould. If the trapped air is not allowed to escape, it is compressed by the pressure of the

o

incoming material and is squeezed into the corners of the cavity, where it prevents filling and causes other defects as well. o Sides of the part are typically angled slightly called “draft angle” to ease release of the part from the mould. Insufficient draft can cause deformation or damage. The draft required for mould release is primarily dependent on the depth of the cavity: the deeper the cavity, the more draft necessary. The mould is usually designed so that the moulded part reliably remains on the ejector side of the mould when it opens, and draws the runner and the sprue out of the injection mould side along with the parts. Ejector pins is a circular pin placed in either half of the mould (usually the ejector half), which pushes the finished moulded product, or runner system out of a mould.

After designing the mould care should be taken during mould filling process because it affects the shape and size of the component required. TECHNICAL REQUIREMENTS IN MOULD FILLING PROCESS: Mould filling is an important part of the injection moulding process and influences many of the properties of the finished moulding, including the following:  Surface finish.  Direction of shrinkage.  Mechanical properties.  Susceptibility to stress cracking and Heat resistance

MACHINE    INJECTION UNIT CLAMPING UNIT CONTROLS    

MOULD RIGIDITY GATE TEMPERATURE CONTROL

MOULDING

MOULDING COMPONENT     FORMULATION GRANULE SHAPE MOISTURE CONTENT  

ENVIRONMENT MAN

PERIPHERAL INSTRUMENTS CLIMATIC CONDITIONS

The moulding process involves three different kinds of pressures namely: 1. Injection pressure 2. Holding pressure 3. Cavity pressure 1) Injection pressure: It is the pressure exerted on the mixture in the plunger during the injection cycle. The plunger is used for discharging the mixture into the mould using the nozzle. 2) Holding pressure: It is the pressure exerted on the moulded part during a secondary pressure stage. Both the magnitude and the duration of the holding pressure are critical for dimensional accuracy. 3) Cavity pressure: As long as the gate is not yet frozen changes in the magnitude and the duration of the holding pressure will have an effect on the cavity pressure. After the gate is frozen no further influence can be exerted. It is not sufficient just to fill the cavity. Additional material must be pushed into the cavity to compensate for the volume change during cooling. This pushing is referred to as the „‟cavity or packing‟‟ pressure. DESIGN AND FABRICATION OF THE DIE: Design of the injection moulding die is the most crucial element in the successful production of CIM products. The die forms the cavity into which the IM feedstock id injected under suitable pressures and temperatures so as to fill the cavity. REQUIREMENTS IN DESIGN OF THE DIE:  The die layout should be planned in such a way as to facilitate easy opening and closing of the dies and smooth removal of the component after injection moulding.  The die cavity formed by closing the die before injection should remain dimensionally accurate at the injection pressures and temperatures.  The die material should not contaminate the injection moulding mix because of the flow erosion of the die; the material of the die should be hard enough to withstand erosion by the powder mix  The die cavity also should have an extremely fine surface finish to prevent injection moulding mix material sticking to the surface; this will also facilitate component ejection after moulding.

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