Manufacturing Process Of Injection Moulding Engineering Essay
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Published: Mon, 5 Dec 2016
Injection molding is a manufacturing process for making parts from both thermoplastic and thermosetting plastic materials. First, materials is pass into a heated barrel, mixed and forced into a mold cavity and then it cools and hardens to the configuration of the mold cavity. After sometime, a product is designed, generally by an industrial designer or an engineer, molds are made by mold maker (toolmaker) form metal, either steel or aluminum and precision machined to form the features of the desired part. Also injection molding is commonly used for developed a varity of parts, from the smallest components to whole body panels of cars.
Alexander Parkes was first man who invented plastic in Britain in 1850 .He publicly demonstrated it at the 1862 International Exhibition in London; calling the materials he produced “Parke Sine”. Parke Sine could be heated, molded, and retain its shape after when cooled and he derived from cellulose. It was highly flammable, expensive to making and prone to cracking.
Most polymers may be also used including all thermoplastics, some elastomer and some thermosets. In 1995 injection molding was used approximately 18,000 different materials and that number was gone up at an average rate of 750 per year. Alloys or Blends are previously developed materials meaning that product designer can choose from a huge selection of materials, ones that exactly the right properties.
Injection molding machines consist of a material hopper, an injection ram or screw-type plunger, and a heating unit and they are known as presses. In this presses hold the molds in which the components are shaped. Presses are expresses the amount of clamping force which tares by tonnage and the machine can exert. Also mold keeping closed by this force during the injection process. When the tooling used to produce plastic parts in molding, Mold or Die are the common terms.
One more side, injection molding involves a nucleate that is upstream of a pressurized mold and also including extrusion system with reciprocating screw for forming a single phase solution of non nucleated blowing agent and polymeric material. Another aspect injection molding involves very thin walled microcellular material and very thin walled polymeric material.
In Injection molding, we can produced thermoplastic and thermosetting products, they are consists of injection molding molten plastic material from heated cylinder into a closed mold. After the mold heating and after some time they allowing cooling down and solidifying. At the end the part ejecting from the mold. Injection molding is good manufacturing process to making mass production. In this process; we can making typical injection molded parts like automotive parts, packing, consumer electronics goods and toys, and household articles.
Injection molding is a big buiseness in whole over the world and major part of the plastic industry. It is in second place of extrusion, which consumes approximately 36 wt% (1, 3, and 7). In the United States alone there are about 80,000 IMMs and about 18,000 extruders operating to process all the many different types of plastics. In the industry an IMM is not regarded as an extruder; however, it is basically a nanocontinuous extruder and in some operations is even operated continuously. ew extruder. It is used to making the melt.
In the market, we can see the many plastics extrusion processes. For example, a film blowing or wire coating. This process is continuous steady processes under ideal operating conditions. Films, tubes profiles, pipes, and sheets are suitable for making object with reproducing cross-section. In injection molding, 3-D shape and dimension control is feasible because it is cyclic and unsteady state process. The most advantages is that they gives significant degree of complexity to the injection molding process. In injection molding process, thermal, and thermodynamic properties are described by the same physical and constitution chemical relationships.
INJECTION MOLDING MATERIALS
It is a very scary task to selection the right material for each application and also they require more deep knowledge of the injection molding resins available. Recommending the right type and grade of resin hinges on a thorough understanding of the product’s application. Before a determination is made, all application characteristics need to be carefully weighed. We do not limit the types of materials we process as per engineering and Prototyping Company.
Below is merely a sampling of the most commonly requested injection molding materials.
Injection Molding Materials
Engineering and general grade resins are available within 3-5 days.
Major Resin Suppliers:
Sabic Innovative Plastics (was GE)
LNP Engineered Plastics
Specialized Materials and Materials Processing:
Styrene (crystal clear and opaque)
Engineering Grades (typical):
GE Noryl GTX
Classification Of Plastics
By Some criteria, Plastics can be classified. In general an initial rough classification can be made according to their chemical structure and intial differentiation is between cross-linked and non cross-linked materials. Moreover, cross-linked materials like Elastomer and Thermo sets and non cross-linked materials like Thermoplastics.
Typical Properties of Plastics
Range of densities
Wide rang of mechanical properties
Easy process ability
Modifiability by additives
Low thermal and electrical conductivity
High chemical resistance
Low energy consumption for raw material production
Additives for Plastics Materials
There are few instances where a designer will have the leverage or lead time to work with materials suppliers on the development of an entirely new polymer for a particular application. Additives are commonly used to enhance certain specific properties (e.g. UV stability, stiffness, color etc ) that the base polymer is lacking. Additives include processing stabilizers, antioxidants, UV stabilizer, internal or external lubricants, Flame retardants or any number of other organic/inorganic used alone or in combination.
When the additives are used high concentrations at that time additives do have side effects which are sometimes important. For example, the reinforced materials are typically more difficult to process when considering problems such as achievable part surface finish, equipment/mold abrasion, weld (knit) line quality, and fiber orientation and also the addition of glass fibers to a material will typically improve properties such as modulus, strength and thermal conductivity.
The Molecular Structure of Plastics
The structure of the macromolecule is classified by the mechanical behavior of the material processed, as well as its processing properties. That’s why; we will take a closer look at the chemical structure of plastics.
In the simplest case, macromolecular materials are made of a single type of chain macromolecule, each of which consists of at least several hundred to thousand of atmos. Such a macromolecule is created when the same or different base units are linearly joined with main valence bonds between the links. The resulting macromolecules can be of different length. The length is described by the molecular weight. Usually the macromolecules of a polymer have a specific molecular weight distribution, which influences the processing and mechanical properties.
The type of copolymer depends on the chemical production process used to make it. The two monomers like alternating and block copolymers are introduced into the reactor simultaneously, for side chain copolymers the monomers are fed consecutively.
The Injection Molding Process
An injection molding machine can be divided into the below components:
Control system and
Tempering device for the mold
injector1.gif (62719 bytes)
Basic Injection Molding
In injection Unit’s, the main goal is to heat the material to the specified temperature until it reaches a viscosity that will allow the material to flow into the mold while under force. All injection machines there are two numbers that will gives an idea as to the size of the machine. The first number is the injection capacity and this will tell us what size part might be able to make with regard to the volume of plastic that can be injected.
Purpose of the Injection Unit
The injection unit must contain many components and present many duties that contribute to the performance of these duties.
The Heating Cylinder:-
The heart of the injection unit is the heating cylinder, also called the barrel. It is making in the form of a long, round tube and is made of an inexpensive grade of steel. The inside of the tube is lined, usually with a thin sleeve of high-quality hard tool steel that can withstand the abrasive nature of the injection process. In general, the sleeve has high chromium content.
In barrel, the outside part has heater bands strapped to it. The bands are placed along the entire length of the barrel with minimal space between them and are electrically activated. Also there are three heater zones: rear, center, and front. Each zone contains three or more heater bands (depending on the length of the injection cylinder) and each zone is individually controlled by an electrical unit located in the control panel of the machine. Each temperature control unit is fed temperature information by a thermocouple in a hole in the wall of the heating barrel in the area of the zone it is controlling and the control unit then decides whether more heat is required and, if so, energizes the heater bands in that zone. When the selected temperature is reached, the thermocouple informs the control unit, which stops sending electricity to the heater bands until the temperature drops again, at which point the cycle repeats.
The Basic Hopper:-
Basic Hopper Design
In hopper, raw plastic pellets are stored before they are introduced to the heating cylinder. In figure, it can be seen that this unit has tapered sides to facilitate dropping fresh material into the barrel. The hopper is designed to hold approximately 2 hours’ worth of raw material for the specific machine. The amount is based on average part weight and normal cycles usually produced on a machine of that size.
The base of the hopper should contain a magnet either an external drawer magnet that can be pulled out and cleaned while the machine is running its normal cycles, or a loose magnet that is placed inside the hopper and must be pulled out of the hopper for cleaning. While the latter is less expensive, the former is much easier to clean. The main aim of the magnet is to trap any loose particles of metal that may have been placed in the raw plastic either by accident or intentionally. Metal particles can ruin the sleeve of the surface of the screw or the injection barrel.
The Injection Screw:-
The screw is placed inside the heating barrel and an auger shaped rod. The main function of the screw is to auger fresh material from the hoper area into the heating area of the barrel. A secondary function is to mix and homogenize the molten plastic. The screw also generates heating friction which raise the temperature of the plastic and the friction is created because there is just a slight clearance between the surface of the screw flights and the inside wall of the barrel, usually only 0.003 to 0.005 in (0.008 to 0.013cm). As the material is brought forward along the screw flights, the plastic is squeezed tighter and tighter. The friction of squeezing generates heat.
The external heater bands used to softening the plastic with the help of more heat. The screw, however, does provide additional heat and this reduces the amount of electricity required to heat the plastic completely. The squeezing action of the screw is called shear. More shears can tear up the plastic molecules and degrade the material, making it inferior or even useless. For this reason, the screw itself can not used to impart all of the heat needed.
Injection Screw Designs:-
There are many different screw designs, with various shapes pf flights, distances between flights, amounts of shearing action, method of shutoff and screw tip geometries.
Screw Tip and check Ring:-
The tip itself is inserted through a check ring and seat designed to keep molten material from flowing back over the screw flights during injection. The tip fits into the face of the screw, usually with a left-hand thread to counteract the natural turning motion of the screw. Aright-hand thread would tend to unscrew as a result of the turning action of the screw. The screw tip angle lengths are determined by the viscosity of the plastic being molded. The material supplier or screw manufacturer can help make the final decision. In some cases, a general-purpose screw and tip can be utilized for a variety of similar materials, but it is better to use a specific design for a specific group of materials.
The usual injection machine uses a reciprocating screw. This simply means that screw pushes forward and pulls backward ( reciprocates ), acting a plunger to inject the molten plastic.
Nonreturn Valves and Ball Shutoffs:-
The purpose of the check-ring nonreturn valve mechanism is to keep molten plastic from escaping back over the screw as the mold. The check ring then is allowed to move forward as the screw augers fresh material forward to prepare for the next cycle. The action of the check ring allows that material to move in front of the screw tip. The sequence of the nonreturn is : –
The screw pushes forward, injection a charge of molten material into a mold.
The check ring is forced back against the screw tip seat and seals against it, preventing material from passing back over the screw.
The screw stops pushing and begins to turn (bringing new material forward).
The check ring slips forward under the influence of the pressure buildup.
Molten plastic flows into the space in front of the screw tip.
Sliding Ring Valve Ball Check Valve
Positive Shut-Off Valve
In both the check ring and ball shutoff cases, the plastic material is restricted, even when the nonreturn devices are in the open flow position. This restriction may causes degradation of the plastic with high viscosity or heat sensitive materials. So, non return mechanisms are generally not used when molding these materials and also they are not usually required due to the high viscosity of heat-sensitive plastics.
One final item makes up the complete injection unit. The nozzle of the machine is a two-piece, tube-shaped component that bolts to the face of the injection barrel.
The nozzle cap has an internal taper that matches that of the screw tip. Also, there is a tapered hole through the nozzle tip itself. The radius on the nozzle tip fits up against a matching radius in the sprue bushing of the injection mold. Also there is heater band on the nozzle tip. This is called the nozzle heater and it is controlled much like the other heater bands on the injection barrel.
(a) Nozzle with barrel in processing position (b) Nozzle with barrel backed out for purging
There are some nozzle designs that incorporate shutoff devices in the form of needles, springs, sliding balls, or combinations of these. Their purpose is much the same as the nonreturn valve in the screw tip; they shut off the flow of plastic for those materials that are not highly viscous, such as nylon, and that tend to drool from standard nozzles.
It is better to prevent noise generation in machinery during the design stage than to try to reduce it later. There are injection molding and auxiliary equipment machines built with exceptionally low noise levels. However, at times noise reduction by external means is preferred. Design changes to reduce noise sometimes decrease efficiency. Although this is relatively unimportant in small, fractional horsepower equipment, it becomes costly and wasteful in large, high power machinery that has been designed for maximum performance and efficiency.
One of the best ways to reduce machinery noise by external means is to place it in an acoustic enclose. Such enclose provide more dB industrial noise control. For this reason many are in use today, and they are very efficient when designed and installed correctly. A good acoustic enclose can easily reduce noise by 20 to 30 dB and more; a very simple design, by 10 dB.
The performance of an acoustic material can be described in terms of its transmission coefficient T, which is defined as the fraction of incident sound power transmission through the material. Materials with low transmission coefficients isolate noise better than materials with higher coefficients. If the material has say a transmission coefficient of 0.01 when airborne sound strikes one side of a wall, only 1% of the sound comes out the other side. Of course, the sound dose not “goes through” the wall; it makes the wall vibrate, and this radiates the sound again. Sound coefficient varies with frequency.
The clamping unit accommodates the mould and must perform the below functions:
The closing of the mould
Building-up the clamping force and locking the clamping unit
Retention of the locking force
Opening the mould
Ejection of the molded part
Clamping mechanism design be of some designs, either hydraulic, mechanical (toggle), or hydro mechanical.
Toggle Clamping Unit:- (1) Open and (2) Closed
Hydraulic Clamping Unit:- (1) Open and (2) closed
Toggle clamping unit:-
Toggle clamps contain various designs. An actuator moves the crosshead forward, extending the toggle links to push the moving platen toward a closed position. At the starting of the movement, mechanical advantages is low and speed is high; but near the end of three stroke, the reverse is true. Thus, When toggle clamps are desirable at that time they provide both high speed and high force at different points in the cycle. They are actuated either by hydraulic cylinders or ball screws driven by electric motors. Toggle-clamp units appear most well-matched to comparatively low-tonnage machines.
Hydraulic clamping unit:-
Hydraulic clamps are used on higher-tonnage injection molding machines and they available in the range 1300 to 8900 kN (150 to 1000 tons). More then, these units are also more flexible than toggle clamps in terms of setting the tonnage at given positions during stroke.
Hydraulic presses have historically been the only opportunity available to molders until Nissei introduced the first all electic machine in 1983.
Hydro mechanical clamping unit:-
Hydro mechanical clamps unit are used for large tonnages, generally above 8900 kN (1000 tons); they operate by three way and First is using hydraulic cylinders to rapidly move the mold toward closing position, Second is locking the position by mechanical means, and Three is using high pressure hydraulic cylinders to finally close the mold and build tonnage.
A system may use a combination of electrical and hydraulic and to make advantage of their distinct benefits.
Comparison of Clamp Design:-
Over the years many arguments have been presented showing each clamp design concept to be superior to the others. In reality each concept has its place, and the final deciding factor is usually cost.
The straight hydraulic design has proved over he years to provide long-terms reliability, excellent low pressure mold protection, and exact control of tonnage. It will not allow the clamp to be overstressed by high injection forces.
The hydromechnical clamp tends to have the advantages of the straight hydraulic, whereas the toggle is more complex because of the block action required.
The clamping tie-bars used for the fixed and movable platens on which the mold is attached. When the mold is closed at that time, they serve as uniformly loaded tension support member of the clamp. The open distance between tie-rods through which the mold must fit determines the maximum outside dimensions of the mold that can be used.
Platens are the precision, very rigid plates on which a mold is fastened and where subsequent clamping takes place. Injection molding machines can have two or more platens. The basic injection molding machine in the past usually had three platens; one to support a pressure clamping system applied to the mold and two for closing and opening the mold. Since the 1990s, injection molding used only two platens and become popular.
Two-platen press in comparison with more conventional hydraulic presses and the two-platen press may offer improved technical performance, cost advantages, reduced floor space, reduced weight, significantly, reduced clamp speed resulting in shorter cycle time, and reduced tonnage. Moreover, a three platen system may still be required when stability is important to ensure molding accuracy as in meeting repeatable tolerances on molded products. Different technical devices, usually located in the back of a platen and tie-bars constitute the pressure clamping system as discussed above.
There are different types of the platens used in injection molding and there are discuss as below:-
Clamping platens parallel and flat :-
It is important for a molding press to maintain the platen surface parallel to each other and flat when clamping pressure is applied. Bellowing is likely to occur with molds that have small cross-sectional area. Where this potential exists, one must use large support plates located between the molds and platens to distribute the load.
Floating Clamping Platens:-
A floating, or center platens is sometimes stacked between the main two platens in multiday light press machines. There can be more than one floating platen. Each daylight opening between any two platens permits inserting a mold. The total clamping pressure of the injection molding is applied equally via each platen on each mold. Thus, a multiday light machine has two or more movable platens that can handle two or more molds simultaneously during one machine operating cycle.
Pivoted floating Platens:-
Milacron has a patented multishot over molding process that uses a center platen that pivots( usually 180 degree but also 90 degree ) between shots. Makers of mold for such systems include Gram Technology and Ferromatik Milacron.
The conventional two shot process using conventional injection molding machine requires a larger-platen machine with higher clamp tonnage so that a shuttle or turntable action can be used. After shooting the first melt, the mold with this shot pivot and is positioned against a different mold half to accept the second shot, which is delivered from a second injection unit. This pivot design can also permit a four-sided, 90 degree indexing center platen with up to four different injection units.
Shuttle Clamping Platens:-
In injection molding machine, two platens are moved so that one mold is positioned to receive plastic material and then moves sideways, permitting the adjoining mold to receive the next shot, whereupon the shuttle cycle is repeated.
The result is to permit insert molding shorten the molding cycle. Horizontal injection molding machine can be used but more often vertical injection molding machine are used so that the shuttled molds are on a horizontal table (platen).
Book Opening Clamping Platens:-
The conventional way for a press to open is for the two platens to remain parallel from open to close to open. Book-action presses (also called Tilting Presses) use instead a motion of the platens that resembles that of a cover of a book. They are used principally in compression molding, reaction injection molding and printing.
Since the 1930s, they have been popular when they were introduced in rubber compression molding.
Rotary Clamping Platens:-
This system is also called a carousel system when the platens operated horizontally, or a Ferris wheel when they are operated vertically. It can be used to over mold two or more materials into a single part. For each plastic, a separate injection feed unit is then required. It is important to recognize that the stability of the rotary table system determines the quality.
Two or more mold halves are arranged in a circle on the moving platen with the matching mold halves attached to the fixed platen. The process starts with the first closed mold cavity receiving a shot of plastic. Upon opening, that cavity, with the plastic partially solidified, is rotated into the next position, where its matching mold cavity is recessed to receive the next shot.
If there are three or more plastic, the procedure continue. Thus when tha platens close after the initial startup, each cavity is simultaneously injected with the required plastic.
Molds For Injection Molding
A mold for injection molding is basically composed of the following five components:
Parts to be mounted on the injection molding machine;
Pathways for molten plastic flow composed of sprue, runners, and gates;
Parts composed of cavities and cores to form molding products;
Parts to cool down molded products;
Parts to eject molded products.
Function of Mold Components:-
Stationary and Movable Mounting Plates:-
Both the stationary and movable mounting plates are used for connecting the mold main body (cavity type and core) to the injection molding machine. The stationary mounting plate is provide with a hole to mount a locating ring, while the movable mounting plate is arranged with holes for ejection rods. Thermal insulation boards are attached to the mounting plates, depending on the molding requirements.
Stationary and Movable Mold Plate:-
The stationary mold plate constitutes the main part of the mold, when it is mounted on the injection molding machine. The stationary mold plate contains some cavities to hold the material during molding. Since the cavities have great influence on the appearance of the product, they should be polished to a mirror finish or in many cases chrome plated.
The movable mold plate has a core mold in it, as the main part of the mold. When the mold opens, after molten plastic injected into the cavities and core, molded products are designed to stick to the core side and de-mold easily. The mechanical strength of the movable mold plate should be high enough to prevent distortion caused by high pressure, flash, and warpage of the product.
Backing-plates are used for supporting and reinforcing the movable mold plate. The thickness of the backing plate should be calculated from injection pressure and the projection area of the product.
Ejector-Pins and Ejector Plates:-
Ejector pins are used for the ejection of the product from the mold. Round pins are commonly used, in view of the machinability and dimensional accuracy, but square shape and stepped pins are used in special cases. Since the ejector pins are fixed to an ejector plate and slide during knocking out products, the abrasion resistance and dimensional accuracy are important.
The ejector plate consists of two flat plates to hold ejector pins tightly. They should have sufficient wall thickness to avoid distortion due to high ejection force.
A spacer block is used for control the ejection distance for molded products. Its length should be equal to the summation of the ejector plate thickness and ejection stroke.
Return pins work to make the ejector plate return back from the ejected position to the initial position. The return pins, together with the ejector pins, are fixed to the ejector plate and guide it to the normal position through the stationary mold when the mold is closed.
Sprue Lock Pins:-
A sprue lock pin is attached at the end of a sprue. If the sprue is fixed to a sprue bushing during mold opening, molded products are not taken off smoothly. The sprue lock pin should be designed to hold the end of the sprue until products are ejected from the mold.
Hot Runner Mold System:-
The hot runner systems are classified into two major types:
The hot runner section to be heated is the area from the nozzle contact portion to the gates, i.e. from the sprue bushing to hot nozzles through a manifold.
The insulated Hot Runner,(b) Internally heated Hot-Runner System,(c) Externally heated Hot-Runner System.
External heating type:- This hot runner system is heated externally by means of a heating source located outside the runners. This allows less pressure and molten plastic stagnation, because molten plastics can smoothly flow in the whole channel. However, precise thermal insulation between the runner and cavities is more important, in comparison with the internal heating type.
Internal heating type:- This system has a relatively small heating source in the center of runners to heat up gently from inside. The runner is usually in the shape of a tube. A molten plastic layer is formed in the inner side of the runner close to the hot element, while a solidified plastic layer is formed in the area close to the cavity. Therefore, neither thermal insulation nor polymer leakage prevention is needed, owing to the formation of the solidified plastic layer. However, higher-pressure losses occur due to the decrease of the cross section of the flow channel.
A hot runner is classified into valve-gate or open-gate type. The valve gate type furnishes a mechanical open/close device at each gate.
Open Gate Type:- This comes with no mechanical open/close device. Therefore. If the gate diameter is too large, some gate-cut problems such as “stringiness” or “drooling” easily occur. The open-gate type leads to pressure loss at the gate than the valve-gate type. This structure is s
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