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    05. Plastic Injection Mold Design

    Production Process III 1

    Module 05: Plastic Injection Mold Design

    1. Introduction

    Injection molding (British origin: moulding) is a manufacturing technique for makingparts from both thermoplastic and thermosetting plastic materials in production. Molten

    plastic is injected at high pressure into a mold, which is the inverse of the product's shape.After a product is designed, (usually by an industrial designer or an engineer) molds are made

    by a moldmaker (or toolmaker) from metal, usually either steel or aluminium, and precision

    machined to form the features of the desired part. Injection molding is widely used for

    manufacturing a variety of parts, from the smallest component to entire body panels of cars.

    Injection molding is the most common method of production, with some commonly made

    items including bottle caps and outdoor furniture. Injection molding typically is capable oftolerances equivalent to an IT Grade of about 914.

    Injection molding is the one of the most commonly used manufacturing process for

    the plastic components. It is used to manufacture thin walled plastic parts for a wide variety

    of shapes and sizes. In this process, the plastic material is melted in the injection chamber andthen injected into the mold, where it cools and finally the finished plastic part is ejected.

    When a plastic material begins its journey through the injection molding process, the

    first thing that is considered is how the material is delivered and stored until it is used. The

    next step is to determine how the material will flow to the individual machines for molding,

    and finally, what process is needed to prepare the material so that it can be molded. Other

    side processes, such as color and additive feeding, also need to be considered if these apply.

    2. Materials UsedThe injection molding process can be used to process materials such as Acetal,

    Acrylic, Acrylonitrile Butadiene Styrene (ABS), Cellulose Acetate, Polyamide (Nylon),

    Polycarbonate, Polyester, Polyether Sulphone (PS), Polyetheretherketone (PEEK),Polyetherimide, Polyethylene, Polyphenylene Oxide, Polyphenylene Sulphide (PPS),

    Polypropylene (PP), Polyvinyl Chloride (PVC), and Elastomers.

    3. Construction

    3.1MATERIAL FEED PHASE

    AA) Drying of material

    The chemical structure of a particular polymer determines whether it will absorbmoisture. Due to their nonpolar chemical structures, a number of polymers (e.g., polystyrene,

    polyethylene, and polypropylene) are nonhygroscopic and do not absorb moisture. However,

    due to their more complex chemistry, materials such as polycarbonate, polycarbonate blends,acrylonitrilebutadienestyrene (ABS) terpolymers, polyesters, thermoplastic polyurethanes,

    and nylon are hygroscopic and absorb moisture.

    BB) Hopper

    The hopper is the section of the injection molding machine that stores material just

    before it enters the barrel of an injection molding machine. The hopper also has a holding

    area for the material as it is fed from its bulk storage (gaylords, railcars, etc.) and awaits any

    preconditioning of the material that may be needed, such as drying. Hopper size is a criticalelement in determining how to make the injection molding process efficient.

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    3.2MELT-CONVEYING PHASE

    CC) BarrelThe barrel is defined as an open-ended cylinder that controls the linear direction of the

    melt-conveying process, from the hopper to the mold. Barrel provides a frictional surface for

    the plastic material to assist in the melting of the plastic which is moving from granular (orpellet) form to molten form and it results in moving the material in a basically linear direction

    from the hopper to the mold. Material used for manufacturing of barrel is steel with a

    bimetallic liner and liner is made from a steel alloy.

    Figure 5.1: Injection molding machine

    DD) Heater bands & ThermocoupleSeveral types of heater bands are used for heating a barrel. These include tubular

    heaters, cartridge heaters, band heaters, and natural gas heaters. A thermocouple is used to

    measure and control the amount of heat being applied to the barrel by the heaters.

    EE) Screw

    The screw forces the pellet then melts material and forwards it to mold through

    nozzle. The key factor is that the material must adhere to the inside wall of the barrel.

    Otherwise, the screw will rotate in one spot without any forward movement. Traditionally,

    the screw is divided into three parts: (1) the feed section, (2) the transition section, and (3) the

    metering section.

    In the feed section, the material in pellet form moves from the hopper section of the

    injection molding barrel toward the nozzle and mold section. The pellets here are still in solid

    form, but there has been some initial softening. The channels of the screw are deep in this

    area to allow the pellets to convey down the barrel. Temperature settings of the barrel are thelowest in this section, to avoid premature melting of the pellets, which can cause degradationor interfere with material feed into the barrel.

    In the transition section the pellet material begins to melt and mix with non-melted

    pellets. In this section the channel depth of the screw becomes shallow, and this degree of

    shallowness increasing down the transition section. This increasing shallowness causes the

    meltpellet mix to compress against the inside of the barrel wall. Frictional heat builds up,

    and in combination with the heat generated by the barrel heater, creates more melt to be

    formed within the screw flight channels. The melt pool formed as you go down the transition

    section increases. As the pellets reach the section where compression takes place, the volumeof material inside the screw flight channel decreases until the metering section is reached.

    The metering section of the screw of the standard injection molding screw acts as the

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    pumping mechanism for the melt, forcing molten material forward accurately and completingthe melting process. As the material goes forward to the front of the screw, force is generated

    to push the screw back in the direction of the hopper to the original, set position of the shotsize. As the screw rotates and pumps the molten material through the non-return valve, the

    molten material that is accumulating in front of the valve is pushing and reciprocating the

    screw.

    FF) Nozzle

    It is the last section of the melt-conveying phase. It guides the melt of the material

    into the sprue bushing and then into the mold. The purpose of the nozzle is to maintain the

    temperature of the molten material after it has been plasticated by the screw and barrel andbefore it enters the mold to be formed into the final part. The nozzle typically is kept short to

    avoid overheating the material by increasing the residence time in plastication.

    Figure 5.2: Nozzle

    It is important that the orifice or opening of the nozzle tip match the opening of the

    sprue bushing. If these areas match up, material will get caught and hang up in this area,

    leading to material degradation due to excessive shear.

    3.3MELT-DIRECTING PHASE

    When molten polymer leaves the barrel through the nozzle, its flow path begins to be

    guided toward its final destination i.e. mold cavity. Molten material takes path which goes

    through a series of turns, twists, and restrictions as it approaches the mold cavity.

    GG) Sprue

    The sprue is the first channel that molten material is exposed as it goes from the barrel

    and nozzle into the mold and begins directing the molten material toward the mold cavity.The main interface between an injection molding machines nozzle and the runner systemthat starts the melt-directing process is referred to as the spruebushing. The design of this

    bushing is not standard to every molding process. The sprue bushing is designed as a shelf

    item, available through a number of manufacturers

    Another part of the sprue, which is sometimes neglected in its design, is the cold slug

    well. The wellis located at the end of the sprue at the interface of the sprue and runner and is

    in the direct line of the sprue.

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    Figure 5.3: Cold slug well

    The cold slug well plays an important part in material directing and has two functions.(1) The cold slug provides a location for cold material that is entering the mold to be directed

    to, which allows the hotter material to flow straight to the cavity. Otherwise, introduction ofcold material into the mold cavity can cause surface defects, such as a blemish or cold flow

    mark or may create a weakness in the part causing the part to fail prematurely. (2) To provide

    an anchor to cause the sprue to be pulled away from the sprue bushing and ultimately be

    ejected from the mold after the part is cooled.

    HH) Runner

    1. It is a series of channels to direct molten polymer into mold cavity.

    2. Must provide shortest, most direct route for molten polymer.3. Viscosity & Temperature determines runner length and diameter.

    4. Runner system must be balanced.5. Freezing occurs if runner length is large & diameter is small.

    6. Excess grinding is required if runner system is too large.

    Figure 5.4: Channel types

    Several cold runner cross-sectional geometries are used in injection molding, including thefull round, half round, trapezoidal, and modified trapezoidal.

    i) Full-round runner design is the most efficient type of runner system and is widely used.

    Full-round runners are easy to eject and are easily machined using a standard end mill.However, this type of runner needs to be machined into both halves of the mold and can be

    more expensive to machine. Also, matching both halves of the runner is critical to the

    functioning of this design

    ii)Half-round runner design allows for machining on one side of the mold with a circular end

    mill. However, a low volume-to-surface area is present in this runner design.

    iii) Trapezoidal runner design is less expensive to machine than a full-round design since;

    machining takes place on one-half of the tool (mold). The trapezoidal runner should bedesigned with a taper between 2 and 5 per side, with the depth of the trapezoid equal to its

    base width. This configuration will provide excellent volume-to-surface area. However, if

    sharp corners are used at the base of the trapezoidal runner, part ejection may be hindered.

    iv)Modified trapezoidal runner system is another variation of the standard trapezoidal runner

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    system. It offers the same features as those of the trapezoidal runner system but includes aradius base. This provides ease of part ejection and is also easy to machine. Modified

    trapezoidal runners have been used with a great deal of success with semi crystallinematerials such as polyethylene, polypropylene and both nylon 6 and nylon 6, 6.

    Two basic types of runners are used in injection molding, namely cold runner and the hotrunner.

    Table 5.1: Cold Vs Hot runner

    Cold Runner System Hot Runner System

    1. Consists of 2 or 3 plates Consists of 2 plates

    2.Runner is ejected from mold after the

    part is cooled.

    No runner is ejected. (Runner-less system)

    (Runners are in separate plate of mold)

    3.Different temperature and viscosity of

    molten polymer in barrel and runner.

    Same temperature and viscosity as in the

    barrel, so material stays in molten state until

    it reaches the cavity.

    4. NA(Not Applicable)

    Hot runner tool operates with a system of

    heater bands located inside the tool and

    heaters, called manifold heaters.

    5. NA Act as an extension of the barrel

    6. Slower cycle time Potentially faster cycle times

    7. Colour changes can be made quickly Cant done easily

    8. Plastic waste from runners Eliminate runners and potential waste

    9. Will not give balanced cavity filling Gives balanced cavity filling

    10. Simple working in nature Complex working

    11.Comparatively higher injection pressures

    are involvedLower injection pressures involved

    12.Comparatively higher clamping

    pressures are involvedLower clamping pressures involved

    13.Greater shot size is required because of

    runner system

    Shot size is reduced due to reduction in

    weight of runner system

    Hot runner uses two systems by which heating of channel is done and these are

    [1] Insulated hot runner systemInsulated hot runner system allows the molten polymer to flow into the runner and

    then cool to form an insulating skin of solid, cooled material along the walls of the runner.

    This insulating layer decreases the diameter of the runner and helps maintain the temperature

    of the molten polymer as it awaits injection into the mold cavity. The insulated runner is

    designed so that while the runner volume does not exceed the cavity volume. This full

    consumption of molten material is necessary to prevent excess buildup of the insulating skinand to minimize any drop in melt temperature.

    [2] Heated hot runner system (or Conventional)

    [a] Externally heated hot runner system

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    Figure 5.5: Externally heated system

    Externally heated hot runner channels have the lowest pressure drop of any runner

    system (because there is no heater obstructing flow and all the plastic which is in

    molten state).

    There are no places for material to hang up and degrade so externally heated systems

    are good for thermally sensitive materials.

    Thermal Efficiency is less

    Good for thermally sensitive materials

    [b] Internally heated hot runner system

    Figure 5.6: Internally heated system

    Internally heated runner systems require higher molding pressures.

    There are many places for material to hang up and degrade so thermally sensitive

    materials should not be used.

    Internally heated nozzles offer better gate tip control than externally heated nozzles.

    Thermal efficiency is high

    High pressure drop

    II) Gate

    The gate is the last major passageway for material to flow from the injection molding

    machine barrel to the mold cavity. The gate directs the flow of molten material from the

    runner channel system into the mold cavity. The location of the gate on the molded part plays

    a major role in how the part will perform as well as the quality, properties, and performance

    of the part. A number of items needed to be considered when selecting a location of gate on a

    part.i) The gate needs to be located so that gases built up during processing can escape through a

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    parting line, ejector pin, porous insert, or vented area without leaving a burn mark or poorsurface finish.

    ii) The gate should be located where the material can flow into a wall, core pin, or other partfeature rather than an empty space, to prevent jetting or worming of the polymer into the

    part surface.

    iii) The location and size of the gate vestige or scar on the part should be in a location wherepart functionality is not compromised.

    iv) Gating is recommended at the thickest section of the part to allow flow to go from a thick

    section to a thin section, which can cause part defects such as voids.

    v) The gate is an area of high stress and should be located in a part that is not exposed to high

    forces or stresses in its end use.vi) Gates should be located so that flow does not occur around core pins, depressions and

    holes leading to the formation of weld or knit lines.

    Type of Gates;-

    {a} Edge gates are used most often in large part designs and also where thin walls are used

    in a part. Examples of where these types of gates have been used are in business machine andelectronics enclosures and in automotive glove box doors. One of the advantages of edge

    gates is that it provides the widest molding window since, due to its design, low shear rates

    are found. This gate is placed along the side or width of a part and the width can range

    anywhere 12.7 to 305 mm. The recommended thickness of an edge gate is approximately0.40 to 0.50 times the nominal wall thickness where the edge gate is located.

    Figure 5.7: Edge gate

    {b} Fan gate:

    Figure 5.8: Fan gate

    Fan gates are used in applications such as automotive trim parts and electronics covers and

    enclosures, provide reduced pressures and clamp tonnage over other conventional gate

    designs and are excellent when flow lengths are short. Fan gates allow for a wide process

    window and reduce over packing issues since the pressure is lower than that found in tunnel

    gates. The disadvantages of using fan gates include the inability to trim off the gate since alarger area must be trimmed through leaving a large gate vestige. Increased scrap may also be

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    found, due to the difficulty of trimming off fan gates. One solution to reducing the vestigeand alleviating the trim issue is to use a chisel cross section for the fan gate. This allows the

    fan gate to break off from the part cleanly and evenly. Figure 5.8 shows a fan gate and achisel-type design to reduce gate vestige issues.

    Application: Electronics covers and enclosures

    {c} Pinpoint gates are used mostly in single or multicavity three-plate tools or wheremultiple gates are used in a part. Pinpoint gates are also used with a thin nominal wall

    thickness but for a part with a large surface area. One big advantage of pin point gates is their

    ease of degating from a part without the use of special degating tools or fixtures to remove

    the gate. However, pinpoint gates have the potential to create high shear on the molten

    material. It is suggested that the recommendations given by a material supplier for a givenmaterial to be molded using a pinpoint gate be followed.

    Figure 5.9: Pinpoint gate

    Applications: Electrical switches and consumer applications such as childrens toys.

    {d} Tunnel gates convey material below a parting line of the mold and tunnel into the

    cavity. This type of gate is used on smaller parts and thin walled parts due to its small size. A

    big advantage in using tunnel gates is its ease in separating the part from the runner and gate

    system upon the part ejecting from the mold cavity. Similar to pinpoint gates, its big

    disadvantage is that due to the high shear rates, which can cause the material to degrade, the

    injection molding window is narrowed. Once again, It is suggested the material suppliers

    recommendations as to the optimum gate design to use for a selected material.

    Figure 5.10: Tunnel gate

    Application: Electronic connectors and small parts for medical applications

    {e} Diaphragm or disk gate: Material flows from the cylindrical core to its perimeter. These

    gates are used mostly for single-cavity tools in fabricating single-shaped parts such as

    cylindrical parts that have small or medium sized internal diameters. One advantage of

    diaphragm gate is the reduction in core pin shift when molding tube-shaped parts.

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    Figure 5.11: Diaphragm or disk gate

    Applications: toner bottles for business machines and industrial applications such ashydraulic fluid reservoirs.

    3.4MELT-FORMING PHASE

    When the molten polymer travels through the channels that guide its flow path such as

    the sprue, runner, and gate, it arrives at its destination i.e. mold cavity. In this location, the

    shape, size, and design of the cavity take the configuration of the final part to be molded. The

    molten material flows into this cavity & takes the shape of this chamber and it is cooled from

    a molten material to a solid mass of polymer (As simple as this sounds, there are many

    aspects to forming the molten material into its final solid mass that need to be considered to

    fabricate a part for its final end use)

    JJ) The ClampFunction of a clamp is to close the mold, hold it closed under pressure during

    injection of the molten material and during cooling of the material until formation of a solid

    part and to open the mold so that the part can be ejected & removed from the mold. In

    injection molding, four different types of clamping are used: hydraulic, mechanical,hydromechanical, and electrical.

    (i) Mechanical clamp

    The concept of a mechanical clamping system utilizes a mechanical linkage called asa toggle. It develops a clamping force needed to hold the mold closed during the injecting and

    cooling portion of the injection molding cycle.

    Figure 5.12: Mechanical toggle clampA mechanical clamp consists of 3 platens, 4 tie bars and a toggle system which is

    activated by a hydraulic cylinder. Two types of mechanical toggles are used; single toggle

    leverand the double toggle lever. The single toggle lever is used for machines which are built

    for lower clamping forces (25 to 50 tons) but can be used with machines built up to 200 tons

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    of force. The double toggle lever is used for clamping forces as high as 1000 tons. In thesingle toggle lever system, a small hydraulic cylinder is used with the single toggle lever for

    closing the clamp. The cylinder travels at a constant speed with a slowdown built in as thetwo mold halves close. This system allows for only short opening strokes. For the double-

    toggle lever system, a center hydraulic driving system is used, and larger opening strokes are

    realized, depending on the length of the driving system.In a mechanical toggle system, the hydraulic cylinder causes the toggle to stretch or

    collapse the toggle mechanism (similar to human elbow extends or contracts the human arm).

    The mold is fully closed once the toggle has stretched and is locked in place. At the

    beginning of the movement, mechanical advantage is low and speed is high but near the end

    of the stroke, the reverse is true.To assure that the toggle is not overstretched or not stretchedenough, after the mold is installed between the platens, the clamp is moved forward until the

    mold actually snaps the mold halves to a closed position. This is adjusted at the rear of themachine using the die height adjustment. If the toggle is overstretched, the mold may open

    slightly, causing material to flash excessively between the mold halves. To prevent

    overstretching, machines with toggle systems are equipped with a limit switch that will

    switch off the hydraulic valves operating the linkages.

    (ii) Hydromechanical clamp

    It operates by mechanical means for closing and opening of a mold under high speeds.

    It consists of two small fast-travel cylinders and one large central clamp cylinder.

    Figure 5.13: Hydromechanical toggle clampFirstly, the mold closes using the two small, fast cylinders until it fully extends. At

    this full extension point, a switch indicates the correct pressure of the pressure column and

    then moves the locking pad into position behind the large central clamp cylinder. The clampsystem moves forward and locks the mold platens in place and holds the mold tool in place

    through the injection and cooling cycle. Upon opening of the mold, the clamp cylinder

    pressure is reduced, the travel cylinders move forward, and the locking pad moves out of

    position. At this point the small fast travel cylinders open the mold. a locking pad is used

    behind the large central cylinder, built for clamp tonnages similar to those of a hydraulicclamping system. The purpose of using a locking pad is to reduce the size of the large

    cylinder.

    (iii) Electric clamping system

    The electric clamping system operates similar to the mechanical clamping system but

    in this case, no main hydraulic cylinder is used to move the toggle. An electrically driven

    motor is used to move the toggle forward and backward into position using a ball screw

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    mechanism. The electric clamping system provides an energy-efficient mechanism toaccomplish all machine functions and it is totally mechanical.

    3.4.1 Cooling SystemAfter injection of molten material (polymer) into the mold cavity, next objective in

    the injection molding process is to turn the molten polymer into a solid mass. This is done by

    decreasing the temperature of molten material by cooling the injection mold. There are threemethods of heat transfer: conduction, radiation, and convection. Conduction is the most

    important in the injection molding process.

    The mold material is responsible for the conduction process. The mold material

    moves the heat from the molten polymer material to the mold and the cooling lines. Heat is

    taken away from the molten material by the mold, and this heat is taken away by the water orcooling medium flowing through the cooling lines. Once the process of heat flow is

    completed and the equilibrium of the temperature is achieved, the heat flow works backwardto hold the temperature of the part at the desired temperature. The temperature of the cooling

    medium can affect the kinematic viscosity of the cooling medium.

    3.4.2 Cooling line positioning

    Location of the cooling lines is critical to achieving efficient cooling of the part andimproving part productivity. One point to keep in mind is that the cooling channel diameter

    should be large enough to have reasonably low pressure drop but not so large that excessive

    flow rate is needed to obtain maximum cooling efficiency via turbulent flow.

    Figure 5.14: Cooling efficiency

    Figure 5.15: Guidelines for layout of cooling channelsThe guidelines to follow for determining a proper cooling line layout are

    Keep the cooling lines at a uniform distance from the part walls. Otherwise cooling

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    will not be consistent across the part.

    Avoid more than 10 bends in any cooling line circuit, to minimize the pressure drop

    Distribute cooling in such a way to match the amount of heat in various sections of a

    part. For example, a non-uniform thick area needs more intensive cooling than a thin

    area.

    Avoid a metal to metal interface between cooling channels and the mold surface.Install channels directly in cores and cavities, not into the back plates alone.

    Limit hoses for cooling lines and fitting restrictions. The inside diameter should be at

    least the size of the cooling channel inside diameters.

    To avoid losing heat to ambient temperature surroundings, insulation should be usedon all pipes and lines.

    Many times it is difficult to accommodate cooling channels in the smaller cores with

    difficult geometry. In such cases, the core should be made of Beryllium copper

    which has high thermal conductivity. These core inserts should be located near the

    cooling channel.

    4. The basic injection cycle is as follows:1) Mold close injection carriage forward 2) Injection of plastic 3) Metering in mold

    cavity 4) Carriage retract 5) Opening of mold 6) Ejection of part.

    Some machines are run by electric motors instead of hydraulics or a combination of

    both. The water-cooling channels that assist in cooling the mold and the heated plastic

    solidifies into the part. Improper cooling can result in distorted molding. The cycle is

    completed when the mold opens and the part is ejected with the assistance of ejector pins

    within the mold.

    Figure 5.7: Injection cycleThe resin (or raw material) for injection molding, is most commonly supplied in pellet

    or granule form. Resin pellets are poured into the feed hopper, a large open bottomed

    container, which is attached to the back end of a cylindrical, horizontal barrel.

    A screw within this barrel is rotated by a motor, feeding pellets up the screw's

    grooves. Channels of the screw do not have a constant depth. The screw at the hopper end of

    the barrel will be deep, and moving forward toward the mold end of the screw, the depth of

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    the channel becomes shallow.As all this is taking place, the inside opening of the barrel stays at a constant diameter.

    So, in terms of conveying, material is fed at the deep channels and conveyed into shallowerchannels, which causes the material to compress and pack together. This compression process

    increases the friction of the material against the inside wall of the barrel, providing frictional

    heat. In addition to this, heaters are spaced on the outside diameter of the entire length of thebarrel, providing additional heat. Therefore the frictional heat of the material in the screw

    plus the heat applied on the outside of the barrel together provide enough heat to convert

    material in pellet form at the hopper end of the screw and barrel to material in a melt form

    midway down the length of the barrel to the end of the barrel and screw.

    The channels through which the plastic flows toward the chamber will also solidify,forming an attached frame. This frame is composed of thesprue, which is the main channel

    from the reservoir of molten resin, parallel with the direction of draw, and runners, which areperpendicular to the direction of draw, and are used to convey molten resin to thegate(s), or

    point(s) of injection. The sprue and runner system can be cut or twisted off and recycled,

    sometimes being granulated next to the mold machine. Some molds are designed so that the

    part is automatically stripped through action of the mold.

    5. Feeding systemThe main elements of feeding system are

    i) Sprue ii) Runner and iii) GateEarlier we have discussed in brief so we are moving to next point.

    6. Cooling System

    6.1 IntroductionInjection moulding process is a cyclic in characteristic. Cooling time is about 50 to

    75% of the total cycle time. Therefore, optimizing cooling time for best performance is very

    important from quality and productivity point of view. Cooling time is proportional to square

    of wall thickness. Therefore part design should ensure more or less uniform wall thickness

    throughout the part. Part design should also ensure that the melt flow is uniform in all

    direction from the gate and melt should reach the boundary of the part more or less at thesame time

    6.2A mold cooling system typically consists of the following items- Temperature controlling unit

    - Pump

    - Supply manifold

    - Hoses

    - Cooling channels in the mold- Collection manifold

    The mold itself can be considered as a heat exchanger with heat from the hot polymer melt

    taken away by the circulating coolant.

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    Figure 5.8: A typical cooling system for an injection molding machine

    6.3Types of cooling channels

    6.3.1Parallel cooling channels

    Parallel cooling channels are drilled straight through from a supply manifold to a

    collection manifold. Due to the flow characteristics of the parallel design, the flow rate along

    various cooling channels may be different, depending on the flow resistance of eachindividual cooling channel. These varying flow rates in turn cause the heat transfer efficiency

    of the cooling channels to vary from one to another. As a result, cooling of the mold may not

    be uniform with a parallel cooling-channel configuration.

    Typically, the cavity and core sides of the mold each have their own system ofparallel cooling channels. The number of cooling channels per system varies with the size

    and complexity of the mold.

    6.3.2 Serial cooling channels

    Cooling channels connected in a single loop from the coolant inlet to its outlet arecalled serial cooling channels. This type of cooling-channel configuration is the most

    commonly recommended and used. By design, if the cooling channels are uniform in size, the

    coolant can maintain its (preferably) turbulent flow rate through its entire length. Turbulent

    flow enables heat to be transferred more effectively. Heat transfer of coolant flow discusses

    this more thoroughly. However, you should take care to minimize the temperature rise of the

    coolant, since the coolant will collect all the heat along the entire cooling-channel path. Ingeneral, the temperature difference of the coolant at the inlet and the exit should be within

    5C for general-purpose molds and 3C for precision molds. For large molds, more than oneserial cooling channel may be required to assure Cooling-channel Configuration uniform

    coolant temperature and thus uniform mold cooling.

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    Baffles and bubblers are sections of cooling lines that divert the coolant flow into areas thatwould normally lack cooling. Cooling channels are typically drilled through the mold cavity

    and core. The mold, however, may consist of areas too far away to accommodate regularcooling channels. Alternate methods for cooling these areas uniformly with the rest of the

    part involve the use of Baffles, Bubblers, or Thermal pins, as shown below.

    Figure 5.9:Baffle, bubbler, and thermal pin

    6.3.3Baffle

    A baffle is actually a cooling channel drilled perpendicular to a main cooling line,

    with a blade that separates one cooling passage into two semi-circular channels. The coolantflows in one side of the blade from the main cooling line, turns around the tip to the other

    side of the baffle, then flows back to the main cooling line.

    This method provides maximum cross sections for the coolant, but it is difficult to

    mount the divider exactly in the center. The cooling effect and with it the temperaturedistribution on one side of the core may differ from that on the other side. This disadvantage

    of an otherwise economical solution, as far as manufacturing is concerned, can be eliminated

    if the metal sheet forming the baffle is twisted. For example, the helix baffle, as shown in

    Figure 5.10 below, conveys the coolant to the tip and back in the form of a helix. It is useful

    for diameters of 12 to 50 mm, and makes for a very homogeneous temperature distribution.Another logical development of baffles is single or double-flight spiral cores, as shown in

    Figure 5.10 below.

    Figure 5.10: (Left) Helix baffle & (Right) Spiral baffle.

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    6.3.4 Bubblers

    A bubbler is similar to a baffle except that the blade is replaced with a small tube. The

    coolant flows into the bottom of the tube and bubbles out of the top (like fountain). Thecoolant then flows down around the outside of the tube to continue its flow through thecooling channels.

    The most effective cooling of slender cores is achieved with bubblers. The diameter of both

    must be adjusted in such a way that the flow resistance in both cross sections is equal. The

    condition for this is

    Bubblers are commercially available and are usually screwed into the core, as shown in

    Figure 5.11 below. Up to a diameter of 4 mm, the tubing should be beveled at the end to

    enlarge the cross section of the outlet; this technique is illustrated in Figure 5.11. Bubblers

    can be used not only for core cooling, but also used for cooling flat mold sections, whichcant be equipped with drilled or milled channels.

    Figure 5.11: (Left) Bubblers screwed into core & (Right) Bubbler beveled to enlarge outlet

    6.3.5 Thermal pins

    A thermal pin is an alternative to baffles and bubblers. It is a sealed cylinder filled with afluid. The fluid vaporizes as it draws heat from the tool steel and condenses as it releases the

    heat to the coolant, as shown in Figure 5.12. The heat transfer efficiency of a thermal pin is

    almost ten times as great as a copper tube. For good heat conduction, avoid an air gap

    between the thermal pin and the mold or fill it with a highly conductive sealant.

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    Figure 5.12: Thermal pin heat transfer efficiency

    7. Ejection system

    The basic function of the ejection system of a mold is to eject the part as fast as

    possible without distorting it. The amount of ejector area needed is depends upon partgeometry, mold finish, material release characteristics & part temperature at the time of

    ejection. To prevent damage during ejection, thin walled parts generally require larger

    ejectors and greater ejector area than comparable parts with thicker walls.

    Figure 5.13: Ejection housing

    At the most basic level, mould consists of two main parts: the cavity and core. The core

    forms the main internal surfaces of the part. The cavity forms the major external surfaces.

    Typically, the core and cavity separate out as the mould opens, so that the part can be

    removed. This mould separation occurs along the interface known as the parting line. Theparting line can lie in one plane corresponding to a major geometric feature such as the part

    top, bottom or centerline, or it can be stepped or angled to accommodate irregular part

    features. Choose the partingline location to minimize undercuts that would hinder or prevent

    easy part removal. Undercuts that cannot be avoided via reasonable adjustments in the partingline require mechanisms (Slide Mechanisms) in the mould to disengage the undercut prior to

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    ejection.Typically, molds have ejector systems built into the moving half (see figure 5.14). Ejector

    travel must be sufficient to clear the moulding from fixed members in the mould. Undercuts

    or pickup ribs may be machined into mould members to ensure that the molded part

    remains on the ejection side of the mould. Parts may be removed from a mould using the

    common type of ejector or knockout system. Ejectors actuated by an ejector bar must containpush backs or safety return pins to reposition the ejector pins prior to the start of the injection

    or mould filling cycle.

    Figure 5.14: Two plate mold showing sectional view(right hand side)

    7.1Various Types of Ejection system used in Injection molds are

    1. Pin ejection

    Cylindrical pins are used for ejection purpose, in case of square and rectangular componentsminimum four pins at the four corners are required and in case of cylindrical component

    minimum three pins at 120 apart is required based on the component profile, size and area of

    ejection the number of pins to be increased. Visible ejection marks will remain oncomponent.

    2. Sleeve ejection

    This type of ejection is preferred for only cylindrical cores and core has to be fixed in bottom

    plate. This ejection is limited to cylindrical core due to manufacturing constraints, when

    ejection assembly is moved the sleeve will slide over the core and eject the component. No

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    visible ejection marks will be there on component.

    Figure 5.13: Sleeve ejection

    3. Stripper plate ejection

    Figure 5.14: Stripper plate ejection

    A stripper plate is used when ejector pins or pressurized air will not be enough to ejecta part off a core. Examples of parts using a stripper for ejection are caps, containers and lids.

    Stripper plates are very common in thin wall injection molding because by their nature theseparts are weak so the ejection method requires full contact with the outer edge of the part to

    remove it off the core. This ejection is preferred for component with larger area, an additional

    plate (stripper) will be provided in between core plate and cavity plate. In order to avoid flash

    the stripper plate will be in contact with cavity plate and gap is maintained between cavity

    and core plate. No visible ejection marks will be there on component. A stripper plate will

    eject parts quickly 100% of the time. Stripper plates can be used for both single cavity and

    multi-cavity injection molds.

    Making a mould with a stripper plate is a lot more difficult than making a mould with

    ejector pins. If it is not designed and made right there will be constant part quality issuessuch as flashing. Cycle time will also suffer. Molds with stripper plates require more mould

    maintenance than molds without. There is always a waxy residue which builds up over timebehind a stripper plate and this must be cleaned on a regular basis usually every 48 hours of

    production. If cleaning is not done part quality issues will result sooner rather than later.

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    4. Blade ejection

    This type of ejection is preferred for thin rectangular cross sections, rectangular blades willinserted in cylindrical pins or cylindrical pins will be machined to rectangular cross section

    till ejection length for easy accommodation of ejection pin head in counter bore provided in

    ejection plates

    5. By rotation of core (internal threaded components)

    This method of ejection is required for threaded components were component is

    automatically ejected by rotating the core insert.

    6. Air ejection

    This method is used to actuate the ejection pin fitted in core using compressed air; retraction

    of ejection pin in core is done by spring.

    Figure 5.15: Air ejection

    8. 2-plate & 3-plate molds

    8.1 2-Plate mold

    This consists of two halves fastened to the two platens of the molding machine's

    clamping unit. When the clamping unit is opened, the mold halves separate out. Molds can

    contain multiple cavities to produce one or multiple parts in a single shot. The parting

    surface is the surface shared by the two mold halves.

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    Figure 5.16: Two plate mold

    8.2 3 Plate mold

    This design (see figure 5.17) has some advantages. The molten plastic flows through a gate

    located at the base of the cup-shaped part, rather than at the side. This allows more even

    distribution of melt into the sides of the cup. In the side gate design in the two-plate the

    plastic must flow around the core and join on the opposite side, possibly creating a weakness

    at the weld line. Secondly, the three-plate mold allows more automatic operation of the

    molding machine. As the mold opens, the three plates separate; this forces the runner to break

    from the parts, which drop by gravity or using air-blower into collecting containers put under

    the mold

    Step 1: Mold opens at Secondary (or Runner) parting line.

    Step 2: Runner drops off.Step 3: Mold opens at primary parting line.

    Step 4: Ejectors eject the part.

    Step 5: Mold closes for next shot and injection & cooling takes place.Step 6: Cycle repeats

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    Figure 5.17: Three plate mold

    9. UndercutsIn manufacturing, an undercut is a special type of recessed surface. In turningit

    refers to a recess in a diameter. In machiningit refers to a recess in a corner. In moldingit

    refers to a feature that cannot be molded using only a single pull mold. Inprinted circuit

    board construction it refers to the portion of the copper that is etched away under the

    photoresist. Inwelding it refers to undesired melting and removal of metal near the weld

    bead.Undercut is any indentation or protrusion in a shape that will prevent its withdrawal from a

    one-piece mold.

    Undercuts on molded parts are features that prevent the part from being directly ejected from

    an injection molding machine. They are categorized into internaland externalundercuts,where external undercuts are on the exterior of the part and interior undercuts are on the

    inside of the part. Undercuts can still be molded, but require aside actionorside pull. This is

    an extra part of the mold that moves separately from the two halves. These can increase the

    cost of the molded part due to an added 15 to 30% cost of the mold itself and added

    complexity of the molding machine.

    If the size of the undercut is small enough and the material is flexible enough a side action isnot always required. In these cases the undercut is stripped or snapped out of the mold. When

    this is done usually a stripping plate or ring is used instead of stripper pinsso that the part is

    not damaged. This technique can be used on internal and external undercuts.

    The biggest problem of using undercuts is in removing the part from the mold. Sometimes thepart can bend enough to eject the part from the mold without damaging the part or the mold,

    depending on the depth and shape of the undercut together with the plastic materials

    flexibility, or flexural modulus. Undercuts can be ejected from a mold only if they are located

    away from stiffening members such as corners or ribs. The part must also have enough room

    to flex and deform.

    For some filled or reinforced plastic materials, such as nylon 6 and nylon 6,6, collapsible

    https://en.wikipedia.org/wiki/Manufacturinghttps://en.wikipedia.org/wiki/Turninghttps://en.wikipedia.org/wiki/Machininghttps://en.wikipedia.org/wiki/Molding_(process)https://en.wikipedia.org/wiki/Printed_circuit_boardhttps://en.wikipedia.org/wiki/Etching_(microfabrication)https://en.wikipedia.org/wiki/Photoresisthttps://en.wikipedia.org/wiki/Weldinghttps://en.wikipedia.org/wiki/Injection_molding_machinehttps://en.wikipedia.org/w/index.php?title=Side_action&action=edit&redlink=1https://en.wikipedia.org/w/index.php?title=Stripper_pin&action=edit&redlink=1https://en.wikipedia.org/w/index.php?title=Stripper_pin&action=edit&redlink=1https://en.wikipedia.org/w/index.php?title=Side_action&action=edit&redlink=1https://en.wikipedia.org/wiki/Injection_molding_machinehttps://en.wikipedia.org/wiki/Weldinghttps://en.wikipedia.org/wiki/Photoresisthttps://en.wikipedia.org/wiki/Etching_(microfabrication)https://en.wikipedia.org/wiki/Printed_circuit_boardhttps://en.wikipedia.org/wiki/Molding_(process)https://en.wikipedia.org/wiki/Machininghttps://en.wikipedia.org/wiki/Turninghttps://en.wikipedia.org/wiki/Manufacturing
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    cores or split-cavity undercuts are used and are recommended to reduce high stresses in thepart. Mold temperature can affect the amount of undercut allowed.

    Undercut designs are often used to create threaded parts such as screw-on bottle caps, Snap-on products such as lipstick containers, and a variety of consumer, medical, automotive, and

    other products. Threaded caps illustrate well the complexities associated with undercuts.

    After the cap is formed, the threads of the part and the threads of the core are intermeshedand must be disengaged before the core can be pulled out and the cap removed from the

    mold. Molders have developed a variety of methods for molding undercut or threaded parts,

    some as simple as unscrewing the part by hand or machining the undercuts in a separate

    operation that range widely in cost-effectiveness and efficiency.

    9.1Unscrewing mold mechanisms

    Two of the most common methods for dealing with threaded parts are by jumpingthreads or installing unscrewing mechanisms. Occasionally, if the material is flexible enough,

    a molder can simply pull out the core or strip the part, jumping the threads over each other. If

    this isnt an option, unscrewing mechanisms built into the mold can unscrew the part from the

    core as a secondary action.Unscrewing molds are among the most complex of all injection molds, requiring

    considerable technical savvy to build and maintain. They are usually built for many years of

    production and are considered a long-term investment for producing high-volume parts.

    Unscrewing technology has evolved considerably, but it still has a significant number oflimitations. It demands frequent maintenance for issues such as broken rollers, damaged

    racks, and water and oil leaks. Part quality issues such as scuffing, ovality, flash, and greasecontamination can arise as well.

    9.2Collapsible cores

    One technology that has expanded the capabilities of undercut molding more than any

    other is the collapsible core. Rather than jumping the threads or mechanically unscrewing the

    parts, flexing steel collapsible cores function by collapsing radially inward during the normal

    mold sequence. They eliminate secondary operations and complex coring approaches while

    providing dramatic cycle-time reductionsoften as much as 30% faster than withunscrewing mechanisms.

    The segments of a collapsible core are attached to the ejector plate, while its tapered innercenter pin is attached to the back of the mold. When the mold opens, the threaded outer core

    collapses as the ejector plate moves forward. Incorporating only three moving parts, which

    utilize conventional mold movements, a collapsible core enables part designs that previously

    would have been considered impossible to mold.

    Collapsible cores are compatible with other mold components, such as two -stage ejectorsand internal latch locks. These products enable positive control of both the stroke sequence

    and distance in two-stage ejection and of mold-plate latching operation.