Fundamental of Mould Design

Fundamental of Mould Design

Inserts: Core and Cavity:

For moulds containing intricate impressions and for multi-impression moulds, the insert-bolstered assembly is used.

The method consists in machining the impression out of small blocks of steel. This small block of steel is known as inserts. These inserts are then inserted and fitted ino holes in a plate of stell called as bolster.

Types of Cavity & Core Inserts:

1. Rectangular, Screw Down

2. Rectangular, Flanged

3. Circular, Screw Down

4. Circular, Flanged

Bolsters

1. The fundamental requirement of bolster is as follows

2. It must provide a suitqble pocket into which the insert can be fitted

3. It must provide some means for securing the inserts in position

4. It must have sufficient strength to withstand the moulding forces

Types of Bolsters:

Solid bolster: This is suitable for use with both rectangular and circuolar inserts.

Strip-type bolster: Suitable for only rectangular inserts

Frame type bolster: Suitable particularly for circular inserts.

Chase bolster: This type is used in conjunction with “split” inserts.

Bolster plate: This is used in particular circumstances with certain type of both rectangular and circular inserts.

Solid-Bolsters

1. Basic Rectangular Pocket Type

2. Pocket made by Slotting technique

3. Basic Circular Pocket Type        

Bolsters: Other Types

1. Strip Type

2. Rectangular Frame Type

3. Circular Frame Type

4. Open Channel Type

5. Enclosed Chase Type

6. Bolster Plate Type

Basic concept: Melt plastic-flow into mould -take part shape-cools-product ejected.

Basic Mould Function

Shaping the Product: The plastic product gets shape in the mould so the mould must be having provisions to give shape to the mould, the place where the plastic product gets shape in the mould is called as impression. The impression is formed by the assembly of core plate and cavity plate.

Ducting the plastics: The melt plastic will flow from injection molding machine to the impression through the nozzle, sprue and gate.

Guide Pillar and Guide bush: For the correct alignment of the core and cavity, guide pillar and guide bush is provided. 

Cooling the Plastic: The mould also functions like a heat exchanger so that to cool the plastic product,

To take the product from the mould all the heat from the plastic must be removed.

The mould must also be designed in consideration with the heat generated by the melt plastic.

Ejecting the Plastic product: The plastic product after cooling stays inside the mould and the product has to be released  out of the mould easily by means of a system called ejector system.

The mould must also be designed in consideration with the features of the product so that the plastic product is ejected from the mould with ease.

Two plate mould: it is single daylight mould. It has only two plate i.e. core & cavity. It has economical. Product finishing is not so good compare to three plate mould.

Three plate mould: it is multi daylight mould. It has an additional plate i.e. steper plate. Product finishing is very good as compare to two plate mould.

Maximum daylight: The maximum distance b/w the fixed platen (injection side) and moving platen (ejection side) is called maximum Daylight.

Min daylight: The minimum distance b/w the fixed platen and moving platen is called min daylight.

Ancillary parts: of mould are played roles in the correct alignment of two moulds half as well as to work satisfactorily. These ancillary items include.

Guide pillar and bushes: They are type, 

Leader pin: Its is the simplest design of guide pillar and bush, which is primary.

Standard type: It is a standard design of guide pillar, which is most commonly used to alignment of mould half correctly. In this types of guide pillar, the working diameter than the flitting diameter.

Surface fitting: An alternative method of fitting the guide pillar and guide bush is to fit both of components is called surface fitting guide pillar &bush. It is similar to that of standard type guide pillar.

Sprue bush mounted: in this type of register ring the inner bore of register ring is accurately located on the external diameter of the sprue bush while the outer diameter is located in the bore of injection machine platen.

Front plate mounted register ring: As the name it is clear that this register ring is attach to the front plate of the mould.

Reduced diameter type: Hear, the injection machine platen bore is smaller than the mould plate recess.

Constant diameter type: This design is adapted when the platen bore is equal to the mould recess.

Shot Capacity: Maximum weight of the material that may be injected per shot.   Ns = 0.85 Ms – Rw / Cw

Plasticising Rate: Maximum material that the machine can bring per hour to the moulding temperature.

Np = (0.85 Mp x Tc) – Rw / Cw

Clamping Force: Force available on the machine platen to prevent the mould from opening during injection.

Nc = Mc – RA Ip / CA x Ip 

Cycle time optimisation: Total time taken to complete one cycle of operation of the mould in seconds.

Mould closing time (a), Injection time (b), Mould cooling (Hold-on time) ( c ), Mold opening & Ejection time (d)

Total cycle time T = a + b + c + d

Injection time: When machine is limited by plasticizing capacity, Tc =m X 3600 / p X k

Cooling time: Minimum cooling time for a molding may be estimated from a following formula:

S =    - t ² /2((  log e   ( Tx – Tm ) /  4 ( Tc – Tm )

Shape and Cross section: Various shapes and cross sections are: Circular, Semi- circular, Trapezoidal, Modified Trapezoidal, Hexagonal, Square, and Rectangular.

Venting: If venting is not provided, the following defects may occur in the moulding: Discoloration, Sink mark, Incomplete filling.

Position of the Vent: At the point where flow paths are likely to meet, at the bottom of the projection, At the point of further most from the gate on symmetrical moulding.

Shrinkage: Shrinkage value is higher for crystalline material than amorphous material. Depend upon: Plastics material, Processing condition, Product design, Mould design.

Heat road: this system is used for cooling slender type of core insert where other types of cooling circuits are not proffered. In this a method a cylindrical copper rod is inserted into an accommodating hole machined in i.e. core insert for conduction of heat from the impression.

Heat pipe: sometimes heat pipes are used instead of heat rods. It is very similar to that of heat rod. These are very good heat transfer agent/device, which is amble of transmitting heat from core insert. It works on the principle of convection, because liquid is used on the heat transfer medium.

Ancillary parts: of mould are played roles in the correct alignment of two moulds half as well as to work satisfactorily. These ancillary items include.

Guide pillar and bushes: They are type, 

Leader pin: Its is the simplest design of guide pillar and bush, which is primary.

Standard type: It is a standard design of guide pillar, which is most commonly used to alignment of mould half correctly. In this types of guide pillar, the working diameter than the flitting diameter.

Surface fitting: An alternative method of fitting the guide pillar and guide bush is to fit both of components is called surface fitting guide pillar &bush. It is similar to that of standard type guide pillar.

Register ring: The register ring is also called locating ring is a circular member fitted on the front face of the mould. Its main purpose is to locate the mould in correct position on the injection moulding machine. Sprue and front plate mounted register ring.

Reduced diameter type: Hear, the injection machine platen bore is smaller than the mould plate recess.

Constant dia type: This design is adapted when the platen bore is equal to the mould recess.

Dry-cycle time: When the injection moulding machine operated without any plastic material, the cycle time is as dry cycle time, because establishing the total shot height of the mould. To understand the locking mechanism, ejection mechanism and other facture available on the madding machine.

Compression pressure: The pressure needed to mould a particular article depends on the flow characteristics of the material, the cavity depth and the projected area of the piece part.

Type of heater’s: Frames, Taps and cartridge, band heaters

Heat equipment and leather capacity: Heater taps have diameter from 50 to 500mm are available with different width this heating powder amounts up to 300watt/cm2. The cartridge diameters are 17 and 19mm and the cartridge length is gernally 100mm. the required no. of cartridge may be calculated from the current rate. The heating powder must compress to the weight of the mould. Alternating current of 20 volt and several hand red ampere caves eddy current that heat up the mould.

Calculation no. of impression: Clamping force available in machine / Clamping force required for impression.

Cycle time optimisation: Total time taken to complete one cycle of operation of the mould in seconds.

Mould closing time (a), Injection time (b), Mould cooling (Hold-on time) (c ), Mold opening & Ejection time (d)

Total cycle time: T = a + b + c + d

Plasticizing capacity: weight of moulding X no of moulding. Kg/nr

Heating of compression moulds: Thermosetting material which are used in compression moulding are cured by heat and pressure teaching of moulds using thermosetting materials suves two purpose. Heat must soften the material sufficiently to allow to flow under the influence of the pressure into any opening into the mould to the desired shape. Enough heat must be applied to bring about the chemical changing or polymerize the material into hand, infusible finished state.

Injection pressure: It is defend as pressure required pushing the molten plastic material into the cavity feed system of the mould.

Projected area: The max. Area which is coved by the moulding is known as projected area. The projected area is the total area of the moulding as seen when viewed in the direction that it will be placed on the press in the plane normal to the press opening.

Taper location: In some moulding close accuracy of location b/w the mould half is necessary. To get an even walled articles. The alignment of guide pillar and bush is not sufficient, to provide accurst. Location taper location is provide. It is employed for minimum wall thickness moulding is 0.6 mm. 

Cam track Actuation: In this method the cam track is machined into a steel plate attachment to the fixed mould half. A boss is fitted to the both sides of splits which runes in the cam track. The movement of split accurately controlled by specific cam track design. A radius or taper should be provided at the erdrance of the boss. The angle of cam-track plate is 10⁰-40⁰. M=La tan Φ – c.

Frame type ejector grid: This is form type in cross-section made up of rectangular steel block. Advantage: simple and cheep to manufacture, it provides good support to mould plate, it allows for rectangular ejector plate assembly.

Circular support pillar: In this design of ejector grid, circular support pillar are used to support the mould plate. This system is support the mould plate. This system is used for large mould.

Amorphous: A plastic that has no crystalline component, no of know order or pattern of molecules distribution and no sharp melting point called amorphous. In the amorphous plastic shrinkage is uniform in all directions. Shrinkage is very low compared to crystalline. It required less heat.

Crystalline vs amorphous: Some plastics are partially crystalline and partially amorphous in molecular structure, giving them both a melting points (the temperature at which the attractive intermolecular forces are overcome) and one or more glass transitions (temperatures above which the extent of localized molecular flexibility is substantially increased). The so-called semi-crystalline plastics include polyethylene, polypropylene, poly (vinyl chloride), polyamides (nylons), polyesters and some polyurethanes. Many plastics are completely amorphous, such as polystyrene and its copolymers, poly (methyl methacrylate), and all thermosets.

Mid range polymers || PC, PPC, COC, PMMA, ABS, PVC

Alloys || PEX, PVDC, PBT, PET, POM, PA 6,6, UHMWPE

Commodity polymers || PS, PVC || PP, HDPE, LDPE

Crystalline and semi-crystalline: A plastic material having liner arrangement of molecules are called crystalline. E.g. - Acetal, Nylon, polyester, PP and PE etc.

Product design concept: Design is the universal terms being used in every make and comes of world. It is nothing but the cost of given shape to our ideas. It deals with conversion of ideal relative and form of human activity. If we design any product, it should have a definite shape, size and good appearance.

Creep properties: When a part or structure is subjected to a given load is a corresponding predictable deformation of the deformation continuous to increase without any increase in load is called creep or cold flow.

Mechanical properties: are mostly important properties all the properties become their majority of end uses verity of application.

Rapid prototyping: Any process to generate free from prototype directly from 3D- cad data is called rapid prototyping. Use: The concept prototype can be made faster. Once the production tool is made, it reduces no. of modification. Excellent properties to produce new component. Advantage: Resulting pot will be accurate as computer model. Appearance can be checked. Tool cost is low. Type: Laminated object manufacturing. Select laser sintering. 3 D printing. Sterolithography.

Sterolithography: light to solidify photo curable resins. Parts are built up on elevator. A laser beam is used in this process. Then plat farm is lowered to liquid flows interlay over the top surface. After elevation is to remove visual material.

Casting: This process starts with liquid resin that can be catalyzed and polymerized. A flexible mould is cost form a machined model of part. Advantage: The material which is selected for casting has low shrinkage, it has equal electrical properties, tool used for casting has low cast. Disadvantage: The cost of equipment is high, A limited plastic materials.

Machining: Machining is a process through which we machined the cost stock to provide prototype pots.

Thumb rules for plastic: Runner cure and can’t be efficiently reclined, Gate leave wintness mark. Part flash nearly. Cavity fills and parts will frescon. Simplicity of design, use of economical, use of economical manufacturing methods, easy of assembly, by using standard and specifications.

Hinges: are similar to insert, which is used for had two plastic parts together. By the help of hinge the parts of product i.e. top and buttom are enclose together.

Bulk factor: The ratio of the volume of the loose plastic powder to the moulding is called bulk factor. Flash thickness allow for flash thickness in compression moulds using thermosetting compounds are:

1. Rag-filled light impact compound - 0.025mm

2. Cotton-flock compound in large mould - 0.02mm

3. Wood-flour compound is small mould - 0.01mm

4. All other mould and for all other component - 0.13mm

Because of the flash thickness that we are considering in the mould design the depth of cavity become.

Depth of cavity = minimum dimension folding + shrinkage of compound.

The flash thickness and the total thickness of the two parts and this thickness must be sub traded from the basic cavity depth in order that the finished piece may have the desired thickness.

Layout of Cavities:

For efficient and Economical Layout the following points to be considered during Layout of the Cavities.

Optimum disposition of Cavities: Minimum Runner length, Balanced Layout 

Optimum disposition of Cavities: Reduce the Mould size, reduce the Mould cost.

Minimum Runner length: Reduce the pressure drop; Fill all the cavities with required pressure and temperature.

Balanced Layout:  Attain uniform clamping; prevent local flashing of the mould. 

Design of Feed system:

Feed system connects: The flow of the material from nozzle to the cavity

Feed system contains: Sprue alone (Direct sprue gate) or Sprue, runner and gate in multi-impression moulds

Feed system is not proper: Difficult to get the product with optimum quality.

Split mould: A mould in which two or more parts of cavity closed together in a chase bolster by using locking hell during injection time is called split mould. A mould in which the cavity is formed by two or more parts hold to gather by a chase bolster during the injection phase. A two or more steel blocks containing the impetration which can be opened, normally at right angle to the moulds axis, to facility the moulding of external undercut type components. Guiding the splits in the desired directions. Actuating the splits. It securely was locking the splits in position prior to the material being injected into the mould.

Split cavity: The split cavity is similar to split core the only difference treeing in it is used for those component which having external undercut.

Cam-track actuation: A hardened steel member fitted to one mould pate for the purpose of operation splits and side core. This member of actuation utilizes a cam track machined into a steel plate attached to the fixed mould half. A boss fitted to both side of splits runs in this track. The movement of splits can thus be accurately controlled by specific cam design to ensure smooth operation a generous should be incorporated at each point where the cam-track from changes. A radius or taper should also be included at the entrance to from a lead-in for the boss as it re-enters the track.

External undercut: The moulding which having recess or projection on the outside surface component is known as external undercut.

Internal undercut: The moulding which having a recess or projection, inside surface of the component is called internal undercut.

Side core: It is a local core insert which is normally mounted at right angle to the mould axis for forming a hde or recess in the side of the moulding.

Side cavity: It is also a local insert which performs similar operation. They only difference being, it is used for external projection.

Feed system: The flow ways which connect from machine nozzle to each impression is called feed system. The feed system comprises: Runner, Gate, and Sprue the molten material passes through the sprue, main runner, branch runner and gate before entering impression. It is desirable to keep the distance that the molten material travel down to minimum to reduce pressure and heat losses. It is necessary to provide a flow way in the injection mould to connect the nozzle of the injection machine to each impression. This flow way is termed as the feed system. The feed system comprises sprue, runner and gate.

Sprue: The sprue receives the molten plastics from the machine nozzle and transferred to the impression through a tapered hole within a bush. It is defined as the solidified plastic material formed in the tapered passage of the sprue which connects the machine nozzle to the mold’s parting surface. The maximum diameter of the sprue should be 6mm. The minimum diameter of sprue should be greater than nozzle orifice by 0.5 to 1.0mm. Recommended sprue length should be at least 100mm.It should be kept as short as possible. The sprue included angle should be 4 to 6 degree.

Mould Cooling depends on: Temperature difference between plastic and coolant, Heat travel distance from hot plastic inside the cavity to cooling channel, Heat conductivity of mould material, Dirt in coolant, Rust and Mineral deposits in cooling channels, Specific heat of coolant, Mould temperature and Coolant temperature, Layout of cooling channels, Size of cooling channels, Methods of cooling systems (Direct, In-direct & embedded copper tubes)

Types of Cooling Systems: Channelling of plates, Figure shows diverting plugs and pipe plugs in cooling channels, Cavity block with three level cooling channels, Flow pattern used on a deep cavity block with the channels running length  wise, Circular flow at various levels around a deep cavity.

Plate Cooling: The hot runner plates should include cooling lines to reduce wear and stabilize the plate temperature. Proper cooling in the hot runner plates prevents the transfer of heat to the mold or the machine platens. In the absence of cooling, plate heating also can reduce the sealing force and, through thermal expansion, cause misalignment of mold components. Also, heat generated by the hot runner can heat the machine stationary platen. Non-uniform heating of the machine platens can lead to premature tie bar wear.  Ideally, the cooling circuit should run in close proximity to the locations where the heated components contact the plates. This allows the heat conducted to the plates by contact of the components to be removed efficiently with minimal thermal expansion. Additionally the cooling circuit should be balanced within the plates to maintain a uniform temperature distribution throughout the plates. 

Dog cam: This method of actuation is required/used where a greater spit delay is required then finger cam actuation. The dog leg actuation which is of a general rectangular section is mounted in fixed mould plate each splits incorporate a rectangular hde. The operating face has a corresponding angle to that of the cam.

Finger cam: in the finger cam actuation system, hardened circular pins termed as a finger cams, are mounted at an angle varies from 10-25⁰ in the fixed plate. The splits mounted in guiding on the moving half plat, have corresponding angle circular holes to accommodate these finger cam.

Temperature Control (Cooling System)

Cooling system is necessary: To solidify the hot plastic material injected inside the cavity.

Efficient cooling in necessary: For efficient production ( Less Cycle time ), For quality moulding, To prevent moulded in-stresses, strains, blisters, war page, sink mark, poor surface appearance, varying part dimensions etc. on the finished product.

Cooling efficiency will depends on: Temperature difference between plastic and coolant, Heat travel distance from hot plastic inside the cavity to cooling channel, Heat conductivity of mould material, Dirt in coolant, Rust and Mineral deposits in cooling channels, Specific heat of coolant, Mould temperature and Coolant temperature, Layout of cooling channels, Size of cooling channels, Methods of cooling systems (Direct, In-direct & embedded copper tubes)

Types of Cooling Systems: Size and shape of the product, Design of the mould.

Cooling systems layout in a mould depends on: Part geometry, Number of cavities, Ejector and Cam systems, Part quality, Dimensional accuracy, Part surface appearance, Polymer etc.

Types of Cooling Systems: Channeling of plates, Figure shows diverting plugs and pipe plugs in cooling channels, Cavity block with three level cooling channels, Flow pattern used on a deep cavity block with the channels running length  wise, Circular flow at various levels around a deep cavity.

Cooling for core plate:  Angled hde system: In this system water flows through the circuits making an angle from the undercut of the core. 

Baffled system: In this system water pass through a no of holes machined at right angle to the rear face of the core plate. Baffles are fitted inside each hde in Oder to allow water. 

Steeped circuit system: the circuit can be used where cooling channels are positioned close the top surface of core.

Deep champer design: in this cooling method the portion of the core insert is machined to form a deep chamber. Water from the insert pass through the pipe fitted on the centre of the chamber and after circulation comes out through “outlet”.

Baffled hole system: in this system baffled holes are used. The holes are drilled into the rear-face of the insert, may either-be right angle to the base or parallel to the outside wall of core.

Baffled hole system for small insert: In this design i.e. impartation are arranged in line. The insert are circular in cross-section and fitted into bolster. Each inserts incorporation a chamber which is connecting with bolster by drilling.

Bubbler system: This system is similar to the baffled hde system except that the inlet and outlet passage are different. It gives the uniform cooling of core insert.

Helical channel design: provide more efficient cooling for a core insert. In this system the water is flowing in a helical path machined into steel or brass block, which is fitted insert the chamber of insert.

Cavity inserts cooling: Rectangular: if the shape of the cavity is rectangular the cooling method is very simple. We make a u circuit by drilling hole and circulating the water for cooling.

Circular insert: coolant annulus method: in this method a coolant annulus is machined on the periphery of insert. For multi imprecation mould, the inserts can be position inline.

Insert connecting groove method: in this method the coolant annuls are incorporated as a groove machined into the mould plate. This method are adapted when insert are arranged in line or a pitch circle dia.

Coolant sleeve method: it is very important and very similar to coolant annulus method. The only difference being. It has an additional coolant sleeve into which cavity insert is fitted.

Runner less mould: In this process of injection moulding, the runners are kept hot in order to keep the molten plastic in a fluid state at all time. In this effect this is a “runner less” moulding process and is some-time called the same. In that moulds the runner is contained in a plate of its own. Hot runner moulds are similar to three plate injection mould, except that the runner section of the mould is not opened during the moulding cycle. The heated runner plate in insulated from the rest of the cooled mould. Other than the heated plate of the runner, the remainder of the mould is a standard two-plate die.

1.) Hot runner mould (most expensive)   2.) Insulated runner mould.

Insulated hot runner mould: The insulated hot runner mould is the simplest of all the hot runner design polymer material flow from a standard nozzle into large diameter runner and finally into. Impression by means or reuse taped sprue and gate. In this type of moulding the outer surface of the material into the runner acts like an insulator for the molten material to pass through. In this insulated mould the moulding material remains molten by retaining its own heat sometimes a torpedo and hot property are added for more flexibility. This type of mould is ideal for multicavites canter gated parts. 

Backing Plate: The backing plate supports the hot half of the mold to the stationary platen. The backing plate contains the clamp slots, mounting bolt locations or quick mold change geometry required to mount the mold to the stationary platen. In addition, the backing plate may contain air or hydraulic lines for a valve-gated hot runner. The functionality of the backing plate hinges on how well it is secured to the manifold plate. Ideally, plate bolts are located near each drop to oppose the plate separation forces generated by thermal expansion. It is recommended that for systems with two to eight drops, three bolts should be positioned at each drop, forming a triangle. Triangulation places an even force on the components and prevents distortion. On larger systems, a shared bolt pattern is necessary due to space limitations. It is critical during assembly of a hot runner system that the backing plate bolts be torque from the center of the plate. This will prevent the plate from bowing and allow it to sit flat on the manifold plate. 

Plate Material: Hot runner plates are manufactured from either a stainless steel or P20. The corrosion resistance of stainless steel adds value to the plates by extending life. Hot runner plates are in close contact with both water vapor and potentially corrosive off-gassing from heated plastics. 

Advantages: The feed system can easily be stripped and cleaned, resulting in very little material a color contamination occurring. Mould start up-times are faster when compared to other hot runner mould system. Moulds are very much cheaper to manufacture than the other hot runner mould design. Thermally unstable polymer may be processed using such a system.

Manifold Plate Design: The manifold plate has three main functions - support and aligns the hot runner components, provide surface area for backing plate bolting, and back-up the cavity plate and its components. Manifold plate design must consider these three functions for successful operation. Plastic melt travels from the machine nozzle to the gate through precisely aligned melt channels. Misalignment of the melt channels can cause any number of problems - from poor color change and burning to complete leakage and flooding of the hot runner. Manifold plate design must provide suitable support for the components responsible for alignment. These components include locating insulators, installation bolts and nozzles. Plate manufacturing also must utilize tools and methods that can maintain the tight tolerances required for these components to function properly. 

Bill Gunn: Proper design of the hot runner plates is critical to molding success. The hot runner plates must perform the function of a rigid and stable support while being exposed to high mechanical loadings from both the hot runner components and the molding machine. Hot runner plates consist of a manifold plate and backing plate that when attached together form the structural shell of the hot runner system. As an integral part of a successful hot runner, the design and manufacture of the hot runner plates deserves discussion. Hot runner systems utilize thermal expansion to develop a sealing force between components. The sealing force is created when the bond between the manifold and backing plate resists yielding to the thermal expansion of the manifold components. The sealing force must be sufficient to prevent plastic leakage at maximum machine pressures and can exceed 12,000 lbf for each nozzle.

Mini fold: It is a type of plate. It is a placed in hot runner mould. It is hardened plate and use for feeling the material in the cavity. It is use for feeding only feed plate is hot. 

Advantage: Reduce cycle time as a result of having a component cooling requirement cooling the runner and feed system remain molten above the quickly frozen gate. Material saving result from having no sprue bush or runner system to grate. Labor and post moulding finishing cost are significantly reduced without the need for degutting of moulding. The ability to grain greater control over the moulding filling and flow charterisitis of the molten polymer during the filling phase of the moulding cycle.

Conclusion: Hot runner plates play a major role in the performance of the mold. Along with supporting the hot runner components, the hot runner plates can impact mold life and the quality of the molded product. Poorly designed plates can lead to damaged cores, worn slides and vents and ultimately higher costs to keep the mold in operating condition. It is important to the mold designer and processor to understand the function of the hot runner plates and the design options that need to be considered.