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Design for Injection Molding

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Chapter 8 Design for Injection Molding Dr. Mohammad Abuhaiba * 8.8 MOLD COST ESTIMATION Mold Base Costs Selection of appropriate mold base is based on: depth of part ... – PowerPoint PPT presentation

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Title: Design for Injection Molding


1
Chapter 8
  • Design for Injection Molding

2
Outline
  • 8.7 MOLDING CYCLE TIME
  • 8.7.1 Injection Time
  • 8.7.2 Cooling Time
  • 8.7.3 Mold Resetting
  • 8.8 MOLD COST ESTIMATION
  • 8.8.1 Mold Base Costs
  • 8.8.2 Cavity and Core Manufacturing Costs
  • Geometrical Complexity Counting Procedure
  • 8.9 MOLD COST POINT SYSTEM
  • 8.10 ESTIMATION OF THE OPTIMUM NUMBER OF CAVITIES
  • 8.11 DESIGN EXAMPLE
  • 8.12 INSERT MOLDING
  • 8.13 DESIGN GUIDELINES
  • 8.14 ASSEMBLY TECHNIQUES
  • 8.1 INTRODUCTION
  • 8.2 INJECTION MOLDING MATERIALS
  • 8.3 THE MOLDING CYCLE
  • 8.3.1 Injection or Filling Stage
  • 8.3.2 Cooling or Freezing Stage
  • 8.3.3 Ejection and Resetting Stage
  • 8.4 INJECTION MOLDING SYSTEMS
  • 8.4.1 Injection Unit
  • 8.4.2 Clamp Unit
  • 8.5 INJECTION MOLDS
  • 8.5.1 Mold Construction Operation
  • 8.5.2 Mold Types
  • 8.5.3 Sprue, Runner, and Gates
  • 8.6 MOLDING MACHINE SIZE

3
8.1 INTRODUCTION
  • Injection molding (IM) technology consists of
  • Heating thermoplastic material until it melts
  • Forcing this melted plastic into a steel mold,
    where it cools and solidifies.

4
8.2 INJECTION MOLDING MATERIALS
  • Polymers that are capable of being brought to a
    state of fluidity can be injection-molded.
  • Polymers can be divided into two categories
  • thermoplastic
  • thermosetting

5
8.2 INJECTION MOLDING MATERIALS - Thermoplastic
polymers (TP)
  • Capable of being softened by heat and of
    hardening on cooling.
  • This is because long-chain molecules always
    remain separate entities and do not form chemical
    bonds to one another
  • Most TP materials offer
  • high impact strength
  • good corrosion resistance
  • easy processing with good flow characteristics
    for molding complex designs.

6
8.2 INJECTION MOLDING MATERIALS- Thermoplastic
polymers (TP)
  • Thermoplastics are generally divided into two
    classes
  • Crystalline (CP)
  • Amorphous (AP)
  • Crystalline polymers
  • ordered molecular arrangement
  • sharp melting point
  • Because of the ordered arrangement of molecules,
    CP reflect most incident light and generally
    appear opaque.
  • High shrinkage or reduction in volume during
    solidification.
  • More resistant to organic solvents
  • have good fatigue and wear-resistance properties.
  • are denser and have better mechanical properties
    than amorphous polymers

7
8.2 INJECTION MOLDING MATERIALS- Service
temperature
  • Heat deflection temperature
  • Temperature at which a thermoplastic can be
    operated under load.
  • This is the temperature at which a simply
    supported beam specimen of the material, with a
    centrally applied load, reaches a predefined
    deflection.

8
8.2 INJECTION MOLDING MATERIALS - Thermosetting
  • Chemical bonds are formed between the separate
    molecule chains during processing.
  • Referred to as cross-linking, is the hardening
    mechanism.

9
8.2 INJECTION MOLDING MATERIALS
10
8.3 THE MOLDING CYCLE
  • Stages of injection molding
  • injection or filling
  • cooling
  • ejection and resetting

11
8.3 THE MOLDING CYCLE
  • During 1st stage, material in molten state is a
    highly nonlinear viscous fluid.
  • It flows through mold passages and is subject to
    rapid cooling from mold wall, on one hand, and
    internal shear heating, on the other.
  • Melt then undergoes solidification under high
    packing and holding pressure.
  • Mold is opened, part is ejected, and machine is
    reset for next cycle to begin.

12
8.3 THE MOLDING CYCLEInjection or Filling Stage
  • Forward stroke of plunger to facilitate flow of
    molten material from the heating cylinder through
    nozzle and into mold.
  • Gradual increase in pressure.
  • As soon as cavity is filled, pressure increases
    rapidly, and packing occurs.
  • During packing part, flow of material continues,
    at a slower rate, to account for any loss in
    volume of material due to partial solidification
    and shrinkage.
  • After packing, injection plunger is withdrawn and
    pressure in mold cavity begins to drop.
  • At this stage, next charge of material is fed
    into the heating cylinder in preparation for next
    shot.

13
8.3 THE MOLDING CYCLECooling Stage
  • Cooling starts from 1st rapid filling of cavity
    and continues during packing and then following
    withdrawal of the plunger, with the resulting
    removal of pressure from the mold and nozzle
    area.
  • Upon pressure removal, gate of mold may still be
    relatively fluid.
  • Because of pressure drop, there is a chance for
    reverse flow of material from mold until material
    adjacent to the gate solidifies and the sealing
    point is reached.
  • Reverse flow is minimized by proper design of
    gates such that quicker sealing action takes
    place upon plunger withdrawal.
  • Following the sealing point, there is a
    continuous drop in pressure as material in cavity
    continues to cool and solidifies in readiness for
    ejection.
  • Length of sealed cooling stage depends on
  • wall thickness of part
  • material used
  • mold temperature
  • Because of low thermal conductivity of polymers,
    cooling time is usually the longest period in the
    molding cycle.

14
8.3 THE MOLDING CYCLEEjection and Resetting
Stage
  • During this stage
  • mold is opened
  • part is ejected,
  • mold is then closed again in readiness for next
    cycle to begin.
  • rapid movements may cause
  • undue strain on the equipment
  • damage the edges of the cavities.
  • Adequate time must be allowed for mold ejection.
  • This time depends on part dimensions
  • For parts to be molded with metal inserts,
    resetting involves reloading of inserts into
    mold. After resetting, mold is closed and locked,
    thus completing one cycle.

15
8.4 INJECTION MOLDING SYSTEMS
  • Components of injection molding system
  • injection unit
  • clamp unit
  • mold

16
8.4 INJECTION MOLDING SYSTEMSInjection Unit
  • The injection unit has two functions
  • to melt pellets or powder
  • to inject the melt into a mold.
  • Most widely used types of injection units
  • conventional units consists of a cylinder and a
    plunger
  • reciprocating screw units a barrel and a screw
    that rotates to melt pump the plastic mix from
    hopper to end of screw and then moves forward to
    push the melt into mold.

17
8.4 INJECTION MOLDING SYSTEMSInjection Unit
  • Injection units are usually rated with two
    numbers
  • First rating No. Shot capacity
  • Second rating number plasticating rate
  • Shot capacity max volume of polymer that can be
    displaced by one forward stroke of injection
    plunger or screw.
  • recommended shot sizes 20 to 80 of rated
    capacity.

18
8.4 INJECTION MOLDING SYSTEMSInjection Unit
  • Plasticating rate amount of material that can be
    softened into a molten form by heating in the
    cylinder of machine in a given time.
  • It is usually expressed as No. of pounds of
    polystyrene material that the equipment can heat
    to molding temperature in one hour

19
8.4 INJECTION MOLDING SYSTEMSClamp Unit
  • Clamp unit has three functions
  • open and close mold halves
  • eject the part
  • hold mold closed with sufficient force to resist
    melt pressure inside mold as it is filled
  • Required holding force 30 to70 MN/m2 of
    projected area of part

20
8.4 INJECTION MOLDING SYSTEMSClamp Unit
  • Magnitude of initial opening force required
    depends on
  • packing pressure
  • Material
  • part geometry (depth and draft)
  • is approximately equal to 10 to 20 of nominal
    clamp force.

21
8.4 INJECTION MOLDING SYSTEMSClamp Unit
  • Two common types of clamp designs
  • Linkage or toggle clamp
  • very fast closing and opening actions
  • lower in cost than alternative systems
  • clamp force is not precisely controlled
  • Hydraulic clamp units
  • long term reliability
  • precise control of clamp force
  • relatively slow and expensive compared to toggle
    clamp systems.

22
8.4 INJECTION MOLDING SYSTEMSClamp Unit
  • Force required to eject the part depends on
  • Material
  • part geometry
  • packing pressure
  • less than 1 of nominal clamp force

23
8.5 INJECTION MOLDS
  • Functions of a mold
  • impart the desired shape to the plasticized
    polymer
  • cool the molded part
  • A mold is made up of
  • the cavities and cores
  • the base in which the cavities and cores are
    mounted

24
8.5 INJECTION MOLDSMold Constructionand
Operation
25
8.5 INJECTION MOLDSMold Constructionand
Operation
  • Fixed Clamping Plate
  • Runner Stripper Plate
  • Cavity plate
  • Movable Cavity Plate or Cavity plate
  • Back up Plate
  • Spacer Block
  • Ejector retainer plate
  • Ejector Plate
  • Movable Clamping Plate

26
8.5 INJECTION MOLDSMold Construction and
Operation
  • Mold basically consists of two parts
  • a stationary half (cavity plate)
  • a moving half (core plate)
  • Parting line separating line between the two
    mold halves
  • The injected material is transferred through a
    central feed channel, called the sprue.
  • In multi-cavity molds, sprue feeds polymer melt
    to a runner system.

27
8.5 INJECTION MOLDSMold Construction and
Operation
  • Core plate holds the main core.
  • Purpose of main core is to establish the inside
    configuration of the part.
  • The core plate has a backup plate.
  • Backup plate in turn is supported by pillars
    against the U shaped structure known as the
    ejector housing, which consists of the rear
    clamping plate and spacer blocks.
  • The U-shaped structure, which is bolted to core
    plate, provides the space for the ejection stroke.

28
8.5 INJECTION MOLDSMold Construction and
Operation
  • During solidification part shrinks around main
    core so that when mold opens, part and sprue are
    carried along with moving mold half
  • Both mold halves are provided with cooling
    channels
  • Mold cavities incorporate fine vents (0.02 to
    0.08mm by 5mm)

29
8.5 INJECTION MOLDSMold Types
  • Most common types of molds
  • Two-plate molds
  • Three-plate molds
  • Side-action molds
  • Unscrewing molds

30
8.5 INJECTION MOLDSMold Types - A two-plate mold
  • consists of two active plates (Fig. 8.3) (cavity
    and core plates) into which cavity and core
    inserts are mounted, as shown in Fig. 8.4.
  • Runner system, sprue, runners, and gates solidify
    with part being molded and are ejected as a
    single connected item.

31
8.5 INJECTION MOLDSMold Types - A two-plate mold
32
8.5 INJECTION MOLDSMold Types - three-plate mold
  • Consists of
  • Stationary or runner plate, which contains sprue
    and half of runner
  • Middle or cavity plate, which contains other half
    of runner, gates, and cavities and is allowed to
    float when mold is open
  • Movable or core plate, which contains cores and
    ejector system.
  • Facilitates separation of runner system and part
    when mold opens

33
8.5 INJECTION MOLDSMold Types - Hot runner system
  • Three main plates
  • Runner is contained completely in the fixed
    plate, which is heated and insulated from the
    rest of the cooled mold.
  • Runner section of the mold is not opened during
    molding cycle.
  • There are no side products (gates, runner, or
    sprues) to be disposed of or reused
  • There is no need for separation of gate from part.

34
8.5 INJECTION MOLDSMold Types - Side-acting
molds
  • are used in molding components with external
    depressions or holes parallel to the parting
    plane.
  • Undercuts prevent molded parts from being removed
    from cavity in axial direction.
  • The usual way of providing the side action needed
    to release the part is with side cores mounted on
    slides.
  • These are activated by angle pins, or by air or
    hydraulic cylinders that pull the side cores
    outward during opening of the mold.

35
8.5 INJECTION MOLDSMold Types - Side-acting
molds
  • The slide, which carries the secondary side core
    pin, is moved by the angle pin mounted in the
    stationary half of the mold.
  • As the two halves of the mold move apart during
    mold opening, the slide, which is mounted on the
    moving plate, is forced to move sideways by the
    angle of the pin.

36
8.5 INJECTION MOLDSMold Types - Side-acting
molds
37
8.5 INJECTION MOLDSMold Types - unscrewing molds
38
8.5 INJECTION MOLDSSprue, Runner, and Gates
  • Fig. 8.4
  • Sprue an inlet channel for molten material from
    the heating chamber into the mold or runner
    system.
  • The gate a constriction between feed system and
    mold cavity, serves several purposes
  • It freezes rapidly and prevents material from
    flowing out of cavity when injection pressure is
    removed.
  • It provides an easy way of separating moldings
    from runner system.

39
8.6 MOLDING MACHINE SIZE
  • Determination of appropriate size of an injection
    molding machine is based primarily on required
    clamp force.
  • This in turn depends upon projected area of
    cavities in mold and max pressure in the mold
    during mold filling.
  • For a 15cm diameter plain disk, projected area is
    176.7 cm2.
  • If the disk has a single 10cm diameter through
    hole in any position, projected area is 98.2 cm2

40
8.6 MOLDING MACHINE SIZE
  • Size of runner system depends upon size of part.
  • As a first approximation, these figures will also
    be applied to give projected area of runner
    system as a percentage of projected area of part.

41
8.6 MOLDING MACHINE SIZE
  • 50 of pressure generated in machine injection
    unit is lost because of flow resistance in sprue,
    runner systems, and gates

42
8.6 MOLDING MACHINE SIZEExample
  • A batch of 15 cm dia disks with a thickness of 4
    mm is to be molded from ABS in a 6-cavity mold.
    Determine appropriate machine size.
  • Projected area of each part 177cm2.
  • Table 8.2 increase in area due to runner
    system 15.
  • Total projected shot area 6x1.15x177 1221.3
    cm2
  • Table 8.5 injection pressure for ABS 1000 bars
  • Max cavity pressure 500 bars 500x105 N/m2
  • Max separating force F (1221.3x10-4)x500x105 N
    6106.5 kN

43
8.6 MOLDING MACHINE SIZEExample
  • A batch of 15 cm dia disks with a thickness of 4
    mm is to be molded from ABS in a 6-cavity mold.
    Determine appropriate machine size.
  • Table 8.4 appropriate machine would be the one
    with a max clamp force of 8500kN.
  • Required shot size volume of six disks volume
    of runner system 6x 1.15 x (177x0.4) 489 cm3,
    which is easily within max machine shot size of
    3636 cm3.

44
8.6 MOLDING MACHINE SIZEExample
  • A batch of 15 cm dia disks with a thickness of 4
    mm is to be molded from ABS in a 6-cavity mold.
    Determine appropriate machine size.
  • For the 8500 kN machine, machine clamp stroke
    85 cm
  • This stroke is sufficient to mold a hollow part
    up to a depth of approximately 40 cm.
  • For such a part, the 85 cm stroke would separate
    the molded part from both the cavity and the core
    with a clearance of approximately 5 cm for the
    part to fall between the end of the core and the
    cavity plate.
  • This stroke is excessive for molding of 4 mm
    thick flat disks.

45
MOLDING CYCLE TIME
  • Molding cycle
  • injection or filling time
  • cooling time
  • mold-resetting time

46
MOLDING CYCLE TIMEInjection Time
  • Initial flow rate gradually decrease as mold is
    filled
  • flow resistance in mold channels
  • constriction of channels as polymer solidifies
    against the walls
  • Flow rate suffers a constant deceleration to
    reach a low value at the point at which mold is
    nominally filled.
  • Under these circumstances, the fill time would be
    estimated as
  • Pj injection power, W
  • Pj recommended injection pressure, N/m2
  • Vs required shot size, m3

47
MOLDING CYCLE TIMEInjection Time - Example
  • For the 15 cm dia disks molded in a six-cavity
    mold, required shot size is 489 cm3.
  • Recommended injection pressure for ABS is 1000
    bars, or 100 MN/m2.
  • Available power at injection unit of the 8500kN
    machine is 90 kW. Thus estimated fill time is
  • tf 2 x (489 x 10-6) x (100 x 106)/(90 x 103)
    1.09 s

48
MOLDING CYCLE TIMECooling Time
  • Mold opening and ejection are assumed to be
    permissible when injected polymer has cooled to
    the point where the highest temperature in mold
    (at thickest wall center plane) equals Tx,
    recommended ejection temperature.

49
MOLDING CYCLE TIMECooling Time
  • Cooling time is given by
  • hmax max wall thickness, mm
  • Tx recommended part ejection temperature, C
  • Tm recommended mold temperature, C
  • Ti polymer injection temperature, C
  • a thermal diffusivity coefficient, mm2/s

50
MOLDING CYCLE TIMECooling Time
  • Eq. (8.5) tends to underestimate cooling time for
    very thin wall moldings.
  • For such parts thickness of runner system is
    often greater than parts themselves and greater
    delay is needed to ensure that runners can be
    ejected cleanly from the mold.
  • 3 s be taken as min cooling time even if Eq.
    (8.5) predicts a smaller value.

51
MOLDING CYCLE TIMECooling Time
  • Eq. (8.5) applies only to a rectangular slab
    which is representative of main wall of an
    injection-molded part.
  • For a solid cylindrical section a correction
    factor of 2/3 should be used on diameter.
  • a 3mm thick flat part with a 6mm dia cylindrical
    projection would have an equivalent max thickness
    of 2/3 x 6 4 mm.

52
MOLDING CYCLE TIMEMold Resetting
  • Resetting time sum of
  • Mold opening
  • part ejection
  • mold closing
  • Resetting time depends upon
  • amount of movement required for part separation
    from cavity and core
  • time required for part clearance from mold plates
    during free fall.

53
MOLDING CYCLE TIMEMold Resetting
  • Part size influences resetting time in two ways
  • projected area of part together with No. of
    cavities determines machine size and power
    available for mold opening and closing.
  • depth of part determines amount of mold opening
    required for part ejection.

54
MOLDING CYCLE TIMEMold Resetting
  • Dry cycle time time required to
  • operate injection unit
  • open and close an appropriately sized mold by an
    amount equal to max clamp stroke
  • Dry cycle time is based on an empty injection
    unit, and it takes only ms to inject air through
    the mold.
  • no required delay for cooling
  • machine clamp is operated during both opening and
    closing at max stroke and at max safe speed

55
MOLDING CYCLE TIMEMold Resetting
  • If depth of part is given by D cm, then the clamp
    stroke is adjusted to a value of 2D 5 cm.

56
MOLDING CYCLE TIMEMold Resetting
  • Mold opening usually takes place more slowly than
    mold closing.
  • Rapid mold opening may result in warping or
    fracture of molded part.
  • It will be assumed that opening takes place at
    40 of closing speed

57
MOLDING CYCLE TIMEMold Resetting
  • It will be assumed that for a given clamp unit
    velocity profile during a clamp movement will
    have identical shape irrespective of adjusted
    stroke length.
  • Under these conditions, time for a given movement
    will be proportional to square-root of stroke
    length.

58
MOLDING CYCLE TIMEMold Resetting
  • If max clamp stroke is Ls for a given machine and
    dry cycle time is td, then time for clamp closing
    at full stroke will be assumed equal to td/2.
  • If a part of depth D is to be molded, then
    adjusted clamp stroke will be 2D 5 cm and time
    for mold closing will be
  • Using the assumption of 40 opening speed and a
    dwell of 1 s for molded part to fall between
    plates, then this gives an estimate for mold
    resetting as

59
MOLDING CYCLE TIMEMold Resetting
  • Example plain 15cm dia cylindrical cups, with a
    depth of 20 cm, are to be mfg from ABS in a
    six-cavity mold.
  • Machine size is 8500 kN
  • From Table 8.4
  • dry cycle time, td 8.6s
  • max clamp stroke, Ls 85cm
  • D 20, Ls 85, and td 8.6 into Eq. (8.7)
    gives an estimated resetting time of 12.0 s.

60
MOLDING CYCLE TIME
61
8.6 MOLDING MACHINE SIZE
62
8.6 MOLDING MACHINE SIZE
63
8.8 MOLD COST ESTIMATION
  • Mold cost can be broken down into
  • Cost of prefabricated mold base consisting of
    required plates, pillars, guide bushings, etc.
  • cavity and core fabrication costs.

64
8.8 MOLD COST ESTIMATIONMold Base Costs
  • Mold base cost is a function of
  • surface area of selected mold base plates
  • combined thickness of cavity and core plates
  • Data in Fig. 8.7 can be represented by
  • Cb cost of mold base,
  • Ac area of mold base cavity plate, cm2
  • hp combined thickness of cavity and core plates
    in mold base, cm

65
8.8 MOLD COST ESTIMATIONMold Base Costs
66
8.8 MOLD COST ESTIMATIONMold Base Costs
  • Selection of appropriate mold base is based on
  • depth of part
  • its projected area
  • number of cavities required in mold
  • In addition to cavity size, extra allowance has
    to be given for molds with mechanical action
    side-pulls and other complicated mechanisms, such
    as unscrewing devices for molding of screw
    threads.

67
8.8 MOLD COST ESTIMATIONMold Base Costs
  • Min clearance between adjacent cavities between
    cavity surface and edges and rear surfaces of
    cavity plates should be 7.5 cm.
  • Side-pulls or side unscrewing devices require
    twice min clearance from edges
  • Rear unscrewing devices require a doubling of
    material at rear of cavity.
  • One side-pull will increase plate width or length
    by an additional 7.5 cm
  • Four or more pulls, one or more on each side of a
    part, will require a plate that is 15 cm larger
    in both length and width.
  • Use of two side-pulls restricts mold design to a
    single row of cavities
  • use of three or more usually implies
    single-cavity operation

68
8.8 MOLD COST ESTIMATIONMold Base Costs - Example
  • 10 cm dia plain cylindrical cups with a depth,
    Dd, of 15 cm are to be molded in a six-cavity
    mold.
  • A 3 x 2 array of cavities with clearances
  • Ac required plate area 2550 cm2
  • Combined cavity core plate thickness hp hd
    15 30 cm.
  • Mold base cost parameter Achp0.4 9940 cm2.4
  • Fig 8.7 estimated mold base cost 5500

69
8.8 MOLD COST ESTIMATIONMold Base Costs - Example
  • If two diametrically opposed holes in the side
    surfaces and an internal thread, estimated plate
    size increases will be as follows
  • cavity plate will now hold a single row of six
    cavities
  • Using 15 cm clearance along each side of cavities
    to house side core mechanisms
  • plate area 112.5 x 40 4500cm2
  • To support unscrewing device, combined plate
    thickness increases to an assumed value of
    37.5cm, which results in a new value of Achp0.4
    equal to 19,179cm2.4.
  • Fig. 8.7 mold base cost 9500.

70
8.8 MOLD COST ESTIMATIONCavity and Core
Manufacturing Costs
  • Mold making starts with purchase of a
    preassembled mold base from a specialist
    supplier.
  • Purchase price of mold base should be doubled to
    account for custom work that has to be performed
    on it.
  • Number of ejector pins used was found to be
    approximately equal to square root of x-sectional
    area
  • Ne number of ejector pins required
  • Ap projected part area, cm2

71
8.8 MOLD COST ESTIMATIONCavity and Core
Manufacturing Costs
  • 2.5 mfg hours for each ejector pin.
  • Additional of mfg hours for ejection system of
    a part
  • Geometric complexity of a part is handled by
    assigning a complexity score (0 to 10) for both
    inner and outer surface of part.
  • Number of mold mfg hours, associated with
    geometrical features of part, for one cavity
    matching core(s)
  • Xi and Xo inner and outer complexity of part

72
8.8 MOLD COST ESTIMATIONGeometrical Complexity
Counting Procedure
  • Count all separate surface segments on part inner
    surface.
  • Inner surface is surface that is in contact with
    main core.
  • complexity of inner surface is given by
  • Nsp number of surface patches
  • Small connecting blend surfaces should not be
    counted
  • When counting multiple identical features on
    surface of a part, a power index of 0.7

73
8.8 MOLD COST ESTIMATIONGeometrical Complexity
Counting Procedure - Example
  • FIG. 8.8 Surface segments of plain conical
    components
  • Inner surface comprises
  • Main conical surface
  • Flat base
  • Xi 0.1 x 2 0.2
  • Outer surface comprises
  • Main conical surface
  • Flat annular base
  • Cylindrical recess in the base
  • Flat recessed base
  • X0 0.1 x 4 0.4

74
8.8 MOLD COST ESTIMATIONGeometrical Complexity
Counting Procedure
  • for parts with very simple geometry the number of
    mfg hours for one cavity and core can be
    represented by
  • Ap part projected area, cm2
  • Sum of point scores from Eqs. (8.10), (8.11), and
    (8.13) provides a base estimate of number of mfg
    hours to make one cavity and core and ejection
    system for a part of given size with a known
    degree of geometrical complexity.

75
8.8 MOLD COST ESTIMATIONGeometrical Complexity
Counting Procedure
  • In order to complete a mold cost-estimating
    system six additional important factors need to
    be considered
  • The need for retractable side-pulls or internal
    core lifters
  • The requirement for one or more unscrewing cores
    to produce molded screw threads
  • Surface finish and appearance specified for the
    part
  • Average tolerance level applied to part
    dimensions
  • The requirement for one or more surfaces to be
    textured
  • Shape of surface across which cavity and core
    separate

76
8.8 MOLD COST ESTIMATIONGeometrical Complexity
Counting Procedure
  • Mfg hours for side-pulls, internal lifters, or
    unscrewing devices will be assumed to correspond
    to be175-290 h
  • Cost of texturing is proportional to both
    complexity and size of part and that a fairly
    good estimate is obtained by allowing 5 of basic
    cavity mfg cost.
  • Shallow lettering can be considered equivalent to
    texture.

77
8.8 MOLD COST ESTIMATIONGeometrical Complexity
Counting Procedure
78
8.8 MOLD COST ESTIMATIONGeometrical Complexity
Counting Procedure
  • Part tolerance affects the time estimate for
    geometrical complexity given by Eq. (8.11).

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8.8 MOLD COST ESTIMATIONGeometrical Complexity
Counting Procedure
  • Flat bent parts or hollow parts whose edge,
    separating inner and outer surface, does not lie
    on a plane, parting surface should be chosen from
    six classifications given in Table 8.8.

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8.8 MOLD COST ESTIMATIONGeometrical Complexity
Counting Procedure
  • Additional number of mfg hours required to mfg
    mold is approximately proportional to square root
    of cavity area
  • Ap projected area of cavity, cm2
  • fp parting plane factor given in Table 8.8
  • Ms additional mold mfg hours for non-flat
    parting surface

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8.9 MOLD COST POINT SYSTEM
  • Mold mfg cost is determined by equating each
    point to one hour of mold mfg.
  • Cost to mfg a single cavity and matching core(s)
    total point score times appropriate average
    hourly rate for tool mfg.

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8.9 MOLD COST POINT SYSTEM
  • Projected Area of Part (cm2)
  • Eqs. (8.10) (8.13), points for the size effect
    on mfg cost plus points for ejection system.
  • Geometric Complexity Eq. (8.11)
  • Side-Pulls
  • Identify of holes or apertures requiring
    separate side-pulls (side cores) .
  • Allow 65 points for each side-pull.

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8.9 MOLD COST POINT SYSTEM
  • Internal Lifters
  • Identify of internal depressions or undercuts
    requiring separate internal core lifters.
  • 150 points for each lifter.
  • Unscrewing Devices
  • Identify of screw threads.
  • 250 points for each unscrewing device.
  • Surface Finish/Appearance
  • Table 8.6 percentage value for required
    appearance category.
  • Multiply by (i) (ii)

84
8.9 MOLD COST POINT SYSTEM
  • Tolerance Level
  • Table 8.7 value for required tolerance
    category.
  • Multiply by (ii)
  • Texture
  • 5 of (i) (ii)
  • Parting Plane
  • Table 8.8 parting plane factor, fp
  • Use fp to obtain point score from Eq. (8.14).

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8.9 MOLD COST POINT SYSTEMExample
  • 2,000,000 plain hollow conical components are to
    be molded in acetal homo-polymer. Material volume
    78 cm3 and a projected area in direction of
    molding 78.5 cm2.
  • Projected Area 43 h
  • Substitute Ap 78.5 cm2 into Eqs. (8.10) and
    (8.13)
  • Geometrical Complexity Xi 0.2 and Xo 0.4,
    apply Eq. (8.11)
  • of Side-Pulls 0
  • of Internal Lifters 0
  • of Unscrewing Devices 0

86
8.9 MOLD COST POINT SYSTEMExample
  • Surface Finish/Appearance 11.5 h
  • (Opaque high gloss Table 8.6 add 25 of 43 3)
  • Tolerance Level 0h
  • Category 1 Table 8.7 insignificant effect for
    low complexity
  • Texture 0h
  • Parting Plane (category 0) 0
  • Total point score 57.5
  • 40 / hour for mold mfg
  • cost for one activity and core 57.5 x 40
    2,300.

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8.10 ESTIMATION OF THE OPTIMUM NUMBER OF CAVITIES
  • When multi-cavity mold is used, 3 principal
    changes occur
  • A larger machine with a greater hourly rate is
    needed.
  • Cost of mold is getting larger.
  • Mfg time per part decreases in approximately
    inverse proportion to number of cavities.

88
8.10 ESTIMATION OF THE OPTIMUM NUMBER OF CAVITIES
  • Machine hourly rate is
  • F clamp force, kN
  • K1, m1 machine rate coefficients

89
8.10 ESTIMATION OF THE OPTIMUM NUMBER OF CAVITIES
  • If cost of one cavity and matching core is given
    by Q, then cost, Cn, of producing identical sets
    of the same cavity and core can be represented by
  • m multi-cavity mold index 0.7
  • n of identical cavities

90
8.10 ESTIMATION OF THE OPTIMUM NUMBER OF CAVITIES
  • Savings occur in mold base cost per cavity when
    increasing of cavities.
  • Savings depend upon cavity area
  • smaller cavities being associated with larger
    savings.
  • A power law relationship similar to Eq. (8.16)
    applies equally to mold bases and with the same
    value for the power index
  • .
  • Ccl cost of single-cavity mold
  • Ccn cost of n-cavity mold
  • n number of cavities
  • m multi-cavity mold index

91
8.10 ESTIMATION OF THE OPTIMUM NUMBER OF CAVITIES
  • cost, Ct, of producing Nt molded components can
    be expressed as
  • t machine cycle time, h
  • Cm cost of polymer material per part,
  • f separating force on one cavity

92
8.10 ESTIMATION OF THE OPTIMUM NUMBER OF CAVITIES
  • Substituting Eq. (8.19) into (8.18) gives
  • Min value of Ct will occur when dCt/dF 0
  • optimum number of cavities
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