Transcript Chapter 8 Design for Injection Molding 20 May 2016

20 May 2016
Chapter 8
Design for
Injection Molding
1
Dr. Mohammad Abuhaiba
20 May 2016
2
Outline
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
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
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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.
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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:
1.
2.
thermoplastic
thermosetting
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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.
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8.2 INJECTION MOLDING
MATERIALS- Thermoplastic polymers (TP)

Thermoplastics are generally divided into two classes:
1.
2.

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
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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.
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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.
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8.2 INJECTION MOLDING
MATERIALS
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8.3 THE MOLDING CYCLE

Stages of injection molding:
1.
2.
3.
injection or filling
cooling
ejection and resetting
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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.
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8.3 THE MOLDING CYCLE
Injection 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.
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8.3 THE MOLDING CYCLE
Cooling 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:
1.
2.
3.

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.
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8.3 THE MOLDING CYCLE
Ejection and Resetting Stage

During this stage:
1.
2.
3.

rapid movements may cause:
1.
2.



mold is opened
part is ejected,
mold is then closed again in readiness for next cycle to begin.
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.
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8.4 INJECTION MOLDING SYSTEMS
Components of injection
molding system:
1.
2.
3.
injection unit
clamp unit
mold
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8.4 INJECTION MOLDING SYSTEMS
Injection Unit


The injection unit has two functions:
1.
to melt pellets or powder
2.
to inject the melt into a mold.
Most widely used types of injection units:
1.
conventional units: consists of a cylinder and a
plunger
2.
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.
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8.4 INJECTION MOLDING SYSTEMS
Injection Unit
 Injection
units are usually rated with two
numbers:
1.
2.
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.
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8.4 INJECTION MOLDING SYSTEMS
Injection 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
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8.4 INJECTION MOLDING SYSTEMS
Clamp Unit
 Clamp
1.
2.
3.
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
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8.4 INJECTION MOLDING SYSTEMS
Clamp Unit
 Magnitude
of initial opening force
required depends on:
1.
2.
3.
packing pressure
Material
part geometry (depth and draft)
 is
approximately equal to 10 to 20%
of nominal clamp force.
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8.4 INJECTION MOLDING SYSTEMS
Clamp Unit
Two common types of clamp designs:
1. Linkage or toggle clamp:



2.
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.
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8.4 INJECTION MOLDING SYSTEMS
Clamp Unit
 Force
required to eject the part
depends on:
1.
2.
3.
Material
part geometry
packing pressure
 less
than 1% of nominal clamp force
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8.5 INJECTION MOLDS
 Functions
1.
2.
A
1.
2.
of a mold:
impart the desired shape to the
plasticized polymer
cool the molded part
mold is made up of:
the cavities and cores
the base in which the cavities and cores
are mounted
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8.5 INJECTION MOLDS
Mold Construction
and Operation
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8.5 INJECTION MOLDS
Mold Construction
and Operation
1.
2.
3.
4.
5.
6.
7.
8.
9.
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
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8.5 INJECTION MOLDS
Mold Construction and Operation




Mold basically consists of two parts:
1.
a stationary half (cavity plate)
2.
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.
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8.5 INJECTION MOLDS
Mold 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.
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8.5 INJECTION MOLDS
Mold 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)
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8.5 INJECTION MOLDS
Mold Types
Most common types of
molds:
1.
2.
3.
4.
Two-plate molds
Three-plate molds
Side-action molds
Unscrewing molds
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8.5 INJECTION MOLDS
Mold 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.
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8.5 INJECTION MOLDS
Mold Types - A two-plate mold
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8.5 INJECTION MOLDS
Mold Types - three-plate mold
 Consists
1.
2.
3.
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
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8.5 INJECTION MOLDS
Mold 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.
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8.5 INJECTION MOLDS
Mold 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.
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8.5 INJECTION MOLDS
Mold 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.
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8.5 INJECTION MOLDS
Mold Types - Side-acting molds
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8.5 INJECTION MOLDS
Mold Types - unscrewing molds
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8.5 INJECTION MOLDS
Sprue, 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:
1.
2.
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.
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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
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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.
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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
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8.6 MOLDING MACHINE SIZE
Example
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
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8.6 MOLDING MACHINE SIZE
Example
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.
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8.6 MOLDING MACHINE SIZE
Example
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.
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MOLDING CYCLE TIME
Molding cycle:
1.
2.
3.
injection or filling time
cooling time
mold-resetting time
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MOLDING CYCLE TIME
Injection 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
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MOLDING CYCLE TIME
Injection 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
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MOLDING CYCLE TIME
Cooling 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.
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MOLDING CYCLE TIME
Cooling 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
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MOLDING CYCLE TIME
Cooling 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.
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MOLDING CYCLE TIME
Cooling 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.
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MOLDING CYCLE TIME
Mold Resetting
 Resetting
time: sum of
1. Mold opening
2. part ejection
3. mold closing
 Resetting time depends upon:
1.
2.
amount of movement required for part separation
from cavity and core
time required for part clearance from mold plates
during free fall.
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MOLDING CYCLE TIME
Mold Resetting
 Part
size influences resetting time in two
ways:
1.
2.
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.
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MOLDING CYCLE TIME
Mold 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
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MOLDING CYCLE TIME
Mold Resetting
If
depth of part is given by D cm,
then the clamp stroke is adjusted to
a value of 2D + 5 cm.
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MOLDING CYCLE TIME
Mold 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
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MOLDING CYCLE TIME
Mold 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.
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MOLDING CYCLE TIME
Mold 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
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MOLDING CYCLE TIME
Mold Resetting
Example: plain 15cm dia cylindrical cups, with a
depth of 20 cm, are to be mfg from ABS in a sixcavity 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.
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MOLDING CYCLE TIME
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8.6 MOLDING MACHINE SIZE
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8.6 MOLDING MACHINE SIZE
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8.8 MOLD COST ESTIMATION
Mold cost can be broken down into:
1. Cost of prefabricated mold base
consisting of required plates, pillars,
guide bushings, etc.
2. cavity and core fabrication costs.
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8.8 MOLD COST ESTIMATION
Mold Base Costs

Mold base cost is a function of:
1.
2.

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
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8.8 MOLD COST ESTIMATION
Mold Base Costs
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8.8 MOLD COST ESTIMATION
Mold Base Costs


Selection of appropriate mold base is based on:
1.
depth of part
2.
its projected area
3.
number of cavities required in mold
In addition to cavity size, extra allowance has to
be given for molds with mechanical action sidepulls and other complicated mechanisms, such as
unscrewing devices for molding of screw threads.
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8.8 MOLD COST ESTIMATION
Mold 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
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8.8 MOLD COST ESTIMATION
Mold 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
Dr. Mohammad Abuhaiba
20 May 2016
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8.8 MOLD COST ESTIMATION
Mold 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.
Dr. Mohammad Abuhaiba
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20 May 2016
8.8 MOLD COST ESTIMATION
Cavity 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
Dr. Mohammad Abuhaiba
20 May 2016
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8.8 MOLD COST ESTIMATION
Cavity 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
Dr. Mohammad Abuhaiba
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20 May 2016
8.8 MOLD COST ESTIMATION
Geometrical 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
Dr. Mohammad Abuhaiba
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20 May 2016
8.8 MOLD COST ESTIMATION
Geometrical Complexity Counting Procedure - Example


FIG. 8.8 Surface segments of plain conical
components
Inner surface comprises:
1.
2.


Main conical surface
Flat base
Xi = 0.1 x 2 = 0.2
Outer surface comprises:
1.
2.
3.
4.

Main conical surface
Flat annular base
Cylindrical recess in the base
Flat recessed base
X0 = 0.1 x 4 = 0.4
Dr. Mohammad Abuhaiba
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20 May 2016
8.8 MOLD COST ESTIMATION
Geometrical 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.

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20 May 2016
8.8 MOLD COST ESTIMATION
Geometrical Complexity Counting Procedure

In order to complete a mold cost-estimating system six
additional important factors need to be considered:
1. The need for retractable side-pulls or internal core lifters
2. The requirement for one or more unscrewing cores to
produce molded screw threads
3. Surface finish and appearance specified for the part
4. Average tolerance level applied to part dimensions
5. The requirement for one or more surfaces to be textured
6. Shape of surface across which cavity and core
separate
Dr. Mohammad Abuhaiba
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20 May 2016
8.8 MOLD COST ESTIMATION
Geometrical Complexity Counting Procedure
 Mfg
hours for side-pulls, internal lifters, or
unscrewing devices will be assumed to
correspond to be:175-290 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.
Dr. Mohammad Abuhaiba
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20 May 2016
8.8 MOLD COST ESTIMATION
Geometrical Complexity Counting Procedure
Dr. Mohammad Abuhaiba
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20 May 2016
8.8 MOLD COST ESTIMATION
Geometrical Complexity Counting Procedure

Part tolerance affects the time estimate for geometrical
complexity given by Eq. (8.11).
Dr. Mohammad Abuhaiba
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20 May 2016
8.8 MOLD COST ESTIMATION
Geometrical 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.
Dr. Mohammad Abuhaiba
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20 May 2016
8.8 MOLD COST ESTIMATION
Geometrical 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
Dr. Mohammad Abuhaiba
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20 May 2016
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.
Dr. Mohammad Abuhaiba
20 May 2016
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8.9 MOLD COST POINT SYSTEM
i.
Projected Area of Part (cm2):

ii.
iii.
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.
Dr. Mohammad Abuhaiba
20 May 2016
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8.9 MOLD COST POINT SYSTEM
iv.
Internal Lifters


v.
Unscrewing Devices


vi.
Identify # of internal depressions or undercuts requiring
separate internal core lifters.
150 points for each lifter.
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)]
Dr. Mohammad Abuhaiba
20 May 2016
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8.9 MOLD COST POINT SYSTEM
vii.
Tolerance Level


Table 8.7; %value for required tolerance
category.
Multiply by (ii)
viii. Texture

ix.
5% of [(i) + (ii)]
Parting Plane


Table 8.8: parting plane factor, fp
Use fp to obtain point score from Eq. (8.14).
Dr. Mohammad Abuhaiba
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20 May 2016
8.9 MOLD COST POINT SYSTEM
Example
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.
i.
Projected Area: 43 h

ii.
iii.
iv.
v.
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
Dr. Mohammad Abuhaiba
20 May 2016
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8.9 MOLD COST POINT SYSTEM
Example
Surface Finish/Appearance: 11.5 h
VI.

VI.

VII.
VIII.
(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.
Dr. Mohammad Abuhaiba
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20 May 2016
8.10 ESTIMATION OF THE
OPTIMUM NUMBER OF CAVITIES
When
multi-cavity mold is used,
3 principal changes occur:
1.
2.
3.
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.
Dr. Mohammad Abuhaiba
20 May 2016
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8.10 ESTIMATION OF THE
OPTIMUM NUMBER OF CAVITIES
Machine hourly
rate is


F = clamp
force, kN
K1 , m 1 =
machine rate
coefficients
Dr. Mohammad Abuhaiba
20 May 2016
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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
Dr. Mohammad Abuhaiba
20 May 2016
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
Dr. Mohammad Abuhaiba
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20 May 2016
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
Dr. Mohammad Abuhaiba
20 May 2016
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:
Dr. Mohammad Abuhaiba