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Plastic Product Design
• Product should meet design requirement
• Economical Process
• Quality should be built into the system
• Should be flexible and responsive to new technology• High productivity: est utili!ation of man" material"
machine" capital and available resources#
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Steps in $odern $anufacturing
Production drawings;
Instruction manuals
Conceptual design and
evaluation Feasibility study
Design
analysis;codes/standards
review; physical and
analytical models
Prototype production
testing and evaluation
Definition of product need,
marketing information
aterial !pecification;process
and e"uipment selection;
safety review
Pilot Production
Production
Inspection and "uality
assurance
Packaging; marketing and
sales literature
Product
C#D
C# and C#PP
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$
Introduction % Concept is simple
• Melt plastic, fow into mold and take part shape, cool, demold
% Injection molding makes parts in discrete (discontinuous) process % More injection molding machines used or plastic processing than
any other equipment
% Almost all thermoplastic and some thermosets materials can einjection molded
% !rocess is automated and highly repeatale parts % Injection molding parts are "nished with little post molding
operations
% #ery comple$ parts can e made
% Machines are e$pensi%e
% Molds are e$pensi%e, usually !&' steel
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Ad%antages o !lastic
• ow Cost
• *asily made into any +hape
•ight weight• ecyclale
• -ide range o color
• Comple$ .esign can e made• /nreakale
• 0on&erosi%e
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imitations o !lastic
• ow +trength
• 1igh initial cost
• 0on&thermal staility• 2rittle at ow temperature
• ow 3hermal and *lectrical
conductors• It destroys soil source 4 not eco
riendly
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!lastic !roduct 5eatures
Corners and *dges is
2oss
1oles
+trengthening the parts
.rats
&ny automobile interior ' exterior thermoplastic partcomes across different types of design features# Some
of the most important design features are below#
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Corners and *dges
In the design o injection moldedparts, sharp corners should ea%oided6
+harp corners act as stress risersor concentrators, reducing partstrength and causing prematureailures6 +harp corners may alsoe7ect plastic fow, producingparts with ojectionale suracefow patterns6
3he inside radius should e atleast hal the part wall thickness
3he outside radius should equalthe inside radius plus the partthickness
(i6e6 68 wall and inside radius o69 equals outside radius o689
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Corners and *dges
As seen in the chart, thestress concentration actoris quite high or :3 %aluesless than 696 5or %alues o
:3 o%er 69 the stressconcentration actor getslower6
3he stress concentrationactor is a multiplier actor,
it increases the stress6Actual +tress ; +tressConcentration
5actor < $ +tress Calculated
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is
Many times the sti7ness o a part mustincrease ecause o the load applied to thepart design6 -ithout ris, the thickness has
to e increased to increase the endingsti7ness =ne o the easiest ways to cure thisprolem is change the part geometry yadding ris6
3he use o ris is a practical way andeconomical means o increasing the endingsti7ness i6e6 the structural strength o a part6
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is
i thickness should eless than wallthickness6
A ri thickness o >?to @? o nominalwall thickness isrecommended6
i should e attached
to the ase withgenerous radiusing atthe corners6
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is
5or a gi%en sti7ness, itis etter to increase
the numer o ris,not the height6
3he height o the ri
should e limited toless than $thickness
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is
3he ri orientation isased on pro%idingma$imum endingsti7ness6
.epending on orientationo the ending load, withrespect to the partgeometry, ris oriented
one way increase
sti7ness6I oriented the wrong way
there is no increase insti7ness6
2 i l i i
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%$
2oosting structural integritywith ris
• +tructural integrityB the goal o e%ery design % 3he major component o designing or structural integrity,
in many cases, is to design the structure to e sti7 enoughto withstand e$pected loads6 Increasing the thickness toachie%e this is sel&deeating, since it willB
• Increase part weight and cost proportional to the increase inthickness6
• Increase molding cycle time required to cool the larger mass omaterial6
• Increase the proaility o sink marks6
% -ell&designed ris can o%ercome these disad%antages with only amarginal increase in part weight6
2 i l i i
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%&
2oosting structural integritywith ris
• 3ypical uses or ris % Co%ers, cainets and ody components with long, wide suraces
that must ha%e good appearance with low weight6
% ollers and guides or paper handling, where the surace muste cylindrical6
% ear odies, where the design calls or wide earing suraceson the center shat and on the gear teeth6
% 5rames and supports6
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%'
is .esign• .esign ules• i geometry
% i thickness, height, and drat angle are relatedB e$cessi%e thickness willproduce sinks on the opposite surace whereas small thickness and too greata drat will thin the ri tip too much or acceptale "lling6
% is should e tapered (drated) at one degree per side6
• ess drat can e used, to one&hal degree per side, i the steel that ormsthe sides o the ri is careully polished6
• 3he drat will increase the ri thickness rom the tip to the root, y aout68D9 mm per centimeter o ri height, or each degree o drat angle6
• 3he ma$imum recommended ri thickness, at the root, is 6@ times thethickness o the ase to which it is attached6
• 3he typical root thickness ranges rom 69 to 6@ times the ase thickness6
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%(
is .esign
• .esign ules•
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%)
ecommended .esign !arameters6• +ee 5igure 8 or recommended design parameters6
i i l
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%*
is .esign ules• ocation o ris, osses, and gussets
% is aligned in the direction o the mold opening are the least e$pensi%e design option totool6
% As illustrated in 5igure 8, a oss should not e placed ne$t to a parallel wallE instead, o7set
the oss and use gussets to strengthen it6 % ussets can e used to support osses that are away rom the walls6 3he same design rules
that apply or ris also apply or gussets6
• Alternati%e con"gurations % As shown in 2elow, ris can take the shape o corrugations6
% 3he ad%antage is that the wall thickness will e uniorm and the drat angle can e placed
on the opposite side o the mold, therey a%oiding the prolem o the thinning ri tip6 % 1oneycom riing attached to a fat surace pro%ides e$cellent resistance to ending in all
directions6
% A he$agonal array o interconnected ris will e more e7ecti%e than a square array,with the
same %olume o material in the ris6
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2osses
2osses are used or locating, mounting,and assemly purposes6 3here are ossdesign guidelines that must e ollowed
to insure the highest quality in moldedparts6 3he wall thicknesses should e less than
> ? o nominal wall to minimiFe sinking6
1owe%er, i the oss is not in a %isilearea, then the wall thickness can eincreased to allow or increased stressesimposed y sel&tapping screws6
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2osses
+ule of thumb the wall thickness around a boss design feature -t.
should be (0 of the nominal part thickness -1. if that thickness is
less than %/*23
If the nominal part thickness is greater than %/*2 the boss wall
thickness should be &0 of the nominal wall3
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2osses
3he oss can estrengthened y
gussets at thease, and yattaching it toneary walls with
connecting ris6
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1oles
1oles are easy to produce in molded parts6Core pins that protrude into the mold
ca%ity make the holes when the part is
molded6 3hrough holes in the moldedparts are easier to produce than lindholes, which do not go all the waythrough a part6
2lind holes are made y core pinssupported on one end only6
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1oles
A good rule o thumB thedepth o the lind holeshould e aout twice thediameter o core pins upto :8>G,and up to ourtimes the diameter ocore pins o%er:8>G6 3heguidelines or lind andthrough holes are seenelow6
2lind 1ole (shown withdrat)B
; '. or .iameters essthan :8>G dia6 core pins
; H. or .iameters Morethan :8>G dia6 core pins
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1oles
As shown the distancerom the edge o a holeto a %ertical surace(i6e6 ri) or the edge o
a part should e twicethe part thickness (ormore), or at least onediameter o the hole6
3his same rule appliesetween holes & at leasttwo part thicknesses orone hole diametershould e speci"ed6
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1oles
5or e%ery cored or molded hole there wille a weld line6 3he weld lines are causedy the fow o the melted plastic around
the core pins6 3hese weld lines are not asstrong as the surrounding plasticmaterial, and also may detract rom theo%erall appearance o the molded part6
3he part designer should consider thesepoints when designing holes in a moldedpart6
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+trengthening the !art
In the design o injectionmolded parts, sharpcorners should e a%oided6
+harp corners act as stressrisers or concentrators,
reducing part strength andcausing premature ailures6+harp corners may alsoe7ect plastic fow,producing parts withojectionale surace fowpatterns6
In addition to reducingstresses, "llet radiusespro%ide streamline fow!aths or the molten plasticresulting in easier "lls6
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.rat
.rats (or taper) in a mold,acilitates part remo%alrom the mold6 3he amounto drat angle depends onthe depth o the part in the
mold, and its required enduse unction6
It is est to allow or as muchdrat as possile or easyrelease rom the mold6
As a nominalrecommendation, it is estto allow 8 to ' degrees odrat6
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!lastic !roduct .esign
Apart rom the earlier discussion.esigners should ha%e to consider
ate .esign
unner +ystem .esign
Cooling +ystem .esign
+hrinkage and -arpage .esign
+ t
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45
unner +ystem
• +e%eral types o runners
% single part runner % multiple part runner
• symmetrical runner
• non&symetrical runner
% runner&less designs with hot maniolds
+ t
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unner +ystem
• unner siFe considerations
% Although properly siFing a runner to a gi%en part and molddesign has a tremendous pay&o7, it is oten o%erlooked sincethe asic principles are not widely understood6
• !ros and cons o large runners % -hile large runners acilitate the fow o material at relati%ely
low pressure requirements, they
% require a longer cooling time, more material consumption andscrap, and more clamping orce6
• !ros and cons o small runners
% .esigning the smallest adequate runner system will ma$imiFeeciency in oth raw material use and energy consumption inmolding6 At the same time, howe%er, runner siFe reduction isconstrained y the molding machineJs injection pressurecapaility6
+ t
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$%
unner +ystem
• unner 2alancing is an essential or a alanced
"lling pattern with a reasonale pressure drop6• !ayo7s o good runner design
% A runner system that has een designed correctlywillB
• Achie%e the optimal numer o ca%ities
• .eli%er melt to the ca%ities
• 2alance "lling o multiple ca%ities
• 2alance "lling o multi&gate ca%ities
• MinimiFe scrap
• *ject easily
• Ma$imiFe eciency in energy consumption
• Control the "lling:packing:cycle time6
1 t + t
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$4
1ot unner +ystem• 3he ideal injection molding system deli%ers molded parts o
uniorm density, and ree rom all runners, fash, and gate stus6
• 3o achie%e this, a hot runner system, in contrast to a cold runnersystem, is employed6 3he material in the hot runners ismaintained in a molten state and is not ejected with the moldedpart6 1ot runner systems are also reerred to as hot&maniold
systems, or runnerless molding6
FI67+8 %3 9ot runner system types -a. the insulated hot runner, -b. the internally heated hot:runnersystem, and -c. the eternally heated hot:runner system
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$$
unner uidelines• Common runners
% 5ull&round runner
% 3rapeFoidal runner
% Modi"ed trapeFoidal runner (a comination o roundand trapeFoidal runner)
% 1al&round runner
% ectangular runner
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$&
ate .esign
• ate .esign =%er%iew % +ingle %s6 multiple gates
• +ingle gate is usually desirale ecause multiple gates ha%eweld lines
% ate dimension• 3he gate thickness is usually two&thirds the part thickness6
• 3he gate thickness controls packing time
• Chose a larger gate i youJre aiming or appearance, lowresidual stress, and etter dimensional staility6
% ate location• !osition the gate away rom load&earing areas6
• !osition the gate away rom the thin section areas, orregions o sudden thickness change to a%oid hesitation and
sink marks
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$'
ate .esignate .esign =%er%iew
% ate 3ypes• Manually trimmed
% equires an operator to separate parts rom runnersduring a secondary operation
% 3ypes include sprue, ta, edge, o%erlap, an, disk, ring,"lm, diaphragm, spider
• Automatically trimmed gates % Automatically trimmed gates incorporate eatures in the
tool to reak or shear the gate % +hould e used to
( A%oid gate remo%al as a secondary operation
( Maintain consistent cycle times or all shots
( MinimiFe gate scars
% 3ypes include !in, +umarine, hot&runner, and %al%e
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$)
ate .esign• .esign ules
% ate ength• ate length should e as short as possile to
reduce an e$cessi%e pressure drop across thegate6 anges rom 8 to 869 mm (6H to 6>
inches)
• 3he gate thickness is normally 9 to @ percent othe gated wall section thickness6 !in andsumarine gates range rom 6'9& '6 mm (68K&
6@K)• 3he reeFe&o7 time at the gate is the ma$ e7ecti%e
ca%ity packing time6
• 5ier&"lled materials require larger gates to
i i i k th "
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