Injection Molded Part Design (an introduction)

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Injection Molded Part Design(an introduction)

• TEC 315
• Dr. Lou Reifschneider

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Guidelines for Injection Molded Part Design

• Nominal Wall Thickness
• Uniform wall thickness ? less material cost,
  faster cooling time, uniform shrinkage less
  warpage
• Corner Radii reduced stress, less warpage.
• Draft Angles easier demolding.
• Core thick areas to reduce wall thickness
• Gradual thickness change less warpage.
• (Gate part so the melt flows from thick to
  thinner areas to insure adequate packing of
  cavity.)
• (Cooling time varies as the thickness squared.)

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The Importance of Radii in Product Design
Force
Wall Thickness
If radius lt .25 thickness, then high stress
concentration.
Radius
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Good (v) Corner Design, and Bad (X)

• v corner design
• generous radius
• uniform wall thick
• minimal shrinkage, minimal stress concentration

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Wall Thickness Uniformity
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Good Plastic Part DesignCore Out Thick
Sections, Add Radii Where Possible
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Wall thickness in IM parts
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Part Ejection Draft
Cavity Draft Angle 1? minimum, Increase draft
with longer draw and surface texture.
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Guidelines for Injection Molded Part Design

• Ribs Bosses
• Root thickness lt nominal wall, less stress
  sink.
• Short height - ease demolding.
• Adequate space between - faster cooling, less
  sink.
• Part Ejection
• Draft walls min. 1/2 degree, 2 degrees nom.
• Form holes with shut off instead of side action.
• Build area for KO pin contact KO pin PAD.

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Sink Mark Formation
Hot mass cools, shrinks - pulls skin inward.
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Rib Root Design 0.6 wall max. to
control sink mark formation
.6 W
W
W
W
1.5 W diam.
1.2 W diam.
Good
Poor
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Designing Ribs with Equal Strength
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Part Ejection Issues Ejector Pin Pads
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Boss Design
2D
D
H 2 to 5 T
R .25 T (min.)
T
t .5 to .7 T to prevent sink marks
W
sink marks when W gt .6 T
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Boss Design for Self-Tapping Screws
Figure 11-7a Plastic Injection Molding, Vol. II
by Douglas M. Bryce, SME publications.
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General Design Guidelines
Figure 12-1 Plastic Injection Molding, Vol. II
by Douglas M. Bryce, SME publications.
r 1.5 x w
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Part Ejection Issues Undercut and Holes
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Holes made by Shut-Offs
Hole on top/bottom with simple shut off
CAVITY
Large hole on side without side action
CORE
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Four Ways to Make Snap Features
Core Pin
Side Action (next slide)
Lifter - next slide
Stepped Parting Plane
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Schematic of Lifter operation used to mold
undercuts
Mold Opened
Mold Closed
Undercut
Lifter
KO Pin
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Schematic of Cam Actuated Side-Action used to
mold undercuts
Mold Closed
Undercut
KO Pin
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Part Design Affects Cycle Time
Cooling Time vs. Thickness
80.0
60.0
HDPE
Seconds for Average Part
Temperature to Reach HDT
40.0
PS
PC
20.0
0.0
0.00
0.05
0.10
0.15
0.20
0.25
Thickness (inches)
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Some additional pointers

• Assembly basics (an intro)
• Product Shrinkage - mold size adjustment.
• Molding force pressure x projected area
• Projected area the shape of the part cavity as
  you look down the barrel at the part cavity

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Bolt Assembly Guideline
Figure 9.16 Designing With Plastic by Hoechst
Celanese
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Ultrasonic Inserts and Studs
Advantages - Excellent performance. (good hold
strength) Very Fast. Very little
induced stress. Disadvantages - Requires
expensive equipment to install.
Figure 9.18 Designing With Plastic by Hoechst
Celanese
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CommonSnap-fitDesign
Figure 11-12 Plastic Injection Molding, Vol. II
by Douglas M. Bryce, SME publications.
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Computing Mold Cavity Size Nominal Mold
Shrinkage of Some TP (in/in)
Material 0.125 thick 0.25 thick
PC 0.005 0.007 PC (30GF)
0.001 0.002 HDPE 0.017 0.021 HDPE
(30GF) 0.003 0.004 PA 66
0.016 0.022 PA 66 (30GF) 0.005 0.005
Slower cooling due to thickness
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Computing Mold Cavity Size (cont)
Rearrange definition of Shrinkage
How long should mold be to make a 6.5 inch long
part if S.02 in/in?
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Clamp Force Calculation
Cavity Pressure at 4,000 psi Diameter of Cup is 3
inches
Pressure acts outwardly in all directions. The
lateral forces cancel each other, leaving
only the force acting in the projected area.
Clamp Force?
Clamp Force P ? Areaproj
Areaproj (3.14D2) / 4
Force 4,000 ? (3.14 32) / 4 28,273 lb
14 tons
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Product Design Resources

• Designing With Plastics, Hoechst Celanese Design
  Guide.
• GE Plastics Design Guide, available as free Adobe
  download at geplastics.com
• Plastic Part Design for Injection Molding,
  Robert A. Malloy, Hanser, 1994, 460 pgs.
• Glenn Beall Seminars, A Designers Guide to Part
  Design for Economical Injection Molding (50 for
  students!!)