Basic Part Design for Injection Molding

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Basic Part Design for Injection Molding
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Why Should You Care?
25 of all thermoplastics are injection molded
Part design is greatest influence on part cost
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What You Will Learn (in 50 minutes)
The basic concepts you will need to apply to
successfully design/develop an injection-molded
part
Now for the fine print
Note These concepts are only
guidelinesthere are always exceptions to every
ruleone needs to consult specific material
literature, consider past experience (yours or
others), and, most importantly, apply common
sense.
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Outline

• Primary Wall
• Draft
• Projections (Ribs, Gussets, and Bosses)
• Undercuts and Holes
• Corners, Fillets, and Radii

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Part Design

• Primary Wall

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Primary Wall

• Normally less than 5 mm / 0.2 in
• Manufacturability issues
• Injection pressure required to fill the cavity
• Cooling time
• Structural, functional, and aesthetic issues

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Constant Wall Thickness

• Primary objective of part design for IM
• To maintain even shrinkage
• To avoid warpage (your worst enemy!)
• To avoid residual stresses (your hidden enemy!)
• To avoid sink marks and voids
• To simplify filling pattern
• To avoid hesitation during filling
• To avoid gas traps (race track or picture
  frame effect)
• To avoid weld lines (discussed later)

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Polymer PnT Behavior
(Amorphous)
(Semi-crystalline)
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Complex Filling Pattern (Hesitation)
Thin
Thick
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Race Tracking
Thin
Thick
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Constant Wall Thickness (Transitions)
Bad
Better
Betterbut
Recommended (gradual with draft)
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Core Out
Use coring to eliminate material masses in the
part
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Core Out - Examples
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Goal Minimize Wall Thickness

• More cost-effective production
• Less material (often more than 1/2 the part cost)
• Reduce cooling time
• Thickness determination
• Amount and type of loading
• Electrical, sound, thermal insulation
  requirements
• Clamp force available (higher pressure to fill)

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Effect of Wall Thickness (Cooling Time)

Cooling time s
Wall Thickness mm
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Effect of Wall Thickness(Fill Pressure)
Specialized molding machines

Fill Pressure psi
Wall Thickness mm
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Surface Finish

• Mold should be polished in the direction of draw
  (ejection)
• Certain polyolefins and elastomers requires a
  coarser finish (400-grit) do not want mirror
  finish
• Surface finish classes (SPI)
• Class A diamond buff (expensive, optical
  components)
• Class B grit paper
• Class C stone (very commonly specified)
• Class D dry blast (non-draw surfaces)
• Apply only what is required!

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Outline

• Primary Wall
• Draft
• Projections (Ribs, Gussets, and Bosses)
• Undercuts and Holes
• Corners, Fillets, and Radii

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Part Design

• Draft

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Taper or Draft Angle

• Applied on vertical walls
• From 1/4 degree to several degrees (depending on
  material coefficient of friction, mold surface
  finish, material shrinkage, and product
  functional requirements)
• Draft on inside and outside wall should be
  equal/parallel (avoid wall thickness variations
  constant wall thickness rule)
• Allows for ejection of part

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Draft Example
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Draft Dollars

Draft
No Draft

3
3
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Metal-to-Metal (Proper Draft) -1
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Metal-to-Metal (Proper Draft) -2
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Metal-to-Metal (Proper Draft) -3
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Outline

• Primary Wall
• Draft
• Projections (Ribs, Gussets, and Bosses)
• Undercuts and Holes
• Corners, Fillets, and Radii

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Part Design

• Projections
• (Ribs, Gussets, and Bosses)

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Ribs

• Increase wall rigidity
• Localized
• Maintain minimum primary wall thickness
• Manufacturing restrictions
• As short as possible, to ease ejection
• Drafted (tapered on each side), to ease ejection
• Polished (draw), to ease ejection
• Ejector pin-pads, to ease ejectionand vent!
• Radius, to minimize stresses

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Recommended Rib Design
2-3 W (min)
¼ min
0.010 (min)
2.5-3 W
W
0.50 W Semi-crystalline 0.75 W Amorphous or filled
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Sink Mark or Void(Due to Ribs)

• Causes
• Thinner wall freezes first
• Increased volume (A vs. B)

A
B
Sink mark?
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Sink Mark or Void Which One?
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Gussets

• Reinforce local feature
• Side wall
• Boss, etc.
• Manufacturing restrictions
• Triangular, to prevent gas trap
• Drafted, to ease ejection
• Radius, to minimize stresses

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Recommended Gusset Design
0.50 W Semi-crystalline 0.75 W Amorphous or filled
Gusset
Rib
2W
BAD
4W
W
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Bosses

• Used for assembly
• Solid or hollow round (generally)
• Supported with ribs or gussets
• Manufacturing restrictions
• Height (cooling difficulties, pin deflection)
• (consider coring from both sides of tool)
• Drafted, to ease ejection
• (interior draft can step if necessary)
• Radius
• Lead-in chamfer
• Venting (particularly on non-moving side of tool)

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Recommended Boss Design
D
2 D
45chamfer
½ (min)
2.5 D
R
W
R
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Outline

• Primary Wall
• Draft
• Projections (Ribs, Gussets, and Bosses)
• Undercuts and Holes
• Corners, Fillets, and Radii

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Part Design

• Undercuts and Holes

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Undercuts

• Increases difficulty of ejecting the part
• Could be stripped off the mold (last resort)
• (material and design dependent)
• Might require mold side-action ()
• (increased maintenance potential)
• Try to eliminate/redesign any feature which
  requires side-action (or undercut) to create

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Recommended Design for Undercuts (Rigid
Thermoplastics)
r
Where e is dynamic strain limit for the
material remember time/temperature-dependent
nature of polymers! (strain limit variable)
Dr
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Removing Undercuts/Side-Actionsthe Bypass
Shut-off
IMPORTANT
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Recommended Design for Holes
A Hole diameter B 2W (min) C 2W (min)
W
C
B
A
Holes form knit/weld lines,,, Knit/weld line
position can be critical in both structural and
cosmetic applications
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Weld or Knit Lines
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Outline

• Primary Wall
• Draft
• Projections (Ribs, Gussets, and Bosses)
• Undercuts and Holes
• Corners, Fillets, and Radii

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Part Design

• Corners, Fillets, and Radii

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Corners, Fillets, and Radii

• Sharp corners/transitions should be avoided
• Most plastics are notch sensitive (stress
  concentrator)
• Flow problems during fillingextensional
  effects, increased pressure drop
• Make sure your CAD model has radii in place
• Radii at corners maintenance of wall thickness

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Recommended Corner Design
W
Not recommended
A
B
Betterbut
High stresses/warpage potential
r
B
Recommended
B
B
R r W
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Sharp Corner Warp or Internal Stress
trouble (get out your checkbook)
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Outline

• Primary Wall
• Draft
• Projections (Ribs, Gussets, and Bosses)
• Undercuts and Holes
• Corners, Fillets, and Radii

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Summary

• Think of most injection-molded parts as a
  primary wall features
• Use common sense and apply basic principles
• - Constant nominal wall with smooth transitions
• - Apply ample draft and proper surface finish
• - Consider material type when sizing features
• Use supplier manuals as a rule-of-thumb
• - Eliminate undercuts (features requiring mold
  side-actions)
• - Add radii where appropriate (NO SQUARE
  PARTS!!!)
• Strive for the best design possible you will
  have to live with it
• Consult with toolmakers/engineers, process
  engineers, and material suppliers early in part
  development

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In your future Call RTP (meor, better yet,
Mark) with questions/applications happy
to help!