Injection Molded Part Design (an introduction)

1. Injection Molded Part Design(an introduction) TEC 315 Dr. Lou Reifschneider

2. 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.)

3. The Importance of Radii in Product Design Force Wall Thickness If radius < .25 thickness, then high stress concentration. Radius

4. X corner design • very sharp radii • high stress concentration • moderate shrinkage • X corner design • inside/outside mismatch • non-uniform wall thick. • maximum shrinkage Good (√) Corner Design, and Bad (X) • √corner design • generous radius • uniform wall thick • minimal shrinkage, minimal stress concentration

5. Stepped Transition Poor Design Tapered Transition Better Design Gradual Transition Best Design Core Out Thicker Area (if possible) Wall Thickness: Uniformity

6. Improved Part Design • Thinner wall sections • More uniform wall sections • Inside and outside radii (where possible) • Original Part Design • Very thick wall sections • Non-uniform wall sections • Sharp inside and outside radii • (GOOD FOR MACHINING) Good Plastic Part Design:Core Out Thick Sections, Add Radii Where Possible

7. Wall thickness in IM parts

8. Cavity Cavity Retainer Plate Draw Core Retainer Plate Core Part Ejection: Draft Cavity Draft Angle 1 minimum, Increase draft with longer draw and surface texture.

9. Guidelines for Injection Molded Part Design Ribs & Bosses • Root thickness < 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.

10. Sink Mark Hot Mass cooling cooling Thick Rib Root Smaller Sink Mark Smaller Hot Mass cooling cooling Thin Rib Root Sink Mark Formation Hot mass cools, shrinks - pulls skin inward.

11. 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. Poor Good

12. 2W W W 3W 2W .6 W W Poor Better Best Designing Ribs with Equal Strength

13. Part Ejection Issues: Ejector Pin Pads

14. 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 > .6 T

15. Boss Design for Self-Tapping Screws Figure 11-7a Plastic Injection Molding, Vol. II by Douglas M. Bryce, SME publications.

16. General Design Guidelines Figure 12-1 Plastic Injection Molding, Vol. II by Douglas M. Bryce, SME publications. r = 1.5 x w

17. Part Ejection Issues: Undercut and Holes

18. Holes made by Shut-Offs Hole on top/bottom with simple shut off CAVITY Large hole on side without side action CORE

19. Four Ways to Make Snap Features Core Pin Side Action (next slide) Lifter - next slide Stepped Parting Plane

20. Mold Opened Schematic of Lifter operation used to mold undercuts Mold Closed Undercut Lifter KO Pin

21. Mold Opened Side Action Schematic of Cam Actuated Side-Action used to mold undercuts Mold Closed Undercut KO Pin

22. 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)

23. 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

24. Bolt Assembly Guideline Figure 9.16 Designing With Plastic by Hoechst Celanese

25. 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

26. CommonSnap-fitDesign Figure 11-12 Plastic Injection Molding, Vol. II by Douglas M. Bryce, SME publications.

27. Computing Mold Cavity SizeNominal 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

28. 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?

29. Molten plastic at 4,000 psi D 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 = (3.14*D2) / 4 Force = 4,000  (3.14* 32) / 4 = 28,273 lb = 14 tons

30. 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 Designer’s Guide to Part Design for Economical Injection Molding ($50 for students!!)