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Design for Cast and Molded Parts

Design for Cast and Molded Parts

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Design for Cast and Molded Parts

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  1. Design for Cast and Molded Parts Team: Terese Bertcher Larry Brod Pam Lee Mike Wehr

  2. Design for Cast and Molded Parts Revision Team: Seamus Clark Scott Leonardi Gary Meyers

  3. Lecture Topics • Basic Casting Design Guidelines • Injection Molding Process • Gating Considerations • Case Study – Corvette Brake Pedal • Case Study – M1 Abrams Tank

  4. Lecture Topics • Basic Casting Design Guidelines • Injection Molding Process • Gating Considerations • Case Study – Corvette Brake Pedal • Case Study – M1 Abrams Tank

  5. Basic Casting Design Guidelines • Visualize the Casting • Design for Soundness • Avoid Sharp Angles & Corners • Minimize the Number of Sections • Employ Uniform Sections • Correctly Proportion Inner Walls • Fillet All Sharp Angles • Avoid Abrupt Section Changes • Maximize Design of Ribs & Brackets • Avoid Using Bosses, Lugs & Pads

  6. Visualize the Casting • It is difficult to follow section changes and shapes from blueprint. • Create a model to scale or full size to help designer to: • See how cores must be designed, placed or omitted • Determine how to mold the casting • Detect casting weaknesses (shrinks / cracks) • Determine where to place gates and risers • Answer questions affecting soundness, cost and delivery

  7. Simplification of Die Configuration

  8. Simplification of Die Configuration

  9. Simplification of Die Configuration

  10. Simplification of Die Configuration

  11. Design for Soundness • Most metals and alloys shrink when they solidify • Design components so that all parts increase in dimension progressively to areas where feeder heads (risers) can be placed to offset shrinkage. • Disguise areas of shrinkage when unavoidable

  12. Design Rules: Disguising Sink Marks

  13. Avoid Sharp Angles & Corners • When two or more sections conjoin, mechanical weakness is induced at the junction and free cooling is interrupted – most common defect in casting design. • Replace sharp angles with radii and minimize heat and stress concentration • In cored parts avoid designs without cooling surfaces • A rounded junction offers uniform strength properties

  14. Incorrect Corner Design Good Corner Design • Generous radius • Uniform wall thickness • Smooth flow transition • Very sharp radii • High stress concentration • Sharp flow transition Incorrect Corner Design Incorrect Corner Design • Outside corner and inside radius • Non-uniform wall thickness • Non-uniform flow transition • Shrinkage stress / voids / sinks • Inside / outside radius mismatch • Non-uniform wall thickness • Non-uniform flow transition Sink Design Rules:Corners & Radii

  15. Minimize the Number of Sections • A well designed casting brings the minimum number of sections together at one point. • Staggering sections (where possible) • Minimizes hot spot effects • Eliminates weakness • Reduces distortion • Where staggering sections is not possible use a cored hole through the center of the junction. • Helps to speed solidification • Helps to avoid hot spots

  16. Employ Uniform Sections • Thicker walls will solidify more slowly. • This means they will feed solidifying inner walls. • Results in shrinkage voids in the thicker walls • Goal is to design uniform sections that solidify evenly. • If this is not possible, all heavy sections should be accessible to feeding from risers.

  17. Improved Part Design • Thinner wall sections • More uniform wall thickness • Inside and outside radii (when possible) • Original Part Design • Very thick wall sections • Non-uniform wall thickness • Sharp inside and outside radii Design Rules: Wall Uniformity

  18. Correctly Proportion Inner Walls • Inner sections of castings cool much slower than outer sections. • Causes variations in strength properties • A good rule of thumb is to reduce inner sections to 90% of outer wall thickness. • Avoid rapid section changes • Results in porosity problems similar to what is seen with sharp angles.

  19. Part gated from “thin to thick” hinders packing of thicker sections and can create flow problems. Internal runner to assist / improve the ability to pack the thick section when gating from “thin to thick” is necessary. Gating from “thick to thin” when possible to improve flow and allow thicker sections to be packed. Design Rules: Wall Uniformity

  20. Correctly Proportion Inner Walls • Whenever complex cores must be used, design for uniformity of section to avoid local heavy masses of metal. • The inside diameter of cylinders and bushings should exceed the wall thickness of castings. • When the I.D. is less than the wall it is better to cast the section as a solid. • Holes can be produced by cheaper and safer methods than with extremely thin cores

  21. Fillet All Sharp Angles • Fillets (rounded corners) have three functional purposes: • To reduce the stress concentration in a casting in service • To eliminate cracks, tears and draws at re-entry angles • To make corners more moldable by eliminating hot spots • The number of fillet radii in one pattern should be the minimum possible.

  22. Fillet All Sharp Angles • Large fillets may be used with radii equaling or exceeding the casting section. • Commonly used to fulfill engineering stress requirements • Reduces stress concentration • Note: Fillets that are too large are undesirable – the radius of the fillet should not exceed half the thickness of the section joined.

  23. Fillet All Sharp Angles • Tips to avoid a section size that is too large at an “L”, “V” or “Y” junction. • For an “L” junction : • Round an outside corner to match the fillet on the inside wall. (If this is not possible the designer must make a decision as to which is more important: Engineering design or possible casting defect) • For a “V” or “Y” junction: • Always design so that a generous radius eliminates localization of heat.

  24. Design Rules: Fillets & Corners

  25. Avoid Abrupt Section Changes • The difference in relative thickness of adjoining sections should not exceed a ratio of 2:1. • With a ratio less than 2:1 the change in thickness may take on the form of a fillet. • Where this is not possible consider a design with detachable parts.

  26. Avoid Abrupt Section Changes • With a ratio greater than 2:1 the recommended shift for the change in thickness should take on the form of a wedge. • Note: wedge-shaped changes in wall thickness should not taper more than 1 in 4. • Where a combination of light and heavy sections is unavoidable, use fillets and tapered sections to temper the shifts.

  27. Tapered Transition Better Wall Thickness Transitions Gradual Transition Stepped Transition Poor Design Best Core out thicker areas where possible Design Rules: Section Changes

  28. Maximize Design of Ribs & Brackets • Ribs are only preferable when the casting wall cannot be made strong or stiff enough on its own. • Ribs have two functions: • They increase stiffness • They help to reduce weight • Common mistakes that make ribs ineffective: • Too shallow • Too widely spaced

  29. Maximize Design of Ribs & Brackets • The thickness of the ribs should be approximately 80% of the adjoining thickness and should be rounded at the edge. • The design preference is for ribs to be deeper than they are thick. • Ribs should solidify before the casting section they adjoin. • The space between ribs should be designed such that localized accumulation of metal is prevented.

  30. General Guidelines for Rib Dimensions* • Component wall thickness: h • Draft per side(0): 0.5º  1.5º • Rib height (L):  5h (typically 2.53.0h) • Rib spacing (on center):  2h  3h • Base radius (R):  0.25h  0.40h • Rib thickness (t): 0.4  0.8h • *Exact rib dimensions are material specific Design Rules:Rib Dimensions

  31. Shrinkage Voids Excessive Radius Rib Sink Mark Radius (fillet) Part Wall Excessive Rib Wall Thickness Correct Proportions Design Rules:Rib Wall Thickness

  32. Maximize Design of Ribs & Brackets • Generally, ribs in compression offer a greater safety factor than ribs in tension. • Exception: Castings with thin ribs in compression may require design changes to provide necessary stiffening and avoid buckling. • Thin ribs should be avoided when joined to a heavy section or they may lead to high stresses and cracking

  33. Maximize Design of Ribs & Brackets • Avoid cross ribs or ribbing on both sides of a casting. • Cross ribbing creates hot spots and makes feeding difficult • Alternative is to design cross-coupled ribs in a staggered “T” form. • Avoid complex ribbing • Complicates molding, hinders uniform solidification and creates hot spots.

  34. Maximize Design of Ribs & Brackets • Ribs meeting at acute angles may cause molding difficulties, increase costs and aggravate the risk of casting defects. • “Honeycombing” often will provide increased strength and stiffness without creating hot spots.

  35. Design Rules: Rib Manufacturability

  36. Design Rules: Rib Design

  37. Maximize Design of Ribs & Brackets • Brackets carrying offset loads introduce bending moments-localized and in the body of the casting. • Tips to avoid this problem: • Taper “L” shaped brackets and make the length of contact with the main casting as ample as possible. • Brackets may frequently be cast separately and then attached, simplifying the molding.

  38. Maximize Design of Ribs & Brackets • A ribbed bracket will offer a stiffness advantage, but avoid heat concentration by providing cored openings in webs and ribs. • The openings should be as large as possible • The openings should be consistent with strength and stiffness • Avoid rectangular-shaped cored holes in ribs or webs. • Use oval-shaped holes with the longest dimension in the direction of the stresses

  39. Sharp corners, small radii H  T H > T core out underside Ribs inside, good distribution of metals for all purposes. May complicate die construction May complicate die construction Good distribution of stresses External ribs may cause poor distribution of stresses Generous draft and fillets, angular transitions Sharp corners, small radii, little draft Recommended Configurations

  40. Avoid Using Bosses, Lugs & Pads • Bosses and pads can have adverse effects on castings: • They increase metal thickness • They create hot spots • They can cause open grain or draws • If they must be incorporated into a design you should blend them into the casting by tapering or flattening the fillets.

  41. a c A B d b Reducing Heavy Masses & Die Simplification

  42. a B C A c d b Reducing Heavy Masses & Die Simplification

  43. A B Reducing Heavy Masses & Die Simplification

  44. Avoid Using Bosses, Lugs & Pads • The thickness of bosses and pads should be less than the thickness of the casting section they adjoin but thick enough to permit machining without touching the casting wall. • Exception: Where a casting section is light the following should be used as a guide:

  45. Avoid Using Bosses, Lugs & Pads • Bosses should not be used in casting design when the surface to support bolts may be obtained by milling or countersinking. • A continuous rib instead of a series of bosses will permit shifting hole location. • Where there are several lugs and bosses on one surface, they should be joined to facilitate machining. • A panel of uniform thickness will simplify machining • Make the walls of a boss at uniform thickness to the casting walls

  46. Poor Boss Designs: result in the potential for sink marks and voids. Sinks / Voids / Cooling stresses Improved Boss Designs Gussets reinforce free standing bosses Thick sections cored out Bosses attached to the walls using ribs Design Rules: Boss Design

  47. Design Rules: Boss Design

  48. Lecture Topics • Basic Casting Design Guidelines • Injection Molding Process • Gating Considerations • Case Study – Corvette Brake Pedal • Case Study – M1 Abrams Tank

  49. Injection Molding Process • The injection molding process is a high speed, automated process that can be used to produce plastic parts with very complex geometries. • A typical die casting machine is shown in the next slide. Due to the combined effects of flow through both the machine and the mold, large pressure drops associated with mold filling can occur.

  50. Injection Molding Process