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Rev. 11-20011 of 69 Design for Cast and Molded Parts Team: Terese Bertcher Larry Brod Pam Lee Mike Wehr.

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Presentation on theme: "Rev. 11-20011 of 69 Design for Cast and Molded Parts Team: Terese Bertcher Larry Brod Pam Lee Mike Wehr."— Presentation transcript:

1 Rev of 69 Design for Cast and Molded Parts Team: Terese Bertcher Larry Brod Pam Lee Mike Wehr

2 Rev of 69 Design for Cast and Molded Parts Revision Team: Seamus Clark Scott Leonardi Gary Meyers

3 Rev of 69 Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank

4 Rev of 69 Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank

5 Rev of 69 Basic Casting Design Guidelines 1.Visualize the Casting 2.Design for Soundness 3.Avoid Sharp Angles & Corners 4.Minimize the Number of Sections 5.Employ Uniform Sections 6.Correctly Proportion Inner Walls 7.Fillet All Sharp Angles 8.Avoid Abrupt Section Changes 9.Maximize Design of Ribs & Brackets 10.Avoid Using Bosses, Lugs & Pads

6 Rev of 69 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 Rev of 69 Simplification of Die Configuration

8 Rev of 69 Simplification of Die Configuration

9 Rev of 69 Simplification of Die Configuration

10 Rev of 69 Simplification of Die Configuration

11 Rev of 69 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 Rev of 69 Design Rules: Disguising Sink Marks

13 Rev of 69 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 Rev of 69 Design Rules:Corners & Radii Good Corner Design Incorrect Corner Design Generous radius Uniform wall thickness Smooth flow transition Very sharp radii High stress concentration Sharp flow transition Inside / outside radius mismatch Non-uniform wall thickness Non-uniform flow transition Outside corner and inside radius Non-uniform wall thickness Non-uniform flow transition Shrinkage stress / voids / sinks Sink

15 Rev of 69 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 Rev of 69 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 Rev of 69 Design Rules: Wall Uniformity Original Part Design Very thick wall sections Non-uniform wall thickness Sharp inside and outside radii Improved Part Design Thinner wall sections More uniform wall thickness Inside and outside radii (when possible)

18 Rev of 69 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 Rev of 69 Design Rules: Wall Uniformity Part gated from “thin to thick” hinders packing of thicker sections and can create flow problems. Gating from “thick to thin” when possible to improve flow and allow thicker sections to be packed. Internal runner to assist / improve the ability to pack the thick section when gating from “thin to thick” is necessary.

20 Rev of 69 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 Rev of 69 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 Rev of 69 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 Rev of 69 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 Rev of 69 Design Rules: Fillets & Corners

25 Rev of 69 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 Rev of 69 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 Rev of 69 Design Rules: Section Changes Wall Thickness Transitions Tapered Transition Gradual Transition Stepped Transition Core out thicker areas where possible Poor Design Better Best

28 Rev of 69 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 Rev of 69 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 Rev of 69 Design Rules:Rib Dimensions 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

31 Rev of 69 Design Rules:Rib Wall Thickness Correct Proportions Radius (fillet) Sink Mark Shrinkage Voids Excessive Radius Part Wall Rib Excessive Rib Wall Thickness

32 Rev of 69 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 Rev of 69 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 Rev of 69 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 Rev of 69 Design Rules: Rib Manufacturability

36 Rev of 69 Design Rules: Rib Design

37 Rev of 69 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 Rev of 69 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 Rev of 69 Recommended Configurations May complicate die construction External ribs may cause poor distribution of stresses May complicate die construction Sharp corners, small radii H  T H > T core out underside Good distribution of stresses Sharp corners, small radii, little draft Generous draft and fillets, angular transitions Ribs inside, good distribution of metals for all purposes.

40 Rev of 69 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 Rev of 69 Reducing Heavy Masses & Die Simplification A a c B b d

42 Rev of 69 Reducing Heavy Masses & Die Simplification A BC a dc b

43 Rev of 69 Reducing Heavy Masses & Die Simplification AB

44 Rev of 69 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: Casting Length:< 1.5’Min. Boss Height:.25” 1.5’< X < 6’.75” > 6’1.00”

45 Rev of 69 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 Rev of 69 Design Rules: Boss Design 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

47 Rev of 69 Design Rules: Boss Design

48 Rev of 69 Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank

49 Rev of 69 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. Injection Molding Process

50 Rev of 69 Injection Molding Process

51 Rev of 69 Injection Molding Process

52 Rev of 69 Injection Molding Process Conventional Injection Molding Gas Assisted Injection Molding Sink Gas Channels

53 Rev of 69 Video Clip of Injection Molding Process

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58 Rev of 69 Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank

59 Rev of 69 Gating Location and Constraint Considerations Spoke Gating (2 spokes)Diaphragm or disk gateSpoke Gating (4 spokes)

60 Rev of 69 Gating Considerations Cavity Spruce Puller (and cold slug well) Gate Core Runner Part Spruce

61 Rev of 69 Gating Considerations Two plate single cavity mold Three plate mold configuration (multi cavity) Single parting line Primary spruce Pin Gate Parting Line 1 Parting Line 2 Secondary Spruce Spruce Gate

62 Rev of 69 Gating Considerations Reverse Injection Cavity (stationary half) Core (moving half) Standard Configuration Alternatives to Reverse Injection Tunnel gating through knockout pin Cavity (stationary half) Core (moving half) Logo..placed At gate location

63 Rev of 69 Gating Considerations Single top center gate Hot manifold for a stack mold Cold edge gate fed by hot manifold Direct lateral gating of several cavities Center gating of several cavities Cold edge gating of several cavities fed by hit manifold Multiple top gating of single cavity

64 Rev of 69 Gating Considerations Two Gates Improved filling pattern and pressure distribution Formation of one weld line Three Gates Filling pattern and pressure distribution are better Formation of two weld lines Fill is complete Sections remain unfilled Weld

65 Rev of 69 Gating Considerations Three gates and flow leaders Most uniform filling pattern and pressure distribution Requires wall thickness variation or diagonal ribs Spruce gated box shaped molding Uniform wall thickness Corners: last to fill Flow leaders / internal runners Local increases in wall thickness promote flow, uniform pressure drop extend from gate to corners of part Filling pattern without flow leaders (uniform wall thickness) Max Flow length (highest  P) Overpacking and changes flow direction Improved filling pattern with flow leaders (non-uniform wall thickness) Sides fill early 

66 Rev of 69 Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank

67 Rev of 69 A Design Study in Aluminum Casting The Brake Pedal for the Chevrolet Corvette Casting\Corvette Case Study.pdf

68 Rev of 69 Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank

69 Rev of 69 A Design Study in Steel Casting The Ice Cleat for the M1 Abrams Tank Casting\ice_cleat M1 Abrams.pdf

70 Rev of 69 References The case studies were obtained from the Engineered Casting Solutions website. –URL: Modern Casting, May 2001 v91 i5 p50., “Basics of Gray Iron Casting Design: 10 Rules for Engineered Quality”


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