Design for Cast and Molded Parts Team: Terese Bertcher Larry Brod Pam Lee Mike Wehr Rev. 11-2001

Design for Cast and Molded Parts Revision Team: Seamus Clark Scott Leonardi Gary Meyers Rev. 11-2001

Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank Rev. 11-2001

Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank Rev. 11-2001

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 Rev. 11-2001

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 Rev. 11-2001

Simplification of Die Configuration Snap beams with inward facing 90° return angles cannot be ejected using a conventional ejector system due to the undercut associated with the hook. Special mold actions are required. Rev. 11-2001

Simplification of Die Configuration Certain snap fit geometries can be ejected by simply creating an angled interface and stripping the part form the core. This is most appropriate for parts produced with flexible, ductile polymers, where tolerance demands are relatively low. Rev. 11-2001

Simplification of Die Configuration A lifter is used to assist in the ejection of a cantilever snap beam Rev. 11-2001

Simplification of Die Configuration Ribs, steps, bosses or other design features cannot be placed in the immediate area of the snap beam as they can interfere with lifter movement. Rev. 11-2001

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 Rev. 11-2001

Design Rules: Disguising Sink Marks A variety of methods can be used to disguise the sink marks that occur opposite ribs and bosses. Rev. 11-2001

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 Rev. 11-2001

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 Shrinkage stress / voids / sinks Sink Ideally corners should have both internal and external radii. The uniform wall thickness can be maintained if the external radius is equal to the internal radius plus the wall thickness. Thick wall sections at the corners will lead to sink marks, voids, shrinkage stresses, and the potential for flow hesitation defects. Rev. 11-2001

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 Rev. 11-2001

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. Rev. 11-2001

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) Thick wall sections should be cored out to reduce the nominal wall thickness. Results in cycle time reduction along with an overall quality improvement. Again, ribs are used to stiffen the part where necessary. Rev. 11-2001

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. Rev. 11-2001

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. Parts with variable wall thicknesses should be gated at the thicker sections whenever possible to reduce the potential for sink marks or voids. If the part must be gated from “thin to thick”, internal runners (possible ribs) can be added to facilitate packing of the thicker sections. Rev. 11-2001

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 Rev. 11-2001

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. Rev. 11-2001

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. Rev. 11-2001

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. Rev. 11-2001

Design Rules: Fillets & Corners Fillets, which connect interior surfaces of a component, are molded at the juncture of external die surfaces. Properly sized radii enhance structural integrity by reducing stress concentration, and promote good metal flow with reduced erosion in the die Corners, generated by the junction of exterior surfaces of a component are molded at the juncture of internal die surfaces. Properly sized radii relieve stress concentrations in the die and ensure that molten metal fills corners properly Rev. 11-2001

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. Rev. 11-2001

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. Rev. 11-2001

Design Rules: Section Changes Wall Thickness Transitions Tapered Transition Gradual Transition Stepped Transition Core out thicker areas where possible Poor Design Better Best Parts having a variable wall thickness should be avoided whenever possible due to the shrinkage stresses that build up in the transition zones. When variable walls are necessary, tapered transitions are recommended. Rev. 11-2001

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 Rev. 11-2001

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. Rev. 11-2001

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 The specific rib design configurations that are feasible to mold are contingent on the properties of the individual material grades. Material processing variables such as melt viscosity and mold shrinkage can have large impact on the design. Rev. 11-2001

Design Rules:Rib Wall Thickness Correct Proportions Radius (fillet) Sink Mark Shrinkage Voids Excessive Radius Part Wall Rib Excessive Rib Wall Thickness Ribs with excessive wall thicknesses or fillet radii can result in the formation of shrinkage voids or sink marks due to the local increase in wall thickness Rev. 11-2001

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 Rev. 11-2001

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. Rev. 11-2001

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. Rev. 11-2001

Design Rules: Rib Manufacturability Rectangular ribs must be modified to improve manufacturability. Draft angles must be added, and generous radii to prevent shrinkage defects, and to ease part ejection from the die. Rev. 11-2001

Design Rules: Rib Design Rev. 11-2001

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. Rev. 11-2001

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 Rev. 11-2001

Recommended Configurations May complicate die construction External ribs may cause poor distribution of stresses 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. Rev. 11-2001

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. Rev. 11-2001

Reducing Heavy Masses & Die Simplification B b d The heavy mass of metal trapped in the design of the boss at A may be reduced by using a metal-saver core as shown at B. The direction of die pull may be either in the direction a-b or c-d. Rev. 11-2001

Reducing Heavy Masses & Die Simplification B C a d c b Boss A is undesirable because of the heavy mass of metal in the walls. Boss B is cast with the parting plane at a-b, and the interior is formed with movable cores, thinning the walls. Boss C, formed with the parting plane at c-d, achieves thin walls without requiring core slides. Rev. 11-2001

Reducing Heavy Masses & Die Simplification B Design B, at right, allows the component to be die cast without movable cores or core slides. Rev. 11-2001

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” Rev. 11-2001

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 Rev. 11-2001

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 Whenever possible, bosses should be free standing, gussetted, or attached to sidewalls using ribs in order to minimize the potential for sink marks and shrinkage voids. Rev. 11-2001

Design Rules: Boss Design Thick wall sections at the base of reinforcing ribs can result in sinks or shrinkage voids. Rev. 11-2001

Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank Rev. 11-2001

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. Rev. 11-2001

Injection Molding Process Rev. 11-2001

Injection Molding Process The large pressure drops can be reduced by providing a fixed short volume of melt, and then applying an inert gas to the central core of the melt to hasten complete filling of the mold. This process is called “Gas Assisted Molding”, and is used for large or complex multi-cavity mold configurations. Rev. 11-2001

Injection Molding Process Conventional Injection Molding Sink Gas Assisted Injection Molding Gas Assisted Injection Molding Gas channel layouts are perhaps the most critical design decision associated with gas assisted molding. Properly positioned, they allow the final part to avoid many of the defects associated with conventional molding. Channels are typically oriented in the general direction of flow and positioned near the last area of fill. Gas Channels Rev. 11-2001

Video Clip of Injection Molding Process Rev. 11-2001

Rev. 11-2001

Rev. 11-2001

Rev. 11-2001

Rev. 11-2001

Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank Rev. 11-2001

Gating Location and Constraint Considerations Spoke Gating (2 spokes) Diaphragm or disk gate Spoke Gating (4 spokes) Gating schemes can have a large impact on the dimensions of the plastic part. The rim-like part shown below is to be produced in a single-cavity mold. Options for gating are internal spoke, and disc gating. The disc gate ensures uniform filling and packing without weld line formation. Rev. 11-2001

Gating Considerations Cavity Spruce Puller (and cold slug well) Gate Core Runner Part Spruce Typical cold runner system with two-plate multi-cavity mold configuration Rev. 11-2001

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 Sprue gates which gate into the top of a single cavity mold, while three plate molds with two parting lines, provide a means of top gating multi-cavity molds. Rev. 11-2001

Gating Considerations Reverse Injection Cavity (stationary half) Core (moving half) Standard Configuration Alternatives to Reverse Injection Tunnel gating through knockout pin Logo..placed At gate location Options for relocating or disguising objectionable gate vestiges. Rev. 11-2001

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 Hot runner systems can be viewed as extensions of the molding machine nozzle into the tool. When hot runner systems are used, the frozen layer effects associated with cold runner fed systems can be eliminated. Hot runner gates are available in a variety of configurations and shapes including the most common open circular and annular geometries. Rev. 11-2001

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 Increasing the number of gates improves the flow pattern and pressure distribution, however, it also creates weld lines. In order to achieve a balanced fill, the pressure drop with each flow path from the gate must be equal Rev. 11-2001

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  Local increases in wall thickness are known as flow leaders or internal runners. These can be used to alter the mold filling pattern in order to achieve more balanced mold filling. A more uniform filling pattern can be achieved when three gates and diagonal flow leaders are used. Rev. 11-2001

Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank Rev. 11-2001

A Design Study in Aluminum Casting The Brake Pedal for the Chevrolet Corvette Casting\Corvette Case Study.pdf Rev. 11-2001

Lecture Topics Basic Casting Design Guidelines Injection Molding Process Gating Considerations Case Study – Corvette Brake Pedal Case Study – M1 Abrams Tank Rev. 11-2001

A Design Study in Steel Casting The Ice Cleat for the M1 Abrams Tank Casting\ice_cleat M1 Abrams.pdf Rev. 11-2001

References The case studies were obtained from the Engineered Casting Solutions website. URL: http://www.castsolutions.com/ Modern Casting, May 2001 v91 i5 p50., “Basics of Gray Iron Casting Design: 10 Rules for Engineered Quality” Rev. 11-2001