Design for Cast and Molded Parts

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

Lecture updates Revisions & Edits provided by: Seamus Clark, Scott Leonardi, Gary Meyers, Joe Torres, Beatriz Dhruna, John Fraser, Craig Jozsa, Dwayne Mattison, Darcy McClure, Kevin O’Callaghan, Norm Opolsky, Henry Gasahl, Rolf Glaser * of 69

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

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 * 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 * of 69

Die Casting vs. Sand Casting
Pros and Cons of using Die-Casting Pros Thinner walls Closer Dimensional Limits Reduced labor in setup Smoother surfaces with a potential for less stress risers Lower finishing costs Good solution for high volume applications that have stable designs and design life. Cons High initial die investment Limited # of casts (<100,000 typically) Limit to size of cast part (approx. 70 lbs for aluminum) High temperature casts can reduce life of dies Cannot be used on hollow parts such as exhaust manifolds Minor casting updates difficult and costly. Die casting prevents the usage of stronger alloys tolerated by sand-casting The designer must decide what casting best suits their specific application as one type is NOT necessarily better than the other!

Simplification of Die Configuration
Die Opening Direction Cavity Part Core 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. Die Opening Direction * of 69 *

Add a chamfer to the part Die Opening Direction Cavity Core 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. Die Opening Direction * of 69 *

Ejector pin pushes the part out of the core as the lifter slides away from the snap undercut. Support plate is stationary. It pushes lifter to the left as ejector plate moves upward. Lifter moves away from undercut portion of snap A lifter is used to assist in the ejection of a cantilever snap beam * of 69 *

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. * of 69 *

Design for Soundness Most metals, alloys, and plastics 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 The mold and pattern should be made larger than the casting by the amount of shrinkage Shrinkage of casting varies not only with material but also with shape, thickness, casting temperature, mold temperature and mold strength Thicker areas will cool slower than thinner areas. Areas of transition between thick and thin (ribs, walls, embosses, etc) will be prone to sink marks. Different tool shops and different materials will require a certain rib-to-wall thickness ratio. * of 69

Design for Soundness The table below shows an average amount of shrinkage for important cast metals * *

Design Rules: Disguising Sink Marks
A variety of methods can be used to disguise the sink marks that occur opposite ribs and bosses. * of 69 *

Avoid Sharp Angles & Corners
When two or more sections conjoin, mechanical weakness is induced at the junction interrupting free cooling (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 a more uniform distribution of strength * 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 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. * 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 * of 69

Employ Uniform Sections
Thicker walls will solidify more slowly. This means they will feed solidifying thinner 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. * 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) 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. * 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. * 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. 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. * 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 * 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 Improves flow of material The number of different size fillet radii used in a pattern should be the minimized Jozsa - added improved flow to functional purposes *

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. * of 69

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. * of 69

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 * 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. * 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. * 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 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. * 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 * of 69

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. Preferably the ribs connect the attachment to the loading point. * 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 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. * of 69 *

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 * of 69 *

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 * of 69

Avoid cross ribs & 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 * of 69

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 * of 69

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. * of 69 *

Design Rules: Rib Design
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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. * of 69

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 * of 69

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. * 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. * of 69

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. * of 69 *

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. * of 69 *

B Design B, at right, allows the component to be die cast without movable cores or core slide. * of 69 *

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” * of 69

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 * 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: Bosses attached to the walls using ribs Thick sections cored out Gussets reinforce free standing bosses 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. * of 69 *

Design Rules: Boss Design
Thick wall sections at the base of reinforcing ribs can result in sinks or shrinkage voids. * of 69 *

Injection Molding Process
The injection molding process is the most commonly used plastic molding process. It is a high speed, automated process for producing parts from both thermoplastic and thermosetting plastic materials, and is used to create a large variety of products with complex shapes and sizes. Advantages: high production rates (multi-cavity tooling, low cycle times) repeatable high tolerances low labor cost minimal scrap losses little need to finish parts after molding Dhruna edit * of 69 *

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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. * of 69 *

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 * of 69 *

Video Clip of Injection Molding Process

Rev * of 69

Gas-Assisted Injection Molding Process
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Common Defects with Injection Molding
Knit lines - Caused by the melt-front flowing around an object standing proud in a plastic part as well as at the end of fill where the melt-front comes together again. Can be minimized or eliminated with a mold-flow study when the mold is in design phase. Sink marks – Excessive material thickness (in local area) or rib thickness too large for wall thickness. Flash - Parting line on the tool is damaged, too much injection speed/material injected, clamping force too low. Parting lines should be placed in non-visible locations, especially in class-A surfaces. Dealing with these: Die draw, parting lines, boss locations, can all be assessed while the interior surfaces are still in the clay design phase. Suggestion is to add a case study around an IP or Console clay in the studio development phase, and how it can be used to assess injection molding capabilities. Dhruna add * of 69 *

Impact of Design Guidelines on Molding Process
Steel / Product Conditions Risk if violated Minimum Wall Thickness 2.5mm Increase Cavity Pressures Short Shots Wall stock designed with variation of more than +/- 15% from normal wall Product stress caused by uneven pack and cooling Inconsistent gloss levels due to “hot spots” 40% of normal wall to be maximum rib thickness at wall stock intersection Sink marks Gloss variation 1 degree per side minimum draft on ribs Sinking in mold Drag marks Distortion of “A” surface Ribs deeper than 17mm without ability to insert and / or vent mold at bottom of ribs Gas traps and /or short shot Gas Burns Steel erosion at bottom of rib due to “Off Gassing” Minimum Rib thickness of 1mm at base of rib Short shots due to inability to fill thin sections Increase cavities pressures Over packing of part causing inaccurate shrink Injection Pressures in excess of 12,000 psi, as shown on mold flow (Mold flow pressures are typically 15% / 25% lower than actual machine pressures in production) Excess stress to cavity or core steel possible causing metal fatigue leading to failure Inconsistent cavity fill and pack on older equipment Rev

Impact of Design Guidelines on Molding Process
Steel / Product Conditions Risk if violated Hot drop quantity and / or positioning that limit mold cooling Internal stress in plastic Gloss variations do to lack of cooling Increase cycle time Standing “Fingers of Steel” that do not maintain a 1:1 height to base ratio (i.e. 10mm high x 10mm square or Dia. Base) Possible failure of steel do to fatigue caused by molding pressure or shutoff loads Minimum draft of 1.5 degrees for an un-textured surfaces and 1 degree / .001” (.025mm) of Texture Depth for grained surfaces Drag marks Damage surface Grain scuffing Class “A” surface parting lines should be designed with deliberate offset to facilitate matching and graining As a little as .001” mismatch of steel is visible on class “A” surface Texture Mask (Shut-Off protection) is visible Split parting lines on Class “A” surface should be designed into a flat surface, if the parting line is designed to a tangent of a radius the surface will deteriorate rapidly Parting line deterioration “Frog Hair” Flash Heavy Surface Flash Deep “V” product designs that create “notch” conditions in injection mold (a deep “V” is defined as anything less than 60 degrees included angle and with a depth of more than twice the width Cracked or broken molds Catastrophic mold failure with loss of production Tolerances that are tighter than are repeatable in normal mold cycles. General rule of thumb is +/- 0.5mm first 200mm than +/ mm per mm of length or width after Molded product that may not meet capability requirements Rev

Injection Mold Design Considerations
Min draft on all faces → 0.5° (without grain) Draft as low as 0.3° for deep features (without grain) Gate into thick regions of the part Maintain acceptable rib-to-wall ratio (to avoid sink) Use action in tooling to avoid undercuts Consider draft angle requirements for intended grain (if parts are to be grained)

Rib Design Guidelines - Injection Molding
Rib thickness at base should be no more than 40-50% of wall thickness to avoid sink marks Rib height should be set to achieve 1mm MIN thickness at peak (avoid “razor edge”)

Tooling Action - Injection Molding
Red hatched lines represent undercut area when using the die draw direction specified. Adding a slide to form this feature will alleviate the undercut issue. Example: ”Witness lines” can be seen on the part in the area where a slide was used to form the clip tower Action (i.e. slides/lifters) should be used to avoid undercut

Part Graining - Injection Molding
Molded parts can be grained to improve appearance and reduce gloss level. Each grain type has draft requirements for grain depth (i.e. 60%, 80%, 100%, etc.). 100% depth requires maximum draft angle for all grained surfaces. General requirement = 1° draft / 1 micrometer of grain depth

Advanced Injection Molding Techniques
Pulsed Cooling Traditional techniques of cooling have circulated fluid through the mold at a constant rate, and the local surface temp of the mold was therefore somewhat uncontrolled. Advanced pulse cooling controls the temperature of the mold on a localized basis by opening and closing valves which controls the mass flow of the fluid and therefore the rate of heat transfer out of the mold. Traditional Cooling Pulsed Cooling

Induction Heating Traditionally, the temperature of the mold during the filling cycle was a function of the temperature of the resin, the facility, the temperature and flow of the coolant, and length of time since the last shot. With the use of induction heating, the mold temperature can be raised above the temperature of the resin before and during the filling phase. This can have many benefits including: lower injection pressure and clamping requirements improved flow lengths reduction of internal part stresses reduction or elimination of weld lines, Reduction of jetting, silver streaks, or sink marks. Simple Induction heating of rod

Live-Feed Injection Molding The live-feed injection molding process applies oscillating pressure at multiple polymer entrances to cause the melt to oscillate The action of the pistons keeps the material in the gates molten while different layers of molecular or fiber orientation are being built up in the mold due to solidification This process provides a means of making simple or complex parts that are free from voids, cracks, sink marks, and weld-line defects

Multi-Material Molding – There are several types Once Step Process Co-Injection Sandwich molding Multi-Step Process Overmolding Two-shot molding

Multi-Material Molding – Sandwich Sandwich molding is typically used to mold multi-layered plastic packaging articles having a laminated wall structure. Each layer is typically passed through a different annular or circular passageway in a single nozzle structure and each layer is partially, sequentially, injected through the same gate. Ref US Pat #7,559,756

Multi-Material Molding – Co-Injection Co-Injection molding is similar to Sandwich molding, however in co- injection the materials may not come out in layers as in Sandwich. The same basic manufacturing principle applies – Muli-Materials are simultaneously input into a single mold, and combined together to form one part Multiple Sprues & Materials

Multi-Material Molding – Over-Molding Overmolding (or two-shot molding) is for parts where more than one material is needed (i.e. plastic housing molded around metal plate) In these processes, only part of a product is molded in one material, and that molded piece is manipulated so the second material can be molded around, over, under, or through it to complete the final part. Overmolding is differentiated from two-shot in the fact that a part that is overmolded may be done in a completely separate operation, or may be made of a material other than plastic, so it is not a two shot process, but still an overmold. Overmolded Knife – Polymer Handle

Multi-Material Molding – Two Shot Two shot molding is very similar to overmolding, only the molding is done within a single manufacturing process. There are several processes of two shot molding, shown below is an indexing process. Shot one is completed in cavity A, then the parts are rotated, part A is transferred into cavity B. After Shot B is complete, the final part is ejected.

Multi-Material Molding – Combined Processes In this example, Overmolding and Two Shot molding are combined Polymer C is overmolded to the Metal Ring Polymer B is overmolded to the Metal Ring / Polymer C Polymer A is then overmolded to complete the assembly Polymer A Polymer B Polymer C Metal Ring

Multi-Material Molding – Combining processes In the diagram below, this shows how an overmolded (2 shot) part and a sandwich molded part are brought together via a hot plate weld. Material C Material B Material B Hot Plate Weld Joint Material A Material B US Patent # - 7,559,756

Extrusion Blow Molding Image Source: Typical applications of blow molding: -Bottles -Automotive rear seat back panels (sedans) -Fuel tanks -HVAC ducts

DISAMATIC Casting Process
Mold parting is vertical. All metal in gates, risers and casting cavities are contained within the flaskless sand molds. There are casting size and weight limitations due to the hydrostatic pressure built up within the mold. There is reduced flexibility for gating and risers. *

Sand Bin Cavity Swing Pattern RAM Pattern Core Pouring cup Mold Pattern Squeeze Piston Sand Mold Mold Chamber *

Sand Blowing Sand Shot *

Mold Squeeze *

Supply sand Swing Pattern drawing and swinging *

Swing Pattern drawing and swinging *

RAM pattern drawing *

RAM pattern closing *

Core Mask Core Core Setting *

Core Setting *

Cycle time : seconds/mold *

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. * *

Spruce Puller (and cold slug well)
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 * 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 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. * of 69 *

Standard Configuration Alternatives to Reverse Injection
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. * of 69 *

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. * of 69 *

Sections remain unfilled
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 * of 69 *

Three gates and flow leaders Spruce gated box shaped molding
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 ΔP) 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. * of 69 *

Gating system design is crucial in controlling the rate and turbulence in the molten metal being poured, the flow of liquid metal through the casting's system, and the temperature gradient within the metal casting. good gating system will create directional solidification throughout the casting, since the flow of molten material and temperature gradient will determine how the casting solidifies. Source:

Superheat (The difference between the solidification temperature and the pouring temperature of the metal is called the superheat.) increases fluidity of the material for the casting increases gas porosity, increased oxide formation, and mold penetration.

Insulate Risers Since the riser is the reservoir of molten material for the casting it should be last to solidify. Insulating the top will greatly reduce cooling in the risers from the steep temperature gradient between the liquid metal of the casting, and the room temperature air.

Sections of the Casting The flow of material is very important to the manufacturing process. Do not feed a heavy section through a lighter one.

Connection Between Riser and Casting Must Stay Open If the passage linking the riser to the casting solidifies before the casting, the flow of molten metal to the casting will be blocked and the riser will cease to serve its function.

Die Casting vs Other Processes
Die casting vs. plastic molding - Die casting produces stronger parts with closer tolerances that have greater stability and durability. Die cast parts have greater resistance to temperature extremes and superior electrical properties. Die casting vs. sand casting - Die casting produces parts with thinner walls, closer dimensional limits and smoother surfaces. Production is faster and labor costs per casting as well as finishing costs are lower. Die casting vs forging - Die casting produces more complex shapes with closer tolerances, thinner walls and lower finishing costs. Cast coring holes are not available with forging. Die casting vs. stamping - Die casting produces complex shapes with variations possible in section thickness. One casting may replace several stampings, resulting in reduced assembly time. Source:

A Design Study in Aluminum Casting
The Brake Pedal for the Chevrolet Corvette Casting\Corvette Case Study.pdf Need to post/add this case study * of 69

A Design Study in Steel Casting
The Ice Cleat for the M1 Abrams Tank Casting\ice_cleat M1 Abrams.pdf Need to post/add this case study * 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” * of 69

Design for Cast and Molded Parts

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