Casting DefectsMetal casters try to produce perfect castings.

A few castings, however, are completely free of defects.

Modern foundries have sophisticated inspection equipment which can detect small differences in size and a wide variety of external and even internal defects. For example, slight shrinkage on the back of a decorative wall plaque is acceptable whereas similar shrinkage on a position cannot be tolerated.

No matter what the intended use, however, the goal of modern foundries is zero defects in all castings

Scrap castings cause much concern.

In industry, scrap results in smaller profits for the company and ultimately affects individual wages.

Scrap meetings are held daily. Managers of all the major departments attend these meetings. They gather castings that have been identified as scrap by inspector. The defect is circled with chalk. An effort is made to analyze the cause of the defect, and the manager whose department was responsible for it is directed to take corrective action to eliminate that specific defect in future castings.

There are so many variables in the production of a metal casting that the cause is often a combination of several factors rather than a single one.

All pertinent data related to the production of the casting (sand and core properties, pouring temperature) must be known in order to identify the defect correctly.

After the defect is identified attempt should be to eliminate the defect by taking appropriate corrective action.

CASTING DEFECTS

SURFACE METALLIC PROJECTION Swell, Crush, Mould Drop, Fillet Vein DEFECTIVE SURFACE Erosion Scab, Fusion, Expansion Scab, Rat tails, Buckle, Seams, Gas Runs, Fillet Scab, Rough Surface, Slag Inclusion, Elephant Skin CHANGE IN DIMENSION- Warped castingINCOMPLETE CASTING- Misrun, Run outCAVITY- Blow Holes, Shrinkage cavity, PinholesDISCONTINUITY-Hot Cracking, Cold Shut, Cold CrackingSUBSURFACESUBSURFACE CAVITY- Blow Holes, Pin Holes, ShrinkagePorosity, Internal Shrinkage, SevereRoughness

INCLUSIONS- Gas Inclusions, Slag, Blow Holes

DISCONTINUITY- Cold ShutsNITC

Repairability

FINS OR FLASH ON CASTINGS -AsMetallic Projections

Joint flash or fins. Flat projection of irregular thickness, often with lacy edges, perpendicular to one of the faces of the casting. It occurs along the joint or parting line of the mold, at a core print, or wherever two elements of the mold intersect. Possible CausesClearance between two elements of the mold or between mold and core; Poorly fit mold joint. RemediesCare in pattern making, molding and core making; Control of their dimensions; Care in core setting and mold assembly; Sealing of joints where possible.

Flask was disturbed while investment was setting. Base was removed too soon. Flask was allowed to partially dry before dewaxing. Incorrect dewaxing or a furnace malfunction. Flask burned out and allowed to cool below (260oC) before casting reheating, flask allowed to cool between dewax and placement in preheated oven. Flask was improperly handled or dropped. Speed was set too high on centrifugal casting machine. Patterns were placed on one plane. The should be staggered on top rack. Incorrect water powder ratio was used. Not enough investment was placed over the patterns. Flask was placed too close to heat source in burnout oven. Flasks were not held at low burnout temperature long enough.

DEFECTS IN CASTINGS- CAN BE ELIMINATED/MINIMISED BY PROPER DESIGN, MOLD PREPARATION, PROPER POURING.NITC

DEFECTS IN CASTINGS- AS HOT TEARS - DUE TO CONSTRAINTS IN LOCATIONS, CASTINGS CANNOT SHRINK FREELYNITC

Cavities

Blowholes, pinholes. Smooth-walled cavities, essentially spherical, often not contacting the external casting surface (blowholes). The largest cavities are most often isolated; the smallest (pinholes) appear in groups of varying dimensions. The interior walls of blowholes and pinholes can be shiny, more or less oxidized or, in the case of cast iron, can be covered with a thin layer of graphite. The defect can appear in all regions of the casting.

Possible Causes

Because of gas entrapped in the metal during the course of solidification: Excessive gas content in metal bath (charge materials, melting method, atmosphere, etc.); Dissolved gases are released during solidification.

In steel and cast irons: formation of carbon monoxide by the reaction of carbon and oxygen, presents as a gas or in oxide form. Blowholes from carbon monoxide may increase in size by diffusion of hydrogen or, less often, nitrogen.

Excessive moisture in molds or cores.Core binders which liberate large amounts of gas.Excessive amounts of additives containing hydrocarbons.Blacking and washes which tend to liberate too much gas. Insufficient evacuation of air and gas from the mold cavity; -insufficient mold and core permeability. Entrainment of air due to turbulence in the runner system.

RemediesMake adequate provision for evacuation of air and gas from the mold cavityIncrease permeability of mold and cores Avoid improper gating systemsAssure adequate baking of dry sand moldsControl moisture levels in green sand molding

Reduce amounts of binders and additives used or change to other types; -use blackings and washes, which provide a reducing atmosphere; -keep the spree filled and reduce pouring height

Increase static pressure by enlarging runner height.

Discontinuities

Hot cracking. A crack often scarcely visible because the casting in general has not separated into fragments. The fracture surfaces may be discolored because of oxidation. The design of the casting is such that the crack would not be expected to result from constraints during cooling.

Possible CausesDamage to the casting while hot due to rough handling or excessive temperature at shakeout.

RemediesCare in shakeout and in handling the casting while it is still hot; Sufficient cooling of the casting in the mold; For metallic molds; delay knockout, assure mold alignment, use ejector pins

Defective Surface

Flow marks. On the surfaces of otherwise sound castings, the defect appears as lines which trace the flow of the streams of liquid metal. Possible CausesOxide films which lodge at the surface, partially marking the paths of metal flow through the mold.

RemediesIncrease mold temperature; Lower the pouring temperature; Modify gate size and location (for permanent molding by gravity or low pressure); Tilt the mold during pouring; In die casting: vapor blast or sand blast mold surfaces which are perpendicular, or nearly perpendicular, to the mold parting line.

Incomplete Casting

Poured short. The upper portion of the casting is missing. The edges adjacent to the missing section are slightly rounded, all other contours conform to the pattern. The spree, risers and lateral vents are filled only to the same height above the parting line, as is the casting (contrary to what is observed in the case of defect).

Possible CausesInsufficient quantity of liquid metal in the ladle; Premature interruption of pouring due to workmans error.

RemediesHave sufficient metal in the ladle to fill the mold; Check the gating system; Instruct pouring crew and supervise pouring practice.

Incorrect Dimensions or ShapeDistorted casting. Inadequate thickness, extending over large areas of the cope or drag surfaces at the time the mold is rammed.

Possible CausesRigidity of the pattern or pattern plate is not sufficient to withstand the ramming pressure applied to the sand. The result is an elastic deformation of the pattern and a corresponding, permanent deformation of the mold cavity. In diagnosing the condition, the compare the surfaces of the pattern with those of the mold itself.

RemedyAssure adequate rigidity of patterns and pattern plates, especially when squeeze pressures are being increased.

Inclusions or Structural Anomalies

Metallic Inclusions. Metallic or intermetallic inclusions of various sizes which are distinctly different in structure and color from the base material, and most especially different in properties. These defects most often appear after machining.

Possible CausesCombinations formed as intermetallics between the melt and metallic impurities (foreign impurities); Charge materials or alloy additions which have not completely dissolved in the melt; Exposed core wires or rods; During solidification, insoluble intermetallic compounds form and segregate, concentrating in the residual liquid.

RemediesAssure that charge materials are clean; eliminate foreign metals; Use small pieces of alloying material and master alloys in making up the charge; Be sure that the bath is hot enough when making the additions; Do not make addition too near to the time of pouring; For nonferrous alloys, protect cast iron crucibles with a suitable wash coating

INCLUSIONS (FOREIGN PARTICLES) IN CASTINGS

Patterns were improperly sprued to wax base or tree or not filleted, causing investment to break at sharp corners during casting. Flask was not sufficiently cured before placing into burnout oven. Improper dewaxing cycle was used. Flask was not cleaned from prior cast. Loose investment in sprue hole. Molten metal contains excess flux or foreign oxides. Crucible disintegrating or poorly fluxed. Improperly dried graphite crucible. Investment was not mixed properly or long enough. Contaminants in wax pattern. Flask was not held at low burnout temperature long enough. Flask was placed too close to heat source in burnout oven.

POROSITYPattern is improperly sprued. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage porosity. Not enough metal reservoir to eliminate shrinkage porosity. Metal contains gas. Mold is too hot. Too much moisture in the flux. Too much remelt being used. Always use at least 50% new metal. Metal is overheated. Poor mold burnout.

ROUGH CASTINGSA poor quality pattern Flask was not sufficiently cured before placing into burnout oven. Flask was held in steam dewax too long. Metal, flask or both were too hot. Patterns were improperly sprued. Flask was placed too close to heat source in burnout oven.

BUBBLES OR NODULES ON CASTINGSVacuum pump is leaking air. Vacuum pump has water in the oil. Vacuum pump is low on oil. Investment not mixed properly or long enough. Invested flasks were not vibrated during vacuum cycle. Vacuum extended past working time.

SPALLING (an area of the mold wall flakes into the mold cavity)Flask was placed into a furnace at low temperature (below 150oC) for an extended period. Flask was placed too close to the source of heat. Sharp corners are struck by metal at high centrifugal velocities. Improper burnout cycle was used.

NON-FILL OR INCOMPLETE CASTINGSMetal was too cold when cast. Mold was too cold when cast. The burnout was not complete. Pattern was improperly sprued, creating turbulence when casting in a centrifugal casting machine. Centrifugal casting machine had too high revolution per minute.

GROWTH-LIKE ROUGH CASTING THAT RESISTS REMOVAL IN PICKLING SOLUTIONBurnout temperature too high. Mold temperature was too high when casting. Metal temperature was too high when casting.

SHINY CASTINGSCarbon residue was left in the mold, creating a reducing condition on the surface.

AVERAGE SURFACE ROUGHNESS VALUES BY VARIOUS PROCESSESNITC

DESIGN CONSIDERATIONSCAREFUL CONTROL OF LARGE NUMBER OF VARIABLES NEEDED- CHARACTERISTICS OF METALS & ALLOYS CASTMETHOD OF CASTINGMOULD AND DIE MATERIALSMOULD DESIGNPROCESS PARAMETERS- POURING, TEMPERATURE, GATING SYSTEMRATE OF COOLING Etc.Etc.

NITC

Poor casting practices, lack of control of process variables- DEFECTIVE CASTINGS

TO AVOID DEFECTS-Basic economic factors relevant to casting operations to be studied.General guidelines applied for all types of castings to be studied.

DESIGN CONSIDERATIONS

NITC

CORNERS, ANGLES AND SECTION THICKNESSSharp corners, angles, fillets to be avoided Cause cracking and tearing during solidificationFillet radii selection to ensure proper liquid metal flow- 3mm to 25 mm. Too large- volume large & rate of cooling lessLocation with largest circle inscribed critical. Cooling rate less shrinkage cavities & porosities result- Called HOT SPOTS

NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- AVOID SHARP CORNERS MAINTAIN UNIFORM CROSS SECTIONSAVOID SHRINKAGE CAVITIESUSE CHILLS TO INCREASE THE RATE OF COOLINGSTAGGER INTERSECTING REGIONS FOR UNIFORM CROSS SECTIONSREDESIGN BY MAKING PARTING LINE STRAIGHTAVOID THE USE OF CORES, IF POSSIBLEMAINTAIN SECTION THICKNESS UNIFORMITY BY REDESIGNING (in die cast products)

NITC

LARGE FLAT AREAS TO BE AVOIDED- WARPING DUE TO TEMPERATURE GRADIENTSALLOWANCES FOR SHRINKAGE TO BE PROVIDEDPARTING LINE TO BE ALONG A FLAT PLANE- GOOD AT CORNERS OR EDGES OF CASTINGDRAFT TO BE PROVIDEDPERMISSIBLE TOLERANCES TO BE USEDMACHINING ALLOWANCES TO BE MADERESIDUAL STRESSES TO BE AVOIDED

ALL THESE FOR EXPENDABLE MOULD CASTINGS.NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- AVOID SHARP CORNERS TO REDUCE STRESS CONCENTRATIONSNITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- MAINTAIN UNIFORM CROSS SECTIONS TO AVOID HOT SPOTS AND SHRINKAGE CAVITIESNITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- GOOD DESIGN PRACTICE NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- STAGGERING OF INTERSECTING REGIONS NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- SECTION THICKNESS UNIFORMITY MAINTAINED THROUGHOUT PARTNITC

DESIGN MODIFICATIONS TO AVOID DEFECTSNITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- USE OF METAL PADDING (CHILLS) TO INCREASE RATE OF COOLING NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- MAKING PARTING LINE STRAIGHT NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS-IN DESIGNNITC

INSPECTION OF CASTINGSSEVERAL METHODSVISUALOPTICAL- FOR SURFACE DEFECTSSUBSURFACE AND INTERNAL DEFECTS THROUGH NDTs & DTs

PRESSURE TIGHTNESS OF VALVES BY SEALING THE OPENING AND PRESSURISING WITH WATER

EXERCISE

PROCESS FLOW CHARTRECEIPT OF ORDER(REVIEW)ARE THE TERMS ACCEPTED? NO COMMUNICATE- NEGOTIATEYESPREPARE WORK ORDERWORK ORDER TO Q.C, INSPECTION, PLANNING, METHODS, PRODUCTION AND DESPATCH

PRODUCTION PLANMETHOD DRAWING, QA DATA, PATTERN PLAN MOULDINGWORK ORDER, CORE MAKING, HEAT CONFORMATION MELTING AND POURING FOR THESE, LAB TEST REPORTSKNOCK OUTSTAGE ISPECTION- NOT OK, REJECT OK, SHOT BLASTING, GAS CUTTING/ARC CUTTING ASTM STANDARDSHEAT TREATMENTROUGH FETTLING, FINISH FETTLING, INSPECTION

NDT- CUSTOMER REPORT, NOT OK, WELDING & RECTIFICATION WELDING LOG SHEET RE-INSPECTION, NOT OK- REJECTMACHINE - IF REQUIREDSTRESS RELIEFHYDRAULIC TESTS Etc.TEST CERTIFICATE DESPATCH DOCUMENTS, PACKING, Etc. Etc.

Largest Vertically Parted Mold Machine Built for Denmark. DISA Group (with U.S. headquarters in Oswego, Illinois) received a contract for the delivery of a jumbo-sized DISA 280B vertical green sand molding system. Molds measure 850 x 1200 mm, making it the largest vertically parted green sand molding system ever produced. This new jumbo-sized molding system provides a space-saving, high-capacity and high-quality alternative to conventional horizontal molding lines. NEWS FROM NET

It works on the vertically parted molding principle of blowing green sand into the mold chamber, which is subsequently squeezed into flaskless molds. In the inaugural installation, the DISA 280B is replacing a horizontal green sand molding line of a similar flask/box size. The foundry is making the switch from its existing horizontal molding arrangement to the large-scale vertically parted machine to gain competitiveness through productivity gains and enhanced casting quality. These improvements are fundamental to the foundrys success in its market sector. NEWS FROM NET

The MM Casting Process Design Considerations Metal Specifications Tooling Machining

Supply of Design Information Inspection / Quality Assurance Case Studies - Automobile Telephone Chassis - Cassette Slide Ratchet - Casting for Satellite TV - Clamp - Computer Data Backup System - Gauge Body - Hand Held Breathalyzer - Light Shield - Regulator Valve - Roller Carrier Slide - Small Metal Connectors - Switch Gear - Twister

MM-Micro Metalsmiths in the field of Investment casting

NEWS FROM NET

The contract includes the design and changeover of existing horizontal patterns to a vertical orientation. This assures that the foundry will be able to continue to use more than 98% of its existing patterns. The foundry expects to begin production without any compatibility problems as the new line takes the same position of the existing mold line on the foundry floor. .

NEWS FROM NET

The MM casting process Precision investment casting process

The aim of this guide is to enable the designer to take advantage of the technical and commercial benefits offered by the MM casting process. Also included in the guide are design ideas and considerations to help and stimulate the design process. NEWS FROM NET

NEWS FROM NETMicro Metalsmiths continues to develop its process so as to meet ever more exacting market challenges. For over thirty years Micro Metalsmiths has refined its techniques for excellence in the production of copper and aluminium alloy investment castings. Micro Metalsmiths has combined its impressive investment casting technology with best practice CNC machining, so allowing many of the constraints placed on engineering design to be overcome. All this is carried out within a BSI accredited ISO 9001 Quality System.

NEWS FROM NETWhy use the MM casting process

Reduce overall costs by Designing for Manufacture: simplify an assembly by combining a number of parts: eliminate joining processes: reduce the part count whilst improving the integrity of the finished product.

Reduce time to market through use of Rapid Prototyping technology and obtain casting, machining, assembly and packaging from one source. Finished metal parts are deliverable from concept in less than four weeks.

Design flexibility, with quick modifications to tooling that costs 90% less than for a pressure die casting, whilst maintaining economic batch quantities of up to 500,000 parts per annum.

What the MM casting process can deliver: High quality finish, detail and specification. Thin walls as fine as 0.2 mm, typically 1-3 mm. Light weight components, weighing as little as 1g can be produced using techniques to minimise material usage whilst retaining component integrity. Maximum size of 190 mm x 160 mm x 160 mm. A length of 250 mm is possible providing no other dimension exceeds 100mm. Complex detail on internal and external features. Superfine surfaces with 0.8 micrometres being the as cast finish. Zero draft anglesNEWS FROM NET

The Sure-Trak shot control system is a real-time, all digital, closed-loop system to increase die-casting quality. Mercury Castings has seen success in reducing air entrapment during the slow shot phase on a variety of parts, including these outboard engine covers www.moderncasting.com/infoNEWS FROM NET