Section two Risering System Design - Sorelmetal

1 19 Section twoRisering System DesignPlease note: Risering must be done before gating System can be shaped (Heine) risers are now the riser of choice in themajority of Risering System Objectives: castings without shrinkage defects economic production maximize casting Essential Components Riser always blind (closed top).Riser contact generally as short as dimensions always measured at thenotch. Gate thin and wide for fast freezing (see p. 28). Vents to assist fast mould The Following are Suggested by Research andSupported by Industrial Experience Volume change patterns of cooling and solidifyinggraphitic irons result in net volume increase of ironin the mould. The net volume increase can produce liquid pres-sure in the mould of several hundred (2 MPa). This pressure always exceeds the elastic limit ofthe mould, except for very rigid moulds, leading tomould enlargement and swollen castings, oftencontaining shrinkage defects.

2 Green sand moulds are not considered to be rigidin this context. Riser function is very sensitive to pouring tempera-ture and pouring time. Volume change pattern is not constant but variesaccording to cooling rate and liquid iron process-ing route (superheat, charge composition, meltingmethod, inoculation, etc). Due to the high pressures experienced by themould during pouring and solidification, mouldhalves should be clamped together. Weightingalone is not Typical Volume Change Patterns General volume change pattern for steel, whiteiron, brass, etc. Volume change patterns for graphitic irons. Cooling liquid initially contracts then expands. Towardsthe end of solidification, last remaining liquid solidifieswith contraction. Shape of volume change pattern influenced bycooling rate and by changes in liquid iron process-ing.

3 This directly affects the extent of contractionand PlanningThe detailed Design principles will be presented inthe following order: Determine significant modulus of the castings (MS). Evaluate mould and iron quality, then select appro-priate Risering method. Determine corresponding liquid transfer modulus(MN) and number of risers required for each casting . Select riser type and compute dimensions (MR). Select riser contact (neck) type and computedimensions. Check that available feed volume in riser(s) is suffi-cient for casting s requirements. Select pouring temperature based on selectedrisering (Liquid)ContractionRate of solidState ContractionTemperature Intervalof SolidificationTemperature of theLiquid after CompletedPouring.(Ty)Temperature ( C, F)Solidification (Freezing)ContractionSpecificVolume(cm3/ g)(in3/ lb) Cooling Rate casting weight or wall thickness not sufficiently ac-curate to describe cooling rate.

4 Simple shapes: cube, plate, bar etc, all 1 inch (25 mm)thick but all cool at different rates. Use modulus (M) to describe cooling. Modulus =volumeeffective cooling surface area More complicated shapes should be broken downinto simple shapes and the moduli of the individualsimple shapes, determined. Note in the example that the connecting surfacesbetween adjacent segments are not considered tocontribute to cooling (variable c below).Where:V = total casting = total cooling surface area of the castingExample for the calculation of Modulusa = any sideb = any sidec = non-cooled side2. Modulus = VM =a bCSA2 (a + b) c all dimensions in cmM1= 5 = 5 3 = cm10M3= 5 4 = cm11 Significant Modulus = M3= cmNote: See example on page When hollow sections are involved, the coolingeffect of cores may be approximated as TOTHE COOLING SURFACE AREAIf d <1/3D, ASSUME 0% COOLING FROM COREIf d >1/3D and d <2/3D, ASSUME 50% COOLINGFROM COREIf d >2/3D, ASSUME 100% COOLING FROM Mould Quality Objective is to avoid enlargement of the mould fromhigh liquid pressures exerted by the cooling andsolidifying graphitic iron.

5 Green sand and shell moulds will not withstand thesolidification pressure. Chemically bonded sand moulds will resist solidifi-cation pressure if they are properly prepared. Thisrequires mechanical compaction of sand duringmould preparation and adequate curing. Cement sand and dry sand moulds will normallywithstand the iron solidification Liquid Iron Processing All aspects of iron processing have some influenceon the magnitude of volume change during coolingand solidification, hence the shrinkage characteris-tics of the iron. Some of the factors which increase shrinkage ten-dency: high melt superheat temperatures long holding times in the furnace high proportion of foundry return scrap or steelscrap in the charge presence of carbide stabilizing elements in meltchemistry (including high Mg) variable carbon equivalent of the iron inadequate inoculation.

6 Combined effect of these (and other) process vari-ables can be assessed, very approximately, bymeasuring nodule count of standard test piece(Nodule count increases with faster cooling). Irons which show low tendency to shrinkage alwaysseem to show low tendency to form as-cast car-bides they graphitise well. Such irons are saidto possess good metallurgical quality . The presence of any type of carbides in the as-caststructure should be considered as an indicationthat the iron has poor metallurgical quality. Conse-quently problems with shrinkage defects should Plot shows range of expected nodule counts forgood metallurgical quality ductile irons in depen-dance of modulus (cooling rate). For example, a 1 in (25 mm) Y block has a modu-lus of in (8 mm). For good metallurgical qualityiron, range of nodule counts is 140-280/mm.

7 See also and Selection of Risering Method CONVENTIONAL Risering The test bar blankor Y block is one example. Use of a large (open)riser encourages directional solidification ensuringdefects appear in the riser not the test bar blank(parallel sided portion). Problem with conventional Risering is low yield. Inthis example, about 23%. Not economical. APPLIED Risering Use this family tree to select Risering method foryour production cmMODULUS COUNT per mm20 0,3 0,8 0,9 1,2 1,5 1,8 2,1 2,4 INCREASEDSHRINKAGETENDENCYEXCESSIVEPRESS URECREATEDRISERINGLIQUID CONTRACTIONWITH GATING SYSTEMRISERINGLIQUID CONTRACTIONWITH RISERPARTIAL RISERINGWITHGATING SYSTEMPRESSURECONTROLRISERINGRISERLESSDE SIGNDIRECTLY APPLIED RISERINGSAFETY RISERNO RISERAPPLIED Risering METHODSMOLDWEAKMODULUS in.

8 > 3/16< 3/16 STRONGMODULUS in.< 1> 1 Selection based on mould strength and castingmodulus. Methods take advantage of the fact that graphiticirons expand during cooling, unlike steel, whiteiron, malleable iron etc. WEAK MOULD:Green sand, shell, non-compactedchemically bonded sand. STRONG MOULD:Well compacted chemicallybonded sand, cement sand, dry sand, permanentmould. There are three basic applied Risering methods: pressure control Risering (PCR) or bottle riser directly applied Risering (DAR) riserless Application of each method: when mould is weak and casting modulus isgreater than in. (4 mm) use PCR. when mould is strong and casting modulus isless than in. (25 mm) or when mould is weakand casting modulus is less than in. (4 mm)use DAR. when mould is strong and casting modulus isgreater than in.

9 (25 mm) use Pressure Control Risering Most green sand and shell moulded castings shouldbe risered by this method. Objective is to control the pressure generated dur-ing cooling and solidification, between a minimumpressure level, which will prevent the occurenceof secondary contraction defects and a maximumlevel, at which the mould will enlarge. Principles of PCR (necks not used to simplify):A. after pouring completed, liquid riser compensates for liquid when expansion starts, mould deformation avoi-ded by pressurized liquid from casting , bleed-ing back to refill the (blind) ideally riser should refill just before expansion ceases. this puts all remaining liquid under slight positivepressure and prevents secondary shrinkage defect. Design Sequence: determine casting significant (largest) modulus(MS) ( Section ).

10 Determine Modulus Riserneck (MN) determine Modulus Riser (MR) see Card #3 metric or #3 Relationship between significant modulus (MS), riser-head neck modulus (MN) and riser-head modulus (MR) inpressure-control riser- System Design . Includes factor (f).See page 28. select blind riser type and compute dimensions. Also see Section bottle riser Design . main riser dimensions expressed in terms of diam-eter, D; height = x D or with neck located indrag riserheight = x D + neck heigth. Find riser neck dimension on Card #4english ormetric. Round or square necks = 4 x MN Rectangular necks = 3 x MN+ 6 x CONTROL Risering METHODS ignificant Modulus (MS) cmRiser Neck Modulus (MN) cmRiser Modulus (MR) cm (MR = MN x ) II Poor Quality Good Quality Riser neck dimensions are measured at the bottomof the radius between riser and casting .