Designing feeding systems for investment castings.
In any casting method, when an alloy is poured into a mold, it starts to shrink or contract in volume as it cools and subsequently solidifies. Foundries compensate for these two volumetric contractions by providing reservoirs--feeders--as parts of the mold cavity design. The solution for a complete feeding system design includes the dimension, shape, location and method of attachment of feeders to the casting and the materials used. Similar approaches to what is used in sand casting can also be applied to investment casting.
Variations in the design of feeding systems for investment casting components arise because of differences in melting, molding and pouring methods used in foundries. Nevertheless, some general guidelines in procedure for feeding syste design can be followed.
An approach is suggested for combining the feeder dimensioning concepts based o Chvorinov's rules with the experimental, theoretical and practical data available on feeding systems used for investment castings.
Through this procedure, calculating the proper size for feeding systems can be straightforward and repeatable using a modulus-based technique. Modulus is the ratio of the volume to the cooling surface area of the casting (or a part of th casting) or the feeder. To show how the procedure is used in practical applications, an example of an investment cast clamping arm is included.
1. Design modifications--Examine casting geometry to see if freezing provides directional cooling gradients toward the feeder. If needed, work with the customer on design modifications to improve temperature gradients for directional solidification, attaching feeders and quality control purposes.
2. Alloy selection--Alloy normally is selected by the designer from standard specifications and alloy handbooks. To meet process requirements, the designer and foundryman must work together. If several alloys are feasible, carry out a value analysis.
3. Feeding system--Using the modulus method, identify the number and location o feeders from the casting design. Select an appropriate gating and feeding system. A grid consists of a framework of vertical and horizontal members attached to a pouring basin and patterns. A standard grid, modified grid or nongrid feeder system may be designed. Detailed adjustment to feeder location i made later.
4. Cutoff and cleaning--Examine the casting design and proposed feeding system to ensure access is provided to the feeders in subsequent cutoff operations. Adhering ceramic and cores must be removed by suitable cleaning.
5. Feeding properties--Working with the designer, make any needed adjustments t the alloy's composition (additions of or dilution in alloying elements) to improve feeding characteristics such as freezing range, eutectic volume and casting fluidity.
6. Melt treatments--Feeding behavior can be enhanced by influencing freezing nucleation and growth behavior. Check on the specified testing procedures for alloy melt quality controls: inoculation, gas and inclusion controls, alloy superheat and pouring temperatures, and obtaining test bars for mechanical property tests.
7. Product quality-- Final cast quality is influenced by mold phenomena such as mold atmosphere, flow of metal, cooling directions and rates due to mold materials. The production of porosity-free castings depends on the success of the whole feeding system. Check on specified product quality control tests (types of procedures for nondestructive or destructive tests in relation to the casting design, the alloy used and product applications).
8. Patternmaking and assembly--Examine how the gating and feeding system affect making pattern dies and pattern assembly. Explore alternative methods of clustering patterns on the grid (location, orientation) or in the mold to produce favorable feeding temperature gradients during mold preheating and allo pouring, while ensuring adequate cluster rigidity. Remember, although each casting is freezing on its own, the whole grid acts as a single cooling system.
9. Gating--Select the gating system to optimize flow for favorable feeding temperature gradients, which will minimize flow turbulence and maximize feeding pressure. Explore the feasibility of horizontal, vertical and bottom gating. Review the feeding requirements in relation to pattern assembly while optimizin gating system functions. Design the gating and feeding systems by considering jointly the flow and feeding requirements. For grid systems, identify parts of the grid that simultaneously act as feeding and gating elements.
10. Mold materials--Examine whether standard mold, feeder wall materials, moldmaking and preheating techniques should be altered--using insulating and chilling techniques--to improve feeding temperature gradients.
11. Feeding pressure--Total feeding pressure on the feeding liquid flow is vita in the last stages of feeding when flow channels become fine. With proper grid design, the fullest possible benefits of atmospheric and metallostatic pressure on the feeding liquid can be obtained to eliminate porosity.
Feeding Elements Dimensioning
12. Number of feeders--Using knowledge from steps 2, 3, 6, 8, 9, 10 and 11, establish whether directional feeding of the whole casting can be obtained from a single feeder. Calculate the freezing moduli of sections that must be fed separately. Exclude the calculation of all "parasite" sections (thinner casting sections, such as bosses, fed by thicker adjacent sections) attached to the feeding parts. Exclude noncooling surfaces (casting surface not facing the mold in modulus calculation. For complex sections, apply the shape substitution principle.
13. Feeders shape and location--From steps 2, 4, 8, 9, 10, 11 and 12, select feeder shape and location. For grid systems, verify that the elements of the gating system that take part in the feeding flow also will obey the freezing modulus rules in step 14.
14. Feeder volume from freezing time criterion--Calculate the feeder volumes from: [M.sub.f] = k [center dot] [M.sub.c]. The freezing moduli of casting ([M.sub.c]), obtained from step 12, while "k's" value (ratio of modulus) is selected from steps 1, 8, 10 and 11 and feeder shape from 13.
15. Feeder volume from volumetric criterion--Compare feeder volume obtained fro step 14 with that derived from: [V.sub.f] = [V.sub.c] [center dot] ([Beta]/[Eta]- [Beta]). [V.sub.c] is the total volume of all parts, including parasite sections, which are fed from a single feeder; [Beta] is total (liquid and freezing) contraction percent; and [Eta] is the available feeding liquid factor, such as the feeding volume delivery percent of a feeder arising under conditions 8 and 13.
16. Feeder neck--Calculate feeder neck dimensions from [M.sub.n] = [k.sub.n] [center dot] [M.sub.c]. The value of [k.sub.n] and neck shape are selected from points 1, 3, 4, 6, 8 and 9.
17. Feeding distance--Verify from 3, 8, 10, 11 and 12 that the required temperature gradients would be obtained to meet the necessary feeding distance.
Guidelines in Practice
To illustrate these guidelines, Fig. 1 shows the essential dimensions for an investment cast clamping arm. The customer specified Gunmetal alloy, with a composition of Cu 88%, Sn 8%, Zn 4%. Alloy shrinkage is estimated at 6%. At a volume feed delivery of 18%, its volume ratio (calculated by volume of feeder/ volume of casting) is 0.50. This criterion sets the minima for feeder sizes required to ensure volumetric adequacy in the supply of feed liquid.
After dividing the casting into four sections and calculating the modulus, it i seen that the arm and rib solidify ahead of the smaller and large cylinder. A feeder is required to feed the big cylinder, which in turn can feed the arm and rib. A separate feeder is needed to feed the small cylinder, since thinner sections#2 and #3 cut off feed supply to it from feeder #1.
To take advantage of the substantial taper in the arm and rib, this component can be cast in a vertical orientation. In this orientation, gravity provides needed pressure to feeder #1 (and the cylinder #1) to feed the long arm (#2) an the rib (#3).
Feeder #1 is designed to feed three sections--the big cylinder, the arm and the rib. With freezing ratio k at 1.2 and [M.sub.c], the highest modulus among fed sections is that of section #1, [M.sub.f] = 14.2.
Modulus of a square bar feeder of side d, [M.sub.f] = d/4, when this feeder is considered part of the grid and two side surfaces are noncooling. Hence, the side of feeder #1, d = 4 [center dot] [M.sub.f] = 57 mm.
At 70 mm length (effective feeder length in the grid is assumed to be 120% of larger dimension, such as the diameter of attached section #1 and #4, respectively), [V.sub.f] = 227,430 cu mm. This feeder has a volume ratio of 1.0 if attached to one casting. Therefore, it's advantageous to attach a second casting on the other side of the same feeder. This results in a volume ratio of 0.52, (greater than 0.50), thus satisfying the volumetric criterion.
Feeder neck sizes are calculated for this feeder. Minimum neck size (frustrum o cone) = 4 [center dot] 1 [center dot] 1 [center dot] 11.8=52 mm diameter. This allows a 3 mm wide annulus on the casting surface (called a "witness"), which provides a reference surface for finishing operations. The length of the feeder neck, shown as 15 mm, allows for easy cutoff.
Minimal size for the second feeder (#2) is determined in a similar way: [M.sub.f] = k [center dot] [M.sub.c], where [M.sub.c] is the modulus of the cylinder #4. Side d, of a square bar feeder, is found to be 38 mm.
To check if feeder #2 has sufficient volume at a length of 38 mm (effective length of feeder in this grid is assumed to be 120% of larger dimension, diameter of attached section), feeder volume is found to be 54,872 cu mm, and 1.14 volume ratio on feeding cylinder #4 on either side. Thus, it easily satisfies the minimum volume criterion [is greater than or equal to] 0.50.
Feeder neck size for feeder #2 is found similarly. Minimum neck size is found t be 4 [center dot] 1 [center dot] 1 [center dot] 7.8) = 34.3 mm.
To verify the feeding distance, [f.sub.d] of the feeders: [f.sub.d] = 10 [cente dot] t = 250 mm, using the combined thickness (t = 25 mm) of arm and rib. This shows that feeder #1 can feed cylinder #1 effectively, as well as the complete length of the arm #2 and rib #3. The second feeder #2 can feed the adjacent section #4 effectively.
Two alternative designs for the feeding grids are given as examples in Fig. 4-- four-piece cluster that can be successfully cast with an alloy charge weight of 42 lb (19 kg) which would give a casting yield of 42.8%; or an eight-piece cluster with an alloy charge weight of 70 lb (32 kg), giving a casting yield of 51.4%.
Other alternative designs, such as six-piece clusters, are possible and are again fitted with the optimal sizes of the feeders. An optimum feeding grid can be selected based on the available foundry equipment, such as in shell-making o melting. For instance, the eight-piece cluster mold would require robotic dipping for making the shell, while manual dipping is adequate in making the four-piece cluster.
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|Date:||Sep 1, 1994|
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