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1、 Chapter 13 Resin bonded sand Chemically bonded sand A wide variety of chemical binders is available for making sand moulds and cores. They are mostly based either on organic resins or sodium silicate, although there are other inorganic binders such as cement, which was the earliest of the chemical
2、binders to be used, ethyl silicate, which is used in the Shaw process and for investment casting, and silica sol, which is also used for investment casting. The binders can be used in two ways: As self-hardening mixtures; sand, binder and a hardening chemical are mixed together; the binder and harde
3、ner start to react imme- diately, but sufficiently slowly to allow the sand to be formed into a mould or core which continues to harden further until strong enough to allow casting. The method is usually used for large moulds for jobbing work, although series production is also possible, With trigge
4、red hardening; sand and binder are mixed and blown or rammed into a core box. Little or no hardening reaction occurs until triggered by applying heat or a catalyst gas. Hardening then takes place in seconds. The process is used for mass production of cores and in some cases for moulds for smaller ca
5、stings. Self-hardening process (also known as self-set, no-bake or cold-setting process) Clean, dry sand is mixed with binder and catalyst, usually in a con- tinuous mixer. The mixed sand is vibrated or hand-rammed around the pattern or into a core box; binder and catalyst react, hardening the sand.
6、 When the mould or core has reached handleable strength (the strip time), it is removed from the pattern or core box and continues to harden until the chemical reaction is complete. Since the binder and catalyst start to react as soon as they are mixed, the mixed sand has a limited “work time” or “b
7、ench life” during which 168Foseco Non-Ferrous Foundrymans Handbook the mould or core must be formed (Fig. 13.1). If the work time is exceeded, the final strength of the mould will be reduced. Work time is typically about one third of the “strip time” and can be adjusted by controlling the type of ca
8、talyst and its addition rate. The work time and strip time must be chosen to suit the type and size of the moulds and cores being made, the capacity of the sand mixer and the time allowable before the patterns are to be reused. With some binder systems the reaction rate is low at first, then speeds
9、up so that the work time/strip time ratio is high. This is advantageous, particularly for fast-setting systems, since it allows more time to form the mould or core. Stripping is usually possible when the sand has reached a compression strength of around 350kPa (50psi) but the actual figure used in p
10、ractice depends on the type of binder system used, the tendency of the binder to Figure 13.1Typical hardening curve for self-hardening sand: Tw= work time Ts= strip time Tc= casting time Tmax= time to achieve maximum strength. Resin bonded sand169 sag before it is fully hardened, the quality of the
11、pattern equipment and the complexity of the moulds and cores being made. It is advisable to strip patterns as soon as it is practical, since some binder chemicals attack core box materials and paints after prolonged contact. The properties of chemical binders can be expressed in terms of: Work time
12、(bench life): which can be conveniently defined as the time after mixing during which the sand mixture has a compressive strength less than 10kPa, at this stage it is fully flowable and can be compacted easily, Strip time: which can be defined as the time after mixing at which a compressive strength
13、 of 350kPa is reached, at this value most moulds and cores can be stripped without damage or risk of distortion, Maximum strength: the compressive strength developed in a fully hardened mixture, figures of 30005000kPa are often achieved It is not necessary to wait until the maximum strength has been
14、 achieved before moulds can be cast, the time to allow depends on the particular castings being made; usually casting can take place when 80% of the maximum strength has been reached. Testing chemically bonded, self-hardening sands Units Compressive strength values may be reported in: SI unitskPa =
15、kN/m2 cgs unitskgf/cm2 Imperial unitspsi = lbf/in2 Conversion factors: 100kPa (kN/m2) = 1.0197kgf/cm2 = 14.5038psi (lbf/in2) 1kgf/cm2= 98.0665kPa = 14.22psi (lbf/in2) 1psi (lbf/in2)= 6.895kPa (kN/m2) = 0.07032kgf/cm2 170Foseco Non-Ferrous Foundrymans Handbook Conversion table kPa (kN/m2)kgf/cm2psi (
16、lbf/in2) 100.101.5 500.517.3 1001.0214.5 2002.0429.0 3003.0643.5 4004.0858.0 5005.1072.5 6006.1287.0 7007.14101.5 8008.16116.0 9009.18130.5 100010.20145.0 200020.39290.1 300030.59435.1 400040.79580.1 500050.99725.2 The curing properties (work time, strip time and maximum strength) are measured by co
17、mpression tests using 50mm diameter specimen tubes with end cups, or AFS 2in diameter tubes, with a standard rammer. Sand is mixed in a food mixer or small core sand mixer; catalyst being added first and mixed, then the resin is added and mixed. Measurement of “work time” or “bench life” Mix the san
18、d as above, when mixing is complete, start a stopwatch and discharge the sand into a plastic bucket and seal the lid. After 5 minutes, prepare a standard compression test piece and immediately measure the compressive strength. At further 5 minute intervals, again determine the compressive strength,
19、stirring the mixed sand in the bucket before sampling it. Plot a graph of time v. strength and record the time at which the compressive strength reaches 10kPa (0.1kgf/cm2, 1.5psi); this is the worktime or bench life. The sand temperature should also be recorded. For fast-setting mixtures, the streng
20、th should be measured at shorter intervals, say every 1 or 2 minutes. Resin bonded sand171 Measurement of strip time Prepare the sand mixture as before. When mixing is complete, start a stopwatch. Prepare 610 compression test pieces within 5 minutes of completion of mixing the sand. Cover each speci
21、men with a waxed paper cup to prevent drying. Determine the compressive strength of each specimen at suitable intervals, say every 5 minutes. Plot strength against time. Record the time at which the strength reaches 350kPa (3.6kgf/cm2, 50psi), this is the “strip time”. The sand temperature should al
22、so be recorded. Measurement of maximum strength Prepare the sand mixture as before. Record the time on completion of mixing. Prepare 610 specimens as quickly as possible covering each with a waxed cup. Determine the strength at suitable intervals, say 1, 2, 4, 6, 12, 24 hours. Plot the results on a
23、graph and read the maximum strength. The sand temperature should be held constant if possible during the test. While compressive strength is the easiest property of self-hardening sand to measure, transverse strength or tensile strength being used more frequently nowadays, particularly for the measu
24、rement of maximum strength. Mixers Self-hardening sand is usually prepared in a continuous mixer, which consists of a trough or tube containing a mixing screw. Dry sand is metered into the trough at one end through an adjustable sand gate. Liquid catalyst and binder are pumped from storage tanks or
25、drums by metering pumps and introduced through nozzles into the mixing trough; the catalyst nozzle first then binder (so that the binder is not exposed to a high concentration of catalyst). Calibration of mixers Regular calibration is essential to ensure consistent mould and core quality and the eff
26、icient use of expensive binders. Sand flow and chemical flow 172Foseco Non-Ferrous Foundrymans Handbook rates should be checked at least once per week, and calibration data recorded in a book for reference: Sand: Switch off the binder and catalyst pumps and empty sand from the trough. Weigh a suitab
27、le sand container, e.g. a plastic bin holding about 50kg. Run the mixer with sand alone, running the sand to waste until a steady flow is achieved. Move the mixer head over the weighed container and start a stopwatch. After a suitable time, at least 20 seconds, move the mixer head back to the waste
28、bin and stop the watch. Calculate the flow inkg/min. Repeat three times and average. Adjust the sand gate to give the required flow and repeat the calibration, Binders: Switch off the sand flow and the pumps except the one to be measured. Disconnect the binder feed pipe at the inlet to the trough, e
29、nsuring that the pipe is full. Using a clean container, preferably a polythene measuring jug, weigh the binder throughput for a given time (minimum 20 seconds). Repeat for different settings of the pump speed regulator. Draw a graph of pump setting against flow inkg/min. Repeat for each binder or ca
30、talyst, taking care to use separate clean containers for each liquid. Do not mix binder and catalyst together, since they may react violently. Always assume that binders and catalysts are hazardous, wear gloves, goggles and protective clothing. When measuring liquid flow rate, the pipe outlet should
31、 be at the same height as the inlet nozzle of the mixer trough, so that the pump is working against the same pressure head as in normal operation. Mixers should be cleaned regularly. The use of STRIPCOTE AL applied to the mixer blades, reduces sand build-up. Sand quality In all self-hardening proces
32、ses, the sand quality determines the amount of binder needed to achieve good strength. To reduce additions and therefore cost, use high quality sand having: AFS 4560 (average grain size 250300 microns) Low acid demand value, less than 6ml for acid-catalysed systems Rounded grains for low binder addi
33、tions and flowability Low fines for low binder additions Size distribution, spread over 35 sieves for good packing, low metal penetration and good casting surface Pattern equipment Wooden patterns and core boxes are frequently used for short-run work. Epoxy or other resin patterns are common and met
34、al equipment, usually aluminium, may be used for longer running work. The chemical binders Resin bonded sand173 used may be acid or alkaline or may contain organic solvents which can attack the patterns or paints. STRIPCOTE AL aluminium-pigmented suspension release agent or silicone wax polishes are
35、 usually applied to patterns and core boxes to improve the strip of the mould or core. Care must be taken to avoid damage to the working surfaces of patterns and regular cleaning is advisable to prevent sand sticking. Curing temperature The optimum curing temperature for most binder systems is 2025C
36、 but temperatures between 15 and 30C are usually workable. Low temperatures retard the curing reaction and cause stripping problems, particularly if metal pattern equipment is used. High sand temperatures cause reduction of work time and poor sand flowability and also increase the problem of fumes f
37、rom the mixed sand. If sand temperatures regularly fall below 15C, the use of a sand heater should be considered. Design of moulds using self-hardening sand Moulds may be made in flasks or flaskless. Use of a steel flask is common for large castings of one tonne or more, since it increases the secur
38、ity of casting. For smaller castings, below one tonne, flaskless moulds are common. Typical mould designs are illustrated in Fig. 13.2. The special features of self-hardening sand moulds are: Large draft angle (35) on mould walls for easy stripping Incorporation of a method of handling moulds for ro
39、ll-over and closing Means of location of cope and drag moulds to avoid mismatch Reinforcement of large moulds with steel bars or frames Clamping devices to restrain the metallostatic casting forces Use of a separate pouring bush to reduce the sand usage Mould vents to allow gas release Sealing the m
40、ould halves to prevent metal breakout Weighting of moulds if clamps are not used Use of minimum sand to metal ratio to reduce sand usage, 3 or 4 to 1 is typical for ferrous castings Foundry layout With self-hardening sand, moulds and cores are often made using the same binder system, so that one mix
41、er and production line can be used. A typical layout using a stationary continuous mixer is shown in Fig. 13.3. The 174Foseco Non-Ferrous Foundrymans Handbook Figure 13.2Typical designs of self-hardening moulds. From Foundry Practice Today and Tomorrow, SCRATA Conference, 1975.) (a) Method of mouldi
42、ng-in- steel tubes for ease of handling boxless moulds. (b) Sockets moulded into boxless moulds for ease of lifting, roll-over and closing. (c) Steel reinforcement frames for handling large boxless moulds. (d) Method of locating mould halves and preventing runout. moulds may or may not be in flasks.
43、 Patterns and core boxes circulate on a simple roller track around the mixer. The length of the track is made sufficient to allow the required setting time, then moulds and cores are stripped and the patterns returned for reuse. For very large moulds, a mobile mixer may be used. Resin bonded sand175
44、 Sand reclamation The high cost of new silica sand and the growing cost of disposal of used foundry sand make the reclamation and reuse of self-hardening sands a matter of increasing importance. Reclamation of sand is easiest when only one type of chemical binder is used. If more than one binder is
45、used, care must be taken to ensure that the binder systems are compatible. Two types of reclamation are commonly used, mechanical attrition and thermal. Wet reclamation has been used for silicate bonded sand. The sand is crushed to grain size, water washed using mechanical agitation to wash Figure 1
46、3.3Foundry layout for self-hardening sand moulds. 176Foseco Non-Ferrous Foundrymans Handbook off the silicate residues, then dried. The process further requires expensive water treatment to permit safe disposal of the wash water so its use is not common. The difficulty and cost of disposing safely o
47、f used chemically bonded sand has led to the growing use of a combination of mechanical and thermal treatment. Mechanical attrition is used to remove most of the spent binder. Depending on the binder system used, 6080% of the mechanically reclaimed sand can be rebonded satisfactorily for moulding, w
48、ith the addition of clean sand. The remaining 2040% of the mechani- cally treated sand may then be thermally treated to remove the residual organic binder, restoring the sand to a clean condition. This secondarily treated sand can be used to replace new sand. In some cases, all the used sand is ther
49、mally treated. Mechanical attrition This is the most commonly practised method because it has the lowest cost. The steps in the process are: Lump breaking; large sand lumps must be reduced in size to allow the removal of metal etc. Separation of metal from the sand by magnet or screen. Disintegration of the sand lumps to grain size and mechanical scrubbing to remove as much binder as possible, while avoiding breakage of grains. Air classification to remove dust, fines and binder residue. Cooling the sand to usable temperature. Addition of new sand to make up losses and ma
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