UNDERSTANDING INVESTMENT CASTING WAXES - PATTERN-MAKING
Maruti Protectives, B-13/125, GIDC colony, Makarpura, Baroda 390 010
To realize best results from a pattern - wax blend for investment casting, careful study of wax quality summarized in the form of supplier’s data-sheet is advised. Relevance of this data-sheet information to operating parameters as well as performance indicators is explained. Three types of wax formulations viz. straight, resin - filled or water emulsified waxes are described under broad classification to explain how differently they perform. Concept of wax conditioning is dealt with in detail mentioning what it may signify with respect to these three types of waxes. Even with proper injection conditions and with a perfectly conditioned wax, each individual job (die) may demand specific consideration of die design factors like mould opening and release mechanisms, die cooling systems, location and size of gatings etc. to optimize the wax pattern quality.
Keywords : Pattern - wax, data-sheets, wax conditioning
Waxes are a universal choice as pattern material for investment casting, barring few exceptions. As the ultimate quality of castings in this process depends primarily on quality of wax patterns, it is necessary to understand how best performance of wax - blend can be achieved. For the purpose, we begin with a look into basics of wax moulding process.
Process of pattern - making requires either gravity pouring of molten wax or injection moulding of wax under pressure. Gravity pouring is practiced when it is not possible to exert pressure on the mould (or only nominal pressure is permitted). In this case pouring is at comparatively high temperature to ensure proper fluidity. Mould material must be specially formulated to achieve low shrinkage values and cavitation to account for this high temperature.
Injection moulding in general, is preferred with semi-liquid or paste form of wax. Preference is driven from the fact that for wax pattern to replicate the die, it must be filled at as low a temperature as possible to contract minimum on solidification. Although for special requirements, billet injection is carried out by pressurizing solid blocks in to die, practically, considering mechanical set-up requirement etc. semi-liquid or the paste form is the best choice.
2. TECHNICAL DATA – SHEETS
Selection of a wax blend for pattern - making must take into account two things. For one, it should be suitable for injection and other processes in the present plant set-up and secondly, the pattern performance on injection must satisfy the quality requirement of castings. Also, as patterns are subjected to several steps in the process such as ‘tree making’, preparation of shells, dewaxing, shell-firing, etc., many qualities of wax may affect the overall performance of the casting process. Manufacturers of blended pattern - wax summarize the indicators of these qualities in the form of ‘Technical data-sheets’. For the benefit of investment casting industry, British Investment Castors’ Trade Association  as well as Investment Casting Institute  have published literature on what data could be relevant for evaluation of wax performance and also update this information as and when new requirements emerge with technological developments. Procedures set for tests are normally standard test procedures as per petroleum standards for evaluation of waxes. Suitable changes with reference to investment casting application are specifically mentioned in BICTA or ICI guidelines.
When considering a wax or a wax blend as pattern – material, it is very important to remember that each one is unique in itself. No two brands of blended waxes may have same constituents or their proportions. Hence, before using a particular wax blend, supplier’s data-sheet must be carefully studied and the conditions for injection trials etc. must be planned accordingly. Also, it may clearly indicate if the wax is not suitable for a particular plant set-up or if the performance may not match the product requirement.
Several quality parameters can be evaluated for wax blends and listed either to set the plant operating parameters or to view performance indicators. Wax supplier’s data-sheets may have varied formats and any listing of these parameters can never be complete, since it is difficult to cover perfectly all wax quality aspects, with even the most exhaustive list of tests. Here, we list generally considered quality parameters, and their relevance to process applications.
(1) Congealing Point
(2) Melting or R&B Softening Point
(3) Viscosity – Temperature chart
Approximate range of injection temperatures
Dewaxing temperature range
- injection condition relating to pressure and flow
- dewaxing conditions
(1) Expansion charts of molten wax
(2) Penetration – surface hardness at different temperatures
(3) Ash content
(4) Specific gravity
- dewaxing conditions
- some indication of pattern shrinkage data
- pattern dressing convenience
- mechanical strength of patterns
- indicates the perfection with which shell cavities will be cleaned after shell firing to accommodate castings free of defects resulting from inclusions etc.
- besides the quantitative estimation of requirement of wax, it serves to standardize the filler content estimation of a particular wax blend
Besides the above generally mentioned details, data-sheets may give mechanical properties like modulus of rupture, impact resistance, (self) deflection over a fixed period as also the thermal conductivity. The test injection may be performed at the supplier’s end and the pattern performance in terms of contraction, cavitation, surface finish etc. reported for a test die under laboratory injection conditions.
Having said all this, it must be remembered that the relation between the test data and the actual plant performance is not necessarily similar for all wax formulations. For example, the injection temperature range in general may be 4 – 5°C below the congealing point, but, some type of waxes may inject far below their congealing points. Again, none of the data being reported as above are absolute properties. They are reported as the ones observed under prescribed set of test procedures. These test procedures must be carefully followed otherwise the results will be totally different. For example, it is important to know the rate at which temperatures must be increased or decreased (heating or cooling rate) in determination of melting, congealing or softening point. Test procedures for determining ash percentage may also make mention of burning rate, along with final temperature reached, when ash was checked. It should then be understood that as the shop floor conditions are rarely uniform with respect to operations to which a test data relate and hence no precise parameters could be drawn for practical working. Thus, supplier’s data-sheet clearly serves the following purposes :
1). It helps in initial decision making whether that particular wax blend is worth trying for a specific plant condition or product requirement.
2). It gives broad guidelines for determining range of plant operating conditions like temperature and pressure conditions for injection etc. It will demand an elaborate effort of trial-error sequence to fix up precise working set-up. Also, different types or sizes of job may suit individual set of conditions and hence it is a practice to maintain job cards for guidance to machine operators for each die.
3). It is a tool for evaluation of a particular blend and an excellent measure for quality control, rather a quality agreement between the wax blend supplier and the user foundry.
3. STRAIGHT, RESIN-FILLED & WATER EMULSIFIED WAXES
One needs to know how the wax will perform in the shop floor condition and what is required to achieve its optimum performance. To answer these questions, we have to understand the nature of formulation of pattern - waxes.
Normal straight formulation of pattern - wax is made from constituents, each of them melting and forming a homogeneous melt. As the molten wax solidifies, it sets with characteristic dimensional shrink and cavitation or dishing effect. To reduce these shrinking effects, a component may be added which does not melt but remains in the fine particulate suspension in the wax melt. Such a component is termed as filler. Proportion of volume of liquid wax that has tendency to shrink reduces by the amount of filler, thereby reducing the shrinkage percentage. For this application, several natural or synthetic chemical entities have been tried in past. But developments till now have resulted in the choice of synthetic resins, often styrene based homo or copolymers. There is always scope for invention of new filler materials which stand the test of performance as filler. Usually the filler cost is a substantial percentage of total wax cost. This concept is slightly modified and some formulations are made so that with suitable emulsifiers, water added in the molten wax forms emulsion in the melt and fine droplets remain dispersed throughout the wax volume. Addition of water reduces the proportion of liquid which solidifies, reducing the shrinkage by that ratio. Such waxes are known as ‘water emulsified waxes’ in contrast with waxes with resins as fillers – conveniently called ‘resin-filled waxes’. It can be said that water acts as filler in water emulsified wax.
It is intended to discuss here how different types of waxes classed as straight, resin - filled or water emulsified waxes perform, from the view point of pattern making. It is natural that shrinkage of filled waxes will be lower compared to straight unfilled wax (base wax without filler, in particular). Other advantages of using filler are dimensional stability of patterns, lower die-setting time, minimum or practically nil sinkage, improved mechanical strength etc. It is necessary to understand that any proven ‘filler’ for some formulations is not necessarily a ‘filler’ for all. For a wax formulation, compatibility criteria for a ‘filler’ are : balance of relative bulk densities of base wax melt and filler, filler particle size distribution, consideration of viscosity of base wax for uniform dispersion of filler and most important – thermal stability of filler in the range of temperatures the wax is being subjected to, from injection till right up to dewaxing operations. Also one cannot expect any of the properties to enhance in proportion to increase in filler content. Simply increasing the filler quantity, besides adding to basic cost, may also end up in an unbalanced formulation when settling of filler may cause serious problems like clogging of injection nozzles.
In case of water emulsified waxes, since water functions as filler, one expects elimination of high initial cost of resin filler and later filler replenishment cost in subsequent cycles for reuse of wax after dewaxing. Following analysis may give an account of some problems associated with resin filled waxes and how the water emulsified waxes may perform in those respects.
1). Water in an emulsified pattern - wax acts as filler and hence affords all advantages achieved by addition of resin fillers, like low shrinkage, elimination of cavitation, minimal statistical variance in production lots, etc.
2). Resin fillers limit the quality of surface (finish) depending upon their particle size and suspension characteristics. Water in emulsified wax does not remain particulate to degrade the surface finish.
3). Recovery for reuse after dewaxing is simpler - boiling off the water, filtering off the (ash) sediments – as can be done for unfilled wax, and adding the required quantity of water for proper emulsion formulation. Resin fillers create problem in recovery process, particularly if reconditioning for reuse as pattern - wax is desired.
4). Dewaxing is really efficient. Heating of shells converts water to steam in wax melt inside the shells and creates positive pressure for wax to be thrust out and clean the shells faster and more efficiently.
With this general understanding of the pattern - waxes broadly classified as straight, resin - filled or water emulsified pattern - waxes and the information given by the data-sheet of a particular blend, it may be possible to visualize how the wax may perform.
4. CONCEPT OF WAX CONDITIONING
Although proper conditions are maintained, unless the wax melt attains homogeneity, good patterns cannot be expected. Concept of wax conditioning arises from the poor thermal conductivity of waxes. Total wax mass must attain the same physical form viz. semi-liquid, paste etc. and throughout maintain the same temperature. Usually, it is attained by gradually cooling wax molten at higher temperature under continuous agitation. Care must be taken to prevent air entrapment in the process. This process to achieve the thoroughly homogenized phase of wax is termed as ‘conditioning’. This being a very slow process, needs to be carried out before feeding into injection machine. Original system of conditioning in cylinders and charging the conditioned cylinder onto wax machine is replaced by agitated thermally controlled tanks for preparing bulk of conditioned wax to feed the injection machine. Although feed tanks of injection machines are capable of melting, one can hardly afford to waste ‘running time’ of machine on sluggish process of wax conditioning. For operations of modern integrated wax machines with multiple temperature controls from feed tank till nozzle outlet, with pressure and flow regulation, following guidelines may help maintain or improve pattern quality.
1). Feed to the machine tank must invariably be conditioned wax at the temperature within 4 – 5°C of expected injection temperature.
2). Feed tank level may never be allowed to drop below half of the total height of the tank.
3). Any make-up wax added to the feed tank must also be ‘conditioned’ and in such small proportion so as not to disturb equilibrium conditions in the tank. In other words, agitation should be able to accommodate make-up addition and immediately attain uniformity as the injection process continues.
4). Special care may be taken to prevent settling of filler on shut down or power failure in case of filled waxes.
‘Conditioning’ aspect may seem trivial, but as shop floor conditions demand speedy operations, it is observed that in several occasions it is overlooked. Also, in a bid to initially save on the extra equipment like ‘conditioning tank’, concept is taken for granted. With proper treatment, wax may be graded excellent, but in other case it may be injecting patterns of rejection quality.
In case of filled wax, uniform particulate suspension of filler in wax melt is a prime requirement. Similarly, for emulsified waxes, perfect emulsion of water in wax melt is necessary. Emulsion may be obtained at substantially low temperature and can be identified by change in colour of melt as it cools.
5. GENERAL CONSIDERATIONS
1). Usual design parameter for linear contraction is 1 % for wax-to-die dimensions. But the contraction norm of 0.5 % or even less is feasible with several waxes. Filled waxes and emulsified waxes, as explained earlier, have more reliable performance in this aspect, coupled with elimination of cavitation problems. Actual contraction data, however in a particular section of a die, will depend on its geometrical location etc. And may vary from section to section even in a single die.
2). For the pattern - waxes having test-injection free of cavitation problems, if the production patterns show cavitation, it could either be due to improper filling or due to air entrapment. Injection pressure, wax flow rate, conditioning of wax etc. need to be attended to.
3). When the penetration values and the mechanical properties indicate favourable properties at low temperatures (may be in the range of 25 - 30°C), pattern dressing and tree making etc. should also be carried out below the recommended temperature. Such waxes are likely to be low temperature injecting and have very good performance e.g. some varieties of water emulsified waxes.
4). Surface finish could be achieved ideal when injecting liquid wax but with proper injection conditions, there should be no problems in paste injection as well. Particle size of fillers does affect surface finish. Mould release agents used may also play a role in spoiling the finish.
5). Wax injection speed or the ‘flow setting’ must be considered an important parameter, as besides ensuring the complete filling, will take care of venting of air as the wax flows.
6. DIE DESIGN FACTORS
In an effort to improve the ultimate casting yield, sometimes ‘gating’ size is reduced to a point when the wax flow is difficult to maintain for filling of die. In this case, the patterns apparently looking acceptable may be undersized or in extreme case with minor unnoticeable defects.
Depending on the volume and intricacies of wax to be filled, dies need to have jacket cooling provisions and may even need chilled water circulation. But excessive efforts to speed up cooling results in surface checking of patterns and also cracking after injection.
Pattern quality, even from a good wax, is a complex function of injection conditions, wax conditioning, die design considerations, and most important – dedicated shop floor practice to maintain all this without any lapses.
1. BICTA, ‘Acceptance Tests for Wax Pattern Materials’.
2. Investment Casting Institute, ‘Applied Wax Technology’.