Processing of castings using burnt wax models of components 

Why the processing of castings using burnt wax models of components determines the reliability of the assembly

Processing of wax casting of components is not just the final stage of production, but a critical stage on which the geometric accuracy and fatigue strength of the finished part depends by 80%. In our practice, we have repeatedly encountered a situation where customers received ideal castings directly from the furnace, but after machining, the defect rate reached 15-20% due to incorrect basing or overheating of the metal when removing the sprues. Many suppliers hide this fact by selling “raw” castings at a low price, shifting the risks of modification to the buyer. Our goal is to reveal the real technology of post-processing so that you understand what you are paying for and how to avoid hidden defects that will only appear under load.

We have been working in this field for over 14 years and have seen how attempts to save money in the finishing stage have led to the failure of turbine blades and high pressure valves in the first months of operation. If you're purchasing components for critical components, you need to understand the difference between simple grinding and full engineering processing, including heat treating and precision milling. This article was written by process engineers who daily solve problems of bringing roughness parameters to Ra 0.8 and below, maintaining IT7-IT8 tolerances.

Our experience is based on the company's workWuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd.. We specialize not only in casting, but also in the full cycle of creating complex heat transfer and power equipment: from titanium shell-and-tube heat exchangers and ASME standard high-pressure apparatus to corrugated tube bundles made of 316 stainless steel and C46400 alloys. It is the requirements for corrosion resistance and work under high pressure in oil refining, the chemical industry and shipbuilding that dictate the most stringent quality standards to us. When we produce C70600 alloy tubesheets or N06625 nickel components, the cost of error is too high, so our approach to casting post-processing is shaped by actual field conditions rather than theoretical tolerances.

Technological nuances of removing the gating system and primary cleaning

The first and roughest stage of processing casting using burnt wax models of components begins immediately after the mold has cooled. It is a mistake to believe that sprues can simply be beaten with a hammer or cut off with a grinder without consequences for the structure of the metal. At the connection point between the sprue and the part, a heat-affected zone and a possible microcrack are formed, which becomes a source of corrosion or fatigue failure. We use waterjet cutting or high-speed CNC circular saws to separate parts from the wood, which ensures that thin-walled parts will not warp.

After separation, the sandblasting stage follows. It is important not to overdo it here: air pressure above 0.6 MPa can change the geometry of thin stiffeners, especially on parts made of aluminum alloys such as AK7ch or magnesium casting. In our workshop we strictly control the abrasive fraction: for rough grinding we use electrocorundum with a grain size of 0.4-0.6 mm, and for finishing surface preparation before testing we use glass microspheres with a diameter of 0.1-0.2 mm. This allows residual molding sand to be removed from complex internal cavities without damaging the base metal.

One of our clients faced a serious problem: a batch of sandblasted pump casings showed leaks during hydraulic tests. The investigation revealed that the contractor used an abrasive that was too coarse, which “clogged” the micropores of the surface layer, creating a false sealing effect that disappeared after the first thermal cycle. To avoid this, we are implementing a two-stage cleaning system with mandatory visual inspection under x10 magnification after each stage. Remember: high-quality cleaning is not about shine, but about the cleanliness of the surface from inclusions.

Geometry control after rough machining

Immediately after removing the sprues, the part undergoes initial measurement on a coordinate measuring machine (CMM). At this stage, we check the compliance of the basic dimensions with the drawing, taking into account the shrinkage of the material. For different alloys, the shrinkage coefficient varies: for stainless steel 12Х18Н10Т it is about 1.8-2.0%, and for aluminum alloys - up to 1.3%. If these parameters are not taken into account when designing tooling, subsequent machining may reveal leaks or leave an allowance of less than 0.5 mm, which will make the part defective.

It is important to note that the GOST 26645-85 standard regulates tolerances for unprocessed surfaces, but for critical components we are guided by internal standards that tighten the requirements by 1.5 times. For example, the flatness of the seats should not exceed 0.1 mm per 100 mm of length even before milling begins. This allows us to guarantee the stability of the part's location in the machine at the following stages. Ignoring this rule often leads to the fact that the part “leads” when clamped in the chuck, and the final size goes into minus.

Mechanical processing and achieving the required roughness

The main value of the lost-wax casting technology is the ability to obtain a part of complex shape with minimal allowance for machining. However, the term “minimal” does not mean “absent.” Processing of castings using burnt wax models of components necessarily includes milling of landing planes, boring of holes and cutting of threads. The allowance is usually from 0.3 to 1.5 mm, depending on the dimensions of the casting. This layer must be removed strategically so as not to compromise the integrity of the surface layer, which often has increased hardness due to rapid cooling in the mold.

We use TiAlN-coated carbide tools for machining high-temperature alloys, as they maintain cutting properties at temperatures up to 900°C. For aluminum alloys, we use polished cutters with a large number of teeth to prevent metal sticking. The spindle feed and rotation speed is selected individually for each alloy grade. For example, when processing VT6 titanium, the cutting speed should not exceed 40-50 m/min, otherwise there is a risk of material tempering and loss of strength.

Particular attention is paid to obtaining the required roughness class. For sealing surfaces of hydraulic equipment, Ra 0.4-0.8 µm is required. It is impossible to achieve such an indicator in one pass. We use the following scheme: rough milling → semi-finish milling → grinding → lapping. Using only milling often leaves traces of vibration (risk), which become pathways for leaks of working fluid. In our practice, there was a case when a batch of distributors failed after a week of operation precisely because of micro-risks on the spool mirror left by an incorrectly sharpened tool.

Problems of processing hard-to-reach areas

The most difficult part of the process is processing the internal channels and curved surfaces that were formed by the ceramic core in the wax model. Often, after the rod is burned out, ceramic residues remain inside, which are extremely difficult to remove mechanically. We use specialized diamond tip flexible shafts and chemical etching systems to clean these areas. Simply rinsing the part with water is not enough - silicate residues can cause abrasive wear of the moving parts of the mechanism in the future.

To control the quality of internal processing, we use endoscopy with image display on a monitor. This allows the operator to see the surface condition in real time and adjust the tool entry angle. Standard visual inspection does not work here, since the human eye cannot penetrate channels with a diameter of less than 4 mm. If your supplier can't show you a video endoscopy of the inside of your part, that's a red flag: there's likely some molding sand left in there.

Heat treatment and stabilization of metal properties

Investment casting often creates residual stresses in the metal due to uneven cooling of thick and thin sections. Without proper heat treatment, the part may become deformed (“failure”) during operation or even during storage. Processing of castings using burnt wax models of components necessarily includes a cycle of annealing, hardening and aging, depending on the grade of the alloy. For stainless steels this is the solution of carbides and stress relief at 1050-1100°C, followed by rapid quenching in water or oil.

For aluminum alloys, artificial aging is critical. Subheating the oven by just 10-15 degrees or reducing the holding time by 30 minutes can reduce the yield strength of the material by 20-25%. We use ovens with forced convection and temperature accuracy of ±2°C. Each batch is accompanied by a protocol of temperature conditions, which we keep in the archive for 10 years. This is a requirement of the ISO 9001 standard, but for us it is also a matter of reputation: we know what will happen to the metal after 5 years, because we controlled every degree of its heating.

An important nuance that many competitors are silent about: after heat treatment, the surface of the part is oxidized and covered with scale. This requires repeated light sandblasting or acid etching before final inspection. Skipping this step will cause the measuring tool to slide across the scale, giving false dimensional readings. We include the operation of descaling in a mandatory technological route, even if this increases the cost by 3-5%.

Microstructure and hardness control

The final step of the thermal cycle is to check the hardness and microstructure. We carry out Rockwell (HRC) or Vickers (HV) hardness tests at three points on each sample part. The spread of values should not exceed 2-3 units. If in one zone the hardness is 45 HRC, and in another 38 HRC, this is a sign of a violation of the hardening regime or heterogeneity of the chemical composition of the charge. We send such parts for remelting without the risk of transferring them to the client.

Metallographic analysis of a thin section under a microscope allows you to see the grain size. Large grains indicate overheating of the metal during melting or holding it in the furnace for too long, which reduces impact strength. Fine grain is a sign of correct technology. According to the ASTM E112 standard, for critical parts the grain size should be no worse than 7-8 points. We provide photo reports of the microstructure upon request, as this is the only way to prove the real quality of the metal, and not just a paper certificate.

Comparison of finishing methods: sand, shot or polishing

The choice of surface finishing method directly affects the performance characteristics of the part and its cost. Below is a comparative table of the main methods we use for processing burnt wax castings of components.

The table shows that there is no universal solution. For hydraulic spools, we choose electrochemical polishing as it removes micro-roughness without mechanical stress that could lubricate the profile. For turbine blades operating at high speeds, shot blasting is required to create a protective layer of compressive stresses that prevents crack growth. An attempt to save money and use a vibrating tumbler for a sealing pair will lead to instant leakage, and manual polishing of a shot-blasted part can destroy the entire hardening effect.

In our practice, there was a case with a customer from the oil and gas industry, who insisted on mirror polishing of all valve surfaces for “beauty.” We explained that on work surfaces in contact with an abrasive medium (sand in oil), a mirror surface will fail faster due to the setting effect than a matte surface with a certain texture. In the end, we found a compromise: the working pairs received a satin finish of Ra 0.8, and the outer cases were polished. This saved the client 15% of the budget without loss of functionality.

Quality control and certification according to international standards

Quality assurance in investment casting production is based not on luck, but on a multi-stage control system. Processing of castings using burnt wax models of components is completed only after passing all the tests provided for in the technical specifications and applicable standards (GOST, ISO, ASTM, EN). We understand that for the Russian market it is critically important to have GOST certificates of conformity and quality passports, and for export to Europe - reports according to EN 10204 3.1.

As a manufacturer of equipment that operates in the extreme environments of seawater desalination and petrochemicals, we take particular care in material selection and control. Our products, including C46400 brass and copper-nickel tube sheets, are designed to withstand harsh environments for decades. Therefore, non-destructive testing (NDT) is a mandatory step for all our critical parts. We use three main methods: penetrant testing (color flaw detection) to detect surface cracks, ultrasonic testing (UT) to look for internal cavities and radiographic testing to analyze the structure of welds or complex castings. The sensitivity of penetrant testing allows you to detect cracks up to 1 micron wide. If the supplier says that “everything is visually clean,” run away from him: 90% of casting defects are invisible to the naked eye.

Each batch is accompanied by a full package of documents: chemical analysis with a spectrometer (OES), mechanical tests of witness samples (tensile, impact), NDT results and a dimensional report. We store witness samples from each heat for 5 years. This allows, in the event of a complaint, to conduct an independent examination and accurately determine the cause of the failure: whether it was an error in the casting technology, a violation of the heat treatment regime, or improper operation on the part of the customer.

Typical mistakes when accepting castings

When accepting products, customers often make mistakes, which later become costly. The first mistake is selective control instead of complete or statistically based control. By checking one part out of a hundred, you can miss systemic defects associated, for example, with wear of the mold of wax models. The second mistake is ignoring packaging requirements. Parts with high processing precision (IT7) must be transported in individual containers with anti-corrosion protection. Shipping loose in a wooden crate will turn your precision shafts into a collection of scratches.

The third mistake is the inconsistency of the measurement technique. Measuring a part that is heated after processing or lying on an uneven table will cause an error. The temperature of the part during control should be 20±2°C (standard laboratory temperature). We equipped our quality control department with a climate control system to eliminate the influence of seasonal temperature changes on the measurement results. If you receive a part in an unheated warehouse in winter, remember: when heated to room temperature, its dimensions will change and it may not fit into the assembly.

Economic efficiency and production time

Many people consider lost wax casting to be an expensive proposition. This is only true for simple parts that are cheaper to make on a CNC machine from rolled stock. However, for complex spatial shapes, especially from difficult-to-process alloys (titanium, heat-resistant steels, cobalt-chrome), this technology is economically unbeatable. Обработка литья по выжигаемым восковым моделям комплектующих позволяет сократить расход металла в 2-3 раза по сравнению с механической обработкой из поковок или плит, так как отливка максимально приближена к чистовой форме.

Сроки изготовления складываются из времени на создание мастер-моделей (7-10 дней), изготовление рабочих пресс-форм для воска (10-14 дней) и самого цикла литья и обработки (15-20 дней). Таким образом, первый образец вы получаете через 35-45 дней. Для серийного производства цикл сокращается до 2-3 недель, так как формы уже готовы. Мы рекомендуем планировать закупки с учетом этих сроков, чтобы не останавливать конвейер. Экспресс-заказы возможны, но стоят на 30-50% дороже из-за необходимости перестройки графиков печей и станков.

В долгосрочной перспективе стоимость владения такой деталью ниже. Высокая точность и однородность структуры металла означают меньший процент брака при сборке узла и больший ресурс работы. Один наш клиент, производитель компрессоров, подсчитал, что переход на наши отливки с полной обработкой позволил ему снизить гарантийные расходы на 40% за первый год, несмотря на то, что цена закупки детали выросла на 15%. Качество, заложенное в металл на этапе литья и обработки, окупается надежностью.

Frequently Asked Questions

Какой минимальный тираж выгоден для заказа обработки литья по выжигаемым восковым моделям комплектующих?

Economic feasibility occurs in batches of 50 pieces. При меньшем количестве стоимость разработки оснастки (мастер-модели и пресс-формы) ложится тяжелым бременем на цену единицы продукции. Однако для опытных образцов или уникальных ремонтных деталей мы предлагаем технологию 3D-печати восковых моделей, которая позволяет делать партии от 1 штуки без дорогой металлической оснастки, хотя цена за штуку будет выше. Для серий от 1000 штук цена снижается на 20-30% за счет амортизации форм и оптимизации загрузки печей.

Можно ли получить деталь сразу с качеством поверхности Ra 0.4 без механической обработки?

Нет, это физически невозможно при данной технологии. Керамическая форма сама по себе имеет шероховатость, которая отпечатывается на металле. Даже самые лучшие формовочные смеси дают поверхность не лучше Ra 3.2-6.3. Кроме того, наличие литниковой системы требует ее удаления, что неизбежно оставляет следы. Достижение Ra 0.4 возможно только путем последующего шлифования и полировки. Заявления некоторых поставщиков о «готовой поверхности» обычно означают лишь хорошую зачистку, но не прецизионную гладкость.

Какие сплавы вы обрабатываете чаще всего и есть ли ограничения?

Мы работаем со всем спектром литейных сплавов: углеродистые и легированные стали, нержавейка (304, 316, 12Х18Н10Т), алюминий (АК7ч, AlSi10Mg), титан (ВТ6, Ti6Al4V), медные сплавы и суперсплавы на основе никеля (Inconel 718). Ограничения касаются только тугоплавких металлов вроде вольфрама или молибдена, где требуются специальные условия плавки в вакууме, что доступно не на каждом заводе. Также сложно лить очень крупные детали весом свыше 50 кг методом ЛВМ из-за рисков деформации керамической формы под весом металла.

Как вы гарантируете отсутствие внутренних раковин в ответственных деталях?

Гарантия обеспечивается комбинацией правильного литникового моделирования (мы используем программное обеспечение MagmaSoft для прогноза заполнения формы) и обязательным рентгеновским контролем каждой критической зоны. Параметры плавки (температура, вакуумирование) фиксируются автоматически. Если в партии выявляется даже одна деталь с раковиной недопустимого размера (согласно стандарту ASTM E186), вся партия подвергается 100% контролю или бракуется. Мы не полагаемся на «авось», так как цена отказа такой детали может исчисляться миллионами.

Conclusion and next steps

Обработка литья по выжигаемым восковым моделям комплектующих — это сложный симбиоз искусства литейщика и точности инженера-механика. Только полный контроль над всем циклом, от воска до полировки, позволяет получить изделие, которое будет работать десятилетиями без нареканий. CompanyWuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd.приглашает вас не просто купить деталь, а внедрить надежное решение в ваше производство. Мы объединяем глубокие знания в металлургии титана, никеля и специальных сплавов с передовыми технологиями обработки, чтобы предоставлять индивидуальные решения для глобальных заказчиков в энергетике и нефтехимии.

Если у вас есть чертежи или 3D-модели, отправьте их нам для бесплатного аудита технологичности. Мы подскажем, где можно оптимизировать конструкцию для снижения цены без потери прочности, и рассчитаем точную стоимость партии с учетом всех этапов обработки. Не рискуйте надежностью вашего оборудования — доверьте производство профессионалам с 15-летним опытом и сертифицированным производством по стандартам PED и ASME.

Contact us today, чтобы обсудить ваш проект и получить коммерческое предложение в течение 24 часов. Мы готовы стать вашим стратегическим партнером в мире высокоточного литья и производства сложного оборудования.

Узнайте больше о наших возможностях в разделеполный цикл литья по выплавляемым моделямили изучитекаталог обрабатываемых сплавов.