The Best Wax For Investment Casting: A Comprehensive Guide

Investment casting, also known as lost-wax casting, is an ancient metal-forming technique that has been used in various forms for thousands of years. The process involves shaping a wax model, which is then covered in a ceramic shell. The wax is melted and removed, leaving a cavity into which molten metal is poured and hardened. This method can be used to create highly detailed and intricate shapes and is particularly useful for thin-walled applications.

The type of wax used in investment casting is an important consideration. Common waxes include filled and non-filled pattern wax, runner wax, sticky wax, and water-soluble wax. Filled pattern wax, which contains additives such as ceramic or metal powders, is often used for large or complex patterns where strength and rigidity are crucial. Non-filled pattern wax, on the other hand, is a simpler formulation that is widely used for standard investment casting applications where cost and ease of use are priorities. Other types of wax, such as runner wax and sticky wax, have specific properties that make them useful for ensuring proper mould filling and assembling wax patterns, respectively. Ultimately, the choice of wax depends on the specific requirements of the casting process.

The types of wax used for investment casting

Investment casting, also known as lost-wax casting, is an ancient metal-forming technique that has been used in various forms for over 5,000 years. The process involves creating a wax model or pattern, which is then coated in a ceramic or plaster shell. The wax is melted and removed, leaving a cavity into which molten metal is poured to create the final product.

The type of wax used for investment casting depends on the specific application and can vary from natural beeswax to more advanced wax compounds. Here are some of the most common types of wax used in investment casting:

Beeswax

Beeswax was the earliest form of wax used in lost-wax casting and is still used today, particularly by jewellers. It has good carving and finishing properties and burns out well. It is also quite pliable and slightly sticky, making it easy to work with.

Paraffin Wax

Paraffin wax is another option for investment casting, especially for those just starting out, as it is inexpensive and readily available. It can be used on its own or mixed with other types of wax to improve its properties. Paraffin wax is quite soft and sticky, so it can be melted with plastic to make it easier to machine. However, it requires careful handling as it can flash and burn if heated too much.

Microcrystalline Wax

Microcrystalline wax is often preferred for modelling due to its pliability and slightly sticky texture. It has a medium-soft consistency that makes it easy to work with and is commonly used in lost-wax casting.

Filled Pattern Wax

Filled pattern waxes are commonly used for large or complex patterns where strength and rigidity are crucial. They contain additives such as ceramic or metal powders, which enhance mechanical properties and dimensional control. One common additive is bisphenol-A (BPA). These waxes can be more costly and may be trickier to process and recycle than non-filled waxes.

Non-Filled Pattern Wax (Unfilled Wax)

Non-filled pattern wax is a simple wax formulation without any additives. It is widely used for standard investment casting applications where cost and ease of use are priorities. Non-filled waxes are easier to work with, have good flow properties, and can be completely recycled and reused. They offer exceptional surface quality but tend to have lower strength and dimensional stability compared to filled waxes.

Runner Wax (Gating Wax)

Runner wax has excellent flow properties and is crucial for ensuring the proper filling of moulds. It has good adhesion to mould surfaces, ensuring leak-free gating channels. Runner wax also has a lower melting point, which helps it to melt out completely without requiring higher temperatures that could damage the shell.

Sticky Wax

Sticky wax is a tacky wax used for assembling various wax patterns. It is easy to manipulate and shape, making it versatile for different applications. However, it is only suitable for pattern assembly or repair tasks and can be tricky to remove, requiring extra cleaning.

Water-Soluble Wax

Water-soluble wax offers a sustainable alternative to traditional pattern wax materials. It is particularly useful for creating parts with intricate geometries and cavities. The cores created from water-soluble wax can be placed within a pattern wax mould and easily dissolved, leaving behind a clean and residue-free mould.

The process of investment casting

Investment casting, also known as precision casting or lost-wax casting, is a manufacturing process that creates precise, intricate metal parts. The process involves several steps, from creating a wax pattern to the final casting. Here is a detailed overview of the investment casting process:

Creating the Wax Pattern

The first step in investment casting is creating a wax pattern of the desired final product. The wax pattern is made using a metal injection die, resulting in a precise replica of the final part. The wax pattern must be durable to withstand the mould-making process. The type of wax used can vary, including filled pattern wax, non-filled pattern wax, runner wax, sticky wax, and water-soluble wax. Each type of wax offers unique advantages and is selected based on the specific requirements of the project.

Assembling the Wax Patterns

Once the individual wax patterns are created, they are assembled into a tree-like structure. This assembly, known as a "pattern tree," involves attaching multiple wax patterns to a central wax gating system. The patterns are carefully arranged to optimise the casting process. The assembly is then coated with a refractory material, such as silica or ceramic slurry, through a process of dipping and drying. This step is repeated until the desired thickness of the coating is achieved.

Creating the Mould

After the wax assembly is complete, it is time to create the mould. The coated wax assembly is again dipped into a slurry, this time made of a refractory material such as silica or ceramic. This step is crucial as it forms the mould that will define the final product's shape. The assembly is then allowed to dry completely, which can take up to 48 hours. During this time, the coating hardens and strengthens, creating a durable mould.

Melting the Wax

Once the mould is dry, it is placed in a furnace or autoclave to melt and remove the wax. This step requires careful control as the wax expands when heated, which can cause stress on the mould. Therefore, rapid heating is employed to minimise these stresses. The wax is melted and drained out, leaving behind a hollow mould with the desired shape.

Pouring the Metal

With the wax removed, the mould is now ready for the metal casting. Molten metal is carefully poured into the mould, filling the cavity created by the wax pattern. The type of metal used can vary depending on the application, including stainless steel, aluminium, brass, carbon steel, or alloys. The metal is allowed to cool and solidify within the mould.

Removing the Casting

After the metal has solidified, it is time to remove the casting from the mould. The mould is broken or chipped away, revealing the metal casting. This process can be done manually, with water jets, or using techniques like sandblasting. Once the casting is removed, it is separated from the gating system, and any remaining imperfections are addressed.

Finishing the Casting

The final step in the investment casting process is finishing the metal casting. This step involves grinding, polishing, and additional treatments to ensure the final product meets the required specifications. Heat treatments may also be applied to harden the metal casting. The finishing process ensures the casting has the desired surface quality and dimensional accuracy.

Investment casting is a versatile and precise manufacturing process capable of producing intricate metal parts with exceptional surface finishes. By following these steps, manufacturers can create complex shapes with fine details, making it a valuable technique for various industries, including aerospace, medical, and jewellery.

The advantages of investment casting

Investment casting, also known as lost wax casting, is a process that has been used to cast metal for thousands of years. It is a highly versatile method that can be used to create objects with thin walls, intricate details, and complex geometries. Here are the advantages of investment casting:

Superior Surface Finish

Investment casting can produce a much smoother and more precise surface finish, averaging a 125 RA surface finish. This reduces the need for additional surface finishing, making it a good solution for industries that require precise parts, such as the medical industry.

High Accuracy and Tight Tolerances

The process can produce highly accurate parts with tighter tolerances, making it suitable for industries that require precise and high-strength components, such as the transportation, agricultural, and medical industries. The standard tolerance for investment casting is +/-.005 per inch.

Material Diversity

Almost any metal or alloy with varying melting points can be used for investment casting, including steel, aluminium, bronze, cobalt, brass, and more. This versatility allows for a wide range of applications.

Complex Shapes and Size Variation

Investment casting can accommodate various sizes and shapes, from tiny to large models. It is particularly suitable for creating complex shapes and geometries, as it does not require any draft, unlike other casting processes. This makes it ideal for incorporating design features such as logos.

Reduced Machining and Labour Costs

The accuracy and tight tolerances of investment casting reduce the amount of machining required after casting, leading to faster production and lower labour costs. The process is also less manual and dangerous, further reducing labour expenses.

Affordable Tooling

Investment casting requires less expensive equipment, which helps to keep costs low. The process is particularly cost-effective for medium to large-volume production of high-strength parts.

The history of investment casting

Investment casting, also known as lost-wax casting, is an ancient process that has been used for thousands of years. The basic procedure has remained largely unchanged, but modern technology has allowed for advancements in the materials and machines used, resulting in improved surface quality, tighter tolerances, and higher production efficiency.

One notable example of early investment casting is the Late Cycladic (17th century BC) gold ibex sculpture, which was excavated on the island of Santorini. This sculpture, measuring about 10 cm in length, showcases the advanced techniques employed by ancient artisans, with its intricate details captured through the lost-wax process.

In Pre-Columbian Mexico, the Aztec civilisation also utilised the lost-wax process to create elaborate jewellery. Unfortunately, much of their work did not survive, as their masterpieces were melted down by the conquistadors. However, the writings of Friar Bernardino de Sahagun provide valuable insights into the Aztec's methods and their mastery of the process.

Another glimpse into the early history of investment casting is offered by the brass smiths of Benin, Nigeria, who continue to use a method passed down through generations. They begin by shaping clay into the approximate size and shape of the desired article. This core is then covered with beeswax, which is carefully modelled into the exact shape. Once the wax form satisfies the artist, it is coated with layers of clay, with a thin roll of wax added to create a channel for pouring the molten metal. Thicker layers of clay are then added to completely cover and create the mould. After heating the moulds to melt and pour off the wax, they are filled with molten brass and then broken open to reveal the final object, which is cleaned, filed, and polished.

In Europe, following the Dark Ages, the sculptor and goldsmith Benvenuto Cellini (1500-1571) employed the lost-wax method of casting. He learned this process from the writings of the monk Theophilus Presbyter, whose Schedula Diversarum Artium, written around 1100 AD, is the earliest known text describing the investment casting process. In his autobiography, Cellini provides a detailed account of the casting of his famous sculpture, "Perseus with the Head of Medusa", which was completed in 1554 and still stands in Florence, Italy.

While investment casting fell out of favour for a period, it experienced a resurgence during World War II. The demands of the war effort required the quick and precise production of metal parts, and investment casting provided an efficient solution. This process continued to evolve and expand into various commercial and industrial applications in the post-war period, including the automotive, aerospace, and medical industries.

The disadvantages of investment casting

Investment casting, also known as lost-wax casting, is a metal casting method that has been used for thousands of years. Despite its longevity and versatility, the process has several disadvantages.

Firstly, there are size limitations with investment casting. This technique is best suited for small, intricate components, and parts exceeding 75 pounds are better suited to other methods. The size and depth of investment casting shells are also restricted, with minimum and maximum measurements of 1.6mm and 1.5 times the diameter, respectively.

Secondly, the investment casting process is complex and labour-intensive, requiring a significant amount of preparation and specialised equipment. As a result, upfront costs are high, especially for low-volume production runs. The process involves many steps, which means that each casting cycle takes a long time to complete.

Additionally, it is difficult to cast parts that require cores using the investment casting method. Very small holes can be challenging or even impossible to cast.

Finally, due to the nature of the process, cycle times are much longer than with other casting methods.

Frequently asked questions