What is investment casting?

What materials can be cast with investment casting?

Investment casting, which is sometimes referred to as lost wax casting, is a way of producing near-net-shape (NNS) components in metals. Almost any metal or alloy can be cast, including aluminium, magnesium alloy, brass, bronze and stainless steel. In addition, the process is widely used by jewellery designers for casting gold, silver, platinum and palladium.

What are the steps in the investment casting process?

Investment casting is today used to manufacture state-of-the-art components but it is actually one of the oldest manufacturing technologies, dating back around 5000 years. Beeswax is no longer the material of choice for patterns but otherwise the process is not too far removed from that used by the ancient Egyptians and Greeks.

The starting point can vary but is typically a 3D CAD model, an existing component or a hand-carved object. If there is a 3D CAD model, this can be converted to an STL file so a pattern can be 3D printed by SLA (stereolithography).

On the other hand, if a physical component or hand-carved object is 3D scanned, the point cloud can be manipulated to create an STL file for 3D printing.

Alternatively, a silicone mould can be taken from a physical item, then this mould can be used to produce wax patterns.

If more than one casting is required, multiple patterns can be 3D printed or made from one mould, then assembled together on a sprue in a tree-like structure so they can all be cast at the same time.

Note that wax patterns can easily be hand-finished to remove parting lines or other surface imperfections, so the cast parts need only minimal post-processing.

Once the patterns are ready, they are coated in refractory materials, layer by layer. Afterwards, the resultant mould is heated so the wax patterns melt. The molten wax is poured out and is usually collected for reuse. Note that if the pattern is 3D printed using the QuickCast technique, it has to be burnt out because this material does not melt like wax.

The next step is to fire the mould to burn off any remaining residues from the pattern(s), drive out moisture and harden the refractory materials.

With the mould now ready, it is preheated and the molten metal poured in to fill the cavities.

When the metal solidifies, the parts and sprues are demoulded, the parts separated from the sprues and any post-processing carried out.

How are investment casting and 3D printing used together?

Here we are talking about 3D printing of patterns for investment casting, not metal 3D printing of near-net-shape parts. 3D printing using SLA is an excellent way to produce patterns, whether the starting point is a designer’s 3D CAD model or a point cloud generated by scanning a physical part.

The QuickCast SLA process uses Accura ClearVue to 3D print investment casting patterns that have a lightweight, hollow structure. Not only does this minimise the material requirements but, more importantly, it allows the pattern to collapse in on itself when heated, so it does not crack the mould during the burn-out phase.

Compared with using wax patterns, the QuickCast 3D printing technique offers better accuracy. This is because two sets of shrinkage allowance have to be applied for wax patterns, one for the pattern and the other for the metal casting. With QuickCast, just the metal shrinkage has to be considered.

Another advantage of 3D printing patterns using QuickCast, particularly for prototyping, is that multiple variants, or different design iterations, can be 3D printed quickly and cost-effectively to reduce the development time and cost.

What are the advantages of investment casting?

Investment casting is a versatile way of producing near-net-shape metal parts in almost any size. Coupled with 3D printing of patterns, it can be very useful for prototyping and the production of end-use parts.

Almost any metal or alloy can be cast, which is an advantage compared with, say, laser sintering of metallic powders, which has a limited choice of materials.

With investment casting, the moulds are made of refractory materials, which can resist the extremely high temperatures encountered when casting steel or stainless steel. The high melting point of steel and stainless steel means that other near-net-shape production technologies – such as pressure die casting – can be unsuitable.

Accuracy and surface finish are excellent, and parts can have finely detailed features and sharp edges – provided the wax pattern is produced to the required standard. Through-holes, alphanumeric text, logos, QR codes and similar features can all be cast.

Another benefit of investment cast parts is that they are isotropic and non-porous.

Compared with other casting techniques and injection moulding, there is no need for draft. It is not necessary to consider where parting lines will be or in which direction the mould will open, as the refractory mould is destroyed and removed in pieces. This also means that undercuts can be cast.

Finally, as investment casting is a near-net-shape process, it is very efficient in terms of material usage and time saved by minimising post-processing and secondary operations.

What are the limitations or disadvantages of investment casting?

Investment casting is a labour-intensive process, so it can be costly. For larger batch sizes, other manufacturing technologies may be more cost-effective.

Because of the specialist nature of investment casting, it is not a technology that is easy to bring in-house.

As we have seen above, investment casting is excellent for casting parts with complex geometries. However, if cores are required, the process may not be suitable.

Although accuracy is very good, it is not as good as CNC machining. Furthermore, as with any manufacturing technology involving high temperatures, a shrinkage allowance has to be applied when making the pattern. There are also risks associated with the high temperatures. First, parts can distort on cooling due to residual stresses and, second, if the part is constrained by the mould as it cools, the part can crack when it is prevented from distorting.

Another risk is that cold shuts can occur if the part geometry is such that two fronts of molten metal meet while the mould is being filled. If cold shuts occur, the metal does not fuse properly and the part will have an inherent weakness.

Although parts are not generally porous, the nature of pouring liquid metal into a mould means that it is possible for air to be trapped, depending on the geometry.

What applications is investment casting good for?

If a part is required to be made of metal and it would be difficult to CNC machine, then investment casting could be a good option.

Typically, investment casting is used in industries including automotive, aerospace, defence, marine, energy, oil and gas, medical and dental, jewellery, sports equipment, and specialised machinery such as for the food, beverage and pharmaceutical industries.

As we have seen above, investment casting is suitable for metals and alloys with high melting points, including steel and stainless steel.

Investment casting can be an excellent stop-gap when ramping up production, prior to die casting moulds being ready.

What applications is investment casting not as good for?

High-volume parts may be more cost-effective if manufactured using other technologies such as pressure die casting. Also, parts requiring cores are difficult to investment cast, and large parts can be unsuitable for investment casting. Finally, some geometries cannot be investment cast, such as long, thin cylindrical forms, or very thin-walled parts.

When is investment casting better than metal 3D printing?

In general, investment casting produces parts with tighter tolerances and a better surface finish than metal 3D printing.

If multiple parts are required, investment casting is likely to prove more cost-effective, particularly if sufficient patterns can be assembled on a sprue and cast as a single batch.

For parts that needs to be manufactured from a metal or alloy that is not available as a feedstock for 3D printing, then investment casting might be the only feasible manufacturing technology.

There are several alternative technologies for metal 3D printing but some of these produce parts that are slightly porous. Consequently, investment cast parts might have superior mechanical properties, or be better for applications where cleanliness and hygiene are paramount.

For medium-sized parts with relatively simple geometry, casting can be more cost-effective than 3D printing. However, bear in mind that cost may not be the primary concern, particularly for prototyping.

When is metal 3D printing better than investment casting?

If timescales are very tight, metal 3D printing will be quicker than investment casting, provided the technology is suitable in other respects.

For a one-off part or very small quantities, metal 3D printing might be more cost-effective. However, it depends on factors such as the part geometry, so quotes would have to be requested for both technologies to be sure.

If the part has very complex geometry, such that it would be difficult for molten metal to fill the mould fully, then metal 3D printing is likely to be better.

When is investment casting better than CNC machining?

For parts with very complex geometry, such as pump impellers, investment casting tends to be the preferred option. Investment casting is also more suitable if parts have internal features or passageways that would be difficult or impossible to machine.

Investment casting and CNC machining can have similar part costs but it depends on volumes. As batch sizes increase, investment casting can become more cost-effective.

Remember that investment casting and CNC machining are not necessarily exclusive processes. For example, a part might be investment cast then CNC machining might be used to add tightly toleranced features or threaded holes, for example.

When is CNC machining better than investment casting?

If a part has relatively simple geometry, it is likely that CNC machining will be cheaper. However, as volumes increase, near-net-shape technologies such as investment casting can become more cost-effective.

If time is of the essence and the part can be CNC machined, this will be quicker than investment casting.

When high precision is necessary, CNC machining is generally better than investment casting. Note that we quote a general tolerance of ±0.1 mm for CNC machined parts but the technology is inherently accurate and repeatable, so the tolerances achieved are usually much tighter.

Talk to us

As we said at the start, while we don’t offer investment casting, we do work with several foundries that can support such projects. Please contact us on 01763 249760 for more information.