posted in Process Selection & Design Information
You can achieve a high degree of shape complexity with a sand casting as the mould can be made from multiple pieces to form undercuts, holes, bosses and protrusions. This means intricate components, with internal channels and gateways unachievable by other methods can be produced quite readily.
Tooling costs can be very low and modifications to tools are typically quite simple and cost effective. For this reason alone, sand casting makes a great choice for early production and prototyping whilst the design may still be changed.
Modern sand castings using dry sand (chemically bonded sand) methods can be extremely accurate, and produce surface finishes which in some cases are in the range of 1.6 Ra - although 3.2 Ra and above is more typical.
Limitations, tolerances and process norms
Production volumes: components may be made in volumes of up to 1000-5000 per year economically before die casting becomes a competitive solution. However sand casting can remain a viable production solution at over 1,000,000 units per year for components not suitable for other processes such as die casting.
Typical minimum wall thickness: 3mm for light alloys - closer 5-6mm for steel and other ferrous alloys. This is highly dependent on the exact alloy - so it’s worth talking to the foundry for their recommendations.
Maximum wall thickness: thick sections are possible in excess of 100mm
Draft angles: draft angles of 1-5 Degrees are required
Achievable linear tolerances: +/- 0.4-0.5mm with additional 0.2-0.25mm over mould parting lines and core joints
Typical linear tolerances: +/- 1mm with additional +/- 1mm over mould parting lines and core joints
Flatness: typically 0.1mm per 25mm (excluding localised pitting)
Surface finish: 3.2-25.0 Ra (Micrometers)
Machining allowance: typically 0.5-1.6% or 1.5-6mm
Size Limitations: 20g to many hundreds of tonnes in weight are possible.
Related Tolerance standards
ISO 8062-1:2007 - Geometrical product specifications (GPS) -- Dimensional and geometrical tolerances for moulded parts -- Part 1: Vocabulary
ISO/TS 8062-2:2013 - Geometrical product specifications (GPS) -- Dimensional and geometrical tolerances for moulded parts -- Part 2: Rules
ISO 8062-3:2007 - Geometrical product specifications (GPS) -- Dimensional and geometrical tolerances for moulded parts -- Part 3: General dimensional and geometrical tolerances and machining allowances for castings
How long will Sand Casting tooling last?
Tooling material choice is down to overall production volume anticipated - and likelihood of design change. Polyurethane board can be relatively quick to machine and can easily have dimensional or feature modifications made to it. Tooling life would likely be around 5000 units, but could be significantly higher depending on the complexity of the pattern.
The next logical step from PU board would be aluminium tooling, offering relatively easy machining, whilst being more hard wearing than Polyurethane with tool life typically 50,000-100,000 units.
Which type of sand casting is for you?
Within sand casting there are sub-processes. Green sand casting is the most common as it’s the most cost effective form of sand casting, however tolerances are greatest. dry sand (chemically bonded sand) casting on the other hand produces more accurate castings with a better surface finish - but due to the extra processing of the mould, it’s inevitably more expensive. This cost is all relative to the entire project, as more accurate castings could reduce machining and tolerance allowances, and reduce subsequent operations - thus reducing overall cost.
Mould cores, whether used with green or dry sand moulding are typically made using the dry sand moulding process or a variation of it. The extra stability allows the mould cores to be handled during the mould assembly process.
What are Green Sand and Dry Sand?
In green sand casting the moulds are made of sand bonded with clay and water. The sand is easily recyclable, and collapsable making demoulding and reprocessing cheaper and easier. Dry sand casting forms a stronger mould. This strength is gained by enhanced bonding between the sand grains, this can be through use of heat with a combination of heat curing resin, or gas injection - with chemically reacting resins, or simply air curing resin mixed with the sand. As a result of the resin content typically less of the sand can be reclaimed than with green sand moulding. Demoulding is also more time consuming due to the moulds increased strength.
Dry sand casting covers a range of mould types, all are reliant on a chemical reaction usually the hardening of a resin mixed with the sand. Dry sand casting can also be called air set, boxless, cold-box, gas-set, no-bake, or chemically bonded sand moulding.
Limitations - Green Sand vs Dry Sand Casting
Green sand casting is carried out within a mould box or flask - therefore the size of the component is limited by the flask or box size. The box size is usually a set standard for your foundry - it’s worth asking what the standard size is. A single mould box may contain multiple mould cavities which are poured via the same gating system - therefore the number of components you can fit in a mould box will directly impact your production rate, and cost.
Dry sand casting is not limited by box size, as the mould pattern is made to suit the component being cast - there isn’t a fixed volume in which the pattern needs to fit as with green sand casting.
Green sand casting with it’s lower mould surface strength doesn’t lend itself to long and thin geometries such as fins or deep pockets. This is because the pressure of the molten metal as it enters the mould could be enough to deform or wash away certain features.
Dry sand casting on the other hand has a lot more dimensional stability under the pressure of the molten metal as it enters the mould, meaning finer detail with long and thin sections are possible.
Dye Penetrant testing can be used to check for porosity and cracking on the finished casting, as can magnetic particle inspection on steel castings.
X-Ray examination is commonplace to evaluate castings for internal defects with a permanent record of the examination. Ultrasonic testing can also be used, but doesn’t provide a permanent record. Obvious problems and known defects within particular components can be identified readily by visual inspection.
Related Testing Standards
ISO 4986:2010 Steel Castings - Magnetic Particle Inspection
ISO 4987:2010 Steel Castings - Liquid Penetrant Inspection
ISO 4992-1:2006 Steel castings -- Ultrasonic examination -- Part 1: Steel castings for general purposes
ISO 4992-2:2006 Steel castings -- Ultrasonic examination -- Part 2: Steel castings for highly stressed components
ISO 4993:2009 Steel and iron castings -- Radiographic inspection
ISO 11971:2008 Steel and iron castings -- Visual examination of surface quality
Pressure testing is also commonplace with sand castings - there will inevitably be a level of porosity in the casting as the molten metal relies on gravity alone to fill the mould. With a well designed gate, runner and feeder system this can be minimised - your foundry will usually provide this aspect of design.
Heat treatment and subsequent processing
Apart from the obvious removal of the gates, runners and feeders further post-processing is required once sand mould has been broken away from the casting. Fettling, sanding and grinding removes flash, and any remaining witness marks from gates, feeders and runners.
Castings then typically need a further cleaning process, usually in the form of shot blasting. This gets rid of discolouration and provides an even surface finish across the parts surface. This can also done after any heat treatment that results in surface discolouration.
Both Ferrous and Non-Ferrous castings are routinely heat treated for a number of reasons, including stress relieving increased machinability and increased strength.
Turning your design into a Sand Casting
Once you have a 3D model of your required finished geometry there’s still a fair deal of work required to turn that into castable geometry. The addition of; machining allowance, allowance for tolerancing and shrinkage and the design of the runner, gate and feeder system.
Casting process simulation
There are many pieces of software available for the simulation of the filling and cooling of the mould during the casting process. Areas of porosity, problematic features and anticipated shrinkage can all be analysed to help refine and prepare the geometry for casting.
Your foundry will typically increase dimensions on your component to allow for shrinkage, this can be anywhere between 5-20% depending on material. There is no design requirement for you here, but is worth considering especially when communicating with your foundry.
Gates, Runners and Feeders
Your foundry will take your component geometry and locate the gates, runners and feeding system based initially on their experience, then optimisation takes place with simulation and/or calculation. Your gate and runner system may need to deliver molten metal to more than one mould cavity, this has to be carefully designed to provide consistent fill of each mould cavity.
Drawbacks of Sand Casting
Surface Inclusions and Machining allowance
It’s accepted that within the surface material of the casting inclusions will exist in the form of sand grains. This is due to loose sand being picked up as the molten metal flows through the mould. This needs to be taken into consideration when specifying machining allowance if it is critical that the machined surface be completely free of sand grains.
When metal is molten it occupies a higher volume than when it’s in solid form, as a result - even when the mould cavity is full, it’s not really “full”. This is partially overcome by using feeders to allow the pulling of metal still in liquid form into the mould cavity, but eventually the viscosity of the metal overcomes the pulling force of the shrinkage. This means the metal continues to cool and shrink without material being added, the result is porosity.
Gas bubbles within the molten metal will seek to rise up through the molten metal, therefore porosity is more likely at the top of the sand casting, even when well designed for the process.
Therefore there have to be acceptable levels of porosity within the design. Porosity doesn’t necessarily impact the functionality of a component, especially if limited to surface porosity which can later be machined away. And although a surface may be porous, it’s overall sealing integrity could still be sufficient for the design - this can be established by leak test. However, even if a casting passes a leak test - subsequent machining and handling can cause pores to interconnect - leading to leakage
If on the other hand porosity is a problem, but the design permits, a casting can be resin impregnated. In this process resin is drawn deep into the casting, typically under vacuum, filling the microscopic pores not just at the surface.
Things to bear in mind with casting impregnation
Porosity can also cause problems for components which are never intended to be “leak tight”. Powder coating in particular can be problematic, as the component “outgasses” during the baking or curing of the powder coat. The trapped gasses within the pores simply expand and push their way to the surface, causing poor adhesion of the powder coating.
Sand Casting Design Guidance from a Designer
Design guidance can be fairly general - this is because sand casting is a very versatile process with the ability to create highly complex shapes and internal geometry.
The finer details of design should involve the foundry, but the earlier you can get advice on how best to adapt or tune a design for sand casting the better.
As the process involves the flowing of molten metal, it makes sense that the geometry have smooth flowing contours. This prevents turbulence during mould fill, and a whole host of potential defects that can arise.
However - with increased Radii comes increased localised material thickness, this is also bad and can cause defects when the material cools and shrinks.
Therefore general guidance, for the mould surfaces, and any cores is to provide smooth corners with a Radii which is appropriate for the thickness of material being moulded.
Typical Internal Radii = 1 - 1.25 x wall thickness
Typical External Radii = Int. Radii + Wall thickness
It goes therefore without saying that an internal radius should be accompanied with an external one to prevent a localised thick section in the corner of the casting.
When different thickness walls are joined at a corner, the radius may be calculated from the thinnest wall section - but be prepared to fine tune this with your foundry.
Section or “Thickness” Changes
Now you get advised that everything should have a uniform wall thickness - but you also get told to include a draft angle - two seemingly contradictory pieces of advice in some cases. So let’s clear this up.
A uniform wall thickness is impractical in many cases - but the principle here is not to leave thicker sections of the casting isolated when cooling. The thicker section will cool last and if all the metal around it has solidified, as it too solidifies it cannot “feed” from the sections around it - leading to defects such as porosity or tearing.
That doesn’t mean to say isolated sections that are thicker aren’t possible - using chills and or feeders the mould can be designed to overcome the issues - but these are all slight additions to cost. Therefore if it can be avoided, do so.
"design your component so any increases in thickness are all moving in the same direction"
This means if your component has a wall thickness of 3mm at the bottom, it’s fine to increase that as you progress towards the “top” of the component. The material will “feed” from top to bottom and you’ll avoid porosity.
That is a massively oversimplified example - as components are far more complex than the above. So I suggest talking to your foundry, discussing what orientation the sand casting is likely to be within the sand mould - then work at designing the walls to allow proper feeding.
When thickness changes are required for design or manufacturing purposes, they shouldn’t be dramatic, instead use a taper to provide the thickness change.
As a general guide:
"If doubling the wall thickness - Minimum Taper - 1:4"
When it comes to thickness changes, it’s worth discussing with your foundry what’s achievable in the context of your material.
A smart strategy for increasing localised stiffness in a sand casting is to use ribs or webs, these can increase stiffness significantly in comparison with simply increasing wall thickness. Minimum wall thickness and radii rules equally apply to ribs and webs you design in.
Draft angles are provided purely to enable the removal of the sand mould from the pattern. Typical draft should be in the region of 2 degrees - 1 degree is very possible on both internal and external surfaces. But the length or depth of a feature is also key to determining the angle possible - the longer the feature, the steeper the angle required.
If not design critical, a steeper draft angle of 3-5 degrees will help significantly in producing consistent sand moulds.
When using dry sand casting it is possible to have small draft angles over a significant depth, as the moulds are far stronger than with green sand moulding - the exact sand casting process your foundry uses will therefore be a significant factor in the draft angles that are achievable.
Junctions are areas of natural localised thickness - like T-Junction or an X-Junction. Therefore you should take care when designing features which result in these “Junctions”. Typical examples of junctions are internal features, ribs and webs or external protrusions such as fins or brackets.
Less obvious junctions can appear when external and internal geometry, which are functionally unrelated are located on the same part of the casting wall. For example a boss or lug on the outside of the casting occupying the same part of the casting wall as an internal rib.
Ideally you would move these functionally unrelated features away from each other on the wall of the casting - reducing or even eliminating localised thickening.
So be careful and mindful of the hidden junctions that you didn’t even realise you had.
Parting line considerations
Once again your foundry will be best placed to advise on a suitable parting line, as it’s location will have a significant impact on the aspects of design they will take care of.
However, it’s good to be aware of where a parting line may sit even when conceptually designing, as it determines a lot of limitations for your design.
Things to think about when locating a parting line:
Sand Casting is a highly versatile process, and there are many variables that can be optimised in the manufacturing process to achieve desired tolerances, finishes and cost targets. Early understanding of the manufacturing route, as always, will yield a design which is inherently more cost effective - both in piece part price, and product development cost. Hopefully this article has provided you with some guidance on what to look out for during design, and also provides some vocabulary which can significantly aid communication with your foundry.
The UK has many Sand casting foundries - offering both airset and green sand casting (and a mixture of everything in between). If you are seeking a manufacturer for your project, head on over to the Supplier Search Engine - where you can search for your manufacturing solution provider, offering the combination of processes you require to simplify and optimise your supply chain.
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