When we quote MFG RFQ, clients always ask,what if we choose cast molding , what’s the cost different than CNC Machining ?
To answer this question, we need to compare those 2 from beginning ;Following this brief introduction to the key features of machining and casting techniques, a more in-depth exploration of their differing costs, lead times, quality and other factors will be detailed below.
Cost
When deciding between the two methods, cost is typically a key factor for consideration. Variations in cost can be influenced by production volume, tool costs, material savings and the size of the part.
Production volume is one of the simplest factors to define. On the whole, if the production run for the life of the part is fewer than 100 parts, machining is a suitable option, while if the lifetime volume is more than 100 parts, casting makes more sense.
In general, CNC machining is less costly for low volume runs and prototyping, but eventually, a crossover point is reached where molding is more cost-effective for larger production volumes.
For cast molding, the cost of the tool is lower than what is often expected. Depending on the volume, the casting tool can be formed from aluminum, filled epoxy, silicon, or even high-density foam.
To compare, metallic tools have the highest costs, but are most likely to last for the full lifetime of the program, while composite tools cost only around one third of the price of metallic tools, but may need repair over time.
Material savings have an impact on the cost of the processing method. Subtractive manufacturing, such as machining, consists of producing a product by removing, cutting, and drilling away excess material to yield the preferred shape. This process does, of course, result in the production of excess waste. This waste is raw material that is included in your costs and must now be discarded.
Alternatively, formative manufacturing employs methods such as casting, injection molding, and stamping to produce or cast materials directly into the shape of the final product. With this method minimal excess waste is produced, thus, depending on the size of complexity of the part produced, there may be substantial material savings.
Subtractive manufacturing, like machining, involves cutting out the desired component from a block of material. Whereas formative manufacturing, like molding, involves forming the part with very little material waste.
Lastly, the size of the component has an impact on the final decision between machining versus casting. For out-sized components, in which the casting tool would be even greater in size, it may be less costly to opt for machining rather than molding the part.
Nonetheless, outside circumstances can have an impact on this. Recently, General Plastics worked alongside a company in the oil and gas industry who were in need of support to produce large flotation modules for subsea pipe supports.
It was assumed that it would be necessary to machine the floatation modules from large foam blocks, as the cost of producing a tool of that size for casting or molding would be prohibitive.
However, following a number of calculations, it was found that the expenses and time factors linked with machining were in fact, not appropriate and as such, a large molded part would be more suitable for the client’s needs. There were no other known instances of anyone creating a custom casting of polyurethane foam of that size, so this was considered a unique case.
By switching from machining to casting, the project accomplished substantial savings in both cost and materials. The key point to bear in mind is that each application is different, and thus consulting with a materials specialist such as General Plastics at an early point is highly beneficial.
Lead Times
From the initial customer communication to the first part delivered, CNC-supplied components offer shorter lead times than molded parts. On the other hand, if the tool has already been produced and the order is simply a delivery of additional parts, the molded components will be must quicker.
To offer an example of timelines for tool production, the quickest and least complex tools produced from rigid foam may take around 40 to 120 hours to produce, from design to first pour. Conversely, more complex tools will take upwards of 200 hours to create. Further time should be factored in for more complex designs.
There are some differences between the finished parts created by machining and molding which are worth noting. Since it is tricky to produce detailed textures with machining, components produced in this way will have a smooth, uniform surface. As a result of the machined part’s cellular foam surface, there may be a need for additional sealing and painting.
In contrast, since a molded part comes away from the tool with a smooth or textured skin that is already sealed, such parts have a superior finish. It is even possible to pre-apply paint inside the mold. For post-processing, all that is required is to wipe off the mold release and get rid of the parting line, and the product is fully ready.
If an application requires a product to be both durable and waterproof, casting is likely to be the most suitable option.
Each process has advantages and disadvantages when it comes to tolerances. On the whole, machining is able to attain greater tolerances than molding, but there is also higher variability within both parts and lots compared to molding.
On many occasions in machining, a program is set up on a machine and then removed to allow the machine to be employed in a different project. When the program needs to be re-loaded, the positioning may differ slightly, which results in minor part consistency issues between lots (although all within acceptable tolerances).
In contrast, with casting, tools are designed to suffer minimal wear, so molded parts are almost identical between batches.