Rapid prototyping is a rather mysterious field, known only to a few designers or experts – at least that's the case here in China.
As a manager of rapid prototyping projects, I am often asked: “What is prototyping? As many may know, a prototype can be interpreted as the first product of a new design or as an adapted sample of a new design.”
Are processing costs always high?
Industrial designers often make prototypes to see what the design looks like in reality: does the design have flaws? How does this design fit in with other pieces? What is it like in reality? Are the materials and dimensions correct? A well-designed part often needs to be changed and tested many times before it can be released to the market as a fully developed product.
Therefore, it is common knowledge that a prototype must be created before spending a lot of money on parts that have major flaws and defects. It's about saving money. As prototype manufacturers, we are always looking for ways to save processing costs for our customers. In this article I will talk about one of the many ways to save on machining costs when producing prototypes.
Photo 1.1: CNC milled surface from PMMA VS Non-CNC milled surface from PMMA
Tips to Save CNC Prototype Machining Costs
This economical method does not involve machining (for lack of a better word) the surfaces of the part. We can usually use this method on plate-like structures and there are often no strict tolerances for height or thickness.
Figure 1.2: Original material of the PMMA block (photo taken from the internet)
Case Study: How to Save CNC Prototype Machining Costs
This is particularly the case when CNC machining transparent prototypes.
When producing transparent prototypes, we generally use CNC machining. We have other processes for producing transparent parts, such as vacuum casting (or urethane casting) and 3D printing. Both methods have their advantages and disadvantages, which I will not go into detail in this article.
To produce transparent prototypes, material selection is limited to polycarbonate or acrylic (PMMA) and the process is limited to CNC machining. Of course, injection molding can produce high-quality transparent parts, but whether it is an aluminum or steel mold, the initial cost would be extremely high, sometimes costing millions of dollars. The processing time would also be quite long. Therefore, it is not advisable to produce some prototypes using injection molding.
CNC machining is a subtractive machining process where we take a block of original material and chisel out the unwanted material until we obtain the desired part.
The original PC and PMMA block material is a standard material with a certain wall thickness or height, and these original transparent materials are themselves transparent (see photo 1.2). After processing the surface of this transparent plastic, the entire part looks quite blurry and rough (see photo 1.1). To make it clear, manual sanding and polishing operations must be carried out.
It is known that the cost of machining a part is quite high when manual labor is used in the machining process. Even in China, where manual labor is not as expensive as in the EU and USA, the cost of manual polishing is understandably high, as it is quite time-consuming and laborious work.
For example, if a machined PMMA part needs to be polished until clear (surface roughness Ra 0.05-1μm), our team will first sand with coarse 320 grit sandpaper and then gradually switch to 1000 grit sandpaper for polishing The piece.
If a machined PMMA part needs to be visually polished (surface roughness Ra 0.025-0.05 μm, see photo 1.3), our employees first sand with coarse 320 grit sandpaper, then gradually switch to 2000 grit sandpaper for fine sanding and then polish the piece with the polishing paste until it is optically transparent (maximum transparency). This is a long and laborious process. It usually takes a skilled worker several days to complete a part.
Even if the original PMMA material has some scratches or imperfections, polishing it would be much easier than polishing the machined surface.
Therefore, we always consider the thickness or (height) when evaluating each project. When faced with projects close to standard original material, we recommend not machining the surface to save machining costs (sometimes a significant amount) if the thickness (height) of the part does not have a strict tolerance or the thickness is not that important.
This also applies to metallurgy. Using standard thick material saves machining time and costs. However, it generally doesn't save as much money as clear plastic. Because we usually only grind the metals a little or send processed metals to our customers as needed.
Photo 1.3: CNC milled and optically polished (the “teeth” area is not polished) PMMA part
Summary
In short, if you need to produce clear prototypes and there is a standard material whose height or thickness is close to your part, choosing not to machine the surfaces can be a good way to save machining costs.
