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How to CNC machine acrylic for a bottle model

Perfume bottle presentation models that look like glass

One of the specialities of design bureau Sinot Design Associates in Eemnes, the Netherlands, is the design of new perfume bottles. The bottles are designed using Alias CAD software, capable of calculating superb presentation renderings. However, in order to present a new design to the customer for evaluation, a physical prototype is needed as well. SDA's model shop uses DeskProto and three desktop CNC milling machines to produce these models.

Milling perspex
Perfume bottle
Perfume bottle
Machining transparent perspex, and two presentation models: 'Van Gils' and 'European'.

These perfect presentation models have been machined from two sides in transparent perspex (acrylic), after that polished to high-gloss, and after that furnished with lettering and cap. Presentation models of glass objects cannot be manufactured using any layer oriented RP technology.

Other DeskProto users with famous brand names in this application field are:
Avon Cosmetics in New York,
Lancome in Paris.

Milling acrylic needs special conditions in order to achieve a good result.
Below you can find a few tips for machining acrylic, by Delft Spline Systems.

Milling in acrylic
Two bottle models in acrylic
Milling acrylic with coolant air, and the result with and without air.

We have machined two models of the DeskProto example perfume bottle (see lesson 2 of the Tutorial). The material used is a block of cast acrylic. The first model was machined in a standard way, the second model with special 'acryl-care'. See the picture shown above for the difference: the first model was rather rough, with a milk-like surface. The second model was very smooth and already almost transparent: very easy to polish.

Differences in conditions were:
  • STL file of a higher resolution (more triangles).
  • special cutters for acrylic (different grinding angles) from Hufschmied in Germany, versus standard cutters for steel with the first model.
  • climb milling (when meandering you can clearly see a quality difference between the climb and the conventional movements).
  • continuous coolant air (compressed air) during machining: for cooling and for removing the chips. For cooling acrylic plain water is even better, however our desktop machine cannot handle liquid coolants.
Cooling can also be improved via the cutting tool: HSS (High Speed Steel) cutters are better conductors for heat than carbide cutters, which will help to keep the temperature low.
Finally: the shorter the cutter and the shorter the cutting length of the cutter, the less vibrations during machining. This applies to any material. Nevertheless: a long cutter may be needed in order to reach the complete part.

We also have a few tips for you about polishing acrylic.
This can be done by hand, using finer and finer sanding and polishing papers with polishing gel. Alternative for the gel is car polish, which works great as well.

A nice alternative for this manual labor is flame polishing: treatment with a flame to melt the material just on the surface only. The result will be glass-like: great for bottle prototypes.
For more information please search on "acrylic flame polishing".