PMMA Clear Parts

Parts often have interior features. But how can you test their functionality on a physical prototype? The answer here lies in transparent material. Transparent parts with interior Features are used to verify the flow of gas or liquid through the interior surfaces. A lot of materials were created for the sake of creating transparent prototypes. One of them is reviewed in this article. It is Polymethyl methacrylate or better known as acrylic glass.

PMMA Properties 

PMMA is formally considered to be a variation of glass despite being a polymer. It's main advantage when compared to other plastics is transparency. This feature makes PMMA a natural substitute for glass, which is even better since PMMA is not as brittle. Its main polymer rival is polycarbonate.

PC has more strength, chemical and thermal resistance then PMMA. But acrylic glass has its own advantages that make it indispensable in modern prototyping. It is more lightweight than glass and even PC, its yield strength is the highest among the three materials. Another advantage of PMMA the ability to be formed and machined easily which will be described in detail later.

PMMA Uses

Due to a number of its properties, acrylic glass has a wide range of uses in almost every modern industry.

·       Construction. Acrylic glass is used all over the world to create safe very-hard-to-break windows. For example, used in hockey stadiums or large water tanks

·       Medicine. As it turns out, PMMA is compatible with human tissue making it useful in creating lenses and even some temporary prostheses

·       Automotive industry. PMMA is a transparent shatterproof glass. Including a thin sheet of it in the windshield makes it stay whole even in case of breaking, thus increasing the safety of the driver. Using colorants while molding PMMA makes the glass colored but still transparent. That is why it is used in headlights

·       Prototyping. Acrylic glass is ideal for parts with internal features, for example for blades with cooling channels or some complex tubing systems. It allows one to see how liquid or gas flows through the features and enables a quick and efficient correction of designs and a shorter lead time and production costs of the product.

Molding and Casting of PMMA

Different PMMA variations melt at temperatures between 85 to 165°C, which is a convenient temperature for different processing methods. Being a polymer, Acrylic glass plasticizes at forming temperature and becomes viable for injection molding so you can get high-quality parts with fine surface finish.

Cell casting is often used to make high-quality transparent sheets. The process consists of pouring molten plastic between two hardened glass sheets. After that, the sheets are pushed together and their edges are covered with rubber to prevent plastic from flowing out. The sheets are heated again and are left to cool down. This way a sheet of acrylic glass is formed. The obvious disadvantage here is that the quality of acrylic sheet greatly depends on the quality of the glass sheet. That is why due to good plasticity, extrusion is often used to create clear transparent parts.

Extrusion process involves pushing plasticized PMMA through a special press-form or dye with a specially designed hole in order to create a part with the required cross-section. Extrusion enables another important prototype manufacturing process: 3D-printing.

Additive Manufacturing Using PMMA

Additive manufacturing or Rapid Prototyping has been including more and more materials into its arsenal during the last decade. PMMA, being a polymer that has great properties for forming and a comparatively low melt point, has been one of the first materials to be implemented for 3D-printing.

The main technology using PMMA is Fused Deposition Modelling. It involves extruding (as was described later) a thin thread of plasticized material and laying a single layer of it as a section of the produced part. Atop, another layer, which corresponds to a higher section of the part, is laid. That way, the part is created layer-by-layer.

The advantage of 3D-printing is that no tools or special fixtures are required. You just make the model and download it into the printer, basically. The result is a part that is very close to the one that you need (depending on the precision and surface finish, FDM, in average, yields IT9 and Ra 20). Often, in order to get a completely clear part, some post-processing is required. It will be described in the next paragraph.

PMMA Postprocessing

Postprocessing PMA is required when an exceptionally fine surface finish or dimension precision is required. It is usually done on conventional metal machine tools because they provide fast and vibration-free cutting. The tools and fixtures are different, however.

The tools must account for an increased thermal expansion of PMMA and for its low melt temperature. The main point here is to continuously produce thin chip as it carries all the heat. That is why the instruments have an increased nose radius (at least 0.5 mm for turning tools) and decreased profile angles (for example for steel, instead of 120 degrees, it is 80-90 degrees, which makes the drill sharper and thinner).

Fixtures must account for the lower hardness of PMMA compared to metals, that is why special soft chucks are required and fixing forces must be much smaller.

Cutting parameters are also different. In order to make chip removal as fast as possible and make the chip itself thinner, certain corrections must be made. Firstly, cutting speed must be about ten times that of cutting steel, that makes chip removal faster. Secondly, cutting tool feed must be very small since it makes chip thinner.

Having machined acrylic glass, a surface finish of RA 0.32 and a tolerance of IT7 or smaller is achievable.

 Conclusions

PMMA is a very efficient material for prototyping and developing end-parts. It is lightweight, strong, easily-machined and, most importantly, transparent. It enables the designer to have insights into the structure of his product and see the internal occurrences of the mechanism he had created. Implementing PMMA for Rapid Prototyping decreases the design and verification phase of creating product even further, making the designing company more flexible in creating new product and thus more competitive.