Everything you need to know about polycarbonate machining
Polycarbonate (or PC) is a very useful polymer that was developed quite some time ago but has quickly conquered its place among industry grade plastics. We may not always notice but I am quite positive that each and every one of you meets PC at least once a day. I’ve flown on a plane just a couple of days back and what do you know? The windows on the plane were made of PC. It is much more reliable than glass but just as transparent. You get it, right? The main use of optically clear polycarbonate is to substitute glass where shattering is not an option. One of the best ways to manufacture PC parts is polycarbonate machining. Let’s see what it is about.
Polycarbonate Machined Part Applications
There is actually a number of different ways to manufacture polycarbonate parts, however, PC machining offers much more opportunities than other processing methods. That’s why it is used to manufacture a lot of mass-produced PC parts and for optical prototyping. Typically, polycarbonate is issued in a transparent form, however, it is possible to tint it up to opacity. It is quite rigid when plastics are concerned and its thermal stability (up to 135 degrees) makes it one of the few plastics available for machining. Therefore, it is usually milled or turned to the desired form and is later polished to get optically clear polycarbonate parts. If we add the corrosion, scratch and shock resistance, we’ll find a lot of areas where PC machined parts can prove useful.
- Car lighting systems
- Shatter-proof glass parts
- Plane windows
- Demonstration models
- Special types of lenses
- Heat-loaded plastic parts
- Electric circuits
- Specialized insulation materials
Polycarbonate Machine Tool Configurations
In order to get good machining results, some nuances of polycarbonate machining must be taken into account. For instance, a simple metal machine tool such as a lathe or a mill is perfect for processing polycarbonate but it should be mounted with specialized jigs and fixtures. Otherwise, the plastic blank can be damaged during setup because PC is susceptible to cracking under high tensile load.
Choosing the Coolant
Despite being a heat resistant industrial polymer, it can still melt and if the molten chips weld to the actual part, you’ll have to start over. That’s why you have to think about the coolant lubricant. The basic choice is high-pressure air current. It is great for deleting the chips from the cutting zone and we all know that they may contain up to 80% of the heat. If you need to machine deep grooves then you will need something more efficient. Water with soap is okay and even some specialized coolants, however, watch out for the contents as pc may react with some oils that are added into cooling fluids.
Choosing the Right Cutting Tool
The first and the most important rule to choosing the correct cutting tool for PC lies in the sharpness of its cutting edge. When we choose a cutting tool for processing steel we may have to consider taking a blunter but thicker tool for longer life but when we cut plastic, it’s all about minimizing the deformation. A blunt tool will simply push the chip away instead of cutting it off. The resulting heat will be much higher and the final part will have plastic distortions. It is recommended to use new cutting tools made of carbide without any coatings to make the edge grain as thin as possible. HSS tools are great for their small grain as well.
Polycarbonate Machining Specifics
The machining parameters to polycarbonate processing have some specifics as well. The optimum spindle speed is around 15000 rpm, a high number, used in aluminum final cutting operations with small cutting depth sometimes. Such high rpm requires a greater feed as well. Typically, the feed for cutting PC is three times higher than that of aluminum alloys.
I should additionally note that similar advice is viable for drilling holes. However, some things are different. Since the heat dissipation, while drilling is extremely bad, you need to keep the heating of the workpiece to a minimum. To achieve that, the drilling speed must be minimal and the feed – maximized. Up to 4 times of aluminum with Avery sharp drill would be perfect. Otherwise, cheap removal is not high enough, chips get smaller and sinter back to the surface of the hole.
Polycarbonate Finishing Operations
If you want to get those transparent prototypes, then you’ll definitely need to think about ways to get a high-quality surface finish on your PC machined parts. Besides, you can use Pmma injection molding to get clear prototypes.There is a number of ways it is typically done.
Vapor Polishing Polycarbonate
Vapor polishing is a chemical method of obtaining near the perfect surface finish in polycarbonate parts. The method uses a special solvent vapor. The most commonly used solution is called the Weld-on 4 by the Weld-On corporation. It is a highly flammable toxic reagent that melts the outer microscopic layer of PC and acrylic. In order to vapor polish polycarbonate, the vapor is poured over the part, it melts the outer layer of the part and its microscopic scratches are smoothed out making it transparent. The method is very easy, fast, and can be used to process complex inner surfaces so it is widely used in optical prototyping. However, it requires annealing, a heat treatment where PC is heated up to 135 C to relieve inner tensions and held at this temperature for 3 hours per inch of thickness. It is slowly cooled down afterwards. This prevents microscopic cracks from appearing.
Buffing PC is a kind of polish job with some nuances to fit the polycarbonate. The method consists of using a cotton polishing wheel revolving at a high speed and the PC machined part administered with a cutting paste. The paste is usually a specialized acrylic or plastic buffing or polishing agent developed specifically for the purpose. Buffing offers good results for large parts with simple outer surfaces.
Manual Polishing Polycarbonate
The last method of finishing is the most ancient and depends solely on the competence of the worker. It consists of consecutively using sandpaper with smaller and smaller grit. After the finest grit had been used, the part is polished with a towel and a special polishing paste. This method is slower than vapor polishing polycarbonate and buffing but it requires almost no equipment and tooling. In addition, an experienced worker will see which areas require less polishing and which ones need to be worked on more.