Due to the diversity of plastic materials in the industry today, it often becomes very difficult to avoid some kind of machining problem. The following article on http://www.onsrud.com/ focuses on the most common problems you will face with plastic routing. We’ve summarized the findings below.
Not all plastic is created equal, a simple change in colour can drastically alter the way plastic material reacts to a cutting tool. We will later go over available resources that will help you sort through these differences, but as a beginning point, plastic can be categorized as soft or hard plastic. To determine this, look at the flexibility or rigidity of the material. When using the proper routing tools, soft plastic will curl a chip, while hard plastic produces a splintered wedge. This may seem simple, however sometimes there can be soft and hard plastic characteristics within a generic group. Point being, not all plastics are the same and ignoring this fact can create problems from the beginning.
With the diversity of plastic materials, cutting tool geometry is vital. Plastic machines much differently than other materials, and through the years, tools specifically made for plastic routing have been developed. Today, plastic fabricators have literally thousands of tools at their disposal, and utilizing the soft and hard plastic categorization, a general tool selection process can be developed. Soft plastic utilizes “O” flute router tools in straight or spiral configurations (Figures 1 & 2), and hard plastic tools can use either “V” flute tooling or “O” flute spiral tooling with hard geometry considerations (Figure 3 and 4). Whether or not you use straight or spiral tooling is based upon the direction, you as the user want to influence the chip or part. A neutral effect can be achieved through straight tooling, whereas spiral tooling can move chips in an upward or downward direction. We will now go through a couple of these common plastic cutting problems, with the assumption that the correct tool is being used.
1. Welding of plastic material
Welding of the chip during plastic routing is the most irritating problem that can be encountered in the industry, as it can be costly in terms of time and scrap rate, and is entirely avoidable. Common reasons that cause this occurrence include improper chip load, small tool diameter size, influencing the chip improperly or direction of cut. The key to plastic routing is producing the adequate sized chip to remove heat, while also accommodating finish requirements. The chipload (thickness of the chip) formula is as follows, chipload=feed rate/ (RPM x # of cutting edges). As shown in the formula, there are several ways to adjust chipload.
2. Poor Finish
Arguably the most important consideration in the plastics industry is the surface finish of the final product. This is especially the case with plastic products such as exhibits, signs or POP displays, where the product is constantly in public view. As mentioned above, the selection of the proper tool is imperative to a good edge finish, however the finish is also heavily influenced by the chipload. Usage of a properly sized chip will eliminate excessive knife marks in soft plastic and a cratered finish on hard plastic, and the optimum range of chipload seems to occur between 0.004 and 0.012. Other than chipload, a poor finish can be caused with the improperly holding of parts, and the condition of the CNC machine itself. Even the best plastic cutting tool in the market today will not perform properly unless the machine and collet system are maintained to the industry standard.
The key to avoiding the most common problems of welding and poor finish is to understand the material being machined, select router tools with geometry specific to plastic, and apply them with proper chipload recommendations. Proper maintenance procedures and rigidly holding parts will further enhance the process.