Plastics Machining

Finding the Sweet Spot in Plastic Cutting Tools

FINDING THE SWEET SPOT

There are a number of factors to consider when setting up to machine plastics.  Most importantly, plastic has a higher thermal expansion rate than most other materials, such as metal or wood products.  Thermal expansion is the tendency of the material to change volume in response to the heat introduced during the formation of chips created during the cutting process.   Reducing thermal expansion is key to producing clean and effective cutting in any plastic cutting operation.

Following are some key tips to reducing thermal expansion and improving your cutting success.  The first thing to consider is tool geometry and quality.  This is true for any type of cutting tool:  Router bits, saw blades and wing-type cutters.  For this article we will be discussing router bits, but the techniques discussed are applicable across the board.

Plastics fall into two main categories: Hard and Soft.  Harder plastics, or those with a higher durometer rating, generally have a lower rate of thermal expansion.  Softer plastics will obviously have the opposite properties.  Softer plastics tend to be more difficult to cut due to the fact that it will tend to “push” out of the way of the cutting edge of the tool, increasing friction and thus heat, introduced to the chip formation.  When the threshold of heat exceeds the melting point of the material the chips being formed, the chips will melt causing a problem.

Generally we select tooling with higher shear or helix angle when cutting softer plastics.  The higher shear angle can reduce cutting pressure, thus reduce heat created.  However, factors such as material thickness and hold down will limit the ability to introduce high shear tools into the cut.

One thing we have recently introduced to our line of plastics bits is a highly polished flute.  High polish dramatically reduces friction created during the cut.  This will create cleaner cuts in most applications.

Selecting the proper chip load is another important factor for proper cutting.  Hitting the “Sweet Spot” is one of the most critical factors for successfully cutting.  The window for the proper chip load is much smaller for plastics than in any other material.  Typically, one or two thousandths of an inch will make the difference for plastics where wood, for example, can machine well across tens of thousandths of an inch.  Chips load is calculated using the following formula:

 

Chip Load = Feed Rate (IPM)/(# Cutting Teeth X RPM)

 

Chip loads for plastics are generally between .004” to .008”.  The important point is that plastics will have a very small “window” of acceptable chip load and all conditions of the cut will factor into what chip load will be successful in a particular application.  Learn to calculate chip loads in your application and make small incremental adjustments to “Dial in” what works best for your particular application.

Tool deflection is another factor that needs to be addressed in order to reduce friction created at the cutting site.  Router tooling, cutting tool diameter, geometry and tool holding all play a role.  In general, larger tool diameter will deflect less, thus reducing tool deflection.  Work with your supplier to select the best tool geometry/diameter that will work for your application.  The tool itself is driven by the tool holding device.  There are numerous options for driving your cutting tool.  The important point is to realize that the more substantial your tool holding is, the better the results you will see in cutting.

The final area to consider to improve your cut quality is “Part Hold Down”.  Again, the more rigidly the part is held, the better your results will be.  When fixturing parts, it is imperative that the part be held securely as close to the cutting site as possible.  This is especially true for thinner materials.  If the part can fluctuate due to part instability, it can make the difference between a good cut quality and a poor one.  In a vacuum hold down situation you want your gasket seal as close to the cut edge as possible.  If you are clamping parts, the same is true.  Basically, the more you invest in fixturing, the better your results will be.

In closing, to increase your cut quality in plastics there are four things to concentrate on.  First: select the proper tool with the optimal tool geometry for your application.  Second: make sure that your tool is run at the optimal parameters for your application (feed rate).  Third: Review your tool holding devices and make sure to optimize as best you can.  And finally, make sure your parts are held in the best way possible.

(Reprinted with permission of Mike Serwa, VP-Vortex Tool Co.)

Plastics Machining

What is Casting?

Casting involves introducing a liquefied plastic into a mold and allowing it to solidify. In contrast to molding and extrusion, casting relies on atmospheric pressure to fill the mold rather than using significant force to push the plastic into the mold cavity. Some polymers have a viscosity similar to bread dough even when they are at elevated temperature so they are not candidates for the casting process. Examples of this are polymers like POM, PC, PP and many others. Casting includes a number of processes that take a monomer, powder or solvent solution and pur them into a mold. They transition from liquid to solid by either evaporation, chemical action, cooling or external heat. The final product can be removed from the mold once it solidifies.

Casting has several advantages:

-Cost of equipment, tooling and molds are low.

-The process is not complex.

-Products have little or no internal stress.

Casting can have some disadvantages:

-The output rate is slow and has long cycle times.

-Dimensional tolerances are not very good.

-Moisture and air bubbles can be difficult to manage and may cause problems.

There is a vast array of materials than can be cast. Nylon Type 6 is one of the most popular and commonly used cast products. You can see some of our product offering by clicking on this link. Polyurethanes are another example of cast materials we offer. Either way, the casting process is often times the preferred method providing large size stock shapes for machining parts. Make the most of this process by optimizing your part blank with a near net cast shape, give us a call and we can explain how.

Plastics Machining

Fear and Insanity in the World of Plastics Machining

We have all heard the old saying: “The definition of insanity is doing the same thing over and over again and expecting a different result.” Sometimes, we secretly can hope for things to be different. Fear can get in the way of making that hope a reality. We all know that fear can either motivate great change or cause great paralysis. Sometimes, fear can keep us taking a course of action we know deep down inside could make things better.

As it relates to machining plastics, many job shops keep machining the same material for the same application the same way just because that’s the way it’s always been done, even though the scrap rates are too high, the machine time is too long, and the material is always hard to find (this last one may not apply to one of our customers!). If you’ve grown to accept situations like these, please stop, and let some one know. Get help! The advances in technology and material science may offer a newer, faster, better way to do what you need to do with your process or application. Here at Plastics International our inventory and offering is constantly changing to reflect the changes in the market and the available technology of current material manufacturing. Sometimes what seemed impossible last year, last month, or last week is now the new standard.

Don’t be afraid of change. A little work and effort up front may make your business more successful in the long run. If you are not sure where to begin, feel free to give us a call and let us know what you are doing. We will be glad to listen and share ideas of what you might be able to do to improve your process. Our website has an entire section on how to work with some hard to machine materials. See our Material Resource Center. Plastics International can easily provide samples and resources to help you figure things out or test the ideas and concepts. The best part is, if something doesn’t make total sense or even seem remotely beneficial, you don’t have to do it. But don’t let the fear of change stick you with a material or process that you only wish could be or would be better and you do nothing about it. Looking at some of your existing business may not only help your business but may also offer your customer a better product or solution in the long term.