Plastics Information, Plastics Materials

Beware… Plastics Aren’t Always What They Seem To Be

Here’s a real life experience we would like to share with everyone:

Plastics International has had a long history of stocking 30% GF Ultem for one of our OEM customers. Their Machine Shop vendors were directed to order the material from Plastics International to ensure the material would be consistent for quality, responsibility and traceability for the life of the project.

A couple years into this project, one of the machine shops had quality problems with their 30% GF Ultem parts.   When these parts failed to pass QC at the OEM, the machine shop sent the rejected parts back to us for credit and replacement material. The OEM Company requested that Plastics International supply a corrective action response for the cause of the machine shop’s material failure. We had the rejected parts evaluated by the test lab and discovered that an unknown filler had been added to the 30% Glass Filled Ultem’s base resin. Since our 30% GF Ultem did not contain the unspecified filler, we knew it wasn’t purchased from Plastics International.

Plastics International called the machine shop and discussed the findings of our evaluation process and asked them to check their purchasing records. After checking their records, the machine shop apologized because their buyer had mistakenly ordered the defective material from Coyote Plastics.

(Coyote Plastics is a fictional name we use to represent those Plastics Distributors who not only sell plastics to machines shops…they also have in house machining…or they own their own machine shop.  Coyote Plastics uses their internal machining capabilities to compete for fabrication business against their machine shop customers.)

The filler was present as the result of Coyote Plastics using a lower cost manufacturer who provided a modified 30% GF Ultem resin. In doing so, Coyote Plastics was able to accomplish 2 positives and 1 negative. Unfortunately, it was a Big Negative.

1st Positive: The filler made the GF Ultem easier to extrude…possibly at faster speeds and with less abrasion on extrusion dies.

2nd Positive: The added filler also lowered the overall resin cost of the GF Ultem, allowing Coyote Plastics to sell it at a cheaper price.

Big Negative: The filler material also lowered the published and specified physical property values of the GF Ultem, resulting in part failures from the intricate machining and application requirements. As a result of trying to lower their material costs, the machine shop ordered from the wrong vendor, received the wrong material and wrongly certified to the OEM’s specifications. It was a costly mistake. As a consequence the machine shop’s reputation was damaged and it negatively impacted their business.

We can quote lower cost resins too. However, before doing so, please consider the risks in using lower cost resins (which can lower the material’s physical properties and increase the chances of non-conforming parts or failures in the field). Instead, Plastics International can help you with our large inventory of quality materials, cut to size yielding and no minimum order to meet your most demanding specification requirements with minimal risk.

There are reasons why plastics can vary significantly in cost and performance. Resins and extruded materials are available in different price ranges and from numerous sources. To protect yourself before problems arise, request material certifications and make sure they match your customer’s specifications. Rejected or replacement parts are expensive and Plastics International pays close attention to material certification requirements so your raw material meets your end user specifications.

You can find technical specifications to help with your plastics application at www.plasticsintl.com.

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.)