How to get a smooth surface finish on Engineernig Plastic Material

I have responded privately to this request. milo

Try polishing with very hot air on surface you need shining--while the other areas remain cool--kind of hot-air-brush

You do not say which plastic. Sounds like a thermoplastic - if so try heat - some respond to a wash with acetone (butyrates)

UHMWPE (VHMWPE) - ultrahigh molecular weight polyethylene

You may find this article helpful: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332004000800029&lng=pt&nrm=iso&tlng=pt

You have broken the thin outer skin of the material by making your internal hole and you can not replace the surface finish of the outer skin internally by polishing. Try two things: 1. make your internal hole by a very low RPM and then ream it to size. Try sub-freezing the material prior to machining. 2. Use a flame to reflow the ID to form a new skin after making the internal hole. Be aware that you will probably lose the tolerances needed in this approach.

This material is not new for joint replacement. Do a search and you will find many sites that discuss the problems associated with UHMWPE. Good luck.

Hi Richard,

Please send a couple of your reground inserts to:

Conicity Technologies

131 Turnberry Circle

Latrobe, PA 15650

We will apply the EMG variable edge prep to smooth out the grind marks on the cutting edge. Typically for plastic we put 0.0002" +/-0.0001" on the edge. This will retain the sharpness you need and remove the microscopic imperfections. The variable edge prep reduces the chip thickness at the point of contact where it separates from the part and helps give a smoother surface finish. Sample tools are free of charge and we'll work with you to resolve your machining issue.

http://cyberprep.net/sample_request_form.html

Thanks....Steve

Hi richardorthopaedic,

your problem is very likely resulting from a dull tool.

PE of any type is requiring very sharp tools with rake-angle and free-angle similar to the angles that are used for wood turning.

Inspect your cutting tools with a 10x magnifier and if you see a rounded or not sharp cutting edge then regrind (by hand if no suitable machine is existing) until very very sharp.

Do a test of the cutting edge by cutting against the surface of your fingernails, if the tool takes some small chips without any feelable force in the direction perpendicular to the fingernails surface then it is done with a good sharpness.

Have success.

We grind some experimental tools with a sharpness down to 0.2µm (8microinch).

RHABE,

I agree with you 100%. I have machined a lot of this material. Soft thermoplastics cannot be pushed, they must be sliced with very sharp cutters preferably with polished edges and flutes. You cannot polish them as a secondary operation with much success or without wasting a lot of time. SHARP CUTTERS with small corner radii.

You're working with ultra high molecular weight polyethylene. The best finish, if you aren't going to run this thru a slot die or such, is to try this: (Be aware that UHMPE does not melt and flow like normal polyethylene!): heat the surface to the point where the polymer just flows. Try rubbing on a heated metal plate. This will give you a glossy, smooth surface. You might try an inexpensive "iron" (as in laundry ironing) and try that. Mechanical finishing usually doesn't do well on this type polyethyelene. (1) Most linear polyethylenes are in the weight average MWs of 100,000 to 200,000. (2) High MW PE is between 300,000 to 500,000 MW, whereas (3) ultra high MW PE (UHMWPE) is typically between 3 million (3,000,000) and 6 million (6,000,000) MW. This trpe of polyethylene has exceptional surface abrasion resistance as well as impact resistance and high wear resistance applications. You might try flame impingement also. Contact a manufacturer of UHMW polyethylene and ask for their best recommendation.

Just had another thought. It sounds like you are getting a good finish everywhere except the bore. I assume that you are pre-drilling , then boring. If you are not leaving at least .003-.005 per surface, your boring bar may not have enough material to pull a good chip and so not leave a good finish. Also, make sure that the boring bar has enough back clearance so that you are not dragging chips across the finished surface. The less of the tool that touches the part the better. Another trick that may help would be to bore exiting the part instead of entering the part.

You may try a fluorocabon based polymer treatment, make sure that use a biocompatible material.

Presuming the tool is sharp, try adjusting the shape of the cutting edge.

What type of tool are you using?

What is the cutting speed and feed rate?

What is the diameter and depth of the hole being cut?

Hello Snave,

Thanks for your reply. Actually, we are using a Diamond Insert with VBMX 110302010-5 to machine the inner spherical radius of the plastic cup. We've measured the rounded tip of the insert and it turns out to be R=.2 radius. We tried to regrind the insert to make the cutting edge very sharp and put it into testing but had no success. It appears the surface finish of inner diameter doesn't seem to improve after several regrinding.

FYI, the cutting speed of CNC Lathe runs within S200 and S250 with a feedrate of 0.001. Further, the Inner Spherical radius should come 0.554±.002 in. and its depth of cut is 0.001 in. As you know, we are having problems on the surface finish so we are figuring out what went wrong and not sure how to proceed from here - but if you could provide any suggestions, we should have no problem.

If you have any ideas, please let me know.

Regards,

Richardorthopaedic

Since you are using UHMWPE and broke through the outer skin you will not be able to achieve the original surface finish by simply machining, regardless of how sharp your cutting tool becomes.

hi richardorthopedic

VHMWPE????? UHMWPE should be the correct definition!

Machining: From Consolidated Form to Implant

Orthopedic manufacturers generally machine UHMWPE components into their final form. Even components that are direct molded may be machined on the back surface to accommodate a locking mechanism. However, the actual morphology of machining marks depends upon the manufacturing conditions as well as the type of UHMWPE material (e.g., conventional vs. highly crosslinked) [1]. An example of machining marks in an as-machined (never implanted) GUR 1050 UHMWPE component is shown below.

Machining of UHMWPE components consists of milling and turning operations for both roughing and finishing steps. In some cases, the resin converter may supply the stock in a shape that approximates the cross-section of the finished implant. Such pre-shaping or pre-forming offers advantages of efficiency and speed to the manufacturer. Since UHMWPE can be damaged by excessive heat, the feed rate, tool cutting force, and spindle speed must be closely monitored during manufacture. Close mechanical tolerances generally require the manufacturing environment temperature to be carefully controlled. Milling machine spindles in the early to mid-1980s could develop up to 4000 rpm, while newer machinery can develop up to 8000 rpm; the latest machines develop speeds of 12000 rpm. Overheating of the UHMWPE is avoided by proper optimization of the feed rate and tool cutting force during machining operations.

The actual cutting speeds, tool feed rates, and depths of cut used to machine UHMWPE components are proprietary, and hence little information is available in the literature about the effect of machining parameters on the tribological properties of UHMWPE [2]. Song et al. have proposed an idealized model for the surface morphology of as-machined UHMWPE as a triangular wave, in which the peak to peak distance, d, is given by the following equation:

d = f/(2s)

where f is the tool feed rate and s is the cutting speed. Song's [2] model also proposes that the base angle of the machining marks is related to the geometry of the cutting tool. Given the numerous (potentially interrelated) factors influencing the surface topology of as-manufactured components, it is difficult (based on measurements of the surface alone) to quantify the machining conditions used to produce the surface finishes of a randomly sampled component.

Cooper and colleagues have classified abrasive wear in UHMWPE as involving "macroscopic" or "microscopic" asperity contact between the sliding surfaces . Because of the initial difference in surface roughness at the UHMWPE and metal counterfaces [3] , the initial wear rate involves the removal of the larger "macroscopic" asperities on the UHMWPE surface, whereas the long-term wear rate is governed by the "microscopic" asperity size of the metal counterface [3]. Thus, changes to the surface roughness of UHMWPE components may be expected to affect the initial wear rate, as removal of machining marks will occur within the contact zone during the first stages of wear in an orthopedic bearing [4].

1. Kurtz SM, Turner J, Herr M, Edidin AA. Deconvolution of surface topology for quantification of initial wear in highly crosslinked acetabular components for THA. JBMR (Applied Biomaterials) 2002; In Press.
2. Song J, Liu P, Cremens M, Bonutti P. Effects of machining on tribological behavior of ultra-high molecular weight polyethylene (UHMWPE) under dry reciprocating sliding. Wear 1999; 225-229: 716-723.
3. Cooper JR, Dowson D, Fisher J. Macroscopic and microscopic wear mechanisms in ultra-high molecular weight polyethylene. Wear 1993; 162-164: 378-384.
4. Wang A, Stark C, Dumbleton JH. Role of cyclic plastic deformation in the wear of UHMWPE acetabular cups. J Biomed Mater Res 1995; 29: 619-26.

if you need more information just ask!!!!