Sep 06 2007

The Part is to Have a Bright Shiny High Polish

Published by at 3:19 pm under Uncategorized

What is a bright shiny high polish?  When you specify the surface finish on a part you are actually specifying the finish on the mold.  This is a problem in communication.  Does everyone know what you mean by Bright? Shiny?  High?  Here’s a way to do it.
Actually I’ve seen this specification on designs.  OK experts, what is “Bright”? Does this imply a high IQ or some glow-in-the-dark property?  What is “Shiny”? Shine comes in varying degrees; even things that are textured can be shiny.  Shiny actually has to do with gloss.  What is “High Polish”?  Is the polisher located in the mountains somewhere?  Is the polisher smoking illegal cigarettes? We also need to get down to the short strokes: are we talking shiny parts, or the finish on the mold, or shiny when compared to other parts?

As you can see the definition of Bright Shiny High Polish or however else you’d like to specify is quite complex.  While I tend to frown on this practice; this is where ‘quality by hostage’ makes sense.  The Society of the Plastics Industry looked at this problem and solved it the most practical way possible:  They have produced a ‘standard’ plastic plaque segmented in to twelve squares each with a designation for the degree of ‘Bright Shiny High Polish’.  You purchase two plaques, send one along to your customer and then specify your designation accordingly. (These plaques can be purchased from SPI [the Society of the Plastics Industry] –part number AR-106 –  Your mold builder should already have one of these.

Plastic isn’t metal.  Many engineers who come from snooty East Coast colleges are quick to point out metal finishes are defined by their polish.  Polish is defined by its smoothness and can be adequately measured using high resolution stylus-type profilometers or non-contact optical interferometers.  The idea is that being able to measure the depth and length of the scratches on a surface will give you a measure of the polish.  The problem in plastics is ‘finger printing’ – will the plastic replicate the metal’s surface finish?

Without getting into the finer points of polishing; the specifying tooling engineer need only use the SPI (Society of the Plastics Industry) definition and ideally use steel the can take a good polish for your cavity steel (keep in mind you can polish lead if you want to but it won’t hold it’s finish).  The general rule of thumb is that the harder the steel the better it takes a polish.  Further, a quick call to your mold builder or steel supplier will get you a variety of steels for your mold cavity that are specifically alloyed to produce excellent polished surfaces.

It is important to remember that polishing, especially polishing contours or completely flat surfaces require almost the patience of a saint and the precision of an artiste – read: Per square centimeter, polishing to a mirror finish is really spendy.  When specifying a part most engineers want the Bright Shiny blah blah on the exterior.  This must preclude a general notation saying ‘Part Surface finish is to be SPI A1’ otherwise the core and cavity will have the same polish. The only time this notation is appropriate is when you are specifying something that you’ll be looking through.  Most product requirements for the Bright Shiny blah blah finish is usually an A2 or A3 for the exterior.  These aren’t a shiny as an A1 (lens quality) but are sufficient for the purpose.  The A series of finishes are always the finish of diamond grit comounds.

The interior of a part at most tend to fall into the B1, B2 or B3 polish.  These B finishes are from 320-800 paper finish.  The C designations are for a 320-600 stone finish with the D finishes being dry blast grit finishes.

The C finishes are usually where you knock off the EDM scale.  However with the proper EDM machines if it isn’t your desire to remove the EDM layer a ‘fine EDM’ finish will approximate a fine stone or paper finish.

Some plastics have a property where they leave a film on the mold.  This property is sometimes called varnishing, scaling or the generic ‘buildup’.  It comes from the hot additives in the plastic that you see as the smoke when you purge condensing on the cold mold steel.  Some bozo-brained people also touch the mold – leaving finger prints.  Hence the term ‘Finger Printing’ (Yes, you can actually mold your finger print in plastic.)

The ‘price’ of a scratch’ is huge with an A1 finish.  Removing buildup with alcohol and coarse toilet paper can leave scratches that will take 15-20 tool room hours polish out.  Obviously the coarser the finish the less chance your buildup removal will damage the mold.  With all that said, it’s always easier to maintain a finish than it is to restore it.

Tool Makers and Setup Techs square off when it comes to removing buildup.  However the techniques with the least damage inflicted are the best.  The best solvent is rubbing alcohol. The best wiping media is some kind of lint free cloth or tissue (Clean room rated).  If there are hairline scratches minor polishing can be done with Q-tips and White Diamond paste (OR Semichrometm polishing compound, OR Wenol polishtm )

The trick in polishing out scratches or removing buildup is light pressure and a lot of patience.  This is why when the engineer first looks at the product rendering its good to make the suggestion for having the mold finished with a light grit blast or a texture because maintaining the part finish is extremely difficult after it comes out of the molding machine.

Unless the customer is interested in gift wrapping his parts, using custom packaging and is willing to pay for your high yield loss specifying BRIGHT SHINY HIGH POLISHED SURFACES is not necessarily the best product choice. Buy the plaques, each of you keep one then go with your customer to one of the Mega Stores and look at the surface finishes of stuff for sale.  With the exception of mirrors, magnifying lenses, and sun glasses most things are not as Bright Shiny and highly polished as your customer might think.

After dimensional tolerances, surface finish is the highest source of rejects but oddly enough something that tends not to get addressed in the feasibility phase of the project.  You’ll address it before you begin shipping parts or at the time of your first reject.

This article is virtual and not does not have BRIGHT SHINY HIGH POLISH.  However you can use it to dazzle your customers or your boss.  OR, if you’d like you can not scare anybody with the threat of high scrap on a product and just sit back and watch the fun.

Your choice.

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