Plastic resins are made by major petrochemical companies. The choicest cuts are sold by their direct sales force or through their "prime" distributors. So who sells the near miss or transitional resins which are not exactly ready for prime time ?
Starting wiith definitions is difficult because there are no written industry standard definitions of most terms pertaining to resins in this realm. H L Mencken defined conscience as "that little voice that says somebody somewhere might be watching." There is no controlling legal authority which oversees the distribution of off grade resins. Caveat emptor.
A "major" is short for a major petrochemical company which manufactures plastic resin. A "broker" is an entity which takes title to the resin which the major deems to be not "prime" resin which falls within the target criteria for the pick of the litter. The majors sell prime through their direct sales force and prime distributors.
A data sheet lists certain key properties which define the characteristics of a resin. The first - melt index or melt flow - describes viscosity. This tells the processor the most about how the material will behave and how it will affect the outcome of the part or material. There are other criteria but going into depth would be too inside baseball and geeky.
Each of these criteria has a target and acceptable range for what is considered to be "prime". These are not rigid and fixed. Rather, they are at the discretion of the product manager at the resin producer. The parameters change depending on the available supply. When resin is plentiful, the parameters tend to tighten up. When resin is "tight", they widen. It's all a function of what the product managers believe they can get away with.
When a product manager gets a report of fresh lots, she decides which ones are prime and which are considered "off grade" or "near prime". The prime material gets "certs" and is sold through their direct sales force. What falls outside the parameters is called "off grade" or euphemistically "near prime", "pencil prime" or "excess prime". Other lots are called "transitional" which means resin made transitioning from one grade to another.
Off grade, or OG for short, is sold through "brokers ". By selling to a third party, the origin is obfuscated. Brokers are supposed to be discreet and sell off grade to customers who are not direct customers of the same majors and preferably into a different application than originally intended ( It gets interesting when brokered resin finds it's way to prime customers ). A business model which relies on somebody's mistakes at first sounds like a scary business model. When resin is "tight" or when the majors are trying to raise prices, the brokers don't have much to sell. The truth is there will always be material available which is rejected by processors. We reject several railcars annually produced by ISO 9000 certified major resin companies.
The most ethical and truthful brokers graduate to become what is known as "prime" distributors. They distribute prime to smaller processors not called on by the major's direct sales force.
It's easy to spot a newbie resin buyer. They are too smart by half. They make the rookie mistake of shoving an invoice from a distributor in the face of the direct sales rep. " I can buy the exact same thing from your off grade broker for 4 cents a pound less !! " Maybe so for one car. After that, their reputation precedes them and they get no more deals. Experienced buyers know the importance of emptying / returning a car quickly so the direct sales rep does not see the car which came from his own plant.
Fun fact: there are no prerequisites to becoming a resin broker.
Many brokers misrepresent material while others are ethical and disclose fully the good, bad and ugly about specfic lots. Processors operating on razor thin margins are tempted to save money with resin that's "not exactly" and brokers are tempted to misrepresent to move their inventory. They hope the processor won't know the difference. Problem: the processor lives with the material and does know when the raw material is not as advertised. Here is an example:http://www.plasticsnews.com/article/20150924/NEWS/150929945/suit-alleges-processor-was-sold-the-wrong-resin
The ramifications go beyond the relationship between the processor and the resin producer. The finished properties of the plastic film or plastic part within a lot or lot-to-lot
are affected with variability of raw material. FDA approval
is meaningless. Just change a few words in a word document and the broker transforms off grade into FDA approved plastic resin. It's legal and goes on every day.
So how can you use this information ? When doing the due diligence on a supplier, ask them preferably in person if the resin they use is "prime". Don't be shy. Ask to see FDA letters and invoices ( for the resin they use to make your product along with "track and trace" records ) with the price of the resin blocked out. If the letterhead is not from a major or authorized prime distributor, it's circumspect. There is an exception. Some brokers distribute "generic prime" which has been tested and sometimes blended to fall within specific ranges.
For non-critical parts, a generic prime will suffice.