Skip To Content

Glossary of Terms

Additive – Anything that can be added to plastic during a molding or Compounding process.  This can be in the form of a powder, a fiber, or a pellet.

Certificate of Analysis (CoA or Cert) – This is a document that contains the test results of various physical properties for a specific Batch or Lot Number (Lot#) of a given resin.  A Cert may contain only one test result (typically Melt Index) or it may contain results from multiple types tests.

Color Concentrate (Concentrate or Pigment) – This is a pellet or powder that, when added to plastic during the molding process or in a Compounding process, turns the plastic into a different color.  The Concentrate is added at a specific percentage or Let Down Ratio (LDR) to add the proper color while still maintaining the properties of the plastic.

Compounding – This refers to plastic that has been melted and mixed together with other ingredients (Color Concentrate, glass fiber, carbon fiber, other fillers, or other plastics) to make a new plastic.  This is done to create a new resin with a specific set of properties.  The feedstocks for a Compounding process can be recycled Regrind resin or Virgin resins.

Let Down Ratio (LDR) – This is how a ratio of additive to main resin is expressed.  An Additive the needs to be added at a 4% rate would have an LDR of 25:1.  That means that for every 25lbs of main plastic, you add 1lb of the Additive.  25:1 and 50:1 are common LDRs.

Melt Index (Melt Flow Index (MFI), Melt Flow Rate (MFR), or Melt Flow) – This is a measure of how easily a plastic flows when it is melted.  This affects how well a plastic can fill an injection mold or how well it can hold its shape during an extrusion process.  The MFI is tested by putting plastic in a heated cylinder with a specific amount of weight on top of the plastic.  The test is run over a period of time and however much melted plastic that comes out is weighed.  The MFI is expressed in grams per 10 minutes (g/10min).  Higher Melt Flows run more like water and can more easily fill a large mold or one with thin walls to the part.  Lower Melt Flows are more like molasses and will hold together better.  These are suited to extrusion processes where the plastic has to hold a shape while it cools in order to form the sheet, pipe, or other profile that is being produced.

Pelletizing – The process of taking melted plastic and turning it into a pellet.  This is accomplished by means of a heated screw and barrel that melts and pushes the plastic through a die face (plate with holes in it) at the end of the barrel.  This creates a series of long strings (like spaghetti) that are then cut into pellets via two main methods, Underwater Pelletizing and Strand Cut Pelletizing.

Pelletizing – Strand Cut Pelletizing – This is a particular pelletizing process wherein the melted plastic is pushed through the die face and the resulting strand of plastic is run through a water bath to cool the plastic. The strands exit the water bath and proceed over a series of rollers while the plastic continues to cool.  The strands then enter a cutting chamber where they are cut to a specified length.  This creates a cylindrical pellet.  This is the most common method of Pelletizing for engineering resins.

Pelletizing – Underwater Pelletizing – This is a particular pelletizing process wherein the melted plastic is pushed through the die face directly into a cutting wheel that is fully encased in a pipe that contains a moving stream of water.  This water grabs the freshly cut small rod of plastic and starts cooling the plastic while rounding off the edges of the pellet.  This produces a round or nearly round pellet.  This process is common for polyolefins like Polyethylene and Polypropylene.  It is not used for many engineering resins.

Pigment – See Color Concentrate.

Regrind – This refers to granules or flakes that are produced by granulating plastic scrap, sprues, runners, or other forms of hard plastic.  This is recycled plastic that can be used in non-critical applications.  It can also be good feedstock for Compounding operations.

Resin – Another way to refer to plastic.

Virgin – This refers to fresh, unused plastic that has been produced from base chemicals by a manufacturer.  This is how all plastics start out.  Virgin plastic is extruded into pellets at the end of the chemical production process.  The pellets can be a round shape, produced by an Underwater Pelletizing process (polyethylene and polypropylene for example) or a cylindrical shape, produced in a Strand Cut Pelletizing process wherein long strands of plastic are chopped into shorter cylinders.

Virgin Prime or Prime Resin – This refer to Virgin pellets that are in the original manufacturer’s packaging and have a Lot# or Batch# on the packaging

Virgin Prime with Certs (V/P/C) – This is the highest level of categorization.  This means that the pellets are in original packaging with Lot#s and a Certificate of Analysis (CoA or Cert) from the manufacturer.   The Cert contains test data from tests performed on the specific Lot# by the manufacturer. Resins with this classification can be used in critical applications including medical, aerospace, and other engineering applications.