Resin Flow within the Plant

As you learned earlier in the series, a continuous extruder converts the resin into a fluid-like mass, and ultimately a parison, by melting and mixing the pellets. The parison is then closed in moulds and blown out to a given shape.
When the moulding is complete, two things remain; the finished hollow plastic article and some extra plastic which was the part of the parison pinched outside of the mould. This extra or excess plastic is referred to as ‘flash’.

Flash
As a normal part of the continuous extrusion process, around thirty percent of the plastic put into the extruder ends up as flash. A higher percentage can be expected when moulding handled or irregular shaped containers – sometimes as much as fifty or sixty percent.

Since resin is so expensive, it is advantageous and feasible to reclaim virtually all of the flash produced. This is done by granulating, or grinding, it down to pellet size and feeding it back into the process. Off-spec parts and purgings made by the extruder during startups, shutdowns, and adjustments can also be reclaimed.

Recycling by Granulating and Grinding
Let’s examine the flow of resin throughout a blowmoulding facility in greater detail. The closed-loop resin handling system begins with a vacuum unit to transfer the virgin pellets out of storage. This usually consists of a major network of pipes and vacuum pumps so that different types of resins can be transported to different locations.
The first stop for the virgin pellets is the ‘blending station’. This area also contains the granulated reclaimed material called ‘regrind’. And since most resin in its virgin state is colourless, colouring agents are also added to meet the containers cosmetic specifications.
The blender is equipped with controls and systems to automatically combine a set percentage of the virgin, regrind and colorant. The finished blend is distributed to the vacuum loader finds its way to the extruder hopper – where it is ready to begin its trip through the blowmoulding process.
At the end of the trip, the resin either leaves the closed loop as a finished product or is fed back into the loop to make another trip.
Flash is normally vacuumed directly to a granulator. Defective parts and extruder purgings are usually hand fed into the grinder. This material is chopped up to a size similar to the pellets and is then blown to the blending station.
The equipment does the bulk of the mixing and conveying, but it is the good resin management habits of the production personnel that ultimately maximize the use of the material.