How to solve aesthetic problems in the production of recycled HDPE bottles

The demand for regenerated HDPE for blow molding has seen a strong surge in recent years, certainly finding some producers not totally prepared to manage the recycled granules in their machines.

It was not just a question of the type of granule which may differ slightly, from a technical point of view, from the virgin raw materials in their behavior in the machine, but problems related to the tone of the colors, stress cracking and the tightness of the welds had to be addressed , micro holes and other minor issues.

In previous articles we have addressed the genesis of recycled HDPE in bottle blowing and the correct choice of recycled raw materials, while today we see some aesthetic aspects that could arise using 100% recycled HDPE granules.

There are four aspects, from an aesthetic point of view, which can negatively impact the good production result:

1) A marked porosity called "orange peel" which forms mainly inside the bottle but, not rarely, is also visible on the outside. It appears as an irregular surface, with the presence of continuous micro cavities which give a wrinkled appearance to the surface.

Normally the problems are to be found in the granule, where a possible excessive presence of surface humidity does not allow perfect laying of the HDPE wall coming out of the mould. In this case the problem can be solved by drying the material in a silo so that it reaches a level of humidity that will not negatively affect the surfaces. Generally speaking, it is always a recommended operation when you want to produce using 100% regenerated material.

2) Streaks on the bottle are another aesthetic problem that occurs for different reasons, especially if you use an already colored granule. The causes may depend on a different percentage of plastic inside the HDPE granule, even in minimal percentages, between 2 and 4%, since, since the plastics have different melting points, the aesthetic behavior on the wall of the bottle can be slightly different, affecting the color in the dough.

It is important to note that the streaks of tone should not be confused with the streaks of structure, which are normally created by the bottle mold due to wear or dirt that accumulates while working. Another reason may depend on the heat resistance of the master used, as it is not uncommon that at temperatures that are too high, both during extrusion of the granule and blowing of the element, a color degradation phenomenon can be created with the creation of small streaks on the walls of the bottle.

3) Perfect weldability in a bottle is extremely important as any detachment of the walls, once the bottle has cooled and filled, causes serious damage with costs to be incurred for the loss of the packaging, the substances contained and the replacement of the material with significant logistics costs.

The newly produced bottle normally does not present the possible defect as the temperature at the exit from the machine "hides" the problem a bit, but once the bottle has cooled down, filled and subjected it to the weight of the pallets that are stacked on top it, a welding defect can present itself in all its problems.

The cause of this problem must normally be sought in the percentage of polypropylene that the HDPE granule may contain due to a non-optimal selection of raw materials upstream of the production of the granule.

A poor selection of the bottles among themselves, but above all from the caps they contain, can increase the percentage share of polypropylene in the granule mixture. There are machines on the market with optical selection of the washed ground coffee that help to substantially reduce this percentage, bringing it back below 1.5-2%.

When purchasing a load of recycled HDPE it is always a good idea to ask for a DSC test to check the composition of the granule for production. The effect of an excessive percentage of PP has as a direct consequence the prevention of effective welding of the contact surfaces that form the bottle.

In addition to working on the granule, it would be a good idea, if you wish to use 100% recycled raw material, to slightly increase the overlap thickness of the two sides of the bottle to favor the correct welding point.

4) The presence of micro or macro holes in a bottle , visible directly through an inspection or, for smaller ones, through the air tightness test, may depend on the presence of impurities inside the granule, when washing and the filtering of the raw material was not done to perfection.

Another reason may depend on poor cleaning of the screw of the blowing machine which can accumulate residues of degraded polymer and subsequently transport them outside towards the mould. Especially if you use recipes with mineral filler, the problem may arise immediately after changing the recipe from one without filler to one that contains it.

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