How to solve aesthetic problems in the production of recycled HDPE bottles: porosity, streaks, detachment and holes

The production of bottles for detergents , for industrial and agricultural liquids, until recently were produced with virgin materials despite some shapes and colors allowed the use of a recycled HDPE granule.

The media impact of plastic pollution dispersed by humans in the environment, has moved the conscience of consumers putting pressure on states, which deal with environmental legislation, but also producers of substances contained in bottles that cannot, for commercial reasons, to lose the consent of its final customers.

The demand for regenerated HDPE for blow molding has had a strong surge in the last years, surely finding, a part of the producers, not totally prepared to manage the recycled granule in their machines.

It was not just a question of the type of granule that may differ slightly, from a technical point of view, from virgin raw materials to machine behavior, but problems with color shades, stress cracking and seal welding had to be addressed. , to 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 occur using recycled HDPE granules at 100%.

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

1) A marked porosity called “orange peel” which is formed mainly inside the bottle but, not infrequently, is also visible on the outside. It appears as an irregular surface, with the presence of continuous micro-cavities that give a rough 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 a perfect laying of the HDPE wall coming out of the mold. In this case the problem can be solved by drying the material in a silo so that it reaches such a degree of humidity that it will not negatively affect the surfaces. In general 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 an already colored granule is used. The causes may depend on a different percentage of plastic inside the HDPE granule , even in minimum percentages, between 2 and 4%, since, having the different plastic melting points , the aesthetic behavior on the wall of the bottle can be slightly different, influencing the color in the dough.

It is important to note that you should not confuse the streaks of shades with the streaks of structure, which are normally created by the mold of the bottle due to wear or dirt that accumulates by working. Another reason may depend on the heat resistance of the master that is used, as it is not infrequent that at too high temperatures, both in the extrusion phase of the granule and in the blowing of the element , a phenomenon of color degradation 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 due to the loss of the packaging, the substances contained and the replacement of the material with important logistics costs. The bottle just produced normally does not present the possible defect because the exit temperature from the machine “hides” the problem a little, but once the bottle has cooled, filled and subjected to the weight of the pallets that are stacked above it, a welding defect can present itself in all its problems.

The cause of this problem normally must be sought in the percentage of polypropylene that the HDPE granule can contain due to a selection of the raw materials upstream of the non-optimal granule production. A poor selection of the bottles between them, but above all from the caps that they contain, can increase the percentage of polypropylene in the granule mixture.

There are commercially available machines with optical selection of the washed ground which help to substantially reduce this percentage, being able to bring it back below 1.5-2%. When buying the recycled HDPE cargo 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 an 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 wanted to use 100% of the recycled raw material, 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 , directly visible through an inspection or, for smaller ones, through the air tightness test, may depend on the presence of impurities inside the granule , when the washing and the filtering of the raw material was not done in a workmanlike manner. Another reason may depend on poor cleaning of the screw of the blowing machine which can accumulate residues of degraded polymer and transport them, subsequently, to the mold. Especially if you use recipes with mineral charge, you may have the problem immediately after changing the recipe between one without charge and one containing it.

The use of mixed recipes between virgin and regenerated material can mitigate some of these points but not completely solve any problems if you do not have the foresight to follow the supply chain of the recycled granule.

Related articles:

• HDPE: PRODUCTION OF BOTTLES WITH RECYCLED PLASTIC | SOME ADVICES 

Automatic translation. We apologize for any inaccuracies. Original articles in Italian.