Polyethylene Powder

Grinding of plastic samples is pretty tricky. Because for such elastic samples, the main grinding force applied should be shearing, the achievable fineness is limited. Even a very accurate gap setting of knives with 0,1 mm, samples only need to comply in one dimension to pass the gap. Because of this, often only brittle samples can be ground < 100 µm.

Finest grinding results will be achieved by our Variable Speed Rotor Mill PULVERISETTE 14 premium line which can be converted into a cutting mill. With a speed up to 18.000 rpm, such a mill is much faster than classic cutting mills (usually up to ~ 3000 rpm). With a narrow gap between the knives, often samples can be ground to < 500 µm; with a few different samples, even < 0,2 mm and finer results are plausible.

With our first trial, we wanted to check the grindability of the HDPE sample. The mill was converted into a cutting mill and the high-performance cyclone separator was equipped with our sample exhauster. We used a sieve shell set of 0,2 mm trapezoidal perforations first.

We used a batch of 40 grams of HDPE and checked the level of fineness by optical microscopy. We found plenty of fragments around 200 µm length or width; only a few larger pieces < 400 µm were spotted.

This batch was slowly fed into the feeding funnel of the mill. The grinding sound was typical smooth. It took us 200 seconds until the complete sample was transferred into the grinding chamber. The temperature of the system did not rose significantly. Surely even higher feeding speeds can be realized. The max. throughput should be evaluated on the customer side.

There was no significant clogging of sample on the sieve shell perforations. Only a bit of electrostatic charged and ground sample was found inside the grinding chamber. Surely higher amounts of sample can be ground.

With higher sample amounts, eventually a Vibratory Feeder could be placed next to the mill to automatize the grinding process.

Cleaning was performed rapidly by vacuuming with a soft brush.

With our second trial, we used the smallest available set of sieve shells. With 0,08 mm trapezoidal perforations, usually only brittle samples like coal or coke can be ground in higher amounts.

We prepared a batch of 45 grams of HDPE and fed the sample material extremely slow into the grinding chamber. We hope that the sample fragments will not remain attached to the sieve shells by the applied vacuum. We hope that the sample is spinning around inside the grinding chamber, until the sample was comminuted fine enough to pass the sieve shell perforations.

After 600 seconds (10 minutes), the sample was batch was fed completely. Also, with this trial, no change in grinding sound was observed. The temperature of the rotor did not rose significantly.

There was no significant clogging of the sieve shells after the batch was comminuted. A lot of electrostatic charged sample in the grinding chamber was merged with the sample inside the cyclones collecting glass. For this, we pushed the ground sample with a brush into the channel of the cyclone while the sample exhauster remained switched on.

As mentioned in result one, the max. throughput needs be to evaluated by trial and error. Surely somehow faster feeding of sample should be possible. With the present feed rate, it would take about 3-4 hours to comminute a kilogram of HDPE powder.