The highly automated production, secure material handling and the associated quality assured grinding of sprues guarantee economic and high grade processing of the materials used. Among others, MDS 230/150 cutting mills (photo 2) are used for granulation of sprues. “Our production with a low level of staff has to work securely and reliably in 3-shift operation. This depends precisely on the trouble-free operation of this apparently subordinate plant component”, explains Stefan Hüser, Plant Manager at Aero Pump.

Production under cleanroom conditions

The modular constructed production plant is capable of extension and is accommodated in a two-storey building. Production of the injection moulded parts in PE and PP, as the essential components of the products, takes place in the upper part in the highly automated and therefore sparsely staffed mould shop (photo 3) on the current 24 injection moulding machines. These parts are ejected from the injection moulding machines under cleanroom conditions into storage boxes, packed and conveyed to an intermediate store. The packing area, raw material store and the grinding area are located in the lower part of the building. Assembly of the individual parts needed for the atomiser systems with additional parts into final products and finishing operations also takes place under cleanroom conditions. Supply of plastic granulate and regrind to the injection moulding machines takes place from the raw material store and the grinding area via an ingenious pipe system.

Economic sprue grinding

24 cutting mills are installed in the grinding area, in accordance with the number of moulding machines. The MDS 230/150 type mills are connected with both the conveyor lines as well as with the central conveying plant via the interface of the communication centre. The start-up parts and sprues and runners that arise fall from the machines via a pipe system directly into the mill hoppers of the cutting mills. The arriving material is shredded and repeatedly granulated between the rotating and stationary blades until the regrind passes through the sieve applied to the mill and falls in the regrind material container located underneath the mill.

Process monitoring – high availability

As the amount of sprues and runners or start-up parts arising can be larger than the maximum weight proportion of that may be dosed with virgin material, the mills have to fulfil several special functions. It has to be mentioned however that dosing of the individual raw material components takes place via automatic dosing equipment installed on the moulding machine. In order to avoid the regrind containers becoming overfull, they have in the first place to be connected to the conveying plant communication system. A filling level limit switch continuously measures the filled level via an interior rotary vane with a friction clutch and is built into each of the cutting mills. Should the amount of regrind material in the regrind container exceed a certain level, a signal requiring emptying of the mill is passed on to the central control system.

The conveyor separator on the machine always has priority in emptying the regrind container. If the conveyor separator has no need for regrind at the time, removal of the excess regrind takes place via a separate conveying system. A second suction pipe is provided at the regrind container in order to remove excess amounts of regrind that cannot be reintroduced to the moulding process. The regrind can be conveyed as desired or needed via the lower or upper suction pipe (photo 4) to its destination.

Cutting mills processing the same material are connected to an appropriatel< available joint pipeline. The regrind arising here fills an Octabin dedicated to the joint pipeline for this purpose at the end of the conveying route. The pure packed regrind here is marketed externally. The filled level limit switch additionally fulfils a safety switch-off function. Should emptying of the regrind container not take place due to a disturbance in the conveying system, the mill switches itself off automatically after a pre-selected time delay. If the mill does not switch off in this case, regrind will continue to be produced. This gets between the sieve and the rotor due to the overfilling of the regrind container.

The regrind becomes melted on as plastic material here due to friction. This inevitably leads to damage that is laborious to remove and that has to be paid for with the expenditure of time and machine downtimes. That the pipeline becomes full with sprues and runners is the obvious lesser evil in this case. These solutions enable recycling without compromising quality and additionally guarantee optimum material utilisation. Production consequently takes place without waste.

Top regrind quality

The large screen area of the cutting mill enables high throughput performance and guarantees a low level of dust as a consequence of the fast material output, whereby this is marked especially by the quality of the regrind produced. The specially shaped sieve hole geometry prevents formation of long, thin granulate and thereby guarantees high homogeneity and easy-flowing properties for the regrind produced, which is very important for trouble-free dosing to virgin material. The sieve, that  encloses the lower half of the grinding chamber, is exchangeable. This way, the sieve perforation size can be adjusted to the required particle size.

Undesirable bridge formation that can disturb the grinding process is prevented by the side location of the noise-insulated hopper above the rotor. An inspection window integrated in the lower area of the hopper enables a view into the grinding chamber and thereby visual control of the grinding process. The machines are fitted with stiff, distortion-free rotors that are made entirely from a single piece of metal and that run with optimum speed of 280 min-1 without getting out of balance, guaranteeing sufficiently high throughput with simultaneous low dust levels. In contrast with open rotors, the closed rotor design enables more consistent introduction of the material to be ground around the full extent ot the rotor and therefore a faster shredding and grinding process.

Stefan Hüser, Plant Manager at Aero Pump says: “The Hellweg cutting mills are characterised by long blade lifetimes. After one year of continuous operation, the blades have only had to be reground once. The regrinding of the three unsegemented rotor blades requires here only a small amount of time to be spent, on account of the very good access. The Hellweg mills are robust and of sturdy design and therefore very stable and steady. The mobile design enables easy and fast change in location. The performance in terms of regrind quality, throughput and input capacity – the machines can also be run trouble-free with a filled hopper – are more than convincing. The simple integration of the cutting mills with the control system of the entire plant was a further, impressive justification for the purchase”.

Aero Pump

Aero Pump GmbH (www.aeropump.de) is a family owned company and was founded in 1976 in Hochheim. The company has 120 employees today. Around 60-65 million atomiser systems per annum for pharmaceutical and technical use are produced in three-shift operation. A modular element system enables production of pump systems with different types of closure and dosing means for the widest possible applications. Aero Pump is certified to DIN EN ISO 9001:2000 and DIN EN ISO 14001.

Hellweg Maschinenbau

Hellweg Maschinenbau GmbH & Co. KG (www.hellweg-granulators.com) is also a family owned company, that has been using its own production facilities to construction machinery and equipment for around 10 years, especially for customers’ specific tasks, for customers in the plastics and recycling industries throughout the world. The production range covers machine-side mills, cutting mills, edge strip granulators, feeding equipment for edge strips, film cutters, extraction and dust removal equipment, including accessories.

Technical data - MDS 230/150 cutting mill

Photo 1: Selection of dosing and atomiser systems
Photo 3: A view of Aero-Pump production, moulding shop
(Photos: Aero Pump GmbH, Hochheim)
Photo 2: MDS 230/150 cutting mill
Photo 4: MDS 230/150 cutting mill, two suction pipes
(Photos: Hellweg Maschinenbau GmbH & Co KG, Roetgen)