Owing to the low specific weight of the plastics and the low weight of the injection-molded component, the total design is lightweight, which is not necessarily desirable for an oscillating machine, as the motion is transferred to surrounding system components (such as the pipes). Pneumatic diaphragm pumps, in particular, are frequently operated without foundation fixing. Their low dead weight often results in the pump “wandering” in an uncontrolled manner.
An injection-molded pump has the advantage of needing less material during construction and usually results in a more favorable price. To be taken into account in the decision, however, is the fact that the manufacturer is then restricted to selecting plastics that are capable of being injection-molded. This also means that the use of abrasion-resistant PE and PTFE—with their high chemical resistances—cannot be used.
Injection-molded pumps can-not be used in potentially explosive atmospheres, since apart from POM, which can only be used in a limited way for the aforementioned reasons, no conductive plastic capable of being injection-molded is currently available for industrial use. Based on a series of intensive tests that were recently carried out relating to electrostatic charges on plastics and the relevant results, as well as some unfortunate experiences, only conductive units may currently be used in potentially explosive atmospheres and for the transport of potentially explosive liquids.
Note, also, that the very expensive injection molds needed for these bulky parts make the response to changed or extended market requirements difficult, and give rise to inflexibility in meeting special application-specific requirements.
All these considerations combine to produce an almost-inevitable answer to the injection-or-weight question. The answer is weight!
Only a solid plastic body can provide the necessary weight for an oscillating pump. The individual components can be designed so that the required thickness of each wall is arranged where it is needed (Figure 3). The mechanical machining of a solid plastic block is economical thanks to modern computer numerical controlled (CNC) technology, which enables far tighter tolerances to be achieved than those with injection-molded parts.
Figure 3. Different wall thickness
Injection-molded parts need compensation for the variations from surrounding parts, such as tightening straps and seals. The occurring loads are absorbed by the solid-plastic component itself.
No external reinforcements (for example, metal) are necessary. This allows the complete external unit to be designed completely without metal, which is always subject to a risk of corrosion.
A pump with a solid design is also better able to withstand external attacks, such as damage due to improper handling. In addition, a solid block absorbs the often disputed, but still existing, loads from the connecting pipes. The external surface is smooth and without grooves. Because all housing parts can be mechanically machined, changed market requirements and special construction parameters can be met quickly and without significant expenditure.
A quick reminder about conductivity: with a solid construction, virtually anything can be realized in design. Both PE, as well as PTFE, can be given a surface resistance (less than 105 Ohm) during the construction process by the introduction of conductive pigments (Figure 4). In other words, electrostatic charge is reliably excluded, provided the unit is earthed. In compliance with the highly restrictive requirements of the FDA in the U.S., it is now possible to achieve an approved conductive pigment admixture below 2 percent, enabling pharmaceutical applications in potentially explosive atmospheres.
Figure 4. Conductive pigments added during construction to give a surface resistance.
Admittedly, a few minor disadvantages can be found in oscillating pumps that use a solid-construction design. The construction process requires a bit more raw material, and that may result in a slightly higher price. However, remember that two of the more popular and reliable types of plastic—PE and PTFE—cannot be injection-molded.
When considering materials of construction for industrial pumps that rely on oscillating-displacement operation for handling a wide variety of sensitive, corrosive or potentially explosive products, plastic is the best choice when compared to steel, stainless or alloys. Also when considering the construction of plastic pumps, thanks to their solid design, added weight, wider array of available material options and overall reliability, pumps constructed of solid plastic are a better choice than those made using the injection-molding manufacturing process.
Pumps & Systems, December 2011