The dialysis solution is pumped into the abdominal cavity by means of a membrane which is activated by triacetin as mediating agent, which is routed to the membrane via a twin-piston pump.

In addition to extracorporeal dialysis (hemodialysis), dialysis technology includes so-called peritoneal dialysis. With this procedure, the dialysate is directly introduced into the abdominal cavity of the patient where it cleanses the blood via the peritoneum. Hence, in a manner of speaking, the peritoneum acts as an exchange membrane. Along with greater ease of handling, peritoneal dialysis provides the benefit of enabling home dialysis, although this procedure can only be used for a limited period of time.

The dialysis solution is pumped into the abdominal cavity by means of a membrane. This membrane is activated by a mediating agent, a type of glycerol called "triacetin". Triacetin, which has a similar viscosity as water, is routed to the membrane via a twin-piston-pump.

Project in three stages

The ultimate development objective of this project, the complete twin-piston pump assembly, was achieved in three stages spanning a two-year period of design and development work.

Stage 1
The development project started with a customer requirement for a triacetin-resistant piston seal to be used under the following operating conditions:

• sealing of a glass cylinder against "triacetin", the critical, highly fluid medium;
• operating temperature: room temperature up to max. 80 °C;
• piston speed: up to 20 mm/s;
• opposite contact surface: 040mm glass cylinder;
• specific seal requirements: very good impermeability, low-friction, stick-slip-free running characteristics, incorporation of a guide into the piston seal, if possible, costefficient solution for volume production, chemical resistance, reproducible level of quality, sealing element with minimal clearance (aseptic solution).

Red inscriptions denote parts manufactured by ElringKlinger Kunststofftechnik, black inscriptions denote parts provided by the customer. Click to zoom!

Following tests of several seal versions, the complete piston solution was found to be the most reliable piston seal. This solution offers the particular benefit of involving no internal static sealing points, because the piston is a single-part design. Consequently, design engineers were able to focus their primary attention on the dynamic sealing point.
The particularly high level of impermeability required for this application resulted in the dual sealing lips. The minute amount of leakage of the primary sealing lip enters the space between the two sealing lips and is returned to the medium space by the reverse stroke of the piston. Additionally, the surface quality of the glass cylinder ensures absolute impermeability. The sealing effect is achieved via the two memory sealing lips. The memory effect of the molded sealing lips ensures that the lips continually apply permanent radial force against the cylinder wail. This wipes off the triacetin film. The memory effect is created by a special manufacturing technique that enables the sealing lips to always "remember" their original state, in other words, the form of a disk. Furthermore, design engineers integrated a guiding-jacket and the fastening of the drive rod into the piston. Initial testing series involved the use of PTFE compounds as well as the special HS 4080 compound (PE- base). In addition to very good permeability compared to pure PTFE, this compound offers additional application benefits:

• excellent memory behavior;
• outstanding wear resistance against water and abrasive media;
• high level of pressure stability in applications up to 250 bar;
• approval for use in medical technology products and foodstuffs (FDA);
• usability up to a temperature of 100 °C.

Due to the critical assembly of the piston in the glass cylinder, it made sense for the customer to have the seal manufacturer perform the assembly work. Consequently, the first engineering stage consisted of the pre-installed HS 4080 piston in the glass cylinder.

Stage 2
The customer fitted the piston with a stainless steel drive gear rack. Because this gear rack had to be sealed against triacetin as well, and there was no ideal solution available for this, the seal manufacturer designed a special spring-energized seal as a rod seal, for which the HS 4080 compound was used as well. The seal design comprises a spring-loaded dynamic sealing lip. The static sealing side was sealed by an elastomeric 0ring made of silicone. One reason for this design was the lower surface quality of the pump chassis compared to the opposite aluminum cover, the other the fact that this enabled the seal to be installed into a cut groove. The entire sealing ring is axially held in place and thus cannot be pulled out of the housing groove during the stroke of the rod. Stage 2 comprised the following individual components and steps: 1.) HS 4080 piston seal and rod seal made by the seal manufacturer; 2.) Purchased parts like the glass cylinder and stainless steel piston rod: installation of the piston into the glass cylinder; assembly of the spring-energized seal on the rod; assembly of the rod to the piston.

Stage 3
In the final stage, the POM (polyoxymethylene) pump casing was offered to the customer as a milled single component. The merger between ElringKllnger Kunststofftechnik and Venus, a specialist in plastics processing technology, enabled economical manufacturing of this plastic housing as well. This third in-house manufactured component and another purchased component, i.e. the aluminum pump cover, paved the way for the complete piston assembly. The entire hydraulic twinpiston pump is now being supplied to the end customer by the seal manufacturer. Another special benefit is that the complete pump is being subjected to a 100% performance test ensuring that it is leak-proof. Prior to shipment, this complex system is provided with a label indicating date of manufacture, lot and part number, thus perfectly enabling the product tobe traced back to its original source. This example serves to illustrate that customers can expect specialized suppliers, like ElringKlinger Kunststofftechnik, to deliver not only single parts or components, but entire systems, like a complete piston pump. Customers selecting a supplier with cross-functional activities will enjoy the benefit of receiving comprehensive consulting support and solutions, not limited to single elements, but based on an extensive product portfolio and full potential of compounds and materials.