Design for Assembly – IVD Application

Preventing Leakage and Simplifying User Experience

One of the engineer’s primary responsibilities is the integration of design, manufacture, and use.  This includes a sometimes forgotten step in the Design for Manufacturability process – Design for Assembly.  The tendency, especially as organizations grow larger, is to divide the labor and separate the responsibilities.  However, when engineers put forth the time and energy to gain a deep understanding of the production process and the end-use, they gain an advantage.

This is one of the reasons why the Wright Brothers beat the Smithsonian to first in flight even though the Smithsonian enjoyed a ten-year head start with seventy times the budget.  While the Smithsonian had one group designing, a separate group building, and a third group test flying, the Wright Brothers performed all three steps themselves and improved the development process with a faster and richer feedback process.

In the world of plastics design and manufacturing, this means engineers who design components, develop manufacturing processes for those components, and test the performance of those same parts are better equipped to achieve optimized part design and process. 

This gets to the heart of why Design for Manufacturability and Assembly (DFMA) is so critical.  The approach might increase the initial cost for a development cycle, but it decreases the number of cycles and time per cycle needed to reach the optimized result, which helps decrease overall project costs.  A recent example with the Natech engineers demonstrates the concept.

The Application – IVD Application with Onboard Reagents

An IVD application with onboard reagents included a collector of patient blood samples.  The collector first collects the sample with a capillary tube.  Then the component directs the sample followed by the buffer reagent onto the test strip.

Operator holding a plastic molded IVD component

The critical requirement here was that once the sample was collected and prior to release of the sample and buffer, the system would have to be a closed, sealed system to:

1. Direct the flow of sample and buffer onto the assay, and

2. To prevent any sample or buffer from escaping the system during this process. 

As pressure is applied the holding force of a parked groove is overcome freeing the collector to move into the mating component to release the buffer.

The Issue

The Natech Engineers spent time with the client to understand the broader system functionality and the collector’s role within that system.  Thanks to this they had an understanding that went beyond the defined requirements and identified a functional risk. 

During assembly when the collector joined with the mating component in the parked position, the collector could wiggle in place and allow for a leak path.  This could happen because, in the parked position, only one of the three radial seals made contact with the mating surface.  If any sample or buffer escaped, it could compromise the soundness of the assay, or it could pose a health risk to the user.

IVD component assembly part drawing

The Natech Engineers also noted that the assembly process would provide no direct feedback for the insertion of the collector into the mating component.  In other words, a person inserting the collector into the mating component would not know when exactly they had reached the correct point of insertion.  This could result in not inserting the collector far enough, which could result in disassembly during transport.  On the other hand, if the collector were inserted too far, it could pierce the buffer container rendering the unit unusable.

The Solution

Once risks are identified, solving them can be much more straightforward.  In the case of the radial seal, simply adding additional seals so that at least two seals engaged with the mating component in the parked position resolved the issue.  These bumps act as o-rings to create a hydraulic lock which can prevent fluid from escaping the system.

IVD plastic part updated assembly design

To address the lack of assembly position feedback, the Natech Engineers designed an increased diameter at the bottom end and created a mating feature to create a firm snap once the component reached the proper parked position.  This groove reduced the risks of unintentional part removal and premature release of the buffer reagent.


Because of the O-Ring Grooves, the Collector stays in place and the only place for the buffer to flow is along the intended flow path.  The Park Groove aligns flat against the surface and provides a tactile snap into place indicating it is in the correct position.  While the finished component may look more complex, the manufacturing process is actually simplified with reduced operational risks.

Problem solving is an important skill for DFMA success.  The risk is identified and the solution is achieved thanks to integration by engineers with experience across disciplines.  An engineer who possesses the relevant knowledge and experience in designing, manufacturing, and using is better equipped to solve the problems that inevitably emerge during the product development process.