BACKGROUND & AIMS
Collection devices, tubes and caps are very fragile with regard to leakage. More so during long storage, in low pressure air freight, or with liquids with low surface tension.
Additionally, plastic creep can weaken the tube-cap tension over time and cause delayed failure.
The aim of this study was to define a polymer tube and cap combination and geometry that could operate without seal or o-ring, for at least 12 months, whilst exposed to internal overpressure of up to 56kPa (e.g. while in plane cargo at 39,000 feet and temperatures ranging from 15 to 30°C.
MATERIALS & METHODS
To design, evaluate and optimize a tight cap-tube closure, 3 types of materials were used:
(i) After selecting high performing solutions in a conceptual design phase, 3D computer-aided design (CAD) simulation models were built to run Finite Element Analysis (FEA) simulations in order to evaluate the efficacy of the concepts.
(ii) In a next step, 3D prints in Polyamide (PA) - with representative mechanical characteristics - were made to assess initial design fit and usability aspects (e.g. closing torque, comfort of shape, etc.),
(iii) And finally, injection moulding tools in 718H and S136 steel were produced to mould parts in High Density Polyethylene (HDPE) for the caps, and in Polypropylene (PP) for the tubes, to verify sealing performance.
The different materials were tested, using these methods:
(i) In the FEA model, different sizes of caps and tubes were virtually assembled, with different ranges of overlap, to simulate future tolerance deviations and related closing forces.
(ii) The HDPE caps and PP tubes were closed without liquid content at 1Atm, before they were submerged in a water filled air clock, with vacuum applied at -85kPa.
The FEA results showed that the smallest tolerated cap with the biggest tube still produces a closing pressure of 30N/mm². Taking reductions due to plastics deformation and creep into account, this connection theoretically would survive negative pressure of 100kPa.
This was confirmed in the conclusive immersion test at -85kPa.
The design - subject of this study - featured a unique internal thread concept with inlying closure ring and proved to be very successful. It is to be expected that prefilled tubes with this tube-cap design (e.g. with preservative) could ensure proper storage and shipment without leakage for 12 months or more.
It is worth mentioning that steel quality of the injection moulding tools and the level of polishing are important parameters for solid sealing.