The accepted wisdom regarding medical paper and Tyvek as a sterile barrier is that harmful microbes are prevented from entering the packs because they get caught up in the “tortuous pathways” created by the random nature of fibre orientation. It is also considered that a hole with a diameter less than 0.5 µm will prevent a microbe from passing through. Although the pore size in paper is often larger than this critical dimension, the pathways protect the sterile barrier and keep it intact.

As extruded films do not have any form of resin or fibre orientation, they offer a short pathway that will not “capture” bacteria trying to pass through a possible pinhole. It is also likely that holes, where they do occur, will have a diameter greater than the critical size.

Further research into this area was started at Michigan State University by Dr Laura Bix and Dr Jane Severin in 2003. I am not aware that any firm conclusions were reached, as elements such as pressure differential during transport were also being assessed, but in the interests of patient safety, I recommend zero tolerance on pinholes in film used to make a sterile barrier.

This question is asked often, and my answer is based on comparing an air-powered syringe system to a positive-displacement syringe system. I’ll answer your question in the order in which you listed the criteria.

Repeatability

Air-powered dispensing is based on a time-pressure system. The operator steps on a foot pedal or actuates a finger switch, thereby sending a pulse of air to the syringe reservoir for a preset period of time, usually measured in milliseconds. The air pushes the fluid through the dispensing tip, a precise amount of which is applied to your part.

In a positive-displacement syringe system, the operator steps on a foot pedal that actuates a motor, usually a stepper motor. The motor is attached to a rod that pushes down on the fluid in the syringe reservoir, usually moving a predetermined distance or motor “steps.”  Once the rod has moved its preset distance, the stepper motor reverses direction, retracting the rod backwards to relieve pressure against the fluid and prevent dripping or excess flow.

Here’s the thing: both systems have their place. If you are working with a two-part epoxy with a limited working time-usually 30 minutes or longer-and you don’t want to make constant adjustments over the working life of the epoxy, then the positive-displacement system is probably the best way to go…for now.  New technology in air-powered dispensing is coming that will allow for automatic changes for fluids that change viscosity. Until then, you can continue to use the positive-displacement syringe system for your two-part epoxy. For all other fluids, I think air-powered dispensers are the way to go. They are easier to use and to set up than positive-displacement syringe systems.  More importantly, both systems are comparable in repeatability. The theory of how positive-displacement syringe systems work is sound; however, there are some flaws that might not make it as accurate as most people would think.

Speed of Operation

Hands down, air-powered dispensers are faster and can provide higher cycle rates than a positive-displacement syringe system…full stop!

Ease-of-Use

This is always a subjective topic. What might be difficult for one person is easy for someone else. Based on feedback from hundreds of companies that have tried both technologies, air-powered dispensers always win the “ease-of-use” debate. Again, this doesn’t mean that positive-displacement dispensers don’t have their place; it just means that a majority of people think that air-powered dispensers are easier to use, easier to set up, and easier to train their operators on.

The simple answer is “it depends.” If you are working with a two-part epoxy that has a working life of approximately 30 minutes or more, and you want to perform an initial setup of the dispenser and not make any changes over the working life of the epoxy, then a positive displacement syringe system would be more accurate than an air-powered dispenser. However, if you are using a standard fluid that does not change viscosity over time, then an air-powered dispensing system and a positive displacement syringe system are comparable in accuracy.

Some people have a difficult time believing this, so let’s take a closer look at how a positive displacement syringe system works. In either an air-powered system or a mechanical positive displacement system, the syringe side is identical: a fluid-filled syringe, a plastic piston inside the syringe that rests on top of the fluid and a dispensing tip attached to the bottom of the syringe. During the operation of a positive displacement system, a rod pushes against the piston inside the syringe to force out the fluid from the dispensing tip. The conventional thinking is that the rod moves down the syringe at a given distance, displacing a specific volume of fluid . . . and that is partly true. The rod does travel a certain distance, but when it gets to the preset distance, it retracts, relieving pressure on the fluid to alleviate any dripping or excess flow from the dispensing tip. To be a “true” positive displacement system, once the rod gets to its preset distance, it would remain there until all of the fluid displaced by that movement has flowed from the dispensing tip. This is virtually impossible because the fluid would flow out of the dispensing tip at an extremely slow rate, which would be impractical in real-world dispensing applications.

A second issue is the compressibility of fluid.  Many arguments have been made regarding what constitutes a compressible fluid, but the reality is that most fluids used in dispensing systems are compressible. Since the dispensing tip provides a restriction at the end of the syringe, the compressibility of the fluid is going to come into play with regards to the accuracy of the system.

The third issue is the retraction of the rod. Again, the rod retracts so that you get the achieved shot size in a reasonable period of time while eliminating excess flow or dripping from the dispensing tip, but the rod retraction is actually “cheating.” As I mentioned earlier, a true positive displacement system would advance the rod a certain distance and remain at that position until all of the fluid has been displaced.

To summarise, although each system uses different means of force (air pressure versus mechanical pressure) to push out fluid from a syringe, the concept for each is the same and their accuracy is comparable. There really is no significant difference between the two systems.