Epoxy 3 + 1.5 = ...3.5?

At my work I've been working on a project for our medical drivers for our assembly department. Synthes issued us the glue to use; Masterbond, a two part epoxy. We've found that a 2:1 ratio of the base (3) to the hardener (1.5), and when they're fully mixed and ready to be absorbed into the syringe, I'm never able to get more than 3.5 units in the syringe. Now, to me 3 + 1.5 should be 4.5, right? Can anyone please explain to me where that other 1 unit is going?

I get the feeling there's more to this than what I'm interpreting but are you obliged to use 3 units of base? 2 and 2/6 of base would fit into a 3 and a half unit syringe.

I get the feeling there's more to this than what I'm interpreting but are you obliged to use 3 units of base? 2 and 2/6 units of base would fit perfectly into a 3 and a half unit syringe.

Sorry, I'm not quite understanding what you're asking. Let me try and clarify our process for you.

We take one syringe and absorb 3 units of the base and empty it into a cup. Using a separate syringe we absorb 1.5 units of the hardener and empty that into the cup with the base already in it. We mix the two until they're fully mixed together and then absorb the contents of the cup into the syringe. I believe the syringe is a 7 unit syringe, but I'm not at the office now, so I'll have to double check on that tomorrow.

My bad. In that case I can only assume the mixed solution is more dense than either one or both of the individual components and takes up less space for the same amount of matter as a result.

I believe it could be something similar to water and alcohol. Alcohol has bigger spaces between the atoms, and so if you add one cup of water to one cup of alcohol, you get less than two cups of the solution.

It can either be that, or by combining your two materials they are reduced to a state that has the same mass but takes up less volume (think hydrogen +oxygen = water, although this isn't a case of gas turning to liquid).

If the final soultion is more dense than the other two, it would explain it.

As others have said, it could be a denser solution, but if this is a solution it's less likely. It could also be that the reaction creates a gas, like CO2 or O2.

It's most likely related to how the two materials interact (and in this case, react). Each of them alone (noting them A and B) has weaker intermolecular interactions A-A and B-B than the interaction A-B.
That way when they are mixed the overall specific volume of the solution is lower than that of A or B.

Since this is an epoxy glue, the base is a linear or branched polymer (not reticulated) with certain chemical functions along its chain, the hardener is a small molecule at least bi-functional with a chemical group that reacts with those of the polymer. When these two are mixed the smaller molecules bridge different polymer chains making a 3D spider web that renders the polymer no longer fluid. This process also reduces the specific volume of the polymer since the molecules are closer together because of the bridges made by the hardener.
Moreover, often times there is a solvent in the base so it's less viscuous and so easier to apply/mix. That solvent also evaporates, contributing to the loss of volume on the final product.