How to measure epoxy by weight with a scale

Most mixing problems come down to the ratio. West System says an incorrect resin-to-hardener ratio is behind most curing problems (West System). A scale removes most of the guesswork, as long as you use the right ratio.

Why weight is more accurate

TotalBoat calls measuring by weight the most accurate method because you know the precise weight of each component (TotalBoat). MAS agrees that measuring by weight is the most accurate, while noting that volume is the most common method (MAS Epoxies).

A scale is not affected by the resin clinging to a measuring cup, by the meniscus, or by how level the cup sits. For some products it is the only option: Smooth-On’s EpoxAcast bulletin specifies weighing the parts on an accurate digital gram scale (Smooth-On).

The one mistake that ruins a weighed batch

The weight ratio and the volume ratio of the same product are usually different numbers, because resin and hardener usually have different densities. TotalBoat’s example is a system that is 1:1 by volume and 100:83 by weight (TotalBoat). Weighing to a volume ratio puts the batch off ratio. Always look for the line on the data sheet that says “by weight”.

Step by step

TotalBoat’s method (TotalBoat):

1. Put a disposable plastic mixing cup on the scale and tare it to zero.
2. Pour in the required weight of Part A.
3. Leave the cup in place and tare the scale again.
4. Pour in the required weight of Part B.

Then mix for the time the manufacturer gives, scraping the sides and bottom. The mix ratio converter gives the Part A and Part B weights for any total.

Choosing a scale

TotalBoat notes that a digital gram scale works well for small quantities and that a food scale set to grams can do the job (TotalBoat). Resolution matters most for small batches: a 1 g error is 5 % of a 20 g Part B but only 0.2 % of a 500 g one. For very small batches, use a finer scale or mix a larger batch.

When the data sheet gives only a volume ratio

Some products publish only a volume ratio. You can still weigh them if the manufacturer publishes the specific gravity (SG) of each part, often in section 9 of the safety data sheets:

weight ratio = (volume share of A × SG_A) : (volume share of B × SG_B)

For example, a 2:1 product with SG 1.11 for Part A and 0.96 for Part B works out to 2.22 : 0.96, or 100 : 43 by weight. The converter does this automatically and labels the result “computed”, so it is never confused with a manufacturer figure. If either SG is missing, it refuses rather than guessing.

Other rules that still apply

• Never adjust the ratio to change cure speed. TotalBoat warns against adding extra resin or hardener to speed up the cure, and West System against altering the ratio to adjust cure time (TotalBoat; West System).
• Warm cold resin first. TotalBoat notes that cold resin is thick and hard to measure accurately, and suggests a warm water bath for the closed container rather than a heat gun or microwave (TotalBoat).
• Use a sturdy, wide cup. TotalBoat warns that thin or narrow cups can concentrate heat (TotalBoat).