What supports and the plastic you bin really cost

Of every part that comes off the printer, some of it goes straight in the bin: the supports, the skirt or the raft, and the purge tower if you print in several colours.

Nearly everybody values that rubbish at the price of plastic: 2 cents a gram with PLA at $20 a kilo.

It costs 3.5 cents. That is 76% more. And the reason is simple: that gram also tied up the machine while it was printing.

What a binned gram really costs

A printed gram is not a bought gram. By the time it leaves the nozzle it already carries power, machine depreciation and its share of the failure surcharge.

There is the 76% difference. Every time you snap a support off and bin it, you are not throwing away two cents a gram: you are throwing away three and a half, because the machine was busy making that instead of something else. The breakdown of that hour is in what an hour of 3D printer time costs.

With that number in hand, the rest of this guide is multiplication.

Supports, in money

Supports are nearly always the heaviest of everything that gets thrown away. How much they are depends entirely on the part and how you place it, so the useful way to see it is as a percentage of the part itself.

A part that needs supports weighing half of it costs 21% more. If the supports weigh as much as the part — and that happens more often than you would think on figures with arms or overhangs — the bill goes up 41% and the machine time nearly doubles.

You can open both extremes: with no supports and with supports at 50%.

The number that counts does not need estimating: nearly every slicer lists support material separately in the summary. Those grams are the ones that go into the table.

How to cut supports without changing the part

The best thing about this line item is that it can be cut a lot without touching the design, just by deciding how the part sits. Four things, in order of what they pay:

• Turn the part. It changes the most and it is the only free one. The same figure lying down, standing up or tilted 45 degrees can go from needing support everywhere to needing almost none. Try three orientations and compare the support grams your slicer reports.
• Raise the overhang angle. The default threshold is usually 45 degrees, and almost every machine prints something flatter than that just fine. Raising it to 50 or 55 wipes out whole supports on chamfered parts.
• Support from the build plate only. A checkbox that stops the slicer planting columns on top of the part itself. It saves material and, above all, avoids marks on surfaces that show.
• Cut the part in two. Split in the right place and glued afterwards, many parts stop needing a single support. It costs you some of your own time, and there you do have to do the sums, because your hour is worth far more than the plastic.

The rule that covers all four: supports are decided when you place the part, not when you slice it.

The purge tower: what multicolour eats

If you print in several colours with automatic changing, there is a second source of rubbish that can beat the supports: every colour change forces a purge of the previous colour, and that gets deposited in a tower that goes into the bin whole.

I am not giving you a figure in grams because it depends enormously on the machine, the profile and how many changes there are. What I will give you is how to know in one minute: weigh it. The purge tower comes off in one piece and fits on a kitchen scale.

If your tower weighs more than the part — and on small four-colour pieces that is normal — you are paying double to print in colour. That figure has to go into the price, or you stop printing in four colours things that do not need it.

What does cut the purge without giving up colour: grouping by colour instead of alternating, and printing several parts on the same plate at once, because the purge is shared between them all instead of repeating on each.

Raft and skirt are small change

Not to overstate it: not everything thrown away weighs anything. The skirt — that loose outline on the first layer — is a couple of grams at most, and at 3.5 cents a gram it does not reach ten cents. As insurance against a badly laid first layer, it is dirt cheap.

The brim, that skirt attached to the part to stop it lifting, weighs a bit more but is still little: it costs less than reprinting the part if it comes loose four hours in.

The raft, on the other hand, does weigh: it is a solid base under the whole part. If your machine grips well you hardly ever need one, and there are real grams to recover there.

Put another way: of the three, only the raft is worth checking the grams on.

Removing supports costs more than the plastic

So far we have only counted material. But supports have to be snapped off, and you are the one doing it, with your hands and your time.

This guide’s scenario already pays labour at $12 an hour. That is the rate the 15 minutes of work in the table above are priced at. At that rate, every minute you spend cleaning a part costs 20 cents.

From four minutes of cleaning onwards, your time costs more than all the plastic you are throwing away. And five minutes a part is nothing unusual on a figure covered in thin supports.

Over a year, with the same two hundred parts, five minutes each is 16.7 hours and $200 of your time: more than the $127 of filament in the previous table.

That is why turning the part pays off more than it looks: it does not just save grams, it saves the minutes of snapping them off.

What you were already paying without knowing

There is a fourth source of binned plastic that this calculator already counts for you: failed prints. It is the 10% that appears in every table on the site.

At two hundred 60 gram parts a year, that 10% is 1.2 kilos of filament and $38. That is slightly less than the same 1.2 kilos as supports, because the failure surcharge is not applied to itself. It is not a pessimistic estimate: it is what happens to nearly everyone, and the causes in order are in why 3D prints fail.

The important thing is not to count it twice. If you already have the 10% surcharge set in the calculator, failures are covered; what is not covered is supports and purge, which have to be added as extra grams in the weight field.

All of it together, over a year

Put on a yearly scale, at two hundred 60 gram parts, the rubbish stops looking like small change.

With supports at 30%, which is not an unusual figure, 3.6 kilos and $127 a year go in the bin. And 330 machine hours: nearly two whole weeks printing things you are going to snap off and throw away.

Add the 1.2 kilos from failures and you have nearly five kilos of filament a year that never end up in a part. That is five spools you have to buy and nobody pays you for.

When redesigning the part pays for itself

If the part is yours, there is a way out that is neither turning it nor cutting it: change the design so it does not need support at all. Chamfers instead of flat overhangs, teardrop holes instead of round ones, walls that do not hang in mid-air.

The question is always the same: is the time I am about to spend worth it? With the numbers above it takes one division. Take what supports cost you in a year and divide by your hourly rate.

With supports at 30%, redesigning pays off if it takes you less than ten and a half hours. And that counts material only: add the cleaning minutes from the previous section and the margin is far wider.

The sums change completely, of course, if you are only going to print the part once. Redesigning only pays on what you repeat, which is why it is worth doing first on the two or three parts you make most.

How to measure yours in five minutes

None of this has to be believed: it is measured, and your slicer already knows.

One. Slice a part of the kind you normally make and find the summary. Nearly all of them separate support material from the part's material. If yours does not, look at the total weight with supports and without: the difference is the supports.

Two. Divide those grams by the part's. That percentage is your row in the table above.

Three. If you print in several colours, keep the purge tower from the next five prints and weigh them all together. Divided by five, that is your average per print.

Four. Add the grams of the part, the supports and the purge, and put the total into the cost calculator. That is the price you have to charge, not the one for the part alone.

It is the difference between believing a part costs you $5.11 and knowing it costs you $6.16.

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