If you landed here directly, check out the first three posts of this series of - The complete industrial designer:
Stage 1: Research & observation
Stage 2: Sketching
Stage 3: Form & proportion
Stage 4: CMF
Stage 5: CAD & surfacing
Types of prototypes
Now that we're in stage six of this development, it's important we hold the thing in our hands for real. And the best way to do that before we get the manufactured part is a physical model.
You get a physical model by prototyping it many ways and each type of prototype can help you get the right insight and can answer different questions about the product.
First, you can do a simple paper cut-out of the current designed version at full size. Keep it next to the bottles we referenced in earlier stages. When you do this simple activity, you'll immediately start observing proportion errors in the shape. Keep doing this till you get the proportions right.
You can now move to foam board or EPS mockups. These mockups give instant clarity on hand-feel, diameter, grip, and proportion in 3D. Make a couple of these prototypes with different diameters. Don't label them or put any badging; simply make enough to hand out to ten people. Ask them if they'd carry the bottle across an office floor?
Now, let's get to FDM 3D printing. You'd now want to interact with a sturdier form of the bottle. You'd want the users to experience and give feedback on a more robust form of the bottle. Not just that, you'd want to check fitment, engagement of cap closer to the real object which aren't possible with foam. In foam, you can't get a thin wall to see the cavity inside but with FDM, you can also check if the cleaning brush would get in comfortably through the neck.
These three prototypes would be ideal progress markers before the first actual material moulded part. But, we're making a transparent bottle now, aren't we? So, we could go to the next level of prototyping with translucent or transparent resin prints. Resin or SLAs are smoother. Once sanded and sprayed with a base clear coat, the surface would become real for the parts meant to be transparent. But a few things to remember before moving on. Clear resin prints would be foggy and yellow-ish. It'll get yellower in less than a week's time. So it may not be clear but close-enough.
It'd work if you intended the body to be frosted. This would look close to the real product and a perfect final form for reviews.
The last thing you could do is sacrifice one of your reference bottle body and use it to review for a close-enough version.
That's on modelling the entire body of the bottle. What you'd need to do now is to prototype the cap. That's where the complication lies. A bottle body is a cylinder at the end of the day. The cap has threads, a loop housing, a spring loaded press button, a hinge to pop the cover open, and such.
You should prototype not just at scale but twice the size first, maybe with multiple different spring stiffnesses, and press each one hundreds of times. Bigger, scaled versions help you see the mechanical movements better. Once you get some confidence, move to full size scale and study differences to converge again.
What to test and do now
Throughout the prototypes, give the bottles to real users. Hand them the bottle and observe. Did they pick up the bottle or grab from yours comfortably or did they do a micro-twitch of their wrists. How did they put it down? How did they pick it up again? They they reach for the loop if they had something else in their hand?
Ask them how would they fill the bottle, or drink from it, or walk with it across the office floor, and then go silent. Resist your urge to fill the silence. Capture hands, take short-videos, and definitely never ask them to score your bottle on a five-point scale.
Don't stop here though. Make look-alike models without any badging but now, one model with every feature on it, an one where you remove three to four features you're least confident on. Maybe people re-pick the one with lesser features and that feels simple than the one overloaded.
You are testing grip, feel, dry weight of the bottle. You're testing if it can be operated with a single hand. One-handed opening while holding it, standing, or while walking. You're testing how easily it slips in the bag and how easy it is to pull out.
You should test if the bottle is tipping or not when filled fully or partially. Record at what angle it tips. Do this test in different common spaces like in a car stationary and moving, at the desk, on a counter top, or a lowered coffee table.
Do a flow rate test. The flow of water from the spout while drinking is important. Time it for 200ml and capture the rate. If the spout is too wide or the space is not comfortable for lips to rest, water would spill by gushing out. On the other hand, if it's too narrow, it'd feel tiring to hold the bottle up high for long and you'd miss having a straw instead.
Another common thing to miss is the fill test. Hold the bottle and fill it under a kitchen drinking water faucet, under a wall-mounted water purifier, an office cooler, a bathroom tap, and even a Bisleri jar spout.
Do the sweat test with cold water by keeping it on an wooden desk, or in a bag with cloth wrapped around. This will also tell you about insulation.
Do the drying test where you wash the bottle and keep it to drain inverted. You'll see how all parts of the bottle need to be placed to drain water. You'll observe where water sticks. You'll know how long it takes for the parts to dry up completely.
And bottles drop too. So do the drop test as well. Typically from slightly above one metre. Drop it on tiles in different angles and see where the parts crack, come apart, scratch.
Speaking of tests, there's a great thing Sir James Dyson had said. If the test doesn't exist, build the machine that does it. If you want to take a break after this stage, watch Dyson's lab videos where there are labs full of rigs built for testing open and close mechanism of hinges, dust testing, and hundreds more.
Anyway, find places that can do the tests or do crude setups that are fairly easy too. Like just washing and putting bottles on a drying rack or a small motor that presses the button and swings the cap open and shut thousands of times.
Doing these tiny bit of practical DIY setups introduces flavour of engineering work in the middle of a design activity.
Modelling and prototyping will eventually get the product direction decisions, not renders.