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Concrete casting with 3D-printed moulds

Design Project 4 weeks Medium build

Design an object in Fusion, turn it inside out into a printed mould, and cast it in concrete. A planter, a speaker body, a desk organizer, or the heavy base of a lamp. The print is the tool; the concrete is the product.

A small cast-concrete planter with sharp facets, sitting beside the two halves of the grey 3D-printed mould it came out of, on a workbench dusted with cement.

Overview

Concrete is cheap, heavy, cool to the touch and takes any shape you can hold it in while it sets. Making that shape used to mean carpentry or silicone. Now the mould comes off a printer overnight, so the design work moves into CAD, where it belongs. You model the object you want, subtract it from a block, cut the block into parts you can pull off, and print those. Pour, wait a day, open it up.

The product can be almost anything that benefits from mass and a hard surface. A planter with a drain hole. A passive speaker that uses the concrete’s weight and stiffness to stop it buzzing. A desk organizer with pockets for pens and a phone slot. The base of a lamp, cast around a threaded rod so a wooden stem screws into it. Pick one you would keep.

The mould is the whole project. A mould with vertical walls holds the cast forever. A mould with no keys casts a shape with a step at the seam. A mould that is too thin bulges under the wet mix and the cast comes out barrel-shaped. Every one of those is a decision you make in Fusion, weeks before you touch cement.

Skills needed

  • Modelling the object, then the mouldThe object is the positive. The mould is the negative, made from it with Combine → Cut, and the cavity has to be reachable and releasable.
  • Draft, split lines and registrationresource comingEvery vertical face leans at least 2°. Where the mould splits, both halves need keys so they line up, and the split has to be somewhere you can hide the seam.
  • Designing the print as a toolThin walls, no infill to speak of, oriented so the cavity surface is the good one. The mould is used once or a dozen times and then it is scrap.
  • Cores and insertsA drain hole, a cable pass, a pocket for a driver or a bulb holder, a threaded insert set into the wet mix. Parameters for every one.
  • Mixing and casting concreteresource comingFine mix, right water ratio, vibrate out the air, cover it, wait. Cement is caustic; gloves, glasses and a dust mask when it is dry.
  • Demoulding and finishingresource comingWhen to open it, how to get it out without breaking either, and what sanding, sealing and felt do to the result.

Assessment

Criterion Developing Proficient Exemplary
Design brief Object chosen, little reasoning Clear use, references studied, concrete’s mass or surface is why it is concrete The brief predicts the hard part of the mould and plans for it
CAD Object modelled; mould made by hand Mould derived from the object with Combine; draft, wall and split as parameters Changing a parameter regenerates a valid mould; cores and inserts modelled and located
Mould design No draft or keys, seam in the wrong place Draft on every face, keys on the split, seam hidden or intentional Releases in one pull; mould reusable; print under budget with a plan for the cavity surface finish
Casting Voids, honeycombing, cast broken on demould Sound cast, air worked out, demoulded whole Surface as good as the print face, inserts true, edges crisp
Function and finish Feature missing or not working Feature works, object level, sealed Object does its job well and looks like something you would buy
Record and reflection Photos only Mix, ratios and times recorded; failures documented Reflection links a mould decision to a result on the cast and says what v2 changes

The cast is judged as a product, not as a demonstration. Casting is where the mould’s decisions show up.

Deliverables

  • A one-page design brief for the object, with three reference photos of cast-concrete products and what you took from each
  • The Fusion file with the object as one body and the mould parts derived from it, with draft, split and wall thickness as parameters
  • The printed mould, with a note on print settings, time and material
  • A casting record with the mix, water ratio, cure time, demould time, and photos of every cast including the failures
  • The finished object, with any inserts working, sealed, and standing level
  • A short reflection on what the seam, the surface and the fit taught you about mould design

Materials

  • PLA or PETG for the mould, 0.4 or 0.6 nozzle
  • Fine-aggregate concrete or a sand-and-cement mortar mix; for smooth surfaces, a rapid-set or countertop mix with no coarse stone
  • Mould release, either a spray or a thin coat of vegetable oil or petroleum jelly
  • Mixing tub, scale, measuring jug, trowel, and a sander or a rubber mallet as a vibrator
  • Threaded inserts, a length of cable or a dowel for cores, silicone or foam for sealing holes
  • Sandpaper 120 to 400 grit, a concrete sealer, felt feet
  • Nitrile gloves, safety glasses, a dust mask, a bucket for wash water that never goes down the sink

Steps

Week 1: Choose, research, model the object

  1. Look at cast-concrete products: planters, speakers, lamps, bookends, tabletop objects. Find three you like and write down for each what the concrete is doing (mass, surface, stiffness, cool touch) and where the seam is hidden.
  2. Choose your object and write the brief: what it is for, who uses it, what it holds or carries, what has to fit into it, and roughly what it should weigh. Concrete is about 2.3 g/cm³; a 1 L object weighs 2.3 kg.
  3. Model the object in Fusion as a single solid. Set parameters first: overall size, wall thickness (12 mm minimum), draft angle, core diameters.
  4. Add the functional feature as geometry: a drain cone, a cable channel, a pocket that matches a measured part. Model the real part as a reference body and check the fit.
  5. Look at every face and ask which direction it pulls. Faces parallel to the pull get draft. Faces that trap the mould get a split or become a core.

Week 2: Derive and print the mould

  1. Make a block that surrounds the object with 4 to 6 mm of clearance. Combine → Cut the object from it. What is left is the cavity.
  2. Decide the pour face. Usually the bottom of the object faces up in the mould, so the top surface people see forms against the smooth mould floor.
  3. Split the mould where it has to come apart: usually one plane through the middle, sometimes two. Add registration keys (a 6 mm hemisphere and a matching dimple) on the split faces. Add bolt tabs or a groove for a band.
  4. Design cores as separate pieces that slide out after demould: a tapered plug for the drain, a dowel for the cable pass, a printed sleeve for an insert.
  5. Slice. Cavity surface up where you can. Two or three walls, 5 to 10% infill, no supports inside the cavity. Add thickness where the mould is tall; wet concrete pushes hard at the bottom. Check the time estimate against the budget. Print.

Week 3: Cast

  1. Assemble the mould dry. Check the keys seat, the bolts pull it tight, and there is no gap at the split you could slip paper into. Seal a stubborn gap from the outside with tape or clay.
  2. Apply release to every cavity surface, thin and even. Too much release leaves pits in the cast.
  3. Weigh out the mix. Follow the bag’s water ratio by weight, on the dry side of the range; a wetter mix is easier to pour and weaker with a worse surface. Mix until there are no dry pockets and it holds a peak.
  4. Pour in stages, a third at a time. After each, vibrate: hold a sander without paper against the outside of the mould for 20 to 30 seconds, or rap it firmly with a mallet all the way round. Watch the bubbles rise. Set inserts and cores now, and check they are square.
  5. Cover with plastic and leave it. Twenty-four hours for a rapid-set mix, 48 for standard. Do not peek. Keep it out of the sun and the cold.

Week 4: Demould, finish, reflect

  1. Open the mould slowly. Bolts out, then work the split with a plastic wedge. If it fights, wait another day. Pull cores straight out, twisting slightly.
  2. Look at the cast before you touch it up. Voids, ridges, a step at the seam, a soft edge: each one points to a mould decision. Photograph it.
  3. Let it cure another 3 to 5 days, damp and covered, before you load it. Then sand the edges and any seam, 120 to 400 grit, wet. Fill any large voids with a slurry of the same mix if you want them gone, or leave them.
  4. Seal it, especially a planter. Add felt to the base. Fit the real part: the driver, the bulb holder, the pens, the plant.
  5. Write the casting record and the reflection. Say what you would change in the mould for a second cast, and if the mould survived, make it.

Stretch options

  • Cast the same mould in two mixes, one with pigment or a different aggregate, and compare surface and weight.
  • Design a two-part product: a concrete body and a printed or laser-cut part that fits it exactly, like a lid, a tray or a stem.
  • Make the mould from a flexible filament for shapes with undercuts, and compare the release to the rigid version.
  • Cast a set of three that nest or stack, with the spacing as a single parameter.