One Part, Two Materials, One Print

Most products that feel good in the hand are made of at least two materials: a rigid structure and a soft surface. A grip, a gasket, a sealed button membrane, a flexible boot around a rigid connector. In production this is achieved with overmolding or two-shot molding, which requires two tools and a coordinated molding cycle.

Multi-material 3D printing produces the same combination in one build, with no tooling. That is a genuine capability, and it is also frequently oversold. Knowing which of the two available process families you are dealing with, and what each actually delivers in terms of hardness range and bond strength, decides whether a multi-material print saves you two weeks or wastes them.

The Two Process Families

PolyJet and material jetting

Jetting machines spray droplets of photopolymer from hundreds of nozzles and cure them with UV light layer by layer. Because each nozzle group can carry a different resin, the machine can place rigid and elastomeric material voxel by voxel and blend them into intermediate durometers.

What that buys you: a continuous hardness range roughly from Shore 27A to a rigid photopolymer, in the same part, with true gradients rather than a hard boundary. Layer resolution around 16 to 30 microns, smooth surfaces straight off the machine, and multiple colors in the same build. For an appearance model that has to look and feel like a molded overmolded product, nothing else comes close.

What it costs you: parts run $200 to $1,500 for a modest assembly, service bureau turnaround is two to five days, and the material properties are photopolymer properties. The soft material tears more easily than real TPE, and everything degrades under UV exposure over months. These are appearance and fit models, not functional test parts.

Dual-extrusion FDM

A printer with two or more extruders lays down real thermoplastic — PLA, PETG, ABS, nylon — alongside a real thermoplastic elastomer such as TPU. Machines start around $600 and desktop-class dual systems are common in engineering offices.

What that buys you: genuine engineering materials with published mechanical properties, parts you can functionally load and test, and a per-part cost measured in dollars rather than hundreds of dollars. A TPU seal printed against a PETG body behaves much more like the production part than a jetted equivalent.

What it costs you: layers around 0.1 to 0.3 mm, visible layer lines, and the central problem of interfacial adhesion. Two dissimilar polymers do not necessarily bond. TPU sticks acceptably to PETG and PLA, poorly to ABS, and hardly at all to nylon. Print speed also drops sharply, since the machine purges material at every tool change — wasting 30 to 50 percent of the filament into a purge tower.

The broader process landscape and where each one fits is mapped in 3D printing technologies compared, and TPU-specific settings and behavior in TPU 3D printing.

When Two Materials in One Print Is the Right Call

Four situations justify it clearly.

  • The interface is the thing being tested. If the design question is how a soft grip transitions into a rigid handle, or how a seal lip meets its housing, printing them separately answers nothing.
  • The soft geometry is captured inside rigid geometry and cannot be assembled after the fact — an internal gasket, a flexure encased in a frame, a boot with an undercut retention feature.
  • You are validating an overmolding design before tooling. A multi-material print reveals whether the mechanical interlock geometry is adequate and where the material boundary lands visually, well before you commit to a two-shot tool. The production process it stands in for is described in overmolding and two-shot injection molding.
  • Appearance models for a customer or investor where the perceived quality of a molded soft-touch surface matters more than mechanical fidelity.

When an Assembly Is Better

Multi-material printing is not the default and often should not be. Choose a two-part assembly when:

  • You need real elastomer performance. Compression set, tear strength, and recovery on a jetted rubber-like material are far below molded silicone or TPE. A seal that must actually seal should be cast, not printed — the workflow is in casting soft silicone parts for a prototype.
  • You want to iterate one half quickly. Separate parts let you print six grip variants against one rigid body in a single day.
  • Cost matters and volume is above a few units. A rigid print plus a bought O-ring or a cast sleeve is usually a fraction of the multi-material price.
  • Adhesion between your two chosen materials is poor. A mechanical interlock — dovetails, through-holes, undercut ribs — printed as two parts and assembled beats a weak printed bond.

Design Rules That Make Multi-Material Prints Work

Do not rely on chemical bonding alone. Design mechanical retention into the interface exactly as you would for a production overmold: through-holes the soft material flows into, undercut grooves, and dovetailed edges. This is good practice for the printed prototype and mandatory for the tooled part.

Give the soft material adequate thickness. Below about 1 mm on FDM, TPU prints inconsistently and tears at the interface. Two to three millimeters is a comfortable minimum for a grip surface.

Avoid long thin soft features spanning rigid regions, which distort during printing, and keep the material boundary on a surface the toolpath can follow cleanly. Specify target hardness in Shore units in your CAD notes so the print matches production intent — how to pick that number is covered in Shore hardness and durometer selection.

Cost and Turnaround Reality

For a hand-sized part with a rigid body and a soft grip: dual-extrusion FDM in house runs $8 to $30 in material and eight to twenty hours of machine time. The same part from a jetting service bureau runs $250 to $900 with a three to five day turnaround. Cast silicone over a printed core lands in between at $100 to $400 including the mold, with better material properties than either printing route.

The sensible sequence is dual-extrusion FDM during functional iteration, cast silicone when seal or grip performance has to be real, and one jetted appearance model near the end. General cost drivers across processes are compared in how much a prototype costs.

Choosing the Right Prototype Route

Projects House builds prototypes across printing, casting, and machining, and picks the process from what the prototype has to prove rather than from what is convenient. Send your part concept and the question you need answered through our contact form.