MECH 502 Midterm - Selection Criteria

Form

Can the process make the shape?

Form

Stronger Processes

• Polymer PBF, binder jetting: no supports (powder carries overhangs);

• Material jetting: fine features, resolution, several materials and color in one part

Form

Weaker Processes

• DED: 2 1/2-D shapes with limited overhangs;

• Melt Extrusion supports past 45-degree overhangs, features limited by nozzle size;

• Metal PBD, Sheet Lamination: supports, trapped powder

Form

Potential Questions (Geared for Tooling)

Are there overhangs, internal channels?

What is the smallest feature?

Does the part fit the build volume?

Does it need more than one material?

Fit

Will the part assemble with mating parts?

Fit

Stronger Processes

• Material Jetting: excellent accuracy;

• Metal Laser PBF: excellent accuracy, tight features;

• Polymer SLS: high accuracy

Fit

Weaker Processes

• DED: poor accuracy, has to be machined to tolerance;

• Binder Jetting: poor accuracy, 20% sintering shrinkage;

• Melt Extrusion: good accuracy, Z steps and road width limit fit

Fit

Potential Questions (Geared for Tooling)

What tolerance is needed?

What is the load direction?

Will critical features be machined after the build?

Finish

Does the as-built surface meet the requirement?

Finish

Stronger Processes

• Material Jetting: excellent surface finish;

• Melt Extrusion: good surface finish

Finish

Weaker Processes

• Powder processes (PBF, binder jetting): grainy surfaces;

• DED: rough, needs to be machined;

• for any process supported/downward-facing surfaces are roughest

Finish

Potential Questions (Geared for Tooling)

Is there an acceptable surface roughness, or does it need to be completely smooth?

Are supports acceptable?

How much post-processing time and cost is acceptable?

Must the part hold up to UV, wear, or sterilization?

Function

Will the part perform in service?

Function

Stronger Processes

• Metal Laser PBF, DED: full density; excellent strength and toughness;

• Binder Jetting: excellent strength after sintering;

• Polymer SLS: good strength, near-isotropic nylon parts

Function

Weaker Processes

• Material Jetting: poor strength, UV sensitive photopolymers;

• Melt Extrusion: good strength in XY but poor in Z;

• Sheet Lamination: interfaces between layers are weak

Function

Potential Questions (Geared for Tooling)

What loads, temperatures, and environment will the part see?

Is full density required?

Which direction caries the load, how anisotropic is the process?

Are fatigue, creep, or biocompatibility requirements involved?

Financials

What does each part really cost?

Cost per part = (machine rate x built time + material + labor + post-processing)/parts per build

Financials

Stronger Processes

• Melt Extrusion: lowest machine and material cost;

• Binder Jetting: fast nested builds, excellent post-processing cost;

• Polymer PBJ, MJF: dense 3D nesting lowers cost per part

Financials

Weaker Processes

• Metal PBF, DED: costly machines, poor post-processing cost;

• Material Jetting: expensive resins and support waste

• for any process the post-processing labor is often the largest hidden cost

Financials

Potential Questions (Geared for Tooling)

How many parts are needed, how often?

What are the machine, material, labor, post-processing costs?

How many parts fit into one build?

What would conventional manufacturing cost at this volume?

Accelerating the Build

Build Time Fundamentals

• thicker layers cut layer count, but error grows with layer thickness;

• raising power lets speed, hatch spacing, or layer thickness rise at the same energy density;

• larger nozzle increases flow, may need to increase nozzle temp;

• layer-wise makes layer time nearly independent of part count

• point-wise layer time increases with total area being built