CNC Machined vs. 3D Printed: Creating the Master Pattern

This article is part of a series on cast urethane manufacturing. You can check out part one here.

In part one, we reviewed the pros and cons of injection molding, citing instances where low volume or short timelines require an alternative production method such as cast urethane manufacturing. Cast urethane manufacturing begins with a master pattern. Usually, that master pattern is 3D printed using PolyJet or Stereolithography (and occasionally Fused Deposition Modeling); however, there are circumstances where a CNC master pattern is required. CNC machined master patterns are preferred over 3D printed patterns when it is necessary to produce more than one or two molds and/ or to meet stricter dimensional tolerances while 3D printed patterns are preferred for more complex parts with faster turnaround timelines.

urethane casting

Consider the following when choosing between CNC machining or 3D printing your cast urethane master pattern:

PART VOLUMES

When creating the silicone mold for cast urethane manufacturing, the master pattern undergoes certain stresses. These stresses can damage the master pattern over time, meaning a single 3D printed pattern may be viable for one mold (and sometimes two) before it begins to lose accuracy. CNC machined master patterns can draw from a more durable and heat resistant material base (nylons, polycarbonate, ABS etc.) than the materials available with PolyJet and Stereolithography, to result in master patterns which retain their accuracy and viability for the production of multiple molds over time. These enhanced materials make CNC machined patterns optimal for cast urethane part volumes that would require making several molds from a single master pattern.

STRICTER DIMENSIONAL TOLERANCES

CNC machined master patterns can also meet tighter dimensional tolerances than 3D printed master patterns. Standard tolerances for CNC machined parts are +/- 0.005” or 0.001”/”, whichever is greater, though even tighter tolerances can be achieved depending on part design. Therefore, CNC machining is often a more accurate choice for part geometries with critical tolerances.

GEOMETRY CONSIDERATIONS

While CNC machined patterns can achieve involved geometries, the more complicated the design the more dependent it becomes on manual fabrication and post-finishing. 3D printed master patterns can complete involved designs in a single build without additional machining. However, it is important to remember the cast urethane part itself cannot mirror the same complexities 3D printing effortlessly achieves; a cast part is still subject to mold release constraints.

ALTERNATIVE MASTER PATTERN TECHNOLOGIES

An additional alternative to CNC machined, Stereolithography or PolyJet master patterns (which, again, are the most commonly used master pattern technologies for cast urethanes) is Fused Deposition Modeling (FDM). FDM cannot achieve the smooth surfaces of PolyJet and SLA without significant post-processing; however, FDM parts can print in multiple, highly heat and chemically resistive materials. FDM additionally has good tolerances with a great accuracy rating; it is not prone to warping during its heated manufacturing process and is widely appreciated for its snap-fit applications. FDM master patterns can be used for longer mold volumes than SLA and PolyJet and is generally faster than CNC machining.

When deciding which master pattern manufacturing process to use, consider the service life, geometry and turnaround. Also, talk to a Stratasys Direct Manufacturing project engineering expert to ensure the best fit to your project. In part three, we’ll discuss what constitutes a complicated mold and some tips for cast urethane part design.

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