3D Printing, Casting and Molding: Finding the Right Process to Manufacture Over-Molded Designs

Designs that incorporate grips, soft touch surfaces or other features for compatibility with manual use (or when incorporating inserts directly into a part), traditionally use over-mold manufacturing via injection molding. This form of manufacturing typically requires larger volumes to account for traditional two-shot molding processes which significantly increase costs. Prototyping over-mold designs are therefore not usually practical given the cost. However, today, 3D printing over-molded parts is possible for prototyping and even low volume needs. By eliminating tooling, and subsequent two-shot molding processes, over-mold 3D printing with PolyJet delivers a cost effective prototyping solution. Another cost effective alternative to injection molding is urethane casting for low volumes of production over-molded parts.

The opaque flexible material has been over-molded into the transparent rigid material. This was achieved via a single print.

Producing Over-molded Units with 3D Printing

PolyJet is one of the only 3D printing technologies capable of manufacturing parts in multiple durometers and colors. It is most significantly used for high fidelity prototypes and models, but its move towards more rugged short-term applications has strengthened in recent years thanks to material developments which include biocompatible options and materials with higher tensile and higher heat deflection mechanical properties.

PolyJet can 3D print in rigid and flexible in a single build directly from a 3D CAD file to created over-molded parts. Through dozens of material combinations, PolyJet delivers parts with shore hardness ranging from 40-90A in multiple colors and opacities. Designers often turn to PolyJet because it can print a design within hours, providing immediacy to a project without increasing the project’s overall cost. Because PolyJet doesn’t require tooling to deliver an over-molded part, it is frequently used during early concept prototyping for designs requiring an elastomeric surface, such as grips or buttons, before moving into final product development stages.

Common PolyJet Over-Mold Applications Include:

  • Concept modeling elastomeric product features

  • Testing out durometers and gauging the right durometer for a new product

  • Creating soft touch surfaces for temporary housings for delicate designs

  • Manufacturing aids with soft touch surface for new prototype design

PolyJet is the only rapid prototyping technology available that delivers immediate over-mold concept realizations that incorporate all material deviations in a single part without tooling or any manual application.

Urethane Casting for Low Volume Over-Mold Parts

For low volume production of over-mold or insert molded parts, urethane casting is the most cost effective process. Urethane casting achieves a similar durometer range and material quality of injection molding; however it eliminates the need for a hard tool. Urethane casting uses 3D printed master patterns to significantly reduce the cost of manufacturing a cast product. The 3D printed master pattern is used to form a soft tool or silicone mold. Urethane casting requires two silicone molds to create the substrate and over-molded elastomeric material. The process achieves similar bond strength to injection molding.

One significant advantage urethane casting offers is its ability to produce micro-welded inserts. Similar to insert molding, micro-welded inserts are cast directly into the part. Micro-welded inserts are on average a 50-100% stronger bond than bonded inserts and over 100% stronger bond than heat staked inserts.

This cast urethane over-molded part features rigid (in black) and elastomeric (in blue) materials. Urethane casting offers strong material bonding for over-mold applications.
Common Urethane Casting Over-mold Applications:

  • Complex geometries that would be cost prohibitive to create steel tooling for

  • Clear resins over-molded anatomical models as training models or samples

  • Large paneling with micro-welded inserts for transportation and medical applications

Urethane casting for over-molded multi-material or insert molded parts is a great resource as both a final prototype before moving into injection molding and a solution for low volume production.

Injection Over-Molding

Injection molding becomes the most cost-effective choice when part volume is high. Because over-molding with injection molding requires more than one tool, the cost for over-molding can quickly become a barrier to part production. The lower the part volume, the more likely urethane casting is the right solution. However, because urethane casting uses a soft tool, its tool life is shorter and therefore higher volumes become less cost sensible to execute with urethane casting. Testing out the prototype through urethane casting – which simulates injection molding without hard tooling – provides for the best design readiness when finally moving into large volume production with injection molding.

Stratasys Direct Manufacturing offers a range of thermoplastic elastomers (TPEs) alongside the most common and hardy injection mold plastics such as ABS, PC and nylon. Our over-mold injection molding solutions are most commonly used to produce consumer goods such as bottles, medical cart manufacturing such as telemedicine robots used in hospitals, transportation goods such as panels and screens, and many similar large volume applications that require frequent human interaction or use and therefore require a soft touch surface of some manner.

Common Multi-Shot Mold Applications:

  • Touch screens and panels

  • Robotic interfaces

  • Consumer goods

  • General large volume multi material production

Multi-shot molding is still the go-to manufacturing solution for large volume manufacturing.

Beginning Your Over-Mold Project

At Stratasys Direct Manufacturing, we’ve found the best way to begin a new over-mold project is to start small. Experiment with material thickness and durometer using PolyJet before moving into injection mold plastics. If you’re unfamiliar with how material thickness relates to softness when it comes to more flexible materials, testing out the rubber-like PolyJet materials is an inexpensive way to get familiar with durometers and determine what your product really needs. Once you’ve finalized design, prototyping through urethane casting provides the fastest turnaround on injection mold quality products. Our customers often move into urethane casting for initial product launches while waiting for their hard tool for injection molding. Wherever you are in your design process, you’re not limited in manufacturing solutions.

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