Injection molding of Bulk Molding Compound (BMC) is a manufacturing process used to form thermoset composite materials into strong, dimensionally stable parts with tight tolerances and repeatable quality. Because BMC is a thermosetting system, it cures irreversibly in the mold, meaning the finished component will not soften or re-melt when reheated, which is a major advantage for high-temperature and electrically demanding applications.
What BMC is and why it matters
BMC is supplied as a ready-to-mold composite “dough” consisting of:
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Unsaturated polyester (or similar) thermoset resin: the binder that flows during molding and then crosslinks during cure.
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Fiberglass reinforcement (typically chopped): increases stiffness, strength, and impact resistance.
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Fillers (often mineral-based): improve dimensional stability, reduce shrinkage, enhance surface finish, and can lower cost.
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Additives: tailored to the application—e.g., mold release agents, pigments, flame retardants, and processing aids.
This formulation provides BMC with a balanced set of properties, commonly including excellent electrical insulation, heat resistance, corrosion resistance, and good mechanical performance. As a result, BMC is widely used in automotive, electrical/electronics, and industrial applications, including housings, connectors, sensor bodies, and under-hood parts.
How the injection molding process works
While details vary by equipment and part design, the process generally follows these steps:
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Material preparation and feeding
BMC is loaded into the molding machine (often via a specific feed system designed to handle its high viscosity and fiber content). Unlike thermoplastics, it is not melted; instead, it is processed in a flowable composite state. -
Preheating (when used)
The BMC may be preheated to reduce viscosity and improve flow into thin walls or complex features. The goal is to enhance mold filling without initiating premature curing. -
Injection into a heated mold under pressure
The machine injects the BMC into a heated, closed mold at high pressure. The mold temperature is set to promote rapid curing while maintaining good surface quality and dimensional control. -
Cure (thermosetting reaction) and part consolidation
Once in the mold, the resin undergoes a crosslinking reaction (thermoset cure). This chemical reaction transforms the material from a deformable compound into a rigid, fully consolidated composite. Because the polymer network is crosslinked, the part becomes permanently set and cannot be reprocessed by melting. -
Demolding and finishing
After sufficient cure, the part is ejected. Any required secondary steps, such as trimming gates/flash, drilling, or adding inserts, are performed depending on design requirements.
What this method is good at
Injection molding of BMC is valued because it delivers:
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Precision and repeatability: consistent dimensions and performance across high-volume production.
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Complex geometries: features such as ribs, bosses, threads (where appropriate), and intricate housings can be molded efficiently.
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High performance in service: good retention of properties under heat and in electrically insulating roles.
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Durability: strong, stiff parts with good environmental resistance, often suitable for demanding industrial settings.
Key point to emphasize
A central distinction is that BMC is a thermoset. The molding process is not simply shaping, it is shaping plus curing. The “set” happens through chemistry (crosslinking), not cooling from a melt. That is why BMC parts maintain integrity at elevated temperatures and why they cannot be re-melted after molding.
MD&M West 2026
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Whether you are exploring new capabilities, scaling advanced manufacturing processes, or seeking customized training solutions, our experts will be on hand to share insights and discuss how Modlex3D can support your goals. We look forward to meeting you in Anaheim and starting a productive conversation about your training needs. Learn more about the event at: https://www.mdmwest.com
Plastics Technology Expo (PTXPO)
The Plastics Technology Expo (PTXPO) is one of North America’s premier trade shows for plastics processing professionals. The event brings together manufacturers, equipment suppliers, material providers, and technical experts to showcase the latest innovations, technologies, and best practices shaping the plastics industry. PTXPO serves as a key forum for education, networking, and hands-on exploration of advanced manufacturing solutions across injection molding, extrusion, automation, and related disciplines.
Attendees are invited to visit Booth #827 at PTXPO 2026 to learn more about Kruse Training, Inc., eLearning and Molding Expert, LLC, and their cutting-edge virtual reality (VR) mold training solutions. These immersive training platforms are designed to enhance workforce development, improve process understanding, and accelerate skill acquisition for plastics professionals at all experience levels. To learn more about the event, including registration and exhibitor details, visit www.plasticstechnologyexpo.com.
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About Kruse Training & Molding Expert, LLC
Kruse Training is a comprehensive, subscription-based online knowledge and training platform developed for professionals involved in plastic product development and manufacturing. It is designed explicitly for part designers, mold designers, and senior process engineers who require both foundational understanding and advanced, practical expertise. The platform is future-oriented, addressing current industry challenges while preparing users for evolving technologies and best practices.
The Kruse Training curriculum is structured to present information in a clear, logical progression, ensuring that complex concepts are introduced, reinforced, and mastered systematically. Its proprietary focused-learning methodology integrates animations, simulations, instructional videos, quizzes, and multimedia presentations. This blended approach accelerates skill development, deepens technical understanding, and builds professional confidence by connecting theory directly to real-world application.
Kruse Training and Molding Expert, LLC shares a common mission: to help organizations develop highly effective, cross-trained teams capable of successfully designing and molding plastic components. When knowledge flows seamlessly from the part designer to the mold designer to the process engineer, and then loops back for continuous improvement, an actual Circle of Knowledge is created. This collaborative, systems-based approach reduces errors, shortens development cycles, improves part quality, and drives overall manufacturing efficiency.
Kruse Training provides the educational foundation that enables this collaboration, while Molding Expert, LLC delivers specialized expertise and a VR APP to support complex molding challenges. Together, they empower professionals and organizations to achieve consistent, high-performance results across the entire plastic product lifecycle.
For more information about Kruse Training, visit www.krusetraining.com.
For more information about Molding Expert, LLC, visit www.molding-expert.com.
