Background: There is a need to find ways of productively engaging K-12 students with engineering design. I investigate a new class of physical models, referred to as Improvable Models, as scaffolds for helping middle school students productively engage in engineering design practices. Purpose/Hypothesis: The purpose of this study is to answer two questions: (a) How do students use Improvable Models? and (b) How does the use of Improvable Models as scaffolds support students' design of engineering solutions? Design/Method: Design-based research was conducted in a sixth-grade classroom. Students used a type of Improvable Model, specifically the Suboptimal System (SS) seed model. Improvable Models make the entire design space accessible and enable iterative design optimization. An SS seed model represents inefficient design decisions and is used as an initiator for iterative design optimization. The study spanned 10 lessons, each 45 min long, totaling 8 days of instructional activity spread across 1 month. Data sources included student conversations, design work, and presentations. The productive disciplinary engagement framework was used to analyze the data. Results: Students used the seed model for productively engaging with five disciplinary practices: decomposing the design challenge into individual design parameters, considering the relationship between design parameters, reasoning through multiple design parameters and making trade-offs, weighing multiple solutions, and using design heuristics for ideation and design optimization. The seed model scaffolded students' productive engagement in various disciplinary practices such as informed decision-making and systematic exploration. Conclusion: This study provides an approach for designing learning environments for productively engaging K-12 students in engineering design practices using a suboptimal model.