INBDE (Integrated National Board Dental Examination) Practice Exam

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What causes back-pressure porosity of metal?

  1. Too thin sprue

  2. Too long sprue

  3. Too short sprue prevents venting of gas

  4. Incorrect temperature control

The correct answer is: Too short sprue prevents venting of gas

Back-pressure porosity of metal occurs when trapped gas within the metal creates interior voids. This is caused by inadequate venting of the gas during the pouring process. A thin sprue (option A) may result in a fast flow of metal, which can cause turbulence and trap gas. A long sprue (option B) may not provide enough pressure to push the gas out of the mold. Incorrect temperature control (option D) can also affect the flow of metal and result in trapped gas. However, the main cause of back-pressure porosity is a short sprue (option C). A short sprue does not allow enough time for the gas to escape during the pouring process, resulting in trapped gas pockets within the metal. Therefore, option C is the correct answer.

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