Injection-Molding Process Control

When manufacturing building bricks from plastic by injection-molding, quality control of the process is done by monitoring the length of the brick. The length is dependent on the filling degree (how dense is the material) of the block and gives a direct feedback on many process parameters. The nominal length of a good-part is 20 mm +/- 0.1 mm. You are supposed to design a measurement system which is able to measure the length of the block up to a precision of +/- 10 µm.

Due to investment-considerations and measurement speed, a camera is no option:

a) Become creative: Describe at least two possible sensors to measure the length of the block! Consider non-manual measurement only, so use an electronic sensor and not manual scale. Make sketches of the arrangement with respect to the blocks given on paper! Describe advantages and disadvantages of your solution.

Finally after some arguments the controlling-department was convinced to implement a camera-measurement in production with a very strong light and a magnifying objective:

b) Considering the variations of color, a contrast level of 70% is considered to be process-safe in the current context. The figure below shows the MTF of the combined camera and illumination system. What is the measurement uncertainty of the camera according to the figure below in micrometer?

c) As an additional parameter the weight of the building brick is measured. There is the following correlation-measurement being done between weight and length-measurement with all errors already included. Identify graphically the measurement uncertainty. What seems to be the nominal weight of the brick?

d) Considering the requirements of the task, would you suggest weight-measurement based on the performance of the weight-sensor in c or length-measurement with the camera for this task? Why?

Points:

In an exam, task a) would get 30% of the points. Task b to d would get 70% of the points.