Plastics processing

(a) In injection molding, shrinkage is defined by the difference in size of the molded part (at room temperature) and the size of the mold cavity. The molding process parameters are set to reduce shrinkage. For each of the following molding parameters, write whether it will Reduce OR Have no effect OR Increase the shrinkage, with reason (3 lines max).

(i) Decreasing the molding temperature

(ii) Increasing the injection pressure.

(b) The figure below shows a problem that occurred during injection molding. The process engineer believes that in the region marked "burn mark" on the part, some air gets trapped in the mold cavity; as the plastic fills the cavity, the trapped air squeezes to a high pressure and temperature. This causes some plastic to oxidize. Assuming this to be the correct diagnosis, suggest a simple way to modify the mold design in order to avoid this problem (give reason why you think it will work).

(c) The figure below shows a small part that we wish to produce using injection molding. The mold must be designed to make 8 parts in each cycle. Assume that the parting direction is as shown in the figure. Make a sketch showing how you will arrange the 8 parts on the mold, indicating (i) the position of the gate(s) for each part, (ii) the relative positioning of the parts on the mold, (iii) the runners, and (iv) the point where the sprue will be joined to the runners. Give reasons for your choices in (i) - (iv).

[Hint: just show a simplified sketch for the part - no need to show details of the geometry] parting planeparting directionparting direction

Q2. Cutting and Process modeling

(a) You are required to cut a taper on a lathe using a single point cutting tool as shown in the figure below. The maximum surface roughness allowed on the tapered surface is Rmax.

Let: Spindle speed = N revolutions per minute;

f = axial feed (mm per revolution)

v = radial feed (mm per min).

Derive the formulas relating Rmax with f and v. Lead cutting edge angle = α°End cutting edge angle = β°Taper angle = θ°Radial directionAxial directionLead direction

Q3. Process Planning

(a) You are given a round bar stock 105mm in diameter and 1000mm long, from which you need to machine the following part. You have access to a saw, lathe and milling machine and any fixture. Make the process plan, listing:

- Each setup (in correct sequence)

- What machine will be used and how you will fixture the part for the setup (sketch if required)

- Machining operations in each setup, with operation sequence and tool used for each operation.

Q4. CNC Programming

(a) You need to machine a 5mm deep pocket in a metal part (as shown in the figures below). Three end-mills are available:

Tool 1: Φ6mm, max feed rate: 20mm per min (at depth of cut=5mm)

Tool 1: Φ10mm, max feed rate: 25mm per min (at depth of cut=5mm)

Tool 1: Φ16mm, max feed rate: 30mm per min (at depth of cut=5mm)

Assume that the top face is Z=0; Slot depth = 5mm; Home position of any tool is at (0, 0, 20); (0, 0)XY(XY

(a) Assume that the milling machine has automatic tool changing, and it takes 60 seconds to change a tool and set it to its home position. You must decide whether to use one, two or all three available tools to minimize the total (cutting time + tool change time). You may ignore the time for rapid motion of the tool.

 (b) For your selected option, sketch the motion of the tool(s), and write the GN-code program for performing the cutting. Write any additional assumptions you need to make clearly.

Q5. Joining and Surface treatment

(a) Give one advantage, and one disadvantage of joining two mild-steel parts using brazing rather than welding.

(b) The body of a tennis racket is made by joining several layers of composite materials together using adhesives. Usually a thermoplastic (acrylic-based) adhesive is preferred over a thermoset (epoxy-base) adhesive. Give one reason for this choice.

(c) In a building window, is it better to use a Silicone-based adhesive or an epoxy-based (e.g. AA superglue) to join the glass to the frame? Give reason.

(d) The outer sheet panels of airplanes are joined to the frame (the skeleton) using rivets, rather than nuts and bolts. Give two advantages of this choice.

 (e) Case hardening techniques (e.g.carburizing, nitriding etc.) are only applicable to steel parts. How can we get a hard coating on an Aluminum part?

(f) In spray painting, a small amount of the spray spreads outside the part to be painted; this is a process inefficiency, or loss. Between pressurized air (atomizing) spraying and electrostatic spray painting, which process do you think is less efficient (more lost paint), and why?