M+P Injection Molded Part Project
For my project I m designing a handle for a can opener that will be injection molded. I am doing an over mold, so there will be two materials, and will add lot more to design considerations. The handle will first be created by injecting Polypropylene into the mold with several units to produce more faster, then they will be broken apart into individual parts, this part will be force fitted onto the metal knob. The Santoprene will then be over molded onto Polypropylene. This is done quite often in consumer products (especially in kitchen products), and the material capabilities allow this to be manufactured easily. the PP provides the support and strength, while the Santoprene provides grip.
my design considerations for this part need to support torque, needs to b kitchen safe, food safe, washable, ergonomic, and force fit the metal shaft. there will only be two ribs on either side of the part being 0.6mm. Shrink rate of PP is 1%-2.5%
the Parting line will run along the edge of the pp part in its flattest dimension. The Santoprene part will have a part line that also runs along the same 2 dimensional plane. The gate is needed and will be placed on the middle and top of the PP part, there will be small knit lines on the outside of either circle, so knit lines are on the out most ends of the part, which get the least amount of torque. It is important that this shaft have no knit lines or week points because the shaft will hold the .25 inch metal rod, a high torque area for the PP molded part. The PP part will have draft angles of 1.5% and the Santoprene will have a draft angel of 2.0 because even though it is a rubbery part that has flexibility to it making it easier to pull out of the mold, santoprene has a higher surface friction coefficient making it less easy to pull from the mold. The mold has a cam to make it strippable, the cam forms the space the metal rod will fit into. Black or clear PP can be any Finnish b/c we wont see much of it, so a glossy would look best but what ever is cheapest will definitely suffice. The Santoprene only comes in black; this will provide the grip, which will look good over the black pp.
Material Choice: I needed a hard, kitchen safe plastic that was washable, and strong. This led me to PP and PE I pick PP over PE because it was stronger and that was most important to me, however it was also slippery finish, so I decided to do a over mold with Santoprene to add a rubbery cushion as well as traction for the user. Over molding these two parts has been done a lot before in kitchen products so I think I will be perfect for this part. Both plastics don’t shrink when cooling very much, and both have around the same melting point, with the over molded plastic being a little lower. PP is stronger and easier to mold than PE, and can be mixed with other things to make even stronger, its FDA approved, food safe, and in production its very energy efficient. PP is inexpensive at standard grade, has a low shrink rate, is corrosion resistant, and Santoprene can be over molded onto it. Santoprene, is Fatigue Resistant, chemical resistant, creep resistance, dimensional stability, heat aging resistant, has a Low compression set, and is Ozone Resistant. It will provide a rubbery black grip; which is perfect for what I want for the knob. Santoprene has a low shrink rate and can also be easily molded.
