Moving right along with this ongoing series of communications about the field of engineering design and how you can be more effective in it, let’s talk now specifically about plastic molded parts. Just look around and note how many parts are made out of plastic. The vast majority of plastic parts are created in a process called “Injection Molding” (IM). Basically, a machine takes plastic pellets, melts them and then injects the molten plastic into a steel or aluminum mold, where the plastic is cooled, the mold then opened and out comes the part in the shape of the “empty space” inside the mold. It is sort of like pouring hot liquid Jell-O into a pan. You then cool the Jell-O, take it out of the pan and the Jell-O has the shape of the inside of the pan.
One needs a good understanding of plastics in general and an excellent understanding of the IM process specifically in order to design parts that are cost-effective and meet all requirements. Starting with a wood or metal part for example, the first tendency is to just substitute the material and make the “same” part out of plastic. This is virtually always the wrong way to go.
First off, you need to know some of the idiosyncrasies of the IM process that makes it very difficult (costly), though not impossible, to obtain features that are obvious and “free” on a machined part. For example, having vertical walls on a machined part is a virtual “given” while on an IM part very costly. Likewise, having ribs and fillets are very expensive on a machined part but “free” using the IM process. Having widely differing wall thicknesses on a machined part is a non-issue, while maintaining a uniform wall thickness on part made using the IM process is a very big “plus.” You need a good and thorough grounding in the basic design considerations of IM to be successful.
Sometimes a slight change in a feature of a plastic part can make a huge difference in the initial cost of a mold, the ongoing maintenance of it and the ultimate quality of the part. I will go into more detail and give you a real-life example of how such a small change saved a company many, many dollars and even more “headaches” had they proceeded with their original design.
Meanwhile feel free to call me for a no-cost, no-obligation appointment, so I can help you with any of your product design or re-design issues or those related to the manufacturing process.
“Big or small, I do it all.”