My strengths are in part design, mold design, mold debugging, part quoting and project management. But I am a wannabe processor. That’s because tooling and molding are so intertwined. Without a thorough understanding of both fields, solving molding problems can be challenging and often frustrating. I reached out to John Bozzelli and asked if he would collaborate on this article to help bridge the two fields. The goal of this article is to help those involved in processing and troubleshooting an injection mold to produce the best parts possible at the fastest possible cycle time.
To keep the length of this column to minimum, many of John’s Plastics Technology columns, as well as some articles authored by Phil Burger of Burger and Brown Engineering, are referenced for additional information. While this series will go into a lot of detail, it’s nothing compared to the multitude of additional factors a processor should consider when trying to establish the best process. I am only going to discuss those items that effect the cycle time.
Many of you know the age-old saying in our industry that that we don’t sell molded parts, we sell press time. That’s not entirely true. The fact is we sell estimated press time, and that is where a molder can either be very profitable or lose his shirt.
At the very start of any project, before a mold is even built, or before a molder receives an existing mold from a customer, the parts have to be quoted. There are at least 14 different factors to be considered when estimating the cycle time. Instead of rehashing them here, they are well covered in the February 2021 Tooling Know-How column, Get Better at Quoting Injection Molded Parts–Part 3. Of those 14 factors, only the cycle time is an estimate—and all too often, a guess-timate.
Even if a flow analysis is performed, the cycle time is still a guess-timate. While a flow analysis may be able to give a fairly accurate prediction of the required fill time and cool time, it cannot predict how well the mold, the machine and, in many cases, the operator will function. It also can’t predict how well the process will be established on any given machine—especially an older model. Therefore, the goal in quoting any injection molded part is to be very critical of the estimated cycle time and add a small percentage to cover any unforeseen issues without pricing yourself out of the job.
If the annual quantity of parts is on the low side, you are not going to lose too much profit if you underestimate the cycle time. But for very high-volume jobs, that cycle-time estimate better be very accurate or you are really going to get hurt. In the real-estate business, the slogan is location, location, location. In our business the slogan should be cycle time, cycle time, cycle time.
Read more: What’s Controlling Your Cycle Time? Part 1