The Ice Cream Spoon, Part 2
In the previous Ice Cream Spoon blog, I wrote about Surface Modeling techniques in SOLIDWORKS. In this installment, I’ll discuss the plastic injection molding analysis of the spoon.
One of the unique design elements on the ice cream spoon is the large rib on the underside of the handle. I have a few ideas as to why the rib is there, but I want to use SOLIDWORKS Simulation tools to verify why the rib was included. The first tool to use is SOLIDWORKS Plastics to verify the injection mold capability of the design. I will run injection molding analysis on two versions of the spoon, with and without the underside rib.
Before I start the analysis, I researched common materials used for injection molded utensils. I found references to polystyrene, polypropylene, and acrylic, with the most common utensil material appearing to be polystyrene. The SOLIDWORKS Plastics materials database contains data for thousands of commercially available plastic resins. When I set up the injection molding analysis, I will specify one of the nearly two-hundred polystyrene resins as the injection material.
To begin the SOLIDWORKS Plastics analysis, I will create a solid mesh of the part to analyze the volume. I could use the shell mesh option, which solves fast and is very accurate for parts with uniform thickness, but it would not allow for the best comparison between spoon versions. Since the spoon with the underside rib has both thick and thin sections, the solid mesh is the better choice at the expense of increased solution time. I created the solid mesh using hybrid tetrahedral mesh settings. This generates two layers of prism elements on the part surface while the internal volume is created with tetrahedral elements.
For each spoon analysis, I will specify a gate size of 0.040” at the end of the handle. The injection fill time will be reduced to 0.3 seconds instead of the recommended ~0.55 seconds. This small change will help to reduce cycle time and is a common practice with injecton molding applications. I will not modify the resin parameters, such as melt temperature or ejection temperature, which may be done during the setup of processing parameters for production molding.
Once the two FILL analyses have been solved, it’s time to compare results. The spoon with the underside rib fills with no warnings and a calculated injection pressure is 5433 psi. The spoon without the underside rib also filled, but at a higher injection pressure, which was expected. The maximum injection pressure for non-rib spoon was 8507 psi, 56% greater than with the rib. The best benefit of the spoon without the rib is the estimated cooling time – 3 seconds instead of 4.8 seconds, which would increase cycle time. From a production standpoint, though, the rib version is better because the part will not be molded one at a time and the lower injection pressure allows for a larger multi-cavity mold tool.
As I and my co-workers gathered the shiny, pink spoons – that’s a code phrase for happily eating ice cream – I thought it was important to determine how large an injection mold tool was being used to manufacture the spoons. In a family mold each cavity often has a small engraving to identify the cavity a part is ejected from. This aids in mold tool maintenance, an important aspect of high-volume molding applications. The highest number on any of the collected spoons was 32. After having eaten copious amounts of ice cream and not received a spoon with a cavity number greater than 32, I think it’s safe to assume that the production mold is a 32-cavity tool.
The final SOLIDWORKS Plastics analysis is for a 32-cavity ice cream spoon mold. I did this as a multi-body part file. I also created a sketch with many line segments to represent the sprue and runner system connecting each of the cavities. In the setup of this SOLIDWORKS Plastics project, the sketch lines are used to generate the sprue and runner system. The shape of the runner path is significant in that the fluid path length from the sprue to any individual cavity is the same. This will help ensure each cavity fill ends at the same time, balancing out the plastic flow.
For this analysis, I added more complexity to gain a better understanding of the spoon molding process. Using SOLIDWORKS Plastics Premium, I included a virtual mold and cooling lines along with the cavities and sprue and runner system. By adding these elements there are some nice benefits with the more complete model.
By adding the cooling lines and mold, I set up a batch run for FILL, then COOL, then another FILL analysis. The initial FILL analysis generates a first pass temperature for each spoon cavity. Then the COOL analysis – with the cooling lines, mold tool and additional processing parameters such as mold-open time – solves the actual temperature of each mold cavity. This, in turn, improves the second FILL analysis and generates more accurate results for estimated cooling time, sink analysis, warp analysis, and other important results. Looking at the results, the plastic flow pattern shows that each of the 32 spoons will complete filling at the same time.
As for the cooling time, most of the volume of every spoon has cooled within five seconds, but not the sprue and runner system. Since I do not design injection mold tools for a living, I likely oversized the sprue and runners even though I followed design guidelines I learned many years ago working with a good friend, John Majors. I would have thoroughly enjoyed talking with John about this blog project, but sadly he passed away earlier this year.
There are a few other things I should note about my 32-cavity spoon mold and my sprue and runner system. I suspect that the real injection mold tool for the ice cream spoon is done with a hot runner system to reduce scrap. My design would use more plastic for the sprue and runners than is used for all 32 spoons. Also, the injection pressure for the 32-cavity mold was approximately 12,000 psi, which is definitely not a linear increase of pressure with the number of cavities. So either I was very good at remembering my discussions with John, guessing the correct sprue and runner system size to minimize pressure drop or it is significantly oversized.
Back to the questions of why the spoon has the rib on the handle. With regards to the injection molding capability of the design, the rib definitely helps improve the molding of the part by lowering injection pressure at the cost of increasing cooling time. In order to build a large, multi-cavity tool for mass production, this is a decent trade-off. In my next blog related to the ice cream spoon, I’m going to investigate the structural performance of the spoon with and without the rib using SOLIDWORKS Simulation Premium. Now go make your products better with SOLIDWORKS Simulation!