by Mike Uhrain, Sumitomo (SHI) Demag, and Nick Sotos
Part lightweighting has become a major trend in plastics. Brand owners are embracing sustainability and eco-friendly initiatives and want to reduce the carbon footprint of their end products. Molders are striving to achieve these goals for their customers by producing lower-weight parts using less plastic, with little or no reduction in part quality.
A proven method for reducing part weight is the use of endothermic foaming agents in the molding process. These additives feature heat-absorbing characteristics, which help with cycle-time reduction and have proven to be effective with both commodity and engineering-grade resins.
In a recent application, endothermic foaming agents were used in an all-electric molding machine to mold shampoo bottle caps with a living hinge.
Machinery and equipment considerations
When embarking on this project, the principals considered numerous items to best address the goals of molding a quality end product using foaming agents. Fast cycle times are necessary for the molding to be effective, and faster cycle times require faster machines that allow overlapping cycles. Today’s all-electric molding machines are more precise and more energy efficient than their hydraulic predecessors, so an all-electric machine was chosen for the project. The specific machine chosen for this study had Flow Front Technology, which allowed for superior balancing of the cavities to ensure equal part weights. This technology works by precisely bringing the screw to a stopped position and allowing the flow front to continue to move forward and naturally equalize the pressure in the cavities before packing out the part (Figure 1). For foaming applications in multi-cavity molds, such technology is important to ensure equal amounts of foam enter each cavity to result in consistent part weights.
When considering foam for an application, a number of equipment considerations need to be addressed, including the following:
- Part design: geometry, surface finish, fill rate
- Application: speed, multi-component, assembly
- Production cell concept: new or existing mold, cavitation, resin delivery methods
- Mold design: runner system, gating methods, single face, stack, cube
- Production cost: cycle-time requirements, energy use, resin, investment, maintenance
- Special process requirements: decompressing hot runner, balance cavity pressure
- Process stability: reliability, repeatability, process optimization
This particular application focused on a flip-top closure (Figure 2), using the following equipment:
Machine: Sumitomo (SHI) Demag SE280EV-AHD C2200 (all-electric machine)
Mold: NyproMold 4-cavity single-face prototype mold
Material: PP MFI 12
Foaming agent additive: iD Additives 6265 iD Foam 70 MFC
The mold was modified from a two-component to inject as a single component to study the effects of the foaming agent on the living hinge. Objectives for the production of the part included reducing part weight as much as possible and reducing cycle time, while maintaining the aesthetics and part quality/performance of the closure.
Seven tests conducted
Seven specific tests were run. The following outlines each test procedure with results:
Test 1. Benchmark: Solid parts without foaming agents were molded at the customer-qualified time of 12.3-second cycles for the single component. The parts were molded without colorants so it would be easier to later observe the effects of the foaming agent.
Observation: Quality parts were molded within the spec of the overall dimensions.
Test 2. Introduce 1% foam by weight: Foamed parts were molded at the same set-up as the benchmark solid parts in Test 1.
Observations: The foam distribution was similar in both part halves. Internal dimensions were slightly smaller than, but almost consistent as, the solid parts.
Test 3. Further reduce hold time: Hold time was reduced to 1.8 seconds, 1.2 seconds, 0.4 seconds and 0 seconds (reduced cycle time as well).
Observations: Energy consumption per cycle decreased as hold time decreased, but part temperatures increased. The 0.4- and 0-second hold times caused parts to stick to the center of the mold core since it was too hot, but the 1.2-second hold time was optimal. Parts were predictably small. A slight energy savings was noted by reducing the hold time, but more interesting was the reduction in cycle time.
Test 4. Lower temperatures: Reduced barrel temperatures, mold temperatures, and screw RPM.
Observations: The maximum part temperature dropped from 174°C to 166°C, and for a 1.8-second hold time, energy consumption per cycle decreased by almost 10%. Even further energy savings could be realized by further optimizing reduced temperatures.
Test 5. Increase foam: Additive was increased from 1% to 1.5%.
Observations: No further reduction in part weight was noted. There is a physical limit to how much additive can foam for a given wall thickness.
Test 6. Check surface quality: Short parts were molded and then parts were molded at increasing injection rates. (Figure 3.)
Observations: Foam resulted in excellent surface quality without sink marks, even for really short shots. This shows that the hold phase is not required, since the foam expansion packs out the part. Excellent surface quality was noted for sub-1 second fill times. Splay was not witnessed until the injection rate was reduced down to 27mm/s, which is much slower than injection rates typically used in production for such parts. Part weight was not reduced at increased injection rates for this particular application.
Test 7. Check hinge performance: An industrial CT scanner was used to inspect the distribution of the foam at the hinge and surface. Molded-in stresses were checked with a polarized lens and, per customer procedure, the hinge was opened and closed 200 times.
Observations: There were no foam voids in the hinge area. A solid skin of resin on the outer 0.005″ of part showed that the foam does not migrate to the part surface. The foamed part had less molded-in stress than the solid part, and there were no hinge failures after flexing, showing that the foam maintains good hinge performance.
Hypotheses were developed and then tested when using foaming agents in the molding process. Results from the testing showed the following:
In addition, by adding foaming agents to the molding process, financial benefits were realized in the form of material and cycle time savings.
- Material savings: 4 percent material cost reduction
- Cycle time savings: 15 percent cycle time reduction
- Other savings: Less mold and machine wear, less vent cleaning, and less cooling water
- One-year savings (assuming 24-cavity production mold running 8,000 hours/56,000 parts per year): Annual material savings of $29,000 and annual production savings of 1,200 hours.
To isolate the benefits of foam, the added savings associated with running the mold in a very precise, energy-efficient, all-electric machine were not included in the above figures.
Note: The complete study with results and samples of the molded parts will be available and on display at the iD Additives booth at the MAPP Benchmarking and Best Practices Conference in Indianapolis, Oct. 10-12, 2018.
Mike Uhrain is Sumitomo Demag’s technical sales manager of packaging for North America and global key account manager. He holds a bachelor of science degree in mechanical engineering from the University of Akron in Ohio, has earned a Professional Engineer License and served as a Board Member for the Society of Plastics Engineers Injection Molding Division.
Nick Sotos is president of iD Additives, Inc. of La Grange, Illinois, a manufacturer of additives for the plastics industry. iD Additives, Inc. launched in 2005 with a line of foaming agents and has since grown to include UV stabilizers, purging compounds and liquid color and additives.