Rotomolded cores: New options for sandwich composites

Permanent or removable, these hollow thermoplastic cores broaden industry offerings.

 

As interest in composites grows, so do opportunities for companies with innovative materials and/or process technologies that boost production efficiency, reduce part mass and cost while improving performance and broadening product design options. Such is the case with a new rotomolded core technology from Italian plastic machinery OEM Persico SpA (Nembro, Italy), now in trials with several European automakers. 

Persico’s process produces symmetrical and asymmetrical hollow cores in a variety of smooth or textured finishes from a range of thermoplastics for use in sandwich-panel constructions. The hollow cores, of course, are lighter and use less material and, therefore, could be priced more competitively than solid cores of the same material. Rotomolding involves injecting or placing a measured charge (shot) of thermoplastic powder in an enclosed, heated mold, which is then rotated simultaneously around two perpendicular axes. The heat and two-axis motion cause the material to melt, flow and coat interior mold walls at a uniform thickness, forming a hollow component. The mold surface is then cooled while rotation continues, causing material to shrink away from the tool (which aids demolding) while retaining the shape. Although rotomolding is slower than other plastics forming processes, cycling 15 minutes to hours for very large parts, it offers precise thickness control and can produce multiple parts per molding cycle in family tools as well as truly large parts, such as multi-thousand-gallon underground tanks, at low tooling costs. For these reasons, the process presents the opportunity to create cores in materials, geometries and sizes that would be difficult or impractical using other processes. 

 

Designed specifically for automotive

Persico’s core concept resulted from the convergence of three factors. First, the company has developed competencies over the past three decades in several “pivotal” aspects of the rotomolding process, says Alberto Carrara, sales manager for the company’s Industrial business unit. “We know how to rotomold a broad selection of polymers — far more than our customers typically need or ask about,” he explains. “In fact, in our lab we keep searching for additional polymers that might be suitable for rotomolding but that aren’t used in the process today. We work with a local partner who helps us prepare powders by grinding granules [pellets].”

Background work on polymers that are atypical of rotomolding has helped guide development of tooling, machinery and processing enhancements, he notes. “Second, based on our proprietary SMART rotomolding machinery controls and on vacuum-assisted rotomolding, we can maintain tight control of local thicknesses on plastic parts during the molding cycle,” Carrara adds. Third, an identified market need also helped. That, Persico found in the automotive segment. 

The auto industry is an important customer base for Persico’s tooling, presses and automated lines used to mold lightweight reinforced thermoplastic (LWRT) and direct-long fiber thermoplastic (D-LFT) composites. Rotomolding is an older but smaller segment of the company’s customer base, which purchases Persico tools and equipment to produce parts for tractors, commercial trucks and other ground transport segments. Cores presented a means to expand rotomolding’s market impact. “We thought there might be a way to introduce our automotive customers to our rotomolding capabilities,” recalls Ottorino Ori, sales manager, Persico Industrial. “Automotive suppliers kept contacting us and asking for an affordable core to use in highly complex 3D parts. As we explored further, the market seemed to be waiting for solutions concerning two kinds of thermoplastic cores — soluble and structural.” That prompted the team to scout for automotive rotomolding opportunities a year ago, which in turn led to work on composite cores.  

The team ran experiments, tested a variety of raw materials and announced preliminary results at the SPE ACCE (Novi, MI, US) in September 2016. Based on positive response from that conference, the company says it is devoting a significant portion of its 2017 R&D budget to mature the technology, with an emphasis on identifying projects and partners to test soluble and structural cores. 

To date, Persico’s structural composites research has focused on a variety of cores combined with autoclave-cured carbon fiber/epoxy prepreg skins or braided carbon fabrics infused with epoxy via high-pressure resin transfer molding (HP-RTM). That work might soon pay off: automakers in Italy and the UK are said to be looking for applications for the technology on two platforms.

 

Versatile technology

To date, the company says it has successfully molded cores from conventional rotomolding resins: polypropylene (PP), polyamide 6 (PA 6), polyvinyl chloride (PVC), liquid and powdered crosslinked polyethylene (XLPE), linear polyethylene (PE), polyethylene terephthalate (PET), polycarbonate (PC), polystyrene (PS), acrylonitrile butadiene styrene (ABS) and the weatherable polymer, acrylic styrene acrylonitrile (ASA). For a greener core, they have rotomolded post-industrial recycled PE, bio-based polyolefins and several water-soluble polymers. 

Persico’s most recent and surprising material is polyetheretherketone (PEEK). Although this very stiff, high-performance aromatic thermoplastic’s melt temperature (343°C) is significantly higher than that of most conventional rotomolding resins, Carrara says Persico’s ability to rotomold it is a function of the efficiency of the electrical heating systems embedded in its SMART tool system, which helps them quickly reach PEEK’s higher temperature. He also credits unique vacuum circuits, which are said to greatly enhance rotomolding of PEEK in powder and micro-pellet forms. Work is underway to optimize processing conditions for PEEK in granular form, and future work might evaluate other high-temperature thermoplastics.

Depending on selection of skin and core materials, the cores can be designed to permanently bond to skins or detach after demolding. Not surprisingly, non-polar olefins offer poor adhesion without surface preparation. On the other hand, powdered PET provides very good adhesion to carbon composites. Similarly, high-gloss cores offer poorer adhesion than cores with textured surfaces. Interestingly, researchers say they’ve seen no correlation between the selected process — at least in terms of autoclave cure vs. HP-RTM — and core adhesion. More study is underway in this area.

So versatile is the technology that there even is the option, when water-soluble polymers are used, to remove cores from skins by dissolving them in hot water. Early work with ethylene vinyl alcohol (EVOH) was found to take up to 48 hours to fully dissolve the cores. However, more recent work, focused on polyvinyl alcohol-polyvinyl acetate (PVOH-PVA), a food/pharmaceutical-grade packaging polymer (Gohsenol, from Nippon Gohsei, Osaka, Japan) has accelerated dissolution times. “More and more we are convinced that an effective combination of soluble plastics and an agile production technique represents a competitive advantage over other methods of producing removable cores in complex shapes,” explains Gaetano Donizetti, sales manager, Persico Industrial. He contends that rotomolded cores can surpass silicone bladders when part shape reaches a certain complexity.

To advance the technology to a pre-commercial phase in 2016, Persico produced a special test tool called a “cuboid” and fitted it to a SMART rotomolding machine. The tool produces rotomolded parts that are 290 mm long by 290 mm wide by 150 mm tall. “We use the cuboid to define the practical limits for our cores, such as maximum and minimum wall thickness, survivable forming pressures and how fast we can dissolve them in water,” adds Donizetti. He notes that when 0.6-kg cuboid cores rotomolded in PVOH-PVA with 2-mm nominal walls were soaked in 40°C water, the cores completely dissolved in 50 minutes. When soaked in 80°C water, they dissolved in 15 minutes. Although they haven’t tested solubility at higher temperatures yet (that’s a future research project), stopping at 80°C makes sense, because any customer with an RTM heating system will have water available at that temperature.

Ori adds that certain thermoplastic core types can be removed via mechanical or thermal means — via side-access openings, with or without pre-softening and folding the core — although such actions carry the risk of damaging the part. That’s why the team is encouraged by how well PET cores adhere to carbon composite. He also says that thin and flexible PET cores produced via rotomolding or blowmolding are another alternative they will explore in the near term.

This past year, autoclave-cured carbon fiber/epoxy prepreg-wrapped cores survived 5-bar molding pressures. More recent testing of PVOH-PVA cuboid cores draped with dry carbon materials and resin injected in HP-RTM withstood 10-bar pressure without collapsing. The team is continuing to explore the forming pressure limitations of the hollow cores.

 

What’s ahead

Persico has ambitions to explore other options. For example, in-mold chemical foaming is often used with rotomolded olefin polymers. This creates a solid outer skin, while the center and inner skin of the hollow parts are foamed, offering opportunities for modest weight and cost savings, albeit at cycle times two-times longer than normal. Researchers think foaming could be used to produce olefin or polyurethane structural cores with novel thermal and acoustical damping properties. Another area of interest is use of chopped glass fiber reinforcement for PA6 cores. 

“Rotomolding ... doesn’t suffer severe limitations in terms of size or geometric complexity,” adds Carrara. “However, for small components, there are some constraints in terms of using sharp edges/corners where powdered materials can stick and prevent correct filling of the tool. Also, for large cores, our SMART machines currently have a physical limitation of 2,800-mm diameter for the machine table.”

How will the company bring the technology to market? Ori says Persico is open to all options. The initial approach will be to co-design parts, build tools and test prototypes with a supplier or OEM partner, who eventually would purchase a SMART machine and produce parts. When the technology matures, Persico will make its know-how available and possibly retain responsibility to co-design and produce tools for the process.

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