Regrind amounts can affect application weld strength

As processors’ resin becomes increasingly expensive, many converting operations are looking at ways to lower costs by using greater amounts of post-production regrind and scrap in applications.

Sonny Morneault, director applications and acoustic technology at welding machinery maker Branson (Danbury, CT) says he is seeing increasing numbers of customers in fields such as consumer electronics and automotive who are attempting to reuse their post-production scrap to the fullest. Yet the amount of regrind, type of filler, and content, says Brian Gourley, technical services manager at welding equipment producer Sonics & Materials (Newtown, CT), can restrict the tolerance of the operating parameters of the welder to produce acceptable product.

Ken Holt, applications manager at Herrmann Ultrasonics (Bartlett, IL and Karlsbad, Germany) says the regrinding process changes the molecular weight distribution of most thermoplastics due to shear and the added thermal cycle. This in turn changes the physical, dielectric, and flow properties of the materials. “Weldability is adversely affected when the impacts of these are high enough to cause [unfavorable] flow conditions such as rates between the materials/parts to be welded,” Holt says.

At automotive parts vendor Visteon (Dearborn, MI), the company says scrap is unavoidable during processing and it is trying to maximize its polymer usage by recycling regrind back into applications. Susan DeGrood Kozora, manager materials and joining technologies engineering at Visteon, says her department wanted some benchmark about just how much regrind could be used in automotive applications before weld strength of polyolefins did not meet specifications.

In trials of welding unfilled polypropylene copolymer (PP) samples at neat, 25%, 50%, 75%, and 100% regrind levels to thermoplastic polyolefin (TPO) in a T-joint geometry, the company was able to simulate production situations. Kozora says that weld strength of PP to TPO was better than PP to PP samples.

“Interestingly, the weld strength of the PP regrind to TPO samples increased as regrind levels increased, where the weld strength of 100 wt% PP regrind to TPO was only 5.6% less than that of the baseline TPO strength,” Kozora says. “On the other hand, the 0 wt% PP regrind to TPO samples exhibited a 13% decrease in weld strength.”

She says that since the grinding process can shorten polymer chains, the increase in weld strength in the 100 wt% PP regrind compared to 0 wt% PP regrind may be attributed to the increase of shorter polymer chain diffusion across the interface. Surprisingly the thermal analysis of the welds of the PP samples did not show significant differences in crystallinity.

She says more research is necessary to find out why the 100 wt% PP regrind weld strength in this case proved to be the highest. Focus of such research needs to make a detailed weld morphology evaluation as opposed to weld strength and thermal analysis alone, Kozora says.

Stephan Heintzsch, managing director at radio frequency welding equipment maker Heinz Schirmacher (Trittau, Germany) says although he has not had specific feedback from customers about problems of regrind levels in welding applications, his company recommends the regrind be exactly specified in terms of type of resin, material properties, and type as well as percentage of pollutant in the material.

At welding producer Leister Process Technologies (Sarnen, Switzerland), product manager laser systems Oliver Hinz says his customers report back a maximum of 20% regrind permitted weldability without problems. Sonics’ Gourley says the company recommends a maximum of 10% post-production scrap.

Herrmann Ultrasonics’ Holt sees no absolute level limit defined by welding but the limit is more defined by the mechanical properties required of the part and the strength required of the design, although his customers tend to “cap” regrind usage at 15%.

Consistency of processed material used has the most affect on welding. For example, if a previously determined part has been designed to use 10% regrind, the processor can’t simply up the scrap amount put into the part the following week if virgin resin prices increase. Doing so would result in brittle molded parts that cannot withstand the same welding forces that the 10% material part can. Welding processes that melt large amounts of material may be less affected by regrind than others, he says.

According to him, ultrasonic welding allows processors to weld quickly and control the welding, thereby compensating for increased regrind percentage through computer-based equipment. Gourley thinks vibration welding may be more advantageous than ultrasonic welding because of the machine’s ability to weld with more power, amplitude, and force.

Nevertheless, he says the majority of applications cannot easily replace one welding method of assembly for another because the application geometry or a variety of other process factors stand in the way. The amount of energy needed to weld by adding regrind will change; typically more energy is needed to accommodate the change in elasticity modulus, density, melt flow index, ductility values, and melt temperature, he says.

Gourley says typically automotive applications use regrind in under-the-hood and interior parts but do not always require a class “A” surface. General Motors, in what appears to be an exception to the rule, equips its GMT800 SUV with a lower engine cover produced from 100% recycled material, welded using a Herrmann Ultrasonics two-headed welding system.

Each application needs to be looked at individually concerning the amount of scrap included, says Branson’s Morneault, but the design of some parts can use up to 40% regrind and still get an acceptable weld.

Welding tips target applications with regrind

Are there precautions processors should take when welding applications with regrind content? Ken Holt, applications manager at Herrmann Ultrasonics (Bartlett, IL) lists his common sense processing recommendations:

• Keep regrind percentages as constant as possible.

• Only use good-quality regrind without dirt, grease, sawdust, and other contaminants.

• Incoming inspections of the flow rates of polymer with regrind, batch-to-batch, should be conducted so the known value is always used.

• Use the same material for each of the two halves or parts to be welded to ensure weld compatibility. A • Make sure the mechanical properties of the materials are suitable for the required weld strength.• Use large, self-centering, and robust weld joint designs (tongue-and-groove energy director types or step joints) while avoiding designs that rely on tight molding tolerances such as angled shear-type welds and flat-to-flat interference welds.• Use accepted plastics part design rules regarding wall thickness, gate locations, and radii in corners to ensure good welds.• Make sure the molding machine temperature/pressure profiles are correct to avoid melt degradation, stress concentrations, weak knit, or weld lines within the parts.• Use a true microcomputer-controlled welding system to allow comprehensive understanding of the progression of the weld cycle and process optimization. BC

BLOWN-FILM BUILDS LAYERS

For blown-film processors, more layers can mean more opportunities, and more headaches. In North America, three layers still dominate, but seven- and nine-layer systems are becoming more prevalent.

“The largest number of machines we’ve sold over the last three years, talking worldwide, is seven layer,” says Bob Hawkins, president of Kiefel’s (Worms, Germany) North American film headquarters in Wrentham, MA. “We’ve installed over 50 machines of five layers and above in the last three years,” Hawkins explains, adding that customer-driven needs in food and medical sectors for economical barriers are promoting layer multiplication.

“Five-layer lines were ’up and coming,’” says Jim Ciolino, extrusion system sales manager in North America for Windmöller & Hölscher (W&H; Lengerich, Germany), “but they were quickly surpassed by the seven-layer lines about eight years ago.”

Philip Kwok, VP worldwide sales for Brampton Engineering (BE; Brampton, ON), says his company has bought into the trend, primarily targeting seven layers and above. “[Seven-layer] lines are more popular for us,” Kwok explains. “It is the end markets for barrier film that drives this interest, but reducing cost of resin is also a main driver for more layers.”

Michael Perrigo, technical director for blown film at Davis-Standard (Pawcatuck, CT), was preparing to ship an 80-inch-diameter, three-layer coextrusion die for an agricultural blown-film system when MPW caught up with him, but he has also seen five- and seven-layer systems stoking greater interest.

“Generally, the three-layer systems are the most common overall,” Perrigo says, “and then we have seen a marked increase in five- and seven-layer barrier products.”

Dave Bartish, sales director at film converter Ampac Flexibles—Performance Films, says his company operates three-, seven-, and nine-layer systems, but it’s the latter that are driving new business.“The seven- and the nine-layer lines open up opportunities to pursue markets where barrier films are required,” Bartish says, listing applications in forming and nonforming films for processed meats and cheeses; bulk packaging for liquid/pumpable foods like sauces and soups; and sealant films used in lamination and printed webs for items like beef jerky and dry food.

Employee buy-in

The move from three to seven layers isn’t without its challenges, however. “The leap from mono to three layer is one step,” Kiefel’s Hawkins explains, “to then go from three to five or seven—it’s not a similar step. It’s quite disproportional, in fact.” Hawkins specifically cites staff training as a challenge, explaining the need for a new mentality, since scrap in a three-layer line, where the predominant resin might be off-spec or regrind: but scrap in multilayer films, where expensive barriers like EVOH and polyamide are applied, is something else.

“You have to have labor that’s both motivated and trained for handling high-quality materials,” Hawkins says. “The operator can’t just go along, pour the wrong material into the wrong hopper and go, ’Oh, that’s no problem, let it flush itself out and away we go.’”

BE’s Kwok says, “Technically, going from three-layer film with a very thick middle core layer to five- or seven-layer film with thin middle barrier layers is much more difficult than from five to seven or nine.”

For Ciolino at W&H, “Getting the process wrong could mean high scrap rates, and most often, that scrap is not easy to put back into the process because of incompatibility issues.”

From a cost perspective, offering what he calls “a very rough estimate,” Ciolino guesses jumping from a three- to a seven-layer line would incur about a 25% price premium, with a slightly higher one to go to nine layers. Kwok at BE estimates going from five to seven layers would increase investment costs by 15%, while Ampac’s Bartish says that beyond the initial investment, processors should expect to pay more in dies and laboratory quality-assurance equipment.

Reaping returns

In spite of these challenges, those involved agree that upgrading layer capabilities will pay dividends. “As end-user requirements drive film technology, more and more layers are required,” Ciolino says.

“Today’s niche is tomorrow’s commodity, so they have to be careful,” Hawkins at Kiefel says, “but not withstanding that, [processors] are seeking more niche markets, which will differentiate them. From that perspective, five, seven, and nine layers give them that opportunity.”