There is an alternative to mold inserts for blow molds and if a Canadian company has its way, the method will be commercially available within the next year.
The method is known as laser cladding, which is an additive metal deposition process derived from laser additive manufacturing (LAM), and it can be used in a blow mold to strengthen wear areas, repair sections or spots, or change the mold's net shape. Blow molded bottles from 4 ounces to 160 ounces produced in high volume are the initial target for this technology, and one official says the turnaround is rapid.
"If the CAD programming is done it would be possible to laser clad two four-cavity molds designed to produce 16-ounce Boston round bottles in less than a day," says Tony Paget, CEO of Garrtech Inc., Stoney Creek, Ontario, a precision blow mold toolmaker.
Pinch-off areas of a blow mold are the points most susceptible to wear, due to repeated clamp up and release during the molding cycle.
"The pinch-off is the area in the blow mold that seals the parison when the mold closes. Aluminum tooling has an inability to withstand the continuous cyclic impact of the molding process; it has no wear resistance, and it does not have sufficient compressive strength," says Paget.
Most blow molds are manufactured from aluminum due to its light weight, ease of machining and thermal conductivity. Blow molders typically order or create an insert composed of harder metals that wear more slowly than aluminum.
"Beryllium copper inserts extend wear resistance around the pinch-offs and have a thermal conductivity equal to aluminum. The other material choice for an insert is a tool steel, which brings excellent wear rates but can slow the molding cycle due to its low thermal conductivity," says Paget.
The issue with inserts
However, molders aren't making the ideal choice when choosing an insert, says Paget, a mechanical engineer with more than 44 years' experience in the design and manufacture of blow molds for injection and stretch, shuttles, wheels and large industrial blow molding machines. He is a senior member of the Society of Plastics Engineers and a faculty associate at the Fraunhofer Institute, a research organization in Munich.
Beryllium copper and tool steels both offer high wear, and both materials enhance the thermal coefficient at wear points in the mold, but there is a natural thermal break that occurs between the aluminum tool and the insert, simply because they are dissimilar materials, says Paget.
"For production, an insert means having to create a space in the tool, hence higher tooling costs; for the process, a thermal break means lower productivity compared to no insert or an insert made from material similar to the substrate; and for the part, the thermal break will be the cause of a witness line," says Paget.
Paget, responsible for new product and process development at Garrtech, holds a patent for single-piece blow molds that focuses on the production of blow molds without inserts.
"CNC technology allows for the manufacturing of a tool from billet to mold that has no design requirements for inserts, hence it can outperform other tools as it does not have any thermal breaks," says Paget.
The laser cladding tooling concept results in a simpler tooling design that is easier and faster to build and can be much less expensive — in some cases up to 50 percent less — than conventional tools with inserts. Market penetration of single blow molds has been slow, mainly due to concern that the aluminum would not meet the rigors of production.
"The challenge is to find a solution that incorporates a single-piece mold concept, does not require an insert that can create a thermal break, yet strengthens wear points and mimics the conductivity of aluminum," says Paget.
Laser cladding concept for inserts
Through separate research collaboration with the London, Ontario, facility of the National Research Council Canada (NRC), Garrtech developed a laser cladding process where a metallic matrix with hard particles is melted then deposited onto the blow mold, in an additive manufacturing step.
The metal matrix composite that is added to the tool is a specially selected group of metals, including aluminum for excellent bonding with the tool, as well as harder, proprietary metals for wear resistance. The deposition process and cladding recipe development was completed by NRC and is the LAM process.
In a four-year pilot program with NRC, tools featuring aluminum-infused cladding performed at a significantly higher throughput rate than a mold with an insert, because the thermal breaks were eliminated. The molds have been running with the laser cladding for two years side-by-side with conventional tooling and have shown no signs of wear or need for maintenance.
"Even if the costs of this tool were 20 percent greater than the tool insert option, the significantly higher throughput rates would justify the choice," says Paget. However, due to the complexity of the cladding material, it is difficult to determine a reliable set of values for toughness or compressive strength.
Matrix material meets additive manufacturing
Laser cladding as an additive manufacturing process (the primary use of the process is in the production of all-metal parts) requires the metallurgical know-how to create the best metal matrix composite (MMC).
"It is a challenge to create an MMC. It is analogous to concrete; everyone knows that a standard concrete mixture uses three parts sand, two parts stone and one part cement. But those are the ingredients, not the process. You have to understand both aspects, because how the mixture is deposited is also important," says Paget.
Garrtech and NRC created its MMC from a mix of ferrous and nonferrous metals.
"There can be as few as two different metals but at least two to four metals will be present, based on what we are looking to accomplish," says Paget. Each material in the matrix has its own specific gravity. The laser cladding device has to be able to melt each of the matrix materials in a way to bind them together without weakening one material or changing the characteristics of others.
Once melted, the MMC is deposited onto the tool through a specialized head fitted to a CNC. The feed rates, temperatures, push-out pressure, speed of the CNC bed, all factor into ensuring that the optimal conditions for material deposition are present.
"Just like concrete, the matrix has certain feed, speed and other process aspects that are important. The deposition must occur in a manner that it will produce the cladding to set up as a single piece; otherwise, if one element in the matrix is poorly conditioned, a thermal break can result," says Paget.
The deposition tool follows a CAD model, but in reverse — a build profile rather than a machining profile — and produces a contoured and specifically dimensioned cladding in a layer-by-layer method. The layer thickness of the MMC to be deposited can be varied, but generally ranges from 0.001 to 0.01 inch, says Paget.
Benefits and challenges
There are other benefits and challenges to applying the matrix to the tool. There is minimal heat as the matrix is deposited onto the tool, so stresses or distortion to the tool do not occur.
"The aluminum in the matrix is melted, not cast, wrought or forged, so it displays different properties," says Paget. The porosity of the material is reduced over that of wrought and cast materials. In combination with the harder metals in the matrix, the reduced porosity makes the cladding a challenge to machine. CNC tooling made from carbides and a boron nitride coating will not effectively machine the laser cladding. Some post polishing or milling is still required.
Garrtech's MMC has demonstrated wear characteristics that exceed that of beryllium copper while demonstrating better thermal properties than tool steel. The majority of the particles of Garrtech's MMC are harder than a beryllium copper or steel tool insert. The keys to success are found with the use of very specific materials and simultaneous control of material volumes and laser deposition rates.
The cladding transfers heat quickly between the aluminum tool due to the incorporation of aluminum in its structure, yet the matrix also retains heat due to the other metals it contains. This retention of heat by the cladding adds heat to fuse material at the parting line, for improved weld strength.
"Laser cladding with Garrtech's MMC materials can be used to repair areas of the mold without the general stresses to the tool caused by welding or differences in porosity," says Paget.
One of the bigger benefits of laser cladding is in its ability to change the net shape of a tool.
"If a molder of a bottle has a customer looking for the same bottle style but with finger grips in it, that addition would typically mean an insert, welding or a new mold. We can change the mold by adding material," says Paget.
Garrtech expects to introduce a laser cladding machine at its facility within the next year.
Mikell Knights, senior correspondent
Contact:
Garrtech Inc.,
905-643-6414, www.garrtech.com
National Research Council Canada,
613-949-3042, www.nrc-cnrc.gc.ca
