Coatings and Finishes for Packaging Molds
Today’s variety of packaging options requires the right surface treatment for the required mold.
When one thinks of packaging a hundred images come to mind. It’s a huge and still growing industry, especially in the plastics sector where PET has become king and companies are finding it more cost-effective to produce packaging using plastic rather than aluminum or paper, for example.
This trend has led to questions about how to better facilitate efficient production of plastic packaging in its various forms and this article will present a few examples where choosing the right finish or coating has helped achieve that goal.
Multiple Packaging Processes Benefit from Electroless Nickel
Surprisingly, aluminum vacuum molds for shrink film packaging and extrusion dies for medical sterile packaging benefit from the use of similar surface treatments. For example, a company which produces toys packaged on a cardboard backing with plastic protecting them, needed a coating that would provide a hard, lubristic surface without compromising the integrity of the mold.
Similarly, a medical manufacturer produces plastic trays with medical instruments that are sealed with a plastic, saran-like cover. Unlike the toy production, the medical packaging is produced in a cleanroom setting where sterility is a major concern. However, both benefited from using either an electroless nickel (EN)-P.T.F.E. or a nickel boron nitride coating that offers a lower coefficient of friction on the mold surface.
In addition, film dies often also do well with simply an electroless nickel coating because it not only protects the tool, it also offers corrosion resistance.
Blow molding of containers for food products or cleaning products is a little more complex because the bottles often begin with an injection molded neck that is threaded with parasin attached. The parasin mold that produces the neck does well when high diamond polished and then flash chromed. In addition, the neck ring inserts that actually form the threads are made from steel and tend to wear. When that happens, micro welding them for repair, followed by polishing and chrome plating to re-establish the original surface properties, has been very successful.
Once the parasin is formed during the injection molding process, it is then transferred to a blow mold to form the bottle. Blow molds are typically made from aluminum and require a high diamond polish to aid lubricity and part appearance. Like most molds, they also require periodic maintenance.
As mentioned previously, aluminum vacuum molds are commonly used for such things as toy packaging, but also are used to mold re-sealable, disposable containers that we use for deli meats or leftovers, and carry-out packaging. In these cases, the molds usually require only a paper finish because the type of plastic being molded takes on the impression of the molds, but doesn’t adhere so tightly to the mold surface that it needs more than a basic draw polish on its side walls to aid release.
After applying a paper finish use either an electroless nickel-P.T.F.E. or electroless nickel boron nitride coating for added hardness and release benefits. These electroless nickel co-depositions are an advantage because they will not plug the vacuum holes in the mold; and, therefore will not hinder the vacuum molding process.
Blown Film Extrusion Dies
Blown film extrusion dies produce the thin film of plastic, or plastic sheeting that is used in vacuum forming. Examples of packaging products commonly molded using blown film are t-shirt bags, barrier food packaging, trash can liners, garbage bags and peat bags. These are often steel co-extrusion dies (though single extrusion dies also are used) that require a heavy deposit of 0.001 electroless nickel over a high diamond (usually an A-2, which is equal to 4-6 RMS) polish on all melt-flow surfaces. Nickel also is used for improved wear and corrosion resistance. This is a very high-speed process, and a single blemish on the die surface can result in thousands of feet of film with a defective line through it. So it’s necessary to perform proper, regular maintenance to maintain a flawless surface finish.
Tooling for cosmetics packaging produces high profile products whose appearance helps sell them to customers; therefore, they require a decorative finish using high diamond polishing to aid aesthetics and add that sparkle to the part. There is a wide array of decorative detail in such things as lipsticks, mascaras and blush or powder compacts, so an A-1 diamond polish is most desired. It also is recommended to use flash chrome to protect that diamond finish, so the mold runs longer between maintenance checks and produces a higher number of flawless parts.
Caps for packaging, while they do contribute to shelf appeal, are approached from a slightly different angle than the cosmetics packaging discussed above. In the world of high production, high cavitation injection molds, any edge that can help reduce cycle times is seen as a tremendous advantage.
Tooling with as many as 240 cavities in a single cycle is possible and with that kind of cavitation, even a small reduction in cycle time can mean a notable increase in both part quantities and profits. One way to achieve this is by using the proper finishes and release coatings because (1) by reducing the coefficient of friction it allows the resin to fill the mold more easily using less injection pressure; and, (2) better release properties equate to fewer imperfect parts and smoother production. These two factors combined help to achieve faster cycle times.
Experience has shown that applying an electroless nickel boron nitride or electroless nickel-P.T.F.E. coating has been particularly beneficial. Some molds also call for a diamond finish or a satin finish depending on the material being molded and the aesthetics of the part.
Single-cavity injection mold tools such as those that produce buckets and coffee cans often have longer cycle times than cap molds. But they can still benefit from better release on the core, corrosion protection on the cavity and core, as well as more efficient material flow.
It has been found that better mold production is experienced when they are paper finished and electroless nickel or nickel-P.T.F.E. plated. It should be noted that these molds can actually benefit from a range of finishes including a diamond polish, a draw polish, a satin finish or even a combination of these. It depends upon the customer’s production goals and objectives.
There are many, many more instances in which coatings and finishes can benefit molders of plastic packaging products. As always, consult with your preferred vendor of these services to find out what will best serve your needs.
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