Annette M. Sharon
This clear, shiny PE flask failed the ink adhesion test without being pretreated. Product decorators use a tape or scratch test to determine ink adhesion, as well as taking into account whether the product is a single-use item or must withstand aggressive conditions, such as dishwashing or outdoor exposure.
When given a product of unknown makeup, product decorators may have to try several different types of inks before they get one that passes the tape and scratch tests normally performed to prove adhesion.
When a molder approaches a product decorator with a finished product and asks, âCan you print this?â there is a degree of incredulity when the answer is, âI donât know.â
While the reason for the answer is far from elementary, it is elementâary.
The surfaces of plastics vary from substrate to substrate, not only in terms of texture but also in terms of surface tensionâa characteristic also referred to as âsurface energy,â or âsurface wetting.â
If the surface tension of a substrate is lower than the surface tension of the ink, the ink wonât stick. Itâs a sign of inadequate chemical covalent bondingââbad ink adhesion,â in laymanâs terms. Likewise, if the substrate is wettable, that is, if it has high surface energy, the ink will adhere properly to the surface, allowing it to be marked or decorated.
One simple test of a plasticâs printability is to drip some water on the surface. If the water beads up, the surface energy of the substrate is lower than the surface tension of the water, and vice versa. If the water sheets, its surface tension is lower than the substrateâs. Beading is bad. Sheeting is good.
The scientific test is to check the surface of your plastic using a dyne kit. These specialized dyne markers contain a fluid that mimics ink adhesion and allows printers to choose the type of ink that will most likely stick to the productâs surface. The ideal surface energy is greater than 40 dynes/cm².
Another important factor for good ink âstickabilityâ is keeping the surface of your molded product free from contaminationâoils, greases, silicones, and condensation, for instance. In fact, it would be a good idea to check that the mold release formula you use doesnât inhibit or prevent ink adhesion.
Printersâ biggest headaches come from two problems: 1) unknown surfactants that repel ink, and 2) unidentified additives that make the plasticâs surface hydrophobic and unable to accept ink.
In the latter case, filled plastics such as talc-filled PP or glass-filled nylon respond to ink differently after molding than before molding due to slip-additive migration. Sometimes when heat is applied to a component, the additives migrate to the surface, changing the surface tension and the productâs ability to be printed.
All is not lost, however, if an ink wonât immediately stick to your plastic. If you arenât lucky enough to be working with polystyrene, vinyl, or other printing-friendly raw or virgin materials, there are pretreatment options available for materials with lousy stickability, like most polyolefins.
For example, PE and PP have a surface energy of 30 dynes/cm2 and usually canât be printed without pretreatment. There are various pretreatment options to raise your partâs surface energy: solvent-based prewipe, a high-voltage discharge corona treatment, bulk plasma treating, or flame treatment.
These techniques change the surface energy of the substrate by altering the electrical charge of the particles closest to the partâs surface.
Pretreatment doesnât last forever, though, and parts should be printed soon after their surface energy is increased to ensure ink adhesion.
The amount of time the pretreatment lasts varies based on a number of factors, including the level of treatment, humidity, and dust. These variables âde-energizeâ the partâs surface, returning it to the stable state that made pretreatment necessary.
There are instances of painted, corona-treated surfaces returning to an âunmarkablyâ low dyne level in as little as 2 hours after treatment.
Also, inappropriate or improper pretreatment can damage the surface of your part by changing its physical characteristics (i.e., melting it), but there are specialty inks that can be cured after the fact by using forced air, infrared, or ultraviolet treatment.
Even with these highly advanced methods of testing, treating, and curing, todayâs plastics can contain a high degree of unknown additives or fillers that can change from batch to batch, thus turning an easy print job into an impossible one for your printer.
While new inks are being developed for these difficult substrates, the explosive use of additives in many plastics can undermine even the most determined chemistâs efforts. Sometimes a particular application demands a particular plasticâwhether for medical, industrial, or automotive needs.
Asking about the elemental characteristics of the plastics at the get-go will save time and frustration when deadlines loom.
So, the best time to plan for printing is right at the beginning, when youâre choosing the plastic. You want to look closely, not only at its contents (its binders, fillers, release agents, etc.), but at its supplier as well. Make sure you have a reputable source that will stand by its product and ensure its batch-to-batch consistency. That way, when you take that finished piece to the product decorator and ask, âCan you print on this?â youâll have a better idea what the answer will be.
Author Annette M. Sharon is lead technical writer at Pad Print Machinery of Vermont (East Dorset, VT; www.padprintmachinery.com). Since 1985, Pad Print Machinery has specialized in developing industrial pad printing solutions for the molding, decorating, and marking industries.