What Do You Need For Printing on Plastic?

Pad printing and inkjet printing are among the top processes for printing on plastic. But plastic is more than meets the eye. Did you know that there are 7 listed different types of plastics, but the 7th option is miscellaneous plastics which include hundreds of alternatives and different mixes? Since many plastics are sourced from outside vendors, understanding exactly which plastic you are printing on may pose a challenge but there are always solutions to these printing roadblocks.

What Printing Methods Work Best on Plastic Parts?

When it comes to printing directly on plastic parts, pad printing and UV digital inkjet printing are the two go-to methods—and each one shines in different situations. Pad printing is your best bet for small, curved, recessed, or oddly shaped parts, and it’s a favorite for logos, text, and one- to multicolor graphics. UV inkjet, on the other hand, is often preferred you need detailed, full-color graphics, and it works on all kinds of surfaces, from flat to cylindrical. So, which one’s right for you? That really comes down to a few factors: the type of plastic you’re working with, the shape of the part, how much space you have to print, your artwork, production volume, and how durable the final product needs to be.

Do Plastic Parts Need Pretreatment Before Printing?

 Some plastic parts need pretreatment before printing, while others can be printed just fine without it. It comes down to the plastic formulation, ink chemistry, and the adhesion you’re looking for. Low-surface-energy plastics like polypropylene and polyethylene typically require treatment to help the ink wet and bond properly to the surface. Pretreatment options can include chemical primers, flame treatment, corona treatment, or plasma treatment.

Adhesion testing is also an important part of the process, because the composition of plastic and silicone products can vary between production batches, even when the finished product looks identical. Changes in resin blends, additives, or other material components can affect how well ink bonds to the surface. That’s why manufacturers should test the actual production part before settling on an ink and pretreatment combination. Testing helps confirm that the process you choose will actually deliver the adhesion and durability needed.

Pad Printing on Plastic

For those who are unfamiliar with pad printing, pad printing is the process of mixing a solvent-based pad printing ink with solvent, hardener, and other additives. This pad printing ink is specifically chosen for the substrate you are printing on. Inkcups carries a dozen different pad printing inks for printing on plastics. The ink is added to an ink cup and placed on the pad printing machine. The pad printing machine essentially acts as a stamp, picking up the artwork from an etched plate or cliché with a printing pad and placing it onto the product. This process is very popular with companies who are looking to print on plastic using 1- to 6-color graphics.

Learn more about the basics of pad printing: Pad Printing the Inkcups Way

When it comes to pad printing on plastic, aside from ink, there are a few things to consider:

Pre-Treatment

• Most solvent-sensitive products will not need pre-treatment. That is because the solvent in the ink will alter the surface tension of the product, making it accept the pad printing ink to adhere to it. Those products that are not solvent sensitive (PP, Nylon, HDPE and other very hard plastics), will need to be pre-treated. In some cases, chemical pre-treatment is plenty. Chemical pre-treatment is typically in the form of some type of wipe of the product to prepare its surface. Other types of pre-treatments for plastic include flame and corona. If you are familiar with solvent-sensitive vs. non-solvent sensitive plastics, make sure to relay this information to your supplier when you are printing on plastic.

Drying

• While pre-treatment prepares the product for printing, a post-cure solidifies the bond. Whether by heat or air, a pad printed plastic product may need to be cured. Be sure to speak to your ink expert about the proper process for printing on plastic to uncover these details and help avoid costly mistakes.

See our line-up of pad printing machines

UV Inkjet Printing on Plastic

The same considerations apply to UV inkjet printing on plastic. UV inkjet printing is a direct-to-substrate process. The artwork is created and manipulated in design software (such as Photoshop or Illustrator) and then sent to the printer to print. Here, a UV inkjet ink is jetted onto the plastic in the form of tiny droplets utilizing cyan, magenta, yellow and black ink to form the image that was created on the computer. As the name implies, the UV inkjet ink is then cured using the UV/LED light that is standard with all machines. Keep in mind that this is not a separate process, this all takes place in one machine.

When it comes to UV inkjet printing on plastic, you will also need:

Pre-Treatment

• With UV printing, an inkjet primer is needed for most plastics. Going further with pre-treatment will depend on your intended use. With drinkware, if the product is a high-quality gift which will be washed often, you may want to pair your primer wipe with an additional pre-treatment step such as flame, corona or plasma treatment. If the product is a simple promotional product, a primer wipe will satisfy the needs of that use. This game changes for flat items. Since most inkjet printed flat items will not go into the dishwasher, a primer wipe may only be needed. There are some plastics that may not need pre-treatment at all due to a specialty UV ink used. For example, our T2 UV ink for Tritan Plastic is a specialized ink for printing on Tritan plastic. This ink was formulated specifically to adhere to this popular plastic without pre-treatment.

Post Curing

• As mentioned above, the final cure takes place within the machine so an additional machine for curing is not needed.

See our line-up of UV Inkjet printers

Pad Printing vs. UV Printing on Plastic

The decision to choose pad printing or UV printing depends on what your desired outcome is. Both processes may need to utilize a pre-treatment method to prep the surface to accept ink. While UV inkjet printing incorporates the post-curing, pad printing on plastics may require drying (either air or heat). If either choice works for you, it is important to first send your product to the machine supplier to test to see which method is feasible. Sometimes, since there can be so much variance when printing on plastic, one method may not work as well as the other.

Inkcups has over 20 years working with different plastics. Our application specialists have expert knowledge in the right combination of inks and pre-treatment but always test to be sure.

Contact us to get started printing on plastic

Frequently Asked Questions

How do you permanently print on plastic?

For permanent printing on plastic, the ink needs to be compatible with the plastic substrate and properly bonded to its surface. Depending on the material, this may call for a primer or a surface-treatment process like flame, corona, or plasma treatment before printing. Pad-printing inks may also need air or heat curing after printing, while UV inkjet inks cure with UV-LED energy right as part of the printing process. Since plastic formulations vary, adhesion testing should always be performed on the actual production part to confirm the durability you need.

What type of printer can print directly on plastic?

Industrial pad printing machines and UV inkjet printers can both print directly onto plastic products. Pad printing transfers ink from an etched plate to the plastic using a flexible silicone pad, making it effective for small or irregularly shaped parts. UV inkjet printers jet digital ink directly onto the surface and cure it with UV-LED light, making them well suited to detailed, multicolor graphics. Depending on the plastic and the required adhesion, either process may require pretreatment before printing.

How do you improve ink adhesion on plastic?

Ink adhesion on plastic can be improved by choosing the right ink for the substrate and, when needed, boosting the surface’s ability to accept and bond with that ink. Common pretreatment methods—primers, flame, corona, and plasma treatment—modify the plastic’s surface characteristics to improve “wettability,” helping the ink make better contact. Proper curing after printing matters too. Since plastic formulations can behave differently, adhesion should always be tested on the actual part before production. 

How do you print on injection-molded plastic parts?

Injection-molded plastic parts can be decorated after molding using pad printing or direct-to-object UV inkjet printing. The right method depends on the resin, part geometry, print location, artwork, and production requirements. Pad printing tends to work well for small, curved, recessed, or irregular surfaces, while UV inkjet can produce detailed, full-color graphics straight from a digital file. Before injection-molding production, the molded part should be checked for surface characteristics and contamination, since pretreatment may be needed to achieve proper ink adhesion.