In the evolving world of product branding and part decoration, pad printing remains a reliable method for applying precise, durable images onto a wide range of plastic components. However, when it comes to printing on “difficult” substrates—such as polypropylene (PP), polyethylene (PE), nylon (PA), or thermoplastic elastomers (TPE)—even seasoned decorators can run into adhesion challenges.

These materials are prized for their chemical resistance, durability, and moldability—but these same properties make them inherently difficult to decorate using conventional ink systems. This article explores the technical factors at play when printing on low surface energy plastics, outlines best practices for ink selection and surface treatment, and introduces specialized ink systems developed to solve these specific challenges.

Most plastics that fall into the “difficult to print” category have one thing in common: low surface energy. Measured in dynes/cm, surface energy determines how well a liquid (in this case, ink) wets the surface. If the ink beads up or fails to spread evenly, it’s a clear sign that adhesion will be poor. Typical printing success requires a surface energy of at least 38–42 dynes/cm, yet untreated PE and PP surfaces often measure below 30 dynes/cm.

This low wettability prevents the ink from bonding properly, leading to poor adhesion, image flaking, or complete failure during durability testing (e.g., crosshatch, rub, or solvent resistance tests).

Before considering specialized ink systems, it’s important to explore surface treatment options that can increase the surface energy of the plastic part. The most common methods include:

• Corona Treatment: An electrical discharge temporarily raises surface energy but can degrade over time—typically within 24–72 hours.
• Flame Treatment: A more permanent solution, flame oxidizes the surface and improves ink anchorage.
• Plasma Treatment: A low-pressure or atmospheric process that modifies the surface at the molecular level for excellent adhesion.

Each treatment method has its benefits and limitations based on the part’s geometry, material, and production throughput.

While surface treatment is often essential, ink selection plays an equally critical role. The right ink system must be tailored to the material properties, part function, and durability requirements. Here’s a breakdown of popular substrate types and recommended ink chemistries:

Polypropylene (PP) and Polyethylene (PE)

These are two of the most common but challenging plastics. Inks for untreated PP/PE often require pre-treatment or a special adhesion promoter.

• Recommended Ink Type:
  Two-component epoxy or polyurethane-based inks with adhesion promoters. Printcolor’s Series 712 pad printing ink for untreated polypropelene or Printcolor Series 752 pad printing ink, when used with appropriate hardener, is a strong candidate for improved bond strength on treated PP/PE.

Nylon (Polyamide, PA)

Nylon is hygroscopic and prone to variable adhesion due to its moisture content.

• Recommended Ink Type:
  Use thermoplastic or epoxy-based inks like Printcolor Series 786 pad printing ink, often with elevated oven curing to enhance adhesion.

Thermoplastic Elastomers (TPE, TPU)

Soft-touch coatings and TPEs used in consumer goods or grips are notoriously difficult due to their oily or waxy surface feel.

• Recommended Ink Type:
  Inks must be specifically engineered for elastomeric adhesion. Printcolor Series 792 UV-curable pad printing ink is ideal for cases requiring flexible, high-adhesion decoration on TPE.

Because plastic formulations can vary even within a single resin type, testing remains essential. Always perform adhesion validation using recognized industry standards:

• ASTM D3359 – Crosshatch Adhesion Test
• ISO 2409 – Crosscut Test
• Rub and Abrasion Resistance Tests – For end-use durability
• Chemical Resistance Tests – Important for medical, automotive, or industrial parts

Be aware that results may vary depending on print pad hardness, cliché depth, ink viscosity, and drying conditions.

• Pre-Treat Consistently: Ensure surface treatments are done inline and just prior to printing for maximum effectiveness.
• Control Ink Viscosity: Pad printing ink viscosity changes throughout the day as solvents evaporate. Systems like microPrint’s Viscomatic can automate thinner dosing, ensuring consistent printability and adhesion.
• Use Hardeners Correctly: Two-component inks require precise mixing ratios and pot life monitoring. Improper ratios will drastically affect adhesion and curing.
• Document and Replicate Conditions: Success in pad printing depends on reproducibility. Log drying times, temperature, treatment settings, and ink batch numbers.
• Collaborate with Suppliers: Work closely with ink and equipment suppliers to test ink systems in real-world scenarios. Many offer lab testing and custom formulation support.

Decorating low surface energy plastics may present challenges, but with the right combination of surface treatment, ink chemistry, and process control, pad printing can still deliver vibrant, permanent images across a wide range of difficult substrates. As materials evolve, ink technology continues to advance—providing more options than ever for meeting the durability, regulatory, and aesthetic demands of modern plastic products.

At Deco Technology Group, we’ve helped hundreds of companies overcome substrate-related print challenges with Swiss-engineered ink systems and technical guidance. Whether it’s medical devices, automotive parts, consumer electronics, or industrial components—we’re ready to help bring your pad printing goals to life.

If you're facing print failures on polypropylene, nylon, or other tough plastics, we can help. At Deco Technology Group, we offer Swiss-made Printcolor inks engineered specifically for low-energy substrates—plus hands-on technical support to get your process dialed in.
📞 Call us at (714) 639-3326
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