Understanding Rub Resistance in Mylar Bag Printing
To test the rub resistance of printing on mylar bags, you primarily use standardized abrasion testing methods like the Sutherland Ink Rub Tester or the Crockmeter, which simulate the friction and wear the printed surface will encounter during handling, shipping, and use. The core principle involves applying controlled, repetitive rubbing motion under a specific pressure to the printed area and then evaluating the degree of ink transfer or degradation. The key is to replicate real-world conditions in a quantifiable way to ensure the print quality meets durability standards for your specific application, whether it’s for food packaging, industrial parts, or retail products. For businesses seeking reliable mylar bags printing, understanding and specifying these test results is crucial for product integrity.
Why Rub Resistance is Non-Negotiable for Packaging
Rub resistance, also known as abrasion resistance or scuff resistance, isn’t just about aesthetics; it’s a critical functional property. Poor rub resistance can lead to significant problems. Faded text can make important information like ingredient lists, barcodes, or instructional text unreadable, potentially causing safety issues or supply chain delays at checkout scanners. Ink transfer, where pigment from one bag rubs off onto another, can ruin entire pallets of merchandise, leading to massive financial losses and brand damage. Furthermore, in applications like medical device packaging or certain food products, the integrity of the printed surface can be a barrier property concern. A robust rub resistance test protocol is your first line of defense against these costly failures, ensuring your brand’s message remains clear and intact from the production line to the end-user’s hands.
Essential Equipment for Professional Testing
While basic “finger rub” tests can give a quick, qualitative indication, professional and reproducible results require standardized equipment. The two most common instruments are:
The Sutherland Ink Rub Tester: This device is a industry workhorse. It operates by placing the printed sample on a flat, reciprocating base. A weighted, four-finger rubbing head, often covered with a specific substrate like unprinted paper or cloth, is lowered onto the sample. The tester then runs for a predetermined number of cycles (e.g., 20, 50, 100 cycles) at a controlled speed and pressure. The abrading substrate is then examined for any transferred ink.
The Crockmeter: Commonly used in the textile industry but applicable to flexible packaging, the Crockmeter uses a reciprocating finger covered with a standard white cotton rubbing cloth. The test is similar but often under different load conditions. It’s excellent for testing resistance to dry abrasion (dry crocking) and can be modified for wet abrasion testing (wet crocking) by moistening the cloth with water or other solvents, which is vital for testing packaging that might encounter condensation or spills.
The choice between them often depends on the specific industry standards you are adhering to. For instance, the Sutherland tester is frequently referenced in ASTM and TAPPI methods for packaging.
Step-by-Step Testing Procedure
A rigorous test follows a meticulous process to ensure accuracy and repeatability. Here’s a detailed breakdown:
1. Sample Preparation: Cut a minimum of three identical samples from the flat, printed area of the mylar bag. The samples must be large enough to fit securely in the testing apparatus. Ensure the samples are conditioned according to the testing standard, typically at 23°C ± 2°C (73°F ± 4°F) and 50% ± 5% relative humidity for at least 4 hours. This eliminates variables caused by ambient temperature and humidity.
2. Instrument Calibration: Verify the calibration of the rub tester. Confirm the weight applied by the rubbing finger is correct (e.g., 2 lbs for a Sutherland Tester). Ensure the rubbing stroke length and speed are set as per the test method (e.g., 60 cycles per minute).
3. Mounting the Sample: Secure the mylar sample firmly to the base plate of the tester, ensuring it is taut and flat to prevent uneven rubbing.
4. Mounting the Rubbing Medium: Cut a fresh piece of standard white test paper or cotton cloth for each test. Secure it over the rubbing finger according to the instrument’s instructions.
5. Running the Test: Set the cycle counter to the desired number. Common industry tests run for 20, 40, or 100 cycles. Start the machine and allow it to complete the programmed cycles.
6. Post-Test Evaluation: This is the most critical phase. Carefully remove the rubbing medium and the sample. The evaluation can be both quantitative and qualitative:
- Visual Inspection: Compare the tested sample to an untested control under consistent, standardized lighting (often D65 daylight simulator). Note any visible scratches, fading, or gloss change.
- Ink Transfer Assessment: Place the used rubbing medium next to a fresh, unused one. The degree of ink staining on the used medium indicates the level of transfer. This is often rated using a grayscale.
- Densitometry/Colorimetry: For a quantitative result, use a spectrophotometer to measure the color density (Delta E, ΔE) of the printed area before and after testing. A ΔE value greater than a specified tolerance (e.g., ΔE < 2.0 for minimal change) indicates failure.
Interpreting Results with Industry Standards
Raw test data is meaningless without a benchmark. This is where industry standards provide the essential framework. The following table outlines key standards and their acceptance criteria.
| Standard | Common Name / Application | Typical Test Conditions | Acceptance Criteria (General Guideline) |
|---|---|---|---|
| ASTM D5264 | Standard Test Method for Abrasion Resistance of Printed Materials (Sutherland Method) | 2 lb weight, 40 rubs, standard test paper. | Minimal visible ink transfer to the test paper; ΔE color difference should be less than 2.0-3.0 when compared to an untested sample. |
| TAPPI T830 | Abrasion Resistance of Printing (Sutherland Method) | Similar to ASTM D5264, widely used for paper and board, applicable to laminated surfaces. | Rating based on a 1-5 scale (5 being no transfer) or a pass/fail against a predefined visual standard. |
| ASTM F2497 | Crockmeter Method (Rooted in AATCC 8) | 9N (approx. 2 lb) force, 10 or 20 cycles, dry and wet crocking cloth. | Rated on a grayscale from 1 (severe staining) to 5 (no staining). A rating of 4.0 or higher is often considered acceptable for commercial quality. |
| Internal QA Standards | Company-Specific | May be a modified version of an industry standard. | Often stricter, tailored to the specific abrasion risks in a company’s supply chain. |
It’s vital to agree upon the specific standard and acceptance criteria with your printer or supplier before production begins. This prevents disputes and ensures everyone is working towards the same quality goal.
Factors That Directly Impact Rub Resistance
The rub resistance of a printed mylar bag is not determined by a single factor but by a complex interplay of the entire printing and material system. Understanding these variables allows you to specify the right combination for your needs.
1. Ink Chemistry and Curing Mechanism: This is arguably the most significant factor.
UV-Curable Inks: These inks polymerize instantly when exposed to ultraviolet light, forming a hard, cross-linked, and highly durable film. They typically offer superior rub and chemical resistance compared to other ink types.
Solvent-Based Inks: These dry through solvent evaporation. While durable, the film formation can be less robust than UV-cured inks unless paired with a protective overcoat.
Water-Based Inks: More environmentally friendly but generally offer the lowest inherent rub resistance and require excellent surface treatment and often an overlaminate for demanding applications.
2. Substrate (Mylar) Surface Treatment: Mylar (biaxially-oriented PET polyester) has a naturally low surface energy, making it difficult for inks to adhere. To solve this, the film undergoes a surface treatment, most commonly corona treatment. This process uses a high-voltage electrical discharge to increase the surface energy (measured in Dynes/cm), creating microscopic pores that allow the ink to mechanically anchor itself. A dyne level of 38-42 is typically recommended for adequate ink adhesion. An untreated or poorly treated surface will result in poor rub resistance, as the ink sits on top rather than bonding.
3. Use of Overprint Varnishes (OPV) or Laminates: Applying a clear protective coating is the most effective way to drastically enhance rub resistance. An Overprint Varnish (OPV) is a liquid coating applied inline or offline that cures to form a protective layer. A laminate involves bonding a clear film (like OPP or another layer of PET) over the printed surface, offering the highest level of protection against abrasion, chemicals, and moisture.
4. Printing Process: Flexographic printing, common for flexible packaging, applies relatively thin ink layers. The ink’s durability is paramount. Digital printing, while offering great flexibility, can sometimes produce layers that are more susceptible to abrasion if not properly finished with an OPV.
Implementing a Practical Quality Control Protocol
For a business, testing shouldn’t be a one-off event but an integrated part of your quality control (QC). Here’s how to build a practical protocol:
Incoming QC: When you receive a shipment of printed mylar bags, randomly select samples from different production batches. Perform a simplified rub test—even if you don’t have a Sutherland tester, a controlled “thumb twist” test (firmly rubbing a thumb over the print) can reveal glaring issues. For critical jobs, retain a pre-approved “golden sample” from the printer to compare against future production runs.
Supplier Qualification: Before placing a large order, require your potential printer to provide certified test reports from an independent lab showing their prints pass specific standards like ASTM D5264 under agreed-upon conditions. Audit their facility to see if they have the necessary equipment and procedures in place.
Defining Your Specification: Your purchase order should be explicit. Instead of saying “good rub resistance,” specify: “Print must pass ASTM D5264 with a 2 lb weight for 40 cycles, with a maximum allowable ΔE of 2.5 and no visible ink transfer on the test medium.” This clarity eliminates ambiguity and holds the supplier accountable.
Environmental Simulation Testing: Beyond standard rub tests, consider simulating the actual environment your bags will face. This might include testing rub resistance after exposing samples to high humidity (to test for wet rub resistance) or elevated temperatures, which can soften some ink films and make them more susceptible to abrasion.