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D3351-93 – Standard Test Method Technical Guide
The ASTM D3351-93 standard establishes a test method for quantifying the number of gels in plastic film. Gels, defined as nodules of oxidised or high-molecular-weight material, are objectionable due to their impact on film appearance and printability. This method uses an overhead projector to magnify film images onto a screen, facilitating easier detection and counting of gels that are typically difficult to observe with the unaided eye.
📐 Specimen Configuration and Test Area
Each test specimen comprises four layers of film, each layer having dimensions of at least 200 mm (8 in.) square. A convenient specimen size is 254 mm (10 in.) square, and the layers are typically stapled together at one corner for handling. Three such specimens are prepared for testing. The effective viewing area, delineated by a 190 x 200 mm (7.5 x 8 in.) rectangle drawn on the projector viewing plate, is observed through all four layers. This yields a total test area of 1520 cm² (240 in.²) per specimen.
The film thickness applicable to this standard is limited to 100 μm (0.004 in.) or less per layer.
⚙️ Test Procedure and Equipment Setup
The apparatus includes an overhead projector with a viewing plate at least 200 x 200 mm (8 x 8 in.), a projection screen of at least 2 x 2 m (6 x 6 ft), and a transparent acrylic plate (230 x 230 mm by 3.2 mm thick) with two standard holes. The acrylic plate has one 0.80 mm diameter hole and one 0.40 mm diameter hole, each approximately 1 mm deep, with smooth edges, and spaced about 12 mm apart.
The projector is set up to achieve a magnification between 8:1 and 10:1. The projection beam should be perpendicular to the screen to avoid image distortion. During testing, the film stack is placed on the viewing plate so that the 190 x 200 mm rectangle encompasses the center of the specimens. The acrylic plate is placed in contact with the top film layer and pressed firmly to remove wrinkles. Ensure that any staples used to hold the film layers do not intrude into the viewing area.
No specific conditioning period is specified, allowing testing on as-received samples.
⚠️ Important Note: When using blown tubing specimens, place two such specimens (each consisting of four layers) to cover the required area. For flat film, use four specimens directly. Always verify that the staple holding layers together does not enter the projected viewing field, as this may obscure gels and affect count accuracy.
📊 Key Measured Parameters and Specifications
| 🎯 Parameter | 📏 Specification | ⚡ Remarks |
|---|---|---|
| Film layer thickness | ≤ 100 μm (0.004 in.) | Per layer, per scope |
| Viewing area dimension | 190 x 200 mm (7.5 x 8 in.) | Drawn on projector plate |
| Total test area per specimen | 1520 cm² (240 in.²) | 4 layers x 190 x 200 mm area |
| Magnification range | 8:1 to 10:1 | To enlarge gels for counting |
| Standard hole diameters (acrylic plate) | 0.80 mm and 0.40 mm | For image size comparison |
| Acrylic plate hole depth | Approx. 1 mm | With smooth edges |
| Hole spacing | Approx. 12 mm apart | Centered on plate |
| Screen size | ≥ 2 x 2 m (6 x 6 ft) | For clear projection |
💡 Technical Tip: The 0.80 mm and 0.40 mm holes on the acrylic plate serve as reference standards for gel size estimation. During testing, operators can compare projected gel images to these standard sizes to classify gels qualitatively.
❓ Frequently Asked Questions
🔍 What is considered a gel in this test method?
According to Section 2.1.1, a gel is a nodule of plastic material present in the film matrix that does not readily blend with the matrix. It is believed to be composed of oxidised or high molecular weight materials, and it is distinguished from contaminants like dirt, carbon, or lint.
💡 What film thickness range is covered by this standard?
As specified in Section 1.2, the standard applies to plastic films with a thickness of 100 μm (0.004 in.) or less per layer.
⚡ How many specimens must be prepared for a test?
Section 6.2 requires that three test specimens be prepared. Each specimen consists of four layers of film, stapled together at one corner for convenience.
📌 Is specimen conditioning necessary before testing?
No, section 8.1 states that no specific conditioning period is required. The film can be tested in its as-received condition.