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D1928-96 – Standard Test Method Technical Guide
This practice, designated D 1928 – 96, provides standardized methodologies for the preparation of compression-molded test sheets and specimens derived from Types I, II, III, and IV polyethylene plastics, as defined in Specification D 1248. Both branched (low-density) and linear (high-density) polyethylenes are covered. The strict control of thermal history during molding is the central principle, ensuring that subsequent physical testing reflects the material’s intrinsic properties rather than its processing history.
📐 Standard Scope, Materials, and References
The practice applies to polyethylene molding and extrusion materials and integrates tightly with several key ASTM standards to provide a complete framework for material specification and evaluation. The table below highlights the critical companion standards referenced within D1928.
| 📚 Standard | 🎯 Title / Relevance |
|---|---|
| D 618 | Practice for Conditioning Plastics and Electrical Insulating Materials for Testing |
| D 638 | Test Method for Tensile Properties of Plastics |
| D 1248 | Specification for Polyethylene Plastics Molding and Extrusion Materials (defines Types I–IV) |
| D 4703 | Practice for Compression Molding Thermoplastic Materials into Test Specimens, Plaques, or Sheets |
🟢 International Alignment: While this practice differs in specific details from ISO 293-1986(E), the standard explicitly notes that data obtained employing either method should be considered technically equivalent.
⚙️ Cooling Schedules (Procedures A, B, and C)
The core of the practice involves heating the material above its melting point to erase prior thermal history, followed by precisely controlled cooling from the melt state. A picture frame mold—a flat metal plate of brass or steel with a precisely machined cavity—is typically utilized to maintain the required sheet dimensions. The three distinct cooling schedules are designed specifically to minimize interlaboratory variability of test results arising from differences in crystallization rates.
| 🟦 Procedure | 📏 Cooling Rate Specification | 🎯 Key Characteristics & Notes |
|---|---|---|
| Procedure A | 5 ± 0.5 °C / hour | Extremely slow cooling for maximum crystallinity. Erases prior thermal history. The standard notes may not be applicable to carbon black-filled materials due to difficulties with void formation. |
| Procedure B | Rapid Chilling (water quench) | Produces a highly amorphous structure. Recommended as an alternative if Procedure A yields void-containing sheets. |
| Procedure C | 15 ± 2 °C / minute (platen cooling) | Controlled moderate cooling rate. Often used for general testing purposes to achieve a standardized crystalline state. |
⚠️ Carbon Black Consideration: It is recognized that Procedure A may present difficulties with materials containing carbon black, frequently resulting in test sheets with internal voids. If a void-free sheet cannot be produced using Procedure A, the practice explicitly directs the user to select Procedure B or C.
📊 Significance, Terminology, and Safety
The fundamental purpose of this practice is to control the crystallization rate of the polyethylene melt, thereby minimizing interlaboratory variability. A picture frame mold is defined as a flat piece of metal (usually brass or steel) with its center portion removed to give the shape and dimension of the final molding, with the thickness of the metal dependent on the desired thickness and material shrinkage.
Specimens prepared using these procedures are typically tested per Test Method D638 for tensile properties. It is the responsibility of the user to establish appropriate safety and health practices prior to use. Specific precautionary statements are located in Section 7.5.1 of the full standard document, and compliance with all applicable regulatory limitations is required.
❓ Frequently Asked Questions
🔍 Why is the cooling rate strictly controlled during sheet preparation?
The cooling rate directly determines the crystalline morphology (size and number of spherulites) of the polyethylene. Varying cooling rates leads to significant differences in mechanical and physical properties, such as tensile strength, modulus, and density. Controlling this rate minimizes interlaboratory variability and ensures test results are representative of the material, not the molding conditions.
💡 What are the exact rate specifications for Procedures A and C?
Procedure A requires a highly controlled, slow cooling of the molten plaque at a rate of 5 ± 0.5 °C per hour. Procedure C specifies cooling the press platens at a significantly faster controlled rate of 15 ± 2 °C per minute. Procedure B involves an immediate rapid water quench.
⚠️ My carbon black-filled polyethylene produces voids with Procedure A. What alternatives exist?
The standard directly acknowledges this challenge in Note 1. If a void-free sheet cannot be achieved under Procedure A, the recommended corrective action is to switch to Procedure B (rapid water chill) or Procedure C (15 °C/min cooling).
📌 What is a “picture frame mold” as defined in this standard?
The standard defines it as a flat piece of metal, usually brass or steel, with its center removed to shape and dimension the final molding. The thickness of the metal is selected based on the desired thickness of the finished molding, taking into consideration the shrinkage of the specific material being molded.