![]() Otherwise, as the film passes the blade shoulders, the additional drag from the blade, combined with web tension, could exceed the film's elastic limit. Hawkins Plastic, Film, & Foil Web Handling Guide). A general guideline is that the web tension in the slitting zone should not exceed 10% of the material’s elastic limit. Since the blade is dragging against the web, this frictional resistance must be factored in with the tension force. Low elongation or thick materials “crack” farther ahead of the blade edge while high elongation or thin materials “crack” closer to the blade edge ( Figure 4).įurthermore, the ratio of web tension to the material’s yield stress must be considered. Edge flaws ( Figure 3) may develop, and uncontrolled tearing or splitting may occur.Īdditionally, the properties of the material yield different crack characteristics. The farther away from the tip of the razor the crack forms, the less stable the process. The physical properties of the material and the shape of the blade edge determine how and where this crack will form. When engaging the web, the razor creates a “controlled crack” ahead of the blade edge. ![]() Razor slitting produces an inferior result primarily due to the fact that it induces lateral tensile stress forces across the web (cross-machine “Y” mode – Figure 2). Let’s take a look at why “razor sharp” is just not good enough. The culprit here is often the slitting process used-razor slitting. The result is rolls that are “blocked” due to solvent flowing through the scalloped edges or visibly poor seam quality due to solvent not flowing to, or being placed on, the very edges of the film. Figure 1.įinished roll edges that are “curled,” like those in the image in Figure 1, significantly jeopardize the bonding of the two edges. ![]() For shrink sleeves, the edges must come together perfectly both to align and create a visually transparent seam, and they must be bonded together with a solvent and not fail or distort during the shrinking process. As such, this edge has more unique requirements than film that remains in a flat form. However, shrink sleeve film has a unique requirement in that the edges of a piece of film are brought together and bonded with a solvent to form the tube, or sleeve, that fits over the product and is shrunk in place. They are inexpensive and they serve the market well for many general film slitting purposes. Today, many films are slit with razor blades. Needless to say, the act of slitting film involves placing a significant obstacle in its path. Due to the “plastic flow” characteristic it exhibits, plastic film reacts to obstacles in its path depending on velocity, density, rigidity, temperature, crystalline structure, and shear characteristics. Plastic films begin in a liquid state that is cooled to become the final product, whether that is polyethylene terephthalate (PET), oriented or bi-oriented polypropylene (OPP/BOPP), polylactic acid (PLA), polyethylene (PE), or some other form. While it is important to understand the properties of all materials in order to achieve good slit quality, plastic film has some unique characteristics. For this article our focus is on how best to control that fracture when slitting shrink sleeve films. However, while most analysis concentrates on how to prevent fractures, in slitting our emphasis is on how to control and focus the fracture. ![]() Fracture mechanics is a field of mechanics that focuses on what causes cracks to propagate in different materials or, more simply, the forces that separate things. When we study how things break or why they break we are in an area of science known as fracture mechanics. Slitting is really just a controlled crack. ![]() When slitting shrink sleeve films, "razor sharp" may not be good enough. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |