The thickness of the plastic film you use is one of the more critical factors in how film looks. While dyes and coatings can certainly affect the appearance, too, it's the thickness that contributes most to the plastic's transmission haze value. The plastic film does not function as one massive piece of pure plastic, which means once light hits the outer layer (or the layer closest to you), it meets a number of obstacles that reduce its clarity almost immediately. Once you know what these obstacles are, you can configure your design to eliminate or increase them as much as possible, given how opaque or transparent you want that plastic to be.
More Material, More Refraction
Thicker plastic means more material that the light has to go through, and the more material there is, the more refraction the light undergoes. If the light doesn't have a straight shot through the plastic, the light is going to lose part of its power to illuminate what's behind the plastic. In other words, more refraction can scatter more light, leading to a hazier view. So if you want clearer plastic, you need thinner plastic because there will be less material to cause additional refraction.
Thicker plastic also means a higher chance of having microscopic contaminants in the plastic. "Contaminant," here, does not mean anything toxic or bad; it simply means something that isn't normally part of the plastic or other ingredients that went into creating the plastic, like dust. But these contaminants, as harmless as they may be, cause additional haziness. If you need clear plastic for your product, you'll need to find a manufacturer who can keep as many contaminants out as possible. You can also get haze testing equipment to help compare the transmission haze of different products.
Also keep in mind that some thicker plastic may not be one block of the same material, but rather layers of the material stuck together. The refraction of light is intensified in this case because there are multiple layers that the light will encounter. Plastic that has these layers doesn't always look like it, so you need to be sure you know how that plastic you're considering is made.
Hazy plastic isn't necessarily bad -- many products use opaque plastic. However, if you want clear plastic, or if you want to know how to increase the opacity of what you're ordering, knowing the additional factors that go into plastic clarity is essential. For questions about what you can do to assess the haziness of your film(s), contact a company like Imbotec Group.