Volumes of production
This is a high-volume method with a capacity to convert 550 pounds of plastic per hour.
Unit price vs. capital investment
High capital costs, but extremely cost-effective for large production runs.
Speed
Up to 430 feet per minute.
Surface
Controlled by various factors, including the material and machinery setup.
Types/complexity of shape
Flat sheets or tubes only.
Scale
Blown films range from 20 inches to 16 feet in diameter, and can be up to several hundreds of feet in length. Films are available in thicknesses of from 10 or 20 microns up to 250 microns.
Tolerances
This process can achieve high tolerances, but you should be aware that some manufacturers offer two grades of blown film—with and without thickness control.
Relevant materials
The most common materials are high-and low-density polyethylene, but other materials, such as polypropylene and nylon, can also be used.
Typical products
Most plastic film products—garbage bags, shopping bags, sheeting, plastic wrap, laminating film, and just about any other type of film you care to mention.
Similar methods
Extrusion (p.96) and calendering (p.90) are both used to produce thin flat sheets.
Sustainability issues
The blown-film process is capable of producing incredibly high volumes of plastic film at a very fast rate for minimal cycle times and therefore minimal energy consumption. However, energy use is still substantial due to the high temperatures involved and the constant high pressures that are required for working. Excess waste material is minimal.
Further information
www.plasticbag.com
www.flexpack.org
www.reifenhauser.com
Exjection®
Product |
An Exjection® sample |
Materials |
POM (although Exjection® can be applied to a range of plastics) |
Manufacturer |
Exjection® |
Country |
Germany |
Date |
Exjection® was first presented in 2007 |
Sample of an Exjection® molding that shows the small supports that cut cross the length. Conventional extrusion does not allow for this type of detail over a continuous shape.
Injection molding and extrusion are both vital manufacturing methods for numerous plastic and metal products. However, there are certain limitations to the types of product each can be used to produce. For instance, extrusion allows for long, thin parts to be created but the entire length of the strip must have a single profile shape, which means variations along the piece cannot be produced. In contrast, complex shaped pieces such as ledges or closures can be created with injection molding, but only in short lengths as the resin will not flow throughout long stretches without evidence of sinking in the final component. Exjection® brings together the benefits of both processes in components that are not only long but have variations and detailing along the length of the part.
The process relies on much the same principles as conventional injection molding where molten plastic is forced into a mold. However, the addition of a movable cavity makes the process a little more interesting. As the molten plastic is injected into the mold it begins to fill the hollow space. As this space begins to fill, the end of the cavity starts to move horizontally along the full length of the mold at the same speed at which the plastic is being injected. As the cavity moves, more of the mold is exposed for the molten plastic to flow into. With the cavity moving and the plastic being forced in at the same speed, a high pressure is maintained which enables the consistent flow of the plastic without shrinkage. It might help to imagine filling a syringe. As you pull the back of the plunger, the pressure pulls the liquid into the container, and the more you pull back, the more area there is for the liquid to fill. The absence of air creates a vacuum which is under high pressure.
When the mold is completely full, the molten plastic is left to cool and set before it is removed and the cavity set back to its starting position for the next cycle. Material type and wall thicknesses have an impact on the speed of production as they affect how quickly the cavity can move.
– Allows continuous profiles to be produced on an injection-molding machine.
– Can cost less than comparable alternatives.
– Limited to a small number of manufacturers.