Volumes of production

Due to the high cost of tooling and the speed at which parts can be made, both rough pulp molding and thermoforming require high volumes of production. Minimum runs of two days (about 50,000 pieces) are generally required.

Unit price vs. capital investment

Tooling costs and setup times are high. The two methods have different tooling requirements; the thermoforming method costs approximately twice as much as the raw method.

Speed

Thickness, and the amount of paper that needs to be dried, determine the speed. As a guide, the molded inserts for four cell-phone boxes take about a minute to produce. This is based on a multiple impression, meaning four components are molded at the same time. These four different molds can therefore produce 960 units per hour.

Surface

Just think of a paper egg carton to get a sense of that uniquely soft, warm, cookie-like surface. The rough pulp process produces one rough side, picking up an impression from the wire mesh, and a smooth surface created by the polished aluminum, or plastic, face of the mold.

Types/complexity of shape

Some fairly complex patterns can be molded, but large draw angles need to be allowed for; forget any complex three-dimensional detailing.

Scale

Standard production allows for up to 60 by 16-inch areas—however, some manufacturers can sustain sizes up to 8 feet long.

Tolerances

Tolerances vary depending on the specific process. Tolerances of ±1/50–1/25 inch are achievable using the thermoforming method. For the rough pulp process, ±1/12–1/8 inch is achievable.

Relevant materials

The raw materials come from two main sources: newsprint and cardboard. The choice of material depends on the final product and the strength that is required. For strong packaging that needs to satisfy drop-test requirements (used, for example, for cell phones, PDAs, and cameras), the long fibers found in cardboard provide the best solution.

Typical products

Conventional rough pulp is used to make wine packs and industrial packaging. The thermoforming process is used to produce more sophisticated products such as cell-phone packaging.

Similar methods

None.

Sustainability issues

Pulp is made from recycled paper products so the process helps to reduce waste and the use of raw materials in the first instance, while the pulp is recyclable at the end of its use. The conventional rough forming of the material requires little energy whereas the thermoforming process involves heat, which significantly increases energy consumption. The main drawback is the amount of water the processes require.

Further information

www.huhtamaki.com

www.mouldedpaper.com

www.paperpulpsolutions.co.uk

www.vaccari.co.uk

www.vernacare.co.uk

Contact Molding

including hand lay-up and spray lay-up molding, vacuum-bag and pressure-bag forming

Contact molding is a method of forming composites by taking plastic reinforcement fibers, layering them, then applying liquid resin over the top to create a hard shell. In its simplest form—the traditional hand lay-up method—the reinforcements are laid over a mold before the liquid resin is brushed or sprayed into it. If you have ever repaired a dent or hole in an old car or boat you will probably have used a simple version of this process. In industry, it is a process for producing large-scale moldings in composite materials, and it is one of the most frequent methods of combining various types of reinforcement fiber with thermoset resins.

The open-form molds used in hand lay-up can be made from any material, but wood, plastic, or cement are the most common. The reinforcement fibers are generally glass or carbon, but other materials, including natural fibers, can be used. A resin is then applied with a brush or by spraying, before rollers are used to squash and to distribute the mixture evenly in the mold. The spray-up method is used when larger areas are involved, using short, chopped fibers that are incorporated into the resin before spraying. In both cases, the thickness of the part is controlled by the number of layers that are applied.

Vacuum-bag and pressure-bag forming are variations of the hand lay-up and spray lay-up methods for forming composites, but they give the molding finer detail and greater strength. The procedure is similar for both variants: in the pressure-bag method, once the materials have been laid over the mold, a flexible bag made of rubber is placed over them and subjected to pressure by clamping it, which compacts the materials, squeezing the resin and reinforcement together; with the vacuum-bag method, the part is cured inside a bag from which the air has been sucked out, forcing the materials together.

With vacuum-bag forming it is possible to achieve similar results to those that you find with autoclave molding (see p.156) but without the need for a pressure chamber. Compared with the hand and spray lay-up methods, both vacuum-bag and pressure-bag forming result in higher fiber content and density because of the use of a vacuum or pressure, which also limits the amount of potentially harmful vapor to a minimum.

–  The use of reinforcing fibers results in high strength.

–  Other performance additives, such as flame-retardants, can easily be incorporated.

–  Versatile in terms of shape and size.

–  Allows thick sections to be produced.

–  Quite a labor-intensive process.

–  Requires good ventilation due to the resins.

–  Other composite-forming methods (such as filament winding, see p.140) offer much higher density and strength-to-weight ratios.