Volumes of production

Impact extrusion is a high-volume production method. Depending on the size of the component, minimum quantities range from 3,000 upward.

Unit price vs. capital investment

Surprisingly, tooling is not as expensive as you might expect for a process that is used for high volumes, but the speed with which it turns out products means that it requires a large minimum order. Unit costs are very low.

Speed

The famous 1-liter (34 oz) Sigg bottles (pictured) are made at a rate of 28 per minute.

Surface

Offers a reasonably high degree of surface finish.

Types/complexity of shape

It is possible to produce thin- or thick-walled containers using backward impact extrusion, either cylindrical or square, that are closed at one end. (The forward process produces solid sections from solid rods of different shapes and sizes.) Both methods are best suited to symmetrical shapes. There are also certain guidelines regarding the ideal proportion of length and width, but you should consult your manufacturer, as these will depend on the material being used.

Scale

Suitable for parts weighing from a fraction of an ounce up to approximately 2 pounds.

Tolerances

High degrees of tolerance achievable by backward impact extrusion. (Obviously, forward impact extrusion offers greater tolerances because the final object is solid.)

Relevant materials

Aluminum, magnesium, zinc, lead, copper, and low-alloy steels.

Typical products

Backward extrusion is a popular method for forming drinks and food cans, aerosol cans, and similar containers. Forward and backward extrusion are used together to make such items as ratchet heads.

Similar methods

Forging (p.187) and extrusion (p.96).

Sustainability issues

Backward impact extrusion gives the metal improved strength and rigidity after forming, to allow for thinner wall thicknesses which can help to minimize material use. It is a cold-working process that requires only one single impact to form the metal into shape so energy consumption is fairly low for a process with such a fast cycle rate. In terms of material use, it is worth noting that aluminum is widely recycled.

Further information

www.mpma.org.uk

www.sigg.ch

www.aluminium.org

Molding Paper Pulp

including rough pulp molding and thermoforming

Product	disposable urine bottle
Materials	paper pulp
Manufacturer	Vernacare

The mesh texture, which is subtly visible on this image, is a testament to how the water was squeezed out through a wire mesh to compact the paper pulp into a finished product. The parting lines on the mold are also visible, and the text on the product shows how the process can achieve a decent standard of surface embossing.

Paper is one of the most efficiently collected and recycled materials of the modern age. Much of what is collected is converted into pulp to make new products for a variety of industries, though these are usually simple sheets or packaging. However, it is the molding of paper pulp using highly unusual mass-production technology that makes it particularly noteworthy.

The manufacture of molded paper products is based on two methods: the conventional rough (or industrial) pulp process and a thermoforming process. Both methods begin by soaking the collected paper in water in a giant tank, with the proportions of paper and water based on the level of consistency needed to achieve the particular end product (typically, the amount of paper can be as low as 1 percent). The resulting gray mixture is churned with a blade to produce the molding compound of “paper mush.”

Unlike most other material molding methods, which involve the mold being stationary, the aluminum or plastic female molds used in molding paper pulp (which have draining holes all over them) are submerged in tanks of liquid paper pulp. The molds are covered with mesh or gauze, which allows the water to drain out, hence the typical mesh impression that you can see on, for example, a standard egg box. A male mold is then used to compress the pulp, and a vacuum draws the water out of the mold, sucking the fibers firmly into the mold. At this point the whole thing is dried, thus forming the final product.

As well as using heat, as its name suggests, the thermoforming process involves the use of transfers and presses. After molding, the component is picked up by a transfer, which is the negative shape of the component, and carried to a heated press that forms the final shape. It offers several advantages, including better quality surface finish but is more costly to set up.

–  Uses recycled and recyclable material.

–  Produces lightweight parts.

–  Requires large production volumes.

–  Only suitable for use with a limited range of materials.