1.4.3 Redistribution: reforming

Reforming: Changing the shape of the material by changing the state i.e. turning from a solid to liquid and back to solid.

Wood is not a material that lends itself to being reformed but in its pulped state it can be mixed with resin to make MDF and it can be formed into a variety of shapes not just sheet material.

Paper and board are also sourced from wood and can be shaped when reformed into egg boxes and other moulded shapes.

Polymers are a commonly formed material due to the ease with which they can be turned into a liquid and back to a solid at relatively low temperatures.

Metals are commonly reformed as they are originally processed in a liquid state and therefore lend themselves to being melted down and reformed.

Reforming Metals

Metal exists in a liquid state when formed so it lends itself particularly well to being moulded again by being melted back into a liquid form.

Sand Casting

Sand Casting is also known as sand moulded casting, is a metal casting process characterized by using sand as the mould material. The term ‘sand casting’ can also refer to an object produced via the sand casting process. Sand castings are produced in specialized factories called foundries. Over 70% of all metal castings are produced via a sand casting process.

In addition to the sand, a suitable bonding agent (usually clay) is mixed or occurs with the sand. The mixture is moistened, typically with water, but sometimes with other substances such as petrol, to develop strength and plasticity of the clay and to make the aggregate suitable for moulding. A common casting sand it known as Petrabond.

The sand is typically contained in a system of frames or mould boxes known as a flask.
The mould cavities and gate system are created by compacting the sand around models, or patterns, or carved directly into the sand.
The sand needs to be broken up to remove the metal object, which in turn needs to be left to cool,
It is quite a slow process!

Die casting

Die Casting is a metal casting process where molten metal is forced under high pressure into a mould cavity. The mould cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mould during the process. Most die castings are made from non-ferrous metals, specifically Zinc, Copper, Aluminium, Magnesium, Lead, Pewter and Tin-based alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.

The casting equipment and the metal dies represent large capital costs and this tends to limit the process to high volume production. Manufacture of parts using die casting is relatively simple, involving only four main steps, which keeps the incremental cost per item low. It is especially suited for a large quantity of small to medium sized castings, which is why die casting produces more castings than any other casting process. Die castings are characterized by a very good surface finish (by casting standards) and dimensional consistency.

There are several different methods which can be used.

Gravity Die Casting

as its name implies, uses gravity to force the molten material into the mould.

Pressure Die Casting

uses a ram to force the molten metal into the former using either a hot or cold chamber.

The additional pressure ensures better accuracy and detail.

Industrial Die Casting

refers to the use of multi slide die castings. Traditional die casting uses just two halves of a die to form the shape making it difficult to produce more complex shapes.

By using four or more slides, complex 3D shapes can be achieved.

**Workshop casting activity - click here

Lost Wax Casting

is the process by which a metal object (often Pewter, Silver, Gold, Brass or Bronze) is cast from an original wax sculpture. Dependent on the sculptor's skills, intricate works can be achieved by this method. Though the process today varies from foundry to foundry, the steps used in casting small pieces are fairly standardized. (In modern industrial use, the process is called investment casting.) Variations of the process include lost mould and waste wax process (or ‘waste mould casting’), because the mould is destroyed to remove the cast item.

Extrusion

Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section. The extrusion process can be done with the material hot or cold.

Commonly extruded materials include metals, polymers, ceramics, concrete, play dough, and foodstuffs. Hollow cavities within extruded material cannot be produced using a simple flat extrusion die, because there would be no way to support the centre barrier of the die. Instead, the die assumes the shape of a block with depth, beginning first with a shape profile that supports the centre section. The die shape then internally changes along its length into the final shape, with the suspended centre pieces supported from the back of the die.

Reforming Polymers

Plastic is perhaps the most commonly reformed material due to the ease with which it can be turned into a liquid and back to a solid at relatively low temperatures.

Injection Moulding

is a manufacturing process for producing parts by injecting material into a mould. Injection moulding can be performed with a host of materials, including metals, glasses, elastomers, confectionary, and most commonly thermoplastic polymers. Material for the part is fed into a heated barrel, mixed, and forced into a mould cavity, where it cools and hardens to the configuration of the cavity. After a product is designed, moulds are made by a mould maker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection moulding is widely used for manufacturing a variety of parts, from the smallest components to entire body panels of cars. Advances in 3D printing technology, using photopolymers which do not melt during the injection moulding of some lower temperature thermoplastics, can be used for some simple injection moulds.

this diagram shows how the Injection Moulding process works using thermoplastic polymers.

Rotational Moulding

involves a heated hollow mould which is filled with a charge or set weight of material. It is then slowly rotated (usually around two perpendicular axes) causing the softened material to disperse and stick to the walls of the mould. In order to maintain even thickness throughout the part, the mould continues to rotate at all times during the heating phase and to avoid sagging or deformation also during the cooling phase. The process was applied to plastics in the 1940s but in the early years was little used because it was a slow process restricted to a small number of plastics. Over the last few decades, improvements in process control and developments with plastic powders have resulted in a significant increase in usage. Unlike other types of moulding, the shape can be hollow yet have a very thick wall.

Extrusion;

similar to Injection Moulding but without the use of a hydraulic ram as the polymer is continually pushed through the die.

Compression Moulding

is the main process used to produce products using Thermosetting Polymers such as Urea Formaldehyde.

The moulding material, generally preheated, is first placed in an open, heated mould cavity. The mould is closed with a top force or plug member, pressure is applied to force the material into contact with all mould areas, while heat and pressure are maintained until the moulding material has cured.

The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. Compression moulding is a high-volume, high-pressure method suitable for moulding complex, high-strength fiberglass reinforcements.

It is one of the lowest cost moulding methods and it wastes relatively little material, giving it an advantage when working with expensive compounds.

Resin and GRP Moulding

is not strictly a reforming process as it effectively involves using a liquid polymer (Resin) which is left to cure and set hard.

GRP is also combined with fine strips of glass to reinforce the resin when it sets. However as the resin changes form from liquid to solid. It is included in this section just for reference.

3D Printing

3D printing is an additive process where you start with nothing and add layers of plastic which build up to form a 3 dimensional shape. As the plastic is molten it is essentially a reformation process but it is a very new process and can print metal, food and even body parts!

Summary

Redistribution is the movement of material in order to shape it. This can be done by deforming the material using heat and/or pressure or by turning the material into a liquid state then reforming it.
Reforming is used primarily with metal and plastics.
Wood can only be reformed into MDF, Chipboard and Maplex but it can be deformed by steam bending or laminating in a press.
Most complex plastic components are made by moulding techniques.
Plastic bottles are commonly made by deforming techniques.
The terms are not rigid so, 3D printing is additive but involves melting the polymer filament so could be considered reforming.

Revision Notes

For each material group, you need to be familiar with a range of forming, redistribution and addition techniques.
In these units we refer to wasting, deforming, reforming and fabrication but they correspond with those same groups.
If you change shape by force/heat it is deforming (forming/redistribution)
If you change shape by changing state, it is reforming (redistribution)
If you join pieces together it is considered fabrication (also additive)
This is just terminology and categorization, what it important is that you understand which processes are possible and effective with each material group.

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