MANUFACTURING

The moulding of objects started in the bronze-age 1400BC to 1000BC when bronze-age man first moulded spear heads and axes. The mould is a hollowed out block which is filled with a liquid such as plastic, glass, metal or ceramic materials. The mould is sprayed with a releasing agent so that the hardened moulded object can be removed easily.

Injection moulding is a process that can use either Thermo plastics or Thermosetting plastic materials. The material is fed into a heated barrel where it is then mixed and when the material is at the right temperature and the right consistency, it is injected into a mould which is usually made from steel or aluminium. It is then left to cool down and take the shape required.

During the course of designing any product, at some point in the process, the manufacturing industry will have to be involved.  Their input during consultations is vital for the product to be successful. Many aspects of manufacturing are not common knowledge to the general public and many “Tricks of the Trade” are unknown to many designers.

The following paragraphs describe some manufacturing processes.

There are various types of moulding techniques:-
  • Injection moulding
  • Reaction injection moulding
  • Compression moulding
  • Transfer moulding
  • Extrusion moulding
  • The Float Glass Process
  • Blow moulding
  • Rotational moulding
  • Thermoforming
  • Vacuum forming
  • Laminating    
  • Electronic Component Parts

The following is a short description of each process and its uses in the manufacturing industry.

 

INJECTION MOULDING

This process involves melting plastic pellets and injecting the liquid under pressure into a mould which is placed in an injection moulding machine. There are three parts to the design of an injection moulded part; the wall thickness, projections from the surface and any holes or recesses.

THICKNESS

The wall thickness should not vary more than 10% over the full area or the moulding, as any higher than this figure could cause weakness in the structure. The design should avoid over thick walls as this means using more plastic and thus, more expense. The design of the part should justify the wall thickness in order that it has integral strength. The introduction of glass of carbon fibres into the injection process produces a reinforced plastic which can be produced with less thickness.

PROJECTIONS

These are usually reinforced ribs in the moulded plastic, but they must be the same overall thickness as the panel or sink marks may occur.

RECESSES OR HOLES

The position of holes and recesses can affect the overall strength of the panel and should be kept to a minimum.

The initial design of the part to be moulded is very important when considering corners. The liquid plastic, when it enters a mould, follows fluid dynamics and it is preferred that the initial design contains no sharp corners. Rounded corners are preferred which allow the flow of the liquid plastic with little resistance. When plastic cools around a square corner a flaw could result. Image courtesy of:- http://www.design-technology.org/injectiondrawing.JPG

REACTION INJECTION MOULDING

 

This process is very similar to standard injection moulding but with the use of a different type of plastic. Thermosetting polymers are used instead of thermoplastics which need to sit in the mould until cured. The end result is a stronger finish product which can incorporate reinforcing material in the initial polymer mixture before moulding.

 

COMPRESSION MOULDING

 

This method of moulding, the pre-heated moulding material if placed in an open, heated mould cavity. The mould is then closed with the second part of the mould, which is called a plug or a top force. Pressure and heat are applied and maintained which forces the moulding material into the mould area, then allowed to cure. Thermosetting resins are used for this process in the form of granules, resin with a putty consistency, and preforms. This method is ideal for high-volume, high-strength, complex fibreglass reinforcements. The disadvantages are the poor product consistency, high occurrence of flashings (waste material produced at mould edges), and not suitable for very small intricate parts.

TRANSFER MOULDING

This method is very similar to compression moulding. A measured amount of thermosetting plastic is inserted in a preheated state into a pot. A plunger is employed to push the material from the pot into the mould cavities via a runner system known as sprue. The mould is heated to a higher temperature than the melting point of the moulding material to ensure a faster and even flow. After the mould has cooled, the part is released along with its sprue, which needs to be removed and finished by hand to produce a completed part.

 

EXTRUSION

 

This process involves the use of plastic beads, often referred to as resin in the plastics industry, which are gravity fed via a hopper into the barrel of the extruder. A rotating screw carries the beads along the extruder barrel which is heated to melt the beads (200°C to 400°C). The beads are heated gradually along the length of the screw. This gradual heating of the beads is necessary to prevent overheating which could cause degradation to the plastic polymer. Once the process has run for a while the heaters can be turned off, the pressure and the friction now keep the plastic in a liquid form. Cooling fans are then used to keep the liquid plastic at the optimum temperature. After leaving the screw the liquid plastic passes through the breaker plate which removes contaminates in the liquid and produces back pressure in the barrel which helps with uniform melting and mixing of the plastic polymer. Holes in the screen pack, which is located within the breaker plate, converts the rotational memory of the liquid plastic as it leaves the screw into a longitudinal memory. The liquid plastic now enters the die which gives the final product its shape. An even flow of plastic is critical at this point as any unevenness in flow causes uneven stress points.
 
THE FLOAT GLASS PROCESS

Glass is produced in its flat state using the float process, invented in 1952 by Sir Alastair Pilkington. It can be produced clear or tinted in thicknesses from 0.4mm to 25mm. The process of floating glass starts with molten glass at a temperature of 1000°C being poured from a furnace onto a bath of molten tin. It floats on the surface of the tin and spreads out to form a flat surface. The thickness of the glass is controlled by the speed at which the solidifying glass is drawn off from the bath. The glass is then cooled.

BLOW MOULDING

 

The blow moulding process uses thermoplastic to produce plastic parts that are hollow. There are three main types of the blow moulding technique: extrusion blow moulding, injection blow moulding and stretch blow moulding.

The process starts by melting down the plastic to make a preform or a parison. A parison is a cylinder of plastic with a hole at one end to allow compressed air to pass through.

 

The process is performed in two phases:

A preform of hot plastic resin is formed into a tubular shape.

A pressurised gas, usually air, is applied to expand the heated preform and pressed into the mould cavity. The pressure remains constant until the plastic resin cools. The detail on the outside of the moulding is easily controlled but not the inside detail, where material thickness can vary and cause an internal shape that is not consistent.

 

ROTATIONAL MOULDING

This technique of moulding plastic was first used in the 1940s, but at that time it was used very little due to the slowness of the process and the limited supply of plastic as a raw material. The process has been improved over the past two decades and the development of plastic powders has resulted in an increase in its usage.

The plastic is held in a puddle in the bottom of a heated mould. The mould then is slowly rotated which causes the molten plastic to flow into the mould shape. The mould continues to rotate after the heating elements are turned off and the plastic is allowed to solidify. This helps maintain uniform wall thickness.
Image courtesy of:-http://en.wikipedia.org/wiki/File:Rotational_Molding_Process.svg
 
THERMOFORMING
 
In this manufacturing technique, a plastic sheet is heated until pliable and allowed to take the shape of the mould form. The unwanted flashings are removed using a trimmer, producing a finished product. Thinner gauge plastics are referred to as films, these are heated in an oven and stretched over the mould and allowed to cool.
 
VACUUM FORMING

This process is exactly the same as the Thermoforming above with the sheet of heated plastic being stretched over the mould but instead of relying on gravity to pull the plastic into the mould, the plastic sheet is held against the mould by applying a vacuum between the mould and the sheet. It is essential that a 3° minimum drift angle is applied to the mould design in order to facilitate easy release of the formed plastic.

LAMINATE

The process of lamination is used throughout the manufacturing industry fin many applications. It is the placing of a material between plastic and gluing them with the use of heat, pressure and adhesive. In electrical engineering, the lamination technique is used to reduce unwanted heating effects due to eddy currents in magnetic cores and transformers.

Other uses of lamination in manufacturing are:-

  • Plastic film on each side of glass for vehicle windscreens.
  • Formica and plywood/chipboard/MDF for the building industry.
  • Photographic paper is a laminate to prevent creasing.
  • Identification cards and credit cards.

ELECTRONIC COMPONENT PARTS

The electronic components are mass produced in many forms, such as:-

  • Resistors
  • capacitors
  • transistors
  • microchips for many functions.
Other components such as:-
  • switches
  • connectors
  • cable
  • trunking
  • fixing brackets
  • set screws and nuts are also manufactured in vast quantities.
The end products of all these separate component parts are the units which will be of interest to most consumers, i.e.
  • computers
  • power supplies
  • cooling fans
  • touch sensitive screens, and wire connectors to connect all the units together. 

There are many considerations to be made when choosing the structural and internal parts of any design and it is important for the designer to be conversant with the manufacturing processes involved before the design is undertaken. Liaison with manufacturing companies is therefore advised throughout the design process.