Moulding and Die Casting

This article based on my professor Archer and I added som more information about photos and explanations.

Mould Structure

Moulding and Die Casting

Introduction

Moulding and die casting are production processes used to manufacture a wide variety of everyday articles.

Four Processes

1. Die casting
2. Compression moulding
3. Blow moulding
4. Injection moulding

Die Casting

Casting metal in moulds is one of the oldest methods used by humans to shape metal. Molten metal is poured into a mould and allowed to cool and harden. Many metals are cast in moulds made of sand, however, non-ferrous metals with low melting points may be cast in metal moulds under pressure. Moulds used for die casting are referred to as dies.

Die Casting Metals

These metals include zinc, copper, aluminum, and their alloys. Some of the earliest uses of the die casting process were in the printing industry, where the rapid casting of type was an important aspect of getting a newspaper out on time. In the early 1900’s zinc was cast, followed by cast aluminum products in 1914. Magnesium and copper based alloys were first cast in steel moulds in the 1930’s.

Advantages of Die Casting

• High speed production.
• Close dimensional tolerances.
• Minimum finishing required.
• Mass production of identical parts.
• High density finished parts.

The Die Casting Process

The process of die casting a zinc alloy is as follows: The two halves of a mould (die) are held together under pressure in a die casting machine. A hydraulically activated plunger forces liquid metal (at up to 10 tons/in2) into the mould (The Shot). The molten metal cools and solidifies. The mould opens and the piece is ejected. The mould then closes ready for the next cycle.

Shrinkage

Die cast metals will shrink as they cool. The size of the die cavity must be adjusted accordingly. For example: cast iron will shrink from 3/32 to 1/8 inches per foot.

Compression Moulding

• Plastic moulding relies on the basic principle that plastic material (resin) becomes fluid under the influence of heat and pressure. The fluid resin is forced into a mould cavity to obtain the desired shape. When cool the part is ejected.
• In compression moulding the thermosetting material undergoes a chemical change. This setting or curing forms a part that is relatively stable and infusible. Unlike thermoplastics, thermosetting plastics cannot be re-melted.

Compression Moulds

• Thermosetting materials are used in compression moulds.
• Thermosetting moulds are made from steel, which is heat treated to resist wear and take a high surface finish.
• The thermosetting material cures (Hardens) under heat and pressure.

Some Thermosetting Plastics

• Phenolics
  • Bakelite
  • Durez
  • Fibrerite
•  Melamine-formaldehyde
  • Melmac
  • Cymel

Mould

Three Common Types of “Compression Moulds”

1. Positive Mould
2. Semi-Positive Mould
3. Flash Mould

Flash on a Positive Mould

• Clearance is between 0.0015 and 0.005 inches.
• Excess material squeezes out forming flash on part.

Positive Compression Mould

• The flash is formed vertically on a positive compression mould.
• Mould is used mainly with materials containing course fillers.
• Disadvantage of this mould type is that cavity walls become scored making it difficult to eject part.
• Flash is removed later by filing, cutting, tumbling or sanding.

Flash on a Semi-Positive Mould

• Excess material squeezes out forming flash on part.

Semi-Positive Mould

• The mould first acts like a flash mould as excess material is allowed to escape; it then acts like a positive mould as the plunger reaches the bottom of the stroke.
• Combines the advantages of the free flow of plastic in a flash mould with a dense finished part.

Flash Mould

• Mould open (top). Mould closed (bottom)
• Excess material is squeezed out over land.
• Landing bars ensure plunger does not crush cavity.
• Land clearance is between 0.002 to 0.005 inches.
• Not normally used with course fillers.
• Not normally used where dense parts are required.
• Good for shallow parts such as plates and saucers.

The Press

• Thermosetting compression moulds are mounted between the platens of a hydraulically operated press.
• The top of the mould is normally stationary in the press.
• Flash is always formed on parts made in a compression mould.

Venting

• Gases form in the mould as the material cures.
• Gas pockets cause incomplete parts, blistering and scorching of the part surface.
• The mould may be opened again for a short time, or grooves may be ground on the periphery of the plunger (0.002 to 0.010 inches deep).

Blow Moulding

• The blow moulding process involves blowing air into a tube of molten plastic to expand it like a balloon to the shape of a cavity. Pop bottles and plastic automobile oil containers are made with this process.

Pros & Cons of Blow Moulding

• Only suitable for mass production.
• Produces dimensionally accurate parts with a high finish.
• High startup costs.
• Fast cycle times.

Injection Moulding

• The injection moulding process normally involves injecting liquid plastic (resin) under pressure, into a closed mould cavity.

The Injection Moulding Process

The process of injection moulding is as follows: The two halves of a mould are held together under pressure in an injection moulding machine. A hydraulically activated plunger forces liquid plastic along the runner system into the mould cavity(s) (The Shot). The plastic cools and solidifies. The mould opens and the piece is ejected. The mould then closes ready for the next cycle.

Moulding Process

Some Thermoplastics

• Styrene-polystyrene
  • Polyco
  • Lustron
• Abs(acrylonitrile-butadrine-styrene)
  • Cycolac
  • Kralastic
• Vinyl, acrylic, nylon, and polyethylene etc.

Strength of Plastics

• Each resin has its strengths and weaknesses.
• One of the thermoplastics materials, in the polycarbonate group, known under the trade name ‘Lexan’, is used for machine guarding and for bullet-proof shields.

Advantages of Injection Moulding

• Efficient and economical method of mass producing products.
• Secondary operations normally not required.
• High production rates. (Parts may sometimes be removed from the mould before they are fully cooled, which decreases cycle time.)
• Little or no wasted material.

Sizes of Injection Moulding Machines

• Shot capacity in ounces. (The amount of polystyrene which the injection plunger can deliver to the mould in one complete stroke).
• The tonnage. (The total clamping force available to hold the mould together as the mould fills under extreme pressure).

Remember

All of these processes involves the manufacture of moulds and equipment that is by its very nature is expensive to produce. The mould cavities are often machined with CNC and EDM equipment. The mould parts are then assembled by skilled mould makers.