Compression Moulding

INTRODUCTION

Moulding is the shaping of raw material using a solid frame of a particular shape called a pattern.It is a process used in the manufacturing of materials and mostly include elastomers,Plastic,metal,glass and ceramic.

It consists of placing a pre cut rubber compound into a two piece heated mold kept in a press.

The pressure applied by the press forces the material to flow and fill the mold.

Compression mold are simple and many rubber products like conveyer belts,seals and gaskets are made by the method.

A simple punching or cutting procedure directly from the rubber slabs produces blanks or alternatively the rubber compound is squeezed into shape with the aid of a plunger or screw through an extruder die. The strip obtained is then cut into suitable lengths.

The supply of heat depends on the rate of heat conduction from the mould to the rubber. Since the heat conductivity of rubber is poor, heating is slow and the curing time is therefore relatively long.

Typical curing times are from 3 minutes for thin goods to several hours for products with thick walls. In the latter case, it is more efficient to pre-heat the rubber. This can take place in a hot air oven or in a microwave oven up to temperatures of approximately 100 °C. When heated over 100 °C the risk of scorching becomes excessive.

Compression moulding was first developed to manufacture composite parts for metal replacemen application.. Compression moulding presses are mostly hydraulic drive, where the hydraulic pump powers a hydraulic cylinder that drive ram or slide.Some are pneumatically powered presses.

Stroke:The amount of possible ram travel.Stroke is the total distance that the ram can travel,from full extension to full retraaction.

Shut Height: The distance between the bed bolster and the ram bolster when the ram is fully extended.This is commonly known as the closed height.As standard this dimension is usually within 0.25 due to assembly processes.

Daylight: The distance between the bed bolster and the ram bolster when the ram is fully retracted.This is also known as the open height.

Bloster: The removable plate that serves as the working surface for the bed and ram.The plate is usually bolted to the bed and rams substructures.The bolsters can be machined with a variety of work holding features such as T-Slots,Drilled and Tapped holes

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Bed Height: Bed height is the distance from the bottom of the hydraulic press structure to the working height or top of the bed bolster.

Remote Power skid: Some press applications may require that the hydraulics be located remotely form the press itself.Other applications may preclude the power system from being able to be installed at the top around the crown structures of the press.In these cases, the power system and even the controls may be designed into a separate unit capable of bing placed adjacent to the press or away from the press.

Heated Platens: These are plates that have heating capabilities.

The compression moulding presses at most are vertical and the moulds can be heateed using electric rods, steam,oil,water or other medium

By varying the ram speed and other operating parameters like time,temperature and pressure , the cycle time for each is optimized.

The advantages of compression moulding :

1.The method is simple and only requires relatively simple presses and moulds. It is an appropriate method for short runs.

2.Allows manufacturing of composite products which contain non-rubber reinforcing material.

3. Suitable for products with large surface or large spreading.

4. Can be used for rubber compounds with high viscosity and poor flow properties.

The disadvantages of compression moulding

1.The preparation of blanks and the insertion of blanks into the mould are time consuming.

Complicated cavities can be difficult to fill out completely.

2.The formation of flash is extensive and the variations of the hot mould cavity dependant dimensions are relatively extensive.

3.The production rate is relatively low.

4.Difficulty arises for rubber metal bonded items and for products with intricate shape.

Trouble shooting of compression moulding.

Air or pick marking
1. May be caused by undercurrent,very hot viscosity or occluded air trapped on the surface.
2. Depending on the cause,bumping the press,use of vacuum,faster cure and higher polymers viscosity may eliminate the problem.
3. The total may need modification to improve venting if vacuum cannot be applied or is ineffective.

Air blisters below the surface:

Adjustment of the bumping program

Back grinding:
This is the phenomenon big tears and distortion at the mold parting line often caused by localized expansion on pressure release.

Excessive shrinkage

Caused by incorrect allowance in the mold design precure during flow or incipient scorch in the compound prior the moulding.

Reduction if molding temperature,incorporation of non reinforcing filers like hard China clay would reduce this problem.

Flow marks:
May be due to nerve,precure during flow,or very soft compound giving excessive flow before full clamping pressure is reached.

Excessive external mold release agents such as silicones and anti stick agents and too much dusting agents on uncured blanks would cause the problem.

Pebbling or orange peel

This surface effects occurs mainly in gum lightly loaded stocks and is typically due to poor accelerator dispersion.if accelerator are added as dispersions the problem may occur.

Types of compression Moulding

Sheet Moulding Compounds:

Placing a reinfocement such as a glass mat, betwen sandwiching layers of a thermoplastic.

Bulk Moulding Compound:

Billet is used with effeciently placed and distributed long reinforcement