Compression moulding is the most common moulding technique you will notice in the rubber industry. The other two are transfer moulding and injection moulding.
Compression moulding is a closed mould process and you apply high pressure on the job. It is best used when you have low to medium production volumes for larger parts where tightest tolerances and flawless finishes are not required. Examples may include diaphragms, valves and gaskets, Dampers, O-rings, seals, rubber wear parts, etc depending upon their areas of application.
As shown in the figure above, in compression moulding, you place an uncured rubber profile in a heated mould. Then you soften the uncured rubber by the heat and press them forcing the rubber compound to fill the mould cavity. The rubber is thus made to conform to the shape of the mould and then you cure it to produce your finished article.
Compression Moulding Press
The machinery you will use is called the Compression Moulding Press.
Compression moulding presses are mostly hydraulically driven, where a hydraulic pump powers a hydraulic cylinder (or cylinders) that drive the ram or slide. (Caution: there are pneumatically powered presses too).
Hence, the terms that apply to a standard hydraulic press are relevant here.
- Stroke: The amount of possible ram travel. Stroke is the total distance that the ram can travel, from full extension to full retraction.
- Shut Height: The distance between the bed bolster and the ram bolster when the ram is fully extended. This is also 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 commonly known as the Open Height.
- Bolster: 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, Lift Rails for Quick Die Change, etc…
- 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 from the press itself. Other applications may preclude the power system from being able to be installed at the top around the crown structure of the press. In these cases, the power system and even the controls may be designed into a separate unit capable of being placed adjacent to the press or away from the press.
- Heated Platens: These are plates that have heating capabilities.
(Disclaimer: The standard industry terms above are borrowed from the website of Beckwood Hydraulic Presses and Machinery).
Hydraulic presses are preferred by your peers in the industry because they give Performance, Reliability, and Versatility. This means you can get full pressing force (or tonnage) anywhere in the range of stroke using hydraulic press unlike a mechanical press.
By varying the ram speed and other operating parameters, you can optimize cycle time for each of your job. Further, you could customize your bed size; say lower tonnage on a larger work area or higher tonnage on a smaller area of work.
You will see that compression moulding presses offered by most reputed manufacturers are vertical and the moulds can be heated using electric rods, steam, oil, water or other medium.
Depending on requirement, your moulds may be fixed to both the platens; or part fixed to a platen, or totally free to be drawn out. When you have free loose moulds, your operators need to load and unload them on a press table manually. You call it charging (loading) and stripping (unloading) by hand. On the other hand, the presence of ejector pins or strippers on fixed mould presses will speed up your operation.
One of the platens is fixed in your compression moulding press. When the lower platen is raised or lowered in the vertical plane, you call it upstroking; while in downstroking, the upper platen is the driven plate (moving up and down).
How will you decide?
Apart from safety and ease of operations, your final choice of a compression moulding press depends on these six important considerations
- Calculation of the proper amount of rubber for your product.
- Calculating the minimum amount of energy required to heat your rubber.
- Calculating the minimum time required to heat your rubber.
- Identifying the appropriate heating technique.
- Predicting the required force, to ensure that stroke attains the proper shape for your rubber.
- Designing the mould for rapid cooling after the rubber has been compressed into the mould.
You try to optimize the Time, Temperature and Pressure – the three critical parameters for your compression moulding process.
Manufacturers compete to offer your robust hydraulic presses with superior drives and controls, advanced levels of hydraulics and instrumentation features.
Another model of hydraulic compression moulding press is as shown below.
Advantages and Disadvantages of Compression Moulding Process
The advantages of compression rubber moulding are lower cost of your moulds, the large sizes of mouldings possible, and the relatively quick changeover between different moulds helping you expedite shorter production runs.
Additionally, there are no tooling features like gates, spruces or runners that could increase the time for your mould to enter the cavity or potentially affect the aesthetics of your product.
The main disadvantage is your output when loaded and unloaded manually. This increases your cost on skilled labour.
There is product wastage too as compression moulding is not precise like injection rubber moulding. Since thermal conductivity of rubber is poor and you often place your profile into the cavity ‘cold’, your cure times are longer.
Additional difficulties you face will be from positioning the blank properly in the cavity and the difficulty to control the ‘flash’ that results from the additional material placed in the cavity. Unsuitable for complex moulding, compared to a transfer moulding or injection moulding, your product contamination is also higher in compression moulding.
Summarizing, compression moulding is the most common moulding technique that you will find in rubber industry and the common machinery for this process is the a hydraulic compression moulding press.
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