Mold Cycle vs. Cylinder Speed

Normally, in an Injection Mould Cycle, the Hydraulic Cylinders start operating during Closing and Opening. As common practice, Mould Users squeeze cycle operation to the minimum possible, for saving time and, therefore, money. The drawback may be a higher stress on the components and an increased requirement for maintenance. Therefore, the right balance must be found.

Below, we report some thoughts referring to the subject of speed in Mold hydraulic cylinders, specifically applied to those manufactured by Vega.

We refer to the chart hereunder, which shows the maximum catalogue speed for all Vega cylinders. By the way, cylinder manufacturers and manuals on hydraulics, suggest that a non-cushioning unit should not exceed 0.1 m/s.

Max Speed

without Cushioning

with Cushioning

V210CR

0,1 m/s

0,7 m/s

V250CE

0,05 m/s

N/A

V220CC

0,1 m/s

N/A

V450CM

0,1 m/s

N/A

V260CF

0,1 m/s

N/A

These speed values are consistent with the delivery reported nearby, in our catalogue. Of course, no one can adjust delivery to each and every cylinder for compliance with the values reported. It would be a crazy task… For that, the real speed of each cylinder will depend on a lot of factors related to the hydraulic circuit, and is very difficult to predict.

So, the speed values marked in the Vega catalogue are a ‘statement of good practice’ in hydraulics, but we know they will usually be overcome in real applications. How much, it’s impossible to say.

We also know very well that most of the competitors have larger oil supply holes, for block cylinders. Is this really a disadvantage for Vega? A small hole, will increase oil velocity at that spot, thus generating a pressure loss. So, it is a kind of ‘built in’ safety device, for avoiding excessive speed.

An example will make things clearer. Let’s consider a cylinder speed of 0.1 m/s, and an oil supply port 1/20 of cylinder bore in diameter. Oil velocity in the port will be 400 (20^2) times higher. So, in this example, it will be some 40 m/s. applying v^2/(2g), we get a piezometric height of 80 m, which is some 8 bars. If cylinder speed is 0.2 m/s, the pressure loss should be some 32 bars. Of course this calculation is far from being really precise, but it gives an idea of the situation. This means that a speed of 0.4 m/s is close to the maximum possible, with a pressure loss close to 130 bars.

If the oil supply port is 1/10 of cylinder diameter, at 0.4 m/s pressure drop will be some 8 bars, and 32 bars at 0.8 m/s. This means that a cylinder without cushioning may trespass the threshold assigned even to a cushioning cylinder!

Of course this kind of oil velocity will start other effects and cylinder speed will not be the calculated one, but this can give an idea of the situation.

What could happen if we exceed the speed limits? Why do others have higher limits?

It mostly depends on cylinder installation. Vega will supply cylinders with larger oil supply holes, upon specific request. Anyway, Vega’s preference is to remain on the safe side, while a higher cylinder speed may be attained with proper installation, low masses connected, and providing end of stroke stops before the piston hits the head or cap. Also, if a shorter lifecycle is accepted, the cylinder can be overstressed. Our tie rod cylinders have large oil supply holes, and no problems are reported with that; however, many of them are bought by the customer with a cushioning system…

What kind of damages could occur? Of course that depends on a lot of factors, but, generally speaking, an overload will result in breakages. The piston or rod may break, as well as the body of the ‘light duty’ V250. However, Vega cylinders usually operate at a speed higher than the rated one, without any problem. Generally, such problems arise only if something is wrong with the installation or the operation.

The only ‘different’ cylinder may be V260. This must reach the end of stroke positions both in and out, and has very large oil supply holes. Too much speed could cause overstresses, both at locking sectors opening and on the internal components at the end of the return stroke. So, speed control for these cylinders is highly advisable.

Anyway, the preferred choice of Vega is to opt for cushioning, and new models will offer it as an option with minimal extra cost. Of course oil ports will be larger. This is not a change in strategy, but the natural development of our traditional strategy, for giving both high speed and a safe operation throughout time.

Comments

  1. LifeinsuranceUK

    Hydraulic cylinders also recognized as a linear hydraulic motor, is a machine that is used to make a unidirectional power by apply a unidirectional stroke. It has many applications particularly in the industrialized division as well as in the area of civil application and manufacturing vehicles.

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