Hi i am trying to make laminates with  carbon fibres using vacuum infusion process. i would like to know is there any calculations involved related to how must vacuum to be set for a given size of laminates to be produced

You need to be pulling as close to full vacuum as your equipment can produce with resin infusion

You might need to adjust the vacuum level if making parts via the wet-layed vacuum bagging process, so as not to make the laminate too dry.

I did fabrication of glass/epoxy composites using vacuum infusion molding. after curing at the centre of the laminate i see poor wetting leaving behind dry patch. any suggestions to overcome it. please guide

To help ensure that your laminate is fully wet out during infusion you can take certain steps to help this, including:

Ensure that you're infusing at a reasonable temperature (ideally at 20 degrees or above). Warming the resin prior to infusion can help lower viscosity, speed up the infusion process and help the resin to flow well to all areas. Be mindful of your pot life, however and measure out the resin in batches if necessary, mixing things up just prior to adding to the resin pot attached to your stack.

Use a resin break i.e. a deliberate gap in the infusion mesh of around 20 mm or so before another strip of mesh incorporating your exit port. This will help ensure that your not having to deal with lots of resin exiting the part before all of the laminate has been wetted-out. If planned well, it should help to make sure that the parts of your mould furthest away from the resin entry line have adequate time to wet through before too much resin enters the exit line. Be sure too, that all elements of your stack are wetted out before clamping-off the exit, including excess release fabric. Resin will try to reach any dry areas even after you close things off which will, in extreme circumstance, cause the resin on the part to become dry in places and possibly leave you with dry spots or voids

Once you've closed off the exit line, leave the resin feed line open for a short while to allow plenty of resin into the stack, even if everything looks completely wetted out. This might make parts ever so slightly 'resin-rich' but it should help to avoid any dry areas being possible. I'd recommend at least 30 seconds on most laminates, but a couple of minutes might be a good idea on large parts

If some parts of your laminate take longer to infuse than others, you can try using extra strips of infusion mesh to, effectively, double-up the mesh in those areas and create high flow channels. It will lead to a little more resin usage, but it can help to ensure that everything wets out completely.