Hi all,

I am currently moulding a bumper and want to ensure I keep things as genuine as possible.

I have a ridge/lip on the top of the bumper and some tabs behind it but am wondering on possible ways to replicate this as part of my split mould.
The ridge is 4mm thick as per pic below and runs along the top of the bumper as per the picture.



I am planning on building the flange for the mould as per this pic: black = bumper, red = flange, green = mould, blue = 2nd part of mould


questions:
a) how would I replicate the ridge using a split mould? Is my diagram the optimal way of achieving the ridge? I don't want to lose the mounting taps behind the top of the bumper as a steel reinforcement bar is located there.
b) how would I get fibreglass into such a small channel when reproducing the part when I did the lay up?
I did think that I would have to gelcoat the blue split mould and then fibreglass some material into the space then squeeze the fibreglass together as it was curing but I am worried that there will be voids between the two lay ups which I want to avoid.

Any help would be appreciated as this bumper is giving me some challenges and ideally I'd like to build a mould which will allow for one session lay up instead of bonding in additional parts.

Thanks
Dave

The way you've drawn will work fine. Makes placing the barriers easy as well, so that will be your best bet. 

thanks for the guidance. I'll try some fine CSM and curve a filler scrapper to get into the channel. Hopefully that will allow me to push CSM into the split mould groove

You can also build the ridge by laying some glass rovings.

I'd really rethink your mould strategy. It is much easier to make that tab separately, either from fiberglass or sheet metal and glue it to the bumper. If the bumper area is accessible from behind when attached to the car you can easily avoid misalignment by attaching the bumper, keeping it in position with lots of duct tape or whatever and THEN adding the tab, either prefabricated or you lay it up against the metal attachment points covered with packaging tape.

If you use a lightweight CSM like 80/100gsm or 225gsm at the heaviest then you won't have an issue getting it into the groove. I've worked with much tighter grooves than that and not had any problems with voids. Once you have filled that groove and are satisfied there are no voids, let it completely cure before continuing with the rest of the layup. This will guarantee that you don't push any air back into the groove during the rest of the layup. 

brainfart - thanks for the feedback. Cutting around the tabs once cured may be additional work so I'll think about bonding in the tabs in.

Hanaldo - thanks for the suggestion. I have made pastes previously with shredded CSM and resin, so will take that into consideration on which option I take.

What I was thinking, was to lay 2x layer of mat into the ridge recess (after gelcoat) then add the paste in between then build the tab out with the main lay up. Or I go with brainfart's suggestion and add tabs onto the main piece. Fun decisions - now looking at a 5-9 piece mould (dependant on how I do this - silly Celica)



I am trying to keep the mould as dimensionally correct as possible. The bracket that fits onto the the tabs holds an additional trim piece between the indicators.