Motorsport Composite Fundamentals: Mould Considerations - Multi-Piece Moulds

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Mould Considerations - Multi-Piece Moulds

05.03

00:00	For any composites project, it's important to have a good understanding of the entire process from as early on as possible.
00:07	Even though we're not at the mould making stage yet, we still need to understand how the mould will work when doing our pattern making.
00:15	Some moulds will be very simple and they won't require much thought.
00:19	However, others could be more complex and involve multiple mould sections.
00:23	This mostly concerns the release of the part from the mould, which we always want to be as easy as possible.
00:29	More specifically, the draft angles of the mould surface, which is essentially the angle of the surface relative to the pull or release direction.
00:37	If the surface is parallel with the pull direction, this will be zero draft.
00:42	As the surface opens up to the pull direction, this is positive draft.
00:47	The opposite is of course, negative draft.
00:49	Think about looking into the mould from the pull direction.
00:52	If the surface faces away from us and we can't see it, this will be negative draft.
00:57	Generally, speaking, a more positive draft will release easier.
01:01	And a negative draft will cause undercut where the part will lock into the mould and prevent release.
01:07	In cases of very minimal negative draft, the part might be able to be flexed slightly while being removed, but this is rare.
01:15	And we're more likely to damage the part or the mould trying to release it.
01:19	We'll use our CR-X fenders for an example here.
01:22	Unfortunately, in this case, we can't just pull a single piece mould directly from the part.
01:26	The top flange surface that bolts to the car under the bonnet or hood forms an undercut that will prevent release.
01:34	So, we need a multi-piece mould design with a split along the transition from the outside of the fender to this mounting flange.
01:41	Essentially, we'll make the mould in two stages.
01:44	After we build some flanges off the pattern, we'll lay up the first side of the mould surface of the fender and the flanges.
01:51	Then, once cured, we'll remove the makeshift flanges and then lay up the other side of the mould before splitting it and pulling it off the parts, of course.
02:00	For clarity, the flanges are the sections of the mould around the perimeter of the part which are used for various reasons.
02:06	We'll show these in detail in the next module, so don't stress too much for now.
02:10	All of this can be hard to visualise initially, but as we see the mould come together, it'll all make more sense.
02:17	The actual design will be different for every part, so it's difficult to apply a golden rule here that works for every case.
02:24	What's important is that when the mould sections are split, we'll end up with a positive draft on all surfaces so the part can be released.
02:32	But we also want to consider where the mould split lines will be.
02:35	These will be visible on the part, usually by way of a small flashing line where the resin has leaked into the split.
02:42	These can of course be removed by sanding and surface finishing, but it's ideal to minimise them as much as possible.
02:48	The other key consideration with multi-piece moulds is the location of the mould sections.
02:53	That means ensuring the sections are in the perfect location relative to each other.
02:58	To achieve this, we add locating features to our flanges, and what's important is that the locating features provide ongoing positive location.
03:07	The basic approach is solely relying on the bolts that we use to fix the mould sections together as the locating features.
03:14	But you'll soon see that the issue here is that it's difficult to drill precise holes in CSM reinforcement fibreglass, and with use, these holes will wear out and the extra slop will mean that they aren't very good locating features.
03:27	An alternative is separating the locating features from the fixing features.
03:32	For example, using small rubber pads designed for the use on the bottom of furniture feet to protect the floor and prevent them sliding.
03:40	They're circular pads around 10mm in diameter and only 3mm tall, with adhesive on one side so we can stick them to the surface of the flange.
03:49	When we laminate over them, this will form a divot or female side in the laminate, and then as we laminate the second stage for the other side of the mould, this will form a key or the male side.
04:00	There are many other features available like split mould clamping bushings that can achieve both the locating and fixing at once.
04:07	But what we've just described is an accessible, easy to source and cost effective option that we'll show in practice in the next module.
04:14	Before we move on though, let's recap the main points to remember from this module.
04:18	Many of the patterns we're planning on pulling a mould from will require multi-piece moulds to ensure the composite parts can be released.
04:26	It's important to design the mould so each section will have a positive draft on the mould surfaces with regards to the pull direction.
04:34	This will be different for every part, but try to keep in mind where the mould split lines will be and plan for the most efficient mould that will give the best results.
04:42	While the sections will typically be fixed together with conventional hardware, we also need to consider the method of locating the mould sections relative to one another.
04:51	It's often worth separating these functions and using features specifically for locating to ensure ongoing positive location with features that won't wear out.