Final production mold ideas and questions, need help

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#1

I have been producing composite rifle stocks for about a year, I have made many mistakes and found many methods that have yielded a better and more repeatable part.

Current process: Stock shells are laid up as a single piece using a two part rigid mold. Materials are E glass, S glass, carbon fiber, and Kevlar, all using epoxy resin. One half of the mold is laid up with the material below the mold mating surface, the second half is laid up with overlap. The overlapped side gets a bladder inserted, mold is clamped together and the bladder is inflated.

Current molds: Molds are made using tooling gelcoat and multiple layers of fiberglass reinforcement using polyester tooling resin, it is then backed with a wood frame and 16 lb rigid pour foam. The current molds do not hold up well over extended mold cycles before they start to chip, which require repair. I have also had difficulty ensuring the molds do no flex during layup. The fiberglass seems to hold up fairly well to the current pressure the bladders are inflated to, but maybe not so well if I attempt pressures well above 40 psi.

The finished product does not require a great deal of attention, I feel the weakest link are the molds. I was considering a few options to improve the molds cycle life, and here is where I need input.

Rigid molds: When choosing a method and material to produce rigid molds, here are the options I am aware of;

  1. epoxy mold, fiberglass reinforcement. What would I choose instead of polyester tooling gelcoat? Can the standard epoxy be used for a primary surface coat?

  2. aluminum molds. Issue is the product has some detailed grip areas (similar to a swim step texture) that would be extremely difficult to machine due to it’s random patterns. Also very costly.

  3. Semi rigid molds, using a thin silicone or urethane mold surface backed with a rigid mold “deck”, aluminum or fiberglass reinforcement.

I keep coming back to the semi rigid mold idea for a few reasons; First, the mold would seal at the surfaces and could allow vacuum/resin channels to be molded in. Second, due to the softer contact surface, minor mechanical locks wold be far less of a concern. Third, mold life, I am assuming the softer material will be a lot less likely to chip, but may be very difficult or impossible to repair.

#2

Here is a thread with some pics of the molds and product. http://www.compositescentral.com/showthread.php?t=10392

#3

Aluminum tooling would be your best bet on a repeatable, durable mold. As far as the grip finish, that should be something a mold/tool&die shop should be able to add. They should either be able to machine it in, or EDM most finishes in to the mold.

#4

I agree, a machined mould such as Aluminum is ideal, and there is no reason they cant machine a pattern in for grip. Considering this is how plastic parts that have texture are usually made. Just beware that aluminum CTE is much higher than fiberglass. If you are curing hot, then likely you need to demould hot. Your end product will be slightly larger then your target model. If final end tolerances are critical then you will need to consider other materials to machine. Steel CTE is lower but significantly heavier. You can also consider machining a master plug, and make a carbon fiber tool off that if you want a high accuracy, low CTE mould.

#5

The thermal expansion of aluminum makes sense, would you see a lot of that if the mold was not heated? Would heating the mold benefit me with cure times?

Here is an aluminum mold similar to what I would need. It is used exclusively for prepreg and a high pressure camera/bladder, and has no texture.

#6

Looking at some of the industry methods, here is an example of a soft mold insert using a aluminum support frame.

#7

Ultimately depends on the resin system you choose. If you are using a low Tg, RT curing epoxy then you probably don’t need to post cure it in mould, however it would probably still benefit from a low out of mould postcure to fully cure the epoxy. If you chose to use prepreg, then you would need to elevate the temp of the tooling for cure. The higher the temperature, the larger the mould grows. So if you dont need to elevate the mould temp much beyond RT then you wont have any issues. However if you cure hot in mould, then things like demoulding hot become necessary in some cases. Also means the part wont fit back in the mould if the mould is cold. So if you ever need to do some type of post in mould assembly, you would have to do the assembly in a hot mould, or another assembly fixture.

#8

I have been using Resin Research 2000 series epoxy, currently no plans to do prepreg due to cost. I have not attempted to post cure, either in the current molds or with the part removed, the part has been allowed to cure for 24 hours before demolding.

#9

Metal is great, don’t worry about the expansion as you can allow for this in your design file. Approx 1mm per m length.

The main thing is your process and techniques.

I have done a number of long pre preg parts from metal moulds and the main issues for me were releasing. At first my mould/part ends were just square so the carbon part would get trapped in the mould and bow the alloy mould until I forced the part out of it. I would demould hot but had to handle everything when it was 90-100 deg C working to undo bolts and nuts fast enough etc.

Once I made a mod to one end of the mould to ease releasing I still had to demould hot but it would pop out on its own.