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Circuit Owner
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Discussion Starter · #1 ·
Hello,

In a moment of madness I bought some resin moulding kit - a silicone for the mould and EasyFlo120 super tough hard resin for making 1:32 chassis.

I have designed a chassis that can be fitted under a Powco Transit for a dead easy conversion (in-line mabuchi, correct wheelbase, space for in-car conversion chip, etc.) so I can build a load and do 6 way transit grand prix on my digital layout! But I have also designed the chassis with a narrow front with a view to developing a rat rod/hot rod body just for a laugh (again to race 6 of them on my circuit).

Does anybody out there have any experiences (good and bad) they can share with me so I can avoid making old/known mistakes and concentrate on making new ones!

Obviously I will also need to cast hot rod body parts at some point but I'm going to concentrate on what I hope will be a relatively straightforward chassis cast before I open up the body panel can of worms.

I'm also going to be casting tyres at some point (also bought some shore 50 urethane resin) because I will be machining up some custom moon wheels in the correct proportions for the hot rods so will require custom tyres. Any advice gratefully received.

Kind regards

Richard
 

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What is your chassis master made of? Has it been painted or primed with any coatings? Some silicones (platinum cure) can be fussy about curing properly against some materials. Tin cure silicones are more forgiving, but generally have a shorter service life.
 

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Is it possible to see a drawing or pic. of the chassis, that might help to highlight any problems you might encounter.
Also if you are using RTV cold cure silicon rubber you shouldnt have any problems with the rubber curing.
cheers steve
 

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Again, it depends on the type of RTV cold cure silicone rubber. Platinum cure silicones don't like materials containing sulfur, which is found in some clays paints and primers. A few coats of Krylon Crystal Clear will generally provide a safe barrier.

And yes, A pic of the molding master would be very helpful.
 

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Circuit Owner
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Discussion Starter · #5 ·
Thanks for the replies - much appreciated.

The chassis exists in drawing form only at the moment. I have some 80 thou (2mm) plasticard coming tomorrow (Royal Mail willing) which will be used to form most of the cross members.

The chassis will be formed using .75mm plasticard as the base. 2mm plasticard will be used to create the peripheral members and some bracing cross members for rigidity. The overall height of the chassis will be 6mm except for the mabuchui mounts and the axle bushes which will stand taller.

I was toying with using Milliput to smooth out the sharp angles but thinking about it I don't know whether smoothing out will make it easier or harder to pull the chassis from the mould - ideally the mould will be usable at least 20 times.

I intend to copy the Scalextric type push fit mounts for bushes and motor. The guide will be an old style push fit so a simple 3/4 circle mount with drilled hole (4.8mm deep) is the order of the day.

My idea is to make the chassis as clean a pull as possible from the silicone. Hopefully the only pinch points will be the axle bush mounts and motor mounts and the silicone should squeeze through these easily enough.

One major concern is mould release - is this likely to be an issue with plasticard? My other major concern is getting the mould level enough to get an even depth of base plate - or should I forget trying to create the base cover?

Also - should the floor have some holes in it to aid pulling from the mould? Potentially high vacuum otherwise I would guess.

I was hoping to be able to create a floor to the chassis to add rigidity and to keep things neat and tidy. I would rather not have to cast the chassis and then fix a separate base plate to it.

Drawings to follow.
 

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Circuit Owner
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Discussion Starter · #6 ·
Chassis

Hopefully you can see this image.

Key:
Green = higher members
Orange = 6mm members
Blue = hole for contrate gear
Pink = circular indents for magnets.

Assume a base plate .75mm thick.
Does the front section ahead of the front magnet position need more cross members for rigidity?

I'm using a polyester resin normally used for rotocasting which is apparent virtually unbreakable and very rigid. Should I lay some fibres in the mould to reinforce - if so where?
Richard
 

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Circuit Owner
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I don't recall having any cure problems with plasticard (styrene) or Milliput. Smoothing out the inside corners will help mold life and will probably help chassis durability. If the mold is to be am open 1 piece mold, be sure to cast it on a dead level surface, or you'll have trouble getting a good run. Mold release can't hurt. It will increase mold life, and if paint is in your chassis future, help prevent silicone contamination of your parts (fisheyes). I use an aerosol wax mold release every other run. I'm guessing you will not be de-airing the silicone, or casting under pressure at this point. No worries, just whack on the table to dislodge bubbles. You don't want any warpage in a chassis, so don't rush de-molding your parts. Offhand, your chassis looks like it should de-mold fairly easily.

As far as laying fibers, what is the pot life of your plastic? You don't want to get caught with the job half done.
 

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Circuit Owner
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Discussion Starter · #9 ·
QUOTE (Datto @ 2 Sep 2011, 02:02) <{POST_SNAPBACK}>...... I'm guessing you will not be de-airing the silicone, or casting under pressure at this point. No worries, just whack on the table to dislodge bubbles. You don't want any warpage in a chassis, so don't rush de-molding your parts. Offhand, your chassis looks like it should de-mold fairly easily.

As far as laying fibers, what is the pot life of your plastic? You don't want to get caught with the job half done.


Hi Datto,

No pressure. Whack to dislodge bubbles in the silicone for sure.

The pot life of the resin is 2.5 minutes!!! BUT it has the viscosity of water so I was contemplating laying fibres or wire if needed then pouring the resin on top - if it is really like water then it should be no problem - I spoke to the manufacturer and they say there is no issue with bubbles in the resin as it is so thin. Curing time is said to be 15 minutes but I shall leave it overnight before pulling.

Thanks for all your advice - very much appreciated. I will post photos of the end result once it is done - I feel more confident now!

Richard
 

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In this warm weather dont bank on 2.5 minutes. it will be less, There will always be issues with resin getting into small spaces and getting rid of the air.Do not worry about having to do it in one shot. mix a small amount and poke it into the crevices likely to trap air and slosh the mix around to make sure all surfaces are covered until it just starts to thicken . leave to set on a flat surface. then when it has gone off, about 5 minutes, mix up the main amount and pour in to the desired level. 30 minutes later you can pop it out. no need to leave it overnight. I have made a whole load of land rover chassis like this with no problem,This info is based on Axson f16 resin which I use but it should be the same for most resins available in the U.K.
cheers steve
 

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Phil Kalbfell
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Some resins will delaminate if poured in two or more goes,so check with the suppier or on the data sheet. I found this out when severaL early bodies delaminated with the resin I was using.
 

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Discussion Starter · #13 ·
QUOTE (Phil Kalbfell @ 3 Sep 2011, 00:13) <{POST_SNAPBACK}>Some resins will delaminate if poured in two or more goes,so check with the suppier or on the data sheet. I found this out when severaL early bodies delaminated with the resin I was using.

Thanks Phil, I was wondering about that (delamination).

Anyway I was going to go for a single pour because my chassis has been designed to be as easy a pull from the mould as possible - I will radius as many angles as possible with milliput - this is to extend the life of my mould and to reduce mental stress on me when pulling! If I have anything to do with it there will be nowhere for bubbles to form with my thin resin. The only undercuts are the slight pinch points for the axle bushes and these are so slight I don't anticipate them being a problem.

HOWEVER I will be casting body panels later so your advice will be remembered when things get more complicated! The resin I have bought is recommended by the supplier for rotocasting so should respond well to being sloshed around a mould to form thin body panels. The challenge I have with the rotocasting/sloshing approach will be consistency of panel thickness - not an issue for most of the parts but a major issue for the mounting points - I was toying with doing it in two parts - casting a "base ring" which is the part that has the fixing points to attach the panel to the chassis - then laying that part in the top (open end) of a mould and sloshing resin around the inner mould to form the panels and hopefully bonding with the base ring. Delamination will be a major concern then! Is there anything I can do to guard against delamination? (even with the right resins I am guessing it can still be an issue). If I use a pre-cast part and then cast again to attach would keying the part help? Would I be better off providing binding posts either moulded into the part or ultra-thin screws (number 0 self tappers) through the first part so I can use the screw threads as binding posts?

Perhaps I'm getting overcomplicated? Advice gratefully received.

Richard
 

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Dont know if it helps but the only time i have experienced delamination was with bodyshells and then that was if the first slosh coat was too thin and therefore the mould is still flexing with handling. Some of the shells I pull have 5 or 6 pours and do not suffer, cheers steve
 

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Discussion Starter · #15 ·
Thanks Steve.

I guess it's time to get cracking on the chassis and start finding out for myself!

If a single pour gives me bubble problems then I will try more than one pour. The cost of raw materials is low enough not to be bothered by a few mess ups.

Thanks for all your advice. Wish me luck!

Richard
 

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Visit my photo album

So far so good - phase 1 - build the chassis - is complete.

Changed the design to simplify the build and hopefully to make it easier to pull from the mould.

Before you ask - yes I attempted to round out some corners with Milliput - won't try that again unless I buy the "fine" stuff.

Might build this again before casting but use this one for my first attempt at making a silicone mould and make the mistakes on this before the final production version!

This will fit nicely under a Transit van with minimal dremelling needed on the van. It should also make a nice base for a hot rod.

All comments gratefully received.

Richard
 

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Circuit Owner
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Discussion Starter · #17 ·
Hello all,

I've been caught up with my track build so have only just come back to the chassis casting...

I have now made my chassis master, I have also made my casting box by routing out some MDF - thickness = the height of the chassis + 6mm. The routed hole leaves 5-8mm gaps around the chassis (yes I have worked out how expensive RTV silicone is!!!)

I need to seal the walls of the box. I also need to fix my master to the floor of the box.

My thoughts are to use an old glass worktop protector as the base of the moulding box - it's clean, very smooth and scratch free and will be easy to get dead level.

I have some PVA glue (Evostik exterior wood glue) which would be ideal to seal the walls of the mdf box. It would also be ideal to use to stick the box to the glass base and to stick the master to the floor of the box. PVA will very easily peel off the glass base and the master so I can re-use at a later date.

Does anybody have any experience with PVA glue in contact with RTV silicone? I have a 550g silicone kit from mbfg.

Thanks
 

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Phil Kalbfell
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I just use hot melt glue to join the moulding box and glue it to the base. Easy and sets straight away. I hols the item to be cast to the bottom with Klean Clay. As I use thin MDF or Corflut for my box's it only takes about 5 minutes to make a box. Fancy box's are good IF you have a lot of same size item to mould.
 

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if you just use PVA the liquid silicon will soften it and the master will detach itself and float to the top. yes it happened to me, once, As a belt and braces idea, take the glass down to your local glaziers and get them to drill a hole in it. then you can use a small screw through to the master to hold it in place as well, as glue.
I found the easiest way to make a moulding box= I use an acrylic sheet as the base as its easier to drill a hole in for the retaining screw and Lego bricks for the walls and use something that wont react with silicon such as Phils suggestion of a hot glue gun or evostik. I dont know if you can get klean clay over here, I have never heard of it. Make sure your master is gap free all round the base otherwise the silicon will creep under it. I use window putty to seal any gaps pushed in with the wrong end of a small paintbrush, hope this helps,
cheers steve
 

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Double sided tape. I've done thousands of inserts using double stick tape to hold the masters to the mold box floor, and, never had any issues.
 
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