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Advice sought on how to make Plastic mould Answered

I’m trying to repair a folding camping bed. See pics

It has a number of feet and other blocks made of some form of HDPE or similar.

I want to recreate these blocks. How can I do so? The material needs to be rigid but not brittle.

I can find plaster or gel to make a mould but what material should I use? Is there some sort of binary mixture such as a flexible filler ?

I have access to basic tools and power tools but not a lathe nor 3D printing

Another option is to buy a block of HDPE and drill and carve the holes, but where to buy the block (in UK)?



22 days ago

Here’s the foot in place - need to add some washers.

la xerra

26 days ago

stupid question: why recreate those bases? what happened to the old ones? lost? broken? (couldnt you weld them etc). from the pictures, as far as i can tell, the design is beautifully insane. what exactly is the function of those suckers? maybe you can send more pictures that give more context. i am sure you can wing it somehow (but then, not everybody is a die-hard chapuzero like me, and some folks take repairs rather seriously. ... my total respect).

anyhow, best furniture on this planet as far as this ape is concerned: a hammock.

Downunder35mla xerra

Reply 25 days ago

These blocks connect the tubing of the frame.
As you can see in the pics, the plastic just breaks or rips apart after some decent use.
No plastic welding works as the entire design is way too weak.
Considering they all coe from asia it is safe to assume they are designed for the use of persons not exceeding 60kg in weight ;)

la xerraDownunder35m

Reply 22 days ago

60kg, eh? ... well, might turn out its less a pain in the exhaustpipe going on a crash-diet than re-casting that sucker.
no, seriously, i havent come across one of those beauties yet, so cant tell, but isnt there a simple way just to rig something?? with wood etc?? there must be a better/simpler design... those photos look gaga


23 days ago

Here’s one option the original foot re enforced with aluminium U section and JB Weld. The next stage is to drill the holes


25 days ago

I'm sorry but that part is a glass reinforced nylon injection molded. if you were to try to cast it it wont be under enough tonnage to make it structurally stable. it will just crumble and break easier than the original part did. What I would suggest is making a bracket that can fit around the original block, from some aluminum maybe? just to replace the bracket ends that broke off of the end. (or there is always the option of having a custom injection mold designed and built for about 15k-25k and I can mold it for you lol)


27 days ago

These things are DESIGNED TO FAIL.
And they are designed to be not fixable.
Back in they day these plastic parts were made from cast aluminium, cost reductions and advancements in plastics provided us with Nylon.
Nylon however is quite costly and tends to absorb moisture, changing the mechanical properties.
The "solution" was to use a mix of HDPE and LDPE, hard enough to tolerate most of the abuse but soft enough so it won't break.
The other way out and used mostly for more costly chairs and beds was fibreglass reinforced plastic.
But no matter which option you choose the material is doomed to fail long before the frame does.
Means now we see chairs and bed made with fabric that won't survive more than one decent summer before the fabric fails due to UV exposure.

The only real way out it to machine these blocks from aluminium and then to use suitable screws - the ones with a smooth shank.
I do have a 3D printer and tried to fix some chairs but even PET, Nylon and what else is available in "structural" plastic is of no use.

If you do want to try a casting:
Use LDPE, like shopping bags, milk bottles and whatever has LDPE on the recycling label.
Melt it down with just enough heat not to burn it and so you can actually bond the pieces to a solid block.
It won't go liquid like a resin, so you need to drill out or machine the voids - trust me it is easier than trying to get this into a mold.
Wherever you can add a few mm in thickness for extra strenght.