Using SMP-kit as poor man rubber
Using SMP-kit as poor man rubber
A small experiment while repairing worn ladder feet.
I could not find a replacement rubber part. I tested whether ordinary SMP construction adhesive could be used as a castable rubber-like material.
The interesting part: adding a small amount of water dramatically accelerated bulk curing.
Mixture
Approximate ratio used:
- 250 g SMP
- 4 ml water
The SMP and water were mixed thoroughly before use.
After two days the result was fully loadable and surprisingly solid.
No significant gas formation was observed. Only minor air inclusion from mixing itself.
Background
Various experiments with SMP and PU sealants were performed over the years.
For general mechanical use, Bloem Sealants MSP Allround proved particularly impressive in tensile strength and toughness.
It compared favorably against several alternatives including:
- low-cost "all in one" sealants
- PU sealants
- Sika products
The Action "all in one" products were surprisingly decent as well.
One downside of SMP in larger masses is extremely slow bulk curing. Surface curing may occur relatively quickly while full internal curing can take many days or even weeks.
The 250 g SMP + 4 ml water ratio used in this experiment appears to significantly accelerate through-curing without introducing problematic gas formation.
Surface preparation
The inside of the aluminum ladder legs was:
- roughened using non-woven abrasive material
- cleaned using isopropanol
This gave excellent adhesion.
Mould construction
Two mould halves were made from sheet material.
The inside surfaces of the moulds were covered with duct tape because SMP does not adhere well to it. This acts as a simple release layer.
A reference line was added to simplify alignment.
Procedure
- Attach the two mould halves around the ladder leg using duct tape.
- Apply SMP mixture inside the aluminum tube.
- Press roughly half the SMP between the mould halves using a putty knife.
- Wrap a thin plastic bag around the mould.
- Press the remaining SMP into the mould and aluminum tube by hand through the plastic.
- Pull the plastic tightly around the mould so the SMP follows the exact shape.
- Wait approximately two days.
- Remove mould and plastic.
Presto.
Observations
The result behaves surprisingly close to solid rubber:
- strong adhesion
- good flexibility
- little shrinkage
- low cost
- easy to shape
- easy to repair later
The added water appears to accelerate curing throughout the full volume instead of only forming an outer skin.
For thick SMP masses this may be a very practical trick.
Photos
Mould part with reference line
Mould with duct tape release layer
SMP after removing mould and plastic
Original ladder foot
Original foot compared with mould part
SMP feet before sanding
SMP feet after shaping
