What to do with silicone leftovers, old moulds and empty PET, HDPE and tinplate packaging. A practical guide to reuse, recycling and end of life.
Anyone working with silicone rubber inevitably has material left over. A mould past its best, a bit of component A that no longer matches the remaining component B, a residue in the bottom of a bottle, and of course the packaging itself. The question that follows is nearly always the same: can this go in the recycling or not.
The honest answer is more nuanced than a simple yes or no. Cured silicone rubber is chemically something quite different from the plastic of a soft drink bottle, and that difference entirely determines what can happen to the material at the end of its life. At the same time, far more can be reused in practice than most users think, certainly if you are willing to cut an old mould into pieces.
This article covers the whole picture, from extending service life and reusing cured material in new casts, through to the correct disposal of leftovers and packaging, for private and commercial users alike.
The short answer
- ✓ Cured silicone rubber is inert and not hazardous waste, but it is also not recyclable in the ordinary plastics stream. It belongs in general or residual waste, at home in the residual bin, commercially in business residual waste.
- ✓ Uncured leftovers should be mixed at the correct ratio and left to cure fully in a disposable cup. After that it is ordinary rubber. Never pour anything down the drain.
- ✓ Empty packaging only goes in packaging collection once it is drip empty. If any residue remains, a different route applies.
- ✓ Reuse is often the best option, cured silicone can be shredded and cast into new moulds as filler material, saving up to roughly a third of your new material.
- ✓ Section 13 of the safety data sheet is always leading, the correct disposal route can differ per product.
Household collection rules differ per country and often per municipality. This article describes the general principles that apply across the European Union; check your local waste authority for the details where you live.
Why silicone is not simply recyclable
To understand why a silicone mould does not belong in plastic packaging collection, you need to know what happens during curing.
A thermoset, not a thermoplastic
Silicone rubber is built from polysiloxanes, chains with a backbone of alternating silicon and oxygen atoms carrying organic side groups. During curing, cross links form between those chains, a process called cross-linking or vulcanisation. The result is a three-dimensional network in which every molecule is chemically bound to the next. More about the structure and the different types is in what is casting rubber and which silicones exist.
That network is exactly what makes silicone so useful: the material stays elastic between roughly minus 50 and plus 200 degrees, is chemically almost inert and barely ages. But it also makes the material a thermoset. Where a thermoplastic such as PET or HDPE melts and becomes reformable when heated, a thermoset does not melt. Heat silicone rubber far enough and it degrades rather than turning liquid.
That removes the entire basis for mechanical recycling. You cannot grind silicone, remelt it and process it into granulate the way bottles are handled. Incineration with energy recovery is therefore the standard route.
What actually happens to silicone in residual waste
Cured silicone rubber that ends up in residual waste goes to an energy from waste plant. There the organic fraction is burned and converted into heat and electricity. What remains is mainly silicon dioxide, or amorphous silica, which ends up in the bottom ash and is partly recovered as a construction material.
That is not recycling in the strict sense, it is energy recovery. Environmentally it is harmless, silicone rubber contains no halogens and burns cleanly, but the material is permanently out of the loop. All the more reason to check carefully first whether reuse is possible.
Reusing cured silicone rubber in your own workshop
This is where the biggest immediate gain sits, both for your wallet and for the environment. Cured silicone is excellent for reuse, provided you know where the limits are.
Method 1, shredded silicone as filler material
The best-known technique among mould makers and by far the most useful. You cut or grind old moulds and casting leftovers into chunks and mix them into freshly prepared silicone rubber. The chunks fill volume, the fresh rubber forms the matrix that binds everything together. On large block moulds this can replace a considerable share of your new material.
Here is how to do it:
- → Select clean material only. Chunks from moulds that have been in contact with plaster, concrete, clay or polyester must be thoroughly cleaned. Residues of release agent, silicone oil, petroleum jelly or wax are unusable, they prevent bonding.
- → Cut or grind to chunks of roughly 5 to 15 millimetres. A sharp knife works fine, an old meat grinder or a coarse shredder is quicker. Finer is not always better, very fine particles thicken the mix too much.
- → Make sure the chunks are bone dry and free of grease. Clean with isopropyl alcohol if needed and let it evaporate completely.
- → Pour a pure face coat first. Apply at least 5 to 10 millimetres of pure silicone rubber against the model, so the chunk material never touches the casting surface.
- → Then build up in layers. Scatter the chunks into the still liquid silicone rubber and pour fresh silicone over them, so every chunk is fully enclosed.
- → Keep the proportion limited. Stay at a maximum of roughly 30 to 50 percent chunk material by volume. More means insufficient matrix and a mould that falls apart.
⚠️ Important limitations, read this before you start
The technique works well, but not everywhere and not always. Bear the following in mind:
Do not mix condensation silicone into addition silicone. Platinum-based addition curing is extremely sensitive to inhibition. Chunks from a tin-cured condensation mould, or chunks carrying residues of release agent, sulphur-bearing clay, latex or certain adhesives, can leave the fresh silicone rubber locally uncured around those chunks. Always run a small test cast if you are unsure.
Mechanical properties drop. A filled mould has lower tear strength than a solid one, because tears propagate easily along the interfaces between chunk and matrix. Use this technique for simple block moulds, not for moulds with thin, fragile sections or deep undercuts.
Not suitable for applications with requirements. For food contact, skin contact or medical applications and for high dimensional accuracy, use only pure, fresh material with the appropriate certification.
Older material bonds less readily. Heavily aged, leached or contaminated chunks bond poorly. Fresh casting leftovers a few weeks old work better than a ten-year-old mould.
Method 2, filler cores instead of chunk material
Often overlooked and technically superior, certainly if you do not want to compromise on quality. Instead of mixing chunks through your silicone, you place a solid filler block in the heart of the mould, made of a material that costs nothing and does not affect curing.
Usable cores include a block of casting foam, a closed piece of PE or PP, a smoothly finished wooden block with a sealed lacquer coat, or an empty sealed plastic container. Position the core so that at least 15 to 20 millimetres of silicone rubber remains all around. You do not pay for the volume the core occupies, while the mould itself stays solid and homogeneous around the model.
More effective still is not making a block mould at all. With a brush-on mould instead of a poured mould you apply only a thin layer of silicone to the model and support the shape with a support shell. That often saves more material than any filling method. The full method is in the guide to making a brush-on or spatula mould.
Method 3, repairing moulds instead of replacing them
A tear in an otherwise good mould is rarely a reason to throw it away. Repair is possible, with one important condition: the repair material must be of the same curing type as the mould. Repair condensation with condensation, addition with addition.
Clean and degrease the torn surface thoroughly, apply fresh silicone rubber of the same type, optionally with a silicone primer for better adhesion, and clamp or tape the parts together until fully cured. The bond of fresh onto old silicone rubber never matches the strength of the original material, but for a mould that still has dozens of casts to deliver it is more than enough. More about bonding and cutting is in working with silicone, bonding, air bubbles, cutting and colouring.
Method 4, a second life as a functional object
Not every piece of cured silicone rubber has to become a mould again. The properties that make the material valuable in the workshop, heat resistance, chemical inertness and a high coefficient of friction, are useful elsewhere too:
- ✓ Anti-slip mats under tools, workpieces or casting boxes
- ✓ Protective caps and edge guards for sharp corners in the workshop
- ✓ Trivets for hot pots, mixing cups and heated plates
- ✓ Spacers, wedges and clamping blocks when aligning moulds
- ✓ Test material for trying new release agents, pigments or cleaners before letting them near a working mould
- ✓ Practice material for students and beginners learning adhesion, demoulding and cutting technique
What you should definitely not do
A few persistent ideas circulate that either do not work or are plainly unwise.
Remelting achieves nothing. As explained above, a thermoset does not melt. Silicone rubber in the oven or under a heat gun produces no liquid material, only degradation and potentially harmful fumes.
Do not burn silicone yourself. Not in the garden, not in a wood stove and not in a fire pit. Combustion outside a controlled installation is incomplete and produces particulate matter and silicon-bearing particles.
Never flush shreds down the drain. Silicone rubber is not biodegradable and not soluble. It passes through the treatment plant and ends up in the sludge or in surface water.
Do not work silicone shreds into soil or compost. The material does not break down and eventually becomes a source of micro particles.
Uncured leftovers, component A and component B
Uncured silicone rubber is an entirely different matter from the cured product. As long as the components are liquid, the hazard properties on the label apply, and these differ sharply per product type. Catalyst components of condensation silicones may contain organotin compounds, some cross-linkers are classified, and in related products such as polyurethane, diisocyanates play a role.
The best route: let it cure
The practical and by far cheapest solution is to move the leftovers from hazardous to inert by simply letting them cure. Mix the leftover of component A with the matching amount of component B at the prescribed ratio, pour it out in a thin layer into a disposable cup or onto a sheet of foil, and let it cure fully in a ventilated space.
A thin layer cures faster and more reliably than a thick lump, and condensation silicones need ambient humidity for the reaction. Once the material feels fully cured and dry, it is ordinary silicone rubber and follows the route for cured material. More on curing times is in how long should a silicone mould cure before demoulding.
When that is not possible
Sometimes curing is not an option, for instance with a large quantity of expired material, a component whose counterpart is missing, or a batch whose catalyst has lost its activity. In those cases the material remains a chemical waste.
Private users hand this in at a household hazardous waste collection point or civic amenity site. Businesses dispose of it through a licensed waste contractor, with the appropriate European List of Waste code and the documentation required in their country. In both cases, check section 13 of the safety data sheet first, which states the disposal route prescribed by the manufacturer.
The best waste is the waste you never create
By far the biggest saving lies not in processing but in prevention. A few points that make the most difference in practice:
- ✓ Calculate your volume in advance. Put your model in the casting box, fill with water or rice to the height you want, pour it off and measure. Then you know exactly how much to prepare.
- ✓ Weigh, do not estimate. A digital scale accurate to 1 gram prevents both over-preparing and failed cures caused by an incorrect mixing ratio.
- ✓ Close packaging airtight immediately. Catalyst components of condensation systems in particular are moisture-sensitive and lose their activity in open storage.
- ✓ Store cool, dark and frost-free. Between roughly 5 and 25 degrees, out of direct sunlight. That extends shelf life considerably.
- ✓ Buy to match your consumption. A large container is cheaper per kilo, but not if half of it passes its date. A small container fully used is more sustainable than a large one half disposed of.
Packaging: PET bottles, HDPE bottles and tinplate cans
Silicones are supplied in various packaging types, and each has its own disposal route. Ours are mainly PET bottles, HDPE bottles and metal cans. The rules also differ between private households and businesses, so we treat them separately.
Step one, actually emptying the container
This is the decisive step, because everything that follows depends on whether any residue remains. The relevant standard is drip empty and drain empty: nothing more comes out when you turn the container over and shake it.
With silicone you have a practical advantage here. Simply let the last residue cure in the bottle or can. Cured silicone can often then be pulled out as a skin, after which the container is genuinely empty. The cured skin goes in residual waste, the empty container into packaging collection.
Do not clean containers with solvent or water unless necessary. That only moves the problem into the waste water.
For private users
Household packaging collection is organised nationally and varies between countries and municipalities. In the Netherlands packaging goes into the PMD stream, in Germany into the Gelber Sack or Gelbe Tonne of the dual system, and elsewhere under an equivalent scheme. The underlying principles, however, are the same everywhere:
| Situation |
Where it goes |
| PET or HDPE bottle, drip empty, cap back on |
Plastic packaging collection |
| PET or HDPE bottle with liquid residue still inside |
Household hazardous waste collection point or civic amenity site |
| Bottle with a dried or cured residue that will not come out |
Residual waste |
| Tinplate can with a dried residue |
Residual waste, the steel is recovered from the incineration residue with magnets |
| Tinplate can, genuinely empty and clean |
Metal packaging collection |
| Cured silicone moulds, casts, shreds |
Residual waste, never in packaging collection |
Three practical points. Screw the cap back on the bottle before disposing of it, which prevents leaks in the collection vehicle and means the cap is recycled with it. Do not nest or stack containers inside one another, because the sorting plant can no longer separate them. And check your local authority, since collection systems differ from place to place.
One more thing worth knowing. Packaging that has held chemicals is never recycled into food or drink packaging, not even PET. That material goes into a technical stream and is processed into non-food applications.
For commercial users
None of the household rules apply to businesses. Packaging waste from a company is commercial waste and goes through a contract with a licensed waste contractor. The full explanation is in how do I dispose of silicone waste as a business. The key points:
- → Coding under the European List of Waste. Clean plastic packaging generally falls under 15 01 02, metal packaging under 15 01 04, and packaging containing or contaminated by residues of hazardous substances under 15 01 10 with an asterisk, which makes it hazardous waste. Which code applies depends on the product and on how empty the container is.
- → Documentation is national, not European. Every member state has its own consignment note, notification and register requirements, and the receiving party must hold the relevant permit. Ask your contractor which regime applies to you.
- → Separating streams pays off financially. A separate stream of clean HDPE containers is considerably cheaper to dispose of than a mixed container classified as hazardous waste. It is worth emptying packaging completely and consistently.
- → Cured silicone as a separate stream. If you regularly produce larger quantities of clean, cured silicone waste, keep it apart. That is the only form in which silicone is likely to become interesting for chemical recycling.
- → Return and deposit schemes. Many suppliers operate return arrangements for bulk containers, drums and IBCs. Ask about them, it saves disposal costs and a cleaned returnable drum is considerably more sustainable than a new one.
The biggest gain is in container size
An often forgotten calculation. Someone buying silicone in one-kilo bottles for a year uses many times the packaging weight per kilo of material processed compared with someone taking the same volume in buckets or drums. The packaging fraction per kilo of product drops sharply with larger containers, and that packaging is what you eventually have to dispose of.
For workshops and businesses with steady consumption, moving to larger containers is therefore almost always the most effective sustainability measure, and at the same time the cheapest per kilo. For occasional users the reverse holds: do not buy more than you will process within its shelf life.
Industrial recycling of silicone, where things stand
The question of whether silicone will ever become genuinely circular comes up more and more often, including from procurement and sustainability reporting. The honest answer is that things are moving, but the sector is still at the beginning.
Chemical recycling and depolymerisation
Because mechanical recycling does not work for thermosets, research is focused on chemical recycling. Catalysts break the silicone network down into shorter chains, oligomers and cyclic siloxanes, which can then serve again as raw material for new silicone of virgin quality. That is a genuinely closed loop, unlike energy recovery.
The scale is still modest. A 2024 review estimated that in the order of 35,000 to 45,000 tonnes of silicone waste were chemically recycled worldwide, against a total market many times larger. In the United States a specialised plant chemically breaks silicone rubber down into polydimethylsiloxane oil, with a reported carbon footprint considerably below that of new material. Comparable initiatives are running in Europe, aimed mainly at industrial production streams.
Why that does not yet apply to consumer waste
The plants that exist run almost exclusively on clean, known, separately collected streams. Think of sprues and edge waste from injection moulding, rejected parts from a single production line, or leftover batches of liquid silicone rubber from a manufacturer. That material has a known composition, a known curing type and no contamination.
An old mould from a hobby workshop meets none of those conditions. The composition is unknown, there is plaster, resin, pigment and release agent on it, and the quantity per supplier is too small to be logistically viable. Until a collection infrastructure exists, chemical recycling therefore remains relevant mainly to industrial parties.
Mechanical reuse as a functional filler
Alongside the chemical routes, research is under way into using ground silicone waste as a functional filler in new silicone elastomers, examining the effect on mechanical integrity and energy dissipation. That is essentially the industrial version of what mould makers have been doing in the workshop for decades, and it confirms that the technique works in principle as long as you accept the limitations.
Working circularly with silicone, ordered by impact
The European waste hierarchy ranks strategies from smart use and life extension at the top down to energy recovery at the bottom. Translated to silicone, that order looks like this, and the sequence is not arbitrary.
1. Extend the life of your mould. This delivers by far the largest saving. A mould that achieves 200 casts instead of 60 replaces two moulds you never had to buy, produce or dispose of.
2. Choose the right product for the application. The right Shore hardness, the right curing type and the right elongation prevent premature failure. See also which material for which job.
3. Minimise leftovers. Calculate volume in advance, weigh accurately, use filler cores and keep the casting box as small as possible.
4. Reuse cured material. Shredded as filler in new block moulds, or as a functional object in the workshop.
5. Repair instead of replacing. A tear is not the end of a mould's life.
6. Dispose responsibly. Cured material to residual waste, uncured material cured first, packaging emptied completely and sent to the right stream.
Extending mould life, the tips that deliver most
Because point one has the greatest impact, here is concretely what makes a mould last longer.
- ✓ Use the right release agent and seal porous substrates. With polyester and polyurethane especially, a good release layer is the difference between a mould that lasts 100 casts and one that bonds shut after 10. See which substrates should I seal.
- ✓ Demould calmly and evenly. Most tears occur during removal, not during casting. Never pull on a thin protrusion.
- ✓ Clean after use. Leftover resin or plaster attacks the surface and degrades detail on the next cast.
- ✓ Store flat and supported. A mould left on its side for months deforms permanently. Lay them flat, if necessary with the model or a support block inside.
- ✓ Keep them out of direct sunlight and dust. UV and heat accelerate ageing, and dust clings to the surface and ends up in your next cast.
- ✓ Mind the mould and casting resin combination. Condensation silicones are clearly more resistant to polyester and polyurethane, addition silicones offer higher dimensional stability and longer life but are more sensitive to inhibition. Read more in silicones and their resistance in practice.
- ✓ Make a mould of your mould. For a model you need often, it pays to cast a durable master model in resin first. When the mould wears out you cast a new one without having to rework the original model.
Another specific question about disposal or reuse?
The questions we receive most often on this topic are answered separately:
Related topics
Less waste starts with the right choice
The biggest saving is not in the bin but at the start, with a silicone that suits your model, your casting resin and the number of casts you want. Unsure which type or hardness you need? Get in touch with our team, we are glad to think through the most efficient approach for your project.
Condensation siliconesPlatinum siliconesAsk for advice
The information in this article is general in nature and describes principles that apply broadly across the European Union at the time of publication. Rules for waste separation differ per country and per municipality. Section 13 of the safety data sheet of the product concerned is always leading for the correct disposal route. Businesses are themselves responsible for correct waste coding, record keeping and delivery to a licensed processor.
