Why Do Wax Melts Sink in the Middle? How to Fix Dips and Sinkholes

If you're making soy wax melts at home or crafting scents to sell, you have probably poured a perfect-looking batch, waited for it to set, and found a dip, crater or hollow in the middle of each melt. With so many makers pouring Christmas stock right now, it is one of the most common faults we hear about. In this guide, we explain why wax melts sink in the middle, how to stop dips and sinkholes, and how to fix melts you have already poured.

Quick answer: Wax melts sink in the middle because wax expands when it is hot and shrinks as it cools. The edges of the melt cool and set first, while the centre stays liquid for longer. When that centre finally hardens and contracts, it pulls down into a dip or cavity. Sinkage is normal and does not affect scent throw or safety. To reduce it, pour within your wax's recommended temperature range, warm your moulds, cool your melts slowly at room temperature and keep a little wax back for a small top-up pour if needed.

What Causes Wax Melts to Sink in the Middle?

Sinkholes (also called dips, craters or sinkage) are caused by normal wax shrinkage as it cools. When you heat wax, it expands. When you pour it into a mould and leave it to set, it cools from the outside in. The wax touching the mould walls sets first and "sticks" there, while the still-liquid centre keeps shrinking. As that centre hardens, it can pull down into a dip on the surface, or leave a small air pocket underneath.

A little sinkage is almost unavoidable in wax melt making. It is more noticeable in deeper moulds, clamshells and snap bars than in shallow tarts, and it can look worse if the room is cold or the moulds were cold when you poured.

Why Do My Wax Melts Dip or Get Sinkholes?

Most dips come down to temperature and how the wax cools. These are the most common causes:

  1. Pouring too hot. Very hot wax has further to cool, so it shrinks more as it sets. That extra shrinkage is what deepens the dip in the middle.
  2. Cooling too quickly. Draughts, open windows, cold worktops or putting melts near a fridge or freezer door can cool the edges much faster than the centre.
  3. Cold moulds. Silicone moulds or plastic clamshells that have been stored somewhere chilly cool the outer wax even faster, which makes the centre more likely to pull down.
  4. Uneven cooling. Crowding melts together, or leaving one side of a tray in a draught, can cool one part of a melt faster than the rest.
  5. Trapped air. Pouring from too high, or pouring wax that has already started to thicken, can trap small air pockets that show up as a hole or soft spot in the middle.

Sinkholes are different from frosting (the white, powdery coating on soy wax) and from sweating (beads of oil on the surface). If your melts look white or cloudy rather than dipped, read our guide on why wax melts frost.

Does a Dip in the Middle Affect the Scent?

No. A dip or sinkhole is a cosmetic issue only. It does not change the amount of fragrance in your melts, and it will not affect the cold or hot scent throw. A melt with a small dip is just as safe to use as a smooth one.

If your melts look fine but do not smell as strong as you expected, the cause is usually something else, such as the melts not having cured for long enough or the fragrance load being too low. Our guide on how long wax melts need to cure explains why curing makes such a difference, and our tips on how much fragrance oil you need for wax melts cover the simple calculation.

How Do You Stop Wax Melts Sinking in the Middle?

You cannot stop wax from shrinking completely, because that is how wax behaves as it cools, but you can reduce dips a great deal. Here are our top tips:

  1. Follow your wax's technical data sheet. For our Kerasoy 4120 Pillar Soy Wax, the data sheet suggests a pouring temperature of around 55 to 65°C. Pouring towards the cooler end of that range usually means less shrinkage. Add your fragrance just before you pour and stir it in thoroughly.
  2. Warm your moulds. Moulds should be clean and at least at room temperature. The Kerasoy 4120 data sheet notes that pre-heating moulds to around 45 to 50°C can be beneficial. A quick blast of warm air from a hairdryer is an easy way to take the chill off silicone moulds. Be careful with plastic clamshells, because too much heat can warp them.
  3. Let your melts cool slowly. Leave them to set undisturbed at room temperature, away from draughts, open windows and radiators. Never put them in the fridge or freezer to speed things up.
  4. Keep the room temperature steady. Pour in a room that stays at a similar temperature all day. A cold kitchen is one of the most common reasons makers start to see deeper dips in autumn and winter.
  5. Pour slowly and close to the mould. This helps to reduce trapped air. After pouring, tap the mould lightly on the worktop to release any bubbles.
  6. Keep a little wax back for a top-up. Reserve a small amount of the same scented wax from the batch. Once the surface has set but the melt is still warm underneath, gently fill any dip with a thin top-up pour. Do not pour higher than the original fill line, or you will get a raised rim around the edge.

How Do You Fix Wax Melts That Have Already Dipped?

A light surface dip can often be smoothed with gentle heat. Pass a heat gun, or a hairdryer on a low setting, quickly over the surface until the very top layer turns glossy and flows into the dip, then let it cool at room temperature. Keep the heat moving, because melting too much of the surface will spoil the shape or detail of your melts. This is best done just before you package or photograph your melts.

If the dip is deeper, or there is a hollow underneath, a small top-up pour works better. Warm a little of the same scented wax to your normal pouring temperature, poke the dip gently with a skewer to release any trapped air, then fill it level with the surface. Let it set fully before you unmould or package.

If a whole batch is badly sunk, you can gently remelt it and pour it again at the right temperature. Heating scented wax a second time may cost you a little fragrance, and the new melts will need to cure again, so test one before you sell the batch.

Are Sinkholes the Same as Cracks or Air Bubbles?

No, but they are often linked to the same causes. Cracks usually appear when melts cool too quickly, for example in a cold room or after a spell in the fridge. Air bubbles often come from stirring too hard, pouring from too high or pouring wax that has already started to set. Stir gently, pour slowly and close to the mould, and tap the mould lightly after pouring. Let melts set slowly at room temperature, and you will see fewer cracks, bubbles and deep dips together.

For more on how different waxes behave as they cool, read our guide to everything you need to know about wax for wax melts.

Which Fragrance Oils Are Best for Smooth Soy Wax Melts?

Every fragrance oil in our range is suitable for soy wax melts at 6 to 10%, and each one is professional grade and highly concentrated, so a little goes a long way. Here are some popular choices for winter and Christmas melts, all available from 10g for testing:

For your wax, our Kerasoy 4120 Pillar Soy Wax is a vegan, biodegradable soy wax made in the UK for wax melts and pillar candles. From £7.99 for 1kg. You'll find it alongside our other waxes in our wax making supplies collection.

You can browse every scent in our wax melt fragrance oils range, or see our seasonal favourites in the Autumn and Christmas fragrance oils collection. For more festive ideas, read our guide to the best Christmas fragrance oils for wax melts.

Why Choose Aroma Ninjas for Wax Melt Supplies?

  • Professional grade and highly concentrated: a little goes a long way.
  • Strong cold and hot scent throw in soy wax melts.
  • Soy wax made for wax melts: Kerasoy 4120 comes with a technical data sheet to help you get your temperatures right.
  • Safety documents for every oil: CLP, SDS, IFRA and allergen documents ready to download from each product page.
  • Sizes for every stage: from 10g bottles for testing new scents up to 1kg for production.
  • Free UK mainland delivery on orders over £125 (excluding VAT).

FAQs

Why do my wax melts have a hole in the middle?

In most cases, a hole or crater in the middle is a sinkhole caused by the wax shrinking as it cools. The edges set first, and the centre pulls down. Pouring a little cooler within your wax's range, warming your moulds and cooling slowly will help. A small top-up pour or a quick pass with a heat gun will usually level the surface.

Is it normal for wax melts to dip as they set?

Yes. Some sinkage is normal and almost unavoidable, because wax always shrinks as it cools. A shallow dip is usually nothing to worry about. Deeper dips are more common when the pour was too hot, the moulds were cold or the room was draughty.

Can I still sell wax melts that have sunk in the middle?

Yes. A dip does not affect the quality or scent of your melts. Many customers will not notice a shallow one. If you prefer a smoother look for photos or markets, use the heat gun or top-up tip above before packaging. Whatever your melts look like, make sure they are fully cured and correctly labelled. Our guide to whether wax melts need CLP labels explains what has to go on the label.

Should I put wax melts in the fridge to stop them sinking?

No. Cooling wax melts in the fridge or freezer makes uneven cooling worse, and it is also a common cause of cracking and frosting. Let them set slowly at room temperature, and allow at least 24 hours before you package them.

Final Thoughts

So, why do wax melts sink in the middle? In almost every case, it is simply wax shrinking as it cools, helped along by pouring too hot, cold moulds or a draughty room. Pour within your wax's recommended range, warm your moulds, let your melts cool slowly and keep a little wax back for a top-up, and you'll see far fewer dips and sinkholes.

If you're new to wax melts, our step-by-step guide on how to make wax melts at home covers the basics, and our tips on making strong-smelling wax melts will help you get a great scent throw once the tops look smooth. Explore our full range today and find the perfect fragrance oil for your wax melts, whether you're crafting for fun or to sell!

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