Week 14 — Molding & Casting

For molding and casting week we need to review the safety data sheet about materials and for the materials themselves I bought some that I found on Taobao.

Here I have:

Tin-based solder alloy

Material Tin-based solder alloy bar (Translation - High-purity 66A solder bar, lead-free tin bar, tin block / soldering bar, leaded soldering tin-coating tool for home use, 500 g): This is a metal solder material, usually made mainly from tin with other metals added. It is designed for soldering, but in this activity it may be melted and used for a small molding/casting test. Supplier didn't provide me with a safety datasheet so I found it on the internet.

Safety Data Sheet: https://www.tme.eu/Document/a983e27d3e3ec8eb1590a33db6220793/SN993CU07LASKI_EN.pdf

Device: Small electric tin melting pot / solder melting pot This is a small electric heating device used to melt tin-based solder materials. It heats the solder bar until it becomes liquid, so it can be dipped, coated, or poured carefully into a mold.

Safety set up: · Ventilation: Work outdoors or with a fan pulling air away from you — fumes are the main hazard

• Temperature: Melts at 217–230°C; use a temperature-controlled pot, not open flame
• PPE: Heat-resistant gloves, splash-rated goggles, long sleeves
• Mold: Must be completely dry — moisture causes violent spattering
• Fire: Keep dry sand or powder extinguisher nearby — no water on molten metal
• Hygiene: Wash hands after; no eating/drinking in the workspace

High-purity tin metal, metal and non-metal powders:

• 金属锡 高纯Sn 高纯锡半球 高纯锡条 — High-purity tin metal (Sn), tin half-spheres / tin bars — 200g — ¥96
• 高纯铁粉铜粉镍粉钨粉钼粉钴粉铬粉 → 高纯碳粉 — High-purity carbon powder (碳粉) — 100g — ¥18
• 高纯铁粉铜粉镍粉钨粉钼粉钴粉铬粉 → 高纯铋粉 — High-purity bismuth powder (铋粉) — 100g — ¥30
• 铜粉电解金属铜粉 高纯铜粉红铜粉紫 — High-purity electrolytic copper powder (铜粉) — 100g — ¥13
• 黄铜粉金属超细工艺品镶嵌紫铜 — Brass powder (黄铜粉), 80–100 mesh — 100g — ¥15.37

Material: High-purity tin block (Sn 99%+) Translation: 金属锡 高纯Sn 高纯锡半球 高纯锡条 — 200g

This is nearly pure elemental tin in solid block/half-sphere form, with no alloying additions. It melts at around 232°C and is the primary casting material for this activity — it will be melted in the electric tin pot and poured into a mold.

Safety setup:

• Ventilation: Work outdoors or with a fan pulling air away — tin fumes are the primary hazard
• Temperature: Melts at ~232°C; use temperature-controlled pot only
• PPE: Heat-resistant gloves, splash-rated goggles, long sleeves
• Mold: Must be completely dry — moisture causes violent spattering
• Fire: Dry sand or powder extinguisher only — no water on molten metal
• Hygiene: Wash hands after; no eating/drinking in the workspace

Materials: Metal powders (handle as dry powders only — not for melting) Carbon powder (碳粉) 100g · Bismuth powder (铋粉) 100g · Copper powder (铜粉) 100g · Brass powder (黄铜粉) 100g

These are fine metal and non-metal powders purchased for surface finishing, coloring, or embedding experiments.

Bismuth (铋粉) — can be melted and mixed with tin to create a lower-melting-point alloy. Tin-bismuth combinations reduce shrinkage during casting. Bismuth melts at 271°C.

Copper powder (铜粉) — melting point 1085°C, cannot be alloyed with tin in a standard melting pot. Use cold only — surface texture or finishing.

Brass powder (黄铜粉) — melting point too high for tin pot. Use cold only — surface finishing or embedding.

Carbon powder (碳粉) — not a metal, does not melt into tin. Use cold only — surface coloring or resin mixing.

Safety setup (powders):

• Inhalation: Fine metal powders are a respiratory hazard — wear an FFP2/N95 dust mask when handling
• Skin/eyes: Avoid contact; wash hands thoroughly after use
• Ignition: Some metal powders (copper, brass) are flammable in fine particle form — keep away from open flame and sparks
• Storage: Keep sealed and dry; label clearly

Plaster:

Material: Dental Model Gypsum Powder (牙科模型石膏粉) Manufacturer: Henan Hongtai Gypsum Industry Co., Ltd. — 25 kg bag

Dental model gypsum (calcium sulphate hemihydrate, CaSO₄·½H₂O) is a fine white powder used to create hard, accurate casting molds. Mixed with water, it sets into a rigid solid through a chemical hydration reaction. In this activity it is used as a mold material for metal casting.

Safety Data Sheet: USG Dental Gypsum SDS (equivalent product) — usg.com Note: No SDS available from the Chinese supplier. The USG document covers the same chemistry (calcium sulphate hemihydrate) and is applicable for safety reference.

https://www.usg.com/content/dam/USG_Marketing_Communications/united_states/sds/usg-coecal-dental-gypsum-cements-sds-en-52000000009.pdf

Mixing ratio:

• Dental plaster requires a water/powder ratio of approximately 0.55 Pocket Dentistry — roughly 55 ml water per 100 g powder
• Add powder to water, not water to powder; mix 1–2 minutes until smooth
• Vibrate the mix to bring air bubbles to the surface Pocket Dentistry before pouring — air pockets reduce mold accuracy

Safety setup:

• Dust: Primary hazard — minimize dust production when mixing or opening the bag; avoid inhalation of dust USG
• PPE: FFP2/N95 dust mask, safety goggles, gloves when mixing dry powder
• Setting: Generates mild heat during setting — normal, not a hazard
• Important for metal casting: Mold must be completely dry before pouring molten tin — any retained moisture causes violent spattering
• Hygiene: Wash hands after use; do not pour waste slurry down drains — it sets solid in pipes

For dental/model gypsum used as a metal casting mold, drying time has two stages:

Initial set (surface hard): 30–45 minutes — the mold feels solid but contains significant internal moisture.

Safe for metal casting: This is the critical point. You need the mold fully dry, not just set. Residual moisture causes steam explosion when molten metal contacts it.

• Air drying at room temperature: 24–48 hours minimum
• Oven drying (recommended): 200–250°C for 1–2 hours — this drives out both free and chemically bound water, and also preheats the mold which improves metal flow and reduces thermal shock

Construction cement:

Material: Construction Cement (建筑水泥)

Standard grey construction cement, sourced locally in Hangzhou. No brand identified — typical general-purpose cement available from local building material suppliers across China. Chemistry and handling properties are consistent with standard cement regardless of brand.

Safety Data Sheet: No supplier SDS available. Standard cement SDS applicable — USG Construction Cement SDS

Mixing ratio:

• Water/cement ratio 0.4–0.5 — 40–50 ml water per 100 g cement
• Add cement to water gradually; mix until smooth and pourable
• Vibrate after pouring to remove air bubbles

Drying / curing time:

• Initial set: 2–4 hours
• Demolding: 24 hours minimum
• Safe for metal casting: 7 days full cure, then oven dry at 150–200°C for 2–3 hours to remove residual moisture

Safety setup:

• Dust: Alkaline — FFP2/N95 mask, goggles, gloves when handling dry powder
• Skin: Chemical burns with prolonged contact — wash off immediately
• Eyes: Flush 15 minutes with water if contact occurs
• Ventilation: Work outdoors or with airflow when mixing dry powder
• Hygiene: Do not pour waste slurry down drains

Note: Cement cures much slower than dental gypsum. For casting tests, gypsum is the faster and more practical mold material.

Silicon Mould and PLA fillament waste.

For my first warm-up casting experience I got some food-friendly silicone mould and I want to use PLA waste from a 3D printer and also low-temp PLA waste from a 3D pen.

Procedure is simple: place PLA waste by some small parts into the mold and melt it by hot air blower. Maximum temperature of hot air blower is 500°C.

During the melting I gradually added some waste PLA, and it melted just fine. Of course it is better to spend more time heating to achieve a smoother shape. Wooden popsicle sticks and a metal spatula did not help to press the melted filament because the filament started sticking to the metal or wood. I used a silicone soldering mat to press the melted filament and it was useful because I still need to apply some force, heat it for longer, or melt it for longer to achieve more liquidity.

On the left red LEGO figure I used low-temperature PLA filament, which can melt faster (60-80°C). Then I used normal PLA filament (220-240°C).

Plaster

I got some plaster and I can experiment with existing silicone molds for plaster. The ratio is two parts of plaster and one part of water, but it's not exactly the same, and better to always see how the paste mixes and how possible it is to pour into the mold because sometimes space can be too dense.

Also, plaster can become solid very fast, so better to mix it within 1-2 minutes and pour it into the mold. It's an exothermic reaction, and the mold and plaster will warm up a little.

Mortar

And the same fan with cement and water, which can create mortar.

Metal casting

Especially for this molding and casting week, I bought a melting device plus five 600 grams of tin. I will use silicon molds because my plaster and mortar molds are still drying, and I don't have options how to speed up with an oven in my lab.

It is better to cut tin into some small pieces, and then it will melt faster.

I set up support with clamps to be sure that the heating part can be raised in the air and I can add tin without any problems with holding it.

Within five minutes I melted almost everything and then poured it into a silicone mold. I just decided to test whether the silicone mold could hold the hot metal and it was successful.

Mould creating

Before I created my 3D statue head from Maker World, I want to use it for molding and casting experiments.

https://makerworld.com/en/makerlab/makeMyStatue?from=makerlab

I found a fast way to use Tinkercad and I can rotate and cut any mesh shape as I want.

First I want to create one side positive mold. Cutting and tinker-cut is easy. I just need to use another shape as a hollow figure and then group two objects together so the hollow figure will cut another figure.

After cutting I reduce the height of the face for better fitting.

Positive Mould.

I can use this mold and fill it by plaster or mortar, which is a pure mix with cement and water.

For negative mode the face object can become hollow and make an empty space in the surface of the rectangle brick.

For wax milling I can use a similar approach and create a negative mold. At first I want to make one side mold and pour concrete and pour mortar and plaster for experiment.

In the Mods Project we can select a milling 3D shape from STL. It will be two passes, first with a 3.175 flat end mill. And second, finish pass with 0.8 mm ball end mill

Milling time: 36 minutes.

For milling finish part, I didn't realize how to set up Mods first, and I used Easel.

The settings are not flexible for beginner level but still after exporting G-code I found that it is easily creating the unnecessary cuts around the shape.

I edited G-code manually and removed this section of cutting the shape around, but still the timing was not so good because for rough milling it's 1 hour 20 minutes, and for finish milling it's 4 hours.

Then I found the way that I can choose the tool dimension in the Mods project, and I can operate more flexibly.

For the finish milling settings a bit different and milling time 2 hours 37 minutes

Also, because I have a shape of my head, it's good to cut it in half on the front and back part and mill together. Then I can use it like a mold and pour plaster mortar and probably tin.

Also for printing in TPU (Positive mould for plaster or mortar mould making – and later for metal)

Negative PLA mould

36 min

2 hours 37 min