Week 14 - Forming and Casting

This week we will learn about Molding and Casting. The core idea of this type of process is to first create a mold, then pour liquid or semi-liquid material into the mold, and wait for the material to solidify before removing the finished product. Compared to direct 3D printing or CNC machining, molding and casting are more suitable for repeatedly producing parts of the same shape, and different materials such as silicone, resin, and plaster can be used to achieve different textures and appearances.

This week\'s group assignment requires reading and comparing the SDS \(Safety Data Sheet\) of all forming and casting materials, then making test pieces using different materials and comparing them, while also comparing different mold-making processes. The individual assignment requires designing a mold based on the process used, making a mold with a surface as smooth as possible and no obvious machining tool paths, and using it to complete the casting production.

Group Assignment

Group Assignment Page: https://fabacademy.org/2026/labs/chaihuo/docs/week14/chaihuo/week14_group_assignment

1. Comparison of Safety Data Sheets \(SDS\)

Before molding and casting, it is necessary to first understand the safety requirements of the materials. The mixing ratios, curing times, odors, skin irritations, and operational risks of different materials vary. Incorrect mixing ratios or insufficient protection may lead to curing failure, sticky surfaces, and even irritation to the skin and respiratory tract.

This group mainly compared four types of materials: industrial silica gel, gypsum, epoxy resin, and food-grade platinum silica gel.

1.1 Industrial Silicone

• *Mixing ratio \(by weight\) *: 100 : 2 \(base rubber : curing agent\)

• Operating Requirements: Stir thoroughly for 1-2 minutes; apply a release agent before casting to facilitate subsequent demolding.

• Full curing time: Approximately 2 hours.

• *Precautions *: The proportion of the curing agent is relatively small, so it is necessary to weigh it as accurately as possible; insufficient stirring can easily lead to incomplete local curing.

1.2 Gypsum \(Calcium Sulfate\)

• *Mixing ratio \(by weight\) *: 3:1 \(gypsum powder: water\)

• Operating Requirements: First add water, then add gypsum powder; stir until it reaches a flowable state; first brush to fill in details, then pour the whole.

• Full curing time: Approximately 1 hour.

• *Precautions *: Gypsum dust is easily inhaled, so dust generation should be avoided during operation; gypsum is brittle after curing and is not suitable for slender or thin-walled structures.

1.3 Epoxy Resin \(Crystal Resin\)

• *Mixing ratio \(by weight\) *: 3:1 \(Adhesive A: Adhesive B\)

• *Operating Requirements *: Weigh proportionally and stir thoroughly to minimize air bubbles.

• Full curing time: approximately 24 hours.

• *Precautions *: Epoxy resin may irritate the skin, so gloves must be worn during operation; direct contact with the skin should be avoided before curing; insufficient mixing may cause the surface to be sticky or the interior to remain uncured.

1.4 Food-grade Platinum Mold Silicone

• *Mixing ratio \(by weight\) *: 1:1 \(Adhesive A: Adhesive B\)

• Operating Requirements: Mix Components A and B in equal amounts, stir thoroughly, and then pour.

• Full curing time: approximately 6 hours.

• *Precautions *: The ratio is simple, suitable for making flexible molds; however, it is still necessary to avoid air bubbles and uneven mixing.

2. Comparison of Material Parameters

Material	Usage	**Mixing Ratio **	**Curing Time **	Features
Industrial Silicone	Make flexible molds	100 : 2	Approximately 2 hours	Fast curing, but high requirements for weighing the curing agent
Gypsum	Make hard castings	3 : 1	Approximately 1 hour	Low cost, fast curing, but relatively brittle
Epoxy Resin	Make transparent / rigid castings	3 : 1	Approximately 24 hours	Good surface effect, but slow curing and requires attention to protection
Food-grade platinum silicone	Make flexible molds	1 : 1	Approximately 6 hours	Simple ratio, good demolding performance, suitable for detailed mold replication

From the comparison, it can be seen that different materials are suitable for different stages. Silicone is more suitable for making molds because it is soft and easy to demold; plaster is suitable for rapid testing; epoxy resin is suitable for making hard, transparent, or decorative finished products, but it requires a longer curing time and more meticulous bubble treatment.

3. Comparison of Mold Manufacturing Processes

The group also compared several common mold manufacturing methods:

**Mold Manufacturing Method **	Advantages	Disadvantages	**Suitable Scenarios **
CNC Milling Wax Blocks / Foam	High surface accuracy and good controllability	Requires cutting tools, machine tools, and machining time	High-precision mold, repeated casting
3D Printed Prototype and Then Mold Replication	Fast production speed, free shape	FDM layer lines are obvious and require post-processing	Rapid Prototyping, Small-Scale Model
Laser Cutting Mold Frame + Prototype Mold Replication	Low cost, flexible setup	Suitable for simple mold opening, not suitable for complex curved surfaces	Planar or bas-relief model

The process used for this individual assignment is ** 3D printing prototype + silicone mold making + epoxy resin casting **. This process is suitable for making small graphic cases, has a low operational threshold, and facilitates observing the mold making and casting processes.

Individual Assignment

1. Design Concept and Process Selection

For this individual assignment, I chose to create a casting with QR code details. Since the model surface contains fine pattern information, the mold needs to retain details as much as possible, and there should be no obvious tool paths or machining marks on the casting surface.

Instead of directly using CNC milling to make the mold, I first used FDM 3D printing to create a prototype, then made a mold using food-grade platinum silicone, and finally cast it with epoxy resin. The reasons for choosing this process are:

• 3D printing is suitable for rapid prototyping;

• Silicone molds are soft, facilitating easy demolding and accurate replication of details;

• Epoxy resin is relatively hard after curing and is suitable for use as a final product;

• Post-processing can reduce the impact of FDM layer lines on the final casting.

To meet the requirement of \"the surface being as smooth as possible and the production tool path being invisible\", I used a smaller layer height when printing the prototype and cleaned and processed the prototype surface before mold making to minimize layer lines and surface defects.

2. Prototype Preparation: From 3D Model to FDM Printed Part

2.1 3D Model Slicing and Printing Parameters

First, import the 3D model into the slicing software, and complete the support settings and parameter preview.

Print parameters are as follows:

• Set the layer height to *≤ 0.1 mm *, improve the surface quality, and try to preserve the QR code details;

• The fill rate is set to ** 20%-30% **, taking into account both structural strength and printing speed;

• Enable ** outer wall first printing **, improve the quality of the outer surface, and reduce the workload of subsequent grinding;

• After printing is completed, check the edges and detailed areas to confirm that there are no obvious material shortages or warping.

2.2 Pre-treatment of FDM Printed Parts before Mold Replication

The surface of FDM printed parts is prone to layer lines, and if directly used for mold making, the silicone will replicate these layer lines. To reduce this issue, I cleaned the model surface before mold making, removed burrs from the edges, and ensured that the pattern area was as clean as possible.

I paste double-sided tape on the flat surface of the model to ensure it firmly adheres to the bottom of the casting container. This way, when pouring silicone, the prototype will not float or move, and it can also prevent silicone from seeping in from the bottom, which could cause deformation of the mold.

3. Silicone Mold Making

3.1 Silicone Ratio and Stirring Preparation

This time, ** food-grade platinum silicone ** was used, with the ratio of A glue to B glue being ** 1:1 ** by weight, and the curing time is approximately 6 hours.

During operation, I first use an electronic scale to weigh glue A and glue B, trying to ensure the ratio is accurate. Then, I pour the two glues into a disposable mixing cup and use a stirring rod to slowly stir along the cup wall while scraping the bottom and the wall to ensure that the silicone on the bottom and the cup wall is evenly mixed.

Stirring time is about 1-2 minutes. Do not stir too fast, otherwise a large number of bubbles will be introduced, affecting the mold details and surface quality.

3.2 Silicone Degassing and Pouring

After stirring is complete, I let the cup stand for 5 - 10 minutes and gently tap the cup wall to allow larger bubbles to rise and burst. If a vacuum defoamer is available, the effect will be better, further reducing bubbles.

To make the detailed area more complete, I first take a small amount of silicone and use a brush to apply it to the QR code detailed area of the prototype. This allows the silicone to first enter the fine concave-convex structure, preventing air bubbles from remaining in the detailed area during direct pouring.

Then slowly pour the remaining silicone from the corner of the mold frame, allowing the silicone to naturally flow over the surface of the prototype. Avoid directly impacting the model when pouring the silicone to reduce the generation of new bubbles. Finally, the silicone level should be about 2-3 cm above the top of the prototype.

3.3 Silicone Curing and Mold Opening

After the silicone gel injection is completed, I place the mold in an environment at 20-28°C and let it stand for about 6 hours. During the curing process, try to avoid vibration and excessive temperature differences to prevent affecting the molding effect.

The bottom of this model is flat, so no complex parting lines are required. After the silicone has cured, I removed the mold from the container and then slowly extracted the prototype to obtain a silicone mold with the details of the prototype.

4. Epoxy resin injection molding

4.1 Epoxy Resin Mixing Ratio and Stirring

Epoxy resin was used as the final casting material this time. The ratio of Component A to Component B by weight is 3:1, and the curing time is approximately 24 hours.

During operation, I first weigh Glue A and Glue B using an electronic scale, then pour them into a mixing cup. Mixing is carried out in two steps: first, stir along the cup wall for 2 - 3 minutes, then scrape the bottom and stir for about 1 minute to ensure there is no unmixed resin residue at the bottom.

After stirring is completed, defoaming treatment is carried out. I reduce bubbles by allowing it to stand for 10 - 15 minutes and gently tapping the cup wall. Due to the limited equipment in the studio, the defoaming effect is not as complete as vacuum defoaming, so a small number of bubbles may still appear in the finished product.

4.2 Mold Pretreatment and Injection Molding

Before casting, I first wipe the inner wall of the silicone mold with absolute ethanol. After the mold is dry, I apply a release agent and let it stand for about 5 minutes. This can reduce the difficulty of demolding and also minimize the risk of resin adhering to the mold surface.

Subsequently, slowly inject the resin into the mold through the gate. Maintain a uniform speed during injection and gently tap the mold wall to assist with air venting. After filling, cover the gate with plastic wrap to prevent dust from falling onto the resin surface.

4.3 Curing and Demolding

Epoxy resin should be left to stand at room temperature \(20\-28°C\) for approximately 24 hours. During the curing process, direct sunlight, movement, and vibration should be avoided.

After the resin has fully cured, I slowly separate the silicone mold and remove the casting. When demolding, do not pull forcefully on the detailed areas, as this may cause damage to the edges of the casting. The final resin part retains most of the details of the prototype.

5. Surface Quality and Problem Analysis

This week\'s individual assignment requires that the mold surface be as smooth as possible and that the tool paths from the production process not be visible. Since I am using FDM printing to create a prototype and then making a mold, rather than directly CNC milling the mold, the main issue to address is the FDM layer lines, rather than the CNC tool paths.

I adopted the following methods to improve surface quality:

• Print the prototype with a smaller layer height;

• Start printing the outer wall first;

• Clean the burrs on the edges before mold turning;

• First brush silicone in the detailed area;

• Slowly pour from the corner during pouring to reduce air bubbles;

• Clean the mold and apply a release agent before injecting epoxy resin.

In the actual results, the silicone mold can replicate the details of the prototype well, but due to limited defoaming conditions, a small number of bubbles may still exist in the resin parts. This indicates that in molding and casting, material ratio and surface treatment are only the basics, and bubble control is equally crucial.

6. Troubleshooting Common Issues

6.1 Silicone Mold Issues

Question	**Possible Reasons **	**Solution **
Air bubbles in the mold	Stirring too fast or directly impacting the prototype	Stir slowly, brush details before pouring, and let stand for defoaming
Incomplete curing	Incorrect A/B glue ratio or uneven mixing	Weigh strictly at a 1:1 ratio, scrape the bottom and walls, and stir thoroughly
Missing details	Silicone did not enter the detailed area	First, use a brush to paint the details, then perform overall pouring

6.2 Epoxy Resin Injection Molding Issues

Question	**Possible Reasons **	**Solution **
Internal bubble	Resin not fully degassed	Let it stand, gently tap the mold, and use vacuum defoaming if conditions permit
Surface sticky	Proportion error or uneven mixing	Strictly follow the 3:1 ratio, stir thoroughly, and allow to cure for a full 24 hours
Difficulty in demolding	No release agent was used or the mold structure has an undercut	Apply release agent in advance and avoid overly deep undercuts during design

7. Summary

Through this week\'s assignment, I have fully experienced the process from prototyping, silicone mold making to epoxy resin casting. Compared to direct 3D printing, molding and casting require more upfront preparation, such as material safety, mixing ratios, bubble control, mold structure, and demolding methods.

This practice has made me realize that the quality of the mold directly determines the quality of the final casting. Layer lines, dust, burrs, and air bubbles on the prototype surface will all be replicated by the silicone mold, which in turn affects the resin finished product. Therefore, in subsequent production, I will pay more attention to the prototype surface treatment and defoaming steps. If conditions permit, using CNC machining wax molds or vacuum defoaming equipment can further improve the surface quality of the mold and casting.

[!NOTE]

AI Assistance：

During the preparation of this documentation, ChatGPT (GPT-4) was used as a language assistance tool.

It helped with sentence polishing and translation from Chinese to English to improve readability and clarity.