Maya Ellison
Plastic mould fabrication is a process that involves creating a custom mould for a product. This is done by injecting molten material, typically plastic, into a mould cavity. The molten material is forced into the mould under high pressure, where it cools and solidifies, taking the shape of the mould. This process is known as injection moulding and is widely used in various industries due to its versatility, cost-effectiveness, and ability to produce intricate designs. There are multiple techniques used for creating plastic moulds, including blow moulding, compression moulding, extrusion moulding, and rotational moulding. The type of mould and resin material are important considerations, as well as the budget and labour availability. Successful mould fabrication ensures precise designs and fulfilment of specific application requirements, contributing to advancements in different fields.
| Characteristics | Values |
|---|---|
| Types of plastic moulds | Injection moulding, blow moulding, compression moulding, extrusion moulding, and rotational moulding |
| Injection moulding process | Injecting molten material (plastic or resin) into a mould cavity at high pressure and speed |
| Injection moulding advantages | Mass production of complex, precise parts with consistent quality and high efficiency, cost-effective, versatile |
| Compression moulds | Lower precision requirements, simpler open and close mechanism, weaker and lighter structure, cheaper cost |
| Rotational moulds | Used for sizable, hollow plastic parts with consistent wall thickness, e.g. tanks, storage containers, playground equipment |
| Mould materials | Silicone, plaster, reusable moulding materials, resin (thermosets or thermoplastics) |
| Mould considerations | Part design, material selection, budget, labour availability, manufacturing demand, regulatory compliance, availability of resin |
| Mould fabrication steps | Precise mould design, material selection, fabrication, surface treatment, assembly, testing, repetition |
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What You'll Learn
Injection moulding
To create a plastic injection mould, you can use a 3D printer or build your own injection moulding machine. If you are using a 3D printer, you will need to use a 3D printing material that can withstand the temperature and pressure of the moulding process. Recommended materials include Rigid 10K Resin and High Temp Resin. You can then import both mould halves of the mould core and the 3D design you want to produce into your CAD tool. Ensure that the object fully intersects with the inlet for the molten plastic during the injection moulding process.
If you are building your own injection moulding machine, you will need to purchase or build a mould frame to hold the liquid epoxy until it hardens. The mould will consist of two halves, so the frame must also be constructed as two halves that fit together. You can then inject molten plastic into the mould.
It is important to note that the mould is placed under the nozzle and raised into place after the cylinder is filled with molten plastic. If the mould is put in place while the plastic is being loaded, some of the plastic will dribble out of the nozzle and clog the sprue of the mould. Additionally, it is easy to apply too much pressure when injecting the plastic, so stop when you first feel additional resistance.
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Compression moulding
There are several types of compression moulds available, including flash, positive, landed positive, and semi-positive moulds, with the flash mould being the most prevalent. The moulds used in compression moulding are constructed to endure repeated use, making this process robust and cost-efficient. The processes within compression moulding are categorized into bulk and sheet moulding. Bulk moulding combines ingredients like fillers, catalysts, stabilizers, pigments, and fibre reinforcers. The significant benefits of using compression moulding for thermoset plastics include greater strength, reduced weight, and excellent resistance to corrosion.
The first step of compression moulding is setting up the mould. The raw material is then heated in the mould until it becomes pliable, while the mould is closed for a specified period. The upper and lower halves of the mould then come together to compress the charge effectively. During this compression phase, the process aims to ensure that the charge fills the entire mould cavity to the intended volume and that the desired density within the finished product is achieved.
After compression, the moulding stage involves cooling the mould or using hardening agents and catalysts to set and solidify the material. The product is then separated from the mould, either manually or automatically. It is common for the moulded product to contain excess material, usually called flashing, which has extruded while being heated and compressed in the mould. This excess material is removed, either manually or automatically, in the de-flashing stage.
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Rotational moulding
The rotational moulding process involves several steps, starting with a heated hollow mould that rotates on two axes. The mould is filled with a polymer powder or resin, typically polyethylene, polyvinyl chloride, or nylon. Pre-compounds for colour and hardness can also be added to the mix. The mould is then closed and moved to an oven, where the heat melts the plastic powder, allowing it to stick evenly to the inside of the mould.
The mould is slowly rotated in various directions to ensure that the plastic spreads evenly across the interior surfaces. The rotation also helps to prevent problems like shrinkage and warping, which can occur during the cooling process. The mould is then cooled, causing the plastic to solidify and take on the desired shape.
After cooling, the solid plastic part is carefully removed from the mould. Proper demoulding is critical to ensuring the quality of the final product. Once the part is removed, it is ready for finishing, inspection, and assembly or packaging.
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Choosing materials
Compression molding is another technique that uses a more straightforward open-and-close mechanism without high-pressure injection. The mold structure can be weaker and lighter, making it more affordable. For this type of mold, you might opt for materials that are more cost-effective and easier to work with, such as aluminium alloys or even composite materials.
Rotational molding, or rotor-molding, is used for creating sizable, hollow plastic parts. The process involves rotating a heated mold on two axes to evenly spread liquid plastic across the interior surfaces. The mold material should be able to withstand the heat and rotation without losing its shape. Materials like cast aluminium or steel might be suitable, depending on the size and complexity of the mold.
The choice of resin or plastic material is also essential. There are two broad categories: thermosets and thermoplastics. Thermosets, such as epoxy, polyamide, polyester, polyurethane, silicone, and vinyl ester, undergo a permanent chemical change when cured, meaning they cannot be reused or returned to their original form. Thermoplastics, on the other hand, can be reused, as excess material can be re-purposed for future production. The choice between these two categories depends on the specific requirements of your project and the desired characteristics of the final product.
Lastly, regulatory compliance is a vital consideration when choosing materials. For instance, if your plastic mold is intended for use with consumable or cosmetic goods, food-grade materials are mandatory. Opt for materials that meet the relevant safety standards to ensure your product's compliance and marketability.
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Surface treatment
Cleaning
First, ensure the mold is thoroughly cleaned. If it's a pre-made mold, wash it under hot water to remove any residue or cornstarch. Dry it with a clean cloth before proceeding.
Coating and Polishing
Apply a release agent to the mold's surface. If using a two-part mold, coat both sides and then reassemble. For superior polishing, diamond compounds offer a comprehensive solution with their varied grades, colors, and base options. This step ensures a precise and polished finish across different materials.
Heat Treatment and Plating
The mold may undergo heat treatment, which involves applying heat to the mold to alter its physical and chemical properties. Plating, another crucial process, involves depositing a layer of material onto the mold's surface to enhance its durability, corrosion resistance, and aesthetic qualities.
Texturing and Engraving
Texturing is the process of creating a specific surface texture on the mold. This can be done to improve the mold's release properties, enhance part functionality, or create a specific aesthetic. Engraving, on the other hand, involves creating indentations or markings on the mold's surface, which can serve functional or decorative purposes.
In-Mold Decoration (IMD)
IMD is a popular surface decoration technology. It involves placing a printed film inside a metal mold before injecting the resin, resulting in a seamless combination of the film and resin in the final product. IMD allows for intricate designs, including decorative graphics, logos, and color patterns, that are resistant to friction and chemical corrosion.
Assembly and Testing
After the surface treatment is complete, the mold undergoes meticulous assembly, where all the components are put together. Finally, thorough testing is conducted to ensure the mold meets the required standards and functions as intended.
The surface treatment process is a critical step in plastic mold fabrication, and the specific treatments applied will depend on the desired outcome, functionality, and aesthetic requirements of the final product.
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Frequently asked questions
Plastic injection moulding involves injecting molten material, typically plastic, into a mould cavity. The molten material is forced into the mould under high pressure, where it cools and solidifies, taking the shape of the mould.
There are several types of plastic moulds, including injection moulds, compression moulds, rotational moulds, blow moulds, and extrusion moulds.
The key considerations when creating a plastic mould are the part design, material selection, budget, and manufacturing process. The size, dimensionality, and complexity of the product will impact the design and material chosen. The budget will dictate the quality of the mould material and whether it is intended for single or multiple units.
Thermoplastics offer the advantage of reusability, as excess material can be repurposed for future production. They are also easier to use since the injection moulding process begins with heating small pellets or powdered resin into a fluid.
