Maya Ellison
Plastic is moulded into shapes through a variety of processes, including injection moulding, blow moulding, compression moulding, extrusion moulding, and rotational moulding. Injection moulding, the most common method, involves forcing molten plastic into a mould cavity, which solidifies into the shape of the mould. Blow moulding is used to make hollow, thin-walled products with a uniform thickness, such as bottles and drums. Compression moulding involves placing heated plastic into a heated mould and pressing it into the desired shape. Extrusion moulding uses a die instead of a mould to produce long, continuous shapes such as PVC piping and straws. Each process has unique advantages and applications, and careful consideration is required to select the most suitable method for a given product.
| Characteristics | Values |
|---|---|
| Blow molding | Used to make hollow, thin-walled custom plastic parts with uniform wall thickness. The process is similar to glass blowing and takes less than two minutes. |
| Injection molding | The most common modern method of manufacturing plastic parts. It involves using a hydraulic or electric machine to melt and inject plastic into a metal mold. It is versatile, flexible, efficient, and consistent. |
| Compression molding | Used to replace metal parts with plastic parts, especially small parts in very high volume. The automotive industry uses this method as the final products are strong and durable. |
| Extrusion molding | Similar to injection molding but produces a long continuous shape. It is a low-cost method as it uses a "die" instead of a "mold." |
| Overmolding | Allows for the production of one-piece tires and wheels by adding a new plastic layer around a previously molded part. |
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Injection moulding
To begin the injection moulding process, a custom mould is required. Professional machinists craft the "tool" or "tooling mould" based on the product design. This step is the most time-consuming and expensive part of the process, taking up to 20 weeks or more.
Once the mould is ready, plastic thermoplastic pellets are fed into the hopper of the injection moulding machine. The pellets move to the barrel, where a reciprocating screw pulls back to make room for them. The pellets are then melted and injected into the mould.
As the molten plastic fills the mould, it takes on the shape of the mould cavity. The plastic is then cooled and hardened, and the moulded part is ejected. The entire process is very quick, taking less than two minutes to produce a piece.
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Blow moulding
There are three main types of blow moulding: extrusion blow moulding, injection blow moulding, and injection stretch blow moulding. The first blow moulding machine was produced and sold in 1938, but the process did not take off until later when the variety and production rates of plastic products increased. The first mass production of plastic bottles using blow moulding was done in America in 1939. Germany started using this technology later but is currently one of the leading manufacturers of blow moulding machines.
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Compression moulding
The process parameters include moulding time, temperature, and pressure. Typically, a 300-400 ton clamp pressure is used. The typical mould is shaped like a clam shell with the bottom being the mould cavity. The mould press resembles a ladle-filled vertical press used for casting aluminium. Compression moulding uses preforms made by an extruder and a wink cutter, where two blades meet at the centre to cut the extrudate to length. Alternatively, it can use a roller die and die cutter.
The process usually involves creating the moulds, setting up the machine, preparing the charge, inserting the charge, compressing the part, and releasing the part. The moulds are usually made of steel or aluminium and have a cavity, an upper mould, and a lower mould. The charge is typically made of fibre-reinforced resin, silicone, or rubber.
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Extrusion moulding
The process begins by heating the stock material for hot or warm extrusion. It is then loaded into a container in the press, with a dummy block placed behind it. The ram then pushes the material out of the die, and the extrusion is stretched to straighten it. If better properties are required, the material may be heat-treated or cold-worked. The extrusion ratio is defined as the starting cross-sectional area divided by the cross-sectional area of the final extrusion. This ratio can be very large while still producing quality parts.
Hot extrusion is done above the material's recrystallization temperature to prevent work hardening and make it easier to push the material through the die. Most hot extrusions are done on horizontal hydraulic presses, with pressures ranging from 30 to 700 MPa, requiring lubrication. Cold extrusion, on the other hand, is done at room temperature or near room temperature. It offers advantages such as a lack of oxidation, higher strength, closer tolerances, better surface finish, and faster extrusion speeds.
Direct extrusion bioprinting applies external pressure to the outside of the cell-containing bioink, extruding fine fibres and depositing them on the receiving platform. Indirect extrusion bioprinting involves extruding a cell-containing bioprinting material into another matrix material, followed by the removal of the bioprinting material and the solidification of the matrix material.
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Overmoulding
The design process for overmoulding starts with understanding the function of the part. It is important to consider what the part accomplishes, what it will be exposed to, and why it is being overmoulded. For example, if you are adding a rubber-like plastic to a handle, it could be for grip, comfort, or vibration dampening. Other design considerations include wall thickness, gate location, sealing features, and mechanical interlock features. Wall thickness should be consistent throughout the part to create even plastic flow and avoid shrink marks and voids. Gate location refers to the point where plastic enters the part, and it may leave a small mark that should be planned for cosmetic parts. Sealing features are important to obtain clean edges between the substrate and overmould.
There are two primary methods for overmoulding: two-shot moulding and pick-n-place moulding. Two-shot moulding uses a single production mould, while pick-n-place moulding uses two separate moulds. In pick-n-place moulding, a batch of substrate parts is manufactured in one mould and allowed to cool before being placed by hand into a second larger mould, which accommodates the substrate parts and leaves room for the overmoulded material to be injected over the substrate. Pick-n-place moulding uses less complex equipment and simpler moulds than two-shot methods, but it can be slower and may result in reduced chemical bonding between overmoulded materials and cool substrates.
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Frequently asked questions
Plastic injection molding is a manufacturing process that melts and injects plastic into a metal mold to set it into a shape. It is the most widely used method for manufacturing plastic parts.
There are several types of molding processes, including injection molding, blow molding, compression molding, and extrusion molding. Each process has its own advantages and is suitable for different types of products.
Injection molding uses a hydraulic or electric machine to melt and inject plastic into a mold cavity, where it solidifies and takes on the shape of the mold. The mold cavity is a fixed part, while the core is a moving part that helps form the final shape.
Injection molding offers flexibility, efficiency, and consistency. It allows manufacturers to choose the mold design and type of thermoplastic, making it suitable for producing a wide range of components, from simple to complex shapes. It is also efficient for high-volume production and energy efficiency.
