Ethan Clarke
Injection molding is the most popular method for manufacturing large plastic parts. This process involves injecting molten plastic into a mold, where it cools and solidifies to form the desired part. However, creating large plastic parts can be challenging due to the significant tonnage and clamping force required. To overcome this, manufacturers often employ gas-assisted molding, which improves quality, reduces cycle times, and lessens the weight of the final part. Additionally, the wall thickness of the mold is crucial, as thicker walls provide more room for the molten plastic to flow and improve insulation. Other techniques, such as rotational molding and blow molding, are also used to create large plastic parts for specific applications. These methods offer advantages in terms of cost, production speed, and the ability to create large hollow parts.
| Characteristics | Values |
|---|---|
| Popular techniques | Rotational molding, injection molding, blow molding, compression molding, extrusion molding, and thermoforming |
| Most popular technique | Injection molding |
| Injection molding | Requires large tonnage, gas-assisted implementation, and thick wall sections |
| Reaction Injection Molding | Superior for large parts due to low viscosity of component chemicals and strength of polyurethane |
| Blow molding | Used for hollow, thin-walled, custom plastic parts |
| Thermoforming | Uses low pressure and inexpensive materials |
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What You'll Learn
Injection moulding
The process begins by loading resin pellets into a barrel, where the temperature is raised until the resin becomes molten. This molten plastic is then injected into a metal tool through a runner system, which feeds the plastic into the mould cavity. The mould tool defines the shape of the final moulded part. The part then cools down, solidifies, and is ejected from the tool.
There are several variants of the injection moulding process, including multi-shot or 2K moulding (where different materials are injected into the same mould), insert moulding (where metal components are incorporated), structural foam moulding (where the material is foamed to reduce density), and assisted moulding (where gas or water is used to reduce wall thickness).
When creating large plastic parts, there are a few key considerations. Firstly, the tonnage of the machinery needs to be sufficient for the size of the moulded part, typically requiring 500 tons of clamping force or more. Secondly, the wall thickness of the mould is important, with thicker walls providing more room for the molten plastic to flow and better insulation. Finally, gas-assisted moulding can improve the production process by reducing cycle times and the weight of the finished part.
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Gas-assisted moulding
External gas-assisted injection moulding (EGAIM) is particularly effective for high-profile and large surface area plastic parts, especially those requiring detailed textures and superior surface aesthetics. In this process, gas packing on the reverse side of the part prevents front-side shrinkage, eliminating warp and sink marks. Additionally, support ribs and bosses can be moulded into the part, enhancing dimensional stability and allowing for the elimination of evidence of support features on the customer-facing side of the parts.
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Wall thickness
The optimal wall thickness can reduce the amount of plastic needed and the weight of the moulded part. Thicker walls can be replaced with thinner ones, which will also reduce raw material usage. Thinner walls also allow for greater resistance to warping during the cooling process. However, thinner walls may cause flow irregularities, leading to increased shrinkage and warping.
To achieve uniform wall thickness, it is recommended that walls in plastic-moulded parts should be no less than 40-60% that of adjacent walls. The recommended thickness depends on the plastic material, the part's requirements, and factors such as mould flow. For everyday plastic items, a wall thickness of 1-6mm is recommended, with a maximum of 8mm. The most typical thickness is 1.8-3mm, but this varies depending on the kind and size of the plastic element.
When selecting a material, it is important to consider the desired attributes of the final product. For example, will the product need to be heat resistant, or will it need to flex under load? The material should be able to be moulded to the dimensions and geometry needed for the project while still meeting engineering requirements.
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Reaction injection moulding
The two main components of the mixture are typically a polyol (resin) and an isocyanate (hardener), which are precisely metered, thoroughly homogenized, and mixed in a specialized mixing head. The most common RIM processable material is polyurethane (known generally as PU-RIM), but others include polyureas, polyesters, polyphenols, and nylon 6.
The RIM process is superior when creating large parts because of two unique characteristics: the low viscosity of the component chemicals and the innate strength of polyurethane. This allows a part to act as both a structural and aesthetic component. The low viscosity of the RIM's component chemicals allows large moulds to be filled quickly and completely, making it possible to mould a part as a single piece, where other technologies require multiple parts to be moulded and assembled.
RIM is widely used for making parts for many industries, including automotive, electronics, marine, medical, aerospace, and consumer goods. It offers design freedom and flexibility and produces parts with excellent strength-to-weight ratios. RIM parts have excellent cosmetic finishes straight from the mould, though they can also be coated, painted, or textured if needed.
RIM is a fast and cost-efficient way to produce a smaller number of units, up to 1,000 parts. RIM moulds do not have to be built to withstand high pressures, so they are less expensive and faster to make. The processing time of most RIM systems lies between 1 to 3 minutes, with cycle times commonly just 30–60 seconds long.
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Rotational moulding
The process involves a heated mould filled with a measured quantity of polymer, usually in powder form. The mould is then slowly rotated, typically around two perpendicular axes, causing the softened material to disperse and stick to the walls of the mould. This results in a hollow part with an even thickness throughout. To achieve an even thickness, the mould rotates continuously during both the heating and cooling phases to prevent sagging or deformation.
Another benefit of rotational moulding is its feasibility for small-scale production, especially when compared to blow moulding. Rotational moulding is ideal for short runs and rush deliveries as moulds can be swapped quickly, and different colours can be used without the need for purging. Additionally, the uniform thicknesses achieved in rotational moulding make large thin panels possible.
However, one of the limitations of rotational moulding is the long cycle time, with typically only one or two cycles completed per hour. This slow process is due to the low rotational speeds and the need for extended periods of heating and cooling to ensure even thickness and prevent deformation.
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Frequently asked questions
Injection molding is the most popular method for creating large plastic parts. This process involves injecting molten plastic into a mold, where it cools and solidifies to form the desired part.
Injection molding is highly efficient and cost-effective for high production volumes. It can also process a wide range of plastics, including Polypropylene, Polyethylene, ABS, Nylon, Polycarbonate, and Acetal. Fillers and additives can be incorporated to increase strength or improve UV resistance, for example.
Large injection molding projects require a lot of tonnage, which affects the clamping force of the machine. The wall thickness of the mold is also important, as thicker walls provide more room for the molten plastic to flow and improve insulation.
Rotational molding, or rotomolding, is a process that uses constant rotation to create even-walled large hollow plastic products. It is ideal for creating larger parts in lower volumes and can create products up to 25 feet in length. It is also more cost-effective than injection molding as it uses less expensive materials and has lower start-up costs.
Reaction Injection Molding (RIM) is a process that uses the low viscosity of component chemicals and the strength of polyurethane to create large molds quickly and completely. RIM is more cost-effective than other processes as the molds do not need to withstand high pressure.
