Sofia Martinez
Plastic moulding is a manufacturing process that involves shaping liquid or pliable material using a rigid frame called a mould or matrix. The process was first introduced in the 1950s for doll heads and has since been used to create a wide range of products. Today, there are several plastic moulding techniques available, including injection moulding, blow moulding, rotational moulding, compression moulding, extrusion moulding, and thermoforming. Each technique has its own advantages and disadvantages, and the selection of the appropriate method depends on various factors such as cost, production rate, uniformity, and the complexity of the desired product.
| Characteristics | Values |
|---|---|
| Molding process | Rotational molding, injection molding, blow molding, compression molding, extrusion molding, thermoforming |
| Molding material | Powder or liquid resin, polyethylene, polypropylene, liquid plastics |
| Mold type | Metal, steel |
| Mold shape | Hollow |
| Molding speed | 2-20 RPM |
| Molding temperature | Ideal ejector temperature |
| Molding cost | Relatively low for extrusion molding |
| Molding time | 20-30 days for custom molds |
| Molding design | Custom, according to the shape of the desired product |
| Molding thickness | 8mm |
| Molding weight | 51 grams |
| Molding color | White |
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What You'll Learn
Plastic injection moulding
The first step of the injection moulding process is clamping. Injection moulds are typically made in two, clamshell-style pieces. In the clamping phase, the two metal plates of the mould are pushed up against each other in a machine press. The plastic, usually in the form of granules, pellets or powder, is then melted down into a complete liquid. This liquid is then injected into the mould at high pressure. Manufacturers should ensure that the temperature remains constant throughout this step. In the dwelling phase, the melted plastic fills the entirety of the mould. Pressure is applied directly to the mould to ensure the liquid fills every cavity and the product is an exact replica of the mould. The cooling stage is straightforward; the mould is left alone so the hot plastic inside can cool and solidify into a usable product that can be safely removed from the mould.
The injection moulding machine is separated into two sections: the clamp unit and the injection unit. The clamp unit facilitates the entry and exit of the die and plastic item ejection. There are two types of clamping techniques: the toggling type and the straight-hydraulic method, where a mould is opened and shut using a hydraulic system. The injection unit melts the plastic using heat and then injects the molten material into the mould. The screw is spun to melt the polymer received from the hopper and collect the molten plastic, and the injection procedure begins once enough molten plastic has been collected.
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Rotational moulding
The process was first used in the 1950s to manufacture doll heads, and it has since been used to create other plastic toys, road cones, marine buoys, and car armrests. In the 1960s, the development of the Engel process allowed for the creation of large hollow containers using low-density polyethylene. This process is still commonly used today and has led to the use of rotational moulding in the creation of fuel tanks and industrial mouldings.
The rotational moulding process begins with loading a measured quantity of polymer, usually in powder form, into the mould. The mould is then locked and loaded into the oven, where it is heated at a controlled rate. During the heating phase, the mould is continuously rotated on two perpendicular axes to ensure even distribution of the polymer and to prevent sagging or deformation. Once the polymer has formed the desired shape, the mould continues to rotate to ensure proper formation of the product surfaces and to prevent air bubbles, which could weaken the structure.
Finally, the mould is cooled using air jets, water mist, or water spray, and the finished product is removed. Rotational moulding is a relatively slow process, with only one or two cycles per hour, but it offers several advantages, including low waste, economic and environmental viability, and the ability to create large, hollow parts easily.
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Blow moulding
The first step in blow moulding is plastic feeding, where plastic pellets are fed into the machine via a hopper or screw. Vacuum pumps are used to draw the pellets from large bags and transfer them into silos or hoppers. The plastic resin is then melted by continuous kneading and heating as it passes through the extrusion machine.
The next step is the preparation of the parison or preform, which is a plastic tube with a hole at one end. This is heated to its melting point and placed inside the cavity of the mould. Compressed air is then used to inflate the parison, forcing the plastic to stretch and conform to the contours of the mould. The amount of plastic and air pressure used determines the thickness of the final product.
Once the moulding is complete, the item is cooled, extracted from the mould, and trimmed. The product is then ready for any further processing steps, such as labelling or printing.
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Compression moulding
There are two types of materials that can be used in compression moulding: thermoset plastics and thermoplastics. Thermoset plastics refer to pliable plastics that, once heated and set to a shape, cannot be changed. Examples include polyurethane, unsaturated polyesters, phenolic, and silicones. Thermo plastics, on the other hand, can be reheated and cooled as much as necessary. Common types of thermoplastics include polyethylene, polypropylene, polystyrene, and acrylics.
The compression moulding process begins with setting up the mould. The raw material is then placed into the mould and heated until it becomes pliable, while the mould remains closed for a specified period. The upper and lower halves of the mould then come together to effectively compress the charge. During this compression phase, the process aims to ensure that the charge fills the entire mould cavity to the intended volume and achieves the desired density within the finished product.
After compression, the moulded product is solidified through cooling or the use of hardening agents and catalysts. This is followed by the ejection phase, where a release agent or coating is applied to the mould to prevent sticking and ease removal. The excess material, known as "flashing," is then removed manually or automatically, depending on the size and complexity of the moulded item.
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Thermoforming
The process begins with a sheet of thermoplastic, which is heated until it becomes pliable and mouldable. This can be done using infrared heating in an oven. Once the plastic is soft enough, it is stretched over a single male mould, or placed between male and female moulds, and shaped using a mould of the provided geometry to get the desired object. The moulding process can be done through vacuum forming or pressure forming. In vacuum forming, the air is vacuumed out from under the mould. In pressure forming, compressed air is used to press the moulds against the plastic sheet.
After the plastic has been formed into the desired shape, it is allowed to cool and then trimmed to create a usable product. The quality of the formed object in thermoforming depends on several factors, including heating time, sheet properties, residence time in the mould, and the deformation mechanism.
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Frequently asked questions
Injection moulding is the simplest way to manufacture a plastic-moulded part. This is because it is well-suited for mass manufacturing goods with complex forms and is widely used in the field of plastic processing.
First, design a custom mould according to the shape of the desired finished product. This type of mould is usually made with steel due to its high melting point and chemical stability. Then, put the mould into an injection moulding machine, melt plastic granules and inject them into the mould. Once the molten plastic has been adequately pressed into the mould, allow it to cool until it sets.
Alternatives to injection moulding include blow moulding, rotational moulding, extrusion moulding, compression moulding, and thermoforming.
