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Plastic Injection Moulding

Plastic Technical Information / Plastic Injection Moulding

Plastic Injection Moulding

Injection moulding is a manufacturing process for making parts from thermoplastic and thermosetting plastic materials using plastic injection moulding machines. Molten plastic is injected at high pressure into a mould , which is the inverse of the product's shape. After a product is designed by an Industrial Designer or an Engineer, moulds are made by a mouldmaker or toolmaker from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection moulding is widely used for manufacturing a variety of plastic products and parts, from the smallest component to entire body panels of cars. Injection molding is the most common method of production, with some commonly made items including bottle caps and outdoor furniture.

Plastic Materials Used:

The most commonly used thermoplastic materials are polystyrene (low cost, lacking the strength and longevity of other materials), ABS or acrylonitrile butadiene styrene (a co-polymer or mixture of compounds used for everything from Lego parts to electronics housings), nylon (chemically resistant, heat resistant, tough and flexible - used for combs), polypropylene (tough and flexible - used for containers), polyethylene, and polyvinyl chloride or PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser). Injection moulding can also be used to manufacture parts from aluminium or brass. The melting points of these metals are much higher than those of plastics; this makes for substantially shorter mould lifetimes despite the use of specialized steels. Nonetheless, the costs compare quite favorably to sand casting, particularly for smaller parts.

Equipment Used:

Injection molding machines, also known as presses, hold the moulds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can generate. This pressure keeps the mold closed during the injection process. Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations.

Plastic Injection Mould

Injection Mould (Tool and mould is the common term used to describe the production tooling used to produce plastic parts in injection moulding. Traditionally, moulds have been expensive to manufacture. They were usually only used in mass production where thousands of parts were being produced. Moulds are typically manufactured from hardened steel, pre-hardened steel, aluminium, and/or beryllium-copper alloy. The choice of material to build a mould is primarily one of economics. Steel moulds generally cost more to manufacture, but their longer lifespan will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel molds are less wear resistant and are used for lower volume requirements or larger components. The steel hardness is typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining. These are by far the superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC). Aluminium molds can cost substantially less, and when designed and machined with modern computerized equipment, can be economical for molding tens or even hundreds of thousands of parts. Beryllium copper is used in areas of the mold which require fast heat removal or areas that see the most shear heat generated. High performance alloys such as MoldMax® and Ampcoloy® have also been developed especially for optimum heat transfer. Such alloys are considered in mold construction when conventional heat removal methods are unsuitable or when cycle time is a critical consideration. Considerable thought is put into the design of molded parts and their molds, to ensure that the parts will not be trapped in the mold, that the molds can be completely filled before the molten resin solidifies, to compensate for material shrinkage, and to minimize imperfections in the parts.

Mould Design

Moulds separate into at least two halves (called the core and the cavity) to permit the part to be extracted. In general the shape of a part must not cause it to be locked into the mold. For example, sides of objects typically cannot be parallel with the direction of draw (the direction in which the core and cavity separate from each other). They are angled slightly (draft), and examination of most plastic household objects will reveal this. Parts that are "bucket-like" tend to shrink onto the core while cooling, and after the cavity is pulled away. Pins are the most popular method of removal from the core, but air ejection, and stripper plates can also be used depending on the application. Most ejection plates are found on the moving half of the tool, but they can be placed on the fixed half. More complex parts are formed using more complex molds, which may have movable sections called slides which are inserted into the mold to form features that cannot be formed using only a core and a cavity. Slides are then withdrawn to allow the part to be released. Some moulds allow previously molded parts to be reinserted to allow a new plastic layer to form around the first part. This is often referred to as overmolding. This system can allow for production of one piece tires and wheels. 2-shot or multi shot molds are designed to "overmold" within a single molding cycle and must be processed on specialized injection molding machines with two or more injection units. This can be achieved by having pairs of identical cores and pairs of different cavities within the mold. After injection of the first material, the component is rotated on the core from the one cavity to another. The second cavity differs from the first in that the detail for the second material is included. The second material is then injected into the additional cavity detail before the completed part is ejected from the mold. Common applications include "soft-grip" toothbrushes and freelander grab handles. The core and cavity, along with injection and cooling hoses form the mold tool. While large tools are very heavy (up to 60t), they can be hoisted into molding machines for production and removed when molding is complete or the tool needs repairing. A mold can produce several copies of the same parts in a single "shot". The number of "impressions" in the mold of that part is referred to as cavitation. A tool with one impression will often be called a single cavity (impression) tool. A mold with 2 or more cavities of the same parts will likely be referred to as multiple cavity tooling. Some extremely high production volume molds (like those for bottle caps) can have over 128 cavities. In some cases multiple cavity tooling will mold a series of different parts in the same tool. Some toolmakers call these molds family molds as all the parts are not the same but often part of a family of parts (to be used in the same product for example).

Plastic Injection Moulding Process

The plastic raw material for injection moulding, is usually in pellet form, and is melted by heat and shearing forces shortly before being injected into the mould. Plastic pellets are poured into the hopper loader, a large open bottomed container, which feeds the granules down to the screw. The screw is rotated by a motor, feeding pellets up the screw's grooves. The depth of the screw flights decreases towards the end of the screw nearest the mould, compressing the heated plastic. As the screw rotates, the pellets are moved forward in the screw and they undergo extreme pressure and friction which generates most of the heat needed to melt the pellets. Heaters on either side of the screw assist in the heating and temperature control during the melting process. The channels through which the plastic flows toward the chamber will also solidify, forming an attached frame. This frame is composed of the sprue, which is the main channel from the reservoir of molten resin, parallel with the direction of draw, and runners, which are perpendicular to the direction of draw, and are used to convey molten resin to the gate(s), or point(s) of injection. The sprue and runner system can be cut or twisted off and recycled, sometimes being granulated next to the moulding machine. Some moulds are designed so that the part is automatically stripped through action of the mould.

Injection Moulding Machines

Plastic injection moulding machine - a machine for making plastic parts. Manufacturing products by injection moulding process. Consist of two main parts, an injection unit and a clamping unit. Injection moulding machines can fasten the moulds in either a horizontal or vertical position. The majority are horizontally oriented but vertical injection moulding machines are used in some niche applications such as insert moulding, allowing the plastic injection moulding machine to take advantage of gravity. There are many ways to fasten the tools to the platens, the most common being manual clamps (both halves are bolted to the platens); however hydraulic clamps and magnetic clamps are also used. The magnetic and hydraulic clamps are used where fast tool changes are required.

Types of injection moulding machines
Plastic injection moulding machines / plastic moulding machines are classified primarily by the type of driving systems they use: hydraulic, electric, or hybrid. Hydraulic presses have historically been the only option available to moulders until Nissei Plastic Industrial Co., LTD introduced the first all-electric injection moulding machine in 1983. The electric press, also known as Electric Machine Technology (EMT), reduces operation costs by cutting energy consumption and also addresses some of the environmental concerns surrounding the hydraulic press. Electric presses have been shown to be quieter, faster, and have a higher accuracy, however the machines are more expensive. Hybrid injection moulding machines take advantage of the best features of both hydraulic and electric systems. Hydraulic machines are the predominant type in most of the world. Hydraulic Injection Moulding Machines, Electric Injection Moulding Machines, Toggle Injection Moulding Machines.

Injection moulding machine - a machine for making plastic parts. Manufacturing products by injection moulding process. Consist of two main parts, an injection unit and a clamping unit. Injection moulding machines can fasten the moulds in either a horizontal or vertical position. The majority is horizontally oriented but vertical injection moulding machines are used in some niche applications such as insert moulding, allowing the plastic injection moulding machine to take advantage of gravity. There are many ways to fasten the tools to the platens, the most common being manual clamps (both halves are bolted to the platens); however hydraulic clamps (chocks are used to hold the tool in place) and magnetic clamps are also used. The magnetic and hydraulic clamps are used where fast tool changes are required.