Injection moulding
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Injection moulding is a manufacturing technique for making parts from thermoplastic material in production. 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 parts, from the smallest component to entire body panels of cars. Injection moulding is the most common method of production, with some commonly made items including bottle caps and outdoor furniture. Injection molding typically is capable of an IT Grade of about 9-14.
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[edit] Materials
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).
[edit] Equipment
Injection moulding 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 mould 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.
Injection moulding machines can fasten the moulds in either a horizontal or vertical position. The majority is horizontally oriented but vertical machines are used in some niche applications such as insert moulding, allowing the 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 hold the tool in place) and magnetic clamps are also used. The magnetic and hydraulic clamps are used where fast tool changes are required.
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 introduced the first all electric 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, with the exception of Japan.
Robotic arms are often used to remove the moulded components; either by side or top entry, but it is more common for parts to drop out of the mould, through a chute and into a container.
[edit] Mould
Mould (or Tool) is the common term used to describ 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 constructed from hardened steel, pre-hardened steel, aluminum, and/or beryllium-copper alloy. The choice of material to build a mould is primarily one of economics. Steel moulds generally cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel moulds 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 moulds are generally vacuum hardened or age hardened 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). Aluminum moulds can cost substantially less, and when designed and machined with modern computerized equipment, can be economical for moulding hundreds or even tens of thousands of parts. Beryllium copper is used in areas of the mould which require fast heat removal or areas that see the most shear heat generated. High performance alloys such as Moldmax have also been developed especially for optimum heat transfer. Such alloys are considered in mould construction when conventional heat removal methods are unsuitable or when cycle time is a critical consideration.
Considerable thought is put into the design of moulded parts and their moulds, to ensure that the parts will not be trapped in the mould, that the moulds can be completely filled before the molten resin solidifies, to compensate for material shrinkage, and to minimize imperfections in the parts.
[edit] 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 mould. 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 moulds, which may have moveable sections called slides which are inserted into the mould 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 moulded parts to be reinserted to allow a new plastic layer to form around the first part. This is often referred to as overmoulding. This system can allow for production of fully tyred wheels.
2-shot or multi shot moulds are designed to "overmould" within a single moulding cycle and must be processed on specialised injection moulding machines with two or more injection units. This can be achieved by having pairs of identical cores and pairs of different cavities within the mould. 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 mould. Common applications include the "soft-grip" toothbrush, the freelander grab handles are formed this way.
The core and cavity, along with injection and cooling hoses form the mould tool. While large tools are very heavy (up to 60t), they can be hoisted into moulding machines for production and removed when moulding is complete or the tool needs repairing.
A mould can produce several copies of the same parts in a single "shot". The number of "impression" in the mould of that part is referred to as cavitation. A tool with one impression will often be called a single cavity (impression) tool. A mould with 2 or more cavities of the same parts will likely be referred to as multiple cavity (impressions) tooling. Some extremely high production volume moulds (like those for bottle caps) can have over 128 cavities (impressions).
In some cases multiple cavity tooling will mould a series of different parts in the same tool. Some toolmakers call these moulds family moulds as all the parts are not the same but often part of a family of parts (to be used in the same product in example).
[edit] Machining
Moulds are built through two main methods: standard machining and EDM machining. Standard Machining, in its conventional form, has historically been the method of building injection moulds. With technological development, CNC machining became the predominant means of making more complex moulds with more accurate mould details in less time than traditional methods.
The electrical discharge machining (EDM) or spark erosion process has become widely used in mould making. As well as allowing the formation of shapes which are difficult to machine, the process allows pre-hardened moulds to be shaped so that no heat treatment is required. Changes to a hardened mould by conventional drilling and milling normally require annealing to soften the steel, followed by heat treatment to harden it again. EDM is a simple process in which a shaped electrode, usually made of copper or graphite, is very slowly lowered onto the mould surface (over a period of many hours), which is immersed in paraffin oil. A voltage applied between tool and mould causes erosion of the mould surface in the inverse shape of the electrode.
[edit] Cost
The cost of manufacturing moulds will depends on a very large set of factors ranging from number of cavities, size of the parts (and therefore the mould), complexity of the pieces, expected tool longevity, surface finishes and many others.
[edit] Injection process
[edit] Injection Moulding Cycle
The basic injection cycle is as follows: Mould close - injection carriage forward - inject plastic - metering - carriage retract - mould open - eject part(s)
The moulds are closed shut by hydraulics or electric, and the heated plastic is forced by the pressure of the injection screw to take the shape of the mould. Some machines are run by electric motors instead of hydraulics or a combination of both. The water-cooling channels then assist in cooling the mould and the heated plastic solidifies into the part. Improper cooling can result in distorted moulding or one that is burnt. The cycle is completed when the mould opens and the part is ejected with the assistance of ejector pins within the mould.
The resin, or raw material for injection moulding, is usually in pellet or granule form, and is melted by heat and shearing forces shortly before being injected into the mould.Resin pellets are poured into the feed hopper, 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 mould machine. Some moulds are designed so that the part is automatically stripped through action of the mould.
[edit] Moulding trial
When filling a new or unfamiliar mould for the first time, where shot size for that mould is unknown, a technician/tool setter should start with a small shot weight and fill gradually until it 95 to 99% full. Once this is achieved a small amount of holding pressure will be applied and holding time increased until gate freeze off has occurred, then holding pressure is increased until the parts are free of sinks and part weight has been achieved. once the parts are good enough and have passed any spc criteria a setting sheet is produced for people to follow in the future.
[edit] Moulding defects
Injection moulding is a complex technology with possible production problems. They can either be caused by defects in the moulds or more often by part processing (moulding)
| Moulding Defects | Alternative name | Descriptions | Causes |
|---|---|---|---|
| Blister | Blistering / Peeling | Raised or layered zone on surface of the part | Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater |
| Burn Marks | Air Burn | Localised burnt zone (often in the yellow/brown tones) | Tool lacks venting, injection speed is too high |
| Color Streaks | Localised change of color | Masterbatch isn't mixing properly, or the material has run out and it's starting to come through as natural only | |
| Delamination | Thin mica like layers formed in part wall | Contamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond | |
| Flash | Excess material in thin layer exceeding normal part geometry | Tool damage, too much injection speed/material injected | |
| Embedded contaminates | Embedded Particulates | Foreign particle (burnt material or other) embedded in the part | Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection |
| Flow marks | Directionally "off tone" wavy lines or patterns | Injection speeds too slow (the plastic has cooled down too much during injection, injection speeds must be set as fast as you can get away with at all times) | |
| Jetting | Deformed part by turbulent flow of material | Poor tool design | |
| Silver streaks | Circular pattern around gate caused by hot gas | ||
| Sink Marks | Localised depression (In thicker zones) | Holding time/pressure too low, cooling time too low, with sprueless hot runners this can also be caused by the gate temperature being set too high | |
| Short shot | Non-Fill / Short mould | Partial part | Lack of material, injection speeds too slow |
| Splay Marks | Splash mark / Silver Streaks | Circular pattern around gate caused by hot gas | |
| Stringiness | String like remain from previous shot transfer in new shot | Gate hasn't frozen off | |
| Voids | Empty space within part (Air pocket) | Lack of holding pressure (holding pressure is used to pack out the part during the holding time) | |
| Weld line | Knit Line | Discolored line where two flow fronts meet | Mould/material temperatures set too low (the material is cold when they meet, so they don't bond) |
| Warping | Twisting | Distorted part | Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the cool side of the tool) |
[edit] History
In 1868 John Wesley Hyatt became the first to inject hot celluloid into a mould, producing billiard balls. He and his brother Isaiah patented an injection moulding machine that used a plunger in 1872, and the process remained more or less the same until 1946, when James Hendry built the first screw injection moulding machine, revolutionizing the plastics industry. Roughly 95% of all moulding machines now use screws to efficiently heat, mix, and inject plastic into moulds.
[edit] See also
[edit] External links
- A History of Plastics
- European Committee of Machinery Manufacturers for the Plastics and Rubber Industries (EUROMAP) – Technical recommendations
- Injection molding machine states
- Injection moulding know how and formulas
- Injection moulding problems and solutions
- Global thermoplastic and thermoset resin manufacturers
- Injection moulded part cost estimator in Java
- Injection moulding cycle & process description
- IRC in Polymer Science and Technology, University of Bradford, UK
- List of thermoplastics and thermoset materials, showing definitions, properties and applications.
- Injection Moulding Part Cost Estimator
- Injection Moulding Information and Illustrations
