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Encyclopedia > Injection molding

Injection molding (British variant spelling: moulding) is a manufacturing technique for making parts from both thermoplastic and thermosetting plastic materials in production. Molten plastic is injected at high pressure into a mold (British variant spelling: mould), which is the inverse of the product's shape. After a product is designed by an Industrial Designer or an Engineer, molds are made by a moldmaker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection molding is widely used for manufacturing a variety of 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. Injection molding typically is capable of an IT Grade of about 9-14. Manufacturing (from Latin manu factura, making by hand) is the use of tools and labor to make things for use or sale. ... This article is about pressure in the physical sciences. ... One half of a bronze mold for casting a socketed spear head dated to the period 1400-1000 BC. There are no known parallels for this mold. ... One half of a bronze mold for casting a socketed spear head dated to the period 1400-1000 BC. There are no known parallels for this mold. ... Industrial design is an applied art whereby the aesthetics and usability of products may be improved for marketability and production. ... Look up engineer in Wiktionary, the free dictionary. ... Moldmaker (mouldmaker) is the designation for a profession in the metalworking industry. ... Moldmaker (mouldmaker) is the designation for a profession in the metalworking industry. ... For other uses, see Steel (disambiguation). ... Aluminum redirects here. ... “Car” and “Cars” redirect here. ... IT Grade refers to the International Tolerance Grade of an industrial process. ...

Standard two plates tooling - Core and Cavity are inserts in a mold base - "Family mold" of 5 different parts

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). Image File history File linksMetadata DSC05440. ... Image File history File linksMetadata DSC05440. ... Monomers in ABS polymer Acrylonitrile butadiene styrene, or ABS, (chemical formula (C8H8· C4H6·C3H3N)n is a common thermoplastic used to make light, rigid, molded products such as piping, golf club heads (used for its good shock absorbance), automotive body parts, wheel covers, enclosures, protective head gear, and toys including... For other uses of this word, see nylon (disambiguation). ... Polypropylene lid of a Tic Tacs box, with a living hinge and the resin identification code under its flap Micrograph of polypropylene Polypropylene or polypropene (PP) is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including food packaging, ropes, textiles, stationery, plastic... This article does not cite any references or sources. ... Polyvinyl chloride Polyvinyl chloride, (IUPAC Polychloroethene) commonly abbreviated PVC, is a widely used thermoplastic polymer. ... Extruded aluminium; slots allow bars to be joined with special connectors. ... Plasticizers are plastic additives, most commonly phthalates, that give plastics flexibility and durability. ...


Injection molding 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 mold lifetimes despite the use of specialized steels. Nonetheless, the costs compare quite favorably to sand casting, particularly for smaller parts. Aluminum redirects here. ... Brazen redirects here. ... Casting is the process of production of objects by pouring molten material into a cavity called a mould which is the negative of the object, and allowing it to cool and solidify. ...

Contents

Equipment

Paper clip mold opened in molding machine; the nozzle is visible at right
Paper clip mold opened in molding machine; the nozzle is visible at right

Injection molding machines, also known as presses, hold the molds 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. Image File history File links Download high resolution version (1869x1344, 297 KB) Summary Photograph taken by Glenn McKechnie, September 2005. ... Image File history File links Download high resolution version (1869x1344, 297 KB) Summary Photograph taken by Glenn McKechnie, September 2005. ... Paper clip mold opened in molding machine; the nozzle is visible at right Injection molding machine (also known as injection press) - a machine for making plastic parts. ...


Mold

Mold (Tool and/or Mold) is the common term used to describe the production tooling used to produce plastic parts in injection molding.


Traditionally, molds have been expensive to manufacture. They were usually only used in mass production where thousands of parts were being produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminium, and/or beryllium-copper alloy. The choice of material to build a mold is primarily one of economics. Steel molds 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 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. A thermoplastic is a plastic that softens when heated and hardens again when cooled. ...


Design

Molds 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. This article is about angles in geometry. ...


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 molds 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).


Machining

Molds 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 molds. With technological development, CNC machining became the predominant means of making more complex molds with more accurate mold details in less time than traditionalben wilson methods. Electrical Discharge Machine Electrical discharge machining (or EDM) is a machining method primarily used for hard metals or those that would be impossible to machine with traditional techniques. ... A lathe is a common tool used in machining. ... For other uses, see CNC (disambiguation). ... A lathe is a common tool used in machining. ...


The electrical discharge machining (EDM) or spark erosion process has become widely used in mold making. As well as allowing the formation of shapes which are difficult to machine, the process allows pre-hardened molds to be shaped so that no heat treatment is required. Changes to a hardened mold 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 mold surface (over a period of many hours), which is immersed in paraffin oil. A voltage applied between tool and mold causes erosion of the mold surface in the inverse shape of the electrode. Electrical Discharge Machine Electrical discharge machining (or EDM) is a machining method primarily used for hard metals or those that would be impossible to machine with traditional techniques. ... Spark erosion is a machining process that is particularly used in the making of plastics moulding tools. ...


Cost

The cost of manufacturing molds depends on a very large set of factors ranging from number of cavities, size of the parts (and therefore the mold), complexity of the pieces, expected tool longevity, surface finishes and many others.


Injection process

Small injection molder showing hopper, nozzle and die area
Small injection molder showing hopper, nozzle and die area

Image File history File links Download high resolution version (2016x1434, 443 KB) Summary Photograph taken by Glenn McKechnie, September 2005. ... Image File history File links Download high resolution version (2016x1434, 443 KB) Summary Photograph taken by Glenn McKechnie, September 2005. ...

Injection Molding Cycle

The basic injection cycle is as follows: Mold close - injection carriage forward - inject plastic - metering - carriage retract - mold open - eject part(s)


The molds are closed shut, and the heated plastic is forced by the pressure of the injection screw to take the shape of the mold. Some machines are run by electric motors instead of hydraulics or a combination of both. The water-cooling channels then assist in cooling the mold and the heated plastic solidifies into the part. Improper cooling can result in distorted molding or one that is burnt. The cycle is completed when the mold opens and the part is ejected with the assistance of ejector pins within the mold.


The resin, or raw material for injection molding, is usually in pellet or granule form, and is melted by heat and shearing forces shortly before being injected into the mold. 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 mold, 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. From left to right, flat, round nose, hollow point and pointed pellets. ...


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 mold machine. Some molds are designed so that the part is automatically stripped through action of the mold. For other uses, see Solid (disambiguation). ...


Molding trial

When filling a new or unfamiliar mold for the first time, where shot size for that mold is unknown, a technician/tool setter usually starts with a small shot weight and fills gradually until the mold is 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 specific criteria, a setting sheet is produced for people to follow in the future.


Process optimization is done using the following methods. Injection speeds are usually determined by performing viscosity curves. Process windows are performed varying the melt temperatures and holding pressures. Pressure drop studies are done to check if the machine has enough pressure to move the screw at the set rate. Gate seal or gate freeze studies are done to optimize the holding time. A cooling time study is done to optimize the cooling time.


Molding defects

Injection molding is a complex technology with possible production problems. They can either be caused by defects in the molds or more often by part processing (molding)

Molding Defects Alternative name Descriptions Causes
Blister Blistering 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/ Gas Burn Black or brown burnt areas on the part located at furthest points from gate Tool lacks venting, injection speed is too high
Color Streaks Localized 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 Burrs Excess material in thin layer exceeding normal part geometry Tool damage, too much injection speed/material injected, clamping force too low
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, gate position or runner. Injection speed set too high.
Silver streaks Circular pattern around gate caused by hot gas Moisture in the material, usually when hydroscopic resins are dried improperly
Sink Marks Localized 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 mold Partial part Lack of material, injection speed or pressure too low
Splay Marks Splash mark / Silver Streaks Circular pattern around gate caused by hot gas Caused by the material (plastic) being damped prior to injection
Stringiness Stringing String like remain from previous shot transfer in new shot Nozzle temperature too high. 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). Also mold may be out of registration (when the two halves don't center properly and part walls are not the same thickness).
Weld line Knit Line / Meld Line Discolored line where two flow fronts meet Mold/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)

Delamination is a mode of failure of laminated composite materials. ... Molding flash is excess material attached to a molded product, which must be removed. ... Look up void in Wiktionary, the free dictionary. ...

History

In 1868 John Wesley Hyatt became the first to inject hot celluloid into a mold, producing billiard balls. He and his brother Isaiah patented an injection molding machine that used a plunger in 1872, and the process remained more or less the same until 1946, when James Hendry [citation needed] built the first screw injection molding machine, revolutionizing the plastics industry. Roughly 95% of all molding machines now use screws to efficiently heat, mix, and inject plastic into molds. John Wesley Hyatt (November 28, 1837 – 1920) was a U.S. inventor. ... Celluloid is the name of a class of compounds created from nitrocellulose and camphor, plus dyes and other agents, generally regarded to be the first thermoplastic. ... A close-up picture of pool balls // US Billiard balls In the US, Billiard balls are balls used to play the game of US billiards. ... James Hendry (September 25, 1885 – September 9, 1945) was Regius Professor of Midwifery at the University of Glasgow from 1943 until his death in 1945. ...


See also

  • Reaction Injection Molding, a similar technique to standard injection molding, enables the use of thermoset polymers to produce large and complex parts.

Reaction injection molding OR RIM Molding is similar to injection molding except that a reaction occurs within the mold. ...

External links


  Results from FactBites:
 
Injection molding - definition of Injection molding in Encyclopedia (789 words)
Injection molding is very widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars.
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, and to minimize imperfections in the parts, which can occur due to peculiarities of the process.
Molds 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 be such that it will not be locked into the mold.
  More results at FactBites »

 
 

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