INTRODUCTION OF INJECTION MOULDING
Injection molding is a manufacturing technique for making part from plastic material. Molten plastic is injected at higher pressure into a mould which is inverse of the desired shape. The mould is made by mould maker from metal, usually either steel or aluminum, and precision machined to from the features of the desired part. Injection molding is very widely used for manufacturing a variety of parts, like cosmetic,food & medicine packaging etc.
Considerable thought is put into the design of molded parts & there moulds , the ensure that the parts will not be trapped in the mould , that the moulds can be completely filled before the molten resin solidities , & to minimize imperfection in the parts which can occur due to peculiarities of the process.
Moulds separate in to at least two halves called the core & the cavity to permit the part to be extracted ; in general the shape of a part to be extracted; in general the shape of a part must be such that is will not be locked in to the mould . For example, sides of objects typically can not be parallel with the direction of draw the direction in which the core & cavity separate from each other. They are angled slightly; examination of must house hold objects made from plastic will show this accept of design, known as draft. Parts that are ˝ bucket – like ˝ tend to shrink on to the core while cooling & after the cavity is pulled away are typically ejected using pins. More complex parts are formed using more complex moulds , which may require moveable sections which are inserted in to the mould to form particular features that can not be formed using only a core & a cavity , but are than with drawn to allow the part to be released.
The resin, or raw material for injection molding, is usually in pellet from, and is electrically melted shortly before being injected in to the mould . The channels through which the plastic flow towards the chamber will also solidify, forming & 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 , & runners , which are perpendicular to the direction of drawn , & the used to convey molten resin to the gates, or points of injection. The sprue & runner system can be cut off & recycled . Some moulds are design such that it is automatically stripped from the part through action of the mould .
The quality of the moulded part depend on the quality of mould , the care taken during the moulding process, and upon details of the design of the parts it self . It is essential that the molten resin be at just the right pressure and temp. , so that it flow easily to all part of the mould . The part of the mould must also come together extremely precisely , otherwise small leakages of molten plastic can from a phenomenon known as flash . When filling a new or unfamiliar mould for the first time , where shot size for that particular mould is unknown , a technician should reduce the nozzle pressure so that the mould fills , but dose not flash . Then , using that now - known shot volume pressure can be raised with out fear of damaging the mould .
Injection molding process
Injection moulding is the process of forcing melted plastic in to a mould cavity. One injection moulding often used in mass production & prototyping injection mould. The first injection moulding m/cs were built in the 1930 s.
There are six major steps in the injection moulding process:
ADVANTAGES OF INJECTION MOULDING
DISADVANTAGES OF INJECTION MOULDING
The process is used to produce large quantities of identical plastic items. One of the most common types of thermoplastics used in injection moulding is polypropylene.
Injection moulding is the most important plastics manufacturing process. It produces such small products as bottle top; sink plugs, cosmetic products,& pharamaseutical products. It is also used to manufacturer larger items such as buckets & milk crates. Familiar products manufactured by injection moulding include cosmatic products ( like fair & lovely cap, gulabri container cap & base, veet spatulla, himamiwashar cap & food packazing products like bornvita cap, horlicks cap, Jemsball top & bottom etc.
The process can be even mould such large items as 1Kg Cap of Bouranvita & medicines box.
Injection moulding used to be operated by people on the factory floor but these days it is a form of highly automated from of production. The whole process is controlled by a central processing unit.
Granulated or powdered thermoplastic is fed from a hopper in to the injection moulding machine.
The injection moulding machine consists of a hollow steel barrel, containing a rotating screw. The screw carries the plastic along the barrel to the mould. Heaters surround the barrel melt the plastic as it travels along the barrel.
The screw is forced back as the melted plastic collects at the end of the barrel .once enough plastic has collected a hydraulic ram pushes the screw forward injecting the plastic through a sprue in to a mould cavity . The mould is warmed before injecting and the plastic is injected quickly to prevent it from hardening before the mould is full.
Pressure is maintained for a short time (dwell time) to prevent the material creeping back during setting. This prevent shrinkage and hollows, therefore giving a better quality product. The moulding is left to cool before removing (ejected) from the mould. The moulding is takes on the shape of the mould cavity. The image below shows an injection moulding machine.
A mould used for injection moulding may cost thousands of moulding can be produced from a single mould. This results in the low cost of each moulding.
It used to be the case that when a new type of casting was needed a brand new die heads to be made. Modern dies and moulds are made from a number of inter changeable parts. This allows the shape of the mould to be altered.
The dies or moulds used in the process are made from ally stainless steel. These special alloy steel moulds are made in a number of parts to allow the moldings to be easily removed. Plastics that are used in injection moulding include low density polyethylene (LDPE), high density polyethylene, Polypropylene (PP).
Key suppliers of raw material
Grades of material :-
PE PE060,CMC PNT
SPECIFICATION OF MACHINE
1) Ferromatic Omega 550
2) Ferromatic Omega 660
3) Polo 250
4) Windser 50/180
Injection moulding machine has been available as a processing method for big plastic products for about 40 years.
With in last 10 to 15 years there has been dramatic change.
Engineers and designers are taking a fresh interest in the process because of the scope it offers for the production of relatively inexpensive, complex shape with low levels of molded in stress. Material suppliers are continually developing new grades of plastics as well as a wider selection of material suited to the process. In addition, machinery suppliers are producing more sophisticated moulding equipment so that the molder now has control over the process that was thought impossible.
The article of bigger size is to be moulded the heating , cooling time will be rised significantly. Heating little but more time will be while cooling time will be around 6-8 sec. The total time for full cycle will be nearly 18 sec.
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