Archive for the Troubleshooting Category

Mold Release in Injection Moulding Mold – By Zafar Kamal         Mold release is affected by a number of factors. Some molding compounds show better mold release than others, but it has been found that these compounds frequently have other disadvantages, such as greater shrinkage and less gloss. It is easy to understand what gloss has to do with the mold release, resins which develop the grainy or frosty surface will release from the mold better than smooth, high-gloss resins. However, even similar materials may vary in mold release properties. This problem may be alleviated by changing the mold design, or one or more of the molding conditions, without effecting the end properties of the molded part. Mold release may be affected if the mold is packed too tightly in an effort to reduce or control shrinkage. Also , a molded piece may stick if the injection time is too long and the part has shrunk on the core, a problem may be alleviated by reducing the injection time. On the other hand, the cycle may be

Part Ejection in Injection Moulding Mold – By Zafar Kamal A very important step in the construction of the injection mold is part ejection. It depends upon the geometry of the part and the material from which it may be molded, and a number of other factors, the ejection system can vary in design and the method of the operation. The method which is commonly used is knockout pin method which contact the molded part at its edges, or a flat area, or both, and are mechanically operated when the mold opens to eject the part. As the part becomes more detailed, the may be limited to contacting the runner system and not the part itself. Sometimes pins have blades or bar to provide more contact area, or the part may be lifted from the core by means of the stripper rings. Some molded part requires very finish outlook therefore, air blast is used to remove the molded part from the cavity or core section of the mold. It is also important to consider the type of mold finish being

Mold Cooling on Injection Moulding – By Zafar Kamal In any consideration of the injection molding process the subject of the mold cooling must not be overlooked. Obviously, if hot polymer is to set up within the mold cavity, the material must be cooled sufficiently to solidify it and allow the molded part to be removed from the mold. This, in essence, is the mold-cooling portion of any cycle, and will vary in time and temperature depending upon the geometry of the part and the choice of thermoplastic material. Mold cooling can be considered as taking place with the mold at any temperature below the inlet temperature of the plastic melt. Most commonly, the mold surface will be maintained at a temperature ranging from 30 to 40 degree F upto 120 to 150 degree F. The successful molder will strive to achieve as uniform temperature across the face of the mold as possible ‘to develop within the mold a uniform shrinkage and thereby reduce the tendency to part warpage. When the injection mold is being designed and constructed, definite consideration

Molding Cycle Time on Injection Moulding – By Zafar Kamal The important thing to remember, while considering the cycle time is that, once the conditions are set into the controlling instruments of the machine, the identical cycle will be repeated accurately for the duration of the production run or until some effective change has been made. In a typical cycle action is started by closing the safety gate at the die area. As it is closed the gate trips two safety limit switches, one for the machine hydraulic system, the other for the overall electrical system. From this point on, machine operation is automatic. Assuming that the machine is equipped with a hydraulic clamp, oil enters a jackram housed within the main ram, causing the movable platen to advance at high speed but under low pressure. Just before the mold halves make contact, a limit switch is tripped, causing oil to be diverted from the jackram to the main cylinder. Platen speed immediately drops off, but pressure increases. As the mold halves make contact, pressure builds up, sensed by the

External Heating of Injection Cylinder – By Zafar Kamal All injection-moulding machines make use of electrical heaters to aid in the plastification of the material being processed. The most common system uses mica heater bands of high wattage. Actually, several of these heaters are used per zone on the injection cylinder and, depending upon the size of the machine, the total wattage available for heating can be quite substantial. Each zone along the length of the barrel is individually control by the thermocouple and a pyrometer. The thermocouple senses the temperature developed within the zone and transmit the information to the controlling pyrometer. When the temperature reaches the set point of the pyrometer, a signal is generated which tells the heater control to cut off the supply of electricity to that set of the heater bands. Likewise, when a given zone is indicated as being below the preset temperature level, a signal calling for a heat is sent back and the power is again applied to the heating bands. There are a number of suitable instruments commonly used sense temperature

– By Zafar Kamal Thermodynamics principle are also valid for plastic materials, which are basic to the chemical structure of each. In the melting of various thermoplastics the quantity of heat required per unit weight may vary significantly depending upon the differences in the heat capacity of the various plastics. Every material has a specific heat, which expresses its ability to absorb or release heat energy. Specific heat is the ratio of heat needed to elevate one pound of material one degree to that for heating water, which has a specific heat of 1, expressed as the number of BTU per pound per degree . Due to complexity in handling specific heat data in calculation of heat input and removal, a mathematical extrapolation, heat content, may be used instead. The crystalline melting points for some materials can be seen by the change in the slope of the curves, making it obvious that a great quantity of heat is required to change the plastic’s state from solid to liquid with the little change in temperature. The more crystalline materials require a

Sprues of Plastic Injection Molds The sprue is the channel along which the molten plastic first enters the mould. It delivers the melt from the nozzle to the runner system. The sprue is incoporated in a hardened steel bush which has a seat designed to provide a good seal with the nozzle. Since it is important that the sprue is pulled out when the mould opens and there is a sprue pulling device mounted directly opposite the sprue entry. This can take many forms but typically it would be an undercut or reversed taper to provide a key for the plastic on the moving half of the mould. Since the sprue, like the runner system, is effectively waste it should not be made excessively long.

Runners of Injection Molds The runner is the flow path by which the molten plastic travels from the sprue (i.e. the moulding machine) to the gates (i.e. the cavity). To prevent the runner freezing off prematurely, its surface area should be small so as to minimize heat transfer to the mould. However, the cross sectional area of the runner should be large so that it presents little resistance to the flow of the plastic but not so large that the cycle time needs to be extended to allow the runner to solidify for ejection. A good indication of the efficiency of a runner is the ratio of its crosssectional area to its surface area.

Different Features of Injection Molds Gates: The gate is a small orifice which connects the runner to the cavity. It has a number of functions. It provides a convenient weak link by which the moulding can be broken off from the runner system. In some moulds the degating may be automatic when the mould opens. The gate also acts like a valve in that it allows molten plastic to fill the mould but being small it usually freezes off first. The cavity is thus sealed off from the runner system which prevents material being sucked out of the cavity during screw-back. Small gates are preferable because no finishing is required if the moulding is separated cleanly from the runner. So for the initial trials on a mould the gates are made as small as possible and are only opened up if there are mould filling problems. In a multi-cavity mould it is not always possible to arrange for the runner length to each cavity to be the same. This means that cavities close to the sprue would be filled quickly

Details of the Process for Injection Molding Machines The earliest injection moulding machines were of the plunger type and there are still many of these machines in use today. A pre-determined quantity of moulding material drops from the feed hopper into the barrel. The plunger then conveys the material along the barrel where it is heated by the conduction from the external heaters. The material is then plasticised under pressure so that it may be forced through the nozzle into the mould cavity. In order to split up the mass of the material in the barrel and improve the heat transfer, a torpedo is fitted in the barrel . But there are few disadvantages in this type of machine i.e. it is difficult to produce consistent mouldings. The main problems are: 1. There is little mixing or homogenization of the molten plastic. 2. It is difficult to meter accurately the shot size. Since metering is on a volume basis, any variation in the density of the material will alter the shot weight. 3. Since the plunger is compressing material which

Injection molding Processing of Plastic One of the most common processing methods for plastics is injection moulding. Nowadays wide range of articles are made by means of injection molding. These include such things as electric drill casing, television housings, gearwheels etc. The original injection moulding machines are based on the pressure die casting techniques for casting metals. The first machine is reported to have been patented in the United States in 1872, specifically for use with celluloid. this was an important invention but probably before its time because in the following years very few developments in injection moulding processes were reported and it was not until the 1920s, in Germany, that a renewed interest was taken in the process. The first German machines were very simple pieces of equipment and relied totally on manual operation. In the late 1930s the next major development in injection molding , i.e. the introduction of the hydraulically operated machines get available. However, these machines still tended to be hybrids based on the die casting technology and the design of injection moulding machines for plastic

Extrusion Processing of Plastic One of the most common methods of processing plastics is Extrusion using a screw inside the barrel. The plastic, usually in the form of granules or powder, is fed from the hopper on to the screw. It is then conveyed along the barrel where it is heated by conduction from the barrel heaters and shear due to its movement along the length of the screw flights. The depth of the screw channel is reduced along the length of the screw so as to compact the material. At the end of the extruder the melt passes through a die to produce an extrudate of the desired shape.