Mould design

The mould design is an essential part of product quality. With the correct mould design and an optimised rubber formulation, it is possible to produce products of world class quality. The choice of press makes that task either easier or more difficult, and has to be made on the individual requirements of the mould and the manufacturing environment. 

The flow ratio depends on MFR, processing conditions, and mould features. It is directly proportional to mould thickness, melt temperature, and injection pressure. Its values are important to establish the number and position of feed points. 

The flow properties of a material can also be investigated using a spiral flow test. The melt is injected into a spiral mould cavity. The melt will freeze at a certain distance along the spiral charmel (spiral flow length) depending on the flow properties and cross-sectional thickness of the spiral. The raw material manufacturer usually gives these values as material specifications. 

Venting is necessary to avoid a short shot or burning of material and also to allow easy and quick mould filling at a lower injection pressure. In some cases the location of vents is quite obvious, while in complex cases mould flow simulation packages can predict the location of vents before machining the mould. Thin-walled mouldings require high injection speeds. In such cases the location of the vents is important. 

The proper design of a taper will ensure easy ejection of a product from a mould - see previously. 

Ejectors leave witness marks on mouldings, and hence it is preferable that they operate on sidewalls, ribs, or bosses. They should have sufficient diameters. A stripper plate acts on the entire wall of a part and so distributes the ejection force. Cylindrical ejector pins are most commonly used, although in constricted areas rectangular pins or blade ejectors can be used. For thin-walled articles, air-assisted ejection can be used. 

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Polymers are not as stiff as metals, so sections have to be thicker. The first rule of mould design is to aim for a uniform section throughout the component. During moulding, hot polymer is injected or pressed into the mould. Solidification proceeds from the outside in. 

A further source of stress may arise from incorrect mould design. For example, if the ejector pins are designed in such a way to cause distortion of the mouldings, internal stresses may develop. This will happen if the mould is distorted while the centre is still molten, but cooling, since some molecules will freeze in the distorted position. On recovery by the moulding of its natural shape these molecules will be under stress. 

Many types of component are produced by this method with little difficulty, but, unless very extreme measures of mould design are employed, some components of an intricate nature defy moulding by compression techniques. The reasons are usually the difficulty or cost of preparing a suitable blank shape, the impossibility of ensuring flow of the required amount of rubber into the cavity, or the inability to retain loose metal inserts for bonding in their correct positions. 

In mould design, flow balance avoids warpage. 

The advantages of the LFRT products are offset by requirements to optimize the whole process chain including extruder screw design, processing parameters and mould design, thus needing higher investment and production costs. 

Tearing Bad mould design, low hot tear strength of compound, overcure and careless removal of moulded article. Avoid sharp edges, improve compounding technique, cure at optimum level and use simple devices for removal. 

Excess flash Excessive stock loading, improper mould aligning, low pressure and improper mould design. Loading blanks of correct weight, increase moulding pressure and replace loose dowel pins in the mould. 

Computer techniques have been and are still being further developed to simulate this complex mould-filling process, so that, for a given geometry and given rheological parameters and solidification behaviour, it can be predicted. This procedure results in a considerable improvement of the product quality, since it enables optimisation of the mould design. 

All these defects can be avoided by taking the specific behaviour of the polymer into account when designing the product. A good mould design is also very important,... 

Pye RGW, "Injection Mould Design", Iliffe Books Ltd, London, 1968. 

Detailed guidelines are presented for the correct moulding of TempRite chlorinated PVC industrial moulding compounds. Information is included for the correct selection of equipment and operating conditions, and includes details of melt preparation, mould design, processing, pressures, startup, process upsets, and troubleshooting. 

A knowledge of the mould-filling process in polyurethane processing thus makes it possible to recognize back at the mould design stage areas of the mould that are at risk from air bubbles and to do something about it by... 

The process is very labour intensive, but survives because it is adaptable to many mould designs and is useful for making relatively large mouldings (since there is no size limitation), which would be cost-prohibitive with other processes (such as injection moulding). Advances in processing include... 

Many moulded components are affixed on the rubber-lined surface such as anode sleeves in mercury cells used in caustic soda industry. These moulded components are either made from natural soft or ebonite rubbers or Neoprene rubber compounds. While moulding, the flow characteristics of the rubber compound and shrinkage need to be taken into consideration. Some aspects of mould designs are described next. 

Started in 1981 the CPMD course is designed to train students in injection mould design for plastics. Course duration is one year part-time. Entry level is NTC-2 (National Technical Certificate) which is described later. The student enrolment is 20-25. 

The duration of the polymer option of this course is 2 years. The curriculum is divided into two parts, namely a grounding in fundamentals in the first year followed by specifics of plastics technology and mould design etc., in the second. The subject matter covered is as... 

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