Barrel injection pressure

In ntosl respects the process is similar to the injection moulding of thermoplastics and the sequence of operations in a single cycle is as described earlier. For thermosets a special barrel and screw are used. The screw is of approximately constant depth over its whole length and there is no check value which might cause material blockages (see Fig. 4.50). The barrel is only kept warm (80-110°C) rather than very hot as with thermoplastics because the material must not cure in this section of the machine. Also, the increased viscosity of the thermosetting materials means that higher screw torques and injection pressures (up to 200 MN/m are needed). 

With PBAs the compressed gases often used are nitrogen or carbon dioxide. These gases are injected into a plastic melt in the screw barrel under pressure (higher than the melt pressure) and form a cellular structure when the melt is released to atmospheric pressure or low pressure. The volatile liquids are usually aliphatic hydrocarbons, which may be halogenated, and include materials such as carbon dioxide, pentane, hexane, methyl chloride, etc. Polychlorofluoro-carbons were formerly used but they have now been phased out due to environment problems. 

High-pressure diesel fuel pumps are not tolerant of dirt, debris, and organic deposits and can be seriously damaged by fuel contaminants. The barrel and plunger clearance is within a 1 to 2 micron tolerance. This tolerance is necessary to ensure that fuel injection pressures are maintained with minimal leakage past the plunger shaft. For this reason, thorough and complete filtration of fuel is required before the fuel reaches the injection pump. 

To increase shot size upsizing the plasticator can be made. A number of items have to be considered for the upsizing process, such as barrel wall thickness, resultant screw L/D, injection speed reduction, screw drive torque, and injection pressure drop. Before considering the upsizing process, one has to determine whether the output can be met properly using the decreased pressure and speeds that occur. The pressure and speeds will decrease directly proportional to the difference in the barrel ID projected areas. If this poses no problems, the L/D and structural integrity of the barrel have to be considered before proceeding. 

Injection pressure in the barrel can range at least from 2,000 to some plastics up to 45,000 psi (14 to 310 MPa). The characteristic of the plastic being processed defines what pressure is required in the mold to obtain acceptable products. Based on what cavity pressure is required, the barrel pressure has to be high enough to meet pressure flow restrictions going from the plasticator into the mold cavity(s). 

Injection Molding The processing and equipment for injection molding of thermosetting polymers differs slightly from the conventional injection machines used for thermoplastics. Reasons are (i) the thermosetting formulation must have a short residence time in the barrel or nozzle and (ii) the injection pressure is reduced after the mold is filled. 

The size of injection molding machines is usually specified in terms of injection capacity and clamp force. These factors are not directly related. Other important variables are the L D (length to diameter) ratio, barrel size, plasticizing rate, injection rate, and injection pressure. Major components of injection molding machines include the injection unit, the clamp unit, and the machine base which contains the power and control units. 

Transparency Cloudiness or haze for clear plastics [contamination] /moist feed/ jmelt too cold] /faulty adjustment of barrel temperature profile/injection pressure too low/back pressure too low/[mold too cold]. Bubbles in dear plastics moist feed/ melt too hot] /injection pressure too low/injection rate too fast/injection hold time too short/booster time too low/[mold too cold] /mold cooling time too short/jcooled too fast]. ... 

Mold conditions wrongfr [mold temperature non-uniform] /injection pressure low/injection forward time too short/injection boost time too short/[barrel too hot] /[nozzle too hot] /inconsistent control of cycle. 

Fig. 2-15. (a) Injection stock temperature vs. injection ram speed, barrel temperature, screw back pressure, injection pressure, nozzle diameter, and screw speed, (b) Injection time vs. injection pressure, barrel temperature, and nozzle diameter. 

When a molding machine is purchased, then the buyer is offered a choice of screw sizes three are usually available and these may be referred to as A, B or C. They have different screw diameters and allow an appropriate match of shot size (the feed system volume plus the molding volume) to barrel capacity. However, because they have different screw diameters, not only will the shot capacity vary but so will other things, for example, injection pressure and plasticizing capacity. Normally t e screws have the lowest shot capacity, and the highest injection pressure, whereas C type screws have the highest shot capacity and lowest injection pressure. 

Barrel temperature in injection molding should gradually increase from the hopper to the nozzle. It should be controlled according to the injection molding machine, shape and size of the product, and the mold structure. Injection pressure should be controlled to mold friU parts with acceptable appearance. Many parameters affect injection pressure, which include injection temperature, product shape, nozzle and gate size, runner dimensions, and mold temperature. It is important that injection pressure should drop off to holding pressure immediately after filling up. 

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