Sprue design

Concerning the technique of sprue design, it should be noted that the venting and positioning of the tool are of the utmost importance. The basic principle should be to fill the tool from the lower side and turn it into such a position that the reactive mixture travels the shortest possible distance from the position of entry to the farthest point of the mould cavity. In this manner, turbulence and air traps are minimized. The vent slots must be cut at the highest point of the tool. Often the best tool position and vent location must be determined empirically and optimized prior to production. 

Example 4.6 The mould shown in Fig. 4.35 produces four cup shaped ABS mouldings. The depth of the cups is 60 mm, the diameter at the is 90 mm and the wall thickness is 1.0 mm. The distance from the sprue to the cavity is 40 mm and the runner diameter is 6 mm. Calculate the clamp force necessary on the moulding machine and estimate how the clamp force would change if the mould was designed so as to feed the cups through a pin gate in the centre of the base (as illustrated in Fig. 4.38). The clamp pressure data in Fig. 4.42 should be used and the taper on the side of the cups may be ignored. 

A problem had arisen with molded parts from a medical products company. Some injection-molded sheets were exhibiting small translucent areas (gels) within the opaque polymeric material, which had been designated as non-melt areas. One theory for the source of the non-melt was that a contaminant was present, while another postulate was that these areas represented a cold slug coming from the sprue. One sheet was chosen for analysis since the area of non-melt within it was largest. Both the non-melt area as well as a "normal" area were tested. 

The final clamping action forces rubber through the sprue holes leaving a thin (0.13-0.25 mm) transfer pad. The mould is designed with load springs to break open the plates and to help support the top plate and plunger when bottom injection is used. 

Many cast loaded items are filled thru the same hole as that into which the fuze is to be inserted. After casting, the sprue is broken off. Although it is a good plan to design the funnel to form a core for the fuze cavity, the problem of funnel extraction limits this practice to some extent. At best, then, the bottom of the fuze cavity is a rough, broken off surface and, generally, the cavity is not as deep as desired. The boring of fuze cavities to the specified depth and surface finish is a routine operation of production... 

There are several key points in these methods to allow proper success. The pot, and if possible the sprues, must be mold-released to ease demolding. The sprues must be designed so that the remaining pad in the pot can be cut off. The taper of the sprues must be such that they are wider on the part side than the pot side. It is normally best to trim the sprues and material in the vents straight after demolding to ease the later trimming and cleaning of the part. 

Fig. 13.30 Four stages of coinjection molding, (a) Short shot of skin polymer melt (shown in dark shade) is injected into the mold, (b) Injection of core polymer melt until cavity is nearly filled, as shown in (c). (d) Skin polymer is injected again, to purge the core polymer away from the sprue. [Reprinted by permission from Design Center, School of Engineering, Santa Clara University, Santa Clara, CA.]...

The Sesame is designed so that molders can use a smaller runner and sprue, which gives them more control over the amount of plastic and pressure used to form the part itself. A smaller runner and sprue also means less material waste. While screw-and-barrel systems waste as much as 99.7% of the shot material, the Sesame wastes less than 80%. This is particularly important when molding expensive materials like biodegradable plastics, which cost as much as 10 per gram. The Sesame can handle any type of moldable plastic, as well as silicone rubber. Super-small medical parts that have been molded by the machine include ... 

Mold design is a decisive factor for the molding success such as dimensioning and location of the sprue gates, dimensioning of shear edges, flow aids, cooling and ejector techniques, etc. 

The primary advantage of these systems is that all material goes into the part, that is, none is lost to scrap in runners and sprues. For molds designed to run millions of cycles, a tremendous cost savings is realized. The main disadvantage is that the molds are more costly to design and build, and they require a separate temperature-control system. 

A three-plate mold design (Figure 2.10) features a third, movable, plate which contains the cavities, thereby permitting center or offset gating into each cavity for multicavity operation. When the mold is opened, it provides two openings, one for ejection of the molded part and the other for removal of the runner and sprue. 

Structural foam Due to low pressures, significant design Good structural integrity Sprue removal Lower tooling cost Vibration welding, ultrasonic... 

The molding compound flowing from the nozzle enters the mold and is distributed into the cavities. This is the task of the sprue- and numer system (see Figure 4-1). It consists of several sections, that may differ in design, subject to requirement. As the melt leaves the nozzle, it passes through the sprue bush and into the runner, which connects to the gates of the cavities. 

A sp>ecial injection mould has been designed such that samples could be cooled at a known cooling rate and imder a known pressure (Brucato et al., 2002, Brucato et al., 2000, La Carrubba, 2001). This heated mould consists of a conical cavity (the sprue), which is located in the fixed platen of the injection molding machine, coupled to a "diaphragm". The front of the cavity is sealed with a high tensile, high thermal conductivity copp>er-beryllium... 

Shear heating of melt] injection rate too fast/injection pressure too high/gates too small/nozzle orifice too small < 0.8 of sprue bushing/nozzle dirty/sharp corners/injection rate too fast/shutoff nozzle used instead of a general purpose nozzle/improperly designed or defective non-return valve. 

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