Runner, flat

An injection moulding is in the form of a flat sheet 100 mm square and 4 nun thick. The melt temperature is 230°C, the mould temperature is 30°C and the plastic may be ejected from the mould at a centre-line temperature of %°C. If the runner design criterion is that it should be ejectable at the same instant as the moulding, eshmate the required runner diameter. The thermal diffusivity of the melt is 1 x 10 m /s. 

Filling a Two-dimensional Flat Mould Through a Point Gate (Runner) 105... 

Filling a Flat Mold through a Slit Gate (Runner)... 

For q(t) determination, now consider the equation of flow rate (mass balance) through the entrance (runner) channel corresponding to the Poiseuille formula for a flat channel ... 

Fig. 11a, b. Kinetic identification of the first stage of filling a flat mould through a round (point) runner a flow is purely radial during some limited time interval, flow front is approximately circular b the radius of free boundary becomes equal to half-width of the forming cavity R(t) = B/2... 

To complete this set of equations, let s use Eq. (4.41) for the flow rate through a flat runner channel where one has to put ... 

It should be kept in mind that two ultimate situations of filling through a short and long runner may be encountered in practice. Both cases were analysed in detail 36 53-54>. For the case of a short flat runner, the following expression for the coordinate of the flow front was obtained ... 

For the case of a long flat runner, the equation for the filling front h(t) has the form 36) ... 

At this stage, one has to take into account the volume compressibility of the material, since upon feed-up the hold-on time of material under pressure is determined by compressibility and slow viscous flow. If the pressure of injection PQ is sufficiently high, then at this stage a liquid may be considered to be Newtonian with viscosity q ,. Keeping this in mind, we may state that the calculation given below will be applicable to various plastisols (of types I and II) with the only difference that for plastisol I q = const, while for plastisol II q = q. For the sake of simplicity, the analysis will be performed for the case of a flat mould filled through a slit runner (Fig. 10 a). 

Fig. 12. Schematic diagram of the experimental set-up for plastisol moulding into thin flat moulds 1 — loading of raw materials 2 — damper 3 — pump unit 4 — plunger 5 — discharge tube 6 — inlet channel (round runner) 7 — metallic bottom half mould 8 — top half mould (of lucite for low-temperature moulding and of steel for moulding into hot mould) 9 — pressure gauge 10 — stub tube for control 11 — drain cock 12 — conical flat tube instead of round point runner bolts for controlling the width of forming cavity within the limits of 0.15-1.5 mm...

ATTRITION MILIS. In an attrition mill particles of soft solids are rubbed between the grooved flat faces of rotating circular disks. The axis of the disks is usually horizontal, sometimes vertical. In a single-runner mill one disk is stationary and... 

The cross-sectional shape of the runners is not critical provided that the cross-sectional area of the runner is not substantially greater than the equivalent round section. Trapezoidal or modified trapezoidal sections have a great deal to offer in terms of the control possible for the position of the runner when the mould opens, when they stay with the greatest surface area in contact with the mould. Flat section runners generate high-pressure differentials and engender the risk of scorch. 

A tab gate is an extension of the part that connects with the runner, thereby locating shear stresses outside the part. For this reason, tab gates are used with flat, thin parts and for materials such as polycarbonate, acrylics, SAN, and ABS, which develop high levels of shear stress during molding. 

A film (or flash) gate permits uniform filling of flat parts. The melt fills a runner and then crosses a wide, but thin, land to enter the cavity. Pin gates are very fine-diameter (typically 0.25-1.6 mm) entrances to the cavity. They are used in three-plate molds or hot ruimer molds to facilitate degating. 

To obtain the requisite tolerances runner surfaces are diamond ground and lapped flat to within 10 helium light bands and checked using a large oj tical flat. The land areas of the discs are finishing likewise. Taper areas, only 25 to 3 m depth, are ground and checked on a Leitz three-dimensional co-ordinate measuring machine. 

A fluid flow thrust bearing usually consists of two flat surfaces having relative motion and separated by a narrow gap filled with lubricant. Normally one of the surfaces (the pad) is stationary whilst the other (the runner) draws fluid into the lubricating film which has a thickness of the order of 25 pm. 

Besides spot welding, ultrasonics can be used to insert metals into plastic, stake materials to plastics, scan weld flat parts, and degate runner assemblies. 

The simple cold runner nozzle with hydraulic needle sealing system, as shown in Figure 7.11 makes possible either direct or indirect injection of silicone parts. Advantages in the application of this system are good thermal separation between the cold nozzle and the heated mould and the flat gate location on the moulded part. 

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