PARTING SURFACE
5.1 Introduction
The parting surface of a mould are those portions of both mould plates adjacent to the impression which butt together to form a seal and prevent the loss of plastic material from the impression. Parting surfaces are classified as flat and non-flat. The non-flat parting surfaces include stepped, profiled, and angled surfaces. If the parting surfaces are not properly matched, the moulding material from the impression will escape through the gap. This escaped melt is called the flash.
5.2 Types of Parting surfaces
5.2.1 Flat parting surface:
The nature of parting surface depends entirely on the shape of the component. Parting line surface must be so chosen that the moulding can be ejected out from the mould.
Straight Stepped Angular
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Consider the moulding shown in figure below. The cavity for this part can be cut in to one mould plate. The position of the parting surface will therefore be at the top of the moulding. Parting surface itself being perfectly flat. For appearance, this is the ideal one as the parting line is not noticeable unless flash develops.
Consider the component in figure given below. In this, the parting line cannot be at top, as this will create an undercut. The only suitable choice for the parting line is on the centre of the double bevel which allows for half of the required form to cut in to each of the two mould halves. So parting surface must be chosen so that the moulding can be removed from the mould without any damage.
Profiled parting surface
An example is shown in the figure below. The component is given left side. It will be noted that while in cross-section, the component form is constant, the general form incorporate curves. As the edge of the component is square with the face, the entire form can be cut into one mould plate. Thus the general form of the parting surface will follow the inside surface of the moulding.
5.2.2 Angled parting surface:
The designer is frequently get problem with a component which, while fairly regular in form, cannot be ejected from the mould if a flat parting surface is adopted. However, by adopting angled parting surface, all parts of the component are in line of draw and it can be ejected.
5.2.3 Complex edge form:
This is implemented when the edge form is not constant. This leads to quite complex parting surfaces. An example is given below. To determine the parting line all we do is draw a number of cross-sections through the brush stock and decide upon the maximum dimension of each when viewed in the draw direction. Parting line will pass round all these points of maximum dimension. Once the parting line has been determined, the moulding’s parting surface can be drawn
5.2.4 Local stepped or profiled parting surface
It is frequently necessary to incorporate a stepped or profiled surface to cater for one or two small irregularities in an otherwise regular form. Normally this is best achieved by localizing the change in parting surface to permit the major portion of the surface to be
kept flat.
5.3 Balancing of mould surfaces:
When the parting surface is not flat, there is problem of unbalanced forces acting in certain instances. This is illustrated in the figure. The figure shows a stepped parting surface. The plastic material when under pressure within the impression will exert a force which will tend to open the mould in the lateral direction. If this happens, some flash occurs on the angled surface. The movement between the two moulds half will be resisted by the guide pillars, but even so, because of the large forces involved, it is desirable to balance the mould by reversing the step so that parting surface continues across the mould as a mirror image of the section which includes the impression. It is often convenient to specify an even number of impression which can be positioned on opposite sides.
Bedding down a parting surface over the entire surface is not practicable because it would be extreme expensive and would also effect the efficient functioning of the mould. Effect of injection pressure and locking force with respect to the area of contact between the two surfaces are given below.
P=F/A
where P = theoretical injection pressure ( Ib./inch)
F = the applied force ( lb. )
A = the area of the injection ram (inch).
The actual pressure exerted with in the impression will be considerably less than the theoretical value for the following reason.
- The melt is non-Newtonian
- The viscosity of the melt progressively increases as it passes through the mould due to cooling.
- The actual pressure within the impression depends on the length of the flow path. That is sprue, runner etc.
In practice, 25% to 450/0 of the value is used. The effective injection pressure is transmitted to the projected area of the impression, the runners and the gates produces a force which tends to open the mould. This tendency to open is registered by a locking force. The clamping force should exceed the opening forCe to safeguard against very high opening force developed due to flash, the parting surface adjacent to the impression and runner is bedded down on Q relatively ~matt are8. The surrounC1ing surfaces are re1ieved. This small area adjacent to the impression and the runner is termed as land.
The corners of the mould are left high in order to with stand the large clamping forces.
5.5 Venting
When plastic material enters the impression, air is displaces. Normally the air can escape between the two mould plates. But if the plates are matched perfectly, the air may be trapped with in the impression resulting in discolouration, sinks, incomplete filling etc. Vents are provided in the mould to allow such air to escape freely.
The vent is a shallow slot not more than 0.05 mm deep and 3mm wide. If the depth is more, the plastic material can pass through the slot and leave a flash mark.
Positions where the vents are required are:
- At the point furthermost from the gate on symmetrical moulding.
- At the point where flow paths are likely to meet and
- At the bottom of a projection.
In the third case, it is necessary to provide the vent through the bottom of the mould plate. This is achieved by incorporating an ejector pin often called vent pin in the required position. The air escapes through the minute gap between ejector pin and mold plate hole.
5.6 Ejection method
The moulding will shrink away from the cavity walls as shown. Thereby permitting a simple ejection technique to be adopted. Example. Jet air. When the moulding has internal form, the moulding as it cools will shrink on to the core and some positive type of ejection is necessary.
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