Parting surfaces of injection mould

General

The parting surfaces of a mould are those portion of both mould plates,adjacent to the impressions,which butt together to form a seal and prevent the loss of plastic material from the impression.

We can classify the parring as being either flat or mom-flat.Examples of the latter type include stepped,profiled and angled parting surfaces.

In gerenal,the flat parting surface is the simplest to manufacture and maintain.It can be surface-ground,and is easily bedded down.

To bed down a pair of mould plates is the process of marrying the two mould surfaces together.This is accomplished by blueing one surface,momentarily bringing the two plates together and subsequently removing andy high spots which will be apparent on the non-blued surface.The plates are said to be bedded down when an even film of blue is transferred fron one plate to the other.

Flat parting surface

The mature of the parting surface depends entirely on the shape of the component for instance,consider the rectangular moulding shown in figure 5.1.The cavity for this article can be die-sunk into one mould plate.The position of the parting surface will therefore be at the top of the moulding,the parting surface itself being perfectly flat.For appearence this is the ideal arrangement as the parting line is not moticeable unless flash develops.Flash is the name given to the wafer of material which escapes from the impression if the two mould halves are not completely closed.

Non-flat parting surface

Many mouldings are required which have a parting line whhich lies on a non-planar or curved surface.In these cases the mould’s parting surface nust either be stwpped,profiled or angled.

*Stepped parting surface

The maximum dimension of this component when viewed in the draw direction occurs at the top of the Z-form.Thus,as this form is stepped,the mould’s parting surface must like wise be steeped as shown 1.Note that as the edge of the component is square with the face(apart from moulding draft)the entire moulding form can be accommodated in one mould half .However,had the edge incorporated a radius,then in addition to the mould giving a stepped parting surface,the required edge form would have to be die-sunk into each of the mould halves.

*Profiled parting surface

The moulding iss shown at(a)It will be noted that while in cross-section the moulding form is constant,the general form(side view)incorporates curves.As the edge of the component is square with the face the entire form can be die-sunk into one mould plate.Thus the general form of the parting surface will follow the inside surface of moulding.(b)To simplify the manufacture of a multi-impression mould it is often convenient to extend the profiled surface completely across the mould.(c)The individual impression can then be die-sunk as required.

*Angled parting surface

The designer is frequently confroned with a component which,while fairly regular in form,can not be ejected from the mould if a flat parting surface is adopted.However,by adopting an angled parting surface all parts of the moulding are in line of draw and it can therefore be ejected.

*Complex edge forms

So far,we have considered only components which have a constant dege form.Now we proceed to consufer other components where the dege form is mot constant.This often leads to quite complex parting surfaces but for the present consider the simple case.

*Local stepped and profiled parting surfaces.

It is frequently necessary to incorporate a stepped or profiled surface to cater for one or two small irregulartites in an oterwise 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.

*Balancing of mould surfaces

When the parting surface is not flat,there is the question of unbalanced forces to consider in certain instance.This is best illustrated by an example.The plastic material when under pressure within the impression,will exert a force which will tend to open the mould in the lateral direction(x)If this  happens some flashing may occur on the angled facd the movement between the two mould halves will be resisted by the guide pillars,but even so,because of the large forces involved,it is desirable to balance the mould by reversingb the step so that the parting surface continues across the mould as a mittor inmage of the section which includes the impression.It is often convenient to specity an even number of impression.It is often convenient to specify an even number of impression when considering this type of mould,as impressions positioned on opposite sides of the mould”s center-line serve to balance the mould.

Relief of parting surfaces

We have,up to now,assumed the parting surface of the mould are bedded down over the entire surface.However,this is not praticable,for not only would it be extremely exensive,but it would also affect the efficient functioning of the mould.

Venting

When plastics material enters an inpression air is displaced.Normally the air can escape between the two mating mould plate.However,should the plates have a very fine lapped finish the air may be trapped within the impression so that the mould defects,such as discoloration,sinks incomplete filling.etc.

It is good design practice vents in the mould to allow air(and other gases when present)to escape freely.It is not usually possible to predeternibe where the vent will be required,so the vent is normalyy machined into the mould plate once the mould has been tried out.

The vents is normally shallow slot,not more than 0.05mm deep by 3mm wide,machined in the land.If a greater depth is adopted there is the likeligood of plastics material passingthrough the slot and the resultant undesirable flash mark being left on the moulding.

Position where a vent is likely to be required are:(i)at the point furthermost from the gate on symmetrical moulding;(ii)at the point where flow paths are likely to meet;(iii)at the bottom of projections.Now the latter case can not be vented by the surface vents discussed above.It is necessary in this case to provide a vent through the mould plate.This is most vonveniently achieved by incorporating an ejector pin in the required position.The minute gap between the ejector pin and the mould plate holes is sufficient to allow the air to escape.