Material entering a mould always follows an almost identical path with successive mouldings. From a sprue gate into a flat plate it forms a spreading disc pattern. From a side gate of proper design into a flat plate, it will flow in an ever-increasing semi-radial pattern.
It will always flow more rapidly along wider and deeper channels, and when these are full it will spread into the narrower and shallower sections. On reaching an obstruction, turbulence will result, and the general flow path will be divided.
The mark of the turbulence will be carried along, sometimes to the end of the flow-path. Some pigmented materials show this very clearly. Flow marks are faults which should be thought about at the very outset of mould design because the location of the gate, its type, and the position of any obstructions in the mould, will influence the type and intensity of any flow marks that form.
If flow marks are inevitable in a moulding, the mould designer should try to make them appear on a surface where they will not be seen or will not cause the mouldings to be rejected. Thus, the siting of a tunnel gate perhaps near the underside of a moulding rather than near the top surface (appearance surface) might produce the more acceptable mouldings.
One of the most common causes of flow marks is lettering on the appearance surface. Whether this is embossed or intaglio makes little difference. The turbulence occurs and generates a flow mark.
In general, flow marks do not affect the performance, but only the appearance of mouldings and perhaps the best thing to do, realizing their inevitability, is to mask the effect with a suitably patterned decoration.
This fault can be caused either by the process or by the mould. It can rarely be traced to machine malfunction, and unless really bad material has been supplied, it cannot be attributed to that cause.
Obviously, the surface finish of a moulding cannot be better than the finish of the surface against which it has been moulded. If a good surface finish has been provided in the first place, this could be damaged by misuse or accident.
Hard tools cause such things as mechanical damage for the removal of a difficult moulding, or damage by rust or by being sprayed with an anti-freeze material as is sometimes used in mould chillers. These sometimes become slightly acidic after long use and etch any metallic surfaces with which they come into contact.
Even if rusting is avoided, hard water evaporating on the surface of a mould will leave a deposit of chalky material which is often difficult to remove. Accidental damage may, of course, occurs if the mould closes on a piece of plastic material. It may also happen if a tool being used on one-half of a mould,s lips and comes into contact with the other part. A protective plate will help in avoiding such an accident.
Many materials, most markedly, perhaps, low-density polythene, give the best surface finish when moulded at fairly high temperatures. The reason is obvious. The lower the melt viscosity of the material,the closer will be its contact with the mould. However, the higher the temperature, the longer will be the cooling time and the slower the output rate.
Obtaining the maximum output rate may entail some sacrifice of surface finish, but so long as this is acceptable and gives a saleable product and one which will not break down in service,everyone is happy.
Filled materials,such as nylon, acetaK polycarbonate, and polypropylene, where glass fibre, a sbestos, or talc are used as fillers, always give the best surface finish when the mould is filled rapidly through a restricted gate.
It seems that this treatment causes each particle of filler to become coated with a thin film of the plastic,w hich prevents any roughness that might be expected if the filler came to the surface of the moulding. As a general rule, the shear rate/viscosity curve for mineral-filled plastics shows the lowest viscosity at the highest shear rates and is much steeper than for unfilled materials. Obviously, the lowest melt viscosity gives the best finish.