Moulds for plastic are used for a wide variety of products, for instance, in average 11% of the total weight of a car is plastic material and much of it is produced by plastic injection moulding. Another large segment is the electronic industry, which constantly changes the models on television sets, computers and mobile phones, to mention some products.
A wide range of cavity sizes and shapes are produced by the die and mould industry when producing plastic components. Radii of 0.25 - 3 mm are typical in such cavities and many moulds require taper angles in the range of 0.5 - 5 degrees to allow the withdrawal of components. The dimensional accuracy required can be down to plus/ minus 5 microns and the positional accuracy of cavities of same order to ensure no mismatch between mating faces in a die set. Surface finish values of Ra 1 micron and less is necessary in many cases.
The steel types most commonly used in mouldmaking are; prehardened mould- and holder steels, through hardening mould steels and corrosion resistant mould steels.
Prehardened mould- and holder steel are mostly used for large moulds, moulds with low demands on wear and high strength holder plates.
Through-hardened steels are used for long production runs, to resist abrasion from filling agent, and additives in the plastic and to counter high closing or injection pressures.
Corrosion resistant mould steels are used if a mould is likely to be exposed to a corrosion risk, then a stainless steel is strongly recommended. Plastic moulds can be affected by corrosion in several ways. Plastic materials can produce corrosive by-products, e.g. PVC, corrosion also leads to reduced cooling efficiency when water channels become corroded or completely blocked. Condensation caused by prolonged production stoppages, humidity, aggressive gases, acid, cooling/heating water, flow rate, plastic material or storage conditions often lead to corrosion.
It is important to have good surface finish in the cooling holes not to get corrosion, which can make the whole mould crack. The threads where the cooling hoses are connected must also be made correct to prevent that corrosion occurs with a cracked mould as a result.
Criterias for mould steel selection
o Moulding method
o Plastic material
o Mould size
o Number of shots
o Surface finish
The mouldmaker is primarily interested in the machinability of the steel, its polishability, heat treatment and treatment properties. The moulder is looking for a mould with good wear and corrosion resistance, high compressive strength etc.
All aspects has to be taken into account, however, good uniform machinability is of outmost importance considering that the cost of machining accounts for a large amount of the total cost of manufacturing a mould. |