Hot Runner & Cold Runner Systems in Injection Moulding of Plastics

21 Nov, 2020

Many plastic parts are made with the help of Injection Moulding Process. Any injection mould has different parts through which hot molten plastic flows after it leaves the machine nozzle, into the various cavities to finally make the moulded part.

Typically the molten plastic gets heated in the injection machine. And after it leaves the heated machine nozzle, it moves through various paths within the mould. Therefore these are called “Cold runner” moulds.

In “Hot Runner Systems”, such mould parts or manifold pathway sections are physically heated, so that the molten plastic continues to be heated even after it leaves from the nozzle, and is thus more effectively transferred into the cavity from the machine.

Thus the main difference between the two systems is that the mould pathways in hot runner systems are heated, while cold runner systems are not heated.

Choosing amongst hot and cold runner moulds
It is necessary to use the right system for any injection mould. This usually depends on many factors like the part complexity, the volume of the parts required, the waste or runners generated while moulding, design of the mould, machine configuration available, cost of manufacturing, maintenance cost involved, availability of labour, amount of regrind allowed in the part etc.

For example, if there are parts with many colours, then using a cold runner mould, will lead to a large pile-up of runners of different colours which may lead to high costs of scrap. The use of hot runner moulds in such cases will reduce the amount of waste generated and thus improve cost efficiency.

But if parts are simple, then a cold runner does turn out to be faster and cheaper to make.

Therefore, cold runner moulds are quite simple but hot runner moulds have a lot of variety in gate style designs like a pinpoint, valve, side gate, gate pin, cluster nozzle holder, nozzle head etc.

Advantages of HOT Runner Moulds:
Hot runner equipped moulds have faster cycle times because of no runner in Mould.
Hot Runner system can assist in easy melt flow movement.
Hot Runner systems benefit in the moulding of multi-coloured parts because of lower wastage owing to zero runners.
Hot runner Moulds require less injection pressure and speed to fill the product.
Any kind of product shape or size, including odd/abnormal ones can be filled with hot runner system.
Hot runner can help to fill larger parts and thereby help increase production.
Hot runner moulds always have good product dimensional consistency and quality.

Disadvantages of HOT Runner Moulds:
Hot runner moulds normally have higher maintenance costs and time.
In case of a breakdown the time to repair is usually higher.
Hot runner equipped moulds cost more because of extra parts and eating elements.
Mould designs are complicated and machines required are also usually of a higher tonnage.
Power consumed is higher.

Making of mould is of a longer lead time since greater attention is required in their manufacturing which leads to relatively higher cost, time and accuracy. Hence Capex costs, as well as maintenance costs, are higher. Some of thermal sensitive materials do not perform well to process with hot runner moulds.

Advantages of COLD Runner Moulds:
Cold runner moulds usually have lower maintenance costs as well as time to repair.
Moulds cost are reasonable since the number of extra heating elements are lower.
Mould designs are not complicated and machine also not needs bigger size.
The lead times of making the mould are relatively faster and better.
Product Colour changes are Not difficult in cold runner moulds.
Thermal sensitive materials are also be processed easily.
Capex Purchase cost of mould and its maintenance cost is not that high.

Disadvantages of COLD Runner Moulds:
Cold runner Moulds have slow cycle times because of presence of runners in the mould.
Multi colour moulding (2 to 3 colours) is not possible.
Cold runner Moulds required relatively higher injection pressure and speed to fill the product material.
It is difficult to service any range of shape / size of products esp odd / abnormal ones.
In fact Cold runner moulds cannot help in filling larger parts.
Product dimensional consistency and quality depends on gate position.
In case of ejection of material, presence of manpower or a robot is required to remove runner from the mould esp since auto ejection may be absent.
Hence based on the above parameters it can be concluded that there are both pros and cons to using hot runner moulds, and it all depends on various situations. In summary the various technical and commercial factors that would be considered in making such decisions are:
The Polymer material proposed to be used for moulding.
Part shape, size, runner wastage tolerated and number of colours being made.
Quality specifications and tolerances desired by customer.
Desired life of the mould.
Production capacity / output desired.
Specification and availability of machines for producing the part and cycle times desired.
Targeted Costs of operation, manufacturing costs, maintenance and capex cost budgeted for the mould.


About Mahesh Soni

Mahesh Soni, is the Head of Development in Parekhplast India. He has a wide experience across many industries and has worked on a number of machines and moulds of different kinds. A dedicated and diligent individual, Mahesh has a keen interest in following trends in the plastic industry