Dear members,
I just needed a help from your side if its possible. I am currently working on a project based upon Injection molding. The problem that I am facing is choosing a runner system for injection molding process (i.e. Hot runner system or cold runner injection molding system). I would be dealing with different kind of materials during my research. I am looking to buy the correct system for my work.

I would be really very thankful if you can answer the questions below

1. What are the guidelines in choosing runner system?

2. How does a manufacturer decides whether to go ahead with hot runner or a cold runner system dealing with different grades of polymers?

3. Is there a kind of algorithm or mathematical model that a company follows which determines choosing the correct system (hot runner or cold runner system) to obtain the final product depending upon the conditions/requirements? What are those conditions are really make a consumer to switch from cold runner system to hot runner system?

5. Are there some grades of polymers that cannot be processed using hot runner system but can be processed using a cold runner system or vice versa??

I am aware of the advantages and the disadvantages of both the systems (hot runner and cold runner systems).

I would be really thankful if you could help me in answering my queries or help me to get in touch with the right person that could help me to answer these questions.

Thanking you

Regards
Chandresh Thakur

Dear Chandresh,
choosing the cold / hor runner system depends to what kind of material you are going to mold. To be synthetic, we can have:

1) Thermosetting resin (EPDM, NBR, SILICON), is a material that irreversibly cures (changing of chemical structure). The curing have to be done by heating, approx. around 180°C - 210 °C, through a chemical reaction (vulcanization).
Tools are heated and could have cold runner system when is needed to inject inside specific moulds, geometry, n° of cavities, EAU, etc. Press is always equipped with a cooled injection chamber to keep row material at low temperature, before injection. Cold runner system on tool couldn't be used if you have a direct injection.

2) Thermoplastic (polypropylene, acrilyc, nylon, etc) is a material that becomes moldable above a specific temperature approx 200 °C and returns to a solid state when cooling. Is recyclable mostly.
Tools are cooled and injected material is heated and keep "liquid" inside the injection chamber. The cavity cooling is basic and de tool design is essential to have good performance when the demold phase (normally using ejectors or robots). For special configuration those molds can have a hot channel system (see for example car headlights) or can have a direct injection without Hot Channel System.

For each system (hot, cold) there are specific issues for molding es well for the manage of whole tool.

In any case (for my knowledge) systems cannot be inverted !

Assuming you are doing thermoplastic injection molding here are my responses, answered exclusively from my personal experience:

1. Every hot runner system I have used was custom designed for the application by the hot runner manufacturer. Criteria for design are: material, shot size, estimated cycle time.

2. Generally you would determine which polymer you are going to make your final product from first. Especially given the range of different shrink rates and fillers, the steel may need cut differently. Personally, if I was going to be running a large range of materials through the mold, I would just use a cold runner. Simpler and cheaper.

3. As far as I am aware any mathematics involved in the cold v hot decision are mostly cost based. High volume jobs are more likely to use a hot runner as they produce less scrap. They are also popular in technical parts as they allow greater control over the melt.

5(4). I am pretty sure you would not be able to process thermoset materials as they would set up in a hot runner system. And then you'd be screwed.

Presumably, since you are a research assistant, you will have a fairly simple mold which would make pieces for you to do testing on. This would explain why you needed to run a large range of materials through the mold also. I would recommend a cold runner system in this application for the following reasons:

1. Hot runner system designs work best when they can be custom designed to account for differing materials, fillers, shot sizes, and residence times. As you will be using a lot of different materials you will run into a problem at some.

2. It is probably unnecessary for the type of work I would imagine an RA does. At Kent we shot our dogbone mold through a cold runner system and never had a problem.

3. Hot runner systems are extremely expensive.

4. They are less reliable than cold runner systems. They are thing which can fail/break. Yes, a cold runner can fail/break, but it is much less likely to occur and the fix will be simpler and cheaper.

Have fun!

Dear Marco,
I really thankful for your feedback. This information helped me to understand and gave me a good idea for things to be included in my project.

I really appreciate all your help

Thank you