Understanding Runner System Molding: How the Right Runner Design Impacts Every Plastic Part

 If you have ever been confused about why two molds that make the "exact same" plastic part come with very different pricing, cycle times, and scrap rates, it's probably due to one unsung hero of the process: the runner system. Runner system molding, or the science and engineering by which the molten plastic journey takes place from the injection unit to the mold cavity, is considerably more critical to production efficiency than most people imagine.

In this article, we take a very hands-on approach to runner systems, discussing how a cold runner injection mold operates, the elements of cold runner solid mold design and at what point a hot runner mold system might be more beneficial on a production run.

Runner system molding is "one thing" people don't talk about but one which influences cost per part as well as scrap rates and part quality. When new mold designs are put out for quotation by toolmakers, the quality of the runner is the first point a highly experienced toolmaker looks at while assessing the overall mold design for cost.

Basics of a cold runner injection molding die, what they are used,

when and why; a guide to cold runner injection molding dies.

When an Injection Mold Will Be a Cold Runner System, how parts from the mold cavity and is then physically removed from the molding after the cavity has been open with a degating tool, with automatically in molding, by degater and manualy is separated from the part with sharp tooling or cutting. The material in the runner is also cooling down along with the part itself; hence the system is called 'cold runner molding'.

Cold Runners Are a good investment when;

* Low volume or prototype quantities are involved. It is less cost to make lower part quantities when tool is cheap.
* The job is changing to multi-material or multi-color, easier to clean out than hot runner.
* the part has got basic geometric shape as the process of inserting plastic will flow with out difficulty.
* Cost budget of the molding job is an issues

Disadvantages; plastic is wasted since the part and runner need to be trimmed, or re-melted and added to future molds or jobs.

 

The Key fundamentals to cold runner mold design

A successful cold runner mold design is more than simply carving a channel between the sprue and cavity-involves design consideration to eliminate defects and inefficiencies. Designers normally focus on:

-Flow balance between cavities in a multi cavity tool, plastic should arrive at every cavity at the same time preventing both over-packed and under-packed parts.
-Runner cross-sectional shape. This should be circular-minimum surface area for volume-reducing heat loss and therefore maintaining smooth flow.
-The dimensioning of runners-taking account of the viscosity of a material - (most materials require bigger diameters the higher the viscosity-and therefore greater pressures-)
-Gate size and design-the type of gate design-placement should eliminate cosmetic issues on the important surface-e.g. Flow lines, jetting or weld lines
-Cold slug wells are essential. This traps the coolest most highly solidified plastics in the runner. This plastic must be trapped before it can enter the cavity.
-Draft and electability-The runners need enough draft angles for them to be readily ejected from the mold without scraping.

Designing for optimal flow, cycle time, and conformity over all the cavities within a mold can vastly improve plastic part production.

Hot Runner Mold Systems: Mass Production Efficiency

Hot runner mold systems eliminate the material-waste issue inherent in cold runners because the plastic never solidifies in the runner channels at any stage during the cycle. The hot runner mold contains a heated manifold and nozzle system which keeps the plastic at processing temperature when it flows from the machine barrel to the mold cavities; thus, there is no runner to trim away after part ejection.

What Manufacturers Select Hot Runners

Reduced or no runner scrap. Since the runner never solidifies it is never discarded at each molding cycle.
Faster molding cycle times. Eliminating runners from the cooling process and ejecting system cuts overall molding cycles in half.
Repeatable part quality. Melt temperatures throughout the system remain consistent.
Lower over all operating costs. Although higher on an initial basis the reduction in material scrap and cycle times will yield greater than expected economies in long-run molding costs per part.
There's also a tradeoff to take into consideration when deciding upon a hot runner mold system: The initial investment costs (tooling, heater/temp control.) for hot runner molding are significantly higher. It's generally harder to maintain, changeover for different molding material, and color.

The bottom line is which runner system to choose comes down to a combination of: volume of parts - higher runs favor hot runners because they help reduce waste, and are quicker - price - if you have tight tooling budgets cold runner designs are often a better choice - material properties - if you are using certain temperature sensitive materials it might be better to use a cold runner mold than a hot runner - part complexity - especially on multi-cavity or high precision parts hot runner molding can provide a more consistent finished product. There isn't necessarily a right or wrong way to go, you just have to match the runner system to what you're trying to achieve. Runners are much more than just background detail to your injection mold – they influence the cost, efficiency, and quality of every part made. If you know the advantages of a cold runner injection mold, some basic design steps for cold runner molds, and when a hot runner mold system is more appropriate, you'll make the better investment in your next project.

 

Frequent Asked Questions (FAQs)

Q1: What actually do runner system molding do in the injection molding process? They control how the molten plastic flows from the machine nozzle to mold cavity, affecting the fill balance, pressure, cycle time and waste of material.

Q2: Can we use hot runner injection molds even today? Yes, cold runner systems are still widely used and have some excellent application, prototyping and medium/low volume production and wherever it is desirable to reduce tool cost.

Q3: What is the major beneficial factor in a good designed cold runner mold? Minimizing the material waste and pressure loss as well as making each cavity fill evenly in the mold can be called a good cold runner mold design.

Q4: Why did Manufacturers change from cold runner to hot runner system in mass production. Forcing runners out from the cavities in high volume can increase the part output and decrease the cost of each piece.

Q5: Does the hot runner system can run on all kinds of plastic resin? While they work with many standard thermoplastics; heat-sensitive materials do need to maintain low and proper temperature in case they degrade.

Q6: How does a cold slug well benefit the mold design when it is working with the runner in cold runner system? A cold slug well takes away and absorbs and the cooling plastic front of the mold runner before it into the cavity; avoiding some material defect which is causing by cold mass that is frozen in the mold runner.

Q7: How could we change between the different runner system in existing tool? No, there is nearly not a simple solution to convert a cold runner mold design to hot runner mold design as one can suppose, just with minor changes to modification. However it might possible sometimes; but usually there is not sufficient space.