A Guide to Hot Runner Injection Molding
Table of Contents
Hot runner injection molding increases efficiency for manufacturers working with plastics. While more expensive than cold runners, the hot runner costs are offset my large volume production due to less waste, faster production speeds, and enhanced product quality.
This guide describes the operation and main parts of hot runner systems, and outlines the situations in which hot runners excel and leave competitors behind. Determine if your plastic injection molding project can benefit from the use of hot runners.ts, and examine the specific scenarios where they outperform the competition. Discover how this system works and if it fits your project.
What is Hot Runner Injection Molding?
Hot runner injection molding is an arrangement of heated units located within plastic injection molds that inject liquid plastic into the cavities of the molds.
Such arrangement is designed to keep the plastic in a liquid state from the machine nozzle to the mold cavity for a prolonged time continuously. This is the contrary of “cold runner” systems. In an arrangement of cold runners the channels that convey the plastic to the part cool off and solidify in between cycles. This results in the formation of a solid scrap plastic block (the runner) which has to be separated and either recycled or thrown away.
Hot runner injection molding avoids the energy consumtion of needing to form and dispose of a solid runner which in turn helps to conserve the resources that that system wastes.
System Components and Functionality
To appreciate how these systems achieve such fine control over the flow of the mold, it is necessary to examine the layout of the assembly. A hot runner system is based on three primary elements which work collectively.
The Manifold
The manifold serves as the distribution hub. It receives the molten plastic from the machine nozzle and splits the flow into multiple channels. It is responsible for even distribution of the material to the different nozzles, thus enabling every cavity in the mold to fill to the same volume and pressure.
Heated Nozzles
The next component in the system is the heated nozzles which the plastic enters after it leaves the manifold. These elements are responsible for upholding the temperature of the plastic up to the ‘gate’ (which is the entry to the mold cavity). This is to ensure that the material does not get frozen off, which would seal the flow and make the part worthless.
Temperature Controllers
The temperature controller is the core of the system, and these devices are responsible for tracking and controlling the heating elements of the manifold and the nozzles. There must be a steady thermal equilibrium maintained here; too much of a change in temperature and the plastic will either deteriorate (burn) or it will freeze, and that would result in loss of time and money on the project.
How Do Hot Runner Systems Work?
During solid phase encapsulation, the flow channel is warmed, the mold cavity is held at lower temperature, and solid polymer injected into the cold mold cavity while the polymer is not specified at the melt flow index.
- Injection: Hot plastic is injected into the heated manifold from the injection molding machine nozzle.
- Flow: The plastic moves into one of the heated channels and is maintained at the melting point of the specified material.
- Filling: The polymer melts via temperature and friction. The polymer retreats to the manifold while the nozzle is filled, and the polymer is injected into the cavity.
- Holding: In contrast to cold runners, the plastic in the runner is not cooled with the part. It stays heated and ready for the next shot.
Maintaining the plastic in a heated state reduces the pressure drop in the system. The machine is able to fill the mold with plastic at lower force, and the system strain, and strain on the part, is lower.
Types of Hot Runner Systems
Not all hot runners are built the same. Engineers choose between specific designs based on the type of plastic being used and the cosmetic requirements of the final product.
Externally Heated Systems
These systems have heating elements on the outside of the flow channel. Since the molten plastic flows through an unrestricted tube, externally heated systems are good for processing delicate materials that may get stuck in “dead spots.”
Internally Heated Systems
In this configuration, the heater is housed in the flow channel, allowing the plastic to flow around it. This increases both energy efficiency and the flow control on the molten plastic. However, they are not color sensitive applications, as material can sometimes get trapped behind the heater and degrade.
Valve Gate vs. Thermal Gate
This is the distinction in how the flow of plastic is stopped at the gate.
- Valve Gates:Offers the greatest control as they employ a mechanical pin to physically close the opening, thereby leaving a clean surface on the part.This distinction refers to how the flow of plastic is stopped at the gate.
- Thermal Gates:Rely on the tip of the nozzle cooling slightly to freeze a tiny plug of plastic, which stops the flow. This is more economical but leaves a small vestige on the part.
What Are the Advantages of Using a Hot Runner Molding System?
The primary advantages are significant reductions in cycle time and material waste.
Cycle Time Reduction
In a cold runner system, cycle time is often determined by how long the thickest section of the runner takes to cool down. Given the volume of pieces produced over a production run, cycle time in injection molding is critical to the overall profitability of the run. Hot runner systems eliminate that time and keep the runner on the injection molding unit, which is a major time saver on every cycle run.
Waste Reduction
For manufacturers of expensive engineering-grade resins, waste is a major cost driver. Hot runners generate no structural waste. There is no solidified runner to regrind and of which to store or dispose, which means that every gram of material purchased, goes toward the finished product.
Quality of Parts Produced
Injection pressures can be lower because the plastic stays hot until it reaches the cavity, and as a result of the lower pressure, the flow is smoother. Lower flow pressure causes fewer sink marks to be present, better dimensional stability, and less internal stress on the part.
Automation
Hot runner molds, due to their specific operational capabilities, present unique opportunities in the realm of automation. Multiple pieces do not need to be individually separated from a runner system, allowing them to fall freely from the mold, or, in the alternative, be easily removed by industrial robots, thereby facilitating the automation of the packaging operation.
How Hot Runner Systems Influence Color Changes?
For hot runners compared to cold runners, color changes in hot runners are more complicated and take more time.
Since the hot runner system is a reservoir for molten plastic, you are unable to switch colors on the spot. The old color must be fully purged from the nozzles and manifold before the new color can be processed. If the purging does not happen, there will be streaks present in the new parts.
To counter this, production managers will often plan on running production in a very specific order to complete the transition in steps from lightest to darkest colors. Moreover, purging compounds that help transition the system quickly are commercially available and can be used.
What Are Examples of Hot Runner Injection Molding Projects?
You will find hot runner technology behind many of the plastic products used daily.
- Automotive Industry:Manufacturers use hot runners for large parts like bumper fascias and dashboards. These parts require long flow lengths, and a hot runner ensures the plastic reaches the end of the mold without cooling prematurely.
- Packaging:High-volume production items like bottle caps and closures almost exclusively use multi-cavity hot runner molds to produce millions of units with minimal waste.
- Consumer Electronics:Mobile phone cases and intricate housings require high precision. Hot runners allow for “direct gating,” which minimizes the visible mark left on the part, ensuring a sleek aesthetic.
Is a Hot Runner System Right for You?
Hot runner injection molding requires a higher upfront investment in tooling and maintenance compared to cold runner systems. However, for high-volume manufacturing, the technology pays for itself through increased speed, efficiency, and material savings.
While they pose challenges regarding color changes and maintenance complexity, they remain the gold standard for efficiency in the plastics industry. Before making the switch, manufacturers should calculate the ROI based on their projected production volume and material costs.
Frequently Asked Questions (FAQ)
Q: What are the distinguishing characteristics of a hot runner versus a cold runner?
A: A cold runner system freezes and solidifies the runner (the channel leading to the part) during every cycle and is thus inefficient. In contrast, a hot runner system keeps the plastic in the runner molten and does not generate waste, resulting in faster cycle times.
Q: Is there a cost difference in hot runner molds?
A: Yes, there is a cost difference. A hot runner mold is more expensive at purchase because of the extra components (manifold, heaters, and controllers of the heating system) that are part of the configuration. For high volume runs, that cost is generally recouped due to the large savings in material costs and the increased profit from the faster cycle times.
Q: Can hot runner systems work with any type of plastic?
A: Yes, with the exception of very heat-sensitive plastics, such as PVC, that tend to degrade if left in the hot runner too long. Most thermoplastics work fine.
Q: How often should hot runner systems receive maintenance?
A: The frequency of maintenance is often dictated by the type of plastic run. In general, during any scheduled routine maintenance of the mold, the hot runner system requires cleaning of the manifold and nozzles and a visual inspection of the wiring. Thermocouples and heaters are scheduled replacement parts typically due for replacement every so often.
