Leafly Mould Blog

All hot runner systems need a heat source to operate as designed, however not all hot runner systems are heated by the same methods. The type of heater technology your hot runner system uses can significantly affect molding performance, molded part quality and system cost. No heating method exists that is ideal for every scenario and each one has their own pros and cons but illustrating their differences may help you in understanding how your hot runner system is constructed and provide you with some additional insight on your next purchase. Some hot runner systems may use a mix of heating methods depending on the components of the system and application but for the purposes of this discussion we will focus on manifold heater elements.

The main types of manifold heating methods include Embedded Heat Sources (Brazing, Pasted, Pushed-in) or External Heat Sources (Heater Pads/Plates). Selecting the right heater element is always a careful balance of performance, reliability and cost (both initial investment and operating). Many factors are also weighed including type of industry, part geometry, mold design, part variables and production requirements.

EMBEDDED HEAT SOURCES

Compared to External Heat Sources, Embedded Heaters offer a range of advantages

• Heater element channels optimized through CAE analysis for enhanced thermal profile
• Enhanced energy efficiency
• Helps protect heater elements from physical damage
• Extended life
• Generally ideal for small to large symmetrical manifold designs with tight or standard pitch dimensions

Brazed-In Heaters

Brazed-in Heaters generally offer the best performance and reliability making them ideal for achieving the highest molded part quality and long term production. Using advanced CAE analysis, heater channels can be optimized for each manifold to achieve unbeatable thermal balance. Thermal balance of the manifold is a critical variable in achieving excellent balance of fill results and therefore superior part quality.

As the element is set into a recessed heater channel, the greater contact area is much more efficient at transferring and maintaining heat than traditional heater methods and helps protect the element from physical damage. The Brazing process also completely eliminates any air gaps that can lead to heater failure through electrical arcing. Brazed heaters are often made to higher quality standards than other heaters and therefore have a very low failure rate. With a lifespan able to exceed 10 years, they have the ability to outlast well beyond the life of the mold. While brazed heaters require a higher initial investment, this is typically offset by higher quality part production reducing scrap rate, eliminating heater replacement costs and minimized downtime. Part quality aside, the higher the cavitation and longer the production run the more it makes sense to invest in Brazed heaters.

Paste-In

Paste-In heaters are considered to be mid-range heaters. They are more economical than the brazed version however there is a trade-off in performance and reliability. Although the paste-in technique attempts to eliminate air gaps, the method is not 100% so there is a relatively higher risk of heater failure which does occur at a higher rate (vs. Brazed). This production challenge is offset by the potential ability to replace the element in the field, although it’s not always possible. Regardless of the potential for increased downtime, Paste-In Heaters are still a great option for molders looking for performance with a more economical initial system price.

Push-In/Flex Heaters

Push Heaters, like Paste-In heaters, are considered to be mid-range heaters but their design favours faster in-field replacement. Since there is no “pasting” step required it helps minimize any servicing downtime. However, with the elimination of the paste, there is less contact area which can increase energy consumption. The element itself also has a more limited bending radius which can restrict its applications. Available in a wide range of standard lengths, diameters and wattages, Push Heaters are an off the shelf item so sourcing spares globally is also quick and easy.

EXTERNAL HEAT SOURCES

Compared to Embedded Heat Sources, External Heaters offer a range of advantages

• Lowest cost for more economical up front system pricing,
• Ability to use multiple heaters to increase number of temperature control zones (ideal for manifolds with long pitch/asymmetrical designs-automotive),
• Easiest to replace,
• High standardization allowing for easy global access to spare parts from multiple sources

Heater Plates

By now, you may be wondering what sense it makes to use Heater Plates when there are several “better” options available. Truth is, although Heater Plates would be considered lower performance under the same circumstances as the other options, Heater Plates are a good choice for a completely different type of mold design. Their ideal applications are on molds that benefit from having additional zones of control. Typically this would be large/high pitch/non-symmetrical manifolds (automotive, and large parts for white goods for example).

Evaluated on a larger scale and utilizing CAE optimization analysis it is possible to achieve a good overall thermal profile, better than if a single continuous element had been used for the same application. Also, Heater Plates are the most economical, the easiest and fastest to replace and are a common standard off the shelf component offered by a wide range of global suppliers. While high reliability is always beneficial, automotive molds have a lifespan limited to 3 to 5 years so the lower system price is better positioned to offset some failure related downtime.

Preventative Maintenance (PM) is a critical task that aims to help ensure your mold(s) run at peak efficiency by strategically investing in service and maintenance before things go very wrong and get very costly. A well executed PM program will help improve productivity and avoid costly downtime including unscheduled breakdowns. However, the trick has always been in finding the right balance to optimize your operation. Too little service and you could still incur breakdowns, among other issues, and too much maintenance you end up throwing money away.

The frequency of a Preventative Maintenance Program is very specific to individual molds. There are numerous factors that have an influence on the scheduling of PM. Some of the most significant are materials and how the processing equipment is being operated.

Materials being molded play a big part in how often a hot runner mold will require maintenance.  Some molds can go months with very little maintenance while others will require daily cleaning of the “gas” residue that builds-up on the face of the mold. Aggressive materials that contain “fillers” (glass) or are corrosive can prematurely wear hot runner components.  These types of materials require that the hot runner be manufactured using special materials that protect the system in order to maximize the “runtime” between PM’s. If your hot runner was not specified to process this type of resin grade your maintenance schedule may be more frequent.

If a mold becomes hard to start-up, difficult to maintain all cavities or has required a gradual increase in nozzle set-points, there needs to be a full evaluation completed to determine the “wear” on gate components (both in the hot runner & mold). The condition of the gate may be a clue as to the need for PM. On valve gated molds, flash around a valve pin may require valve pin replacement and/or gate replacement or repair. Molds with a thermal gate also experience wear on torpedoes or liners in the gate components. Sometimes a frequency for PM can be set up based on the shot count.

Training personnel on the shop floor on the proper start up & shut down procedures is also an important factor in keeping a mold performing at its best. All hot runner systems require a “soak time” for heat to penetrate through the system. Just because the hot runner controller indicates that the set-point has been reached, doesn’t mean that everything is up to a temperature that allows you to safely start up the mold. Knowing the material & the hot runner system allows you to determine what the “soak time” requirements are for each mold. Heat sensitive materials may degrade if it’s left to sit at a normal processing set-point for an extended period of time.  Degraded material can lead to major hot runner maintenance.

If the hot runner system is “On”, the mold cooling must be “On”. If the mold is valve gated and has individual pistons for each cavity, mold cooling should remain “On” for a period of time after the hot runner is shut down.  This protects the actuator seals from being harmed by the residual heat from the hot runner as it cools. This becomes especially important if running a high temperature engineered resin.

For years, natural balance has been the cornerstone of a successful hot runner balance. This means that the melt experiences the same flow length and the same diameter melt channels from when it leaves the machine nozzle until it enters each cavity within the mold.  This approach has served the industry well.

In recent years, much attention has been paid to the effects that shear has on the melt as it flows through a cold runner system. Of specific interest is how shear heated melt is distributed by the cold runner geometry. Research in this field has led to a greater understanding of shear-induced variances as they apply to cold runner systems and led to the introduction of technologies that are aimed to help address molding issues which have irritated molders for years.

It has also led to a greater awareness of the topic by the general injection molding community. While the majority of published research on this phenomenon has been based on cold runner systems, it has naturally called into question the validity of natural balance principles as applied to hot runner systems.

As a hot runner supplier, Mold-Masters has long been aware of the phenomenon of shear-induced variances that can occur within melt channels and the effect that this can have on the balance performance of a mold.

Balance Sensitive Applications

Uniform balance is always desirable. If a mold is significantly imbalanced, it will be difficult to start up and may have a narrow process window. The balance that can be achieved between cavities on a multi-cavity mold will have a bearing on the part to part consistency. That being said, there are some applications that will require a higher degree of balance than others. These will include parts that have a demanding dimensional requirement or parts that will be difficult to eject if they are over packed. Here uniform balance is important to ensure that all cavities are uniformly filled. It is important to recognize applications where balance will be critical.

However, it’s important to recognize that there are some fundamental differences between hot runner system and cold runner system designs. Cold runner systems are more prone to the effects of shear due to their inherent design.

Runner Geometry

It is good design practice to minimize the size of a cold runner. Consequently, in comparison to the hot runner that may be employed for a similar application, the cold runner is small. For a given injection rate (fill time) this means that the shear rates that the material sees in the runner system will be higher in a cold runner system. When you further consider that the shear rate is inversely proportional to the diameter to the power of three, it is obvious that the smaller sized cold runners will have a significantly greater shear rate and consequently accentuate any shear-induced variations.

Another significant difference between a hot and cold runner is that with a cold runner, the effective size of the runner reduces during injection. The first melt to flow into the cold runner solidifies; effectively further reducing the diameter of the runner and further increasing the shear-induced on the melt. The runner continues to reduce in effective size during injection as the runner cools. This is in stark contrast to the hot runner system where the runner wall is maintained at the required processing temperature during the injection molding cycle.

Level Changes

Cold runners are typically restricted to a single face of a mold. This means that a cold runner is typically a ‘single level’ runner. Level changes are more easily incorporated into a hot runner design and can be strategically positioned to assist in uniformly distributing shear-induced variances.

Shear Sensitive Materials

Certain materials exhibit a dramatic change in viscosity in response to shear and in response to temperature change. Such materials will be more susceptible to shear-induced variations than other materials. In order to design the optimum hot runner system and avoid shear-induced imbalances, it is important that the material’s behavior, in response to shear and temperature, is understood.

Summary

In summary, the phenomenon of shear-induced imbalance can occur in a hot runner mold; however, it is much less likely to happen to a significant extent due to lower shear in hot runners compared to cold runners. The phenomenon of shear-induced imbalance is well understood, and its relevance can be anticipated based on the material being molded and critical nature of each application. Hot runner design allows more opportunities to introduce such features as level changes which facilitate uniform distribution of sheared material.

Mold-Masters dedicates its global resources into delivering molds with the best balance performance in the industry. When Mold-Masters technology and decades of experience comes together, anything is possible. Our customers rely on our high-performance capabilities to deliver solutions where others fall short.

Mold-Masters hot runner systems feature iFLOW Manifold Technology, a 2pc brazed manifold where runners are carefully CNC milled utilizing patented melt flow geometry, flow path options, and runner shapes to be able to achieve the best possible results. For instance, iFLOW incorporates curved runner channels, eliminating sharp corners and dead spots, which help overcome the challenges associated with traditional hot runner manifold designs which significantly improves processing results.

Have you ever looked at the bottom of a plastic container and noticed one or more symbols molded into the part? These markings are added at the time of molding with a metal insert placed in the cavity of the mold. When the resin solidifies it bears the mark of the specific insert added. Believe it or not these markings can be extremely important and very helpful for molders and consumers alike as they guide decisions through the life cycle of the plastic part. For the molder, the markings provide key information which can include when the part was made, helping to identify a specific manufacturing lot or batch and ensuring that a reliable quality system exists. For consumers, the markings can show how the finished good can be used and possibly re-purposed through recycling.

High-Temperature Blind Hole Inserts

The use of engineering resins is increasing and since these resins are processed at elevated temperatures, high-temperature inserts are required. One important feature of high-temperature inserts is a blind hole which allows the mold designer to enhance cooling around the location of the insert.

Food Container Inserts

Fast-paced lifestyles and more affordable modern day conveniences, such as microwaves, and dishwashers for example, have changed the food container industry for both the manufacturer and the consumer. Food Container Inserts allow producers to easily identify the material characteristics of the product. These simple pictured images take the guess work out, allowing consumers to quickly visualize appropriate conditions for product use.

Recycling Inserts

As we all know today, green initiatives have become common practice. More and more plastics are being recycled to avoid placement in landfills, ultimately protecting our environment. Through the use of recycling or resin identifier inserts by manufacturers, recyclers can easily sort and recycle post-consumer plastics helping the sustainability of our industry.

The Industry Innovator

The ability to effectively cool the mold is imperative for managing an efficient cycle time. Up until now mold inserts have been an incumbent as they often interfered with the location of the water lines used to cool the mold. Recognizing this gap in performance DME has introduced the first mold insert that is inserted through a blind hole. This added feature allows the mold designer to enhance cooling around the location of the insert and improve the molders productivity. It also eliminates the need to remove the mold when it is time to change the insert. This is a truly major industry breakthrough.

Milacron’s DME brand has enjoyed a leadership position in the supply of mold marking inserts for many years. We offer over 9 styles and hundreds of data combinations suited for manufacturers in all industries. DME is building on this success and ensuring an even brighter future with its launch of its new products expanding the number of applications and improving the productivity of use for its customers.

In today’s world of plastic injection, there are several factors driving long cycle times. Thermal control of the mold is the most influential aspect. With the inability to evenly cool an injected part, the cycle times run long, warpage is increased and the end product is plagued with tension and stress which often ends with parts failing quality control.

Conformal cooling technology provides the industry greater thermal control over injection molds. The increased control is accomplished through the manufacture of conformal cooled inserts custom tailored to fit each and every need. Using cooling channels in places and shapes that conform to the geometry of the part being manufactured ensures greater control over these specific, hard to reach areas. Traditional or conventional tooling simply cannot achieve the shapes, paths, and channel geometries possible with conformal cooling.

Having control of both the hot and now the cold halves you have full process parameter control.  Your design solution places cooling (or heating) channels at the optimal distance from the part surface, allowing the mold to maintain a targeted, consistent temperature for complete thermal control.

Conditions sometimes dictate that heating is required within the mold and conformal heating channels operate in tandem with conformal cooling channels as well. Conformal heating can assist in maintaining a molten flow front of resin as it enters a hard to fill area of the tool. This takes place much like a hot manifold operates. The heating element, in this case high-temperature oil, will run through channels that follow the shape of the part to increase surface temperature of the mold. As the resin passes through, it still maintains the melt temperature needed to properly flow and fill tight areas, such as an automotive speaker grille.

Shown here (on the left) is an example of an insert that originally contained conventional cooling channels. Beneath the image showing the circuits you can see the thermal effect the resin has on the insert in question. To the right you will see the counterpart with conformal cooling channels to address the key warm areas of the mold insert. Beneath the channel image again you can see the thermal effect on the insert. Side by side comparison shows the conformal channels provide much greater thermal control and reduced delta when used with injection molding.

Hot runners are known to offer many processing advantages. However, to maximize your productivity and production quality, maintaining precision temperature control is a critical component for these systems to work as designed. Maintaining precise temperature control is known to affect part quality and processing as temperature directly impacts processing variables associated with the Resin Characteristics and Hot Runner System.

RESIN CHARACTERISTICS

All thermoplastics degrade under elevated temperatures. A well designed hot runner system eliminates hot spots from its design and precise temperature control will help to ensure temperature uniformity of the system during processing. Temperature uniformity is important because it allows you to bring the overall system temperature closer to the lower processing temperature limit. The greater the heat history of the polymer, the more the properties of the polymer will have been weakened.

Additionally, as the plastic melt is forced along the hot runner channel under pressure, it is subject to shear which also generates additional heat. Precise temperature control becomes especially challenging when processing shear sensitive or highly viscous polymers and additives. Otherwise polymer properties will deteriorate and part quality will be affected.

HOT RUNNER SYSTEM

In a multi-cavity system, natural flow balance means the same flow length, same channel diameter and the same number of turns from the machine nozzle to each mold cavity. In conjunction with temperature uniformity across the entire system, this leads to uniform filling and back pressure conditions in each cavity, uniform part quality and the widest processing window. Where changes in material or injection parameters exist (like temperature) these changes will cause quality problems such as poor weight consistency.

The hot runner and mold cut-out are dimensioned so that all hot runner components are perfectly aligned at the specified operating temperature. To allow for heat expansion, sliding or rotational pressure seals are used between hot runner components. If excessive temperature variations exist in the system, it can cause leakage or even component fatigue and/or failure. The nozzle center axis must remain in an absolutely fixed position, in exact alignment with the gate at processing temperature.

Fast Response is Crucial

Injection molding is a cyclical process. The hot runner must maintain accurate and uniform temperature conditions through both the heating (injection) and cooling (hold) phases of the molding cycle. Since the gate is the most critical areas of the hot runner, it is essential for the TC to accurately measure gate temperature. For quick and accurate response to temperature over or under swinging, closed-loop temperature control is required for each and every nozzle. The quality of the molded part is directly related to temperature control response efficiency.

Unlock your Operations Full Potential

Mold-Masters offers precision temperature control through our comprehensive line of advanced TempMaster Hot Runner Temperature Controllers. They have the ability to optimize the performance of any hot runner system.

All TempMaster hot runner temperature controllers feature APS Technology which maintains set point for precision temperature control of 1 to over 500 zones. APS is a proprietary Auto-Tuning algorithm that continuously monitors, learns, predicts and automatically adapts to process variables every 20mS.

Benefits include:

• Precise 1°F Control Accuracy
• Superior System Reliability
• Enhanced Part Quality & Gate Vestige
• Improved Part Consistency
• Reduced Scrap
• Lower Power Consumption
• Maximized Profit Margins