Science of Plastics


Plastics are a group of materials, either synthetic or naturally occurring, that may be shaped when soft and then hardened to retain the given shape. Plastics are polymers. A polymer is a substance made of many repeating units. The word polymer comes from two Greek words: poly, meaning many, and meros, meaning parts or units. A polymer can be thought of as a chain in which each link is the “mer,” or monomer (single unit). The chain is made by joining, or polymerizing, at least 1,000 links together. Polymerization can be demonstrated by making a chain using paper clips or by linking many strips of paper together to form a paper garland.


Naturally occurring polymers include tar, shellac, tortoiseshell, animal horn, cellulose, amber, and latex from tree sap. Synthetic polymers include polyethylene (used in plastic bags); polystyrene (used to make Styrofoam cups); polypropylene (used for fibers and bottles); polyvinyl chloride (used for food wrap, bottles, and drain pipe); and polytetrafluoroethylene, or Teflon (used for nonstick surfaces). Although many polymers are hydrocarbons that contain only carbon and hydrogen, other polymers may also contain oxygen, chlorine, fluorine, nitrogen, silicon, phosphorus, and sulfur.

Natural polymers, such as cellulose and latex, were first chemically modified in the 19th century to form celluloid and vulcanized rubber. The first totally synthetic polymer, Bakelite, was produced in 1907. The first semisynthetic fiber, rayon, was developed from cellulose in 1911. However, it was not until the global disruption caused by World War II, when natural sources of latex, wool, silk, and other materials became difficult to obtain, that synthetics were mass produced. Synthetic rubber was needed for tires, and nylon was needed as a replacement for silk for parachutes. Today synthetic polymers in the form of plastics are in wide use, and the plastics industry is one of the fastest growing in the United States and around the world. The industry produces approximately 150 kilograms of polymers per person annually in the United States.


Monomers can be chemically joined together in two ways: addition polymerization or condensation polymerization. Addition polymerization has three basic steps: initiation, propagation, and termination. In this type of polymerization the monomers join by adding on to the end of the last “mer” in the chain, just like making a chain of paper clips. Polyethylene, polystyrene, and acrylic are examples of plastics formed by addition polymerization. These polymers are often thermoplastic in nature: they can be heated and made soft and then hardened when cooled. They are easily processed, reprocessed, or recycled. See the attached tables, Some Addition Polymers and Some Condensation Polymers, for examples of each type.

During condensation polymerization a small molecule is eliminated as the monomers join together. Nylons, some polyesters, and urethanes are examples of condensation polymers. These polymers can be thermoplastic or thermosetting. Although all plastics are in a liquid state at some point in processing and are solid in the finished state, once a thermoset polymer is formed, it cannot be melted and reformed.

The monomers in a polymer may be arranged in a variety of ways. For example, the monomers may have a linear arrangement like a long chain of paper clips, although the tetrahedral carbon bonds actually give the chain a zigzag configuration. Polyethylene is the simplest example of a linear polymer.

Polyethylene Zigzag Structure


If the monomers not only form straight chains but also form long side chains off the main backbone, the resulting polymer is described as branched and may look like a tree branch or the stems of a bunch of grapes. Another arrangement occurs when the long chains are chemically linked together, forming a mesh-like structure known as a crosslinked configuration. Vulcanized rubber, which is formed by reacting natural rubber (isoprene) with sulfur, is an example of a crosslinked polymer.

The polyvinyl alcohol is cross-linked using borax, Na2B4O7x10H2O (sodium tetraborate).


The polymer molecules can also have different arrangements. If the arrangement has no particular order or form, like the arrangement of spaghetti on a plate, the polymer is said to be amorphous (having no shape). Amorphous polymers are often transparent and, therefore, are used as food wrap, headlights, and contact lenses. These materials also tend to have lower melting points. If the arrangement is in a distinct pattern, the polymer is said to be crystalline. The higher the degree of crystallinity, the less light passes through. Such materials are either translucent or opaque. This quality depends on the degree of crystallization and the presence of additives. Crystalline polymers have greater strength and tend to have higher melting points.

Characteristics of Polymers

Polymers seem to have a limitless range of characteristics along with properties that allow them to be dyed in an endless array of colors. Their properties can be enhanced by additives. Being able to design or engineer polymers for specific applications makes plastics unique materials. Although each polymer has unique characteristics, most polymers have some general properties:

  1. They are resistant to chemicals.
  2. They are insulators of heat and electricity.
  3. They are light in mass and have varying degrees of strength.
  4. They can be processed in various ways to produce fibers, sheets, foams, or intricate molded parts.

The raw material for manufacturing plastic products is called a resin. Some of the most common resins are polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS). These resins are often used in packaging. The Recycling Code chart in the linked PDF shows the recycling code for these resins.

Some Additional Polymers

Polymer name Monomer(s) Polymer Use









Most common polymer. Used in bags, wire insulation, and squeeze bottles


Polypropylene CH2=CH








Fibers, indoor-outdoor carpets, bottles

















Styrofoam, molded objects such as tableware (forks, knives, and spoons), trays, videocassette cases
Polyvinyl chloride





(vinyl chloride)





Clear food wrap, bottles, floor covering, synthetic leather, water and drain pipe





-CF2-CF2 Nonstick surfaces, plumbing tape, chemical-resistant containers and films


Polymethyl methacrylate

(Lucite, Plexiglas)






(methyl methacrylate)








Glass replacement, paints, and household products

(Acrilan, Orlon, Creslan)









Fibers used in knit shirts, sweaters, blankets, and carpets



Polyvinyl acetate





(vinyl acetate)







Adhesives (Elmer’s glue), paints, textile coatings, and chewing gum
Natural rubber  CH3








Rubber bands, gloves, tires, conveyor belts, and household materials

(neoprene rubber)











Oil- and gasoline-resistant rubber

Styrene butadiene rubber1.png

Styrene butadiene rubber1.png

Styrene butadiene rubber













Non-bounce rubber used in tires

Some Condensation Polymers

Polymer name Monomers Polymer Use








Fibers, molded objects

(Dacron, Mylar, Fortrel)




Linear polyesters, fibers, recording tape

(Glyptal resin)




Cross-linked polyester, paints

(Casting resin)




Cross-linked with styrene and benzoyl peroxide, fiberglass boat resin, casting resin







Mixed with fillers, molded electrical cases, adhesives, laminates, varnishes
Cellulose acetate

(cellulose is a polymer of






Photographic film




Water-repellent coatings, temperature-resistant fluids and rubber






Foams, rigid and flexible, fibers

Recycling Codes for Plastic Resins

Recycling code Polymer and structure Uses



Poly(ethylene terephthlate) (PET)

Bottles for soft drinks and other beverages



High-density polyethylene

Containers for milk and other beverages, squeeze bottles



Vinyl/polyvinyl chloride

Bottles for cleaning materials, some shampoo bottles



Low-density polyethylene

May have some branches

Plastic bags, some plastic wraps




Heavy-duty microwavable containers




Beverage/foam cups, toys, window in envelopes


All other resins, layered multimaterials, some containers Some ketchup bottles, snack packs, mixture where top differs from bottom



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The History and Future of Plastics (7)-The Future of Plastics

Despite growing mistrust, plastics are critical to modern life. Plastics made possible the development of computers, cell phones, and most of the lifesaving advances of modern medicine. Lightweight and good for insulation, plastics help save fossil fuels used in heating and in transportation. Perhaps most important, inexpensive plastics raised the standard of living and made material abundance more readily available. Without plastics many possessions that we take for granted might be out of reach for all but the richest Americans. Replacing natural materials with plastic has made many of our possessions cheaper, lighter, safer, and stronger.

Since it’s clear that plastics have a valuable place in our lives, some scientists are attempting to make plastics safer and more sustainable. Some innovators are developing bioplastics, which are made from plant crops instead of fossil fuels, to create substances that are more environmentally friendly than conventional plastics. Others are working to make plastics that are truly biodegradable. Some innovators are searching for ways to make recycling more efficient, and they even hope to perfect a process that converts plastics back into the fossil fuels from which they were derived. All of these innovators recognize that plastics are not perfect but that they are an important and necessary part of our future.

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The History and Future of Plastics (6)-Plastic Problems: Waste and Health

Plastic’s reputation fell further in the 1970s and 1980s as anxiety about waste increased. Plastic became a special target because, while so many plastic products are disposable, plastic lasts forever in the environment. It was the plastics industry that offered recycling as a solution. In the 1980s the plastics industry led an influential drive encouraging municipalities to collect and process recyclable materials as part of their waste-management systems. However, recycling is far from perfect, and most plastics still end up in landfills or in the environment. Grocery-store plastic bags have become a target for activists looking to ban one-use, disposable plastics, and several American cities have already passed bag bans. The ultimate symbol of the problem of plastic waste is the Great Pacific Garbage Patch, which has often been described as a swirl of plastic garbage the size of Texas floating in the Pacific Ocean.

The reputation of plastics has suffered further thanks to a growing concern about the potential threat they pose to human health. These concerns focus on the additives (such as the much-discussed bisphenol A [BPA] and a class of chemicals called phthalates) that go into plastics during the manufacturing process, making them more flexible, durable, and transparent. Some scientists and members of the public are concerned about evidence that these chemicals leach out of plastics and into our food, water, and bodies. In very high doses these chemicals can disrupt the endocrine (or hormonal) system. Researchers worry particularly about the effects of these chemicals on children and what continued accumulation means for future generations.

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The History and Future of Plastics (5)-Growing Concerns about Plastics

The unblemished optimism about plastics didn’t last. In the postwar years there was a shift in American perceptions as plastics were no longer seen as unambiguously positive. Plastic debris in the oceans was first observed in the 1960s, a decade in which Americans became increasingly aware of environmental problems. Rachel Carson’s 1962 book, Silent Spring, exposed the dangers of chemical pesticides. In 1969 a major oil spill occurred off the California coast and the polluted Cuyahoga River in Ohio caught fire, raising concerns about pollution. As awareness about environmental issues spread, the persistence of plastic waste began to trouble observers.

Plastic also gradually became a word used to describe that which was cheap, flimsy, or fake. In The Graduate, one of the top movies of 1968, Dustin Hoffman’s character was urged by an older acquaintance to make a career in plastics. Audiences cringed along with Hoffman at what they saw as misplaced enthusiasm for an industry that, rather than being full of possibilities, was a symbol of cheap conformity and superficiality.

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The History and Future of Plastics (4)-Plastics Come of Age

World War II necessitated a great expansion of the plastics industry in the United States, as industrial might proved as important to victory as military success. The need to preserve scarce natural resources made the production of synthetic alternatives a priority. Plastics provided those substitutes. Nylon, invented by Wallace Carothers in 1935 as a synthetic silk, was used during the war for parachutes, ropes, body armor, helmet liners, and more. Plexiglas provided an alternative to glass for aircraft windows. A Time magazine article noted that because of the war, “plastics have been turned to new uses and the adaptability of plastics demonstrated all over again.”[1] During World War II plastic production in the United States increased by 300%.

The surge in plastic production continued after the war ended. After experiencing the Great Depression and then World War II, Americans were ready to spend again, and much of what they bought was made of plastic. According to author Susan Freinkel, “In product after product, market after market, plastics challenged traditional materials and won, taking the place of steel in cars, paper and glass in packaging, and wood in furniture.”[2] The possibilities of plastics gave some observers an almost utopian vision of a future with abundant material wealth thanks to an inexpensive, safe, sanitary substance that could be shaped by humans to their every whim.

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What is a hot runner- more info?

Mold-Masters hot half


Hot runner technology, introduced to the plastics industry over 50 years ago, revolutionized injection molding processing capabilities by improving molded part quality, enhancing operational efficiencies, reducing scrap and saving money.

A hot runner system is a molten plastic conveying unit used within an injection mold. In other words, a hot runner system consists of heated components (generally via electricity) used inside the plastic injection molds, which brings the molten plastic from the barrel of an injection molding machine into the cavities of the mold. The sizing of hot runner melt channels depends on many factors such as the type of resin, the injection speed, fill rate, and the molded part. A temperature controller (standalone controller or controls from the injection molding machine) heats the hot runner system within the injection mold and the resin inside the machine barrel to processing temperature and injects the resin into the mold. The resin travels through the inlet, down into the manifold which then splits to the various nozzles and through injection points (or gates) into the final mold cavity where the final part is formed. Today’s molds can have anywhere from 1 to over 192 nozzles depending on the plastic parts being manufactured.

Prior to hot runner technology, cold runners were widely used on injection molds. Cold runner molds faced many challenges in conveying the resin from machine barrel to cavities without affecting the flow and thermal characteristics of the resin. With the advancement of resin types and the complexity in mold and part designs, it became more and more difficult to control the molding process via cold runner molds to produce molded parts of acceptable quality.

However, with the introduction of hot runner technology with advanced thermal controls, processing of wider ranges of resin became more practical and convenient to injection molders. Unlike a cold runner mold, the hot runner components are individually heated to ensure the resin maintains the temperature continuously through the mold. The temperature of each hot runner heated component can also be precisely controlled to ensure the process is optimized to the requirements of each type of resin delivering the highest possible part quality. Today, hot runners are capable of producing highly complex parts in a wide range of sizes which are utilized in every industry.

Elements of a Hot Runner

  • Locating Ring – The locating ring aligns the injection mold with the platen of the molding machine. It ensures there is proper alignment of the mold with the machine.
  • Inlet – When resin is injected into the mold, this is the entry port where the resin enters from the injection machine nozzle. Depending on the type of resin and the design of the hot runner the inlet component may be heated in order to optimize the molding process.
  • Manifold – The manifold enables the flow of resin into different nozzles and injection points (gates). Manifolds are normally used where multiple cavities are injected or where more than one nozzle/gate per part is needed. Manifolds are available in a wide range of materials, designs and shapes and are often optimized to improve the molding process using CAE analysis. There are 2 main types of manufacturing techniques, gun drilled and 2 piece brazed. Gun drilled are often ideal for simpler, more economical systems while 2 piece brazed is often favoured when tighter performance criteria is required (balance, faster color change). 2 piece manifolds are also ideal for multi material or multi color molding applications.
  • Nozzles –Nozzles are components where the resin is injected into the cavity through a gate. Depending on the design, nozzles are typically installed into the mold plate with or without a manifold. A wide range of nozzle designs are available, using different materials, in order to achieve the processing characteristics of various resins that best suit the application.
  • Heater Technology – Heater technology is the basis of all hot runner systems and significantly affects molding process and part quality. There are several heating options each with their own pros and cons. Selecting the right hot runner depends on the requirements such as molding process, part performance, reliability and cost. Most common hot runner technologies have heaters with heater bands/plates, paste-in/flex heaters or brazed-in heaters.
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Understanding Differences in Hot Runner Heater Technologies – Manifolds

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.


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 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.


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.

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Hot Runners vs Cold Runners: Why You Should Be Using a Hot Runner System



Plastic components are in use by every industry and manufacturing these components through injection molding has come a long way. A wide range of equipment options exist depending on your application and capabilities. Generally speaking you have a choice between traditional cold runners or the more advanced hot runners. Each option comes with its own unique sets of pros and cons and so understanding the differences and how they relate to your application could have a big impact on your productivity and overall profitability.

Cold Runners

In a cold runner mold, the molten thermoplastic is injected into the mold which fills the runners that distribute the molten plastic to the individual mold cavities. The cold runner mold then cools the sprue, runner, and gate along with the molded part.

Cold runner molds are certainly more economical to manufacture and can be easier to maintain, however they have several major limitations compared to molds with hot runner systems:

  • Longer cycle time
  • Creates waste (sub-runners)
  • Require additional auxiliary processing equipment (robotics, re-grinding machines/employee labor to remove runners, etc.)
  • Secondary operations (degating, removal of cold runners, re-grinding etc.)

Why Choose Hot Runners

While hot runners often come with a higher upfront cost and require some additional maintenance, their more efficient design can often easily provide a valuable return on this investment. Hot runners significantly overcome the inefficiencies of its cold runner counterpart.

Hot runner systems produce less wasted plastic, have shorter cycle times, use less energy, improve gate quality, use fewer auxiliaries and require less manual labor for runner handling, trimming and regrinding.

Wasted Plastic & Energy

Depending on the part design, the cold runner can equal 50% to 250% of the mold part weight with regrind typically limited to 15% at most, so the remaining 85% is waste or has minimal salvage value. Re-grinding also adds a step in the manufacturing process and could decrease the plastic’s mechanical properties

For some markets, this waste could be much higher. The medical market requires 100% virgin resin, so all of the runner would be scrap. The energy consumption of a cold runner can double due to extra heat, cool and regrind wasted.

For many applications, the wasted runner can double the part cost.

Cycle Time

Cycle time is typically dominated by part cooling, with cooling time being dictated by part wall thickness or cold runner thickness. Even optimized cold runners cause typically 50% to 100% longer cycle times than hot runners.

Hot runners offer higher productivity yields due to significantly reduced process cycle times.

Capital Equipment

Cold runners mold with 3 plate design, trimming equipment, regrinding equipment, added chilling/cooling capacity and metering blender. Hot runners only require a manifold, nozzles and plates as well as a temperature controller, which is reusable.

Managing additional overhead and operational factors such as added chilling capacity and the noise and dust related to grinding scrap runners.

Labor Costs

Cold runner costs include runner handling, trimming, re-blending and scrap. They are prone to occasional stick in molds interrupting overall operation. Maintenance is also required on numerous auxiliaries.

Hot runners are highly automated and are ideally suited to scheduled preventative maintenance.  Interruptions are possible with failed heaters or thermocouples but depending on the hot runner manufacturer, these interruptions can be minimal.

Eliminating the cold runner saves the added labor from runner handling, gate trimming and regrinding.

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Prevent Hot Runner Downtime with Scheduled Maintenance

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.

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Hot Runner Balance and the Effects of Shear

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.


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.

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