Common sense about runnerless molding.

Common Sense About Runnerless Molding

The first thing to realize about runnerless injection molding is that almost any commercially available system will work to some level of satisfaction with certain materials and applications, if applied properly. And yet no one system will work universally. Therein lies the source of great confusion and controversy. Proper selection, implementation, and use of the correct system for your specific application can mean the difference between success and disaster.

Unfortunately, in the years I have been involved with runnerless molding products, I have seen very few guidelines published on proper selection of runnerless equipment. Predictably, I have also seen the same basic mistakes being made over and over again by different molders. Only a few processors have really made a science of implementing such systems. And the ones that have been successful at implementing such a program are understandably reluctant to divulge the information that they have so painstakingly learned. This article is intended to help fill that gap.

Most engineers and mold designers depend upon runnerless molding system manufacturers for information regarding proper product selection and implementation. But it's important to note that most manufacturers or distributors of such systems are not molders! Most of their information regarding applicability of their equipment comes from feedback from the field. Since there are many pitfalls in runnerless molding, it is important for you to ask whether the supplier has specific knowledge of products, materials and molds exactly like the ones you propose to use. If not, his advice regarding suitability and adaptation to your mold may be seriously flawed or exaggerated.

I am not, nor have I ever been, a molder myself. However, I have spent more than a dozen years working with molders and moldmakers throughout North America and Asia, advising them on selecting and installing runnerless systems for several hundred applications and mold starts, involving all kinds of products and materials. Most of the information has been learned the hard way and has been seasoned with time and reflection on both the successes and the failures. Although I no longer market or sell any type of runnerless system, I am still frequently asked for advice on this troublesome subject. So I offer here what are intended to be objective and generic guidelines, based on a commonsense approach to defining what all runnerless systems do and how they do it. It is my sincere hope that the basics covered here will be of help to the more accomplished molders and shorten the learning curve of the beginner in runnerless molding.

GETTING DOWN TO BASICS

Before selecting a runnerless molding system for a specific application, you must have a basic understanding of the process and your specific application needs. Resin manufacturers generally recommend that you use a molding method as free as possible of overheating, shearing, stressing and pressure, in order to preserve the molecular structure of the material and, hence, its physical properties. Therefore, runnerless systems which allow processing with low, uniform heating, and have an open flow path where the material can flow at low pressure, are preferred over more restrictive systems with great variances in temperature along the melt path.

However there are many considerations when selecting a system for your specific needs. Some of the general considerations are:

* Usable melt-temperature range of the plastic material;

* Part size and distance the melt must blow within the system (residence time);

* Gate size;

* Part cosmetics;

* Dimensional requirements;

* Projected cycle time;

* Frequency of color or material changes.

Products with more general specifications, and smaller molds with fewer cavities, will be compatible with many more brands and types of runnerless systems than those with very stringent requirements. It is important to know and understand the characteristics of the various types of systems available and their limitations to make a good choice for your specific needs.

UNDERSTANDING MANIFOLDS 

The basic purpose of a manifold system is simply to act as an extension of the injection machine nozzle, to convey molten material to each gate location within the mold. There are generally three basic ways that this is accomplished:

1. The insulated runner method is the oldest and simplest means of conveying melt. It uses large-diameter runner passages between the machine nozzle and the gates. When the plastic material is shot into the mold, the material forms an insulating layer throughout the passages, which acts to hold in the heat from the melt at the center of the flow channel. Since these runner passages are generally split between two plates in the mold, the mold may be opened while in the press and the solidified runner easily removed, facilitating color or material changes.

Since the melt flows through the runner obstructed only by the insulating layer of plastic that has formed on the flow-channel walls, the molecular structure of the material is usually not greatly altered until the insulating layer (over time) becomes so thick as to become restrictive to flow, necessitating removal of the solidified runner. Therefore, some deliberate overheating of the material at the barrel is common to postpone the decreasing-flow-channel effect.

This approach is very basic and can be cost effective; however, it should only be considered for general-purpose materials and simple-to-mold parts with very few hot gates (drops), and where relatively short runs and frequent color changes are prerequisites. This method should definitely not be used where you anticipate any start-up problems or molding interruptions, as the unheated runner requires a constant flow of molten plastic from the molding machine to remain open. Depending on the material, a stop of more than a few moments will require pulling the solidified runner from the mold.

2. Annular heated runners, commonly called distributor tubes or runner torpedoes, are an adaption of the insulated-runner type manifold. Instead of allowing the melt to flow unheated through the manifold passages, annular heaters are installed within the runner to maintain the melt temperature so that the runner will not "freeze off" if molding is interrupted.

Despite the presence of an internal heater, a frozen layer of material will still coat the wall of the flow channel through the cooled mold. Because the melt must flow between the frozen insulating layer of plastic and the heater in the center of the channel, the movement of melt is somewhat restricted. In fact, the melt-flow layer around the heater is usually less than 0.050 in. thick throughout the length of the runners. The longer the distance of flow through such a restriction, the greater the resistance and consequent shear-induced damage to the molecular makeup of the material. Also, because the material is in direct contact with the heater, any nonuniformity in the heater winding will cause temperature variations in the melt. Many annular heaters exhibit more than 50 [degrees] F variance down the length of the heater.

Another factor to consider with annular-heated manifold systems is runner balance. The pressure drop in a typical annular heated flow channel is much greater than in an externally heated or open flow channel, due to the greater drag resulting from the higher ratio of surface area to volume in the flow channel (see Fig. 1). The relatively high pressure drop in an annular-heated channel will exaggerate the effects of an unbalanced runner system. That is, positioning the probes so that the flow distance is not the same to each probe will often cause overfilling of some cavities and underfilling of others (see Fig. 2).

Therefore, flow imbalance will be more critical with annular heating than with externally heated, open flow channels. This certainly does not mean that annular-heated manifolds are unworkable. But it does mean that best results with this type of manifold system will generally be achieved with broad-temperature-range materials and balanced runners with equal flow distances to all drops.

3. The third approach is the hot manifold. Generally, this is simply a solid block of steel with runner channels drilled through it. Heaters are attached to heat the entire block to the recommended melt temperature. While more costly to manufacture and install in the mold, hot manifolds do offer several advantages. Since the entire block is heated to the melt temperature, there is no insulating layer of plastic or annular heater to impede flow. The result is less resistance to flow and more uniform pressure to each drop. The manifold mass also distributes the heat more evenly than do annular heaters, resulting in less temperature variation throughout the runner passages.

Some manufacturers offer hot manifolds that are balanced by runner diameter rather than length. These manifolds are generally more compact and easier to manufacture than manifolds with equal flow lengths. However, while these manifolds may be adequately balanced with a specific material at a given shot size and pressure, balanced runners of equal length are much more versatile and are preferred for all types of manifolds.

HOT-MANIFOLD CONSTRUCTION

Proper construction of the manifold components is essential. The diameter and placement of runner passages, as well as placement of heaters and thermocouples, are critical to temperature stability and material residence time within the system. Processing materials with narrower temperature ranges - such as nylon 66, PET and PVC - obviously requires more consideration of these factors than do g-p materials with broader processing ranges. In all cases, the basics for a good hot manifold are:

* Contoured runner ends and plugs with no "dead spots";

* Uniform heating without heaters crossing above or below runners;

* Balanced runner flow paths to all drops;

* Small physical size.

Since the entire manifold is heated to the processing temperature, it is essential that there be no "dead spots" where material can stagnate and degrade. Removable runner plugs are desirable to facilitate cleaning the manifold runners if material accidently is burned and cannot be purged from the manifold.

Most modern hot manifolds use tubular heaters ("Calrod" type), rather than heater plates or cartridge heaters. The advantages of tubular heaters are that they may be bent to contour with the shape of the runner system (Fig. 3), and therefore need not cross above or below runners, causing hot spots (Fig. 4). Tubular heaters also allow lower watt density per inch of heater and inherently reduced manifold size, providing a more compact manifold package and more even heating, which is critical with narrow-temperature-range materials. More compact manifolds allow greater mold strength and design freedom, and they require less wattage to maintain processing temperatures than large, bulky manifolds. Another benefit of tubular heaters is reduced thickness of the manifold, which allows less stack height in the tool. Finally, tubular heaters give considerably longer service than cartridge heaters.

But regardless of the heaters used, good thermal contact between the heater and the manifold block is essential to prolong heater life and to heat the manifold uniformly.

MOLD CONSTRUCTION AND GATING

The most crucial aspect of a successful runnerless mold is the integration of the manifold system and gating devices into the mold. Each molded product and each type of gating device will impose its own particular requirements. The mold is mainly a heat exchanger. The manifold system is a means of conveying melt to the heat exchanger with minimal heat loss. The two systems are at odds with each other and therefore must be isolated from each other to work efficiently, and yet the gating device must successfully make the transition between the two systems. It must allow the flow of melt into the cavity without stagnation or freeze-off, and yet allow degating of parts without drooling or stringing.

All gating techniques have one thing in common. They must exchange heat from the gate area. Even valvegate systems must exchange some heat from the valve pin when closed, so that the material will not stick to the valve pin when the part is removed. Hot tips, sprue gates and edge gates all depend on a thermal slug or barrier of semi-molten plastic at the gate to allow clean degating of the part. This thermal barrier must be solidified enough to allow the part to break away cleanly and yet be molten enough to re-establish flow upon the next injection shot. Different materials and systems require varying amounts of heating or cooling in the gate heat-exchange area to keep the thermal slug at precisely the correct temperature. Therefore, gate design and cooling are extremely important.

Runnerless equipment manufacturers generally are the best source of information regarding gate configuration and tolerances, especially if the manufacturer has specific experience with the material and product that you will be molding. However, in most cases the manufacturer will not have direct experience with your product requirements and therefore you must design with maximum flexibility. Whenever practical, use individual gate bushings with a baffled, 360 [degrees] cooling channel near the gate. Use a separate cooling circuit for each gate, with sufficient pressure to allow positive flow through the baffles, to give yourself the flexibility of heating or cooling the gates as needed to maintain proper gate slug temperatures, without affecting the rest of the tool (see Fig. 5). If individual gate bushings are not practical, place cooling lines as close as possible to the gate heat-exchange area and feed as few gates as practical on one circuit. Under any circumstances, do not connect in series ("loop") the water lines controlling the gates, since that will produce differences in gate performance from the first gate to the last on the loop.

MORE HEAT-EXCHANGE FACTORS

All manifold and probe contact areas should be minimized to prevent unnecessary "bleeding" of heat into the mold base. Excessive contact area makes the hot half of the mold grow larger than the ejector half, which can cause binding on the leader pins (especially on new molds that are not "worn in"), as well as contributing to molding problems such as part warpage and longer cycle times. It also will cause the heaters to work harder to maintain temperature and cause uneven heating of the manifold block. Many runnerless equipment manufacturers provide large contact areas on pads or bushings that can be reduced or relieved to reduce the contact area. However, too much reduction can lead to problems - you could lose proper support for the manifold or probes, or the seals might leak. You should discuss any modification of manifold or probe contact areas with the manufacturer before proceeding with changes, so that you do not void the warranty on the system.

To further isolate the manifold system from the molding process, a secondary plate should be used between the manifold and the molding plates. Except when molding extremely high-temperature materials that require heating the molding plates, cooling lines should be placed into this secondary plate to prevent bleed-through of heat from the manifold to the molding plates. This secondary plate will also serve as a retainer to hold the manifold and drops together when you are removing the molding plates or inserts.

An important point that's sometimes overlooked is that the manifold should be completely encased in steel on all sides to form an oven. Some makers leave the ends of the manifold chamber open. This is thermally inefficient and wastes energy, and it can be dangerous, because it leaves the hot manifold components exposed. If the manifold is completely enclosed, the heaters will not have to work as hard, the wiring and connectors will be better protected from manifold heat, and the manifold will be more uniform in temperature.

Also, an insulator sheet should be used to form a thermal barrier between the "hot-half" clamp plate and the molding machine platen. This will prevent the platen from draining heat from the manifold, thereby making it easier to maintain the manifold at proper, uniform temperature.

Using a secondary, "retainer" plate between the molding plate and manifold allows the manifold/probe assembly and wiring to remain undisturbed when servicing the rest of the mold. By unbolting the molding ("A") plate from the retainer plate and adding swing latches to bridge the parting line, the A plate may be transferred to the ejector side while in the press to facilitate color changes, removal of debris from clogged gates, or servicing thermocouples and heaters (see Fig. 6). The secondary retainer plate will hold the manifold assembly together and will even allow purging of material through the manifold system with the molding plate removed. Since this procedure eliminates the gate restriction, a great volume of material can be moved at low pressure for color changes or for clearing degraded material from the manifold.

TYPES OF GATING

The most common method of conveying melt from the manifold to the part is via a "hot tip" or probe. There are many different types of probes available, including high-voltage (240-v) and low-voltage (24-v, 26-v) types, and probes that have no heater windings - such as high-amp induction units and probes containing "heat-pipe" type devices. The most important aspect of any probe is how well it conducts heat to the tip. Until enough heat is conducted to the tip of the probe, which controls the plastic material in the heat-exchange area of the gate, you can do nothing. The trick is to maintain the critical temperature balance necessary to allow injection without freezing or drooling and without overheating the material around the rest of the probe. Therefore, how evenly the probe heats along its length and how efficiently it transfers the heat to the tip will determine how suitable it may be for different materials.

Materials with a very narrow processing-temperature range require probes with very good temperature uniformity throughout the body and tip. General-purpose materials are more forgiving in this regard. Dual-element "Spear"-type probes with separate body and tip heaters are good for narrow-temperature-range materials, since the body need not be overheated to maintain the correct gate temperature. However, they are quite expensive and need specialized controls. In most cases, very good results can be achieved by simply using probes with heater windings that are "shaped," putting more windings near the tip and fewer windings in the mid-section of the probe. If these windings have a good thermal contact with the probe, heat will be transferred to the tip, and the overall temperature of the probe will be more uniform, giving the molder a greater margin of safety.

Valve gating is another method that can be very successful for narrow-temperature-range materials. Because the gate is opened and closed mechanically by means of a valve pin, the probe body temperature does not need to be elevated to control the gate. However, some materials require more heat transfer to the gate than others for a clean shut-off. Therefore, separate cooling lines and directly cooled gate bushings are desirable for this gating method, as well.

Most of the valve-gate systems available use externally heated probes with an "open" internal melt channel and valve pin. When used with an externally heated manifold system, there is no insulating layer within the runner system. Since the gate is opened and closed mechanically rather than thermally, the gate sizes may be quite large with little or no gate mark or "vestige." These factors make such a system ideal for large parts, temperature-sensitive materials and jobs requiring frequent color changes.

However, there are disadvantages as well. Valve systems are more costly and require more installation time by the moldmaker, as well as more maintenance over the life of the mold. While most modern systems are positively opened and closed by hydraulics or pneumatics, some systems still use valves opened by a build-up of injection pressure and closed by some type of spring. Considering the hostile environment in which these systems are expected to work (high temperatures, pressures, corrosives, abrasives, etc.), valve systems that are positively opened and closed are much more likely to work over the long term.

EDGE GATING: A SPECIAL CASE

Runnerless edge gating is one of the most difficult areas in which to make recommendations, because so much depends on the specific needs of the molder and on the configuration of the part to be molded. The main advantage of hot edge gating is that as many as four parts can be molded from a single drop. This makes the manifold and control system less costly than direct gating each part with a separate probe. Also, edge gating leaves only a small mark on the part, since the gate material is sheared when the part is removed from the cavity. However, if you want to shut off a damaged cavity or you have a heater failure on the drop, you will lose all of the parts being fed by that drop.

One of the biggest disadvantages to edge gating is that it requires some type of rib or area on the part that will allow a very thin (usually 0.020-.040 in.) gate land from the drop to the cavity (Fig. 7). Molded parts with a protruding rib opposite a core are ideal. (The core opposite the gate is needed to break up the gate slug on the next shot.) It is very difficult on most parts to have such access to the cavity, and it is difficult to get cooling near the gate to control the gate slug temperature. Poor cooling at the gate can cause distortion of the molded part in that area. High temperature and pressure in this thin area can lead to gate land cracking. H-13 steel seems to give the best service for such applications.

Also, since you are trying to control more than one gate with only one heater element, the position of the drop relative to each gate must be exactly the same at operating temperature. Because of these stringent requirements, edge gating is generally not suitable for precision parts or narrow-temperature-range materials. Therefore, projects using edge gating should be chosen carefully.

THERMOCOUPLE LOCATION

One of the most overlooked areas of runnerless molding is thermocouple placement. Absolutely the worst place for a thermocouple is inside a heater element. Such placement can only tell you the temperature inside the heater at whatever point the thermocouple is placed and has no relationship whatsoever to melt temperature in the flow channel! If, by luck of the manufacturing process, the thermocouple is placed near the heater element, your temperature controller will constantly turn on and off, trying to maintain the resistance wire temperature at the controller set point, rather than the temperature of the probe or manifold that is being heated. This is not only hard on the heater, it slows the start-up process and gives you an unreliable source of temperature information.

For hot manifolds, the thermocouple should be placed an equal distance from the heaters being controlled, near the main runner passages in that area of the manifold (Fig. 8). After all, it is the runner temperature that we are concerned with. Spare thermocouples may be placed at similar points on the manifold at the time of installation. In the event of future thermocouple failure or the need to verify readings, the auxiliary thermocouples may be used. This can save many hours of disassembly, or having to run the manifold on manual control, in case of thermocouple failure.

On annular runner heaters, the thermocouple must necessarily be placed within the heater, since there is no mass of metal being heated. Temperature settings considerably higher than the desired melt temperature are normal on such systems, since you are actually monitoring and maintaining an internal temperature of the heater near a resistance wire that could be as hot as 1400 F.

Thermocouple placement in probes or drops is extremely important. As stated previously, the critical factor in thermal gating is consistently maintaining the temperature of the melt slug at the gate. The ability to maintain the temperature balance necessary to prevent freeze-off or stringing/drooling will usually be the determining factor in shot-to-shot repeatability and overall part quality. Thermocouples should be placed as near the gate as possible, and they must be in good thermal contact with the probe or drop mass to quickly read surface-temperature changes. In hot probes, the thermocouple should extend all the way to the tip so that it can sense temperature fluctuations from shear heating or mold coolant temperature changes as soon as possible (Fig. 9).

Temperature controllers which sense temperature acceleration (not velocity) should be used to react as soon as possible to compensate for temperature upsets. Thermocouples located away from the gate, or that are not in good thermal contact with the mass being heated near the gate, are of little use in maintaining shot-to-shot repeatability, because they ignore the thermal conditions during each shot. Because the thermocouple cannot "see" the short-term upsets at the gate, these upsets go unreported. The molded parts will vary in quality and appearance from shot to shot and yet the temperature controller will indicate that the probe temperature has not changed. The controller cannot report what the thermocouple does not see!

MORE ON TEMPERATURE CONTROLS

Temperature controls for use with plastic materials that degrade easily should be integrated into the molding machine's alarm circuits, so that current to the manifold system and drops is reduced or cut off if the machine stops molding for a predetermined period of time. This will help prevent material from burning in the manifold while press or other equipment problems are being resolved. Some temperature-control systems offer a standby mode that can be utilized for such a purpose. Using a delay timer between the machine and the controller will allow a few minutes to pass to correct the alarm condition before turning down the current on the runnerless molding system.

Some manufacturers advocate putting more than one probe or drop on a single temperature controller to save money. In most cases this is not a good bet! The subtle variances in probe position, length, concentricity with the gate, heater output, heat-exchange rate at the gate, etc., prohibit long-term success using the same controller for multiple drops. The end result will be that the coldest gate will dictate the temperature and performance of all other gates on the same circuit. If seven gates must string or leave blush marks on the parts because the eighth probe requires higher heat to prevent freeze-off, you will be stuck with that condition at least until the next mold tear-down. Chances are, you will throw away in rejected parts many times the value of the additional temperature controls you "saved" your self from buying.

Another consideration in selecting equipment is heater life. It makes no sense to spend time and money on runnerless equipment to increase productivity and lose most of the efficiency you have gained to downtime replacing heaters. Temperature controllers with true low-voltage wet-heater bake-out and current-limiting heater protection will help, but selecting heaters that are designed for long life and that have good thermal contact with the mass being heated will also reduce the likelihood of heater failure.

Systems that use low-voltage heaters for probes or drops are known for extended heater-element life. However, there are problems with low-voltage systems. Some low-voltage probes may have trouble reaching the higher temperatures required by many of the newer engineering materials. Low-voltage bushings or externally heated drops have an even more severe problem in reaching high temperatures, due to their increased mass and contact with the mold base. Also, special controls will be needed for the low-voltage heaters, in addition to the high-voltage controls for the manifold.

High-voltage heaters can reach very high temperatures and do not require special controls, but tend to burn out more easily. High-voltage heaters cast in conductive metals such as aluminum or copper to increase heat-transfer efficiency keep the internal temperatures of the heater element lower, offer longer life and more even heating of the probe or drop. And they have the ability to reach virtually all processing temperatures.

ABOUT HIGH-CAVITY MOLDS...

High-cavitation, direct-gated runnerless molds should be avoided unless you have considerable runnerless molding experience, plus R&D time and money. The complexities of directed-gated molds of 32 cavities or more are much greater than molds with fewer drops into sub-runners that feed clusters of parts. Almost every runnerless molding equipment manufacturer has success stories of runnerless molds with 64, 96, 128 or more cavities. However, they do not tell you what the overall success and downtime ratio has been when compared with a smaller system feeding clusters of parts via small sub-runners. They also do not tell you how long it took the customer to perfect the molds or train the molding and maintenance personnel.

My basic feeling is that most often on molds with high numbers of cavities for small parts, a simpler manifold system with probes or drops feeding into a sub-runner to a cluster of parts will out-produce the faster direct gated mold. This is because the sub-runner mold will not have as many runner-system downtime hours, part rejects, or part-inspection costs over the life of the mold. Because the probes or drops feed into a sub-runner rather than directly into the cavities, the gate temperatures become much less critical and therefore easier to maintain consistent quality levels. Also, the residence time in the manifold system will be greatly reduced on the sub-runner mold, which may be a consideration with temp materials.

On programs with extremely high volume and more than one or two high-cavitation molds on which to base production, direct-gated high-cavity molds may be a valid consideration for second-or third-generation molds, after considerable runnerless molding experience has been achieved with that part.

SOME BASIC ADVICE

It's impossible to generalize about what will work best in a specific application. There are too many factors, and the variables are much too complex. Many molders are quite proficient in the use of runnerless molding systems for their products and can afford to be more adventurous in trying new ideas. However, most molders have limited time and money to spend searching for the right combination. And, no matter how experienced the user, new products or mold sizes create entirely new runnerless molding problems that can make existing methods obsolete. For example, jobs that run fine on a four- or eight-cavity, annular-heated system may not be satisfactory at all for a direct-gated, 32-cavity mold using the same runnerless system.

The most important aspect of selecting a system for your application is to pay attention to the basics. The more stringent your product requirements, the more important it is that the entire system be evenly heated and free flowing, with an equal-length, balanced runner system. There is no way to succeed if the equipment cannot meet the basic temperature requirements of the plastic material.

New products are being introduced constantly by the various runnerless equipment manufacturers in their efforts to improve effectiveness and increase the number of applications to which their products apply. However, as I indicated before, few manufacturers are molders; and even the ones who do have facilities to do pre-introduction testing of runnerless molding systems could not possibly test and evaluate all of the various methods and applications for which their equipment may be used. Therefore, for most applications, use equipment and methods that have been proven over several years in the field. New equipment with features that have great advantages for your applications should be tested in an R&D environment or on a limited scale before betting your entire program on it's suitability for your purpose. I have never seen a new runnerless equipment introduction that did not require at least some changes within the first year or two of manufacture. The reasons for the changes were generally learned in someone else's mold!

Finally, use vendors and suppliers that will back up their products and services, not only from the factory, but also in the field. Use suppliers that have demonstrated their abilities in your specific product area, if possible. Demand that they work closely with you through design, installation and mold start-up phases of new programs to ensure that the critical details necessary for successful installation and operation of the equipment are understood by everyone involved.

PHOTO : No runnerless molding system is university satisfactory with all resins and molds. So selecting the right one for your application can make all the difference between success and failure.

PHOTO : FIG. 1 - CYLINDRICAL VS. ANNULAR FLOW PATHS Externally heated, cylindrical flow paths are much less restrictive than annular heated runners, making the former preferable for molds with lots of "drops."

PHOTO : FIG. 2 - BALANCED VS. UNBALANCED RUNNERS Unequal flow distances, while always undesirable, can have especially bad effects with annular heated runners, owing to large pressure drops.

PHOTO : Fig. 3 - Tubular "Calrod"-type heaters can be bent to the contour of the runner system, providing good thermal contact.

PHOTO : FIG. 4 - IMPROPER HEATER PLACEMENT Heaters that cross the runner create a hot spot and potential resin degradation. As shown, manifolds should be enclosed on all sides.

PHOTO : FIG. 5 - CLOSE-UP OF GATE AREA Attention to cooling and gate details will yield best results.

PHOTO : FIG. 6A & B - SERVICING A RUNNERLESS MOLD With secondary retainer plate to facilitate servicing, cavity plate bolts are removed (left) and swing latch readied for transfer of cavity plate to ejector side of mold (right). Then, plastic may be purged freely, gates unclogged, and even probe thermocouples replaced.

PHOTO : FIG. 7 - RUNNERLESS EDGE GATING Edge gating at a rib or corner of the parts provides better cooling and a stronger gate land that resists cracking.

PHOTO : FIG. 8 - MANIFOLD HEATER & THERMOCOUPLE PLACEMENT Note manifold heaters at equal distances from, and parallel to, the runner, and thermocouple equidistant from the heaters.

PHOTO : FIG. 9 - PROBE THERMOCOUPLE PLACEMENT For best response to shear heat and gate temperature changes, probe thermocouples should be as close to the probe tip as possible.