Printer Friendly

Mold design: designing and selecting quality hot-runner systems.

Selecting a system that will consistently produce quality parts at short cycle times and that requires little maintenance begins with an understanding of good hot-runner design.

The advancement of hot-runner injection molding has made it important for the molder to understand what to look for when purchasing a hot-runnered injection mold. Because the molder wants the mold to run unattended, require little maintenance, and produce flawless parts for the duration of its life, he or she should also understand the limits of hot-runner design, why a particular hot-runner was designed the way it was, and what constitutes a quality hot-runner system. The following list of characteristics of a quality hot-runner system will aid in differentiating a good from a bad system.

Cavity-to-cavity consistency Wide molding window Easy start-up Serviceability Long life--low wear Safe to operate and maintain Price - cost effective No cycle limitations Quick color changes Innovative Standardized parts Quality workmanship Acceptable gate quality

The hot runner must also produce a part that has a small gate mark, low stress, and is well packed, not brittle, and has no burning, splay, or jetting. Of course some of these defects can also come from the injection molding machine, the dryer (if applicable), the product design, the mold design, the temperature controller, or the resin.

Significant advances in hot-runner design and modifications to the resins have made it possible to gate directly into the part with almost any type of plastic. The main advantages a hot-runner system has over a cold-runner system are the elimination of any regrind from the runner system; more design freedom on gate location; faster cycles; energy savings; and better part quality. Therefore, hot runners are the quality choice for molders looking to maximize their profitability through reduced scrap and cycle times.

The two key elements that any molder or mold builder should examine prior to selecting a hot-runner are gate/nozzle design and manifold design.

Hot Manifold Design

The function of the manifold is to convey the plastic to the cavities without affecting the resin properties. In order to provide this function effectively, the manifold should have a temperature variation of less than 5|degrees~C to 10|degrees~C over the length of the melt channels. A mold with a poorly designed hot manifold may produce degraded, unevenly filled parts with poor color changing characteristics, and have a small operating window.

Temperature uniformity is primarily achieved by reducing the heat losses, although heating the manifold uniformly is also important. Heat losses can be reduced with well-designed insulators, which are needed to support the manifold in the mold plates. An air gap of 5 to 10 mm between the cold plates and the hot manifold provides good insulation in areas where the insulator pads are not located. The manifold should be located in an enclosed pocket to reduce the amount of heat loss due to convection.

In order to make the manifold heat up and maintain temperature uniformity as quickly and efficiently as possible, the mass of the manifold should be reduced by shaping it to conform to the melt-channel pattern as much as possible. Experiments have shown that the areas with the highest mass and lowest exposed surfaces have the highest temperatures.

To reduce the radiation losses to the surrounding plates, a cover with a very low emissivity could be mounted onto the manifold. The plates surrounding the manifold should be cooled to prevent overheating, control thermal expansion, and to prevent heating of the attached cavity plate.

The heating elements in the manifold and nozzle should be durable, replaceable, formable, efficient, and low cost. External heating of the melt channels should be used for unrestrictive flow, lower shear rates, and minimal hang-up locations. The heaters in the manifold should be located at a minimal but consistent distance from all the melt channels and concentrated at the largest heat losses. To prevent the possibility of local degrading of the resin, the heater should avoid crossing a melt channel.

If tubular heaters are used to heat the manifold, they must be in firm contact with the manifold to enhance heat transfer and heater life. To achieve this, the heaters should be mounted into milled grooves on the manifold surface and surrounded by a thermally conductive cement. A rule of thumb for heater sizing is 2 W/|cm.sup.3~ of mass of the manifold plus the calculated heat loss per contact point. A manifold heater, properly designed into the hot manifold, should not need replacing during the lifetime of the mold.

An embedded or surface-mount thermocouple, plus a spare, and a closed-loop controller must be used to control the heating in the manifold. If the manifold is large, several individually controlled heater zones on the manifold may be required to reduce the temperature variations. The thermocouple(s) should be located at the average temperature point of each controlled zone of the manifold. The fact that the heat from the manifold rises will always make the top of the manifold hotter than the bottom should be taken into account when deciding on the number of heater zones to use on the hot-runner manifold and the location of the thermocouple.

The melt-channel layout in the manifold must be totally balanced. Total balance means that each nozzle drop has the same flow length, pressure drop, and shear velocity. If this is not possible, the number of cavities or the layout should be changed to make a naturally balanced runner system. Multiple cavity systems that are naturally balanced are 2, 4, 6, 8, 12, 16, 18, 24, 32, 48, 64, 72, 96 and any circular cavity layout. The number of intersection points and directional changes in the hot runner should be kept to a minimum because these are the locations where the largest risk of resin hang-ups occur. Therefore, an "X" style melt-channel layout is preferable to an "H" style. A multiple-level hot runner layout should be used to ensure total balance on larger than 4-cavity hot runners and especially on applications where high speeds or large cavitations are being used. A change in level should occur at an intersection where any leg of the runner may receive preferential flow due to differences in momentum or flow length.

To prevent the possibility of hang-ups in the runner system, which can cause severe degradation of the plastic, contoured plugs are essential at any intersection points in the hot runner. The plugs should be leakproof, removable, fixed from rotation, and have no dead corners.

To enhance smooth flow and eliminate the possibility of a skin (stagnant layer) forming on the melt channel walls, the melt channel surface finish should be 0.4 microns or better. This can be achieved by gun drilling the channels and then using an abrasive grit to "polish" the melt channel walls. This polishing also rounds the sharp corners that occur at intersecting melt channels.

A mold with incorrectly sized hot-runner melt channels can reduce the operating window of the mold, degrade the plastic, and affect the appearance of the final molded product. There are basically four criteria to evaluate when determining the melt channel sizes. By using the resin rheology, projected processing conditions, and part and hot runner geometry, these four items can be determined using any commercially available flow-modeling program. The limiting criteria are listed here in order of importance:

1. Pressure drop (|is less than~ 25% of machine capacity).

2. Temperature rise (|is less than~ maximum temperature of resin or |is less than~ 15-20|degrees~C).

3. Residence time (ideally |is less than~ 1 shot or |is less than~ 10% to 20% of time to degradation at a given temperature.)

4. Shear rate and shear stress (uniform in all legs of the hot runner and sufficiently high to wipe the melt channel walls--usually 700 to 1300 |s.sup.-1~). There is some evidence to support the hypothesis that the onset of melt fracture is a good indication that slippage is occurring at the melt channel walls.

Note that a specific resin or its additives may be shear, residence time, or temperature sensitive. This may change the criteria shown above. Also note that not all the criteria may be met for every mold. In these areas, a compromise must be determined that balances the four criteria.

Since the hot runner is mounted into cold plates, the thermal expansion that occurs during heat-up of the hot runner must be accurately accounted for in the design of the hot runner. This is usually calculated so that there is some specified clearance between the manifold and the surrounding plates in the cold condition. When the hot runner heats up, the cold clearance is eliminated, and the hot runner expands to seal off all areas where the melt channels pass through.

Gate Design

The function of the gate is to provide a passage for the resin to enter the cavity. It also functions as a freeze-off point for separating the cavity from the runner. However, the mark of a quality gate is how close to invisible it can be. This often results in a gate's being too small, or of the wrong type, or located in the wrong position. Incorrect gate design can result in higher rejects, faulty parts, part defects, and a narrow operating window.

Gate Sizing

Gate sizing must be absolutely uniform in multicavity systems because this can be the source of the highest pressure drop in the hot-runner system. Unequal gate sizing will cause cavity-to-cavity variations, which reduce the size of the operating window.

Shear-thinning resins, such as PP, PE, PA66, PBT, and POM, can be molded with a small gate in thin-wall applications to take advantage of the lower resistance to flow at high injection rates. However, do not override the critical shear rate where molecular breakdown occurs. Resins that experience large temperature rises when sheared, like PS, can also use small gates to their advantage.

High shrinkage, fast freezing resins, such as PE, PET, PP, PA66, and PBT, generally require larger gates in thick-wall applications to avoid sinks and voids. Crystalline resins, which are fast freezing, need less cooling in the gate area than amorphous materials, but they also require more heat to keep the gate area molten.

A common misconception about gating is that the bigger the gate the better the flow. The shear heating in the gate may be good for filling thin-wall or high L/t containers. Also, making the gate too big may shift the bottleneck from the gate area to the wall section just inside the cavity entrance.

Shear-sensitive resins, such as polyesters and flame-retarded materials, require larger gates to avoid molecular breakdown. For high viscosity resins, sharp edges in the gate area, which allow for easy de-gating, can cause flow disturbances. There are basically two types of hot-runner gates: open and valve gated.

Open Gate

The basic concept of an open gate is to freeze the gate rapidly at the end of injection, but heat the plastic within the runner so it stays viscous on the runner side. This functionality depends on the shape of the gate and nozzle, the heating and cooling in the gate area (thermal separation), and the accuracy of the heat controller.

Three types of open gates that have been used successfully on any grade of commodity or engineering resin are circular, edge, and annular. An annular gate is better than a circular gate in theory because it allows for better temperature control at the gate, easier plastic flow, and a lower gate vestige. However, an annular gate can sometimes produce weld lines in the parts because the flow of plastic must be diverted at the gate to allow the hot probe to enter the gate. A circular gate is identical to an annular gate except that the plastic flows from the front of the nozzle. Annular and circular gates are primarily used for gating on the top of the part. In cases where the gate must be located on the side of the part, an edge gate may be used. Since an edge gate is self de-gating (like a tunnel gate), it usually produces very low vestiges. A single edge gate nozzle drop can be used to gate more than one cavity.

An open annular gate size for multicavity tools is generally 0.4 to 1.5 mm in diameter with a land of about 1 to 2 mm and a straight portion of around 0.1 to 0.2 mm. Amorphous materials generally require smaller gate diameters than crystalline materials. Circular gates as large as 4 to 8 mm can and should be used on low cavitation molds running large, thick (|is greater than~ 2 mm) parts with long cycles. The same gating principles apply for these large gates.

A dimple can be used to aid flow at the gate because it permits a well for the frozen cold slug to be blown into. Note that a dimple is only useful where the frozen slug in the gate is larger than the wall section. This happens most often when a relatively large gate diameter is being used on a relatively thin-walled section. A dimple should have a large radius and produce a section equivalent to the cold slug from the gate.

A clearly defined freeze-off point and a sharp notch should be used at the gate when molding highly elastic polymers because the resin may pull out of the gate.

Valve Gate

In a valve-gated system, the shape and size of the gate are not as important because it is physically shut off when the valve pin comes forward. In fact, valve gates usually have a wider operating window than open gates since the low gate vestige and quality are assured by the valve pin. Valve gates are generally used where the required gate vestige is zero and the gate size needs to be large. In a valve-gated system, it is very important to have effective cooling of the valve stem. This can be accomplished with a tapered shut-off angle on the gate and a cooled gate insert.

Valve gates are often used when a large gate (|is greater than~ 2.5 mm) is used in conjunction with a resin that strings easily (PP, PE, polyesters, polyamides). Valve-gate sizes should generally be less than 4 mm in size to avoid a hot valve pin. Larger sizes may require cooling of the valve pin itself. A valve gate less than 1.0 mm is generally not necessary since an open gate could produce an equivalent gate quality at a lower cost.

Nozzle Design

Nozzle design is critical to the success of a particular gate design. If the nozzle cannot control the gate temperature very accurately, the gate may string from being too hot or freeze off and tear from being too cold. The nozzle must provide just enough heat to the gate to keep it molten but prevent drooling. Therefore, the heat supplied to the gate must be very carefully controlled by either fluctuating power to the gate or precisely balancing the cooling and heating in the gate area. In order to separate the hot nozzle tip from the cooled cavity plate, a space, called the insulating bubble, is machined into the gate detail. This insulating bubble fills with the molten plastic and acts as an excellent insulator.

The nozzle must be sturdy to withstand the high pressures generated from the heat expansion of the hot components, yet is must also have low heat losses to reduce unnecessary heating of the nozzle. The tip of the nozzle should be as close to the molding surface as possible and the nozzle should be made of a wear-resistant material with high thermal conductivity. In cases where an abrasive resin may erode the nozzle tip, a wear-resistant coating may aid in increasing the life of the nozzle tip.

The flow path in the nozzle should be smooth, unrestricted, and free from hang-ups, just as the manifold is. A nozzle with hang-ups can cause streaks or flow lines in the molded parts and severely reduce the ability to conduct color changes. In cases where many color changes are required or the resin is temperature/time sensitive, the plastic that fills the insulating area in the gate area may have to be replaced by a low-conductivity material that does not melt to facilitate flushing of the nozzle area.

The nozzle heaters should be able to provide a sustained given temperature at the tip area while not overheating the resin in the rest of the nozzle. As such, the watt density of the heater on the nozzle should be changed along its length to reflect the heat demands of different areas. A thermocouple should be located in the nozzle, as close to the gate as possible, to provide the closed-loop heat controller with accurate information on the temperature at the front of the nozzle. The temperature should be fed to a fast-cycling Pi|D.sup.2~ controller. With commodity resins such as PP, PE, and PS, the additional cost of the closed-loop controller and thermocouple is not absolutely necessary.

Locating the Gate

How and where to gate depends not only on the geometry of the part but also on the shear viscosity characteristics of the molten plastic, its freezing rate, and the crystallization kinetics of the resin. Keep in mind that a gate cannot compensate for poor part design and it is poor practice to balance the filling of a mold with gate sizes.

Where Not to Put the Gate

The mechanical properties of the resin in the gate area are different because of high shear flow. The high shear flow can cause increased crystallinity and molecular orientation, which in turn can make the gate area brittle. Therefore, the gate should not be located where it would be subject to direct impact in the molded part, in an area of flexing, at a weak spot, or stressed point in the part.

Because the gate mark is considered a necessary defect, it should not be located where the possible flow, blush, or splay marks will be highly visible and objectionable. Because the plastic near the gate experiences very high pressures at the end of injection, the gate should not be located where the force of the injected plastic may cause the deflection of core pins or other mold components.

The gate should not cause any flow defects by its location; therefore the gate should not be located where it will cause an unnecessary knit line or an air entrapment. Do not put the gate in an area where there is no direct cooling. Do not gate onto a rib or into a long unobstructed flow path, as this may cause jetting.

Where the Gate Should Be located

The gate should be the last area to freeze off. Therefore, always gate in the thickest section of the part if possible to provide for the best packing of the part. Gating into the thickest part of the mold allows resin to flow, unrestricted, from the gate during packing so as not to starve any wall sections that are still shrinking and solidifying when the gate area freezes. The exception to this rule is when air can become trapped by a thick-walled frame around a thin-walled section.

The gate should be positioned for symmetry of flow to optimize the flow path. Plastic from the gate should be aimed onto the core to prevent jetting. Because the hot-runner gate is a source of heat, it is essential that the hot runner/mold design provides a dedicated cooling circuit for the gate. Where there is no cooling there should be no gate. Flow modeling can be used as a valuable tool to evaluate different gate locations before the mold is built.
COPYRIGHT 1993 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:includes related article
Author:Catoen, Bruce
Publication:Plastics Engineering
Date:Apr 1, 1993
Words:3293
Previous Article:Flow analysis: effects of finite element meshing on mold filling analysis.
Next Article:SPE Awards ANTEC '93.
Topics:

Terms of use | Copyright © 2016 Farlex, Inc. | Feedback | For webmasters