DESIGN OF EXTRUSION BLOW MOLDS

The actual design of a blow mold will be guided by the standard data available to adapt the mold to the machine and the applicable process. The mold cavities will be replicas of the exterior surface of the blownware product with the essential material shrinkage added to the product dimensions. A typical mold is shown in Fig. 1.

Fig. 1.

The blow mold designer’s initial consideration is the location of the mold parting line. This is determined first by the shape of the article which must be molded so it can be ejected from the mold, and secondly, by its appearance. The mold parting line should be positioned so that there is no tendency for the molded piece to “lock” in the mold. Most thermoplastics used for blow molding are sufficiently hot and pliable at the time the mold opens to permit some negative relief or hook. Care must be exercised in the amount of hook used, for an excessive amount will cause the finished part to stay in one mold half. While some form of manual or automatic ejection can be employed, parts with excessive hooks are subject to severe surface scuffing-Intentional hooks can sometimes be designed into tooling advantageously-For example, opposite and opposed hooks which create a force couple at t e time of mold opening will spin the molded article and allow it to eject freely.

The mold parting line should be hidden in the container design if at all possible. It should also be kept off surfaces which will subsequently be decorated with silk-screen, hot stamp, or paper labels. Any one of these labels will show some irregularity or flaw caused by the uneven surface associated with the mold parting line. The quality and speed of molding should not be sacrificed for appearance design. On a square bottle, for example, it might appear advantageous esthetically to hide the parting line on two corners by having it run diagonally across the bottle. A closer analysis will show the hidden parting line is not worth the irregular wall distribution which will result from the pinch-tube method. The base corners perpendicular to the parting line will be much thinner than those on the parting line, and will cause bottom distortion through uneven cooling.

With the parting line established, the extremities of the mold are selected within the limitations of the blow molding machinery on which the mold will be used. At this time the cavity is positioned most advantageously relative to the extruded parison. For most symmetrical items the cavity and parison centerlines coincide and are parallel. There are, however, times when the wall distribution of the final molded part can be improved by tipping the mold cavity centerline relative to the parison centerline.

Additional Requirement for Molds

(1) Pressure air. Some molds require air pressure for their operation. In general, the designer should be aware that compressed air, especially in large volumes, can be very expensive, especially if it is left to blow for any length of time.

• Blow downs(air jets or air curtains) are often used to assist the products to rapidly clear the molding area. There are several commercial air jets on the market with low consumption of pressure air. Their initial cost is paid back rapidly by savings from wasted air volume.
•    Air-operated actuators.The air volume used is usually small, compared with a blow down. There could be problems with controlling the speed and uniform motion of air actuators, but they are simple and inexpensive.
• Air required for air ejection,which is usually activated on demand, for a very short time. Most of the time, the actuation time is controlled from the machine control panel. The designer must make sure that the intended machine is equipped with sufficient controls and hardware (timers, valves, and large enough supply lines). It may be even necessary to add pump capacity, for the added volume of air that will be required for the planned mold. If much air is needed for short blasts, one or several suitable accumulators could be installed near or even on the mold.

Where pressure air comes into contact with the molded products, for example, in blow downs or in air ejection, the air must be filtered from any oil residues, water (always present in air lines), and so on, before reaching the outlets in or at the mold, to prevent contamination of the products if they are used for food or pharmaceutical purposes. (Unfortunately, most air actuators require lubricated air, unless their seals are selected for dry air.) A low-pressure, high-volume blower with its air intake from the shop environment, or better yet, from within an enclosure built around the molding machine when special “clean room” requirements are specified, is a preferred solution to ensure that there is no oil or water contamination in the air as it comes into contact with the plastic products. In many cases, such blower can be directly mounted on the top of the mold. Another advantage is that the power consumption of this type blower is low, on the order of 0.2kW (1/4hp) or less, and does not require timing or valving.

(2) Auxiliary hydraulic supply. For some operations, compressed air may be not suitable. (a) Air cylinders are often jerky in their operation, especially with long strokes. (b) In cases where several air cylinders actuate one large mold member, the forces can be uneven and the member can jam. (c) In most molding shops the compressed air pressure is fairly low, usually about 600kPa (80psi), and rarely 900kPa (120psi), so large air actuators are needed to produce large forces. It could be difficult to accommodate sufficiently large cylinders within the available mold space, or even outside the mold. In all these cases, the much more powerful hydraulic cylinders would be an alternative. The hydraulic pressure could be taken from the machine system with a pressure reducing valve, and by providing the necessary safety measures to protect against the very high pressures in that system. A preferred method, however, is to use an auxiliary power supply, usually at a system pressure of about 3,500 kPa (500psi). This is much safer and requires much less expensive hardware (valves， hoses，etc.) than that for higher pressure. The motion of hydraulic operators is smooth and the speed can be well controlled.Two points of caution， though. Hydraulic oil (with some special， expensive， exceptions) is highly flammable and there is always the danger of leaks， especially if the leaks were to occur near heated areas of the mold, for example， near a hot runner system. Also， products used in the food or pharmaceutical industry could be contaminated by the oil; this is usually specified as not allowed.

(3)    Cooling water supply. This is a very important area of concern. There is not much sense in designing the mold with very sophisticated cooling circuitry if the cooling water supply is insufficient in temperature，volume，and pressure. An individual chiller unit may be the answer if the plant supply is too small or has not enough pressure. It is also important that the coolant is clean， that is， with a minimum of minerals or dirt，and is not corrosive. Dirty coolant could gradually plug the water circuits or coat the channel walls with a poor heat conducting layer of dirt and lime，thus reducing the cooling efficiency，and could require frequent cleaning of the coolant channels if the mold is expected to maintain high productivity. Corrosive action of the coolant could attack and eat away the mold steels; rust creates insulating layers similar to lime and dirt deposits. It is always good policy for the designer to check with the molder to ensure that there are no such problems with the water supply， and to specify that only clean，noncorrosive coolant is used with the mold.

(4)    Electric power and controls. The electric power supply in North America and in most developed countries is usually sufficiently stable and uninterrupted， except during natural catastrophes， and of not much concern to the designer. This is not the case in developing countries， where power interruptions occur frequently; the effects of such interruptions on the operation of a mold may cause concern. Typically， in the case of a power failure， a machine using a cold runner mold will just stop，but can resume work as soon as the plastic is again up to molding temperature. However，in a hot runner mold the melt will freeze in the manifold and nozzles and it may take much more time to restart (in small molds between 15 and 30 minutes). The expected savings through using a hot-runner mold may become an illusion. The controls (breakers， heat controllers) available to operate a mold on a specific machine must be discussed with the molder when designing a mold that will require additional heat controls; typically， such controls are required for hot runner molds. For safety reasons，heaters in molds are rated at 230VAC or less，and the power consumption may be from as low as 40 W per heater，such as in some nozzle heaters， and up to several thousand watts in hot runner manifold heaters.

Since heaters are often bundled in parallel and operated by designated controls, it is important to ensure that adequately sized circuit breakers and so on are available; some can be controlled with time-percentage controllers or variable (voltage) transformers, whereas some will need thermocouples and heat controllers.

Productivity Features

(1) Shot size (mass per shot). The total calculated or estimated shot size, that is, the total mass (weight) of the products coming from all cavities, plus the mass of the runner system (in the case of cold runners) should be within 30-90% of the shot capacity of the machine. The shot capacity of a machine is given in g/shot of PS, with a specific gravity of about 1.05. The specific gravity of materials such as PE and PP is less (about 0.90 to 0.95); that is, the same mass will have a greater volume. Since shot size is rated in grams (or ounces) but is actually a volume (cross section of extruder barrel times the stroke of the extruder), the shot size of these materials will be less than for PS, by about 10%. These are only approximate figures; exact values should be checked with materials suppliers. What are the practical implications? If, for example, an 8-cavity mold is required to run in a specific machine, but its shot capacity is not large enough, it would not make sense to build it for this machine. This is especially important with cold runner molds, where the mass of the runner can add considerably to the mass of the sum of all molded parts, per shot. A machine could be well suited for a hot runner mold but be unsuited for a cold runner mold for the same number of cavities. (This is a major advantage of the hot runner system.)

(2)    Plasticizing capacity (kilograms per hour). Plasticizing capacity is the amount (mass) of plastic a machine can plasticize per hour, that is, melt the cold plastic pellets into a melt of a specific temperature (and viscosity). Plasticizing capacity is usually given as mass for PS, in kilograms (pounds) per hour. Here, the same applies as with shot capacity. The actual mass of other materials, such as PE, PP, or any other, will be different, mostly smaller, sometimes greater. This should be carefully considered before starting. But, first, the designer must estimate the molding cycle, to find out how much plastic per hour will be required. Dividing 3600 (1 hour equals 3600 seconds) by the number of the seconds of the estimated cycle will give the number of shots per hour (N). Multiplying the total shot weight S (g/shot) calculated in (1) above, with the number of shots N per hour we find the total mass W_x in grams per hour required (Wt = S x N). For best quality of the melt (and the molded piece), it is also suggested to use only between 30 and 90% of the rated plasticizing capacity. If Wt is more than the rated capacity, the machine can still be used but the cycle time will have to be lengthened; in other words, fewer shots per hour can be produced than the mold could yield with a suitable, larger size machine.

(3)    Injection speed (grams injected into the mold per second). This is an important consideration when molding thin-walled products. Because of the narrow gap through which the plastic must flow within the cavity space, the injected plastic will cool rapidly when in contact with the cooled cavity and core walls. As the plastic cools, the gap narrows even more, making it more difficult to fill the mold. To overcome this condition, the melt and/or the mold temperatures could be increased so that the plastic will not freeze before filling the mold. However, this increase in temperature will also cause an increase in the cooling cycle (and a lengthening of the molding cycle), resulting in a smaller output from the mold. This points to two areas for possible remedy: (1) The injection speed and (2) the injection pressure must be increased. But these two are interrelated. The higher the pressure, the faster the melt will be pushed through its paths, from the machine nozzle to the farthest corners of the cavity space. The problem is now that the injection speed depends on the speed with which the hydraulic injection cylinder is filled with pressure oil. Therefore, the speed of the injection cylinder depends on the hydraulic pump output^oil volume per second—entering the cylinder, but it also depends on the size of the associated hardware—hoses, valves, and so on—from the pump to the cylinder.

Most machines for conventional (not thin-wall) products are served sufficiently well by the output of the pump (and the motor driving it). However, the injection speeds required for thin-wall production require the cylinder to be filled more rapidly than what the pump alone can provide. To remedy this, the machine could be equipped with a much larger pump and motor, but in many cases this would be uneconomical or impractical. The preferred solution is to provide the machine injection system with an accumulator, which stores high-pressure oil during the time pressure oil is not used. Additional valving and other hardware is required, which is often sold as an “option” with the machine, called an accumulator package. The accumulator releases the stored high-pressure oil together with the pump output into the cylinder when required for injection. The designer will need to recognize when an accumulator package is necessary for the product for which the mold is to be designed, and must discuss this with the molder to make sure the right machine is available to run the mold.

The two halves of an injection mold are bolted in place to the stationary and movable die plates. Most injection machines operate in a horizontal position. The injection press makes use of two hydraulic cylinders，and the mold is closed by the action of a hydraulic cylinder. Some presses use straight hydraulic action, while others use a toggle mechanism. Figure 2.1 shows a toggle mechanism, but a straight hydraulic action serves the same purpose. The purpose of the mechanism on the left side of Fig. 2.1 is to open and close the mold and to provide the clamping pressure to hold it Closed during the injection part of the cycle. The injection cylinder at the right side of Fig. 2.1 serves to force the compound through the “shot” or injection. This cylinder has nothing to do with the opening or closing of the press. Several methods of heating or plasticizing the material are in use, but none of them have any direct bearing on the type or de-mold required.

ejector plate or knockout bar is operated by an ejector ，which stops the plate as the mold opens and before the end of the open” stroke is reached. Thus, the mold cavity is pulled back after the Rector or knockout pins are stopped so that the molded piece comes free pins extend out of the mold cavity.

The ejector bar and pins are pulled back into molding position by springs, or they are pushed back into molding position by various devices that are described elsewhere, such as early return pins，hydraulic or air cylinders, or safety pins. This return action takes place as part of the “open time” or as part of the “closing time cycle.” This is calledresetting the ejector. Many other operations or actions may be included in the mold design. Some of these are pulling side cores, unscrewing threaded sections automatically or collapsing cores for internal undercuts.

Mechanical Features

(1)    Tie bar clearances and platen size, front to back, top to bottom. Will the planned mold fit on the platens? In some cases it is all right to have the mold larger than these dimensions; it may even overhang the platens, as long as the cavities are located within the area between the tie bars. In some (today rare) cases, it may be necessary to pull one or both top tie bars to be able to install the mold. If this is required, the designer must find out if the planned machines have provisions for easy tie bar pulling.

(2)    Locating ring size，sprue bushing radius. The locating ring centers the injection half of the mold on the stationary (or “hot”）platen. The sprue bushing radius must fit the injection nozzle radius. There are standards, but make sure you have the appropriate sizes. Some of the machines for which the mold is planned may have different sizes, so more than one locating ring (or an adaptor ring) and different sprue bushings may be required.

(3)    Mold mounting holes and slot pattern (Euro, SPI，or other standard?). How will the mold be mounted on the platens? The best method is where the mold halves are directly screwed onto the platens, using standard mounting holes on the platens or clearance holes on the platens with threaded holes in the mold. With this method the full holding force of the screw is utilized. But this is often not possible, especially if the mold must fit several, different machines. In these cases, mold clamps are frequently used, with the clamp screws making use of standard mounting holes or slots in the platens. The disadvantage of this method is that only a portion of the holding force of the screw is utilized.

(4)    Quick mold change features. There are a number of commercial and proprietary systems, and the designer must get the specifications to fit the system before starting to design the mold.

(5) Machine ejector. The ejector force is usually about 10% of the clamp force, which is sufficient for most molds, but there are cases where this is not enough. The mold may have to be equipped with additional ejection means， often built-in hydraulic or air actuators. The machine ejectors are always on the moving platen，but their size and pattern will vary according to the builder’s standards (Euro，SPI，other standard?). If the mold will make use of the machine ejectors it is important to know their size and location when designing the ejection mechanism.

(6)    Shut height. This is the total height of the mold，that is，the distance from the mounting face of the cavity half to the mounting face of the moving half. This distance must not be greater than the maximum distance of the platen surfaces of the machine when in fully closed position. The machine specifications indicate maximum and minimum shut height. If the laid-out shut height is too great，there are several ways to reduce it:

• Investigate whether all the shown mold plates are really necessary. In some molds，for example，the mounting plate under an ejector box can be omitted，by fastening the mold to the machine using the mold parallels
• Reduce the thickness of one or more of the mold plates.
• If neither is possible without compromising the quality (strength) of the mold， a different machine must be selected. This should be discussed with the molder before proceeding.

Conversely，if the shut height is too small，plate thicknesses can be increased， which is not always a good solution because it makes the mold unnecessarily heavy and adds cost to the mold. Some machines are equipped with Bolster plates, or bolster blocks, which are mounted on the moving platen in order to decrease the minimum shut height.

(7)    Clamp stroke. In most machines, the mold clamp stroke is adjustable. For many molds, the suggested minimum stroke should be about 2.5 times the height of the product to ensure that the molded pieces have enough space to fall free between the mold halves during ejection; however, the stroke should not be less than about 150mm (6 inches), so that the mold surfaces can be accessed for servicing while the mold is open. There are exceptions to these two suggested values, for special applications, particularly when using automatic (robotic) product removal methods, which are outside the scope of this book.

(8)    Ejector stroke. This stroke is also adjustable, within the limits of the machine specifications. The designer must make sure that the available ejection stroke is large enough to push the products completely off the cores, in cases where little draft is specified, for example, when molding deep-draw containers. With good draft, it is usually not necessary to do more than push the products some short distance before they fall free, or before air-assist features will blow them away. There are again some exceptions, particularly with robotic product removal methods.

(9)    Clamping force. The designer must make sure that the total projected areas of all cavities, plus the projected areas of any runner system in the same parting plane, multiplied by the estimated injection pressure, will not be greater than the available machine clamping force. As we have seen earlier, the estimated injection pressure depends on the ease of plastic flow (viscosity, temperature) and on the wall thickness of the product. In borderline cases, it is sometimes possible to change conditions, for example, in a very large product, by increasing the number of gates and placing them far apart; it may then be possible to use lower injection pressures, thereby requiring less clamp.

(10)    Auxiliary controls. Some molds may require specially designed air circuits for air ejection or for air actuators. Is the machine equipped for such circuits, to be timed within the molding cycles? In some cases, hydraulically actuated side cores may be required. Has the machine a provision for timed core pulls?