UNDERSTANDING THE MOLDING PROCESS

The injection molding process can be compared to filling a jelly donut. A machine prepares a plastic material (the jelly) and injects it into a closed mold that contains a vacant area (like the donut). The molding machine has two basic functions. First, it must prepare the plastic material using heat to melt the plastic, and inject that molten plastic into a closed mold using hydraulic pressure pushing against a plunger device. Second, it must generate enough clamping force to hold the mold closed against the amount of injection pressure being used to fill the mold. When these two activities have taken place, the mold is held closed until the molten plastic cools enough to form a solid skin on the molded part. Then, the part can be ejected and the next cycle can begin, as shown in Figure 1.

                

                    Mold Closed, Part Being Molded                               Mold Open, Part Ready To Eject                        
                     Figure 1                        

PARAMETERS

There are over 200 different parameters that must be established and controlled to achieve proper injection molding of a plastic part. These parameters fall within four major areas: pressure, temperature, time, and distance, as shown in Figure 2.

Figure 2 Four parameter areas.

To the troubleshooter, all four areas are important. But, the pressure and temperature areas are the ones most commonly considered during the troubleshooting process. Based on the requirements of any particular plastic material, the pressure must be sufficient to inject the plastic material and to hold the mold closed. In addition, the temperature of the injected plastic and mold must be correctly maintained.

In the following section, we will discuss the importance of various facets of the pressure and heat parameter areas. Distance and time will not be discussed here as they are beyond the intended scope of this chapter.

Pressure

Pressure is found primarily in the injection area, but there is also pressure found in the clamp unit of the molding machine. We will address all of these pressure requirements here.

BACKPRESSURE

The first pressure to consider is backpressure. This is pressure that is created during the return action of the screw after injecting material. The screw turns (augers) to bring fresh material into the heating cylinder. This material is placed in front of the screw and nudges the screw backwards. A buildup of pressure is created at the front end of the screw. This pressure is used for better mixing of the plastic (especially if colors are added at the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt. The back pressure setting should start at 50 psi and be increased in 10 psi increments as needed, with a maximum setting of 300 psi. The maximum setting is needed because anything over that will cause too much shearing of the plastic and result in thermally degraded  plastic.

INJECTION PRESSURE

The next type of pressure to consider is injection pressure. This is the primary pressure for injecting 95% of the molten plastic into the closed mold. Normally, the highest pressure and fastest fill rate are the best condition. However, high pressure will increase molded-in stress. And, that stress will be released at some time. There is no question as to its being released, only as to when it will be released. The greater the pressure, the greater the stress, and the greater the reaction when it is released. So, you should determine the minimum amount of pressure necessary to fill the mold, and then use all of it. And, remember, the hotter the plastic, the more fluid it becomes and the lower the pressure can be to fill the mold.

HOLDING PRESSURE

Once the majority of the plastic (95%) has been injected using standard injection pressure, the machine should drop into hold pressure. This pressure is about half of the injection pressure and is used to finish filling the mold by packing the molecules together in an orderly fashion. Hold pressure is required until the gate freezes off, normally in 3 to 4 seconds. Once that happens, hold pressure has no more effect on the molecules on the other side of the gate. If hold pressure is released before the gate freezes, the material in the cavity is still molten and will be sucked back out of the cavity. At the very least, there will be insufficient pressure to pack the molecules together and uneven shrinkage and cooling will take place. If valve gating of a hot runner system is used, holding pressure can be released earlier than with standard surface gating.

CLAMP PRESSURE

At the other end of the machine, we have clamp pressure. The only reason to have clamp pressure is to keep the mold closed against injection pressure. Therefore, the amount of clamp pressure required is based on the material being molded. The easier flow materials require less injection pressure, thus they require less clamp pressure. Conversely, the stiffer flow materials will require more injection pressure, thus more clamp pressure.

To determine how much clamp force is needed for a specific product, find the projected area of the part being molded and multiply it by two to six tons for each square inch of projected area.

Figure 3  Determining Clamp Force Requirement

For example, if we were molding polycarbonate for the part shown in Figure 3, we would need five tons per square inch of area (as published by the polycarbonate supplier). So, the total force required would be 180 tons (36 sq. in. x 5 tons). But, if we were molding nylon 6/6, we would only need two tons per sq. in. (as published by the material supplier) so the total force would be only 72 tons (36 sq. in. x 2 tons). Therefore, we could run the mold in a smaller press, or use only a portion of the tonnage available in the press the mold is in. Reducing the tonnage requirement also reduces the cost to mold the part because we use fewer resources.

There is one factor, however, that must be discussed regarding clamp tonnage calculations. When the material suppliers state a specific clamp tonnage value (such as five tons per square inch for polycarbonate), the supplier assumes there is a proper shutoff land surrounding the cavity image. A typical shutoff land is shown in Figure 4. This shutoff land is simply a wall of steel surrounding the part and is approximately 0.002/0.003 inches in height and at least 1/4" wide.

Figure 4  Shutoff Land

The purpose of this shutoff land is to concentrate all the clamp force to the area surrounding the cavity. That allows us to use less total force than if the clamp force is dispersed over the entire face of the mold base. Without the shutoff land, the amount of clamp force will be 3 to 4 times as much as with the shutoff land.

The calculations shown earlier assume there is a shutoff land. If you use these calculations and the mold flashes easily around the part, chances are there is no shutoff land being utilized.

Heat

The next parameter area we will look at is heat. Heat is used to soften the plastic to the point of being able to inject it, but heat is also found in the mold and in the heat exchanger of the machine. We will investigate controlling all of these areas.

INJECTION UNIT

There are four zones of heat that must be controlled in the injection unit. They are the rear, center, front, and nozzle zones. Each is controlled independently of the others. See Figure 5 for reference.

Figure 5  Injection Unit Heat Zones

The injection unit is designed to drag material through the four zones and to heat it gradually as it travels through the heating cylinder. The heat should be lower in the rear than in the front, and the nozzle should be the same as, or 10 degrees (F) hotter than, the front zone. Modern temperature controls are able to hold a temperature setting to within 1 degree (F).

It is important to understand, however, that the heat is created by heater bands strapped around the outside of the injection cylinder as well as by the turning action of the screw inside the heating cylinder. The heat is provided at a rate of approximately 50% for each of these devices.

The temperature control units do not measure the actual temperature of the plastic being heated, but rather the temperature of the steel of the injection unit's heating cylinder. This is important because the settings for the heating cylinder temperatures must be made higher than the actual melting temperature of the plastic. This is because the plastic is moving through the heating cylinder at a fairly rapid pace and must absorb enough heat during its travel to get to the desired temperature. In most cases, the heating cylinder will be set at 50 to 100 degrees (F) higher than the melt temperature of the plastic.

The plastic must be at the right molding temperature as it leaves the nozzle of the molding machine. We can determine that temperature by sliding the injection unit back from the mold and injecting material into the air. This is called an air shot. When it falls upon a special plate of the machine that is designed to accept this material (called a purge plate) we can use a pyrometer to measure the molten plastic temperature. Figure 6 depicts this action.

Figure 6 Measuring Melt Temperature

The temperature at this point should be the temperature requirement published by the material supplier. You can find this value for some common materials in the area of our site that discusses Melt Temperatures.

 

MOLD TEMPERATURE

The most common method used for cooling the plastic once it is injected into the mold is a set of water lines. These lines are connected to a source of temperature-controlled water that circulates through the mold and pulls out heat that is building up in the mold over time.

One common mistake of most troubleshooters is believing that the water leaving a mold should be hotter than the water entering the mold. The belief is that the water is used to pull heat from the plastic and therefore must be hotter when the job is complete. Actually, the water is being used to maintain the temperature of the mold and should be the same temperature leaving as entering (within 10 degrees F). That is the definition of maintaining the temperature.

The waterlines are sized and located such that the water pulls heat out of the mold as fast as it is being generated. That is true maintenance of temperature. If the water leaving the mold is hotter than that entering the mold it means there is still a lot of heat left in the mold and the waterline design is not adequate to pull heat out as fast as it is being generated.

The material supplier is the source that defines the proper mold temperature and this is published information for mold temperature requirements for some common materials). It must be cool enough to solidify the plastic quickly, but warm enough to keep it from becoming solid too fast. If the plastic solidifies too fast, the molecules do not have a chance to ``bond'' properly and the part will be weak or brittle. This is especially true of crystalline (or semi-crystalline) materials. Each plastic family has a specific mold temperature range within which it should be processed for highest quality parts at lowest possible cost.

HEAT EXCHANGER

The heat exchanger is a sophisticated radiator device that controls the temperature of the hydraulic oil used in the molding machine. This oil usually must be maintained at a temperature between 100 and 125 degrees (F) for proper use. If it is too cool, the machine actions are sluggish and inconsistent, and if it is too hot, the additives in the oil will fall out of solution and clog hydraulic mechanisms causing them to be inoperative or slow to respond. The oil is passed over a series of copper tubes that have water running through them. As with the mold, this circulating water pattern is designed to pull heat from the oil as fast as it is being generated.

If the water leaving the heat exchanger is hotter than the water entering it, this indicates a blockage (such as calcium deposits) in the heat exchanger that is interfering with proper heat transfer. The heat exchanger must then be removed and the copper tubing must be flushed with acid or drilled out with special cleaning equipment.

CONTROL AND CONSISTENCY

The two most important things necessary for the molding of the highest quality products at the lowest possible cost are control and consistency.

Control should be applied to every possible parameter that can be determined. These include those of the four areas mentioned earlier, but also such items as the environment in which the parts are molded, the placement and operation of portable cooling fans, standardization of mold components, operation of secondary equipment, and any other action that is adjustable or variable. There should be no random or wandering motions or actions.

Consistency should be applied wherever possible, but primarily in machine actions. One area of major importance is that of any operator-controlled activity such as the opening and closing of a safety gate (often called the operator's gate). Most machine actions are controlled through electronic devices or computer controls and are extremely consistent. But any operator-controlled action tends to be inconsistent due to the nature of human endeavor. The human being works at varying speeds and distances throughout the day, ranging from quick and short at the start of a shift and after each break or rest period, but slower and longer in between those times. The inconsistent action causes inconsistent cycles and molded parts that vary in quality, not to mention cost. This is the major reason many molders are utilizing robots or other automated methods of molding. They wish to remove the human tendency towards inconsistency thus improving quality and reducing cost of the molded parts.

Human consistency can be achieved, however, through proper training and knowledge transfer. The operators can be instructed in the value of consistent operation and strive for consistency in their personal actions. A conscientious operator (and most are) will find ways of achieving the required consistency by humming tunes, tapping feet, singing songs, reciting poetry, or simply listening for a specific sound produced by the molding machine or mold. When they find a method that works they can be as consistent as the automated molding machine for an entire shift. Once control of as many parameters as possible is established and maximum consistency is achieved, defective parts will be minimized and production costs will be reduced as the quality of the molded parts increases. All of this reduces the necessity of troubleshooting activities as fewer defects are being produced.

(We now provide our popular Molding Machine Operator's Primer seminar in an interactive CD version. To see details of this program (or to order) click HERE )

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