Machine Sizing and Specification for Optimal Mold Performance, Part 2

Last month in Part 1, we discussed the importance of properly sizing machine clamps to ensure that your mold operates effectively throughout its intended lifespan without experiencing excessive wear or damage. This month, we focus on ensuring that the injection unit of the machine is correctly sized. An improperly sized injection unit can lead to a variety of issues with the molding process, including short shots, flash, burning, slow cycle times and incorrectly sized parts. Taking the time to evaluate the machine intended to run the mold during the mold building process will help prevent these problems when the mold is ready for production.

Extruder Shot Sizing

To ensure that the planned machine is optimized for the mold, we need to carefully consider the proper sizing of the extruder system. Extruder size is a crucial characteristic of the molding machine, as it directly impacts the molding cycle. To calculate the required extruder size, it is essential to understand the difference between shot size and rated shot size.

• Rated shot size is the amount (in grams or ounces) of PS (melt density = 0.95 g/cm³ ) that the injection system (ram screw or two-stage) can inject at every cycle and is indicated in all machine specifications.
• Actual shot size depends on the mass (W) of the molded product and cold runner (if applicable) (in grams).

The size (mass) of a cold runner is very important in the shot size calculation. Although the mass of a cold runner can be relatively small — often only a few percent of the total shot weight — it can also be significant. As illustrated in Figure 1, in three-plate molds, and in some two-plate molds, the mass of a cold runner can equal or even exceed the weight (W) of the molded parts, especially when the products being produced are very small and require a large runner system.

To calculate the required extruder shot size, include the part weight, the runner weight (only if using a cold runner), and the melt density of the plastic being molded, as follows:

     _{Maximum extruder shot size = SSR ÷ 0.2
     Where
     SSR = shot size required}

Note: Conversion factor between grams and ounces: 28.3 g = 1 oz.

Example: Calculate the acceptable extruder sizes for a polypropylene printer tray weighing 45 grams molded in a four-cavity hot runner mold.

     _{SSR = (120 x  4) ÷  0.70 x 0.95 = 244 g of PS = 8.6 oz}

     _{Min. extruder shot size = 350 g = 12 oz.}

     _{Max. extruder shot size = 1.2 kg = 43 oz.}

Although this is a simple calculation, it is often overlooked, particularly the distinction between melt density and solid density. Failure to do this calculation could lead to significant issues during the mold's operation. In some cases, it may result in the mold being entirely incompatible with the machine!

It is recommended to use no more than 70% of the machine's rated shot capacity to ensure the following:

• Sufficient shot capacity is available

• A cushion of material remains in the extruder

• Solid plastic pellets have enough time to melt properly

• Enough plastic is available in case the check valve or ball check in the machine experiences any leakage back to the extruder due to wear

Also, when calculating the required extruder shot size, it’s important to ensure that it is not too large for the required (small) shot size. The practical lower limit for the shot size — defined as the distance the screw retracts during the shot — should be no smaller than 0.5 times the screw diameter or less than 15-20% of the machine shot capacity. If the shot size is smaller than this value, injection rates could become inconsistent, as some stroke is necessary to reset the check ring or ball check. In such cases, it is advisable to select a smaller injection unit.  

Extruder Sizing for Plasticizing Capacity

The plasticizing capacity is defined as the amount (mass) of plastic an injection unit can convert per hour from cold pellets into a homogeneous, thoroughly heated and mixed plastic melt, at the required temperature, ready for injection.

While the screw turns, the plastic in the barrel is melted and moves forward toward the screw tip, past the check valve and accumulates there until enough plastic is made up for the next shot (see Figure 2). When the desired shot volume is ready, the screw stops and waits for the signal to inject.  At this moment, the screw is pushed forward to inject the plastic into the mold. The screw does not plasticize during the time when the screw is stopped (and waiting before injecting); while injecting (injection cycle) or while holding the screw forward (low-pressure hold cycle). The sum of these times must be subtracted from the total available time, meaning less time is available for plasticizing. So, the concept of plasticizing per hour is really a guide for general sizing.

To ensure that the machine will do an adequate job of plasticizing for the planned mold, we must consider the hourly throughput based on the type of plastic being used, rather than relying on the figure provided in the machine specifications, which is relevant only for polystyrene (PS).

The plastic pellets enter the extruder as a solid and are melted through shearing and heating in three sections: feeding, transition and metering (see Figure 2). This process is time-dependent, which means that the extruder on an injection molding machine has a maximum throughput specified (in kg/h or lb/h). It is essential to calculate the expected throughput of the mold to ensure it remains within the machine's specified limits.

Note: Typically, higher backpressure results in better melt quality. However, this also means that more power (measured in kW or horsepower) is required from the screw motor and less melt is pushed ahead of the screw. The amount of material plasticized is directly proportional to the speed of the screw (rpm), its diameter, length and design. There are limits to both the motor's available power and the screw speed, as per machine specifications. These limits restrict the amount of material that can be plasticized per unit of time.

If the output of the extruder is less than required by the mold, it means that although the mold can still operate on the machine, it will run slower because the extruder will still be plasticizing a shot while the mold is ready for another injection cycle. Consequently, productivity will be lower than expected since the mold must wait for the extruder to make up a new shot each cycle.

The available recovery time for the screw can be calculated as follows:

     _{RTA (s) =} t_cycle - t_inj - t_hold - t_pullback - t_{decompression}

     _Where

     _{RTA = recovery time available}

Note: If a two-stage injection unit is to be used then the available recovery time (RTA) is t_{cycle  –} t_{transfer to shooting pot}

Once the available recovery time (RTA) has been calculated, the required throughput of the extruder can be calculated as follows:

     _{Required extruder throughput (kg/h) = SSR (g of PS) ÷  RTA (s) x 3600 (s/h) ÷ 1000 (g/kg) ÷ SF}

     _Where

     _{RTA = recovery time available (s)}

     _{SSR = shot size required (kg, g or oz)}

     _{SF = safety factor = 0.8 (recommended)}

Note: The throughput of the mold is calculated based on the cycle time rather than the RTA. So, the throughput will be lower than the required throughput of the extruder, since the extruder can only run at certain intervals during the cycle.  The mold throughput (not needed for extruder sizing, only noted here for clarity) would be as follows:

_{Mold throughput (kg/h) = SSR (g of PS) ÷ Cycle time (s) x 3600 (s/h) ÷ 1000 (g/kg)}

Due to the extruder running only during certain portions of the cycle (RTA), the required extruder plasticizing capacity will be higher than the throughput of the mold. The example below illustrates the point.

Example: A 36-gram HDPE blender casing is molded in a two-cavity mold with a 50-gram cold runner system at 25 seconds (with 15 seconds available for recovery). What is the throughput of the mold? What is the throughput required of the extruder? The melt density of HDPE = 0.76 g/cm³.

     _{SSR = (36 x 2 + 50) ÷ 0.76 x 0.95 = 152.5 g of PS} 

     _{Throughput of the mold = 152.5 ÷ 25 x 3.6 ÷ 0.8 = 22 kg/h}

     _{Required extruder throughput = 152.5 ÷ 15 x 3.6 ÷ 0.8 = 45.75 kg/h}

This value should be checked with the extruder capacity to ensure that the mold will not be limited by the machine.

Alternatively, the following equation may be used to accurately size the extruder, if provided by the machine supplier.

^{_{Required extruder throughput (kg/h) = Instantaneous recovery (g/s) (adjusted for the right resin) x  RTA (s) x  # shots/h  x  0.8}}

If a larger machine or extruder is not available and there is insufficient plasticizing capacity, the mold can still operate, but it won't run at the expected speed. This is because the system will have to wait longer for the shot to be completed, resulting in lower output than planned. Normally, when the machine is limited by the extruder, the operator will extend the cooling time to allow the extruder more time to rotate, helping to maintain a consistent cycle time.

Nozzle Radius and Orifice

The machine nozzle radius and orifice must match the mold sprue bushing radius and orifice to prevent plastic from leaking out during injection. A mismatch in this area can create flow problems in the mold and increase the potential of burning or degrading the plastic before injection.

Type of Machine Nozzle

The machine nozzle is where the injection unit intersects with the mold at the mold sprue bushing (see Figure 2). The type of machine nozzle and the size of the orifice can limit the productivity of the injection molding work cell if the wrong design is used. Since the machine nozzle represents only a small portion of the overall cost of the machine, it is essential to select the appropriate nozzle for the specific application.

All molding machines come standard with open nozzles. This means that the tip of the machine nozzle is open to allow plastic to pass through freely from the end of the screw, whether into open air or into the sprue bushing of the mold during injection.

To achieve effective plasticizing, it is important to apply a controlled low back pressure in the injection cylinder, which acts on the injection piston. This back pressure is typically around 5 to 10% of the injection pressure. However, the pressure is high enough at the tip of the screw, while the screw is plasticizing, to push the plastic through the open nozzle.

With an open nozzle, the screw must not turn (plasticize) unless it is blocked, because:

• With cold runner molds, the nozzle is pressed against the mold sprue bushing. The plastic in the sprue acts as a stopper and the screw can start rotating and producing as soon as the injection (or injection hold) pressure ends. However, when the mold opens, the screw rotation and the backpressure must stop. Otherwise, the plastic will be pushed into the now-empty sprue bushing and into the open mold.
• With hot runner molds, the nozzle is also pressed against the sprue, and the screw can start rotating as soon as the injection (hold) pressure ends. The mold could be opened safely even with the screw plasticizing, but only if all gates were frozen sufficiently to stop the plastic from drooling out of the gates. Otherwise, plastic will drool into the open cavities. In these cases, the screw must also be stopped as soon as the mold opens. With most valve-gated molds, the gates are mechanically closed after injection and a shut-off nozzle is not required.

In both cases, the time available for plasticizing is limited to the cooling cycle. While the mold is open, the screw is stopped. In molds with long cooling cycles, there is usually sufficient time for plasticizing the next shot volume.

• Example: A mold and machine with a 4-second dry cycle, a 2-second injection and hold cycle and a 6-second cooling cycle. The total cycle is 12 seconds. When using an open nozzle, the maximum time the extruder can run is 6 seconds, which is the same length of time as the cooling cycle. If the amount of plastic needed for the shot to be injected can be plasticized in 6 seconds or less, there is no problem with an open nozzle.
  • Note: The screw can run only 50% of the time, so the extruder uses only 50% of its rated capacity (see Figure 3).
    

• Example: Same machine and mold conditions as in Example 1, but this time, not enough plastic can be plasticized in the 6 seconds between the end of injection and the end of cooling. If we assume we need 9 seconds to generate the melt for the next shot, we must increase (unnecessarily) the cooling time by 3 seconds (from 6 seconds to 9 seconds), for a total cycle of 15 seconds. This represents a severe loss of productivity (4 versus 5 shots/min). This extruder uses 60% of its rated capacity. So, we should consider using a shut-off nozzle or a machine with a larger extruder.
  • Note: The shut-off nozzle will only be effective if there is extruder capacity to run during the clamp open/close time (see Figure 4).

It is also important to understand that adding unnecessary cooling time causes the products to eject cooler than necessary, resulting in less shrinkage (they will be larger than expected). This can be significant when molding plastics with high shrinkage factors.

Also, products cooled too much inside the mold may become overstressed in some areas, as they shrink onto the core and fail. They may also stick to the molding surfaces and be hard to eject. To overcome both problems, the melt temperature should be higher to ensure that the product will not be overcooled in the mold. This adds to the product cost because it requires more energy to heat the plastic higher than necessary and to cool it.

Using a shut-off nozzle eliminates the extra cooling time required and can solve the above issues. For short cycle times, a shot-off nozzle can greatly increase the productivity of the mold and machine. The basic principle of the shut-off nozzle is to provide a mechanical stop within the machine nozzle, which controls the flow of plastic from the extruder to the nozzle tip. Shut-off nozzles come in various styles (shuttles, rotary cocks, pins) and are operated by compressed air or hydraulic pressure (oil).

Figure 5 shows a complete shut-off nozzle. In this design, the lever (A) pushes a pin inside the nozzle to close the nozzle opening (B). When injecting, the plastic pressure pushes the pin to the right, opening the nozzle for the plastic to enter the mold. The lever is operated by a link (C) connecting it with a hydraulic or air actuator (not shown).

Even though the shut-off nozzle represents an added one-time expense, their use is very desirable, especially with short cycle times, because it allows the screw to plasticize while the mold is open.

Example: A mold and machine with a 4-second dry cycle, 2-second injection and hold cycle and 6-second cooling cycle, equaling a total cycle time of 12 seconds. Using a shut-off nozzle, the time the extruder can run is 6 seconds (the cooling cycle) plus 4 seconds (the dry cycle), equaling 10 seconds. The screw has now enough time to plasticize 10 out of the 12 seconds full cycle or 83% of the rated capacity. This is a tremendous improvement over the use of an open nozzle. Even a smaller extruder (or machine) could be used for this job (see Figure 6).

Screw Design

Most machines come equipped with a general-purpose or barrier-type screw. However, there are thousands of screw designs used to optimize melt creation. You should check to see what screw is in the machine and confirm that this screw will do an effective job of creating a uniform, homogenous melt for the mold. On many occasions the problems with poor-quality moldings can be traced back to the screw design.

Injection Speed (Average and Peak) and Pressure

The injection unit and the mold are connected via the sprue and nozzle. Injection speed, in this context, is the amount of plastic per second that is pushed out of the machine nozzle into the mold, as the extruder screw (or piston in two-stage systems) moves forward to inject. Injection speed is usually specified in cm³/s (or in³/s).                                                                     

Assuming there is no loss from leakage or poor design of the check valve in front of the screw, the entire volume of plasticized melt awaiting injection will be pushed into the mold. This volume (V) can be easily calculated as the cross-sectional area (A) of the barrel with a bore (D), multiplied by the stroke (S) of the screw:

A (the screw area) is a constant; therefore, the (shot) volume V is directly proportional to the stroke S. Each extruder screw (or shooting pot) has a specified, maximum stroke Smax. The stroke is the same as the stroke of the (hydraulic) injection cylinder, which pushes the screw (or piston) forward during injection.

In most injection molding machines, for the standard size screw (or piston), the ratio of area of the screw (piston) and of the injection cylinder is 1:10. This is so that the oil pressure of the hydraulic oil in the injection cylinder can be simply multiplied by 10 to arrive at the melt pressure of the plastic at the machine nozzle. For example, for an indicated oil pressure of 14,000 kPa (2000 psi), the plastic injection pressure is 140,000 kPa (20,000 psi).

In most machines, the extruders can be easily changed (within limits) for smaller or larger screws and barrels. However, the hydraulic injection cylinder remains unchanged. Using the above formula, a 25% larger screw diameter will result a 56% increase in the maximum shot volume while reducing the maximum possible injection pressure by 36%.

On the other hand, reducing the screw diameter by 20% will result in 36% less volume, but an increase of 56% in the injection pressure. This should always be calculated, as it may be possible to run a mold that does not need high injection pressures on a smaller machine, just by changing the extruder screw and barrel to a larger diameter.

Note: As the screw diameter decreases the volumetric injection rate (cm³/s or in³/s) goes down.

The impact on the extruder can be summarized in Table 1. The injection speed is essentially dependent on four factors:

1. Viscosity of the injected plastic (which varies as a function of the injection rate and the temperature).
2. Available injection pressure.
3. Resistance against flow within the runner system, gates and cavity space.
4. Volume of hydraulic oil flowing into the injection cylinder.

By taking the time to carefully understand and specify the machine in which the mold will run, you can ensure that it operates at its optimal performance level throughout the expected life of the mold.