Cost-Cutting Strategies


A note from R.F. Dray:

This series by leading industry authorities and educators will show the molder the way to substantially reduce cost by improving productivity and at the same time improve molded part quality. The most obvious way to reduce molding costs is to reduce cycle time. Melt temperature is the easiest way to reduce cycle time; lower melt temperature also improves part physical properties. The injection molding machinery manufactures have provided injection units that are virtually in total disregard for proper melt temperatures and polymer quality. The screw designs, L/D, and rpm are at best going in the wrong direction.

Example: viscosity = shear stress/shear rate 

Most machines today require high rpm to achieve required recovery rates. As the formula shows, increasing shear rate (rpm) decreases viscosity and thereby increases melt temperature. As will be shown, melt temperature and proper dispersion can be provided with modern screw designs, not with the short L/D no-purpose designs provided by many machinery OEMs. 

This series describes the advantages of lower melt temperature and also show how, with new injection unit design, molders can routinely:

  • Run regrind (100%) without contamination
  • Run polymers without drying, problem-free
  • Run mono or coinjection on the same unit without machine changes
  • Achieve part weight accuracies (.02g) that eliminate shorts and flash and therefore virtually eliminate scrap

The first section below, by Don Williams, CEO of Opti Temp Corp., shows the cost savings derived from lower melt temperatures and proper cooling methods.

The second section by Mike Sepe, corporate director of technology, Dickten & Masch Mfg., discusses how polymer physical properties are influenced by lower melt temperatures.

The third, authored by John Bozzelli, CEO of Injection Molding Solutions, covers injection molding with lower melt temperatures.

The final section, by R.F. Dray, CEO of R. Dray Mfg., will explore injection unit and screw design solutions for processing polymers at proper melt temperatures and quality.

—R.F. Dray  


Section 1: Improved Mold Cooling

The global economy and the increase in imports from countries with low labor and overhead costs make it more necessary than ever that U.S. molders increase plant efficiencies to be competitive.

In the injection molding machine cycle, about 85% of cycle time is consumed cooling the plastic from a molten to a solid state. This 85%, then, is the largest opportunity in the injection molding cycle for improvement. 

One question arises instantly: Is it worth it? It certainly is. The most cost-effective way to increase profitability is to improve the productivity on existing injection molding machines by decreasing the cycle time on existing molds.

 

More parts or more time—it's all good

When determining the hourly cost of a molding machine, use machine data and financial statements to obtain the parameters above.

Table 1 shows the inputs required to determine the costs and profitability for a 450-ton injection molding machine processing at a base rate of 100 lb/hr of high-density polyethylene, operating 24 hr/day, 5 days/week. The inputs are generally available from the machine setup charts and from the plant's financial statements.

Table 2 shows the the results of cycle time improvement. The example shown is a molder making proprietary parts. Because of the improved cycle time, he can sell 21.3% more parts to his customer without increasing the machine operating hours. An approximate 2-second cycle time improvement was made on this machine. 

From the results, you can see that each second of cycle time improvement was worth almost $49,300 in increased annual operating profit, the total increased annual operating profit was about $96,600, and the annual increase in revenue was almost $204,000.

Another way of looking at cycle improvement would be a custom molder who has contracted for a fixed number of parts on the machine. because of the reduced cycle time, the molder can provide the fixed number of parts in about 1,000 fewer machine hours. These 1,000 machine hours are available to be sold to another customer. 

These profitability improvements will be different for each molder and each machine, and are dependent on machine size, utility costs, plant burden, operating expense level, machine throughput rate, and type of material processed. It is necessary for each molder to do this calculation for each machine in his plant. 

 

Three steps to cool

In this example of what 2 saved seconds of cycle time can do for the bottom line, this molder making proprietary parts can increase productivity by 21.3% without increasing machine operating hours.

Now that we have determined that increases in operating profit are significant when we improve productivity by decreasing cycle time, and that the cooling part of the injection molding cycle offers a significant opportunity, what are some methods that can accomplish this?

 

Reduce the melt temperature of the plastic

The amount that we can reduce the melt temperature depends on operating conditions and material. However, experience has shown that with some materials and screw designs, it is possible to reduce melt temperatures by as much as 100 deg F. If we take the same example of the machine operating above and can reduce the melt temperature by 100 deg F, we see from Table 3 that we have removed 6,400 Btu/hr less heat from the material. If the load on the chiller is reduced by 6,400 Btu/hr, then the molding cycle will be out of balance—i.e., with the chiller operating at the same temperature and flow conditions, the mold surface temperature will be reduced (less heat input to the mold).

Thus, we can speed up the molding cycle to increase the mold surface temperature to the original level, and bring the molding cycle back into balance. By decreasing the cycle to increase the heat input to the mold, we increase productivity 21.3% and decrease the cycle by about 2 seconds. From Table 2, this is worth an additional $96,600 in annual operating profit on the machine for a proprietary molder. Keep in mind that if we are heating the material to a lower temperature, we are also saving about 2 kW of energy per hour on the heating side of the process.

 

Ensure turbulent flow in all mold cooling passages

Increasing throughput rate offers the greatest cycle time improvement.

It has been the practice for many years in the heat transfer field to design heat exchangers so that turbulent flow occurs in the heat exchanger passages. One of the functions of a mold on an IMM is to act as a heat exchanger. Thus, it is now widely accepted that the mold and cooling system be designed for turbulent flow in the cooling passages.

How can this be determined? Osborn Reynolds developed a dimensionless number, using the physical properties of the fluid in 1883 that describes the flow properties in channels based on the value of his number. This number came to be known as the Reynolds number. While there isn't room here for a discussion of the Reynolds number, suffice it to say that this number should be between 5,000 and 10,000 to be sure that turbulent flow is present in the mold passages.

So, how can we determine that we have turbulent flow in the mold cooling channels?

  • First, if ethylene glycol is being used in the mold, remove it from the mold cooling system and replace it with treated water and a corrosion inhibitor. At this time, all cooling channels should be cleaned to remove scale and other mineral buildup that causes resistance to heat transfer. Generally, running the cooling system with treated water (and a corrosion inhibitor) at 50 deg F will provide better heat transfer and high productivity than a system running at lower temperatures with glycol (to prevent freezing). Be sure that the freeze protection circuit in the chiller is set to shut off the chiller if the cooling fluid temperature drops below 45 deg F.
  • Determine the existing flow and required flow in the mold cooling passages. The existing flow can be measured by flow meter or weighed water test, and the required flow by calculation.
  • Review the mold circuiting, so that turbulent flow can be obtained with a reasonable amount of flow and pressure, and be sure that flow and pressure are available from the mold cooling system. Make sure all hoses and supply lines are sized properly for the required flow, and are as short as possible.
  • Check the fluid temperature rise across the mold. This should generally be less than 2 deg F. If the fluid temperature rise is too great, the plastic in the mold will see colder fluid at the entrance of the mold than what the plastic will see at the exit of the mold, which can cause stress in the molded part.
  • A central chilling system in a plant is a network. When a machine is removed from or added to the network, it affects the flow and pressure of all the other machine sin the network. Adding or removing machines from the circuit temporarily and checking the flow and pressure at other machines in the network can determine this. If the flow and pressures do change at the individual machines, it may be necessary to correct the central system design. Methods of correcting this are to adjust the pumping, adjust the piping and sizing, or use isolators (true closed-circuit heat-exchange devices) at each machine to isolate the molding machine from the central system.

 

Design the mold for the efficient heat transfer

It is important that cooling passages are properly sized and positioned such that the mold will remove the heat efficiently. Also, the mold must be designed for balanced cooling—about the same amount of heat must be removed from both halves of the mold. There are computer programs available for analysis of the heat transfer in injection molds. 

If molders in the United States are to remain competitive in the global economy, it is important that they use all of the technology available to them to be sure the injection molding machine is running at optimal performance.

Donald Williams is CEO and founder of Opti Temp Inc. (Traverse City, MI; www.optitemp.com), a supplier of heat transfer solutions such as chillers, heat exchangers, and mold temperature controls.

Section 2: The Problems and Added Cost of Running Elevated Melt Temperatures

Injection molding involves melt processing. The raw material must be converted from a solid to a fluid in the injection cylinder and then back to a solid in the mold. The time required to cool the material back to the solid is the primary factor determining the cycle time, a parameter that has a substantial impact on the cost of the part and the amount of press time that must be allocated in the production schedule. The point at which a part can be ejected from the mold is a function of part geometry and the temperature at which the material becomes stiff enough to withstand the forces of ejection without deforming. The time that the cooling process takes is therefore dependent in large measure on the temperature of the material entering the mold cavity from the barrel.

Melt temperature selection is not an exact science in the world of injection molding today. The range of allowable melt temperatures for a given material can be quite narrow for a thermally sensitive material like PVC or it can be extremely wide for a thermally stable material like polyethylene. Exceeding the recommended upper limit has the obvious consequence of degrading the polymer. This can manifest as changes in color or a loss in properties, particularly impact resistance. But within the recommended melt temperature range there are also hazards and hidden costs associated with running the melt at a higher temperature than necessary, even in a forgiving material. 

Consider polypropylene. Polypropylene materials have a crystalline melting point somewhere between 300-335°F depending upon whether they are homopolymers or copolymers. The published process temperature range for most grades of polypropylene is 375-550°F, which presents the molder with a very broad selection when establishing a new process. The hidden cost of selecting a melt temperature that is higher than necessary is difficult to capture unless a detailed study is performed relating melt temperature to cycle time. When this work is done, the results can be surprising.

 

Productivity, regrind problems

A common polypropylene appliance part chosen for such a study was run at melt temperatures of 400°F and 480°F. The higher melt temperature was being used for no particular reason. When the melt temperature was reduced to the lower value, the minimum achievable cycle time dropped from 46 seconds to 39 seconds. This is a 15% reduction in cycle time, which translates to an 18% increase in output. This increased productivity is the result of achieving the stiffness required to eject the part more quickly because the extra energy required to raise the melt temperature up to 480°F does not have too be removed during the cooling portion of the cycle. Plus, there is an additional cost in running the higher melt temperature that is not related directly to cycle time; this is the energy cost required to raise the melt temperature by 80°F. 

But the problems go well beyond the cost factors associated with cycle time. A melt-flow-rate (MFR) test on the raw material and the molded parts shows that the MFR for the material processed at 400°F increases by only 4% while the MFR for the part produced at 480°F increases by 35%. The increases are associated with the reduced molecular weight of the polymer. Increases of up to 40% are considered to represent an acceptable preservation of the molecular weight of the polymer, so both parts are good.

However, the runners and any rejected parts run at the higher temperature that have to be reground and reclaimed will almost certainly exceed the upper limit after one more heat history. In fact, an orchestrated regrind study using both melt temperatures shows that the cumulative change in MFR from five passes through the process at 400°F is less than the 35% that occurs from the one pass through the molding machine at 480°F.

 

This regrind study revealed another effect of the higher melt temperature. The color of the product started to drift off target after two passes through the molding process at 480°F while the product run five times at 400°F still provided a suitable color match. This color drift is an indication of another process that is not necessarily captured by the MFR test: the chemical reaction known as oxidation. 

The threat to antioxidants

Polypropylene materials rely on small amounts of additives known as antioxidants to survive the molding process and the application environment. A review of the UL relative thermal index data for a variety of polypropylenes shows that some materials can withstand long-term exposure to temperatures as high as 115-120°C (239-248°F) while others are only usable over the same time frame as temperatures up to 60°F (140°F). These materials are not distinguishable by their molecular weight. Instead, they differ according to their antioxidant content. 

Elevated melt temperatures can consume antioxidants that are intended for use when the part is in the field. This will shorten the life of the product. The amount and effectiveness of the antioxidant can be measured in a relative manner by conducting a test designed to promote rapid oxidation in a controlled environment. When this test is performed on the parts molded at 400°F and 480°F, the time required to achieve oxidation is 5 times longer for the product molded at the lower temperature. 

So the use of high melt temperatures has a number of consequences for the molder as well as the end user, and they are all negative. Energy costs increase. Cycle times increase, reducing throughput and extending the length of the production runs. And the parts that do come out of the mold at these elevated temperatures are of lower quality and less suited for their intended uses, particularly if the application calls for long-term field service. 

 

Cycle time reduction = output

Example: PP appliance part

PP process temperature range: 375-550°F

Cycle time at 400°F: 39 seconds

Cycle time at 480°F: 46 seconds

Cycle time reduction: 15%

Output increase: 18%

Results: improved part quality, energy savings, more productivity

 Mike Sepe, corporate director of technology, Dickten & Masch Mfg. 

Section 3: Processing Requirements When Using Lower Melt Temperatures

As discussed, there are advantages such as minimal polymer degradation, better color stability and faster cycles etc. by using lower processing temperatures. Cycle time savings with lower melt temperatures was discussed in Section 1 by Don Williams, advantages of lower melt temperature to eliminate polymer degradation explained in Section 2 by Mike Sepe. As we begin our discussion on processing, let us first establish what lower melt temperatures mean. This is not simply setting the barrel-temperature set points on the controller screw 10, 25 or 50 degrees lower. What we mean by lower temperatures is that the actual melt temperature is within the accepted range as stated by the resin manufacture AND the melt is uniform in temperature and viscosity. Rarely can this be achieved via a standard general-purpose screw design. One of the main reasons many molders use higher temperatures than necessary is that the melt quality is poor, consisting of melted polymer and partially unmelted (solids) polymer.

For processing it is important that we know the melt temperature and that there is melt uniformity. Easy to say, "Measure the melt temperature," but hard to do. It is absolutely astounding that we can put a man in space, find some 3,900 colleges and universities in the United States and neither has provided us a way to measure melt temperature without interrupting the cycle. Considering this industry is the forth largest manufacturing base in the US and this is a critical processing parameter the SPE or SPI should work to resolve this. In the extrusion industry 99% of all extrusion processes measure melt temperature constantly and monitor uniformity by watching temperature and extrusion torque variations. This could be done with the shot-pot ICU design described in Section 4 by Robert Dray.

For measuring melt temperatures on a typical molding machine, the best we can offer is with two methods, and both have problems:

  1. Preheat the probe to a higher temperature than your best guess is for the actual melt and then interrupt a cycle and make a purge patty.
  2. With an infrared device that has the appropriate wavelength, a peak pick mode, has a full target area visibly defined and then interrupt a cycle and take the melt stream temperature as it is being purged. 

For melt uniformity the test is straightforward: As you are running a color, stop feeding the color and note the uniformity of the natural color. Or if running natural add color and check uniformity. Count how many shots it takes before the color shows in the part for an actual measure of residence time. Then take the parts and inspect for color dispersion. if you see streaks of color you do not have melt uniformity. 

 

Keep Fill Time Constant

Once the melt temperature and melt consistency have been properly established, processing at a lower temperature or really any temperature will follow the standard guidelines:

First stage, sometimes called boost or high pressure, is used to fill the part 99% by volume. At the end of the first stage, the part is not packed out and may appear short or contain sink marks. The last area to fill does not see much pressure at the end of the first stage, as the cavity is not full. This may be different from the way you currently mold but it is a critical point in developing a stable process. Know what the part looks like at the end of the first stage by bringing second-stage, or hold pressure to near zero. It should be slightly short. 

Understanding the polymer's flow characteristics during cavity filling is critical in developing first stage. Plastics do not flow like oil or water, which do not change viscosity as a function of flow rate. Plastics change viscosity as injection rate changes. This can be demonstrated through mold-filling analysis, capillary rheology, or using the injection molding machine as a rheometer. 

In this graph plotting viscosity vs. shear rate for a typical plastic, we see that plastics change viscosity relative to injection rate (mm/sec or in/sec). To have a stable process, fill time or injection rate must be kept constant.

Figure 1 is a typical viscosity vs. flow rate relationship for most polymers. It shows how viscosity changes with flow or shear rate (temperature effects are not shown). Because of this phenomenon, we must keep flow rate (injection velocity) constant. If flow rate changes, viscosity changes; if viscosity changes, parts will be different! Therefore, our first strategy of a consistent process is to fill all cavities identically each shot. To do this, we need to keep flow rate (shear rate) constant. We can measure injection velocity by the ram's velocity or by fill time. Since not all machines have velocity measurement capability, we will use fill time. Bottom line: Fill time for a part that is 99% full must be constant shot-to-shot and run-to-run.

This velocity or fill time control is similar to cruise control on a car. Note that we are not worried about a specific machine velocity setting. We have established a universal number based on a plastic variable that will work on any machine: fill time.

 

Know When to Ease Up

To keep constant fill time, or have velocity control, the machine must have more power available than what it is using; delta P must be maintained and abundant pressure used. This is similar to a car on cruise control.

The next question is, how do we keep fill time constant while running production as lots or temperatures change? Most hydraulic machines have a flow control valve that regulates oil to the hydraulic ram (Figure 2). This can be a manual valve, servo valve, proportional, cartridge, and so on. These valves are adjusted manually or electrically, closed loop or open loop, to regulate the flow of oil to the ram; usually the setting on the control is in mm/sec or in/sec.

For these valves to function correctly, they require one common variable to be set correctly: a pressure differential (delta P) across the valve. That is, first-stage pressure must be set higher than the maximum pressure required to push the plastic to fill the part 99% full. If first-stage set pressure on the pump side nears or equals the pressure in the hyrdraulic ram, the injection speed will slow down. If the ram slows down, then the viscosity of the plastic will change (get stiffer) and process variations occur. Bottom line: for velocity control you must operate your machine with a delta P across the flow control device and with abundant pressure on the pump side.

Caution must be exercised in setting this delta P, or the amount of abundant pressure. This is often called firststage
high-pressure limit. We need an adequate delta P to control velocity. We also must protect the mold from the situation that arises if it is a four-cavity mold and one of the cavities blocks off due to metal contamination or unmelt. Then we would be driving four cavities’ worth of plastic into only three cavities. If there were slides in the mold, we could flash them and damage the tool on the next shot.

This presents a dilemma. Without delta P, we have no process control on fill, and parts will vary thanks to viscosity changes (such as temperature, lots, colors, and percentage of regrind). With too much pressure, we run the risk of overpacking the mold and may damage it. The ram must be taken off cruise control before the last area of the part fills out, which is why we end first stage at about 99% full. Otherwise, the mold will likely flash. It’s like driving a boat into a dock; you must cut the power before you hit the dock.

A well-built mold should be able to withstand a certain amount of excess pressure. Overpressurizing the mold is likely to happen for a number of reasons throughout its life. The question is, how much extra pressure is
needed to gain velocity control without damaging the mold? The machine should not be set to full system
pressure unless required for the mold.

 

Securing The Pressure Differential

Methods to find the minimum delta P are available, but not practiced by many molders. One way to find this pressure is to raise the hydraulic pressure limit on first stage as you are making short shots until the fill time stops decreasing and peak pressure during first stage stops increasing. Then you can measure how much you are using to drive the plastic into the part during first stage vs. the first-stage set pressure limit. This is the pressure differential, or delta P. Delta P is the difference between what the hydraulic ram uses and what you have set for first stage (first stage must be higher). First-stage setpoint limit is a relief pressure. 

You must maintain this delta P as lots change or if you change temperatures. You must set the machine with enough first-stage pressure to control velocity for the resin’s typical viscosity range. The ram will use whatever pressure it needs to control velocity, providing there is sufficient delta P. Easy-flow or hotter plastic will require less pressure. With stiffer lots or cooler temperatures, it will use more pressure. This is velocity control, so viscosity is constant shot to shot and run-to-run.

Every shot should take a little different pressure for first stage. Lower temperatures in processing will require higher pressures, and the machine must be capable of providing adequate plastic pressure. If you do run out of pressure, you can downsize the barrel or shot cylinder for the ICU and the intensification ratio will increase. This has an added benefit of providing a longer stroke distance for the shot size, which is better for velocity control. Ideally, I like to see a shot volume of 25-65% of the shot capacity.

Another method of finding the required delta P is to ask what the machine manufacturer recommends. It designed and built the press; it should understand this concept fully. What is the manufacturer’s recommendation for setting first-stage pressure limit? As a general rule, set first-stage pressure limit to 200-400 psi higher than the highest pressure during first
stage for hydraulic machines (1500-2500 psi higher for electric machines). But each machine’s hydraulic architecture is different and required pressure differentials can vary. Also, be sure to find the correct delta P and verify that the mold can withstand the possible overpressurization.

John Bozzelli , founder of Injection Molding Solutions (Midland, MI) and initiator of Scientific Injection Molding.

Section 4: Machinery and Technology Required

Correct polymer processing is the fundamental principle that, more than any other, determines profitability and molded product quality. Unfortunately, this process has been virtually ignored by injection molding machinery manufacturers.

In general, molders have not been made aware of technology already available to substantially reduce molding cost and
improve part quality. This is somewhat understandable, as this technology is not available through injection molding
machinery manufacturers or resin suppliers. However, proper polymer process technology has been used for many years in the extrusion industry. 

Existing injection units are unable to provide the basic melt temperature and torque information that is provided on every extruder. Without proper melt temperature information, the molder does not have baseline viscosity information that he can refer to if there are problems with the molded part. Without accurate torque information, correct barrel zone settings for maximum screw performance cannot be achieved.

Attempts have been made to correct injection unit inadequacies through inmold pressure sensing; this may help (at substantial cost) in filling the cavity, but is unnecessary if the injection unit is providing accurate shot size and consistent viscosity. This band-aid approach attempts to compensate for upstream variations that should not be there in the first place.

 

What’s Wrong With Your Injection Unit?

In Section 3 of this series, John Bozzelli describes his amazement that our industry has not found a way to measure barrel melt temperature without interrupting the cycle. My amazement is even greater: How can this industry virtually overlook the importance of lower melt temperatures in cycle time, part quality, and energy savings?

The answer is fundamental: Reciprocating-type injection molding machines were a mistake from the start. They combine recovery with shot size; but recovery and shot size have nothing in common. Recovery is melting the polymer and shot size is volume.

All reciprocating-type injection units have these failings:

  • They complicate plasticating.
  • They contribute to viscosity variations.
  • Non-return valves decrease their accuracy.
  • They can only provide high melt temperatures, particularly when high recovery rates are required, due to high rpm and antiquated screw designs.

 

How it should work

Low melt temperature and consistent viscosity are requirements in all areas of extrusion, particularly in blown film. Sound like something that Injection Molding needs, too? Follow the physics of injection and you’ll see why a new solution is needed.

 

Screw design

 Low melt temperature and low shear rate are basic needs for more efficient molding and better part quality. Since viscosity=shear stress/shear rate, as shear rate (rpm) increases, viscosity decreases. When viscosity decreases, melt temperature increases. To achieve low melt temperature, the screw must be designed to maximize the pph/rpm, or specific rate. When high specific rate is achieved, only minimum rpm is required. For example, in blown film, rpm is normally less than 100 and the specific rate on a 4.5-inch diameter, 30:1 L/D screw would be 13-16 pph/rpm (.0578-.0711 oz/sec/rpm).

Many current reciprocating injection units are designed for 200 rpm and faster. This rpm and corresponding shear rate not only provide excessively high melt temperature and corresponding melt-flow-rate increases, but also lead to polymer degradation or, in many cases, black specks. Therefore, with current injection units it is difficult and, in many cases, impossible to reduce cycle time by lowering melt temperature.

 

Click here to learn about the downfalls of "General Purpose" screw designs

Click here to learn how to compare barrier screws

 

Purging

Consider the possibilities of an extended sprue bushing containing a passage that allows for the
purge to be captured, so that it can be added as regrind. Covers wouldn’t be required as the nozzle would
always be engaged in the extended sprue bushing.

 

Inline compounding

Used in the extrusion industry for many years, inline compounding should be a part of the Injection Molding process. Molders could incorporate fillers such as fiberglass, talc, calcium carbonate, colorants, and other additives, and side feeding or throat feeding in the extruder would be possible. The savings gained by inline compounding vs. buying precompounded pellets is well known.

 

Regrind usage

All injection machines require screen changers to eliminate contaminants. Reciprocating injection units cannot utilize screen changers normally since resistance increases as the screen fills, which in turn requires increased injection pressure. If injection pressure is not increased, short shots occur. A more logical injection unit design would incorporate a screen changer between the extruder and the injection cylinder. In this position it cannot influence injection. (See second drawing above.)

Contaminants are found even in virgin resin and are virtually impossible to eliminate in regrind. Mold damage and downtime are common with regrind usage and the cost, though normally not calculated, is not insignificant. Proper injection unit screw design should allow for the running of regrind percentages up to 100% with various resins.

 

Vented extrusion

Elimination of drying is a great cost savings. Why don’t current injection units permit trouble-free venting to remove volatiles and moisture, as in extrusion operations?

 

Coinjection

With the addition of a second extruder, coinjection would be an option or the same resin could be used in both extruders for large shot sizes (Figure 1, p. 48). For smaller shot sizes, one extruder could be shut down. System changes would not be required to run mono or coinjection, large or small molds.

 

Accumulator

A better design should be able to accumulate up to 7,000 psi during recovery, enabling faster injection.

 

Accuracy

We could improve shot-to-shot repeatability with the elimination of a non-return valve and a screw design that provides consistent viscosity. To produce shot-to-shot accuracy that eliminates shorts and flash, and enables the molder to select his part weight without changing part size, density must be controlled during the pack and hold sequence. This cannot be accomplished in a large-diameter injection cylinder as only small amounts of plastic need to be injected to compensate for part shrinkage. A better injection unit design would sense plastic pressure in a small secondary accumulator that enables the operator to control part weight in the normal machine sequence by accurately controlling density.

In a two-extruder layout, the second extruder can be used for coinjection or for injecting more of the same resin in large shot sizes.

A standard reciprocation machine does not allow recovery to start until pack and hold is complete. This scenario is the same in shot-pot types. So let’s consider an injection unit with an integral shutoff that can recover at all times, except during long inject times. This would enable the molder to avoid the seconds added to cycle time during pack and hold. Many molders simply skip the pack and hold phase, and therefore overpack. But this is costly because it increases part weight and uses more resin.

 

A possible solution

A more logical injection unit design would enable the molder to reduce cycle time, improve molded part quality,
and reduce scrap by improved accuracy. 

One option is the ICU injection unit, which is provided in either a retrofit or new injection unit design. It consists of an extruder, normally with an electric drive, and a small secondary accumulator for accuracy.

Robert F. Dray is president of R.Dray Mfg. Inc.