Tài liệu Cost Factors Affecting Productivity P2 doc

The injection capacity can also be too large for a required small shot size.The practical lower limit for the shot size the distance the screw retracts for the shot should not be smaller

3.7.6.2 Required Shot Size of the Mold

Shot size is an important characteristic of the molding machine, which affectsthe molding cycle But first, what is the difference between “shot size” and

“rated shot size”?

“Rated shot size” is the amount (in grams or ounces) of polystyrene (PS) that

the injection system (ram screw or two-stage) can inject at every cycle, and isindicated in all machine specifications

“Shot size” depends on the mass (W) of the molded product (in grams).

There are several common methods to determine the mass:

 Weigh a sample (or a handmade model) If the sample (or model) is ofthe same material as the desired product, this is the mass of the product.Otherwise, divide by the specific weight of the sample (model) andmultiply with the specific weight of the desired plastic

 Establish the volume, by completely immersing the sample (or model)

in a graduated container, filled partly with water The difference in fillinglevels gives the volume

 Calculate the volume from the drawing dimensions (this can be verycumbersome and time consuming)

Multiplying the established volume with the specific weight of the desiredplastic gives the estimated mass (weight) of the product (see Appendix forcharts of average specific densities (weights) for various plastics)

At this point it should be determined, which runner system would be mostsuitable for the planned mold If a cold runner system is selected, the mass ofthe cold runner must be added to the shot size And finally, the number (N)

of cavities should be determined All these calculations will have to be repeatedseveral times, with different assumptions, before settling on the final selectionsfor the planned mold

For 2- and 3-Plate Molds Only:

Shot weight SW = (N · W) plus the mass of the cold runner

The runner size (mass) is a very important The mass of a cold runner R can

be small and represent only a few percent of the shot weight But it can also

be quite large; in 3-plate molds, and in some 2-plate molds, R could be asmuch, or even more than N · W, particularly if the products are very smalland a large network of runners is required

Figure 3.21 shows a runner system for a 32-cavity 2-plate mold for caps Themass of the runner system is about 25% of the mass of all the caps per shot.The runner system is fed in the center by a hot sprue (not shown) to avoid

an otherwise large, cold sprue The cavities are tunnel-gated, and the productsand runners are separated after ejection

Figure 3.21 Runner system for a 32-cavity

2-plate mold for caps

Figure 3.22 (a) shows a runner for a 72-cavity 3-plate mold for a small cap.

The runner takes almost as much plastic as the products themselves The

cycle time is controlled by the cooling time of the (clearly visible) heavy

distribution runners A hot runner system for such a mold would reduce the

injected mass by half and the mold would cycle twice as fast, but would be

more expensive to build in the first place This is a typical example where

both methods must be considered, in view of the total requirements of the

product

Figure 3.22 (b) shows the runner and products from a very simple 4-cavity

2-plate mold for a simple but heavy product Note the heavy runners and the

cold sprue This too is a candidate for hot runners (valve gated into the

product) provided the quantities justify the greater expense

For both products in Figure 3.22 (a) and (b), the amount of plastic in the

runner system is large in relation to the mass of products molded (25% in

one, almost 100% in the other case) What could we gain by using hot runners?

The answer is mostly a matter of economics We must consider the price of

about $750 – $1,000 per hot runner drop, plus the price of the manifold at

approximately $5,000 or more On the other hand, the gain in cycle time and

productivity by using hot runners can be considerable A mold as shown in

Figure 3.10 (b) could cycle about twice as fast with hot runners! Also, the

cost of recycling the runners and the percentage of plastic lost during recycling

must be considered A smaller injection unit will be required with hot runners,

because all of the injected plastic will be converted into products There is

also savings in energy, because less plastic material is processed for the same

output The mold sizes are about the same, whether a 3-plate or a hot runner

mold is used Therefore, it is just a question of the quantities to be produced

If the quantities are large, the savings in the cost of the product can easily

justify the higher cost of the hot runner mold

Figure 3.22 Runner for a 72-cavity 3-plate mold for a small cap (a), and a runner for a simple but heavy product (b)

In many cases, a hot runner makes excellent economic sense

As a rule, the shot weight SW should not be more than about 80% of therated shot capacity of the machine This allows for leakage of plastic in thescrew check valve, and for wear in the screw and barrel.

If a material other than PS is used, it is necessary to convert the masses intovolumes, which is really what the machines inject For the same mass, PE hasabout 10% more volume than PS; therefore, if the mass is known, the volumewill be about 10% more than for PS For the same mass, the shot capacitywill be about 10% less than that for PS This must be properly calculated toavoid surprises when the new mold cannot be filled on the selected machine

Hot Runner Molds Only:

Figure 3.23 shows a schematic of a typical hot runner system consisting of amanifold to distribute the melt to the gate and one of each of two types ofnozzles: one an open type nozzle, the other a valve gate type nozzle Thevarious elements are clearly labeled Normally, these two types of nozzleswould not be used in the same manifold

One of the major advantages of hot runner molds is that they do not haverunners to be molded (and ejected) with each shot and therefore make fulluse of the shot capacity of the machine, therefore,

Shot weight SW = N · WThese molds used to be called “runnerless,” which is a misnomer There is arunner, but it remains (molten) in the mold and is not ejected at every cycle.Hot runner molds have the additional advantage of not needing to cool arunner In a cold runner mold, the runner, and in particular the sprue, arethe cycle-limiting factors With a hot runner, the cycle time depends mostly

on the wall thickness of the product and the quality of cooling of the mold

Figure 3.23 Schematic of a typical hot

runner system (Courtesy: Husky)

a b c Wire groove Manifold T/C Center insulator Nozzle housing Manifold heater Insulating air gap Guide pin

Manifold backing plate Plate bolt Back up insultor pad Sprue bushing Locating ring Piston cylinder Manifold Alignment pin Plate cooling

The injection capacity can also be too large for a required (small) shot size.

The practical lower limit for the shot size (the distance the screw retracts for

the shot) should not be smaller than 0.5 times the screw diameter If the shot

size is smaller than this value, injection will be inconsistent because some

stroke is required to reset the check ring or ball check If this is the case, as

smaller machine or a smaller injection unit should be selected

3.7.6.3 Plasticizing Capacity of the Machine

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

Today, practically all machines use an extruder to “plasticize” (or “plasticate”)

the material The extruder consists of a plasticizing screw of appropriate

design, rotating (for plasticizing) inside an externally heated barrel In most

machines, the screw is driven by a hydraulic drive Today, in more and more

machines the screw is driven by an electric motor, which is more efficient

and saves energy costs As the screw rotates, the raw plastic, which enters

usually near the drive end, is pushed against the inside wall of the barrel The

friction generated between the plastic in the rotating screw flights and the

barrel heats the plastic and the “melt” gradually moves forward toward the

end (“tip”) of the screw, where it accumulates while the screw retracts, pushed

back by the pressure exerted by the plastic The screw stops when the desired

shot volume is reached At the “injection” signal, in a ram screw, the plastic

accumulated in front of the screw is pushed out through the machine nozzle

into the mold In 2-stage injection machines, the extruder is used to fill an

injection cylinder (the ”shooting pot”)

Note that as a rule, the heaters surrounding the barrels contribute less than

10% to the plasticizing process The heaters are there mainly to allow starting

up again after a shut down, when the screw and barrel are cold and filled with

cold plastic Heating the screw from the outside and melting the frozen plastic

enables the drive to turn the screw again As a rule, only the mechanical energy

of the drive generates the (frictional) heat for plasticizing the cold pellets

The amount of plastic an extruder can convert depends essentially on, and is

limited by, the size (or power) of the drive motor (kW or HP), torque, screw

speed and screw diameter, and on the design of the screw (screw design is a

specialized area of engineering and not within the scope of this book) Many

machines are equipped with so-called “general purpose” (GP) screws, which

do a fairly good job for most materials, but do not work as efficiently as a

screw designed for a specific material This is an important consideration

when planning production A mold in a machine equipped with the most

suitable screw will perform better (i.e., deliver better quality melt faster and

with less power) than when using a GP screw

Today, most machines are equipped with easy screw-change features Note

also that the condition of the screw is very important A worn screw (and/or

barrel) will have greater clearances than when new, and therefore will produceless than the original specifications indicate.

As with shot capacity, the effect of specific gravity of the selected materialmust be considered and used to adjust the rated figures In addition,allowances for the type of plastic are necessary: some plastics require adifferent L/D ratio (the ratio of the active length of screw over the screwdiameter) and a different compression ratio (ratio of height of screw flightsfrom the feed zone to the final “metering” zone, near the screw tip) If moldsfor plastics other than the most common ones are used, this should bediscussed with suppliers of the plastic intended to be used, with the machinedesigner, or with a plasticizing screw design specialist

Here, we are mostly concerned with the data provided for plasticizing capacity.All machine specifications rate the capacity in kg/h, but that really means the

amount the extruder can plasticize if it runs continuously! But no reciprocating

screw or ram screw (RS) machine can run continuously The screw cannotturn when pushed forward by the high injection forces As we have describedearlier, while the screw turns, the plastic in the barrel is melted and movesforward toward the screw tip, past the check valve and accumulates there untilenough plastic is made up for the next shot When the desired shot volume isready, the screw stops and waits for the signal to inject At this moment, thescrew is pushed forward to inject the plastic into the mold During the timewhen the screw is stopped (and waiting before injecting), while injecting(injection cycle), and while holding the screw forward (low pressure hold

cycle), the screw does not plasticize The sum of these times must be subtracted

from the total available time; therefore, less time is available for plasticizing.The concept of “plasticizing per hour” is really a guide only

Note: Usually, the higher the back pressure, the better is the quality of the melt,but at the same time, more power (kW, hp) is drawn from the screw motorand less melt is pushed ahead of the screw The amount plasticized is directlyproportional to the speed of the screw (in RPM), its diameter, and design.There is a limit to the available power of the motor and to the screw speed (permachine specifications), which limits the amount plasticized per unit of time

It is important to have an idea of how long the screw will be stopped for eachmolding process The low-pressure hold time is usually not required for thin-walled products, which freeze so fast that there is no possibility for the plastic

to enter the cavity space after the original injection But for thicker walledproducts, which are likely to shrink after the cavity space is first filled, it isimportant to keep the flow coming from the machine nozzle by keeping thepressure on the screw

Another important consideration is the injection speed, as discussed in moredetail in Section 3.7.6.7 With faster injection, less time is required to keepthe screw stopped This is of particular importance with large shots, whichcould take several seconds to fill the mold But the designer must also beaware that not all plastics are suitable for very fast injection; they may sufferexcessive shear stresses and lose some of their physical characteristics, whichcould reduce the quality of the product

Plasticizing capacity is typically

given in the machine specifications

in kg/hr polystyrene (PS), using a

universal screw, running

continuously

Assuming that the injection and hold time required will total 1.5 seconds,

the plasticizing capacity of the machine must at least ensure that it can

prepare the required mass per shot in 8.5 seconds, or 108 ÷ 8.5 · 10 =

127 kg/h This will require a machine rated at least 130 kg/h (PS) If the

planned product is, e.g., made from PE, we must still convert for the

different specific gravity and add about another 10%, which demands a

plasticizing unit yielding at least 145 kg/h (PS) Note that these figures

are only achievable with a shut-off machine nozzle With an open nozzle,

a much larger extruder would be needed For explanation, see later

examples for open and shut-off nozzles

What happens if there is no such larger machine available and we do not

have sufficient plasticizing capacity? The mold will be able to run, but it will

not run at the expected speed, because the system will have to wait for the

shot size to be made and will therefore have less output than planned

Light-Weighting the Product and Mold Improvements

The importance of understanding the plasticizing capacity of a machine

becomes clear when planning to redesign a product for less mass

(“light-weighting”) The reduction of mass not only saves plastic, but also decreases

(in most cases) the cooling time thus increasing productivity of the mold

The preliminary questions to ask are: will the new mold require more plastic

per hour? Is the existing injection unit large enough?

But even if no plastic can be saved by light-weighting, better mold design,

and particularly better cooling, better ejection methods and other

improve-ments can result in a substantial decrease in cycle time; in other words, more

pieces per hour Is the plasticizing capacity now large enough for this new

mold, on the same machine as before?

Example 3.2

A product has a mass of 40 g and can be molded in a 12-cavity mold,

running at 4 shots per minute This product is redesigned to have a mass

of 35 g and will be able run at 6 shots per minute

The old design of the product required 40 g · 12 cav · 4 sh/min · 60 min

= 115,200 g (or 115 kg/h) Because the extruder in a common ram screw

injection unit cannot run during injection, we will assume that an extruder

of 150 kg output will be required

Always make sure that the machine

is not the limiting factor when trying

to improve a mold’s productivity

With the redesigned product, we now require

35 g · 12 cav · 6 sh/min · 60 min = 151,200 g or 151 kg/h, which meansthat the machine will require a much larger plasticizing unit

But it is also important to ensure that the dry cycle of the machine is

capable of allowing the faster cycle

Example 3.2 highlights important consequences:

1 The productivity can be increased by 50%:

Before, 12 cav · 4 sh/min · 60min = 2,880 pieces per hour were produced,after redesign, 12 cav · 6 sh/min · 60 min = 4,320 pieces per hour can beproduced, requiring much fewer machine hours, thus also reducing theproduct cost The increase in productivity is significant However, thecost savings could be somewhat less if a larger machine is required

2 The product cost has been greatly reduced by using less plastic, thedifference being 5 g per unit or 5 kg per 1,000 pieces At an estimatedcost for a commodity plastic of approx $1.00 per kg and an annualrequirement of 10,000,000 pieces, this amounts to a saving of $50,000.00per year If it were an expensive engineering plastic, the savings would bespectacular

3 If, at the same time, we also consider to switch from a cold runner to a hot

runner system in the new mold, we must remember that for cold runners,the plasticizing unit must not only provide melt for the products butalso for the runners By selecting a hot runner system, we are in fact

increasing the usable plasticizing capacity by the no longer required mass

of the cold runner system This means that, especially if the runner waslarge, the existing injection unit could possibly be sufficient for the new

mold With hot runners, there is also the cost of material per unit affected,

because we don’t have the mass of runners to consider Even if all plasticrunners could be recycled, there will always be some losses of materialduring recycling Also, there is no cost of recycling

4 If the injection unit is not capable of supplying the new required meltquantities, the whole effort of redesigning for better productivity wouldhave been economically useless and the money wasted The machinewould have to continue to cycle at the old, lower speed

All the above must be considered and the calculations must be done everytime a redesign is contemplated

Figure 3.24 shows sections through a molded tumbler; on the right, the wallthickness before redesign, on the left, after redesign This resulted in areduction of plastic of 20% and at the same time, in a decrease in cycle time

of about 20% At the same time, it eliminated many molded defects caused

by the thick to thin transitions

Figure 3.24 Sections through a molded

tumbler

3.7.6.4 Open Nozzles

All molding machines come standard with “open” nozzles, i.e., the tip of the

machine nozzle is open and will let plastic pass through freely, from the end

of the screw either into the open air or into the sprue bushing of the mold,

while injecting In order to achieve good plasticizing, we must provide some

controlled low backpressure in the injection cylinder, acting on the injection

piston This back pressure is usually only in the order of about

5–10% of the injection pressure, but 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 But as soon as the mold opens, the screw rotation and the

back pressure 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, which

is of course unacceptable 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 (see below)

would not be required

In both these 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 3.3

Let us assume a mold and machine with a 4 s dry cycle, an injection and

hold cycle of 2 s, and a cooling cycle of 6 s The total cycle is therefore

12 s When using an open nozzle, the maximum time the extruder can

run is 6 s, 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 s or less, there is no problem with an open nozzle Note that in this

example, the screw can run only 50% of the time; therefore, the extruder

is used only 50% of its rated capacity

Figure 3.25 Graphic illustration of

Example 3.3

Figure 3.27 Graphic illustration of

Example 3.5

A B

C

Figure 3.28 Shut-off nozzle

Figure 3.26 Graphic illustration of

Example 3.4

Example 3.4

Let us assume the same machine and mold conditions as in Example 3.3,but this time not enough plastic can be plasticized in the 6 s between theend of injection and the end of cooling If we assume we need 9 s togenerate the melt for the next shot, we must increase (unnecessarily) the

“cooling” time by 3 s (from 6 s to 9 s), for a total cycle of 15 s Thisrepresents a severe loss of productivity (4 versus 5 shot/min) This extruder

is used 60% of its rated capacity

We should therefore look for an alternative, either the use of a shut-offnozzle, or find a machine with a larger extruder

It is also important to understand that by adding unnecessary cooling time,the products will eject cooler than necessary and shrink less (they will belarger than expected) This can be significant when molding plastics withhigh shrinkage factors Also, products cooled too much inside the mold maybecome overstressed in some areas as they shrink onto the core and thereforefail early in use 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 not only more energy forheating the plastic higher than necessary, but it will also require more energyfor cooling it The use of a shut-off nozzle (see below) eliminates the extracooling time

Example 3.5

Let us assume the same machine and mold conditions as in Example 3.3,

but here the necessary cooling time is 9 s, for a total cycle of 15 s In this

case, the screw has sufficient time for plasticizing, up to 9 s out of a 15 scycle The extruder can be used up to 60% of the cycle time

This illustrates that with longer cooling cycles, a simple open nozzle isadequate

3.7.6.5 Shut-off Nozzles

For short cycle times, a shot-off nozzle can greatly increase the productivity

of mold and machine The basic principle of the shut-off nozzle is to provide

a mechanical stop within the machine nozzle, which closes and opens the

flow path of the plastic from the extruder to the nozzle tip Shut-off nozzlescome in various executions, such as shuttles, rotary cocks, or pins and areusually operated by compressed air or by hydraulic pressure oil

Figure 3.28 shows a photo of a complete shut-off nozzle In this design, thelever (A) pushes a pin inside the nozzle to close the nozzle opening (B).When injecting, the plastic pressure pushes the pin to the right, thus openingthe nozzle opening to let the plastic enter the mold The lever is operated bylink (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 3.6

Let us again assume a mold and machine with a 4 s dry cycle, an injection

and hold cycle of 2 s, and a cooling cycle of 6 s The total cycle is therefore

12 s Using a shut-off nozzle, the time the extruder can run now is 6 s (the

cooling cycle) plus 4 s (the dry cycle) equals 10 s 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

Example 3.7

This example illustrates an extreme case: A machine with a 2 s dry cycle

runs a mold with a 1 s injection cycle (no hold time) The cooling cycle is

1 s, for a total cycle of 4 s (15 shots per minute) With an open nozzle, the

screw would have only 1 s time to make up for the next shot or can run at

most 25% of the rated capacity With a shut-off nozzle, the screw could

plasticize for 3 s (adding the dry cycle time and the cooling time) The

screw could therefore plasticize during 3 out of 4 seconds, or at 75% of

the rated capacity

Example 3.8

A molder planned to operate a mold in a machine with a 4 s dry cycle, at

a 9 s total cycle, or 400 shots/hour However, there was not enough time

for the extruder to deliver the required shot size in time for the next shot

The cooling cycle had to be lengthened from 3 to 6 s and the total cycle

increased from 9 to 12 seconds; in other words, there was about 25% less

production than expected

When I got involved, I suggested the use of a shutoff nozzle The time available

to extrude could be raised to 7 s, plenty of time for this job

Figure 3.29 Graphic illustration of