Chapter 6 Quantitative Production Equipment

and Mold Maintenance

6.1 Injection Molding Machine

6.1.1 Basic Principles of Injection Molding Machine

The injection and molding is achieved through the injection molding machine and molds. Though in various types, the injection molding machine, no matter which type it is, comprises only two basic functions: 1) to heat plastics and make them fuse; 2) to apply high pressure on the fused plastics, make them eject and fill the mold cavity.

According to the mode of plastication, the injection molding machine falls into the following two

types： 1) Plunger Injection Molding Machine The plunger injection molding machine pushes forward the fused plastics in the charging barrel

Fig.6-1: highlight and no-welding-line injection molding system

1. SP108A injection molding machine; 2. drier and hopper; 3. water tower; 4. water pump;

5. control machine for rapidly heating and cooling injection molding

3

4

1

5

2

through the to-and-fro movement of plunger therein, and the spreader further injects them into the mold cavity through the nozzle. The plastication of plastics in the charging barrel mainly depends on the heat provided by the outer heater of charging barrel. The poor thermal conductivity of plastics results in the unbalanced plastication to the inner and outer layer of materials in the charging barrel and therefore, the plunger injection molding machine is not suitable for the molding of plastic parts with low fluidity, strong heat sensitivity and overlarge amount of injection.

2) Screw Injection Molding Machine The screw injection molding machine differentiates from the plunger injection molding machine

mainly in that the horizontally-pushed plunger in the charging barrel is replaced by a rotary screw which undertakes all the fusion, plastication as well as injection of the materials in the charging barrel. The screw can not only rotate in the barrel but also can move forward and backward, and thereby is able to achieve such functions as sending materials, compacting, plasticating and transmitting pressure. Currently, the screw injection molding machine yields the highest productivity and is also the most widely applied. As indicated in Fig.6-1 is a complete set of super-luminous and traceless injection molding system, wherein Yanhing SP108A injection molding machine is one of the screw type. The subsequent discussion shall be based on the screw injection molding machine.

The course of injection and molding of plastics is a cycling process, wherein the completion of one

cycle represents the completion of one injection cycling period. Each period mainly comprises: quantitative feeding, fusion and plastication, pressurization and injection, mold filling and cooling as well as mold opening and taking out the parts etc. The next cycle follows after the plastic parts have been taken out and the molds have been closed. The completion of each injection cycling period represents the completion of a working cycle for the injection unit and mold clamping unit of the injection molding machine, namely, each moving part of the two units has moved once according to the scheduled sequence. The cycling process is indicated in Fig.6-2.

Fig.6-2: cycling process of plastic injection molding

Three stages shall be needed for the completion of injection and molding: plastication, injection and molding.

1) Plastication The raw materials of plastics fall into the charging barrel from the hopper and are sent forward by

the rotating screw. While in the charging barrel, the plastics, on the one hand, receive heating and warming from outside of the barrel, and on the other hand, are compacted due to the gradual reduction of the volume of spiral flute. Meanwhile, strong stirring and cutting shall be applied to the plastics by the rotation of screw in the charging barrel, resulting in the intense friction between plastics and between plastics and the barrel as well as screw, and producing a lot of heat which leads to the gradual plastication and fusion of plastics while being pushed forward. The screw while rotating also moves

Raw materials

Injection unit moving forward

Preheating mold

Insert

Plastication

Injection unit moving backward

Measurement

Closing mold

Parts Clear-up mold

Opening mold

Injection

Pressure maintaining

Ejecting parts

backward due to the pressure from the fusion of plastics by the head of screw, so that the plasticated fusant can be stored at the top of the charging barrel for use during injection. The backward stroke of screw is determined by the quantity of materials injected as required by each molding of plastic parts.

2) Injection When the screw stops rotating and moves backward, the mold clamping part begins to work to

close the mold and the injection unit subsequently moves forward to make the nozzle and mold spruing gate joint with each other; the injection oil tank plunger drives the screw to move forward rapidly, and the injection pressure and speed as required by the molding technics inject the fused plastics into mold cavity. To avoid refluence of the materials injected into the mold and to supplement the small amount of materials needed due to the contraction caused by the cooling and molding of parts, the screw, upon completion of injection, shall still keep certain pressure on the fused materials, i.e. pressure maintaining, until the materials at the gate have cooled and molded.

3) Molding The fused materials in the mold cavity restore from viscous and fluid state to glassy state after

being cooled. After complete cooling and molding, the plastic parts in the mold cavity are ejected by the ejector unit during mold opening, thereby the whole process of molding of the injected parts being completed.

Fig.6-3: injection molding machine operation process

As indicated in Fig.6-3, centering on the screw, three procedures shall be needed to complete an injection cycling period:

1) The injection tank plunger, under the scheduled pressure, pushes the screw forward to inject the plasticated fused materials;

2) The injection tank plunger, under the scheduled pressure, further pushes the screw forward to supplement the materials and to maintain pressure;

3) The oil pressure motor drives the screw to rotate and to supplement materials to be plasticated;

Rotating screw Hopper Hydraulic motor

Injection cylinder

Heater

Barrel Single check valve

Injection nozzle

Injection

Pressure maintaining

Cooling(Plastication measure)

Screw backing off

Screw rotating

Screw going ahead

Demolding

Closing mold

the screw overcomes the scheduled pressure on the injection tank plunger, moves backward until the work has been completed, and prepares for he next injection; on the other hand, after the cooling and molding of plastics in the mold cavity, the ejection shall be made.

6.1.2 Structural Composition of Injection Molding Machine

A universal injection molding machine mainly comprises such three parts as the injection system, the molding clamping system and the hydraulic control system.

1. Injection System The function of injection system is to plasticate a certain amount of plastics uniformly into fused

state within certain time, and inject them into the mold cavity with sufficient pressure and speed. Above all, the system is able to maintain pressure and supplement materials for the fused materials in the mold cavity after injection.

As indicated in Fig.6-4, the most extensively-applied system is the to-and-fro screw injection device which plasticates the materials through the rotation of a screw that injects the plasticated fused materials into the mold cavity. It mainly consists of the plastication parts (including a nozzle, a charging barrel and a screw), a hopper (including a drying device), a screw rotation device, an injection device, an injection seat as well as a movable oil tank for the injection seat.

1.nozzle；2. front adapter；3.screw head assembly；4.heater band；5.barrel；

6.injection screw；7.hopper；8.bearing；9. hydraulic motor；10.oid tank for injection seat；

11. cylinder rod for injection seat；12. oil tank stopper for injection；13. injection oil tank；14. injection unit base block；

15.nozzle center adjusting bolt；16.driving housing；17. driving housing guide rod；18. guide rod bracket

Fig.6-4: injection unit structure

Its working process: With the continuous rotation of screw, granular materials keep falling into the spiral flute of the screw in the charging barrel from the hopper and are pushed to the front of the charging barrel by the rotating screw, during which the materials are gradually plasticated and fused under the heating by the outer heater of charging barrel as well as the cutting and mixing by the screw,

and then are pushed between top of the screw and the nozzle. With the increase of fused materials at the top of charging barrel, the anti-resistance on the forward-moving materials also grows. When the resistance exceeds that returned by the injection oil tank plunger, and when the stored materials reach the quality required by the parts, the measurement control unit begins working and the screw stops rotating to prepare for injection. The mold at this moment is closed, and the screw pushed by the injection plunger injects the fused materials into mold cavity with a certain pressure and a high speed. To avoid refluence and to supplement the materials contracted due to cooling, a certain period for pressure maintenance shall be needed.

L—Length of screw L1—Length of feeding section L2—Length of plastication section；

L3—Length of measurement section；S—Pitch；D—Screw diameter；e—Width of screw rib；h1—Depth of feeding Section

h2—Depth of plastication section e—Screw rib width R1、R2—Fillet radius of the root screw rib Fig.6-5 Geometric shape and dimension of injection molding machine’s screw

Table 6-1: section lengths of injection screw /mm Types of Screw Feeding Section Plastification Section Measurement Section Notes Morphing Type 30~50 50 20~35 Mutation Type 65~70 （1~1.5）D 20~25 General Type 45~50 20~30 20~30

D is screw diameter

The screw is an important part of injection unit as indicated in Fig.6-5. Its major parameters are as follows: Diameter and Stroke. The injection molding machine’s maximum quantity of injection is

determined by the diameter and maximum stroke of the screw. The diameter proportionates to the stroke, with a ratio of 3~5. If the ratio is too large and the stroke too long, the working length of screw shall be shortened, thereby influencing the plastication quality of materials. Should the ratio be too small and the stroke too short, to ensure the quantity of injection the diameter of screw must be increased and the diameter of injection oil tank shall also be increased accordingly to increase power consumption.

Screw Lift Angle, Rib Width and Radial Clearance.

Generally screw lift angle '4017°=ϕ 。

Screw rib width De )12.0~08.0(= (D is screw diameter). For materials with poor viscosity,

the rib width should be taken as large as possible since if it is too small, overflow shall easily occur, yet when the width is too large, power consumption shall be increased, which may cause over-heating.

Morphing screw ( used by extrusion molding machine )

Mutation screw (used by injection molding machine)

Radial clearance is the fitting clearance between the screw and charging barrel. If the clearance is too large, the plastication capacity of the machine shall be reduced and the refluence of refused materials during injection shall increase; should the clearance be too small, the difficulty for the mechanical processing of screw and charging barrel as well as the power consumption of screw shall

both be increased. Usually the radial clearance De )005.0~002.0(= .

Length-Diameter Ratio and Segmentation The LD radio of screw refers to the ratio between thread length of the screw and the diameter

of screw, generally taking 16~20. The larger the ratio, the better the quality of plastication is, the temperature shall be more uniform and the effect of mixing and fusing shall be better; meanwhile, the rotation speed of screw shall be improved on the premises that the quality of plastication be ensured. If the LD radio is too large, however, it shall be difficult for the mechanical processing of screw and charging barrel. Hence, when the quality is ensured, the ratio should be taken as small as possible, since short screws are not only easy to be manufactured but also can shorten the body of injection molding machine. Furthermore, it is also easy for cleaning.

The segmentation of screw refers to the proportion in the threaded part of the screw as taken by the feeding section, plastication section and measurement section of the screw for injection, wherein the feeding section generally takes 50%, and the compression and measurement section each takes 25%.

Depth of Spiral Flute and Compression Ratio The charging barrel is a container for heating and pressurizing plastics. The feeding of injection

molding machine mostly depends on the deadweight of granular materials which freely fall into the charging barrel through the hopper. Therefore, shape of the inlet of the barrel should be suitable for the deadweight falling of granular materials, so that the transmission capacity during automatic feeding can be ensured.

During injection, the fused materials in the charging barrel, subjecting to the movement of screw, flow rapidly through the nozzle and into the mold cavity, thereby being molded. The nozzle thus plays such part as connecting the charging barrel and the mold as well as endowing the materials with high injection speed. The materials, when flowing through the nozzle, shall undergo large-scale cutting and be further plasticated due to the reduction of the nozzle’s inner diameter.

On the other hand, the moisture in the raw materials of plastics shall directly influence the quality of injected products, especially engineering plastics. It is therefore quite necessary to pre-dry the moisture absorptive plastics, which can not only improve the (products’) surface glossiness, improve the bending strength and tensile strength and avoid occurrence of silver lines, crackles, air bubbles and spots inside the parts, but also can improve the plastication capacity and shorten molding cycle. The moisture in the raw materials of plastics exists into two types. One is that the materials absorb moisture and enter the inside of the matrix, such as ABS and Perspex etc. The other is that the materials is wet on the surface, the moisture only wraps around the surface of the basal body and seldom infiltrates to the inside, such as polyethylene and polypropylene etc. Table 6-2 and 6-3 list the admissible injection molding temperature range for partial plastics and their drying condition. During the drying, different method should be adopted according to the different materials and different forms of moisture absorption. Attention should be made that whichever method is used, the dried materials must be sealed for storage or heated and insulated, since they cannot be exposed in the air for too long time (including in the hopper with a lid thereon), wherein

for sunny days not exceeding 3h and rainy days 1h. Table 6-2: injection molding temperature range of partial plastics

Temperature of Barrel℃ Plastic Types Temperature of Mold℃ Inject#1 Inject#2 Inject#3 PS Types PS HIPS ABS SAN

10~75 10~75 10~80 10~80

200~280 220~270 220~270 220~270

200~260 190~260 190~260 200~250

150~160 150~160 150~160 150~160

LDPE Types LDPE HDPE PP

20~60 20~60 10~80

230~310 230~310 220~325

220~300 220~300 220~290

130~200 130~200 140~180

PVC Types Hard PVC Soft PVC

20~60 20~60

170~200 170~200

170~200 140~200

140~150 130~140

PC 70~115 300~350 280~340 230~270 PMMA 30~70 190~240 190~220 140~170 PA Types PA6 PA66

50~80 50~80

210~230 250~280

210~230 250~280

200~210 190~250

POM 60~90 190~210 175~210 160~180 CA Types CAB CA CP

30~75 30~75 30~80

140~150 180~200 180~210

140~150 150~180 190~220

120~130 130~150 160~180

Table 6-3: drying temperature and time range of partial plastics Plastic types Drying temperature and time range

PS Types（PS-ABS） 60~80℃ 1~4 h PC 100~120℃ 7~8 h PMMA 70~80℃ 6~8 h PA Types 80~100℃ 10~14 h CA-CAB-VP Types 70~80℃ 2~4 h

2. Mold Clamping System The mold clamping device should meet the following requirements: To have sufficient clamping force and to ensure that no overflow of the mold shall occur under

the pressure from fused materials. To have sufficient mold plate area, stroke and space to suit for the molding of plastic parts with

different shapes and dimensions. The running speed of mold plate should be first fast then slow during mold clamping and first

slow then fast during mold opening to avoid collision of molds during mold clamping and to have the plastic parts ejected steadily.

Sufficient intensity of the mold plate should be ensured to avoid distortion or damage to the molds during molding as incurred by the frequent pressure applied thereto, which may influence the steadiness of the products’ dimension and service life of the injection molding machine.

The mold clamping device mainly consists of the mold clamping unit, mold adjusting unit, ejector unit, front and rear retainer plate, moving plate and rod as well as the oil tank etc.

1) Mold Clamping Unit According to the different method for achieving clamping force, the common structural forms of the

mold clamping unit fall into the hydraulic type and hydraulic-mechanical type. a) Hydraulic Mold Clamping Device The hydraulic mold clamping device achieves the opening and clamping as well as locking of

molds through hydraulic pressure. As indicated in Fig.6-6 is the basic structure of a simplest single-tank direct-acting hydraulic mold clamping device, which is mainly applied to mini-injection molding machine. The mold clamping oil tank is also referred to as mold locking oil tank, whose principal function is to push and lock the mold plate through the oil pressure in the oil tank. The front and rear mold plates are supported by the rod and secured by the nut, ensuring that the moving plate can move forward and backward on the rod.

1. mold clamping oil tank; 2.rear fixed mold plate；3. moving plate；4. guide bar；

5.mold；6. front fixed mold plate；7. nut of guide bar

Fig.6-6: single-tank direct-acting hydraulic molding clamping device

The hydraulic mold clamping device comprises the following features: Space between the molds and the range of thickness of the molds are both large. The adjustment of clamping force can be achieved through adjusting oil pressure; the clamping

force can be directly numerated, which is quite convenient. The moving plate can stop at any place within the range of stroke, which is very convenient for

adjusting mold space. The parts can be self-lubricated and is low in abrasion. Injection molding machine with strong clamping force requires large mold clamping oil tank

and high oil pressure, which is disadvantageous for the sealing and manufacturing of hydraulic system.

The plenty of pipeline for hydraulic system tends to be difficult for ensuring no leakage, and therefore the poor steadiness of clamping force shall influence the quality of products.

b) Hydraulic-Mechanical Type As indicated in Fig.6-7, a large clamping force can be gained through small oil tank pushing force

and the enlargement of bar linkage mechanism. Such mold clamping unit combined by hydraulic and mechanical function improves the speed of mold clamping and saves power consumption, which is very advantageous for improving equipment structure and reducing product cost. Current commonly-used types are the single-toggle and double-toggle hydraulic-mechanical mold clamping devices.

The hydraulic-mechanical mold clamping device embodies the following features: Having power enrichment function, which can reduce power consumption. Having self-locking function. Even if the oil pressure is removed, clamping force shall not

disappear. The molds can be securely and steadily locked. The moving speed of the mold plate is variable, wherein during mold clamping it is from fast to

slow and during mold opening from slow to fast then to slow again, which ensures the

steadiness of mold during opening and clamping and avoids the impact and collision of molds when contacting each other.

The toggle link unit is easily worn and requires high rigidity and abrasion resistance. When the thickness of molds varies, the space between moving plate and fixed should be

adjusted, which tends to be troublesome.。 The clamping force must be measured by special instrument and is hard to be adjusted.

1.tie-bar lock unit；2.fixed plate；3.mechanical safety bar stopper；

4.mechanical Safety Stop Bar；5.movable Plate；6. movable plate link；

7.cross head link；8.long link；9.main link；10.end plate link；11.end plate；12.clamping cylinder；

13. clamping cylinder rod；14.cross head；15.ejector cylinder；16.ejector pin；17.movable plate slider；18.tie-bar

Fig.6-7: hydraulic-mechanical clamping unit structure

2) Mold Adjusting Unit The mold adjusting unit is set for the variation of mold thickness, especially for

hydraulic-mechanical mold clamping device. Since the stroke of moving plate cannot be adjusted, a mold adjusting unit must be set to meet the requirements of molds with different thicknesses.

Space adjustment for thread toggle link.。 Space adjustment for movable mold clamping oil tank.。 Space adjustment for rod nut.。 Space adjustment for the connecting nut between movable mold plates.。

3) Ejector Unit The ejector unit is set for ejecting the plastic parts in the mold cavity and therefore should be

provided with sufficient ejection force and stroke. The unit usually falls into three forms: mechanical ejection, hydraulic ejection and pneumatic ejection.

Mechanical Ejection The ejector pin, which is unmovable in itself, is fixed on the frame of the machine. During mold

opening, the moving plate moves backward and the pin spreads through the center port in the plate, acts on the ejector plate of the mold and pushes the ejector plate to eject the plastic parts from the mold.

Length of the ejector pin is determined by the thickness of the molds and adjusted by the thread. Hydraulic Ejection

As indicated in Fig.6-7, an ejection oil tank is assembled at the back of the moving plate and is used to drive the plunger, i.e. the ejector pin, to work. The bolt on the plunger can be used to adjust the length of the ejector pin; during mold opening the mold plate moves backward and ejection oil tank drives the ejector pin which spreads out and begins working. The force, speed, time and stroke of ejection can be adjusted through hydraulic system and can automatically restore. The plastic parts can thereby be ejected during or after mold opening, which is beneficial for shortening the cycle of injection and molding.

Large injection molding machines usually comprise both mechanical and hydraulic ejection, wherein the ejection oil tank is usually located between the moving plate and the mechanical ejector device is set on the two sides of the mold plate.

Pneumatic Ejection Pneumatic ejection is achieved through compressed air which directly blows out the plastic parts

through the many mini holes on the mold. This method can avoid ejection mark on the surface of the plastic parts, yet a pneumatic auxiliary device shall be needed.

2. Hydraulic Control System As indicated in Fig. 6-8, the hydraulic control system of injection molding machine consists of the

power system, execution system, control system, auxiliary system as well as the driving medium oil. The power system mainly provides hydraulic oil (including such parts as the motor and pump unit etc.) for the system. The execution system mainly refers to the various oil tank on the injection molding machine, including mold clamping oil tank, ejection oil tank, integral movable oil tank, injection oil tank as well as the motor for driving oil, which altogether transfer the pressure energy of liquid into mechanical energy and drive the execution unit to apply force onto other parts. The control system mainly controls the pressure, quantity and direction of flow of the hydraulic oil to achieve scheduled working procedures and power parameters. It mostly comprises pressure control valve, quantity of flow control valve as well as direction control valve. The auxiliary system mainly refers to such parts as the oil tank, oil filter, energy accumulator, pipeline, joint and pressure meter etc, which mainly act as auxiliary to help other systems complete their functions. The hydraulic oil is the “blood” of hydraulic system, through which the conversion, transfer and control of energy can be carried out.

During the injection and molding, the fused materials are injected into the mold cavity through high pressure, and therefore, the mold clamping oil tank must be provided with sufficient clamping force to avoid flash of the plastic parts due to expansion of the molds. In addition, the hydraulic system must meet the requirement for speed during mold opening and clamping: first fast then slow during mold clamping and first slow then fast and again slow during opening. The ratio of fast and slow speed is generally very large. Usually such methods as double-pump in parallel connection, multi-pump in hierarchical control and throttle governing are employed to adjust the speed of mold opening and clamping.

The integral movable oil tank for the injection seat should possess sufficient pushing force to drive the seat to move rapidly forward and backward, and meanwhile should ensure the close sealing and secure jointing between the nozzle and the mold gate.

The injection oil tank should be able to flexibly adjust the injection pressure and speed in accordance with the variety of plastics and the shape of plastic parts. For plastics of high viscosity or plastic parts with thin walls, large area and complicated shapes, the injection pressure should be higher

and can be lower if vice versa. The injection speed should be properly selected, since if the speed is too low, the parts can easily get cold joint and the contour of the parts with complicated shapes cannot be ensured; if the speed is too fast, plenty of friction heat may be produced, resulting in the decomposition and color change of the materials, and meanwhile the gas in the mold cavity may not be discharged freely and air bubble will possibly occur in the molded parts. After the injection, the pressure should be maintained to ensure that the fused materials fill the mold cavity and provide supplement to the contracted materials caused by cooling.

The ejector unit should be provided with sufficient ejection force to ensure smooth ejection of the parts. For ejector unit with a plurality of pivots, the ejection force on each pivot should be uniform and the protruding length of the ejector pin should be the same. Meanwhile, the moving speed of the ejector pin should be steady and adjustable.

Fig.6-8: hydraulic schematic diagram

6.1.3 Basic Parameters of Injection Molding Machine

The basic parameters of injection molding machine, as basis for the design, manufacture, purchase as well as use of the machine, include such three aspects as injection, mold clamping and comprehensive performance.

1. Theoretical Quantity of Injection The quantity of injection reflects to a certain extent the processing ability of the injection molding

machine and indicates the maximum weight of the plastic parts that can be produced. It is therefore usually used as a parameter representing the specification of the injection molding machine. Two rendering methods are generally available thereof: the theoretical injection volume and the injection

weight. l) Theoretical Injection Volume When the injection molding machine injects to the air, the volume of fused materials injected during

the maximum injection stroke made by the screw or plunger is represented by cm3. The formula for the theoretical injection volume VL is:

SDVL2

4π

=

Wherein: VL ―Theoretical injection volume, cm3; D ― Diameter of screw (or plunger), cm; S ― Maximum injection stroke of screw (or plunger), cm.

During the injection, on the one hand, density of the fused materials will change with temperature and pressure; on the other hand, small amount of refluence will occur under the pressure, and during pressure maintenance, the materials must be supplemented due to the contraction resulted from cooling; therefore, the actual quantity of injection is less than the theoretical quantity of injection, which needs to be corrected with injection coefficient.

LS VV α=

Wherein: VS ― Actual quantity of injection, cm3; α ― Injection coefficient.

The injection coefficient is related with many factors, such as structure and parameters of screw, injection pressure and speed, back pressure, structure of mold, shape of molded parts as well as nature of plastics etc. It is usually selected between 0.7-0.9.

2) Theoretical Weight of Injection When the injection molding machine injects to the air, the maximum weight of polystyrene (PS)

materials injected during the maximum injection stroke made by the screw or plunger is represented with gram (g). If the materials of the plastic parts differentiate from PS, conversion for the quantity of injection should be made per the following formula:

05.1ρ

×= MW

Wherein: ρ ―Density of the plastics, g / cm3; M ―Quantity of Injection rendered by PS plastics, g.

For example, the density of PP is 0.909/cm3, the standard parameter for the quantity of injection of an injection molding machine is 288g, and hence, the quantity of injection during the production of PP

parts with this machine is g8.24605.1/9.0288 =× .

According to the actual production experience, total weight of plastic parts (total sum composed by the weight of parts and that of gate system) should be preferably controlled within 85％ of the quantity of injection. For noncrystalline plastics, large value can be taken, whereas for plastics with high viscosity, smaller value should be taken.

2. Injection Pressure During the injection of fused materials, the screw (or plunger) must apply pressure large enough

onto the materials to overcome the resistance when the materials flow through the nozzle, the spruing channel and the mold cavity. The pressure thereby applied is called the injection pressure. The formula whereof is as follows:

020

2

020

)(

4

4 PDD

D

PDP == π

π

Wherein: P ―Injection pressure, MPa ; D0 ―Inner diameter of injection oil tank, cm ; D ―Outer diameter of screw (or plunger), cm ; P0 ―Oil way pressure during the operation of injection molding machine, MPa.

The selection of injection pressure during molding is very important. If the injection pressure is too high, such problems to the molded parts as flash, ejection difficulty, influence on the appearance quality of parts as well as large internal stress may occur; the molded parts may even become spoiled products and meanwhile, service life of the machine can be reduced. If the injection pressure is too low, the fused materials may not fill the mold cavity and molding can even be spoiled. Therefore, reasonably selecting injection pressure is an important condition for ensuring precise dimension of molded parts. Generally, the injection pressure of the selected injection molding machine should be higher than that required for the molding of parts; meanwhile, such factors as the viscosity, shape of parts, plastication, mold temperature and precision of parts should be taken into account during the selection of injection pressure. For example, when processing products with sound fluidity of the fused materials, simple structure of the plastic parts, and thick walls, the injection pressure should be generally lower than 70MPa; for products wherein the fused materials are of low viscosity and the requirement for shapes and precision is not high, the injection pressure is generally 70～100MPa; for those wherein the fused materials are of medium viscosity and which require certain precision, the injection pressure should be 100～140MPa; for those which require high viscosity of fused materials, high precision, thin walls of the plastic parts and large dimension, the injection pressure is approximately 140-180MPa; for the injection and molding of precise plastic products with various shapes and high precision, the injection pressure can reach 230～250MPa.

3. Injection Rate The injection rate refers to the amount of fused materials injected from the nozzle in unit time. It

can be rendered with the following formula:

zz

sz vDt

Vq 24π

==

Wherein: qz ―Injection rate, cm3; Vs ―Actual capacity of injection, cm3; tz ―Injection time (the time required by one injection made by the screw or plunger), s; D ―Inner diameter of charging barrel, cm; vz ―Injection velocity, cm / s.

The injection velocity vz refers to the distance moved by the screw or plunger in unit time. The formula whereof is:

zz t

Sv =

Wherein: S ―Maximum injection stroke of the screw (or plunger), cm ; tz ―Injection time, s.

The injection rate can directly influence the quality and productivity of products. Since low injection rate means slow injection velocity and long injection time, it will be difficult for the fused materials to fill the mold cavity and such quality problems as cold joint, uneven density and large internal stress etc shall be found in the products; should the rate be too high and the velocity too fast, plenty of friction heat shall be produced when the materials flow through the nozzle, resulting in the scorch, color change or decomposition of materials; meanwhile, during high-speed injection, the gas in the mold may not be discharged in time and is mixed in the materials, which can influence the appearance quality of products and silver lines as well as air bubbles will occur. The injection rate, therefore, should be selected according to the performance of plastics, the shapes of products, the technical conditions as well as the features of molds. The relations between quantity of injection, injection rate and injection time refer to Table 6-4.

Table 6-4: the relations between quantity of injection, injection rate and injection time Quantity of injection / cm3 125 250 500 1 000 2 000 4 000 6 000 10 000

Injection rate /（cm3/ s）

125 200 333 570 890 1 330 1 600 2 000

Injection time /s 1.00 1.25 1.50 1.75 2.25 3.00 3.75 5.00

The current injection technics not only requires high injection rate but also hierarchical injection

during the process. Specifically, to realize ideal production effect, the flow state during materials’ filling in the mold should be effectively controlled according to the features of plastic materials and the processed products.

4. Plastication Ability The plastication ability refers to the amount of materials (usually based on standard polystyrene)

plasticated by the plastication device in unit time. The plastication device of injection molding machine should be able to provide sufficient fused materials which are uniformly plasticated within required time limit. The device should fit the molding cycle of the injection molding machine. If the plastication ability is high, whereas the time of empty cycle is too long, the ability shall not be played in full. Vice versa, the molding cycle shall be prolonged. Hence, the plastication ability can influence productivity. The plastication ability can be improved through raising the rotation speed of screw, increasing driving power as well as improving the structure of screw.

5. Clamping force Clamping force is one of the most commonly-used parameters for injection molding machines. It

refers to the final clamping force applied by the mold plate to the mold after the clamping unit has locked the mold and when the fused materials are being injected into the cavity. The clamping force to a great extent reflects the ability of the injection molding machine in processing products. Thus, most manufacturers take it as a parameter indicating the specification of injection molding machine.

When fused materials flow into the empty cavity with certain injection pressure and velocity, the clamping force should satisfy the following formula to avoid expansion of the molds by fused materials:

PAF α≥ Wherein: F ― Clamping force, t ;

α ―Safety coefficient, generally taking 1.1-1.6 ; P ―Cavity pressure, MPa ; A ―Projection area of the molded parts on the parting line, cm2。

Cavity pressure refers to the average pressure in the mold cavity, generally taking 20-40MPa, and

specifically should be determined according to such factors as the nature of plastics, requirements of the molded parts as well as the L/T rate of the parts. The distribution diagram of clamping force, injection pressure, projection area of molded parts and cavity pressure see Fig.6-9. Insufficient clamping force shall result in overflow. Therefore, when selecting machines, please ensure that the locking force needed for the processing of products be smaller than the mold clamping force of the machine. Table 6-5 lists the average cavity pressure often selected for processing different molded parts and plastics.

Fig.6-9: distribution diagram of cavity pressure and projection area of molding parts

Table 6-5: the average cavity pressure in common use

The part requirement & material properties The average cavity pressure /MPa Example

Easily molding parts 25 Even-walled commodities and containers made of PE, PP, PS, etc.

Common molding parts 30 Thin-walled containers

Materials have high viscosity and parts have high precision. 35

Mechanical parts and high precision molding parts made from ABS,PC,POM etc.

Materials have very high viscosity and parts have high precision and are hard to fill in. 40 Mechanical parts of high precision

6. Basic Dimension of Mold Clamping Device The basic dimension of mold clamping device includes dimension of mold plate, rod space,

maximum open daylight of molds, stroke of moving plate, maximum and minimum thickness of molds, ejection force as well as ejection stroke etc. These parameters are related with the dimension, assembly and positioning of the molds used.

1) Dimension of Mold Plate and Space between Rods The mold plate is used to retain molds for forming plastics, wherein the dimension of mold plate

is )()( mmVmmH × , and the rod space is )()( 00 mmVmmH × . Fig.6-10 shows the external dimension

of Yanhing SP108A injection molding machine. The dimension of mold plate circumscribes the maximum molding area of the machine, whereas the rod space circumscribes the dimension of mold. Therefore, during the design of plastic molds, the external dimension of molds should match with the dimension of mold plate.

Fig.6-10: mold plate dimensions of Yanhing SP108A

2) Maximum Space between Mold Plates The maximum space between mold plates refers to the maximum distance (including mold

adjusting stroke) between the moving plate and the fixed plate after mold opening, as indicated in Fig. 6-11. The maximum space between mold plates circumscribes the maximum height of molded parts that can be processed by the injection molding machine. To ensure smooth ejection of molded parts, the maximum space between mold plates L is usually 3-4 times of the maximum height of molded parts hmax.

Fig. 6-11: dimensions between mold plates

1.movable plate；2. fixed mold；3.molding part；4.movable plate and moving half mold

L = S + hmax

Wherein: L ―Maximum space between mold plates, mm; S ―Mold opening stroke, mm; hmax ―Maximum thickness of molds, mm.

The maximum space between mold plates of Yanhing SP108A injection molding machine is

595mm. 3) Mold Opening Stroke The mold opening stroke refers to the maximum distance that can be moved by the mold plates.

Stroke of the movable mold plate is limited and is generally related with the thickness of molds. To ensure smooth ejection of the molded parts, stroke of the moving plate should be twice larger than the maximum height of the parts. In actual production, the stroke of mold plates should be adjusted as short as possible so as to shorten each cycling period of molded parts, to improve productivity and to reduce abrasion of the machine as well as power consumption.

The mold opening stroke of Yanhing SP108A injection molding machine is 290mm. 4) Maximum and Minimum Thickness of Molds The maximum thickness hmax and minimum thickness hmin of molds refer to the maximum and

minimum distance between the moving plate and the fixed plate when the former has closed and the required clamping force has been reached. The maximum and minimum thickness of injection molding machine are also referred to as mold thickness, representing the thickness of molds that can be held by the injection molding machine. If the mold thickness is smaller than the minimum thickness hmin, the thickness adjusting block should be added during the assembly, otherwise, the normal clamping force cannot be reached and the parts can even be damaged. If the mold thickness is larger than the maximum thickness hmax, the molds after assembly may not be clamped normally, the required clamping force cannot be realized, and thereby the injection molding machine will not work. The clearance between the maximum and the minimum thickness is the maximum adjustable stroke of the mold adjusting unit.

The maximum and minimum thickness of Yanhing SP108A injection molding machine are 305mm and 102mm respectively, and hence the maximum adjustable stroke is 203mm.

5) Ejection Force and Ejection Stroke Only effective ejection force and ejection stroke can separate the molded products from the molds

successfully. The ejection stroke should be reasonably selected according to the external shape of products and the structure of molds. Generally the maximum ejection stroke of machines is fixed and users can adjust in accordance with specific products. When ordering machines, users should select those with large ejection stroke to suit for the molding of more products.

The ejection stroke of Yanhing SP108A injection molding machine is 74mm.

6.2 Double-Color Injection Molding Machine

Double-color (or multi-color) injection and molding refers to the molding method that applies injection molding machine with two or more injection units to inject plastics of different colors or types into the molds simultaneously or successively. Double-color plastic products and double-color flower-pattern plastic products can be manufactured with special double-color (or multi-color) injection molding machine.

There are various types of double-color (or multi-color) injection molding machine, which mainly differentiates common injection molding machines in the design of injection unit and the movable mold plates. The mostly commonly-used injection molding machines at present are double-clear-color and blended-color types.

1. Double-clear-color Injection Molding Machine The double-clear-color injection molding machine consists of two injection units, one universal

mold clamping unit and double-color molds, which can manufacture plastic products with clearly-marked color in the outer and inner layer. The technical process is: The first injection unit begins to inject, cool, mold and open mold, still affixes the molding parts on the side of moving mold and moves it to the position for second molding. The fixed mold for second molding is provided with space for first molding as well as the newly-added space; after mold clamping, inject fused materials of the second color and take out the products after cooling and molding. The two injection units respectively plasticate materials of different colors; the two cores are identical in shape and size, whereas dimensions of the cavities are different, to respectively mold the bottom and outer shape of products which is ultimately got through the conversion of mold revolving plate during production.

2. Blended-color Injection Molding Machine Injection unit of the blended-color injection molding machine consists of two injection charging

barrels and a universal nozzle, wherein materials of two different colors can be plasticated in the charging barrels. The two barrels are provided with two independent running channels and are controlled by the single-direction valve. In this way, the sequence for the fused materials in the two injection units to enter the molds and the proportion of injected plastics can be adjusted by hydraulic system, whereby double-color plastic products with different blended color arrangements as well as blended-color products with natural transitional colors can be produced. The blending proportion of the two types of injection fused materials is controlled by the stroke, speed as well as time difference of injection. As indicated in Fig.6-12 is a screw blended-color injection molding machine.

1.Nozzle 2.Heater 3. Single-direction valve of barrel B running channel 4.Barrel B

5. Screw B 6. Screw A 7. Barrel A 8. Single-direction valve of barrel A running channel

Fig.6-12: screw blended-color injection molding machine

Technical Features of Double-color Injection and Molding: 1) The double-color injection molding machine consists of two pre-plastication and injection

systems, wherein slight fluctuation of temperature, pressure and quantity of the injected fused materials will influence the color and pattern of products. To ensure uniform appearance of the same batch of products, the technical parameters such as temperature, pressure as well as quantity of injection of the two systems must be under strict control and be kept invariable during production.

2) The blended-color injection molding machine is long and complicated in structure and has turning corners therein; above all, pressure losses of the fused materials are immense, which requires large injection pressure to ensure smooth filling in the molds.

3) To ensure sound fluidity of the fused materials as well as smooth filling in the molds, temperature of the materials should be properly set higher.

4) The high temperature of fused materials and their long stop time in the running channel may result in thermal decomposition. Therefore, raw materials for double-color injection and molding should be taken from thermal-plastic materials steady in thermal stability and low in viscosity.

5) During double-color injection, fused materials of two different colors can be either taken from the same plastics or from different ones. When the latter alternative is adopted, plastics with sound viscosity and compatibility under fusing temperature.

6) During double-color injection, fused materials of two different colors are blended in the molds. Therefore, the welding mark and internal stress of double-color products should be taken into full account during the formulation of technical parameters. High material and mold temperature as well as high injection rate are beneficial for reducing welding mark and internal stress.

6.3 Temperature Control Machine for Rapidly Heating and

Cooling Injection Molding

According to a lot of experiments, necessary viscosity must be ensured in the thermoplastic fused materials during injection and molding in order to avoid welding mark and to ensure sound fusion of the materials. Such measures as adopting proper material temperature, improving surface temperature of mold cavity, shortening the flow length of fused materials prior to their reaching welding mark as well as filling the molds rapidly etc are usually taken, whereby the pressure loss of the fused materials in the molds can be reduced to a certain extent. Although the influence of injection pressure on the intensity of welding mark is not that great, properly high pressure must be ensured.

For many years the mold temperature control machine has been extensively applied as the application range of molded products has been enlarging, and meanwhile, distorted products which fall short of the basic requirements of design have been increasing, combined with the higher and higher requirements for the physical performance, appearance as well as precise dimension of materials. Hence, these factors must be considered during the design of mold temperature control machines which not only aim at reliably improving mold temperature, but more importantly, keeping the mold temperature within a certain range despite the influence from outer air temperature.

To manufacture high-quality products, apart from applying the shape follow-up medium channel mentioned in previous chapters, the key is to improve surface temperature of mold cavity, which can be simply achieved by using common mold temperature control machine or heater. The heater is not ideal in

all other aspects except for partial heating, which is problematic both in efficiency and stability due to the underlying differences between heaters and mold temperature control machines. The most obvious shortcoming of heaters is that they can only heat yet cannot absorb heat. When the mold temperature rises due to continuous operation and exceeds the required temperature, the heaters will not be able to reduce the temperature. In contrast, mold temperature control machines can both heat and disperse heat if it uses water or oil as medium. Moreover, uniform heating and cooling can be achieved with a mold temperature control machine and partial heating like the heaters do can be avoided.

Although high-quality products can also be manufactured with ordinary mold temperature control machines, yet the injection productivity is relatively lower due to the high mold temperature and long cooling time, especially for products of large dimension. Such temperature control machines for rapid thermal and cold injection of molds gradually applied in mold manufacturing circle in recent years will effectively solve the problem.

The temperature control machines for rapidly heating and cooling injection of molds can rapidly improve the surface temperature of mold cavity to above thermal molding temperature of rosin through high-temperature medium (such as steam, water or heat conductive oil), whereby the solidifying layer attached to the mold walls due to the rapid reduction of temperature on running channel or surface of molds can be avoided. Instead, rapid injection and filling can be completed, molding without cooling can be achieved and plastic parts with super-luminous appearance as well as high tenacity and intensity can be obtained. Above all, such deficiencies as distortion, sunken surface, welding mark and ripples can be overcome. Upon the completion of molding, mold temperature can be promptly reduced by switching the valves and bringing in the cooling medium, which greatly shortens injection cycle.

Fig.6-13 shows a temperature control machine for rapid thermal and cold injection of molds with

water as medium, wherein the medium temperature can be heated to C°160 , and the pressure is

2/6 cmkg .

Fig.6-13: internal structure of J11-W-160 temperature control machine for rapidly heating and cooling

injection molding as developed by Shenzhen University Fig.6-14 shows the working principle of this equipment which mainly comprises internal cycle of

thermal medium, external cycle of thermal medium, internal cycle of cold medium, cooling pipeline for thermal medium as well as several safety pipelines. 1. Internal Cycle of Thermal Medium. When the molds are controlled by the external cycle of cold medium, the heating pneumatic electromagnetic valves HV1 and HV2 as well as the electromagnetic valve SFV for cooling internal cycle are closed, whereas the cooling electromagnetic valves FV1 and FV2 as well as the electromagnetic valve SHV for heating internal cycle are opened. An internal cycling loop for thermal medium is thereby formed along the thermal cycling pump, heating resistance wire, the electromagnetic valve SHV for heating internal cycle as well as the pressurizing cylinder, which repeatedly heat the water in the pressuring cylinder to maintain the pre-set temperature. When the temperature exceeds pre-set value, the heating resistance wire automatically closes. 2. External Cycle of Thermal Medium. When the molds need to be heated, the cooling electromagnetic valves FV1 and FV2 and the electromagnetic valve SHV for heating internal cycle are closed, whereas the electromagnetic valve SFV for cooling internal cycle as well as the heating pneumatic electromagnetic HV1 and HV2 are opened. An external cycling loop for thermal medium is thereby formed along the thermal cycling pump, heating resistance wire, heating pneumatic electromagnetic valve HV1, molds, single-direction valve 2, heating pneumatic electromagnetic valve HV2 as well as the pressurizing cylinder, which rapidly improves the surface temperature of molds. 3. Internal Cycle of Cold Medium. When the molds are controlled by the external cycle of thermal medium, the cooling electromagnetic valves FV1 and FV2 and the electromagnetic valve SHV for heating internal cycle are closed, whereas the heating pneumatic electromagnetic valves HV1 and HV2 as well as the electromagnetic valve SFV for cooling internal cycle are opened. An internal cycling loop for cold medium is thereby formed along the pressurizing pump, the electromagnetic valve SFV for cooling internal cycle as well as the water tower. 4. External Cycle for Cold Medium. When the molds need to be cooled, heating pneumatic valves HV1 and HV2 and the electromagnetic valve SFV for cooling internal cycle are closed, whereas the electromagnetic valve SHV for heating internal cycle as well as the cooling electromagnetic valves FV1 and FV2 are opened. An external cycling loop for cold medium is thereby formed along the pressurizing pump, single-direction valve 3, cooling electromagnetic valve FV1, molds, cooling electromagnetic valve FV2 as well as the water tower, which rapidly reduces the surface temperature of molds. 5. Cooling Pipeline for Thermal Medium. When temperature in the pressurizing pump is too high, the cooling electromagnetic valve CV is opened and a loop is formed along the water tower, the cooling electromagnetic valve CV as well as the cooler, inducting cooling water in the water tower into the cooler. Note that there is a flow of thermal internal and external cycle running through the cooler; hence temperature of the thermal medium can be reduced by employing the low temperature of cooling water running through the heat conductive copper sheet between two pipelines in the cooler. 6. Safety Pipelines

1) Exhaust Pipelines. When the hydropneumatic pressure in the pressurizing cylinder is too high, the exhaust electromagnetic valve PV opens and exhausts to the water tower along the pressurizing cylinder, the Y-type filter, the screw pipe and the exhaust electromagnetic valve PV.

2) Pressurizing Pump and Manual Pressure Adjusting Valve. 3) Electromagnetic Valve for Pressure Reduction. 4) Safety Electromagnetic Valve.

Fig.6-14: schematic diagram for J11-W-160 temperature control machine for rapidly heating and cooling

injection molding as developed by Shenzhen University

6.4 Mold Maintenance

6.4.1 Storage and Care of Injection Molds

Injection molds have a limited service life as shown in Table 6-6. Appropriate measure can greatly extend this, however. Such measures can be classified on the basis of maintenance, storage, and care.

Table 6-6: number of molded parts obtainable with various mold materials Material Attainable number Description

Zinc alloys 100,100 Casting Aluminum 100,100 Casting Aluminum 100,100-200,000 Rolled Copper-beryllium 250,000-500,000 Surface hardened Steel 500,000-1,000,000

To be able to quickly fall back on ready-to-use molds, the following demands on storage and care must be fulfilled.

1) Every mold must be stored along with one molded part and a mold card in its own, easily accessible space in the mold store.

2) Only ready-to-use, complete, clean molds may be stored. The purpose of also storing a molded part (usually the last one from previous production) and a mold card bearing the article number and the mold number is to allow the mold to be uniquely identified.

The mold card should also bear all the information needed for setting up the mold and starting up

the injection molding machine. Information in this category includes the following. 1) Mold design. 2) Dimension of the mold and the molded part. 3) Mold mounting equipment. 4) Injection molding machine suitable for production. 5) Shot weight (injection volume). 6) Suitable plastic. 7) Rules on material pretreatment. 8) Mold temperature and heat-control medium (water, oil, etc). 9) Cycle times. 10) Injection pressure, follow-up pressure, and dynamic pressure. 11) Injection speed. 12) Screw speed. 13) Cylinder equipment. 14) Maintenance intervals. 15) Number of pieces produced. This list could be extended and thus matched to the special needs of a factory. Instead of on a mold

card, much of this information, such as the settings for the injection molding machine, could be stored on external data storage media that could be read into the computer prior to production startup.

Mold changes can only be performed quickly if the molds are ready for use when they leave the stores and can go into production without the need for major assembly or cleaning work. Every mold must therefore be a self-contained unit, i.e., it must not be made of parts that are required for other molds. Parts or groups of parts that are loaded or borrowed often disappear or are needed elsewhere just when the mold is scheduled for use. The consequences are unnecessary, incalculable, and often time-consuming downtimes.

Cleaning work also delays the start of production. It should therefore be kept to a minimum. This means that special care has to be taken of the molds and imposes specific demands on the store, its cleanness and particularly the ambient conditions. Damp and unheated rooms promote corrosion. Once rust has begun to attack the mold, maintenance becomes very time consuming and very expensive. Often it is impossible. The mold store should therefore be kept at a constant temperature where possible, and dehumidified. Not much equipment is required for this, and it soon pays for itself.

Important to the accessibility of the molds is also the size of the store. It is essentially determined by the vehicles available in the factory (e.g., forklift) and the maneuvering space.

When a job is complete, the mold may only be returned to storage when its suitability for future use has been checked. The last parts produced with it can provide an indication of its condition. They must be examined for dimensional stability and closely scrutinized. This will provide information about the state of the mold surface, the level of seal in the mold parting line (perhaps flash formation on the molded part) and the working order of the ejectors, etc. If no deficiencies are found, the maintenance work then takes the form of the general care measures described below. 1. Maintenance of Heating and Cooling Channel

Heating and cooling channels must be cleaned thoroughly to eliminate scale, rust, and sludge. Since these deposits decrease the channels, measuring the flow rate is a way of checking the system. A pressure-controlled valve is installed between mold and channels and a defined pressure drop is set,

which has to be same for each examination. If the flow rate was measured with the new mold, a comparison with any new measurement after a production run provides information about the degree of clogging of the channels.

For cleaning, heating and cooling channels are usually flushed with a detergent because mechanical removal of the deposit is generally not feasible due to the geometry of the system. Detergents and special cleaning equipment are marketed by several producers.

The nipples, bridges, bolts and feed tubes outside the mold are also checked for damage and replaced where necessary, provided they stay on the mold. Before the mold is stored, water has to be removed with compressed air and the system dried with hot air. 2. Care and Maintenance of the mold surfaces

After the end of production, the mold must be carefully cleaned of any adhering plastic residue. The work is independent of the type and amount of molding material. It is advisable to use soap and water for removing material remnants and other deposits. The mold then has to be dried carefully. Rust spots from condensed water or aggressive plastics have also to be removed before storage. Depending on the degree of chemical attack, abrasives for grinding and polishing may be suitable. Removal of residual lubricants from movable mold components is also part of the cleaning operation. 3. Care and Maintenance of the Heating and Control System

This work is particularly important for hot-runner molds. After each production run, heater cartridges, heater bands, and thermocouples should be checked with an ohmmeter and the results compared with those on the mold card. A check should also be made to ensure the lines, connections, insulation, and main lead cleats are in proper working order. 4. Care and Maintenance of Sliding Guides The guides on movable mold parts require particularly careful cleaning and must be washed with resin-free and acid-free lubricants. Also check the level of seal in the cylinder in the case of hydraulically actuated slides and cores. 5. Care and Maintenance of the Gate System Start checking at the nozzle contact area, which is subjected to very high loads during operation. Check also any special nozzles belonging to the mold. In the case of temperature-controlled gates that are not generally demolded with every shot, it is necessary, to an extent depending on the plastic processed, to flush the gating system until the end of production with a plastic that has wide processing latitude. 6. Care Prior to Storage

At the end of each maintenance work, the mold has to be carefully dried and lightly greased with non-corrosive grease. This is especially important for movable parts such as ejector assembly, slides and lifters, etc. For extended storage, the mold should be wrapped in oil paper. Greasing and wrapping of the mold in oil paper is crucial when the mold store does not satisfy the demands above and below. All observations and maintenance work are recorded on the mold card.

6.4.2 Repairs and Alterations of Injection Molds

Injection molds can be subjected to extreme conditions during operation. This gives rise to wear symptoms that are due to rolling, sliding, thrusting, and flowing movements. The consequences of wear are dimensional inaccuracy, flawed surfaces and flash on the molded part. Before the damage can be repaired, the cause must be determined. The following are possible:

1) Simple mechanical finishing. 2) Replacement of parts. 3) Deposition of material. Leaky parting planes are typical injection molding damage. When this is not very extensive, it can

be eliminated by grinding. However, this is limited by the tolerances imposed on the molded-part dimensions. Minor damage to the mold surface that can be attributed to impact can be remedied by reboring, remilling, and then setting pins or wedges. Once the flaw has been treated, the mold is heated and the drill hole or groove closed with a cold insert (slight overdimensioned). The repaired spot is then rendered flush with the mold surface by grinding or polishing.

It is important to use the same type of material for this repair work, as the repaired area should have the same material properties as the rest of the mold surface. Damage to functional and mounting parts, such as guide pins and bushings, ejectors, locating flanges, nozzles, etc., should not be repaied. These are normally standard parts available in various dimensions and can thus be replaced cheaply. Doing this means that the molds will function perfectly and avoid any major risks.

Welding is often necessary for a mold repairs. Repair welds to injection molds should always be preceded by heating to keep thermal stress and the formation of internal stress as low as possible. Preheating avoids compression and shrinkage in the weld aone and, above all, preents heat from being dissipated so quickly from the weld area that hardening sets in (as when heated parts are quenchd in oil or water). The preheating temperature (at which the workpiece must be kept during welding) depends on the material to be welded, and in particular on its chemical composition, Steel manufactures provide details of this.

During welding, the workpiece must be kept at the preheating temperature, When welding is complete, it is cooled to between 80 and 100°C and then reheated again to the normalizing temperature. Welding repairs are performed by the TIG method and welding with coated electrode wires. TIG (tungsten inert gas) offers distinct advantage. The following basic rules must be observed for repair welding:

1) The electrode wire material should be of the same composition as the mold material, or at least similar. Ensuing heat treatment of the weld results in equal hardness and structure.

2) The amperage has to be kept as low as possible to prevent reduced hardness and coarse structure.

3) The preheating temperature must be above the martensite-forming temperature. It should not be considerably higher, however, since it increases the depth of burn-in.

4) During the entire welding process, the mold must be kept at the preheating temperature. This is particularly the case for several deposits.

5) At edges, the molten material needs to be supported. This can be effected with copper pieces or copper guide shoes that can be water-cooled if necessary.

Lasers have been used for repair welding of molds. Mostly these are pulsed solid-state lasers, e.g.,

ND-YAG lasers, with laser capacities of 50~200 Watt for hand welding. The great advantage of laser welding over conventional welding is that low amounts of energy are applied with extreme precision to the welding site. Due to the very short welding impulses (1~15 milliseconds max.), the heated zone is very small, in the order of a few hundred millimeters. Thermal stress on the mold is therefore slight. Laser welding is more or less distortion-free.

The electrode wire material is generally

Turn on the chief switch of power supply and that of the injection molding machine. If, upon turning on the machine, the alarm light is on, accompanied with buzzing sound, please switch on the red emergency stop button on the platform of the machine and then turn on the on/off button of motor. The operation panel of SP108A injection molding machine as indicated in Fig.6-15, the functions of some buttons in the figure are as follows:

1) Manual Button: This button is of multi-function, which not only can restore the automatic state into manual state, but also can clear the alarm and abnormal function. The button is actually a reposition button.

2) Semi-automatic Button: Press this button and the machine shall be in automatic cycling. At the beginning of each cycle, the safety door must be switched on and off once so that the next cycle can continue.

3) Electronic Eye Automatic Button: Press this button and the machine shall be in automatic cycling. Check whether the product has fallen off within 4 seconds after each cycle; if no falling off is detected, it indicates that the product still remains in the mold. If the machine stops and alarms, “Ejection fails” will appear on the screen.

4) Time-auto Button: Press the button and the machine shall be in fully automatic cycling. Unless there is an alarm, the subsequent cycle shall follow immediately after the previous one (Electronic eye is void under current status). For automatic operation entered into from manual state by pressing the automatic button, the previous safety door must be switched on and off once to ensure there are no unnecessary parts in the mold before the mold can be clamped.

5) Injection Button: Under the manual state, if the heater switch is “ON”, temperature of the materials has reached the set value and the pre-warming time is up; press this button and the injection shall start.

6) Screw Retraction Button: In the manual operation mode, if the heater switch is “ON”, temperature of the materials has reached the set value and the pre-warming time is up; press this button and the injection shall start. If the screw is located ahead of the position where the screw retraction stops,

Manual button

Semi-auto button

Mold opening button Mold clamping button

Moving platform backward button

Moving platform forward button

Screw retraction button

Motor button Numerical button locking switch

Emergency switch Heater button

Electric eye automatic button

Time-auto button

Automatic purge button

Mold forward button

Mold backward button

Fig.6-15: operation panel of SP108A injection molding machine

Language button

Injection button Charge button

Cursor control buttons Enter Button Cancel button

Manual mold thickness adjustment On/Off button

press this button to give up screw retraction. 7) Automatic Purge Button: To clear the remnant materials in the charging barrel, press this button

to begin automatic purge in accordance with set purge times and time of glue-smelting. 8) Manual Mold Thickness Adjustment On/Off Button a) The first pressing is manual mold adjusting. To make it convenient and safe for installation the

molds, the pressure and speed for mold opening, clamping, injection, glue-smelting, screw retraction as well as the moving of moving platform all use internally-set low pressure and slow speed, which will not change during moving. Yet mold opening, clamping, glue-smelting and screw retraction will stop in set positions, therefore, manual mold adjusting should be employed during the installation the molds.

b) The second pressing is automatic mold adjusting. After the operator has assembled the molds and has set such parameters as the pressure, speed and position required for the opening and clamping of molds, automatic mold adjustment can be made, which shall automatically adjust thickness of molds according to the set clamping high pressure after the safety door is closed, until the set pressure conforms with the actual clamping force. The automatic mold adjusting is thus completed upon an alarm sound.

2. Parameters Settings 1) Thimble Working Mode Setting

Fig. 6-16: operation panel of thimble working mode

As indicated in Fig.6-16, three alternatives are available for the thimble working mode: 0: No return thimble. It can only be operated under semi-automatic injection mode, wherein the

thimble stops immediately after ejection. When the plastic parts have been taken out, the thimble returns only after the delayed timing has completed or the safety door has been closed.

1: Set times. Repeat the ejection as per the set times for ejection (i.e. thimble times). 2: Shaking. Shaking thimble mode, i.e. the ejection will be repeated around the place where it has

terminated according to the set times.

2) Mold Opening/Clamping Parameters Setting (For this set of J08-QMB molds, the operator is only required to set according to the parameters as indicated in the Fig.6-17.)

As indicated in the Fig.6-17, the left upper corner shows current position of the moving plate of injection molding machine.

To ensure steady and highly-efficient mold clamping, the process can fall into the following four sections:

a) Slowly moving forward and low pressure. b) Quickly moving forward and high pressure. c) Slow mold clamping and low pressure. d) Slow mold clamping and high pressure mold locking. Similarly, the mold opening process also falls into four sections: a) Slowly moving backward and high pressure mold opening. b) Quickly moving backward and high pressure. c) Quickly moving backward and low pressure. d) Slowly moving backward and low pressure stop.

Fig.6-17: mold opening/clamping parameters setting

3) Injection Parameters Setting The conversion to pressure maintaining mode after injection falls into two types: a) Converting to pressure maintaining mode when injection time is up. b) Converting to pressure maintaining mode at the injection position. When injection reaches the

termination place (conversion position of pressure maintaining) of section 4, it shall convert into pressure maintaining mode. If the place cannot be reached, automatic conversion to pressure maintaining mode shall be made when the pre-set upper-limit time is up. Therefore, the time can be usually set as larger than normal injection time, as indicated in Fig.6-18.

Fig.6-18: injection parameters settings

4) Charge/Suck Back/Cooling Parameters Settings Fig.6-19 shows the setting for Charge/Suck Back/Cooling Parameters Inverse S.BK mode: If it is set as 1, the screw retraction should not be carried out until cooling has

completed; if it is set as 0, normal standard actions shall be done after material storage. Charge: Controlled by two-section pressure and speed. Screw Retraction: Back pressure and speed needed for screw retraction as well as the distance

required for screw retraction since charge has terminated. Automatic Purge: Operator under manual state can set the times for materials purge and the time for

each purge and charge if he wants to clear the remnant materials in the tubes. The operation can be done by pressing the automatic purge button (wherein the accounting and timing for automatic clearance should not be set under 0).

Cooling time: The products’ cooling time parameter setting starts from charge.

Fig.6-19: charge/suck back/cooling parameters settings

5) Temperature Parameters Settings

When the injection molding machine begins to work, please do check whether there are materials in the charging barrel and meanwhile turn on the heater switch button to pre-warm for approximately one hour until the set temperature almost equals the actual temperature (usually3~5 ) when the inje℃ ction can be started. Relevant data of this instance are indicated in Fig.6-20, wherein section #1, #2 and #3 represents material feeding section, plastication section and measuring section respectively.

Fig.6-20: temperature parameters settings

3. Mold Assembly Before assembling molds, please pay attention to the following: 1) Measure the dimension of molds to see whether they conform with that of the machine. 2) Check the mold positioning ring to see whether they conform with the positioning hole on the

fixed mold plate of the machine. 3) Check whether the inner spheric radius of sprue bush is larger than the spheric radius of the

injection nozzle of the machine; otherwise, leakage or disappearance of pressure may occur during injection.

4) Check the fitting between thimble and molds. If the dimension and length of the thimble are insufficient to eject the products, the thimble needs to be prolonged.

During assembly, lift the molds and slowly move back between the fixed and moving mold plate from top to bottom. The molds should be descended slowly to avoid damages to the chromeplated layer on the rod due to collision. The lifter moves rightwards to insert the mold positioning ring into the positioning hole on fixed mold plate. Secure the molds on the fixed plate with pressing board when eyeballing that the molds have been horizontally placed.

Start the oil pump and select mold clamping function by pressing the setting button. If the signal light is on, it indicates that it is under slow mold clamping state, the speed of which has been set by the operation panel. Gently press the mold clamping button and the reamer begins to expand, and the movable mold plate moves slowly forward until it joints the mold; if the reamer straightens and the moving plate still cannot joint the mold, manual mold adjustment must be made. Mold adjustment can only be made under the setting mode. Gently press the mold opening button and the moving plate shall be under opening state; gently press the forward button for mold opening and the moving plate will move forward, and stop it at proper position through eyeballing. Repeat the aforementioned slow mold clamping operation until the moving plate is closely fitted with the mold and then secure the mold on the

moving plate with pressing board; meanwhile, using two pressing boards to fix the bottom plate of the moving half of mold onto the moving plate, which altogether withstand the mold.

Next, untie the locking parts of suspending mold on the mold, gently press mold opening button and the moving plate will move backward. The mold is thereby opened and the assembly is completed.

4. Pre-glue-smelting Turn on the on/off button for motor, press the glue-smelting button to start glue-smelting. When the

amount of glue-smelting has reached the rated value, press glue injection button to begin pre-glue-injection operation. Ensure that the amount of glue-smelting reaches rated in Fig.6-21 value upon the completion of glue injection.

Fig.6-21: charge parameters settings and overview screen

5. Injection 1) Check whether the front and rear door of injection molding machine have been closed and whether the motor has started. 2) Press the manual button to convert the state of injection molding machine into manual operation. 3) Press the mold clamping button to close the mold.

Set amount of

glue-smelting

Rated amount of

glue-smelting

Equals

4) Press the forward button for moving platform to protrude injection gateof the platform into the major spruing channel of the mold. 5) Wait until the injection gate of moving platform has been protruded into the major spruing channel of the mold, then adjust the position adjustment button and slide it to the bottom of photoelectric limit switch to stimulate the signal. 6) Press semi-automatic button to convert the state of the machine into semi-automatic operation, and use the front safety door for operation to control the injection process of the machine.

Prior to mold opening of the injection molding machine, make sure that the semi-automatic state is not converted into manual state.

6. Stop Procedures 1) Convert the semi-automatic state of injection molding machine into manual state after the machine has opened molds and ejected plastic parts. 2) Daub the machine with anti-rusting oil. 3) Press the mold clamping button to close the mold, and make sure that 5mm space is left between the fixed and moving molds. 4) Press the backward button for moving platform and move the platform to a position far away from the molds. 5) Press the on/off button for motor to shut off the motor. 6) Press on/off button for electric heatin to stop heating the plastics. 7) Screw on the emergency stop button and shut off the general switch for power supply of the machine. 8) Shut off the general switch for power supply of the switchboard.