Alignment and Changing of Molds

For this purpose the mold is equipped with a flange-like locating ring, which keeps the sprue bushing in the mold Figure 13.1 and matches the corresponding opening in the machine platen,

1 3 1 F u n c t i o n o f A l i g n m e n t

Injection molds are mounted onto the platens of the clamping unit of the injection-molding machine The clamping unit opens and closes them during the course of the molding cycle The molds have to be guided in such a way that all inserts are accurately aligned and the mold halves are tightly closed Without proper insert alignment, molded parts would exhibit deviations in wall thickness; they would not have the required dimensions

Because guiding molds with the clamping unit alone is generally not sufficient, injection molds also need a so-called internal alignment It aligns both mold halves with the necessary precision and prevents their convolute joining

1 3 2 A l i g n m e n t w i t h t h e A x i s o f t h e P l a s t i c a t i n g U n i t

Precise alignment is mandatory here Otherwise the nozzle could not be sealed by the sprue bushing, and undercuts occurring at the sprue would interrupt operation Therefore, alignments concentric with the sprue bushing are used almost exclusively For this purpose the mold is equipped with a flange-like locating ring, which keeps the sprue bushing in the mold (Figure 13.1) and matches the corresponding opening in the machine platen, the diameter of which depends on the size of the machine platen [13.1]

Locating rings are readily available from producers of mold standards (see Chapter 17) and are machined from case-hardening steel or water-quenched unalloyed tool steel

The locating ring has a tapered inner bore of appropriated dimensions to allow the nozzle tip to pass through

A mold has usually only one locating ring If both mold halves are equipped with locating rings, then a loose fit is needed on the movable side to better align both sides This is only a help for setting up the mold

1 3 A l i g n m e n t a n d C h a n g i n g o f M o l d s

Figure 13.1 Locating ring Figure 13.2 Split locating ring [13.5]

Machine platen Locating ring Machine platen

Heat insulating sheet

Mold clamping plate

The locating ring is slightly press-fitted into the mold plate and fits the machine platen with a close sliding fit Figure 13.2 shows a locating ring combined with a thermal insulation plate This design is particularly useful for adding an insulating sheet to the mold This is done for processing thermosets or thermoplastics if a high mold temperature is needed for molding precision parts

1 3 3 I n t e r n a l A l i g n m e n t a n d I n t e r l o c k i n g

The mold halves themselves have to be guided internally, by the tie bars of the machine,

to obtain the needed accuracy In small molds this is done with leader pins; pins which protrude from one mold half of the opened mold and slide into precisely fitting bushings

in the other mold half during mold closing This ensures a constant and accurate alignment of both surfaces of flat molds without shifting during injection and the production of moldings In molds with deep cavities, especially those with long and slender cores, a shifting of the core can occur during injection in spite of exact alignment with leader pins This has already been discussed in Chapter 11, "Shifting of Cores" Design examples for such molds were presented with Figure 11.21

Figure 13.3 shows an example for positioning and mounting a leader pin and the appropriate bushing Four leader "units" (pin and bushing) are usually required for proper alignment To facilitate the assembly and to make sure that the mold is always correctly put together, one leader pin is offset or made in a different dimension [13.2-13.5] The latter method may cause fewer difficulties, especially when standard mold parts are used If two leader pins, one diagonally opposite the other, are made

Leader pin

Leader bushing

Locating sleeve

Figure 13.3 Leader-pin assembly

[13.5]

The length of leader bushings depends on their diameter It should be 1.5- to 3-times the inside diameter (Figure 13.6) The corresponding holes in the mold plate have to be drilled according to instructions Figures 13.7 and 13.8 demonstrate the assembly of leader pins and bushings [13.4]

Figure 13.7 shows the system for the design of simple molds and for leading slides

A jig drill is needed for proper alignment of the holes in the various plates

Figure 13.8 represents a system with shoulder leader pins Bores for pin and bushing can be drilled in a single operation (equal diameters)

The system of Figure 13.9 is not used very often It is carefree and ensures a precise and low-friction movement, but at additional expenses

With leader sleeves, mold plates can be aligned and connected with one another which would otherwise not be engaged by leader pin or bushing (Figure 13.10) The diameter

of the sleeve is kept the same as that of the bushing or the shoulder of a shoulder leader pin Therefore, all holes can be machined in one pass The internal diameter is adequately large to permit unrestricted entering of pins The removal of the locating sleeves can be readily effected through the tapped hole at the end by means of an extractor, e.g., leader pin in Figure 13.11

longer, it is easier to slide the two halves together while placing the mold into the machine or during assembly The leader pins are positioned as close to the edge as possible to gain space for the cavity and an adequate number of cooling lines

Effective alignment is possible only if close tolerances are kept between leader pins and corresponding holes This, however, causes considerable wear Therefore it is not prudent to let the pins slide directly into the respective holes of the mold plates As a matter of principle, leader bushings should be used to counteract wear and to enable worn-out parts to be exchanged easily Leader bushings, like leader pins, are made of case-hardened steel with a hardness of 60 to 62 Rc They are commercially available in various sizes and shapes Wear can furthermore be reduced by lubricating the pins with molybdenum disulfide For this purpose pins or bushings have oil grooves Leader pins without lubrication (Figure 13.7) should only be used for rather small molds or special applications in slides or with ball bushings (Figure 13.9) Low-maintenance operation is achieved through the use of leader bushings with solid lubricant depots [13.2-13.4] Leader pins and bushings are commercially available in various designs (Figure 13.5) Attention should be paid to the recommended fits (Figure 13.4) The length of leader pins depends on the depth of the cavity

Leading has to begin before the mold halves are engaged Therefore, a sufficient length must be selected Shoulder leader pins (Figure 13.5) can, at the same time, pin mold plates together Commercial availability is treated in Chapter 17

Figure 13.4 Tolerances for leader

Among the multitude of leader-pin systems, those shown in Figures 13.11 and 13.12 should also be mentioned Both are based on components already described: leader pin, bushing and sleeve The system of Figure 13.11 has pins and bushings with threaded holes and tightening screws, which are propped on the opposite side by head supports Plates not engaged by pin and bushing are lead by sleeves This design is more expensive, however, than the one shown in Figure 13.3 but has some decisive advantages The assembly of the individual plates is not done with screws, and additional drilling of holes is unnecessary The plates are kept together with the tightening screws

At the same time, the mold plate can be utilized better for accommodating cavity and cooling lines [13.3, 13.5]

The system in Figure 13.12 shows a very different design of leader bushings and sleeves and their assembly in the mold The bushings consist of three parts: the bushing proper, a retainer ring, and a ring nut There are two locations for the retainer ring The bushings can be mounted flush with the plate surface or protruding by 5 mm and

Figure 13.6 Shoulder bushings with

common tolerances

Figure 13.8 Shoulder leader pin

[13.4]

Figure 13.7 Leader pin without

oil grooves [13.4]

Vent

Figure 13.9 Leader pin assembly

with ball bushing [13.4]

Figure 13.10 Locating sleeve [13.4]

tightened with the nut In the protruding position any number of plates can be connected

to one another This kind of bushing can take over the job of bushings with or without retainer ring and can also be used as a leader sleeve In this case, no ring nut is used The nut connects the bushings reliably to the mold plates This could save additional plates, which are needed in other systems for supporting the bushings The height of the mold

is lower This may compensate the costs for the more elaborate design [13.32]

To ensure proper operation, no lateral forces should act upon the leader systems If there is no lateral load, the required cross-section of the leader pins does not have to be computed For oblique pins, especially those acting on slides, the necessary cross-section should be calculated The same equations can be used as presented in Section 12.9.2.1 for slide molds

Figure 13.12

Leader pin assembly with several straight bushings [13.6]

Figure 13.11 Leader pin

assembly [13.5]

a Leader pin with tapped hole

for retainer screw,

b Shoulder bushing with

retainer screw,

c Tubular dowel

1 3 4 A l i g n m e n t o f L a r g e M o l d s

Occasionally, no leader pins are used in large and deep molds such as those for buckets and boxes Guiding is left to the tie bars of the molding machine during opening and closing until short of complete engagement Since this accuracy is insufficient for proper alignment, special arrangements become necessary They are all characterized by the fact that the alignment does not begin sooner than shortly before the mold is closed Both mold halves brace one another when closed Of special advantage is the "pot" design (Figure 13.13) and its variations (Figures 13.14 and 13.15) because it also reacts

Figure 13.13 Interlock machined into solid

material [13.4]

Figure 13.14 Interlock with

attached gib [13.4]

Figure 13.15 Interlock with inserted gib

[13.4]

against forces from cavity expansion The inserted ledges in the variations are easily replaceable after they are worn out More variations are presented with Figure 13.16 Frequently bolts are used as aligning interlocks fitted into both mold halves (Figure 13.17) Their center line is not in the plane of the parting line Thus, both mold halves are braced against one another after clamping Figure 13.18 shows such a mold Instead of cylindrical alignment bolts, rectangular interlocks made of shock-resistant tool steel can be employed Alignment of molds by such interlocks calls for high accuracy of machining, because later corrections are not practical Frequently, tapered interlocks according to Figure 13.19 are finally used The long design of the locating bolts and bushings, in contrast to the round design, allows different thermal expansion

on the nozzle and clamping sides to be compensated

If precise alignment and precision locating of the mold halves are necessary, the use

of flat leaders with solid lubricant depots may be expedient (Figure 13.20) These permit

Figure 13.16 Modified interlocks [13.7]

Figure 13.17 Cylindrical

interlock [13.4]

Figure 13.18 Alignment with

cylindrical interlock [13.8]

1 3 5 C h a n g i n g M o l d s

13.5.1 S y s t e m s f o r a Q u i c k C h a n g e o f M o l d s

f o r T h e r m o p l a s t i c s

Injection molds are usually mounted to the machine platens by mechanical clamping devices (conventional mold clamps with bolts) and connected to power- and water-supply lines To do this the mold is either horizontally or vertically brought into the machine by a lifting device Depending on size and weight of the mold and the number

of connections, this leads to shutdown times, which may last from an hour to several days (Figure 13.22) [13.9] Such secondary times affect the productivity considerably, especially in the case of small batch sizes and thus frequent mold changes The development towards automation, the demand for more flexibility and better economics, lead by necessity to systems for a quick mold change [13.10-13.12] In spite of this, such systems have not prevailed so far There are two reasons for this One is the lack of compatibility among the various systems on the market today [13.13, 13.14, 13.16, 13.17] The second one is the need for a change of almost all molds used in a machine and the associated high costs

Figure 13.20 Flat leaders with solid

lubricant depots [13.2]

Figure 13.21 Combinations of flat

leaders with conventional leader elements [13.2]

Figure 13.19 Tapered

interlocks [13.8]

1 Male,

2 Female

precise centering even before the mold is completely closed, as well as compensation of differential thermal expansion of fixed and moving mold half Flat leaders may also be combined with circular (conventional) leader elements (Figure 13.21)

A quick-change system consists of several components which allows changing injection molds either fully automatically or semi-automatically, controlled by an operator Such components serve the function of

- detaching and fastening the mold at the machine platens,

- disconnecting and connecting the supply lines,

- bringing the mold into the clamping unit or taking it out

From this follows the need for these means of a quick-change system:

- quick-clamping devices,

- quick-connection couplings,

- changing equipment

Besides this, some more components are required for automation of mold changing, which have to be combined to one system (Figure 13.23) Only the combined action of all components permits flexible and automated injection molding [13.13]

Mold design is mostly affected by quick-connection couplings Two solutions for quick-clamping devices have prevailed on the market One can distinguish between adaptive and integrated clamping systems, which are usually actuated hydraulically They can easily be inserted into a concept of flexible automation

The adaptive clamping system has hydraulically actuated locking cylinders or ledges with integrated collets [13.9] mounted to the clamping platens of the machine, into which the precisely machined clamping plate of the mold is inserted They are mostly chamfered or provided with a groove (Figures 13.24 and 13.25)

During clamping, the piston or ledge, which is also chamfered, is moved against a corresponding counter chamfer of the mold (Figures 13.24 and 13.25) The counter chamfer is about 5° This angle causes self-locking (as long as no oil has dripped on it) For reasons of safety, clamping elements are therefore equipped with a proximity switch

as a standard [13.9, 13.16-13.19, 13.21]

The integrated clamping system has a hydraulic locking device integrated into the clamping platens It clamps the mold either directly via bolts mounted on base plates of the mold [13.22, 13.23] or via its own bolts, which press flat against the edges of the mold edges (platens) [13.23] (Figure 13.26)

Manual change Automatic change

Time for mold change

(hours) Time for mold change(minutes)

Figure 13.22 Cutback

in down time with rapid-clamp systems for mold changes in injection molding machines [13.9]

Main computer

Machine control

Rapid-clamp system Quick couplings Exchange equipment

Pre-heating location

Mold-handling

system

Mold storage

Figure 13.23 Components of automatic mold change [13.13]

Figure 13.24 Adaptive rapid clamp system

Hydraulic jack (left), Clamping ledge with integrated lugs, mold plate chamfered (right) [13.9]

Figure 13.25 Adaptive rapid-clamp

system A-E Variable dimensions of the individual design,

top: Cylinder with sloped piston assuring

a self-locking clamp with correspondingly sloped clamping face,

bottom: Tilted cylinder causes self-locking clamp with level clamping face