12566_15.pdf

8/11/2019 12566_15.pdf

http://slidepdf.com/reader/full/1256615pdf 1/16

Injection molds represent a major investment for plastics processors. They constitute a

large position of the company's assets and are the basis for production, economic

success, and technical development. For these reasons, injection molds must be in good

working order and ready for use.

In practice, however, situations frequently arise in which defects and improper

maintenance of injection molds cause major disruptions to current production, occurring

particularly during modifications. This reduces the actual working time of the injection

molding machines and continually impedes proper, planned production. Against this

background, back in 1992, an industrial survey by the Institute for Plastics Processing

(IKV) in Germany [15.1] showed that on average almost 7% of possible production time

was lost due to damage to injection molds (Figure

15.1).

Comparison with the results

from 1973 clearly show that this figure has more than doubled in twenty years. As

opposed to that, the proportion of machine-related downtimes is much lower.

Technological developments in injection-molding machines have low ered the propo rtion

by as much as one third.

In view of this situation, it is difficult to understand why injection molding shops,

which would frequently have to maintain as many as 1000 m olds, still employ the fire

brig ade approach, by which is mean t that a mold is only repaired when it has failed. The

industrial survey mentioned above [15.1] showed that only 30% of injection molding

shops carry out preven tive main tenance at fixed intervals. In these sho ps, again, only one

third of the maintenance data is recorded and evaluated systematically. It follows from

this that only 10% of injection molding shops perform preventive maintenance founded

on a sound database [15.1].

1 5 M a i n t e n a n c e o f I n j e c t i o n M o l d s

Fi gu re 15.1 Change in production

downtimes

D

o

w

n

t

m

e

]

Injection modng

machnes

Injection

mods

Year973

1992

6.1%

2.9%

4.4%

6 8

8/11/2019 12566_15.pdf


The situation just described may be used to illustrate the deficits arising during the

maintenance of injection molds (Figure 15.2). The state of the art is such that while

damage and cost data are recorded, they often cannot be combined with each other.

While such information, which represents invaluable experience for mold-making, is

Work

preparation

Design

State of th

art

Maintenance/

mold making

roduction

Data capture

Downtimes

Downtime causes

Inspection

report

Maintenance report

Repair times

Repair costs

Spare parts

E

x

p

e

r

e

n

c

e

f

e

e

d

b

a

c

k

E

x

p

e

r

e

n

c

e

e

e

d

b

a

c

k

Archiving/documentation

Data evaluation/weak point analysis

Choice of strategy

Figure 15 2

State of data acqu isition and evaluation

8/11/2019 12566_15.pdf


archived, it merely serves documentation purposes. The goal should be, however, to use

this invaluable practical experience as a basis for preparatory work and design.

Much as damage to molds is of interest, its causes are even more important. It turns

out that, of the most frequent causes of damage, wear comes top of the list. This is

followed by set-up errors and operating errors. The similarly relatively high proportion

of design errors can, among other things, often be attributed to poor communication

between those responsible for mo ld maintenance and m old design [15.2] (no feedback or

archiving).

Since every injection mold is unique, it is not possible to generalize about main-

tenance. Commonplace are maximum possible standardization, the use of mold

standards, easy accessibility and exchangeability of parts on the injection molding

machine where possible, and the wear-resistant construction of friction pairings. But

there are invaluable hints to be gained for individual molds, particularly from use. It will

be shown below how these signs of weak points taken from production can be recorded

so as to reduce costs and to optimize processes in production and m old-making.

1 5 . 1 A d v a n t a g e s o f M a i n t e n a n c e S c h e d u l e s

Figure 15.3 compares the work processes involved when the fire brigade and the

preventive strategies are employed, in terms of attainable machine utilization and

resultant downtimes.

Examples from shop practice prove the efficacy of performing scheduled preventive

maintenance. Constant m onitoring of the throughput times of maintenance job s or actual

repair times (Figure 15.4) perm it measures to be taken so as to increase efficiency (e.g.,

Disrupton Report Start of repars

Startup

Scrap

Standstill Standstill

Scrap

Passed parts

Fire brigade

strategy

Producton

Eapsed time

till reported

Waiting

for repars

Repairs

Producton

Mantenance job

Startup

Passed parts

Preventve

strategy

Producton

Preventve

maintenance

Startup Producton

Figure 15 3 Time schem e for application of different maintena nce strategies

Producton time ganed

8/11/2019 12566_15.pdf


investment in new machining equipment) and decrease the throughput times for

maintenance and for production through reducing downtimes. This effect is reinforced

by purposeful preparation of the measures to be implemented (e.g., provision of

equipment and spare parts), much as when set-up preparations are made wh en molds are

changed. As may be seen in Figure 15.4, the proportion of quick repairs has increased in

our example while the proportion of longer-lasting ones has receded over the years, as a

result. This clearly illustrates the success of the measures implemented.

1 5 . 2 S c h e d u l i n g M o l d M a i n t e n a n c e

1 5 . 2 . 1 D a t a A c q u i s i t i o n

The choice of molds to examine first necessitates the acquisition of detailed data. Since

various factory studies revealed that a great deal of acquired data are not used, particular

value should be attached to goal-oriented or need-oriented data acquisition. The goal of

data acquisition must be the provision of informative maintenance data in the form of

feedback to staff in design, work preparation and mold making (Figure 15.2).

Data on mold maintenance is essentially required in two areas. The first is the control

and monitoring of the direct and indirect costs that arise. It should be possible to report

on all molds, a particular class of molds, an individual mold, or a functional group. The

second is selective weak-point analysis, which requires detailed data acquisition. Here,

a distinction needs to be made between the damage that occurs and its actual cause

[15.4].

The mold data can be stored in a type of lifetime. As shown in Figure 15.5, the data

for each individual mold should be recorded in the form of a lifetime data record. Item

1 is the mold identification number. To be able to schedule maintenance measures or

intervals, the num ber of cycles needs to be know n as it is a wear-determining factor (item

2).

It is also important to establish if the maintenance measure is scheduled or non-

scheduled (item 3). For referencing purposes, the functional system where the damage

occurred must be noted (item

4 ).

Description of the damage (item 5) and, where possible,

Figure 15 4 Decreasing

repairs over the years

epar tmes [hours]

Up to 5 Up to 10 Up to 15 Up to 20 Over 20

F

r

a

c

t

o

n

[

A

]

1989

1990

1991

8/11/2019 12566_15.pdf


the cause (item 6) shou ld be coded for the weak-point analysis. Space is also required for

a brief comment. The costs are entered into item 8, separated according to direct and

indirect costs, for the purposes of evaluation. The mold lifetime can be kept for all

injection molds by a central unit and forms a good basis for informative evaluations.

To illustrate the need for cost-related data acquisition, two evaluations of a mold

resume will now be presented. In the first, the maintenance activities were assigned to

the various functional groups. The sum of the activities and the relation to the total

instances of damage are shown in Figure 15.6.

In this example, repairs to the demolding system were the most frequent (50%),

followed by mold cavities at 14%. The other functional groups sustained much less

damage, amounting to less than 10%.

How ever, reporting m old dam age in terms of the number of repairs is not satisfactory.

It is important to link each event with the time for repair and the costs incurred. In this

example, it made sense to use the available data to weight the damage susceptibility of

certain modules according to the number of maintenance hours incurred. This afforded

the possibility of making a concrete, value-based evaluation.

Recording an event

in the mold lifetime

Mod lifetime

Unschedued

Date

C y c l e s

I D . N o . :

Key

Category

Abbrev.: Door hande

Ma

i n t .

costs

Direct

Indirect

Sysem date

Cyces:

IScheduec

ate

2

Cavty

3

Temperature control

4

Demodng

5 Leader/locating

6

Power transmsson

lWear

2

Setup error

3

Material failure

1

Jammed

2

Fracture

3...

1

Initiate

job

2

Reverse

job

igu re 15.5 Data in

a mold lifetime

y

2 3 4 5 6

2 3 4 5 7

2 3 4 5 8

2 3 4 5 9

8/11/2019 12566_15.pdf


Application of this approach to the same injection mold yielded the distribution of

maintenance hours that is shown in Figure 15.7. This modified damage distribution is

based on a total of 539 maintenance hours for a mold that carried out roughly 830,000

cycles in the production period concerned.

This analysis differs enormously from that based on the number of activities. While

mold cavity and demolding still constitute the most damage, their ratio is now reversed:

demolding: previously 50% - now 21% ,

cavity: previously 14% - now 4 3 % .

This reversal is logical considering that an ejector can generally be replaced very

quickly, but a repair to what often is a polished or chrom e-plated mold cavity is relatively

time-consuming. From the economics point of view and for the purpose of establishing

a work-benefit ratio of maintenance measures, the analysis shown in Figure 15.7 must

be considered to be more informative.

Conventional data acquisition using forms still serves a purpose, especially if it is

only a temporary measure. A company will resist the unavoidable effort involved until it

recognizes the advantages that this approach has to offer [15.5]. Although computer

support should be the long-term goal, despite the considerable work involved for

evaluation, forms can be used with great effect in a pilot project or for m ultiple instanta-

neous records. Generally, however, there is no extra work involved for the company as

most already perform data acquisition, even if this does not always satisfy the criteria for

an evaluation.

1 5 . 2 .2 D a t a E v a l u a t i o n a n d W e a k P o i n t A n a l y s i s

A major goal of data acquisition and evaluation is to illustrate the failure modes of

injection molds. This goal is served by the answers to the various questions, such as:

- What are the most common types of damage?

- Which functional system of an injection mold is most frequently affected by damage?

- Which molds are the most susceptible?

- What are the most common causes of damage?

- Which types of damage cause the most trouble?

Figure 15 6 Damage frequency for a single

injection mold

Figure 15 7 Maintenance hours expended on

a single injection mold

After evauaton of 830000 cyces

100%

= 539 manenance hours

Demodng 21%

Aer evauaon o a mod lfetme

I

100% = 70 repars

Others 25%

Temperature

control 5

Leader and

locatng

6%

C a v i t y 5

.Gate

1%

Temperaure contro

4%

Demodng

50%

Cavty

43%

Gae 5

Leader

and locating 5

8/11/2019 12566_15.pdf


The financial effects will not be discussed in detail here. Instead, the focus will be on

technical aspects and possible consequences. A determination of the proportion of the

most serious types of damage and their causes can reveal, for instance, that only five

types account for more than 50% of all failures

[15.6].

This provides those responsible

in mold making with a direct starting point for eliminating the weak points.

A Pareto Principle can be derived from this relationsh ip; it states that a small

number of monitored types of damage will incur by far the most costs [15.7]. Also

known as the AB C m ethod , this can be illustrated as shown in Figure 15.8. This tool

can considerably reduce the amount of work involved in that it restricts attention to the

greatest causes of costs incurred by molds on the one hand (Figure 15.8, top) and

investigates only the most important types of damage for these on the other (Figure 15.8,

bottom).

To mak e acquisition and evaluation of the various types of damage ascertained as easy

as possible, a numbering system should be employed for the various types, just as was

done for the various mold parts. For the sake of clarity, initially no more than 10 types

of damage should be identified per functional system. Implemented as a numbering

system, this means that ejector fracture would have a two-digit number (e.g., 41 where

4 = demolding system and 1 = ejector fracture). For five functional systems, this would

allow fifty different types of dam age to be described. Th e particular advantage of this is

unambiguous identification of damage during data acquisition - employees are not using

M

a

i

n

t

e

n

a

n

c

e

c

o

s

t

s

[

]

M

a

i

n

t

e

n

a

n

c

e

c

o

s

t

s

[

]

Cass C -

Cass B

Cass

A

Number o f mods [%]

Cass

A:

10 Mods

75 Coss

Cass B:

25 Mods

15

Costs

Cass

C :

6 5 Mods

10 Coss

Cass C -

Cass B

Cass

A

Mod damage

[%]

Cass A :

10

Damage

75 Costs

Cass B:

2 5 Damage

15

Costs

Cass C :

65 Damage

10 Coss

Figure 15 8 ABC

analysis of damage

8/11/2019 12566_15.pdf


their own descriptions for the same type of damage. Furthermore, the numbering system

allows direct classification and access to the corresponding work plan. It also helps to

smooth the transition to a computer-based support system.

1 5 .2 . 3 C o m p u t e r - B a s e d S u p p o r t

Crucial to the use of a computer system are the functions provided for data evaluation,

which must suit the case at hand. A peculiarity arises from the fact that injection molds

are not fixed permanently in one place, but rather have to be tracked as movable

inventory. A computer system must therefore be able to accommodate the respective

status with more detailed information (e.g., in the mold department for repair until

approx. ...) This information serves production control for planning subsequent

production orders as well as maintenance in the planning of preventive measures.

From the point of view of the prim e aim of a maintenance analysis, a com puter system

must be in a position to provide the following functions:

- acquisition of all relevant data in a mo ld lifetime,

- support for ABC analyses for all molds, mold groups, functional systems, and mold

damage,

- comprehensive support in the evaluation of lifetime data,

- presentation of percentage types of damage for a single mold or mold group,

- possibility of classifying damage within a functional system,

- presentation of the proportion of a certain type of damage in a mold group,

- presentation of the maintenance measures for the service life in cycles,

- comparison of intervals between occurrences of a particular type of damage,

- presentation of the frequency of the damage that has occurred with the goal of weak-

point analysis,

- tracking of repair times, comparison of in-house/external share, etc.

The ultimate goal of the evaluations must be to eliminate primarily those weak points

that incur the highest costs. An example of such a presentation is shown in Figure 15.9.

This allows the costs of the different functional systems of a specific mold to be

compared. If a functional system becomes noticeable because of extremely high

maintenance, it must be possible to call up more detailed information on the proportions

of the various types of damage. The special advantage of this presentation comes to the

foreground when an evaluation can be performed separately on the basis of direct and

indirect costs. In this case, those weak points whose direct costs were not high enough

to cause concern can be uncovered if they lead to indirect costs in the form of lost profit

contributions due to equipment downtimes.

1 5 . 3 S t o r a g e a n d C a r e o f I n j e c t i o n M o l d s

Injection molds have a limited service life (Table 15.1). Appropriate measures can

greatly extend this, however. Such measures can be classified on the basis of:

- maintenance,

- storage, and

- care.

8/11/2019 12566_15.pdf


To be able to quickly fall back on ready-to-use molds, the following demands on storage

and care must be fulfilled:

-

every mold must be stored along with one molded part and a mold card in its own,

easily accessible space in the mold store,

- 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.

Table 15 1 Numbers of molded parts obtainable with various mold materials [15.8]

Material

Zinc alloys

Aluminum

Aluminum

Copper-beryllium

Steel

Casting

Casting

Rolled

Surface hardened

Attainable number

100,000

100,000

100,000-200,000

250,000-500,000

500,000-1,000,000

Figure 15 9

Weak

point analysis based

on

mold lifetime

C

y

c

l

e

m

a

i

n

t

e

n

a

n

c

c

o

s

t

s

y

i

r

e

t

Evaluation

of a mold lifetime

Mold

cycles

ID. No.: 123.456

250.000-500.000

Abbrev.: Door handle

Gate

Cavity

Temperature control

Demolding

Leader and

locating

Damage cause

tables:

Cavity

(4-cavity model)

1

Contamination

2 Corroson

3...

Leader and

locating

Demolding

Temperature control

Gate

8/11/2019 12566_15.pdf


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:

- mold design (split, sliding split, unscrewable mold etc.),

- dimensions of the mold and the molded part,

- mold mounting equipment,

- injection molding machine suitable for production,

- shot weight (injection volume),

- suitable plastic,

- rules on material pretreatment,

- processing temperatures,

- mold temperature and heat-control medium (water, oil, etc.),

- cycle times,

- injection pressure, follow-up pressure, dynamic pressure,

- injection speed,

- screw speed,

- cylinder equipment (sliding shut-off nozzle, non-return valve),

- maintenance intervals,

- number of pieces produced.

This list could be ex tended and thus m atched 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

control unit prior to production startup.

M old 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 m olds . Parts or groups of parts that are loan ed

or borrow ed 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 (discussed later) and

imposes specific demands on the store, its cleanness and particularly the ambient

conditions. Dam p and unheated rooms p rom ote 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.

Imp ortant to the accessibility of the mo lds 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

job is complete, the mold may only be returned to storage when its suitability

for future use has been checked . The last parts produced w ith it can provide an ind ication

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, ejector bushes, etc. If no deficiencies are found, the

maintenance work then takes the form of the general care measures described below.

8/11/2019 12566_15.pdf


Maintenance of Cooling Lines

Cooling lines must be cleaned thoroughly to eliminate scale, rust, sludge, and algae.

Since these deposits decrease the diameter of the channels, measuring the flow rate is

a way of checking the system. A pressure-controlled valve is installed between mold and

water line and a defined pressure drop is set, which has to be the 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 cooling channels.

For cleaning, the cooling lines 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

[15.9,

15.10]. A solution of hydrochloric acid (20° Be) with two parts water and a

corrosion inhibitor has been successfully used.

The nipples, bridges, bolts and feed lines (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.

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 (car polish) may be suitable.

Removal of residual lubricants from movable mold components is also part of the

cleaning operation. Degreasing detergents for this are available on the market.

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 thermoco uples should be checked w ith an ohm meter

and the results compared with those on the mold card. Accidental grounding should be

investigated, too. The control circuits are easily tested with an ammeter installed in the

circuit.

A check should also be made to ensure that lines, connections, insulation, and main

lead cleats are in proper working order.

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.

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.

8/11/2019 12566_15.pdf


Care Prior to Storage

At the end of each maintenance work, the mold has to be carefully dried and lightly

greased with noncorrosive grease (petrolatum). 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 mo ld in oil paper is crucial

when the mold store does not satisfy the demands above and below.

AU observations and maintenance work are recorded on the mold card [15.11, 15.12].

1 5 . 4 R e p a i r s a n d A l t e r a t i o n s o f I n j e c t i o n M o l d s

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. A

survey of the various kinds of wear, their causes and symptoms is provided in

Figure 15.10.

W

i

h

a

n

d

w

i

h

o

u

t

l

u

b

r

c

a

t

o

n

m

e

t

a

l

s

p

l

a

s

t

c

s

,

s

o

l

d

s

)

E

r

o

s

i

v

e

A

b

r

a

s

i

v

e

w

e

a

r

Type

of

wear

Initial condtons

Sliding friction

Rong wear

with

and

without

slppage

Wear

by

shock

Vibrational

wear

Particle,

sliding

and

rolling

friction wear

Sliding

friction wear

Hydroabrasve wear,

radiation wear,

other erosve wear

Manifestation, progression,

results

Seizing, cratering,

grooving, running,

clearance,

chatter marks

Pitting, peeling,

spoiling, rippling,

seizing, groovng

Break

out,

peeling,

pitting

Roughenng

seizing, oxde fluttering,

fretting

Grooving,

break

out,

rolling tracks

a

Grooving, break

out,

embedding, smoothng

b

Flat grooves, washout

Waves, cavities,

piercing, washout

Characteristc

Counter-partcle

furrowing

Particle furrowing

b)

)

Figure 15 10 Overview of types of wear [15.13]

8/11/2019 12566_15.pdf


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.

Rem edial measures require a detailed know ledge of the cause of dam age. The following

are possible:

- simple mechanical finishing,

- replacement of parts or m odules,

- 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 (pits) 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 (slightly

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 repaired. These are normally standard parts

(see Chapter 17) available in various dimensions and can thus be replaced cheaply.

Doing this means that the molds will function perfectly and avoid any major risks.

The repairs described so far will often be inadequate and material will have to be

deposited because, e.g., edges or corners have broken off. Welding is necessary in such

cases.

Repair welds to injection m olds should alw ays be preceded by heating to keep therm al

stress and the formation of internal stress as low as possible.

Preheating avoids compression and shrinkage in the weld zone and, above all,

prevents heat from being dissipated so quickly from the weld area that hardening sets in

(as when heated parts are quenched 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 manufacturers 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

0

C and then reheated again to

the normalizing temp erature [15.14].

Welding repairs are performed by the TIG method and welding with coated electrode

wires.

TIG (tungsten inert gas) offers distinct advantages. The following basic rules mu st

be observed for repair welding:

- 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 [15.14].

- The am perage has to be kept as low as possible to preven t reduced hardness and coa rse

structure [15.14].

- The preheating temperature must be above the martensite-forming temperature. It can

be taken from the respective temperature-time phase diagram for the steel. It should

not be considerably higher, however, since it increases the depth of burn-in [15.14].

- During the entire welding process, the mold must be kept at the preheating tempera-

ture. This is particularly the case for several deposits.

8/11/2019 12566_15.pdf


Figure 15.11 Possible welding depths and seam widths in laser welding [15.15]

The electrode wire material is generally < 0.5 mm in diameter, a small portion of which

is melted onto the mold with every welding pulse. The wire material is available in

different thicknesses and compositions.

The w elding process itself

is

observed through a stereomicroscope fitted with a proper

shield.

Due to the expected and actual difficulties inherent in all forge welding techniques,

the calls for cold metal-deposition pro cesses are und erstandable. One such process is

electrochemical metallizing for depositing all kinds of metals and alloys on almost all

metallic materials.

Dimensional corrections up to several tenths of a millimeter are possible with this

method on flat surfaces, shafts, and in drill holes [15.16].

The steps required in effecting a repair (Figure 15.12) vary with the type and extent

of the damage. Major damage (deeper than 0.5 mm) is first rebored and the hole sealed

with pins. Then the damaged area, e.g., minor damage, is ground out in a hollow and

sandblasted or electrochemically cleaned with a so-called preparatory electrolyte. An

area treated in this way, free of grease and oxide, is optimally prepared for metal

deposition.

Dameter: depth ratio

= 1:3 for sma dameter

=

1:1

for large dameter

Spot dameter

0.2-2

mm

Spot welding

Up to 2 mm

Seam

welding

Up to 2 mm

0.2-2 mm

- 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.

Very recently, lasers have been used for repair welding of molds . M ostly 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 conv entional weld ing is that low

amo unts of energy are applied with extrem e 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 [15.15].

Figure 15.11 shows which welding depths and seam widths are possible with lasers.

Only relatively minor damage can be repaired in one working operation.

8/11/2019 12566_15.pdf


Figure 15 12 Working stages in electrochemical metal deposition [15.16]

1 Flaw; 2 Pins inserted; 3 Cavity ground out; 4 Cavity metallized with rapid-depositing

electrolyte; 5 Leveling of metal filling (mechanical); 6 Transition to intact surface, covered with

hard finishing layer

The repair area is then sealed off with galvanic sealing tape and the ground-out hollow

is filled w ith a fast-depositing me tal such as copper or nickel and mechanically flattened.

The damaged area is then ready for sealing flush to the mold surface with an appropriate

covering metal [15.16].

Sealing is carried out by soaking a graphite anode surrounded with an absorbent

material in the desired high-performance electrolyte and moving it across the area to be

coated. Under direct current, the metal is deposited onto the cathode, i.e., the mold

surface.

There are also micro-cold or deposit welding devices [15.17, 15.18] on the market

that operate on the principle of resistance pressure welding. The most common appli-

cation of this process is spot welding.

Resistance pressure welding uses the heat generated by the electric current in over-

coming the electric resistance at the point of contact with the parts to be welded. At the

points of joining, the parts become pasty and are pressed together without the need for

additional materials [15.19].

For repair we lding of molds, e.g., filling out of hollows, one part for weldin g is

replaced, e.g., by a steel tape which covers the hollow. During welding, the electrode is

rolled along the steel tape, and pressed at the same time against the area to be repaired.

The steel tapes are available in thicknesses of 0.1 to 0.2 mm. For deeper hollows, the

process has to be repeated.

For minor repairs, e.g., to edges or corners, the steel tape is replaced by metal powder

or metal paste [15.17].

The repaired areas can then be machined afterwards and polished to a high finish.

Hardening and coating are also possible.

Metal-depositing processes are risky ways of effecting repairs, require dexterity and

good know ledge of material behavior and the actual process em ployed.

R e f e r e n c e s

[15.1] Micha eli, W.; Feldhau s, A.; Ecke rs, C ; L ieber, T.; Paw elzik, P.: Instandhaltung von

SpritzgieBwerkzeugen - Ergebnisse einer Befragung von SpritzgieBbetrieben. Pro-

spectus, IKV, 1992.

8/11/2019 12566_15.pdf


[15.2] Feldhaus, A.: Instandhaltung von SpritzgieBwerkzeugen - Analyse des Ausfallverhaltens

und Entwicklung angepaBter MaBnahmen zur Steigerung der Anlagenverfiigbarkeit.

Dissertation, RWTH, Aachen, 1993.

[15.3] Hackstein, R.; Richter, H.: Optionale Instandhaltung - u ntersucht am Beispiel von Spritz-

und DruckguBmaschinen. FB/IE 24 (1975), 5, pp. 267 -27 3.

[15.4] Oltmann s, P.: EDV -Unterstutzung zur Instandhaltung von SpritzgieBwerkzeugen,

Unpublished report, IKV, Aachen, 1993.

[15.5] M exis, N . D .: Die Verfugba rkeitsanalyse in der Inves titionsplan ung . Verlag fur

Fachliteratur, Heidelberg, 1991.

[15.6] Wilden, H.: W erkzeugkon zeption. In: Der SpritzgieBprozeB. VD I-Verlag, Diisseldorf,

1979,

pp. 87-109.

[15.7] Taubert, D.: Wirtschaftliche B ewertungskriterien fur die geplante Instandhaltung, VDI-

Berichte, No. 380, 1980, S. 13-19.

[15.8] Rheinfeld, D.: Werkzeug soil in Ordnung sein. VD I-Nachrichten, 30 (1976), 31 , p. 8.

[15.9] Reinigungsgerate fur Ku hlkanale. Kunststoffe, 54 (1974), 3, p. 112.

[15.10] SpritzgieBen-Werkzeug. Technical information, 4.3 , BAS F, Ludw igshafen/Rh., 1969 .

[15.11] Kundenze itschrift. Arburg heu te, 10 (1979), 16, June 1979.

[15.12] Oe bius, E.: Pflege und Instandhaltung von SpritzgieBw erkzeugen. Kunststoffe, 64

(1974),

3, pp. 123-124.

[15.13] Brandis, H.; Reismann , J.; Salzmann, H.; Spyra, W; Klupsch, H.: HartschweiBlegierun-

gen. Thyssen Edelstahl, Technical report, 10 (1984), M l , pp. 5 4- 75 .

[15.14] Rasche, K.: Das SchweiBen von Werkzeug stahlen. Thyssen Ed elstahl, Technical report,

7 (1981), 2, pp. 212 -21 9.

[15.15] Schmid, L.: ReparaturschweiBen m it dem Laserstrahl. Paper presented at the 8th Tooling

conference at Wlirzburg: Der SpritzgieBformenbau im internationalen W ettbewerb ,

Wurzburg 24. 9. 1997-25. 9. 1997.

[15.16] Elektrochemischer M etallauftrag. Prospectus, Baltrusch und Mtitsch GmbH & Co., KG ,

Forchtenberg.

[15.17] Prospectus, Joisten und Kettenbaum Gm bH & Co., Joke KG, Bergisch G ladbach.

[15.18] Fachkunde M etall. Verlag Europ a-Lehrm ittel, Nourmney, Vollmer Gm bH & Co., Haan -

Gruiten, 1990.

[15.19] Prospectus, Schwer & Kopk a Gm bH, Weingarten.

12566_15.pdf

Documents

Transcript of 12566_15.pdf