Maintenance of Injection Molds

15

M a i n t e n a n c e

of

Injection

M o l d s

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 lowered the proportion
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 molds, still employ the "fire
brigade" approach, by which is meant 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 preventive maintenance at fixed intervals. In these shops, 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].

Downtime [%]

4.4%
Injection molding
machn
i es
6.1%

2.9%
Figure 15.1 Change in production
downtimes

1973

Injection
mod
ls
Year

6.8%

1992


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 the art
Maintenance/
mold making

Production

Downtimes

Repair times

Downtime causes

Repair costs

Inspection report

Spare parts

Maintenance report

Archiving/documentation

Data evaluation/weak point analysis

Choice of strategy
Figure 15.2

State of data acquisition and evaluation

Experience feedback

Experience feedback

Data capture


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 mold maintenance and mold design [15.2] (no feedback or
archiving).
Since every injection mold is unique, it is not possible to generalize about maintenance. 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 mold-making.
15.1

A d v a n t a g e s of M a i n t e n a n c e

Schedules

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 monitoring of the throughput times of maintenance jobs or actual
repair times (Figure 15.4) permit measures to be taken so as to increase efficiency (e.g.,

Disruption

Report

Start of repairs

Startup
Passed parts

Scrap
"Fire brigade"
l psed time
Production Ea
strategy
till reported

Maintenance job

Standstil

Standstil

Waiting
for repairs

Repairs

Scrap
Production

Startup
Passed parts

Preventive
strategy

Production

Preventive
maintenance

Startup

Production

Production time gained
Figure 15.3

Time scheme for application of different maintenance strategies

Fraction [0A]

1989
1990
1991

Up to 5 Up to 10 Up to 15 Up to 20 Over 20
Repari times [hours]

Figure 15.4 Decreasing
repairs over the years

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 when 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.
15.2

Scheduling Mold

Maintenance

15.2.1 D a t a Acquisition
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 number of cycles needs to be known as it is a wear-determining factor (item
2). It is also important to establish if the maintenance measure is scheduled or nonscheduled (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,


the cause (item 6) should 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%.
However, reporting mold damage 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
Mold lifetime
Unschedue
ld
Cycles
Date

ID. No.:
Key

Category

Abbrev.: Door handle
Ma int. costs
Indirect
Direct

System date

123.456
123.457
123.458
123.459

Cycles:
ISchedue

lc

n/y 1 Gate

Figure 15.5 Data in
a mold lifetime

2 Cavity
3 Temperature control
4 Demolding
5 Leader/locating
6 Power transmission

lWear
2 Setup error
3 Material failure
1 Jammed
2 Fracture
3...
1 Initiate job
2 Reverse job


Temperature control 4%
Demod
ln
ig
50%

Cavity 43%

Cavity 5%
.Gate 1%
Leader and
locating 6%

Temperature
control 5%

Gate 5%
Leader
and locating 5%

Others 25%
After evau
l ato
i n of a modl lifetime I
100% = 70 reparis
Figure 15.6 Damage frequency for a single
injection mold

Demolding 21%
After evau
l ato
i n of 830,000 cyce
ls
100% = 539 man
itenance hours

Figure 15.7 Maintenance hours expended on

a single injection mold

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:
cavity:

previously 50% - now 21%,
previously 14% - now 43%.

This reversal is logical considering that an ejector can generally be replaced very
quickly, but a repair to what often is a polished or chrome-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 multiple instantaneous 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.
15.2.2 D a t a Evaluation a n d W e a k - P o i n t Analysis
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?


Maintenance costs [%]

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 relationship; it states that a small
number of monitored types of damage will incur by far the most costs [15.7]. Also
known as the "ABC method", 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 make 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 damage to be described. The particular advantage of this is
unambiguous identification of damage during data acquisition - employees are not using

Ca
l ss CCa
l ss B

Ca
l ss A

Ca
l ss A:
10% Molds
75% Costs
Ca
l ss B:
25% Molds
15% Costs
Ca
l ss C:
65% Molds
10% Costs

Maintenance costs [%]

Number of molds [%]

Figure 15.8 ABC
analysis of damage

Ca
l ss CCa

l ss B

Ca
l ss A

Mold damage [%]

Ca
l ss A:
10% Damage
75% Costs
Ca
l ss B:
25% Damage
15% Costs
Ca
l ss C:
65% Damage
10% Costs


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.
15.2.3 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 prime aim of a maintenance analysis, a computer system
must be in a position to provide the following functions:
- acquisition of all relevant data in a mold 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 weakpoint 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.
15.3

S t o r a g e a n d C a r e of Injection

Molds

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.


Evaluation
of a mold lifetime
ID. No.: 123.456 Abbrev.: Door handle
250.000-500.000

Cycle maintenance costs yirect

Mold
cycles

Gate

Cavity Temperature control Demolding

Leader and locating

Damage cause tables:
Leader and locating

Cavity
(4-cavity model)
Figure 15.9 Weak

point analysis based on
mold lifetime

1 Contamination
2 Corrosion
3...

Demolding
Temperature control
Gate

Table 15.1 Numbers of molded parts obtainable with various mold materials [15.8]
Material
Zinc alloys
Aluminum
Aluminum
Copper-beryllium
Steel

Attainable number
Casting
Casting
Rolled
Surface hardened

100,000
100,000
100,000-200,000
250,000-500,000
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:
- 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.


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 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
control unit 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 "loaned"
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 (discussed later) 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, ejector bushes, etc. If no deficiencies are found, the

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


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 thermocouples should be checked with an ohmmeter
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.


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 mold 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].
15.4

Repairs a n d 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. A
survey of the various kinds of wear, their causes and symptoms is provided in
Figure 15.10.

Manifestation, progression,
results
Seizing, cratering,
grooving, running,
clearance, chatter marks

Rolling wear
with and without
slippage

Pitting, peeling,
spoiling, rippling,
seizing, grooving

Wear by shock

Break out,
peeling, pitting

Vibrational
wear

Roughenn
ig
seizing, oxide fluttering,
fretting

Particle,
sliding and rolling
friction wear

Grooving,
break out,
rolling tracks

Sliding
friction wear

Erosive

wear

Characteristic

Sliding friction

Abrasive

With and without lubrication
(metals, plastics, solids)

Type of wear

Initial conditions

Counter-particle Particle furrowing
a) Grooving, break out,
furrowing
embedding, smoothing
a)
b)
b) Flat grooves, washout

Hydroabrasive wear,
radiation wear,
other erosive wear
Figure 15.10

Overview of types of wear [15.13]

Waves, cavities,
piercing, washout


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.
Remedial measures require a detailed knowledge of the cause of damage. The following
are possible:
- simple mechanical finishing,
- replacement of parts or modules,
- 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 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 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 0C and then reheated again to
the normalizing temperature [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 must
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 amperage has to be kept as low as possible to prevent reduced hardness and coarse
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 temperature. This is particularly the case for several deposits.


- 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. 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 [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.

Spot welding

Seam welding

Up to 2 mm

Up to 2 mm

Spot diameter 0.2-2 mm
Diameter: depth ratio
= 1:3 for small diameter
= 1:1 for large diameter

0.2-2 mm

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 welding 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 processes" are understandable. 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.


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 with a fast-depositing metal 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 application of this process is spot welding.
Resistance pressure welding uses the heat generated by the electric current in overcoming 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 welding of molds, e.g., filling out of hollows, one "part for welding" 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 knowledge of material behavior and the actual process employed.
References
[15.1]

Michaeli, W.; Feldhaus, A.; Eckers, C ; Lieber, T.; Pawelzik, P.: Instandhaltung von
SpritzgieBwerkzeugen - Ergebnisse einer Befragung von SpritzgieBbetrieben. Prospectus, IKV, 1992.


[15.2]
[15.3]
[15.4]
[15.5]
[15.6]
[15.7]
[15.8]
[15.9]
[15.10]
[15.11]
[15.12]
[15.13]
[15.14]
[15.15]
[15.16]
[15.17]
[15.18]
[15.19]

Feldhaus, A.: Instandhaltung von SpritzgieBwerkzeugen - Analyse des Ausfallverhaltens
und Entwicklung angepaBter MaBnahmen zur Steigerung der Anlagenverfiigbarkeit.
Dissertation, RWTH, Aachen, 1993.
Hackstein, R.; Richter, H.: Optionale Instandhaltung - untersucht am Beispiel von Spritzund DruckguBmaschinen. FB/IE 24 (1975), 5, pp. 267-273.
Oltmanns, P.: EDV-Unterstutzung zur Instandhaltung von SpritzgieBwerkzeugen,
Unpublished report, IKV, Aachen, 1993.

Mexis, N. D.: Die Verfugbarkeitsanalyse in der Investitionsplanung. Verlag fur
Fachliteratur, Heidelberg, 1991.
Wilden, H.: Werkzeugkonzeption. In: Der SpritzgieBprozeB. VDI-Verlag, Diisseldorf,
1979, pp. 87-109.
Taubert, D.: Wirtschaftliche Bewertungskriterien fur die geplante Instandhaltung, VDIBerichte, No. 380, 1980, S. 13-19.
Rheinfeld, D.: Werkzeug soil in Ordnung sein. VDI-Nachrichten, 30 (1976), 31, p. 8.
Reinigungsgerate fur Kuhlkanale. Kunststoffe, 54 (1974), 3, p. 112.
SpritzgieBen-Werkzeug. Technical information, 4.3, BASF, Ludwigshafen/Rh., 1969.
Kundenzeitschrift. Arburg heute, 10 (1979), 16, June 1979.
Oebius, E.: Pflege und Instandhaltung von SpritzgieBwerkzeugen. Kunststoffe, 64
(1974), 3, pp. 123-124.
Brandis, H.; Reismann, J.; Salzmann, H.; Spyra, W; Klupsch, H.: HartschweiBlegierungen. Thyssen Edelstahl, Technical report, 10 (1984), Ml, pp. 54-75.
Rasche, K.: Das SchweiBen von Werkzeugstahlen. Thyssen Edelstahl, Technical report,
7 (1981), 2, pp. 212-219.
Schmid, L.: ReparaturschweiBen mit dem Laserstrahl. Paper presented at the 8th Tooling
conference at Wlirzburg: "Der SpritzgieBformenbau im internationalen Wettbewerb",
Wurzburg 24. 9. 1997-25. 9. 1997.
Elektrochemischer Metallauftrag. Prospectus, Baltrusch und Mtitsch GmbH & Co., KG,
Forchtenberg.
Prospectus, Joisten und Kettenbaum GmbH & Co., Joke KG, Bergisch Gladbach.
Fachkunde Metall. Verlag Europa-Lehrmittel, Nourmney, Vollmer GmbH & Co., HaanGruiten, 1990.
Prospectus, Schwer & Kopka GmbH, Weingarten.