World's First 3,000 ton Mold

Clamping Force Electric

Injection Molding Machine
SHIKI MATSUO* 1 TAKAMITSU HIMENO* 1
MASAHIRO BESSHO* 1 KENJI TSUTSUI* 1
NAOKI KATO* 2 TAKASHI MIZUNO* 2

Mitsubishi Heavy Industries, Ltd. (MHI) has developed a large 3,000 ton class clamping force electric injec-
tion molding machines, the 3000 em and 3500 em, the first in the world. While meeting the demands for the
world's fastest high-cycle production of large plastic parts such as bumpers for automobiles, the machine is
space conservative and energy efficient by incorporating MHI's unique 2-platen type mold clamping mechanism
and hybrid electric system. Furthermore, it is equipped with a newly developed center press platen and has
electric control of the internal mold pressure waveform, thus reducing flash on products to enable burr-less
molding, contributing to a reduction in customers' running costs.

incorporate MHI's unique, new center press platen and

1. Introduction the electric control of internal mold pressure waveform
In manufacturing plastics products and parts, elec- to enable burr-less molding. The use of the high-speed
tric drive system injection machines have been increasing mold opening and closing system, and nut opening and
their market shares while increasing their size and ca- closing systems accompanied by the high plasticization
pacity because of their cleaner and more energy efficient screw enables the machines to achieve the world's fast-
operation than hydraulic operation systems. The major est production cycle. The first 3000 em series production
issues in the injection molding of large automobile parts machine was delivered in April 2005 and started up as
such as bumpers are to achieve high cycle operation, re- planned and is presently in commercial operation to the
duced flash and a thinner product wall thickness. To satisfaction of the customer.
meet these demands, MHI has developed the world's first
energy efficient and space conservative super-sized elec- 2. Machine configuration and features
tric injection molding machine model the 3000 em with Model 3000 em is the next-generation injection mold-
a 3,000 ton mold clamping force and its sister model 3500 ing machine for large automobile parts and has realized
em with increased mold clamping force, both of which its concept of meeting customer demands for reduced
are equipped with a 2-platen type mold clamping mecha- machining, material and running costs. Figure 1 is an
nism and a hybrid motorized system. These machines outline of the technological developments.

Electric control of
Electromechanical Large capacity High-speed electric
internal mold pressure
split nut mechanism AC servomotor plasticizing unit
waveform

4-point even mold

clamping mechanism MD type UB screw
Center press structure
AC servo mold opening
and closing AC servo drive
(ejector lap action) + low noise eco-pump system

Fig. 1 Outline of technological development

*1 Nagoya Research & Development Center, Technical Headquarters Mitsubishi Heavy Industries, Ltd.
*2 Mitsubishi Heavy Industries Plastic Technology Co., Ltd. Technical Review Vol. 43 No. 3 (Sep. 2006)
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 Table 1 Comparison of specifications

Electric injection molding Hydraulic injection molding

Item machine 3 000em machine 3000 MM
470 470
Screw diameter (mm) 150 150
Screw L/D - 22 18
Mold clamping system - Electro-hydraulic system Hydro-mechanical system
Mold clamping force (kN (tf)) 29 420 (3 000) 29 420 (3 000)
Mold opening force (kN (tf)) 1 824 (186) 1 824 (186)
Daylight, maximum (mm) 3 700 3 700
Mold thickness (mm) 1 000 - 1 900 1 000 - 1 900
Maximum clamping stroke (mm)
Clamping

2 700 2 700
Tie bar space H x V (mm) 2 050 x 1 900 2 000 x 1 800
Platen size H x V (mm) 3 200 x 2 500 2 550 x 2 450
Minimum mold dimensions (mm) 1 290 x 1 290 1 290 x 1 290
Mold opening/closing speed
(mold closed/mold open) (m/min) 50 42/36

Ejector stroke (mm) 350 350

Ejector force (kN/min) 392 (40) 686 (70)
Ejector speed (forward/reverse) (m/min) 12.5 5.6/3.9
Injection stroke (mm) 750 750
Theoretical injection capacity (cm3) 13 200 13 200
Injection mass (0 Z.) 466 466
Screw rotational speed (rpm) 167 118
Maximum injection pressure (MPa) 177 161
(kgf/cm2) (1 800) (1 640)
Maximum injection rate (cm3/s) 2 030 1 275
Injection

Maximum holding pressure (MPa) 147 161

(kgf/cm2) (1 500) (1 640)
Injection horsepower (kW) 359 205
(PS) (488) (278)
Plasticizing capacity (PP) (kg/h) 860 680

Nozzle touch force (kN (tf)) 98 (10.0) 59 (6.0)

Machine dimensions (L x W x H) (m) 15.2 x 4.8 x 4.0 13.2 x 4.8 x 3.9
1.14 compared with hydraulic
Energy consumption (kWh/shot) injection molding machine 43 % 2.63 (Hydraulic system)
(AC servo motor system)

Table 1 compares its main performance specifica- benefit of the high-speed, high-accuracy repeatability of
tions with those of MHI's model 3000 MM III hydraulic the electric system and the benefit of a hydraulic sys-
injection molding machine. tem which is excellent in controlling large operating
The most outstanding feature of the 3000 em is its forces, the machine is space conservative (the machine
employment of MHI's unique, space conservative 2-platen is less than 15 meters in length, about 25% shorter than
type mold clamping mechanism. The machine employs other machines with a toggle link type mold clamping
a hybrid system that combines the mold opening and clos- mechanism) and is energy efficient (reducing energy con-
ing system of the movable platen by a servo motor, and a sumption by 60% compared with hydraulic injection
hydraulic clamping mechanism driven by an energy ef- molding machines), making the 3000 em the world's
ficient, low noise eco-pump system. Thus, given the shortest electric injection molding machine.

Mitsubishi Heavy Industries, Ltd.

Technical Review Vol. 43 No. 3 (Sep. 2006)
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 To reduce customers' running costs by eliminating 3.2 Developing method of mold opening deformation
flash on molded products with burr-less molding, MHI analysis
has developed a new type center press platen which has A method of predicting the gap formation between
MHI's unique center press structure and electric control mold elements is necessary at the design stage of an
of the internal mold pressure waveform that exercises injection molding machine to eliminate flash from
4-shaft synchronous control over MHI's large capacity, molded products. In addition to the conventional
quick-response DD (direct drive) servomotor mounted on strength and rigidity design of each single platen, a
the injection shaft drive mechanism to regulate the mol- model with the mold located between the platens (sta-
ten resin pressure in the mold to low. tionary and movable) was created as shown in Fig. 3 to
The high-speed mold opening and closing and the nut establish a method of analyzing the gap formation be-
opening and closing system and the UB screw equipped tween mold elements by using contact analysis. First,
with an MD melter dedicated to the bumper material the molten resin pressure distribution in the mold ob-
realizes the world's fastest production cycle. The cycle tained by mold flow analysis was analyzed even further
time of 45 seconds of MHI's conventional model has been as boundary parameters for the structural analysis to
reduced by 30% to a fast 30-second cycle, significantly obtain the contact load distribution over the mold part-
improving productivity. ing faces. As shown in Fig. 44, the parting faces are not
in contact with each other over a relatively wide area.
3. Center press platen structure and burr-less molding
Particularly, the area around the gate where the mol-
3.1 Center press platen ten resin pressure is relatively high is predicted to be
Since the 2-platen type mold clamping mechanism in the condition of mold opening. Actual injection form-
applies the clamping force evenly to the four tie bars, ing tests took place using the mold of an automobile
the effect of the mold temperature and molds with an part where the gap formation between mold elements
eccentric cavity profile is less than that with the toggle was measured at the gap sensor positions.
type mold clamping mechanism. This enables the clamp-
ing accuracy to be maintained for a long period of time
among other benefits. However, similar to the toggle Stationary platen
type mold clamping mechanism, the 4-point mold clamp-
Movable platen
ing mechanism has the shortcoming that the bending
deformation of the platen supported at both ends is likely
to adversely affect the mold compared with a center press
straight hydraulic type injection molding machine. More
particularly, this caused the mold to open, making flash-
ing more likely.
Figure 2 compares the mold clamping mechanisms.
MHI conducted a mold opening deformation analysis of
a structure that combines the platen structure and the Z
mold structure. Based on the analysis results, MHI has Y
developed a unique center press platen that employs a X
Movable mold Stationary mold
rib structure to allow the lines of force to flow from the
tie bar loading points through the center of both mov- Fig. 3 A model for analyzing the gap formation between mold
able and stationary platens so that the forces apply elements
evenly in a similar manner to the straight hydraulic type.
The new platen reduces the gap formation between mold
elements, which in turn restricts flashing.

Center press mold Conventional mold Toggle type mold Molten resin Contact force distribution
clamping mechanism clamping mechanism clamping mechanism pressure distribution on mold parting faces

Fig. 2 Comparison of mold clamping mechanisms Fig. 4 Internal mold analysis

Mitsubishi Heavy Industries, Ltd.

Technical Review Vol. 43 No. 3 (Sep. 2006)
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 Figure 5 compares the actual measurements with the 3.4 Burr-less operation by center press platens
results of analysis. The analyzed values are in the range The new method of analyzing the gap formation be-
of -30% to +10% of the actual measurements, confirm- tween mold elements was used to exam a mold that
ing that the newly developed method is practical for simulated a bumper profile to compare the gap forma-
analyzing the gap formation between mold elements tion for the new type center press platen to that of the
quantitatively. conventional platen. As shown in Fig. 7 7, the new center
3.3 Gap formation between mold elements at limit of press platen reduces the gap to nearly zero at the cen-
flashing tral part E of the mold. With this outstanding
To investigate the correlation of the gap formation be- improvement in the gap around the gate, the new center
tween mold elements with the limit of flashing, injection press platen is expected to reduce flashing significantly.
molding tests were done with gap sensors built into the
mold to measure the gap formation between mold elements 4. Electric control of internal mold pressure waveform
when flashing occurred. As shown in Fig. 6 6, flashing oc- To cope with injection molding of thin walls for
curs when the gap formation between mold elements bumpers, the 3000 em employs newly developed multi-
measured by the gap sensors are at a certain level and shaft synchronous control that synchronously drives the
higher. Similarly, the measurement at each point confirms four shafts of the direct drive (DD) servomotors and the
that flashing is related to the gap formation between mold four shafts of the ball screws without using such means
elements. Since flashing occurs when molten resin pen- as synchronous drive belts. This reduces the inertia in
etrates the gap created between mold elements, the gap the injection mechanism and improves the speed which
formation at the flashing limit depends on the viscosity, is essential for thin-wall molding and at the same time
temperature, pressure and other parameters of the resin. eliminates the factors that would otherwise deteriorate
MHI further studied the flashing limit of the polypropy- the control accuracy stability due to the elongation or
lene used for injection molding automobile bumpers. dislocation of the belts.

Accuracy of analyzing gap : Injection pressure (MPa)

formation between mold elements +10% : Internal mold pressure (MPa)
-30 to +10%
Gap formation between

: Gap
-30%
mold elements
Analysis

: Gap sensor
Opening Completing to Opening
maximum hold pressure mold
Actual measurement
No flashing
Fig. 5 Comparison of gap formation between mold elements
between actual measurements with analyzed results Condition No.1: Holding pressure 5%
Mold clamping pressure 40%

: Injection pressure (MPa)

: Internal mold pressure (MPa)
Gap formation between

Conventional : Gap
Points for calculating
mold elements

gap formation between mold clamping

mold elements mechanism
between mold elements
Gap formation

C E
B
D
Limit of flashing
Center press type
mold clamping
mechanism Opening Completing to Opening
maximum hold pressure mold
0
Flash present
B C D E
Condition No.2: Holding pressure 15%
Distance between points B and E Mold clamping pressure 40%

Fig. 7 Comparison of gap formation between mold elements between new Fig. 6 Results of measuring gap formation between
center press platen and conventional platen mold elements

Mitsubishi Heavy Industries, Ltd.

Technical Review Vol. 43 No. 3 (Sep. 2006)
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 In the injection control, the rising of injection speed electric injection molding machine to control the inter-
and switching from charging to pressure-holding control nal mold pressure waveform that was otherwise possible
are critical. To this end, it is necessary to make the in- only with hydraulic injection machines. The electric con-
ertia of the injection ball screw drive mechanism small. trol of the internal mold pressure waveform and
Since the conventional speed reduction mechanism multi-gate filling (sequence valve control) has brought
linked by timing belts and large-diameter pulleys has a about stable, low pressure molding. Figure 10 shows
large inertia which lowers the response, the control the effects of reduced internal mold pressure in the
method alone has a response improvement limit. There- bumper molding. The synergetic effect together with the
fore, mobilizing MHI's high-speed technology with its center press platen structure prevents flashing from oc-
unique motor electromagnetic analysis engineering and curring in the bumper molding.
synchronous control, MHI has developed large capacity
DD servomotors with low rotational speed and high 5. High plasticization and high cycle
torque characteristics dedicated to the injection mold- In the injection molding of bumpers, the highest op-
ing units (see F i g . 8 ) . Instead of a mechanical eration rate of MHI's hydraulic injection molding machine
synchronous drive linked by synchronous belts, the 3000 3000 MM III is 45 seconds per cycle. Achieving an even
em employs synchronous control to drive the four shafts higher rate of 30 seconds per cycle requires a plasticizing
of the ball screw as shown in Fig. 9 9. The 4-shaft direct capacity of 860 kg/h, for a bumper weight of 4.3 kg, com-
drive injection mechanism enables the 3000 em to de- mensurate with the cycle time and a reduction of resin
velop an injection rate of 2,030 cc/s, the highest for 3,000 temperature dispersion at a screw rotation speed as high
ton class machines, at an injection pressure of 1,800 kgf/ as 167 rpm. Particularly, an improvement in blend char-
cm2 while achieving a speed response that compares with acteristics by preventing unmolten resin from being mixed
the hydraulic servo specifications. and a uniform distribution of resin temperatures are es-
The reduced inertia and improved response allows the sential to ensure the required bumper quality.

MHI's high-torque servomotor

(1) Internal mold pressure sensor
Low rotational speed,
high torque (2) For normal injection molding
Torque

(1 point gate)
Speed reduction (3) For control of internal mod pressure
mechanism not necessary waveform (1 point gate)
General purpose servomotor (4) For control of internal mod pressure
High rotational speed, waveform (3 points multiple gates)
low torque (5) Pressure necessary for completely
filling flow end, 80 to 100 kgf/cm2
Rotational speed

Fig. 8 Large-capacity DD servo motor

Injection pressure F/B

Load cell
Movable
PG Position F/B
section (1)
AMP 1
Motor 1 (2)
Internal
Mold Motor 2 PG Speed mold (3)
DSA pressure
Screw command
AMP 2
Synchronous MIU 2 000
control (4)
AMP 3
Position
Motor 3 PG command
(5)

Motor 4
AMP 4 Distance from center
Internal mold PG Position F/B
pressure sensor
Fig. 10 Effect of reducing internal mold
pressure for bumper injection molding
Internal mold pressure F/B

Fig. 9 4-shaft synchronous control system

Mitsubishi Heavy Industries, Ltd.

Technical Review Vol. 43 No. 3 (Sep. 2006)
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 Long barrier zone
MD melter
UB screw
Multi-dam mixing

Liquid phase resin updating Making finer

and thinner
Direction of a 1
A
rotation Solid phase Melt (liquid phase)
Melt
A groove
a 1

Cross-section A-A Solid groove

: Direction of resin flow

a : Flight cut section Development of
1 : Dam groove

Fig. 11 UB screw with MD melter

As shown in Fig. 1 11
1 , the long barrier for the separa-
tion of liquids and solids on MHI's double-flight type UB Plasticizing capacity necessary to
860 kg / plasticize a 4.3 kg bumper for 18 seconds
screw segregates unmolten resin to ensure a homoge-
Plasticizing capacity (kg/h)

1 200
neous plasticizing quality. Furthermore, the UB screw 1 100
has at its end an MD melter for multi-dam mixing to 1 000
improve the blend characteristics during high cycle in- 900
jection molding. The multi-dam MD with a polygonal 800
cross-section creates an optimal shearing and division 700
600
motion to achieve a significantly high level of uniformity
500
in the molten resin. Figure 12 shows the plasticizing 400
capacity of this screw measured when injection molding 100 130 167 30 35 40

bumpers. It has a plasticizing capacity that exceeds the Screw rotational speed (rpm) Injection molding cycle (s)

target value by an ample margin during the high cycle Screw: 150 MD-UB screw
injection of 30 seconds. The reduction in the dispersion Resin: PP
of resin temperatures also measured during the high
Fig. 12 Plasticizing capacity for injection molding of bumper
cycle injection significantly improves the plasticizing
quality of molten resin. Thus, the UB screw equipped
with the MD melter realizes the high speed injection high cycle operation. The machine offers high productiv-
molding of bumpers. ity with reduced running costs to not only the
manufacturers of large injection molding products for
6. Conclusions automobiles but also to plastic injection molding machine
To meet the demands for improved quality and reduced users such as those producing large household electric
injection cycle time for large injection molding products, apparatus. To keep answering customers' needs, MHI is
MHI has developed and put on the market the energy ef- willing not only to improve the machine performance but
ficient, space conservative, super-sized electric injection also to develop new machines to improve the quality of
molding machine model 3000 em which realizes burr-less injection molding products.

Shiki Matsuo Takamitsu Himeno Masahiro Bessho Kenji Tsutsui Naoki Kato Takashi Mizuno

Mitsubishi Heavy Industries, Ltd.

Technical Review Vol. 43 No. 3 (Sep. 2006)
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