Dimensional Study of Thermoplastic Parts Made UsingSequential Injection Molding

Jorge Aisa, Javier CastanyT.I.I.P.—C.S.I.C. Associated Unit, Mech. Dpt of the University of Zaragoza, C/Maria de Luna 3, 50018,Zaragoza, Spain

As a result of new aesthetical and economical require-ments in injection molding, several processes havebeen developed during last years. In spite of conven-tional process, sequential injection molding is a versa-tile technique based on independent control for eachgate opening. This procedure allows part filling withoutweld lines or, at least, these defects can be relocatedin suitable positions. A short discussion about sequen-tial principles is made introducing computer simula-tions. Pressure required for part filling and moldingshrinkage were evaluated using experimental proce-dures with a prototype mold and using conventionaland sequential methods. Several setting up parameterswere also used to analyze the effect on injection pro-cess. Final results show that shrinkage in sequentialinjection is similar to conventional process but somenew effects could be induced by the process asymme-try and should be investigated further. POLYM. ENG.SCI., 49:1832–1838, 2009. ª 2009 Society of Plastics Engineers

INTRODUCTION

Injection molding is the most popular process of plastic

manufacturing for complex parts, because of its economical

advantages for large series, different shapes capabilities,

and high production rates. In the same way, narrow toleran-

ces can be achieved if the mold and the process are prop-

erly designed. All these reasons make the injection molding

process a necessary element for automotive industry, espe-

cially when all companies are interested in offering attrac-

tive parts and lighter components, reducing car fuel con-

sumption. In addition, films or textile layers can be over-

molded, getting a new appearance of aesthetic parts.

Basic rules for injection molding design are well

developed in different research articles and literature since

the 80s [1, 2], and the computer aided engineering (CAE)

changed the traditional sequential development method to

a concurrent one where all the actors involved are work-

ing together from the beginning of the project [3].

Injection molding has many variations [4] over the ba-

sic procedure to produce new parts improving their stiff-

ness or reducing assembly operations, for example. These

special injection methods, not only sequential injection

but gas-assisted or injection-compression, are not always

analyzed with care, giving to their users inadequate

results. For example, gas-assisted technology offered

shorter cycle times but could not be used easily, because

it required specific part design, or gas extends in non uni-

form ways inside the thickness of plastic part, giving a

non adequate aspect [5, 6].

In this way, there are no technical references for sequen-

tial injection molding far from its basic definition and its

general advantages [7–11]: this technology uses independent

gate opening control on each injection point, operated by a

computer system. The mold requires a hot runner system,

Fig. 1 (mold used in this work), and a control device, add-

ing new costs not required in a conventional system (mold

investment plus operating costs). Figures 2 and 3 show a

conventional filling sequence for a rectangular part using

two injection points. Weld line will be placed between

injection gates, and the aesthetical aspect and the mechani-

cal strength will be poor. Figures 4 and 5 show sequential

injection process for the same plastic sample: first, only one

gate is open; when polymer flow reaches the second gate

this will be opened, avoiding in this way weld lines caused

by shock of both flow fronts. After this action, generally

previous gates remain closed. At the beginning of the pack-

ing phase, all the gates will be opened again, to improve

part quality. Figure 6 shows conventional filling of real

parts, and Fig. 7 illustrates the sequential process. Authors

describe in [12] that weld line elimination in sequential pro-

cess, giving interesting results about mechanical behavior of

different samples.

This injection molding technique is much used for

complex parts which can not present visible weld lines as

dashboards or car bumpers. Plastic painted parts are usu-

ally made by sequential injection too due to flaws caused

by weld lines. The designer has to combine weld line

position and general rules during mold design, and many

items are manufactured successfully (as dashboard or car

bumpers).

Correspondence to: J. Aisa; e-mail: jorge.aisa@unizar.es

DOI 10.1002/pen.21310

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2009 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2009

However, designers should be careful with common

rules cited above, because under simple rules, the position

of some gates may increase clamping force required due

to ‘‘overpacking’’ effects [13–15].

This article is focused on results about shrinkage in

injection molding comparing conventional and sequential

samples made using a prototype mold. Other added results

are the needed pressure to fill the cavity and the sample

weight. Both are included to explain some significant dif-

ferences between both technologies. All these data were

obtained at T.I.I.P. research group, Unit Associated to

C.S.I.C., (University of Zaragoza, Spain) with the support

of Foundation a.i.T.I.I.P. (www.aitiip.com), which lent its

facilities for the injection trials.

MATERIALS AND METHODS

For this study, a prototype mold was made. The mold

was planned looking for a frontal weld line and it is also

prepared, changing the cavity plate, for injecting an

adjacent weld line in the future as the literature explains

[16, 17]. Figure 1 shows a scheme of the actual part mod-

eled using C-Mold software (AC-Technology), showing

the hot runner system and the layout of mold cooling

lines. This computer program is used in this article to

illustrate how the flow fills the mold in the types of injec-

tion described.

The specimen tested is a rectangle, 450 3 150 mm,

2.5 mm thick, with two injection points separated

150 mm. A Mold-MastersTM hot runner was used equipped

with two DuraTM hydraulic valve gates. This disposition

generates a weld line very well defined, when conventional

process is used and the defect can be avoided with

FIG. 2. Step 1 of conventional melt front advancement for a rectangu-

lar part using two gates (simulated with software C-Mold).

FIG. 3. Step 2 of conventional melt front advancement for a rectangu-

lar part using two gates (simulated with software C-Mold).

FIG. 4. Step 1 of sequential melt front advancement for a rectangular

part using two gates, Only the first gate is open, second gate remains

close avoiding weld lines (simulated with software C-Mold).

FIG. 1. Prototype mold layout: a single rectangular cavity filling using

two valve gates using conventional or sequential injection molding. Part

dimensions are 450 3 150 mm; thickness 2.5 mm; distance between

gates 150 mm.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 1833

sequential injection. Indirect gate was added between the

hot runner and the cavity to improve the thermal isolate

from the manifold plate, eliminating ‘‘hot spots’’ and to

look for symmetry in the thermal conditions for both

faces of the plastic part. The reader can revise and com-

pare again the melt front advancement simulated with

both injection technologies at Figs. 2 and 3 (conven-

tional); at Figs. 4 and 5 (sequential process); and, further-

more, the real samples injected in the Figs 6 and 7.

Mateu&Sole 340 tons injection machine was used,

equipped with a hydraulic pressure sensor. This device

was combined with three pressure cavity sensors in the

mold. These transducers were mounted in significant posi-

tions: near to one gate, center of the part—where the weld

line is formed, and at the end of filling close to the edge

of one short side. Figure 8 indicates those locations using

an image captured from C-Mold software, reproducing a

sequential process (notice that two different flow direc-

tions can be selected, we will discuss later).

An independent data recorder (Dataflow software, from

Kistler) was used for register the information from

machine and the three cavity pressure transducers.

Under each set-up conditions, eight shots were made to

calculate average values. The sequential injection needs a

control device for valve gate opening, using the controls

of the injection machine or an independent computer

desk. New software was developed by the a.i.T.I.I.P. team

with this intention.

A polypropylene grade for automotive components,

DSM Kelburon 95694, was selected for all experiences,

(this material is used for bumpers, and it has an EPDM

fraction to improve its behavior in the impact tests).

FIG. 5. Step 2 of sequential melt front advancement: second gate is

opened to complete the part (simulated with software C-Mold).

FIG. 6. Conventional injection filling for the specimen used in this

work [12].

FIG. 7. Sequential injection filling for the specimen used in this work

[12].

FIG. 8. Sequential filling, simulated by C-Mold software and cavity

pressure locations.

1834 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen

Setting up conditions were selected around generic

producer recommendations: (a) temperature 215 and

2308C for plastic material and hot runner zones; (b) the

filling of the mold cavity was made under ram speed con-

trol at constant flow rate until 95% of the cavity volume;

(c) two values of filling time were used, 1 and 3 s, and

finally, (d) during the packing phase two constant pressure

levels were applied: 20 and 35 hydraulic bar. The result

of cooling time plus the packing time remained constant

at 45 s.

As we presented before, the sequential injection allows

two directions for mold filling, depending on which valve

gate was operated in first place. So, we are able to com-

pare the experimental results for first sequential direction

(see Fig. 8) and sequential reverse direction (see Fig. 9).

Notice that the pressure transducers were not placed in a

symmetrical way for both directions in mold filling,

because they are fixed in their special locations.

RESULTS AND DISCUSSION

In the conventional process, all the samples show a

visible weld line located in the middle of the part. For the

sequential process, this defect is eliminated in all the

experiences and the appearance of the part is improved.

This is the main reason that forces the designers to use

sequential injection molding. Authors describe in [12] that

some results evaluating the mechanical behavior under

tensile forces for specimens injected using conventional

and sequential methods, which show that weld line reduce

the tensile strength and make brittle fracture in normal-

ized samples, according to Stevenson [9].

Other two basic results were obtained: first, experimen-

tal data from hydraulic pressure and cavity pressure to

show differences between both technologies and to evalu-

ate the integral of pressure; and, in the other hand, weight

and main dimensions for injected samples, using a preci-

sion balance and a digital caliper. The complete results

are not shown in order to make much easier the text; all

the experiences had similar tendencies.

Table 1 shows the selected results of the maximum hy-

draulic pressure required depending on the process type.

These experiences indicate that pressure in the sequential

process was higher than in conventional filling. This is an

expected result because the distance of flow when plastic

reaches the second gate is longer in the sequential pro-

cess. Notice that this maximum pressure does not always

occur at the end of the filling phase, it is connected with

the design of feeding system. This high internal pressure

forces the raise of pressure level to complete the cavity

during packing phase. For example, several samples were

unfilled until an increase for pressure in the packing phase

in order to force the material to reach the end of the cav-

ity (sequential injection case with temperature set at

2158C and filling time at 1 s required an increase of pack-

ing pressure from 20 to 25 hydraulic bar).

Both results indicate that sequential process should not

be considered as ‘‘low pressure process’’ in the technical

literature without further remarks. Only when the mold

maker increases the number of injection points or com-

FIG. 9. Sequential filling, reverse flow, simulated by C-Mold software

and cavity pressure locations. Compare this filling pattern with Fig. 8.

TABLE 1. Compared pressure for conventional and sequential process

under several conditions.

Conventional

process

Setting conditions for both

processes. Screw rotation speed: 55

rpm; Screw diameter; 75 mm. Total

cycle time: 50 s

Sequential

process

Melt

temperature

(8C)Filling

time (s)

Holding

pressure

55 bar 215 1 35 bar 69 bar (þ25%)

44 bar 230 3 35 bar 49 bar (þ11%)

43 bar 230 3 20 bar 53 bar (þ19%)

53 bar 230 1 35 bar 66 bar (þ25%)

Maximum hydraulic pressure required at end of filling stage.

TABLE 2. The value of integral pressure under different setting conditions (bar s).

Conditions

2158C 1 s 35 bar 2308C 3 s 20 bar 2308C 1 s 35 bar

Conventional Sequential Conventional Sequential Conventional Sequential

Hydraulic 457 458 330 343 452 456

Gate 3354 3405 2048 2312 3366 3457

Weld line 2403 2522 784 1193 2608 2792

Cavity end 907 1118 110 332 1149 1338

Hydraulic values show filling parameters accuracy.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 1835

bines the sequential process with additional technologies

as injection—compression, this sentence will become true.

Mold design and operative costs have to be considered as

these restrictions before to plan the manufacture of new

plastic parts using this technique.

Table 2 shows values of the pressure integral for both

processes. This value is strongly connected with related to

dimensional quality and mechanical properties of molded

parts, as it is well established in the literature. The inte-

gral of hydraulic pressure shows that all the experiences,

independently from process type, could be compared

because this value is the same.

Notice that, depending on flow direction, this value of

the integral of pressure is different for the gate and cavity

end position in all the examples (Table 3). The value of

integral of pressure for the conventional process is placed

between sequential ones depending on plastic flow direc-

tion. These differences indicate that local shrinkage will

not be the same depending on selected process, or even

more, depending on flow direction.

This asymmetric effect could be observed from other

perspective regarding Figs. 10, 11, and 12, where we

compared under identical setting conditions pressure

curves from each transducer. In the Fig. 10, the sensor

placed in the weld line shows a trace independent from

filling flow direction, whereas Figs. 11 and 12 pointed out

the effect of flow direction ‘‘close to the gate transducer’’

and ‘‘far from the gate transducer’’. The conventional pro-

cess has no differences because its filling is symmetric

and easily anticipated.

After these results, and according to theory of shrink-

age and PvT diagrams [18, 19], local flaws as sink marks

or voids can not be well predicted or controlled if pres-

sure is not uniform in the mold during process.

In authors opinion, this asymmetric filling is the

most significance characteristic of sequential process

for this mold, and probably it has to be considered for

many others parts as door panels, dashboards, or other

aesthetical automotive parts to get a proper mold

designed.

This conclusion allows suggest a basic rule for mold

designers: a symmetric filling should be regarded to avoid

unexpected shrinkage behavior, or differences will be

shown between both sides of the final component.

FIG. 10. Cavity pressure data for weld line location, sequential injec-

tion in continue line; reverse sequential injection in dot line. Values are

identical due to symmetrical transducer position on part axis. Setting

conditions: temperature 2158C, filling time 1 s, packing pressure 35 bar.

FIG. 11. Cavity pressure data for close gate sensor, sequential injection

in continue line; reverse sequential injection in dot line. Asymmetrical

registers are shown depending on melt flow direction. Setting conditions:

temperature 2158C, filling time 1 s, packing pressure 35 bar.

TABLE 3. The value of the integral pressure under different setting conditions (bar s).

Conditions

2158C 1 s 35 bar 2308C 3 s 20 bar 2308C 1 s 35 bar

Conventional Sequential

Sequential,

reverse flow

direction Conventional Sequential

Sequential,

reverse flow

direction Conventional Sequential

Sequential,

reverse flow

direction

Hydraulic 457 458 460 330 343 347 452 456 456

Gate 3354 3405 3239 2048 2312 1935 3366 3457 3298

Weld line 2403 2522 2494 784 1193 1187 2608 2792 2768

Cavity end 907 1118 863 110 332 179 1149 1338 1147

Flow direction is compared for the sequential process.

1836 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen

For the whole injected part, the measures (length and

width) that we get when we use both processes are similar

as Table 4 shows. The small differences are in the error

limits of the measurement device. Linear shrinkage was

encountered between the material limits (around 2–2.2%)

for all samples, mold—makers can use data from material

suppliers to machining molds.

However, differences were found when measuring the

both sides of the piece in the samples made with the se-

quential process. These changes were not found in con-

ventional process. As we said before, results show an im-

portant divergence for sequential process depending on

flow path, and this reason produces the alterations.

Finally, all the parts were weighed and heavier parts

were made, in the most cases, using conventional injec-

tion. Table 5 shows these values. The conventional tech-

nique seems the best choice under the criterion of higher

weight, although differences are small due to small

weight of the part. As we can imagine, the aspect of big

components could be affected when the sequential pro-

cess was used, if sink marks could appear due to the

presence of ribs under some aesthetical surfaces, or espe-

cially with asymmetrical fillings if there is one main

flow direction.

CONCLUSIONS

Sequential injection is widely used in many molds

nowadays because of high aesthetical requirements

imposed to plastic parts. Weld lines are eliminated or

moved to hidden locations using an adequate sequence of

valve opening. However, mold makers and automotive

suppliers are not always prepared to understand and

assume all new commitments that this new technology

implies.

In this article, a prototype mold was designed and

injected using a 340 ton machine. Comparative values of

shrinkage were obtained using conventional and sequen-

tial processes in plastic parts injected.

For mold shrinkage, similar values were encountered

and data provided for material suppliers can be considered

as correct for both techniques. In spite of this conclusion,

manufacturers could discover unexpected defects when

the cavity will be filled with an asymmetric way using se-

quential injection. These flaws will be much more signifi-

cant when the size of the piece increases, because the gra-

dient of pressure will be higher

Under the weight criterion, the conventional process is

better if the injection conditions are equal. Even more,

using some conditions, some short shots were encountered

with sequential injection molding because higher filling

pressure was needed.

Finally, the filling pressure and, logically, the clamping

force, will be greater for sequential process because of

longer flow path if the design of the tool is not considered

in the mold layout. To reduce pressure and clamp force,

new gates should be added.

New questions are opened now, for example, does this

technology affect to the warpage phenomenon? Do the

criteria of mold design have to change in sequential injec-

tion molding? We hope that the answers are affirmative,

but values need further works.

TABLE 4. Experimental measurements for different samples produced using conventional and sequential injection.

Conditions 2158C 1 s 35 bar 2158C 3 s 20 bar 2158C 3 s 35 bar 2308C 1 s 35 bar

Conventional

Longitudinal measurement (mm) 444.9 443.6 444.6 444.8

First transversal measurement (mm) 147.8 147.3 147.8 147.9

Second transversal measurement (mm) 147.8 147.3 147.8 147.9

Sequential (first or reverse direction) First Rev Rev First Rev Rev

Longitudinal measurement (mm) 444.8 444.8 443.8 444.9 444.7 445.0

First transversal measurement (mm) 147.8 147.8 147.6 147.8 147.8 148.0

Second transversal measurement (mm) 147.9 147.7 147.4 148.0 147.9 147.8

First transversal measurement was considered in the end part sensor side; second transversal opposite side.

FIG. 12. Cavity pressure data for part end location, sequential injection

in continue line; reverse sequential injection in dot line. Asymmetrical

effects were encountered again due to flow advance during filling stage.

Setting conditions: temperature 2158C, filling time 1 s, packing pressure

35 bar.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 1837

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TABLE 5. Experimental weight values for samples produced using conventional and sequential techniques.

Sample conditions 2158C 1 s 35 bar 2158C 3 s 20 bar 2158C 3 s 35 bar 2308C 3 s 35 bar 2308C 3 s 20 bar 2308C 1 s 35 bar

Conventional weight (g) 136.0 123.2 133.9 134.8 123.6 136.1

Sequential weight (g) 134.9 124.1 133.5 133.7 123.7 135.9

For this value, flow direction of sequential filling has no effect.

1838 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen