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Process of Injection molding is described in detail. As a result the main infliuencing factors are identified. There influence on the responses is verified by experiments in small and large scale. Further more, a strategy is given how to use this knowledge to set up the process almost indepent from the machine used. This will be achieved by combining the Idea of the "Oerating Window" and "Statistic Experimental Design" technique.
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Understanding of
Rubber Injection Moulding
Dr. Hans-Joachim Graf
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Understanding of Rubber Injection Moulding
Author: Dr. Hans-Joachim Graf
Author
Dr. Hans-Joachim Graf
Josef Leistenschneider Strasse 27
63628 Bad Soden Salmünster GERMANY
Tel: +49 (0)151 2000 6394
E-Mail: graf.hans-joachim@t-online.de
Publisher
TechnoBiz Communications Co., Ltd.
2521/27, Lardprao Road, Khlongchaokhunsingha
Wangthonglang, Bangkok 10310 Thailand
Tel: +66-2-933 0077 Fax: +66-2-955 9971
Web: www.rubber-industry.org
Copyright (c) TechnoBiz 2018
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Contents
1. Introduction 5
2. The Injection Moulding Machine 8
3. Process Settings of an Injection Moulding Machine 15
4. Strategy for parameter setting process modelling with DOE 27
5. Window in Injection Moulding 42
Literature 54
Author Biography 60
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1. Introduction
Injection moulding is recognized as one of the most important processes for
manufacturing rubber molded parts. It is the preferred method over compression
and transfer moulding, especially if large volumes of parts have to be produced
(1,2,3,4,5,6,7,8,9,10). It also has advantages over compression and transfer
moulding in terms of part quality, cycle times and efficiencies.
Figure 1: Compression Molding Sketch
In the case of Compression Moulding [CM] (Figure 1), preformed pieces of a rubber
compound are placed into a steel mould. The rubber is vulcanized through heat
transfer from the mould. The vulcanization time depends upon the thermal
conductivity and heat capacity of the rubber but can be shortened somewhat by
preheating the rubber before loading it into the mold cavity (11,12).
If Transfer Moulding [TM] is used, the compound is transferred from a reservoir
through short gates into the cavities (Figure 2). This process is carried out during
closure of the mould. The compound can be preheated before putting it into the
transfer cylinder, but the temperature has to be carefully controlled during its
residence time. It is transferred with a slow speed through the gates, causing
minimal heat rise. The part, as with CM, is vulcanized mostly through heat transfer
from the mold (13,14,15,16).
Figure 2: Transfer
Molding Principle
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A variation of TM is the Injection Transfer Moulding (ITM) process. The difference
between this process and TM is the compound is preheated by a plastification unit
and then poured into the transfer reservoir before being injected into the cavities.
Because of this the compound temperature in the reservoir will be higher and more
homogeneous compared to that seen in the TM cylinder; however, its cavity
entrance temperature will still be only a little higher than in TM (17,18,19).
Finally there is the Injection Moulding [IM] process (Figure 3) . This process was
originally developed for the manufacture of thermoplastic materials. This is, why the
machine and the controls are similar for both processes. Processing rubber with this
equipment has been made possible by adjustments to the plastification unit,
specifically to the temperature controls of the screw and plunger cylinder - they are
heated, instead of cooled (1-10, 20, 21, 22). The advantage of the IM process lies
with the plastification unit. It is here that the temperature of the compound is
increased and thus plasticized. It is then injected through a runner and gates into
the cavity under much higher pressures compared to TM and ITM. The length of the
runner causes significant heat rise of the compound.
Consequently, the temperature of the compound is already close to that of the mold
surface temperature as it enters the cavity. Higher mould temperatures can be
applied, because of smaller temperature differences inside and outside the part.
This in fact shortens the vulcanization time. The force of the clamping unit keeps the
mould closed against the forces created by thermal expansion of the injected rubber
stock, which allows to produce parts with high precision.
Figure 3:
Injection
Molding -
Sketch
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