Lattice gate for injection molding of rubber compounds

Details Lattice gate for injection molding of rubber compounds Lattice gate for injection molding of rubber compounds

1999-12-07

2002-04-30

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

Shaping against forming surface

C264S328120, C425S543000, C425S573000

Reexamination Certificate

active

06379603

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an improved gate design for injection molding of rubber compounds. More particularly, the present invention relates to an improved gate design for injection molding of rubber compounds with increased gate heating efficiency and reduced cycle time for curing the rubber part being injection molded.
BACKGROUND OF THE INVENTION
In a typical rubber injection molding process, the uncured, viscous rubber compound is introduced into the elongated barrel of an injection molding machine at ambient temperatures. It is advanced through the barrel towards a mold connected to the downstream end of the barrel, usually by either a rotating screw conveyor or a reciprocating ram or piston disposed in the barrel. As the rubber compound advances, it is heated by heat conduction and mechanical shear heating in the barrel to reduce its viscosity and render it more flowable and amenable to subsequent injection into the mold. Typically, the less viscous the rubber compound, the more easily it flows through the runners and gates and the more easily it fills the mold cavity to produce a satisfactorily molded object.
Since curing of the rubber compound is a “time at temperature” phenomenon, the heating also serves to supply some of the “time at temperature” requirement in the barrel without prematurely curing or scorching the compound in the barrel. This increase in temperature also reduces the “time at temperature” required in the mold and consequently the vulcanization cycle time. As known in the art, most rubber compounds can be cured through either a shorter exposure to a higher temperature or a longer exposure to a lower temperature, and it is this phenomenon that is referred to herein by the term “time at temperature ”.
Cure time in ram injection rubber molding, for example, consists of three separate and distinct “time at temperature ” periods. The first is the “time at temperature” during the compounding and storage of the material prior to introducing the rubber into the upstream end of the barrel of the injection molding apparatus and is referred to as the “process scorch time ”. The second “time at temperature” is the “residual scorch time” or the time permitted in the barrel of the injection molding apparatus before incipient cure occurs. The higher the “time at temperature” during the process scorch time, the shorter will be the “time at temperature” permitted for the residual scorch time in the barrel of the apparatus. The third “time at temperature” is the “vulcanization time” of the compound within the mold itself. The three periods of time together comprise the total cycle time and the nature and degree of the “time at temperature” of the first two periods have an effect upon the third period, i.e., the cure or vulcanization time. Thus, rubber compounds with the same formulation at higher temperatures will vulcanize more quickly than the same rubber compounds at lower temperatures. In most injection molding operations, a smaller portion of the “time at temperature” requirement is supplied in the barrel of the injection molding apparatus, i.e., the residual scorch time, and larger portion of the “time at temperature” requirement is supplied in the heated mold, i.e., the vulcanization time.
In addition to the cumulative effect of the above “time at temperature” periods, there is a critical temperature range for each rubber compound called the “critical residual scorch temperature range” at which the vulcanizing of rubber is initiated. These temperature ranges are known to or can be determined by those skilled in the art. For typical rubber compounds the critical scorch temperature range can be between about 240° F. (115° C.) and 320° F. (160° C.). Just above that temperature range the compound will begin to “scorch” or vulcanize prematurely in some period of time, which may be minutes or seconds. Below that temperature range, vulcanization may require hours.
In a typical rubber injection molding process, the objective is to heat the rubber compound to the maximum temperature, just below the critical scorch temperature range, which will produce the lowest viscosity of the compound at this limited temperature. The inability to supply more temperature or heat energy or “time at temperature” in the barrel so that vulcanization time in the mold can be reduced has been a continuous problem in prior art processes and apparatus. It is toward this problem that the present invention is generally directed.
The rubber compound is usually initially heated by externally heating the barrel of the apparatus electrically, with a steam jacket or from some other such external heat source and transferring the heat by conduction from the hot barrel wall into the mass of the rubber compound moving downstream through the barrel. Additional heat is supplied to the compound by frictional forces and by shearing of the rubber compound which occurs in the barrel and screw, the sprue, and the runners and gate of the mold. In many cases, this additional heat is an important factor upon which the vulcanization depends. Once the compound is in the mold cavity, additional heat is supplied to the compound and the compound is held in the mold for the required “time at temperature” to vulcanize and complete the cure.
Vulcanizing cycle time can be reduced if the compound can be rapidly and uniformly heated to a higher temperature and then quickly injected into the mold so that more of the “time at temperature” required to cure the rubber compound occurs when the compound enters the mold. However, the rubber compound cannot be exposed to high temperatures for even short periods of time in the barrel or undesirable curing or scorching would take place before the rubber compound enters the mold. One difficulty encountered in attempting to quickly and uniformly heat the rubber compound, while it is still in the barrel, stems from the poor thermal conductivity of the rubber compound. This makes it difficult to use external heat to quickly heat the rubber compound to a uniform temperature throughout. To rapidly obtain the desired temperature in the portions of the rubber compound distant from the heat source, e.g., the electricity or steam heated barrel wall, it is necessary for the heat source to have a temperature substantially above that desired in the rubber compound. This produces local hot spots in the rubber compound in proximity to the barrel wall which cause formation of an undesirable skin of scorched rubber compound or prematurely vulcanized rubber compound near the barrel wall. This can produce undesirable pieces of cured rubber compound in the material before it even reaches the mold for final curing of the rest of the product. These pieces of cured rubber compound can clog the sprue and mold runners and ruin the molded product. As a result, the temperature of the barrel wall is usually maintained sufficiently low to avoid such hot spots and is kept below the critical scorch temperature. Consequently, the compound temperature does not become excessively high so that only a relatively small portion of the “time at temperature ” required to cure the rubber compound is provided in the barrel. Furthermore, the temperature of the compound varies throughout, with the compound more distant from the barrel wall cooler than that close to the wall. The result of these factors is that a longer vulcanization cycle is required once the rubber compound is injected into the mold in order to provide the “time at temperature” required to complete the cure of the entire mass of rubber material.
Various techniques have been proposed to more quickly and uniformly heat the compound entering the mold by, for example, heating the rubber compound to high temperatures in the barrel. The heating of the compound increases its temperature and reduces its viscosity to produce a heated, plasticized, more flowable material suitable for injection into the mold. However, since the temperature of the injected material is not uniform, and since the “time at temperature” in the barrel is well below that

Affiliated with

Also associated with

Rating

Lattice gate for injection molding of rubber compounds does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Lattice gate for injection molding of rubber compounds, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Lattice gate for injection molding of rubber compounds will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2853173