Multiple injection compression molding process

Details Multiple injection compression molding process Multiple injection compression molding process

2000-01-27

2002-12-31

Silbaugh, Jan H. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

To produce composite, plural part or multilayered article

C264S328700, C264S328800, C264S002200, C425S130000

Reexamination Certificate

active

06500376

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a multiple injection compression molding process using sequential or simultaneous injection compression cycles to create a multi-layered plastic component having controlled layer thickness.
BACKGROUND OF THE INVENTION
Various methods are known for manufacturing multi-layered plastic components, such as overmolding, co-injection molding, and insert molding of films. One product application in which multi-layered plastic manufacturing is useful is in vehicle headlamp reflectors. Such reflectors are typically metallized on the interior thereof to reflect light forward of the vehicle. Recent styling trends have further complicated the headlamp reflector by the incorporation of “facets” which form the optic prescription used to direct light and form the headlight beam pattern onto the road. In the past, these facets were included on the headlamp lens. The addition of these facets to the headlamp reflector has resulted in the need of a more thermally stable plastic material than what was traditionally used for headlamp reflectors. This plastic material is much more expensive than its predecessor. Accordingly, it is desirable to minimize the use of such material. Therefore, it is desirable to manufacture headlamp reflectors in multiple layers of plastic, with a thin layer of metallizable plastic on the interior of the reflector to minimize the use of such expensive material.
Injection/compression molding has not been used for each and every shot of material injected into a tool to manufacture multi-layered plastic components. Injection/compression techniques can generally be divided into two types: (1) clamp-end injection/compression: Compression induced by movable platen motion, or molding machine clamp-end compression; and (2) auxiliary component injection/compression: Full molding machine clamp-up (no movable platen motion), with mold-cavity compression induced by auxiliary moving components internal to the mold set (usually driven by their own springs or auxiliary hydraulic cylinders).
Each of these injection/compression techniques is discussed below.
1. Clamp-end injection/compression or “coining”:
First disclosed by Martin (U.S. Pat. No. 2,938,232, issued May 31, 1960) but popularized by Engel's “sandwich press” toggle-clamp injection molding techniques (see, for example, Engle brochure A-24-TV-4/75, Ludwig Engel, Canada Ltd., Guelph, Ontario, Canada), this approach in each case generally involves the following process sequence:
a. From their full-open position, the mold platens and mold halves (and opposing male and female cavities formed thereby) are brought together until a predetermined air gap is present at the parting line.
b. At that point, a very low pressure, low-velocity injection fill begins (to prevent molten plastic from splashing through the air gap).
c. After injection fill is completed and the molten polymer mass is allowed to cool for a predetermined time interval, the injection molding machine commences a closing motion of the movable platen. This clamping-up motion mechanically seals off the mold cavity and its partially solidified contents to zero-clearance at the parting line, thus locking up the mold halves for the duration of the molding cycle at some predetermined clamp pressure.
d. Under this clamp pressure, the partially solidified polymer mass is compressed due to the reduced separation of the male and female dies precise mold surfaces being brought nearer together by the air-gap distance existing at the parting line when initial injection started. By eliminating this air gap, the volume of the cavity-and-runner system is proportionately reduced, resulting in compressive forces being exerted upon the partially solidified polymer contents, causing a reorientation and re-flow phenomenon.
e. Held under this clamp-induced compressive force, the mold cavity's contents continue cooling and solidifying, eventually reaching a temperature sufficiently below the glass-transition temperature of that polymer (in the case of polycarbonate, T
g
=296 degrees F.) that the molded article may be safely ejected without risking optical distortion. Then the whole cycle starts again, as the movable platen is moved to the predetermined air-gap position to await injection of the next shot.
While clamp-induced coining offers considerable advantages over straight injection, the current state-of-arts in such clamp-induced coining gives optimum results only within a narrow band of process parameters or setup conditions. Such successful coining is a function of:
a. Initial injection pressure and fill rate;
b. Air-gap dimension;
c. Timing interval between injection and compression; and
d. Final clamping forces.
Especially critical is control of injection pressure and fill rate, along with the timing interval. In order to prevent molten polymer from spilling outside the desired runner-mold-cavity configurations, the injected melt must be allowed to form a surface skin and partially solidify. Otherwise, molten polymer spills or “flashes” into the air-gap, necessitating costly and laborious trimming operations on the molded part. Even worse, if the melt has solidified to too great an extent, compression at ultimate clamping pressures can cause hobbing or deformation of the mating plates at the parting line, thus damaging the mold set. Cooling time interval is additionally critical to achieving acceptable molded part yields, since if the melt is not sufficiently solidified at its most constrictive point (i.e., gate freeze-off has not been completed), then partially molten polymer can still be extruded under this compressive force back out of the cavity and into the runner system, which can then result in an underfilled and underpacked part with badly distorted surfaces. On the other hand, if compression is delayed too long, too much polymer solidification will have occurred when the compressive force trough final clamp-up is initiated, resulting in forcible reorientation of the polymer and “cold working” of the plastic, which in turn produces birefringence and undesirable molded-in stress levels, with resulting localized nonuniform light-bending characteristics.
Illustrative of these problems in the context of optical disk molding is Bartholdesten et al (U.S. Pat. No. 4,409,169), which teaches the need for a slow (up to 3 seconds), low-pressure injection of an oversized shot into a partially open (air gap) mold parting line, then providing for deliberate melt cooling and viscosity-thickening, followed by a short pressing stroke (typically ⅕ to {fraction (1/10)} the disk's thickness, or 0.005-0.010 inch) which initially squeezes out of the reduced mold cavity volume the partially cooled and viscous excess plastic, then as the pressing continues to the fully closed parting line position (zero clearance), this radially extruded overflow is pinched off and full clamping force is thereafter maintained for shrinkage compensation and to assure no prerelease.
Another clamp-induced disk coining process is disclosed in Matsuda et al (U.S. Pat. Nos. 4,442,061 and 4,519,763) wherein, into a slightly opened mold set, a melt is injected and cooled till fully solidified, then reheated till uniformly above the plastic's melt temperature, at which point clamp actuated compressive stroke is conventionally delivered and maintained through this second cooling cycle.
2. Auxiliary Component Injection/Compression:
As noted above, another type of molding process (termed an “auxiliary component” process for the sake of discussion) includes the use of auxiliary springs, cylinders or the like which function to apply a compressive force to the opposing surfaces and which are commonly internal to the mold itself or as peripheral apparatus thereto. The primary difference between “auxiliary component” molding and clamp-end injection/compression, therefore, is that mold compression is provided by a stroke-producing element inherent to known modern injection molding machines (examples of same are the ejector or movable platen driving

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