Method of coinjection molding of thermoplastic and thermoplastic elastomer

Vibration isolators are manufactured by a co-injection molding process. Outer and inner parts of the isolators are molded first from a rigid or stiff thermoplastic material such as polystyrene and intermediate spring-like parts of the isolators are made of a thermoplastic elastomer such as copolymer of butadiene and styrene formed and bonded to the outer and inner parts in a subsequent molding step.

This invention pertains to improvements in vibration isolator technology 
and more particularly to a new form of vibration isolators and a new 
method of manufacturing such device. 
PRIOR ART 
A number of different types of vibration isolators are known. This 
invention is concerned primarily with plate-type and tubular type 
isolators, so called because the former type has a small length to 
diameter ratio and thus is relatively flat while the latter type has a 
relatively large length to diameter ratio. Prior to this invention such 
isolators have usually consisted of inner and outer metal parts and a 
molded elastomeric part extending between and bonded to the two metal 
parts. While this well known form of construction has permitted the 
manufacture of isolators in different sizes and load ranges, the 
manufacturing process entails a number of steps which add to the cost of 
the product and must be carefully carried out for the sake of product 
reliability. Among these steps are the important ones of cleaning the 
metal components, applying a bond conditioner or adhesive to the metal 
parts so that they will bond to the elastomeric part, and then loading the 
components into the mold for fabrication of the elastomeric part. The 
molded product also must be heated to effect and complete vulcanization of 
the elastomer. Thirdly a shear bond strength of about 400 to 500 psi is 
desirable in order to prevent separation of the elastomer from the metal 
parts and permit the isolator to satisfy commercial requirements and 
withstand prolonged use. This level of shear bond strength can only be 
achieved by proper design and strict compliance with manufacturing 
requirements, including proper control of molding temperatures and 
pressures. 
SUMMARY OF THE INVENTION 
The primary object of this invention is to provide a new and improved 
method of manufacturing plate-type and tubular-type vibration/shock 
isolators and new and improved forms of such isolators. 
A second important object is to make possible the manufacture of isolators 
of the type described in a manner which avoids or substantially reduces 
the problems and limitations of the prior manufacturing method. 
Still another object is to provide a method of manufacturing vibration and 
shock isolation isolators which is substantially faster and cheaper than 
methods of like purpose already employed in the art. 
These objects are achieved by making the vibration and shock isolators of 
two different synthetic plastics using a co-injection molding process. One 
plastic is a rigid thermoplastic material; the other is a thermoplastic 
elastomer. The latter is injected after the rigid thermoplastic material. 
This order of injection is initiated in order to achieve proper bonding of 
the two materials.

Referring now to FIG. 1, the article which is illustrated is a plate-type 
vibration isolator which consists of inner and outer parts 2 and 4 and an 
intermediate part 6. Both of the inner and outer parts are made of a 
substantially rigid thermoplastic material, while the intermediate part is 
made of a thermoplastic elastomer. As used herein the term "substantially 
rigid thermoplastic material" means a solid substantially rigid material 
which has the property of fusing (softening to the point of becoming a 
liquid) when heated to a suitable temperature and of hardening and 
becoming a solid and substantially rigid again when cooled to room 
temperature, i.e., 70.degree. F., and the term "thermoplastic elastomer" 
means a solid material which has the property of fusing when heated to a 
suitable temperature and of hardening and becoming a solid which is 
resilient and behaves as an elastomer when cooled to room temperature. 
These thermoplastic materials may consist of a single thermoplastic 
polymer substance or a mixture of such substances, with or without 
additives such as colorants, plasticizers, anti-oxidants, stabilizers, and 
other functional ingredients that suitably modify one or more of the 
physical properties of the thermoplastic substance(s). 
A further requirement of this invention is that the parts 2, 4 and 6 be 
formed by injection molding. Hence the substantially rigid thermoplastic 
material and the thermoplastic elastomer must be made from molding 
materials which are capable of being injection molded. The molding 
materials may consist of or be made up in the majority of one or more 
polymers and/or one or more copolymers. Additionally the material used to 
manufacture the parts 2 and 4 and the material used to form the part 6 
should be compatible in the sense that they are capable of bonding to one 
another by fusion, i.e., by contacting the materials when at least one is 
in a fluid state and then cooling the fluid state material until it has 
solidified and formed a bond with the other material. While the parts 2 
and 4 could be made of different mutually compatible materials which melt 
and solidify at the same or nearly the same temperatures, it is preferred 
that they be made of the same material. Preferably the parts 2 and 4 have 
a flexural modulus in excess of 400,000 psi while the part 6 is a soft low 
modulus thermoplastic elastomer having a Shore A scale durometer value of 
between 35 and 85. By way of example but not limitation, the parts 2 and 4 
are made of polystyrene having a flexural modulus of about 465,000 psi and 
the part 6 is made of a butadiene styrene compound having a Shore A scale 
durometer value of 55. 
The parts 2, 4 and 6 are shown in the drawings as having sharply defined 
boundaries since, as explained below in greater detail, the interfaces 
between those parts are substantially free of any intermixing or 
interdiffusing of the thermoplastic materials. 
Still referring to FIG. 1, the inner part 2 has flat annular top and bottom 
surfaces 8 and 10, a cylindrical inner surface 12 defining an axial bore 
17 and an outer boundary represented as a surface of revolution comprising 
cylindrical end sections 16 and 18 and a double-curved intermediate 
section 20. The outer part 4 serves as a flange for the isolator unit and 
has a cylindrical outer surface 22 and an inner boundary 24 represented as 
a cylindrical surface, and mutually parallel top and bottom surfaces 26 
and 28 which are parallel to the corresponding surfaces of inner part 2 
and extend at right angles to the axis of the inner part. Outer part 4 
also has a plurality of mounting holes 5. The intermediate part 6 has 
inner and outer sections 30 and 32 that are bonded respectively to inner 
part 2 at its boundary section 18 and the part 4 at its inner boundary 24, 
plus a convoluted intermediate section 34 that extends between inner part 
2 and outer part 4. Intermediate section 34 is bonded to the inner part 2 
at the boundary section 20. Intermediate section 34 acts as a spring to 
resiliently locate inner part 2 with respect to outer part 4. 
When the device of FIG. 1 is made by the molding method hereinafter 
described, substantially no diffusion or mixing of one material into or 
with the other material will occur. Additionally no or only minor 
distortions of one material by the other will occur along the boundary 
regions. It has been determined by inspecting cross-sections of isolators 
like those of FIG. 1 made according to this invention with a scanning 
electron microscope to a magnification of 20,000, that the boundaries 
between the butadiene styrene thermoplastic elastomer and the polystyrene 
parts have an interface region (the region of diffusion or intermixing of 
one material into or with the other) with a thickness in the order of only 
1.0.times.10.sup.-6 inch. Nevertheless the bond between the elastomer and 
non-elastomer parts is sufficiently strong for the device to perform 
satisfactorily as an isolator. 
Referring now to FIGS. 2A-2C, the device of FIG. 1 is produced in 
accordance with a preferred mode of practicing the invention by means of a 
coinjection mold that essentially comprises three relatively movable mold 
members 36, 38 and 40 and a center part or core 42 attached to mold member 
36. As seen in FIG. 2A, mold member 36 has a contoured inner end surface 
which comprises four distinct portions 44, 46, 48 and 50, while mold 
member 38 has a flat inner end surface 52 and a cylindrical inner surface 
54. Mold member 40 has a contoured inner end surface comprising sections 
56 and 58 and a cylindrical outer surface 60 that makes a close sliding 
fit with surface 54. The mold members are arranged so that when the mold 
members are in closed position, surfaces 50 and 52 will mate with one 
another while surfaces 44 and 58 and post 42 will form a first cavity 62 
and surfaces 48, 52 and the upper portion of surface 60 will form a second 
cavity 64. Mold member 40 has a center hole 66 in which center post 42 
makes a close sliding fit. A plurality of pins 68 secured in mold member 
36 make close sliding fits in holes 70 in mold member 38. Pins 68 serve as 
cores for forming the mounting holes 5. Mold members 36, 38 and 40 are 
adapted (by conventional means not shown but known to persons skilled in 
the art of injection molding) to move relative to one another along the 
axis of post 42, so that as described hereinafter mold members 38 and 40 
are movable separately and selectively to different positions along that 
axis relative to mold member 36. 
The vibration isolator shown in FIG. 1 is manufactured using the mold 
assembly of FIGS. 2A-2C according to the following method. First the mold 
members 36, 38 and 40 are placed in the totally closed position shown in 
FIG. 2A (the first injection position) and a suitable liquid thermoplastic 
injection molding material capable of solidifying into a rigid or near 
rigid solid (e.g. polystyrene) is injected into mold cavities 62 and 64 
via injection ports 74 and 76 so as to form the isolator parts 2 and 4. 
Then mold member 40 is retracted until the outer edge of its surface 56 is 
flush with surface 52, so as to form a third cavity 78 as shown in FIG. 2B 
(the second injection position). Next a suitable liquid thermoplastic 
injection molding material capable of solidifying into a solid material 
with the properties of an elastomer (e.g. butadiene styrene) is injected 
into cavity 78 via one or more injection ports 80 so as to form the 
isolator part 6. This injection step is conducted after the material 
injected into the cavities 62 and 64 has solidified or become viscous 
enough so that it will not be displaced or distended by the material 
injected via port 80, yet is soft enough to bond to the elastomer 
material. Thus the second injection step is carried out while the material 
in cavities 62 and 64 is still hot but after it sets up as a solid. By 
appropriate choice of materials, it is possible for the cavity 78 to be 
filled within as short a time as one to three seconds after the cavities 
62 and 64 have been filled and still achieve a satisfactory bond between 
the elastomer and non-elastomer parts. Finally, after the part 6 has set 
up as a solid in cavity 78, the mold members 38 and 40 are separated from 
mold member 36 as shown in FIG. 2C, whereupon the finished isolator may be 
removed from the mold and set aside to cool. Thereafter the mold members 
are returned to the position shown in FIG. 2A for the next molding cycle. 
In the preferred mode of practicing the invention, the isolator parts 2 and 
4 are molded of polystyrene which solidifies so as to have a flexural 
modulus of about 465,000 psi and the isolator part 6 is made of a 
butadiene styrene copolymer which solidifies so as to have a durometer 
measured on the Shore A scale of between 35 and 85 (depending upon the 
spring rate desired for the isolator), with the polystyrene preferably 
being the material sold by Shell under the trade name Shell DP-203 and the 
butadiene styrene being the material sold by Shell under the trade name 
Kraton 3000 Series thermoplastic rubber. Adequate temperatures and 
pressures are determined by the characteristics of the materials used, 
i.e., the foregoing polystyrene molding material is injected with a 
pressure of approximately 5000 psi and a temperature of about 390.degree. 
F.; the foregoing butadiene styrene molding material is injected into 
cavity 78 at a pressure of approximately 6000 psi and a temperature of 
about 390.degree. F. The latter injection step should occur about 1 to 3 
seconds after terminating insection of the polystyrene molding compound 
into cavities 62 and 64. The injection materials are maintained at a 
temperature of about 390.degree. F. during the two injection steps, but 
the mold is maintained at a temperature of about 100.degree. to about 
150.degree. F. during the molding process. The mold is opened and the 
finished part is removed about 1 minute after the second injection step is 
completed. The mold part is then set aside and allowed to cool to room 
temperature before being labelled, tested and packaged. The finished 
products exhibit a shear bond strength between the part 6 and parts 2 and 
4 of at least 400-500 psi and usually between about 600-800 psi, in 
comparison with the typical bond strength of about 500 psi between the 
metal and elastomer parts of conventional metal/elastomer isolators. 
It is to be noted that injecting the elastomer material after the rigid 
material has been injected is critical. It has been determined that if the 
elastomer is injected at the same time as or before the rigid material, a 
satisfactory isolator product cannot be achieved since the elastomer is 
incapable of withstanding deformation in cavity 78 under the pressures 
required to be used in injecting the rigid plastic material into cavities 
62 and 64. This is true even if the elastomer has fully cured before the 
non-elastomer material is injected. Only if the elastomer injection is 
delayed until after the rigid plastic material has set up sufficiently to 
withstand deformation under the pressures required to inject the 
elastomeric material is it possible to achieve a strong enough bond 
between the elastomer and non-elastomer parts and also have the isolator 
parts conform exactly to the shape of the three mold cavities. 
A further distinct advantage of the invention is that the spring rate of 
the isolator may be changed by modifying the composition and hence the 
durometer of the material used to form the intermediate part 6. Thus, for 
example, the Kraton molding material is available from Shell as Kraton 
3226 for 35 durometer A scale, Kraton 3202 for 55 durometer A scale, and 
Kraton 3204 for 85 durometer A scale. Other durometer values can be 
achieved by suitably blending any two or all three of the foregoing Kraton 
materials or by the addition or substitution of other thermoplastic 
elastomers. The stiffness of the parts 2 and 4 may be modified by mixing 
some of the elastomer molding material with the polystyrene molding 
material. In this connection it is to be appreciated that the parts 2 and 
4 need not be absolutely rigid; for some isolator applications it may 
suffice or be desirable that those parts be merely stiff, i.e. semi-rigid. 
A further advantageous feature of the foregoing preferred method of 
practicing the invention is that the molding materials are injected 
coaxially rather than by the biaxial method disclosed by I. Martin Spier 
in his U.S. Pat. No. 3,950,483. It has been found that molding by coaxial 
injection is simpler to execute and thus leads to a more reliable product. 
Another advantage of the invention is that isolators of various shapes, 
sizes and vibration-isolating characteristics may be formed. Thus, for 
example, the shape and/or size of the intermediate part 6 may be altered 
merely by modifying the several mold parts. Further by way of example, by 
appropriately molding the mold assembly it is possible to form a flat 
isolator 90 (FIG. 3) consisting of a cylindrical inner part 2A, a 
cylindrical outer part 4A with a flat circular flange 92, and a 
cylindrical intermediate part 6A having flat end surfaces. Also by way of 
example, the invention is adaptable to providing an axially-elongate 
isolator 96 (FIG. 4) where the three parts 2B, 4B and 6B are all 
cylindrical and the length of part 6B substantially exceeds its inner 
radius as well as its outer radius. The two alternative forms of isolators 
have different vibration isolation characteristics than the unit of FIG. 1 
even where made of the same materials as the latter. 
Still another advantage of the invention is that it may be practiced with a 
variety of thermoplastic injection molding materials. Thus, the injection 
molding thermoplastic elastomer material constituting the part 6 could 
comprise or consist of a material other than butadiene styrene known to 
persons skilled in the art. In this connection it is to be noted that the 
term "thermoplastic elastomer" is a term already known to persons skilled 
in the art, as evidenced by Tobolsky et al, Polymer Science and Materials, 
page 277, Wiley-Interscience (1971); and that a variety of such materials 
exist as disclosed by B. A. Walker, Handbook of Thermoplastic Elastomers 
(1979). 
Further, by way of example but not limitation, the stiff thermoplastic 
parts 2 and 4 may be made of a material other than polystyrene, e.g., 
acrylonitrile-butadiene styrene (ABS), poly methyl methacrylate (e.g. 
Plexiglas), a polypropylene polymer and other materials obvious to persons 
skilled in the art, as taught for example, by U.S. Pat. Nos. 3,941,859, 
3,962,154, and 4,006,116. The exact choice of materials used will depend 
upon the characteristics desired and the compatability of the 
non-elastomer and elastomer materials with respect to bonding to one 
another. 
Following are a number of combinations of compatible non-elastomer and 
elastomer materials which may be used to form the parts 2, 4 and 6 
respectively: 
(A) parts 2, 4-Shell DP 203 Polystyrene (density--1.04 gm/cm.sup.3 and 
tensile strength--471 kg/cm.sup.2) with carbon black filler, and part 
6--(1) Phillips Solprene 475P Radial SBS block co-polymer of polybutadiene 
and polystyrene (Shore A hardness--90), or (2) Shell Kraton 1101 linear 
block co-polymer of styrene, butadiene and styrene (SBS) using carbon 
black as filler, or (3) Shell Kraton 3226 linear block SBS co-polymer 
using a clay filler; 
(B) parts 2, 4--Kodak Tenite P2635-72AA, believed to be a polypropylene 
polyallomer containing 30% glass fibers (density--1.12 gm/cc, tensile 
strength--1357 kg/cm.sup.2, and part 6--(1) Union Carbide Bakelite 
Ethylene co-polymer DQDA-3269, Natural 7 Ethylene -propylene co-polymer, 
or (2) Exxon Vistaflex Thermoplastic Rubber, EPDM Terpolymer, or (3) 1,2 
Syndiotactic Cross-linkable polybutadiene (density 0.90 gm/cm.sup.3, 
tensile strength--102 kg/cm.sup.2). 
Other modifications and advantages of the invention will be obvious to 
persons skilled in the art.