Method and apparatus for injection molding plastic articles having solid exterior surfaces and porous interior cores

Method and apparatus for injection molding plastic articles having solid exterior surfaces and porous interior cores wherein a molten mixture of a chemically reactive foaming agent and a thermoplastic resin is injected into a mold so as to fill the mold cavity with unfoamed resin and form an outer solid skin on the molded article. Immediately prior to injection, an activator additive is introduced into the mixture, the additive reacting chemically with the foaming agent after a time delay of no more than a few seconds to provide for cellular expansion within the core of the molded article. The result is a molded body with a solid unfoamed skin which accurately relicates the surface of the mold and a cellular inner core.

BACKGROUND OF THE INVENTION 
The present invention relates to method and apparatus for molding articles 
of structural foam. In structural foam molding, a molded article with a 
solid unfoamed skin and a cellular inner core has long been known to be 
desirable. Conventional structural foam molded parts are sometimes 
objected to on the grounds that they have surface imperfections, such as 
swirls, blisters, pin holes, pot marks, streaks and the like. These 
surface irregularities are produced by the expanding foam gas bubbles, as 
they move across the cold mold surface during filling of the mold, being 
permanently solidified by the colder mold surface. 
In conventional structural foam molding processes in commercial use, the 
gas bubbles start to form when the resin enters the mold cavity, the 
bubble growth rate being a function of the pressure drop between the 
pre-pressurized resin and the mold cavity pressure which is usually at 
atmospheric pressure. U.S. Pat. Nos. 3,268,636 and 3,436,446 illustrate 
conventional processes. 
The blowing agents used in the conventional structural foam molding 
processes can be classified as either physically or chemically generated, 
by which is meant the blowing agent is either a gas that is mixed with the 
resin in the plasticator or the blowing agent is a solid material which 
decomposes in the plasticator in response to heating of the solid blowing 
agent. Both physical and chemical blowing agents are introduced and mixed 
in the extruder and generally dissolve into the molten resin at the 
pressures and temperatures found in the resin molding system. Various 
prior art patents disclose processes utilizing these blowing agents which 
are intended to overcome the problem of surface imperfections on the 
molded parts. 
For example, U.S. Pat. No. 3,988,403 discloses a system in which the mold 
surfaces are heated prior to introduction of the molding resin into the 
mold cavity in order to soften and "smooth out" surface imperfections. 
Such a system is objectionable because thermal cycling of the mold from 
hot to cold has many economic disadvantages such as increased energy 
consumption, increased cycle time and increased mold costs. 
U.S. Pat. No. 4,255,367 attempts to solve the surface imperfection problem 
by selectively adding foaming agents to the melt stream during injection. 
In this way, a solid unfoamed material can be first injected into the mold 
to form the skin and then a foaming agent is introduced in the melt stream 
and injected into the core to form the cellular core structure. Proper 
sequencing of the injection mold fill cycle becomes critical and residual 
foam material must be purged from the system before the solid injection 
phase of the next cycle can begin. Since the foaming agent injection point 
is upstream of the manifold system and the injection nozzles considerable 
volume can exist to store unwanted material between shots. This residual 
foam material may be inadvertently injected into the mold causing 
imperfections in the surface of the part. Also, in some cases, the foam 
core material breaks through the mold surface. The mixing of core and skin 
materials adversely affects the surface of the molded article. 
U.S. Pat. No. 4,255,368 discloses a method for producing a molded part with 
a foamed core and a non-foamed exterior shell by using a physical foaming 
agent such as nitrogen. The foaming agent is introduced into the polymer 
melt directly in the extruder prior to mold filling. A pressurized mold is 
used to retard the growth of foam bubbles during injection, the foam 
bubbles forming only after the mold has been completely filled. This 
process is objectionable because of the necessity to pressurize the mold 
with the attendant requirement for sealing valves and gaskets. 
U.S. Pat. No. 4,247,515 discloses a system capable of forming articles with 
solid skins but not foamed cores. The process disclosed in this patent 
produces parts with solid color and smooth surfaces but there is no 
provision for a foamed core. 
U.S. Pat. No. 4,129,635 shows a process for producing an article with a 
solid skin and a foamed interior. A foaming agent is introduced into the 
resin in the extruder and the mixture is injected into a pressurized mold 
to retard bubble expansion. This structure is objectionable because of the 
requirement for special molds designed for pre-pressurization and also 
capable of withstanding very high mold cavity pressures. 
The object of this invention, therefore, is to provide an improved method 
and apparatus for forming plastic parts with exterior solid surfaces and 
foamed cores which overcomes the above objections. 
SUMMARY OF THE INVENTION 
In the method and apparatus of this invention, a resin, preferably in 
pellet or powder form, is melted and plasticized in a conventional 
extruder which also contains a chemically reactive foaming agent that is 
mixed with the resin. The resin containing the foaming agent is then 
transferred to an accumulation zone where they are maintained in a molten 
state. When a sufficient volume of the molten mixture is in the 
accumulation zone, the molten mixture is ejected from the accumulation 
zone to form a stream which is directed into the mold so as to fill the 
mold cavity with unfoamed molten resin which solidifies upon engagement 
with the mold surface to form a solid skin at the mold-resin interface. 
An activator additive is introduced into the stream for flow into the mold 
and subsequent chemical reaction with the foaming agent resulting in a 
time-delayed generation of gas bubbles in the mixture within the confines 
of the solid skin. The result is in the formation of a molded article 
having a solid exterior surface which is a detailed replicate of the mold 
surface and a porous interior which expands in the mold to maintain the 
solid exterior surface in pressured contact with the mold surface so that 
it will conform in detail to the contour of the mold. 
The particular object of applicant's invention is, therefore, achieved by 
virtue of the interaction of the foaming agent and the activator additive 
in the mold cavity. The result is a molded article having a porous 
interior core and a solid exterior skin which does not contain the 
objectionable surface imperfections heretofore described.

With reference to the drawing, the apparatus of this invention, indicated 
generally at 10, is illustrated in FIG. 1 as including a conventional 
melting extruder 12 having the usual hopper 13 and feed screw 15. The 
resin from which the molded article 14 (FIG. 5) is formed is melted and 
plasticated in the extruder 12. The extruder 12 also contains a chemically 
reactive foaming agent, which unlike conventional foaming agents, is not 
decomposed by heat. Gas evolution of a chemically reactive foaming agent 
is independent of processing tempertures so that such an agent will not 
generate gas prior to being combined with an activator agent. The extruder 
12 provides for melting, plastication and mixing of the resin and the 
chemically reactive foaming agent and forces the mixture through a passage 
16 into a melt accumulator 18. The accumulator 18 includes a piston 20 
slidable in a cylinder passage 22 for molten material received from the 
extruder 12. The accumulator 18 also includes a hydraulic piston and 
cylinder assembly 24 comprised of a cylinder 26 and a piston 28 slidably 
supported therein and connected to the piston 20 by a connecting rod 30. 
As the melt accumulator passage 22 fills with molten material from the 
extruder 12, the piston 28 is forced rearwardly in the cylinder 26 forcing 
hydraulic fluid in the cylinder 26 to flow out of the cylinder through a 
passage 32 through a back pressure valve 34 and, thence, into a tank 36. 
When the melt accumulator 18 has accumulated a shot size batch of molten 
resin, a shut off valve 38 is actuated to close the passage 16 and isolate 
the extruder 12 from the accumulator 18. 
A hydraulic pump 40 is operable to pump hydraulic fluid from the tank 36 
through an injection valve 42 into the cylinder 26. A pre-charged 
accumulator 41 operates in conjunction with the pump 40 to insure a rapid 
injection of fluid forcing the piston 20 to move rapidly in a direction to 
cause the piston 20 to inject molten resin through a supply passage 44 and 
injection nozzles 46 into the mold cavities 48. Two injection nozzles 46 
are illustrated communicating with the passage 44, it being understood 
that operation of the apparatus 10 is the same whether one, two or more 
nozzles are used. Subsequent description will, therefore, deal with only 
one nozzle. 
Simultaneously with injection of the resin into the mold cavities 48, the 
injection nozzles 46 are operated to inject an activator additive into the 
stream of resin-foaming agent mixture that is flowing into each of the 
mold cavities 48. 
As shown in FIG. 2, each of the injection nozzles 46 includes a valve body 
50 formed with a main passage 52 which communicates with the passage 44 
which contains the flowing stream 54 of resin-foaming agent mixture. A 
tubular rod member 56, positioned coaxially within the passage 52, has an 
internal axial passage 58 that communicates at the discharge end of the 
member 56 with a transverse passage 60 which in turn communicates with the 
passage 52. A check valve member 62, positioned in an enlarged portion 64 
of the passage 58, is movable between an open position shown in FIG. 2 and 
a seated or closed position shown in FIG. 3 in which it is engaged with a 
seat 66 so as to close the passage 58. 
The passage 58 is connected to a line 68 which in turn communicates with a 
reservoir 70 for the activator additive which is added to the 
resin-foaming agent mixture at the injection valves 46. A high pressure 
pump 72 is connected at its inlet side to the reservoir 70 and at its 
outlet side to an accumulator 74 for the activator additive and with a 
directional valve 76 movable between two positions. In the position of the 
valve 76 illustrated in FIG. 2, it communicates the line 68 and the 
reservoir 70. In a moved position of the valve 76, communication between 
the line 68 and the reservoir 70 is blocked by the valve 76 and the pump 
72 and its accumulator 74 are connected through the valve 76 to the inlet 
line 68. A flow control valve 78, interposed in the line 68 functions to 
adjust and control the amount of additive that is injected into the 
passage 58 from the accumulator 74. The additive flows through the passage 
58 past the check valve element 62 and into the passage 60 for injection 
into the molten resin. When the flow of additive through the passage 58 is 
discontinued, the check valve member 62 is movable to the closed position 
illustrated in FIG. 3 to prevent back flow of molten resin into the 
passage 58. 
The activator additive requires a predetermined period of time to react 
with the resin-foaming agent mixture and produce gas bubbles 80 (FIG. 5) 
in the mold. This time delay allows complete filling of the mold cavity 48 
with the mixture of resin, foaming agent, and activator additive before 
foaming takes place. 
The flow of additive through passage 58 is shut off by moving directional 
valve 76 to its off position illustrated in FIG. 2 allowing all of the 
activator additive to be purged from the passage 58 before the cylinder 
assembly 82 for the injection nozzle 46 is actuated to move the valve rod 
56 to the closed position shown in FIG. 3 in which the inlet opening 83 
for the mold is closed shutting off further flow of molten material to the 
mold cavity 48. The injection pressure at which the molten material is 
supplied to the mold cavity 48, preferably in the range of 300-600 psi, is 
high enough to provide for pressure engagement of the molten material with 
the mold walls thereby producing smooth surfaces and excellent 
replication. 
During the time delay period between filling the mold and formation of the 
gas bubbles 80, preferably a time period between one and five seconds, the 
unfoamed resin mixture contacts the colder mold walls 84 and solidifies 
forming a solid skin 86 on the molded part 14 (FIG. 4). The core 90 of the 
molded article remains in a molten state due to heat retention. After 
expiration of the time delay period, the gas bubbles 80 evolve from the 
chemical reaction between the foaming agent and the activator additive. 
Since the mold cavity 48 is completely filled and the core 90 is under 
pressure, the gas bubbles 80 remain initially in a microstructure. As the 
resin contracts due to the thermal shrinkage, the gas bubbles 80 grow 
larger, continuing to grow and expand and exert pressure on the surface 
skin 86 urging the skin 86 against the mold walls 84 until the core 90 
cools off sufficiently to make the resin rigid enough to terminate the 
expansion. Thus, the process of this invention takes advantage of the 
combined phenomenons of delayed foam generation and thermal expansion. 
This eliminates the shrink marks that are normally found in molded parts 
obtained from conventional injection molding processes. Maximum achievable 
density reduction is dependent upon the coefficient of thermal expansion 
for each polymer used as the resin in the process of this invention and 
the temperature of the molten mass during injection. 
The provision of the porous core 90 provides for a density reduction in the 
molded article in the range of 10%-30%. Examples of polymers that can be 
used in the process of this invention are high density polyethylene, high 
impact polystyrene, polypropylene, polycarbonate, modified polyphenylene 
oxide (PPO), and most thermoplastics. 
The preferred chemically reactive foaming agent is sodium borohydride 
(SBH). Examples of activator additives are stearic acid, octanoic acid, 
oleic acid, polyacrylic acid, polystyrene sulfonic acid and water. In the 
chemical reaction of the foaming agent and the activator additive, a 
proton (H+) from the additive reacts with hydride (H-) on the BH.sub.4 
anion in the foaming agent to produce hydrogen (H.sub.2) gas according to 
the following formula: 
EQU NaBH.sub.4 +2H.sub.2 O.fwdarw.NaBO.sub.2 +4H.sub.2 
One gram of SBH yields 2.37 liters of hydrogen gas. 
Further by way of illustration of the present invention, there is set forth 
below a specific example of the molding of a part 14 having an exterior 
solid surface skin 86 and a porous core 90. 
High density polyethylene pellets having a solid density of 0.92 grams per 
cubic centimeter at 77.degree. F. and a melt index of 8.0 g/10 min. are 
fed into the hopper 13 of the extruder 12. The extruder 12 has a 21/2" 
diameter screw 15 and the resin is plasticated by the extruder screw 
running at 18.0 rpm. Sodium borohydride (SBH) granules are added to the 
hopper for the extruder 12 in a ratio of one part of sodium borohydride 
for each 400 parts of polystyrene so that the sodium borohydride forms 
approximately 0.25% of the melt in the extruder 12. It is to be understood 
that a range of foaming agent of from 0.1 to 1.0 percent can also produce 
satisfactory results. 
The mold cavity 48 has a volume of 113 cubic inches. At the injection 
nozzle 46, an activator additive consisting essentially of water (H.sub.2 
O) in an amount constituting 0.28% of the melt is added to the mixture and 
the molten mixture is injected into the mold cavity 48 at a pressure of 
400 psi. The resulting article 14 has a solid skin 86 which accurately 
replicates the inner mold surface, a porous core 90 and a density 
reduction of 25% relative to a solid molded part. 
From the above description, it is seen that this invention provides an 
improved method and apparatus for molding a structural foam article 14 
having a solid exterior surface skin 86 and a porous core 90, the surface 
skin 86 accurately replicating the mold surface. The advantages of the 
invention are due to the exploitation of the combined phenomenons of 
delayed foam generation and thermal contraction. By virtue of the 
utilization of a chemical reaction to form the gas bubbles 80, the bubbles 
consist essentially of hydrogen gas which diffuses very rapidly out of the 
molded part after foaming. This enables more immediate post-finishing of 
the part 14 without risking the possibility of blistering caused by 
subsequent diffusing of the gas from the part. Further, the hydrogen gas 
is safely and readily vented by normal industrial safety practices. The 
foaming agent that is preferred in the process of this invention, namely, 
sodium borohydride has the advantage it does not leave any toxic or 
standing residue on the molded part and leaves only the bi-product sodium 
metaborate which is odorless, non-toxic and non-contributory to color 
development of the finished part. 
The reaction rate of generation of the bubbles 80 can be controlled by 
varying the pH of the resin mixture. Increasing the acidity of the mixture 
will accelerate the reaction; increasing the basicity will retard it. The 
presence of certain transition metals (cobalt, nickel, copper) catalyses 
the foaming reaction in polymers basic in composition. Chelating agents 
can also be added to the system to tie up these metals and retard the 
reaction. Thus it is possible in the process of this invention to develop 
resin-foaming systems that have controlled rates of gas evolution over a 
wide temperature range.