New methods for molding better structural-foam parts and for overmolding incompatible materials won the awards for best technical presentations at April’s 2006 Plastics Parts Innovation Conference of the SPI Alliance of Plastics Processors (APP) in Columbus, Ohio. The APP was formerly SPI’s Structural Plastics Div.
APP members gave the Best Paper award to Prof. Chul Park of the University of Toronto for his discussion of research in structural-foam molding using continuous plastication. The first runner-up award went to Barry McGraw, member of the Advanced Materials Applications group at Battelle, Columbus, Ohio, for his presentation on ultrasonic welding of dissimilar polymers.
‘Decoupled’ structural foam
Conventional equipment for structural foam has drawbacks that can compromise part quality and consistency, says Prof. Park of the Univ. of Toronto’s Microcellular Plastics Manufacturing Laboratory, part of the Dept. of Mechanical and Industrial Engineering.
Park cited the example of a conventional structural-foam system with two-stage injection. It has a fixed plasticating extruder that melts the resin and mixes and dissolves the gas that is injected through a port in the barrel. The second stage is a reciprocating injection plunger, separated from the extruder by a shut-off valve and from the mold by a shut-off nozzle.
The drawback with this system, Park said, is that the first shut-off valve does not completely separate or “decouple” the functions of plasticating and injection. This is evident in the fact that when the injection plunger is operating and the shut-off valve between the extruder and plunger is closed, the plasticating extruder must be stopped, since the melt has no place to go. The screw is restarted once injection and cooling are completed, and after the shut-off valve is opened.
Discontinuous start-stop plasticating causes significant pressure fluctuations in the barrel, which affects the amount of injected gas going into the melt stream, Park reported. Pressure fluctuation in the barrel can cause a non-uniform polymer/gas mixture when the injected gas does not completely dissolve into the polymer. This results in a two-phase polymer/gas mixture and not a single-phase gas/polymer solution.
This can form numerous undissolved pockets of gas in the part, especially along weld lines. The ultimate consequences of uncontrolled formation of gas pockets in the structural foam can be less effective density reduction, compromised mechanical properties, and surface defects such as non-uniform surface swirl, visible weld lines, and color contrast across weld lines. The foam cells are larger than desired, cell density is lower than optimum, and cell-distribution is very non-uniform. The achievable void fraction using conventional equipment ranges from 0.08 to 0.20, Park said.
Researchers at Toronto University designed a new structural-foam machine that eliminates intermittent operation of the extruder and decouples the plasticating/gas dosing operation from the injection phase. Park says the patent-pending technology creates a homogenous one-phase polymer/gas solution that improves cell structure, void fraction, and surface finish of structural foam parts.
The new machine accomplishes continuous extruder operation by means of a positive-displacement gear pump and an additional melt accumulator. The gear pump, positioned just after the extruder, is intended to control melt pressure and maintain a consistent polymer/gas ratio. The constant rotational speed of the screw maintains gas solution in the polymer at a steady state. Downstream of the gear pump is a hydraulically operated accumulator, and farther downstream is the shut-off nozzle to the injection plunger. The accumulator collects the polymer/gas solution during the injection process, when the check valve to the plunger is closed.
A second version of the new-style press eliminates the additional accumulator between the gear pump and check valve and instead adds a second injection plunger and shut-off nozzle. A split melt channel feeds both injection plungers in alternating fashion: One plunger barrel fills while the other is injecting.
Join dissimilar resins
Joining of incompatible resins using ultrasonic energy plus a tie layer represents a potential alternative to adhesive bonding, according to McGraw’s study at Battelle. What’s more, this represents a novel application of ultrasonics to bonding two materials in the mold.
Overmolding PE and ny lon is not possible with conventional methods, McGraw said. Ultrasonic energy reportedly can help, because it adds intense localized heating that increases melt flow in the mold and promotes in ter facial bonding between dissimilar materials. However, ultrasonics alone are not sufficient. A tie layer is necessary to compatibilize the non-polar PE and polar nylon.
Battelle studied injection molding of MDPE onto a preformed nylon 66 or nylon 12 insert, and vice versa. The tie layer was an extruded film of ethylene acrylic acid (6.5%) copolymer. The tie layer was 0.51 to 0.76 mm thick.
Each polymer was molded into a typical “dogbone” test specimen, which was then cut in half. One half of the dogbone served as the insert in an overmolding trial. A piece of tie-layer film was placed over the insert in the mold where the solid/melt interface would occur. Then the second material was injected onto the tie layer.
Battelle researchers developed a special mold in which the tip of an ultrasonic horn is positioned inside the cavity near the bonding interface. After a certain amount of cooling of the injected material, the ultrasonic horn is actuated to provide the bonding energy through mechanical vibration.
A 40-kHz ultrasonic transducer is coupled to an aluminum horn whose circular tip is inserted through the mold flush with the inner surface of the mold cavity. Some parts were molded with ultrasonics but no tie layer, and vice versa.
After overmolding, adhesion was tested by pulling the dogbone specimens apart using a tensile testing machine. Tensile yield strength and ultimate elongation were used to assess the bond strength. Average tensile strength for three dogbone samples molded of a single material provided baseline strength values for comparison: 1986 psi for MDPE, 9091 psi for nylon 66, and 5031 psi for nylon 12.
No bonding occurred without both the tie layer and ultrasonics. All samples bonded with both showed cohesive failure rather than breaking cleanly at the joint between the two materials.
When MDPE was injected onto a nylon 66 or nylon 12 insert, PE melt temperature was 450 F. Tensile strength of the bond was 536 psi with nylon 66 and 609 psi with nylon 12, representing approximately 30% of the original PE strength.
Better results were obtained by injecting nylon onto PE inserts with a tie layer. Melt temperatures were 540 F and 390 F, respectively. Tensile yield strengths were 1769 psi for nylon 66 and 1203 psi for nylon 12, or 92% and 63% of virgin PE strength, respectively. The researchers say the higher melt temperature of nylon 66 may contribute more thermal energy to bonding. Also, the lower melt viscosities of the nylons relative to PE, especially under the influence of ultrasonic vibrations, may cause more penetration of the PE surface by the injected nylon as compared with penetration of the nylon by injected PE.
A similar relationship was seen in elongation data. Tests on mono-material dogbones showed that virgin MDPE yielded and then broke at 23.5% ultimate elongation, vs. 16.4% for nylon 66 and 7.8% for nylon 12. When PE was injected onto nylon 66 or nylon 12 inserts, average elongation was 1.5% and 1.9% respectively. But when nylon 66 or 12 was molded onto a PE insert, average elongation was 8.7% and 3.2% respectively.