Bergen International: Molding Tips



	Introduction Machine Parameters Structural Foam Molding Mold Design/Material Adding Foaming Agents Temperature Injection Rates/Pressure Properties
	INTRODUCTION
	The injection molding of structural foam molded parts is a well-established process in the plastics industry. There are a number of processes used for the production of rigid, lightweight structural foam moldings. These parts typically have a higher density skin with a lower density cellular foam core and are produced by introducing a foaming agent to the molten polymer.

	MACHINE PARAMETERS
	In principle chemical foaming agents can be processed on all injection molding machines. Use of a shut-off nozzle has proven to be advantageous to prevent or eliminate drool. If a shut-off nozzle is not available, working with the nozzle adjacent to the mold can help. In order to obtain a uniform foam structure, it is imperative that the gas expand after it has been injected into the mold. Therefore, it is important to run the injection speed as fast as possible. In some injection machines, the injection rate can be increased by means of a gas pressure accumulator.

	STRUCTURAL FOAM MOLDING
	Typically, low-pressure structural foam molding machines are equipped with a plasticizing extruder and a separate accumulator cylinder for quicker injection.

Top
	MOLD DESIGN/MATERIAL
	Mold Design
	Some of the basic considerations for building molds designed for structural foam molding are: The gates and runners should be located in a way to achieve quick and uniform filling with relatively short flow lengths The gates and runners should be relatively large to help ensure that the injection pressure is relatively low It is important that the vents be situated at the end of the flow lengths and large enough to vent the trapped gases

	Mold Material
	Due to much lower mold pressure, molds for structured foam molding do not need to be made of high strength steel. The mold cooling, on the other hand, needs to be more intense to overcome the insulating effect that the foam has on cooling. This is especially important in very thick wall sections to help prevent or reduce post expansion. Due to the lower pressures associated with structural foam molding, many applications can use aluminum as the mold material. In cases where steel is the mold material choice, a high alloy steel containing approximately 13% Cr. is recommended.

	ADDING FOAMING AGENTS
	Chemical foaming agents are typically added to the injection molding machine just like colorants or other additives. Mixing with the plastic granules can be carried out lot by lot in separate mixers or in an automatic blending unit directly on top of the injection molding machine. When processing powder products, the addition of approx. 0.1% adhesive to avoid later separation is recommended. Liquid chemical foaming agents can easily be added directly in the injection cylinder with a dosing pump. For better accuracy the pump should only work parallel to the material feeding controlled in the total cycle of the injection molding machine.
Top
	TEMPERATURE
	Feeding Zone The cylinder temperature for foam molding should be low enough in the feeding section to prevent too early decomposition of the foaming agent in order to avoid gas losses through the feeding hopper.

	Compression Zone
	In the compression zone the melt temperature should be high enough for the whole reaction of the foaming agent so that the total gas yield can be achieved. At the nozzle, the temperature can be reduced again by 10 – 20% in order to increase the back pressure and improve melt strength.

	INJECTION RATES/PRESSURE
	Rates The mold cavity should be filled to allow the dispersed gas bubbles to expand completely only after termination of the injection process. The internal expansion pressure forces the melt against the cold mold wall. This leads to a cellular core with a solid skin. Pressure Injection pressure must be high enough to guarantee a high injection rate. Sometimes a pressure accumulator is recommended. Post-pressure is normally not used for foam injection molding because it can suppress expansion of the foam. It is only required when a thicker solid skin is required.
Top
	PROPERTIES
	The application of chemical foaming agents in injection molding mainly aims at weight reduction in the molded parts thus saving material and money. The maximum weight reduction which can be achieved greatly depends on the individual mold shape. Thicker mold walls enhance the formation of foam. A reduction of 10 – 25% is usually achieved in molds with a wall thickness of more than 4 mm and short flow paths. If circumstances are favorable, reduction of more than 30% can also be possible. Technical housings are mainly foamed to increase wall stiffness with the same quantity of material used.

	Sink Marks When molding high shrinkage resins such as polypropylene, sink marks often occur during cooling. In cases where the wall thickness of the part varies, for instance parts with ribs and bosses, the thicker areas will shrink at a higher rate than the thinner areas causing sink marks. This effect can be avoided by the addition of small amounts of specially formulated foaming agents such as Foamazol 62.

	Injection molded parts which have been produced with chemical foaming agents usually have a mat surface. Glossy surfaces can only be achieved with special processes and/or specifically formulated foaming agents. Bergen International has developed a special grade Foamazol 62, that allows molders to eliminate sink marks without the swirled surfaces associated with foam-molded parts.


Top	Back