Chemical Foaming Agent Molding Guide

Introduction guide to injection structural foam co-injection rotational molding chemical foaming agents
Machine Parameters
Structural Foam Molding
Mold Design/Materia
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.

structural foam molding with chemical foaming agents

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.

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.

temperature in feeding zone molding chemical foaming agents

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.

injection molding cycle rates pressure chemical foaming agents

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

elimination removal sink mark injection molded molding part chemical foaming agents

injection molded parts chemical foaming agent 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.

 

swirl patern structural foam molded part chemical foaming agent 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
structural foam class a surface chemical foaming agent

 


Endothermic CFAs
Endothermic chemical foaming agents are chemicals that take heat away from the chemical reaction. This produces foams with a much smaller cell structure, resulting in improved appearance and better physical properties. Find an Endothermic CFA
Exothermic CFAs
Exothermic chemical foaming agents generate heat during the decomposition process. They liberate more gas per gram of foaming agent than endothermic agents and higher gas pressure. Find an Exothermic CFA
Endo/Exo Blend CFAs
Endothermic and Exothermic blends can have a synergistic effect that offers benefits over the use of either an endo or exo by themselves. Find an Endo/Exo Blend CFA
Custom Formulated CFAs
Bergen International prides itself on our ability to work with you to develop the exact blend that offers the best set of performance/cost benefits for your specific process and polymer. Find out more on custom CFAs
Request a Sample
Contact Bergen International to get a sample of endothermic, exothermic, or endo/exo blend CFA. Request a Sample