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I was wondering if you help me locate some references on Processing HDPE and PP with 1% chemical foaming agent? All the information I find focuses on Structural Foam which is a low pressure process. We primarily use high pressure injection molding, using the the chemical foaming agent for weight reduction and cosmetics.

Since the banning of foaming agents containing CFC’s, the use of chemical blowing agents has obtained an inappropriate stigma in the marketplace. Believe it or not, their use is not as rare as it may seem. Many injection molders will use a small amount of blowing agents to eliminate sinks, lower material costs, and even enlarge the part to help meet dimensional requirements.

In your case, there are many places to find information… when searching online, use terms such as:

or get more specific:

There are also some good articles available online:

or

Since structural foam molders use the largest volume of additives, much of the literature is focused on their needs… but you will find that many of these resources will also provide great high-pressure molding information… click for an example of this from Bergen.

You may have to register to see some of the materials from the Suppliers, but this is typically free.

Keep in mind, your material providers can be great resources if you ask the correct questions. Some additive providers provide agents specifically formulated for your type of application.

Basically, when you are conducting a high pressure molding process with blowing or foaming agents, you would melt and inject the material in a manner similar to a traditional process, but apply many of the packing and cooling strategies of structural foam. Ultimately, the more blowing agent you add… the more the polymer will behave like a structural foam after it is injected. 

-Andy

Your first assumption is most likely correct…

The hydraulic clamp system on your machine uses a large diameter hydraulic cylinder in the center of the movable platen to apply tonnage to the mold. Since the clamping force is centered on the platen… deflection occurs on the perimeter. This deflection reduces the tonnage appied to the perimeter of the mold resulting in improved venting. Adversely, toggle clamps tend to provide a more evenly distributed force to the platen.

Before you make the assumption that the machine is the only cause, keep a few more aspects in mind… 1) The clamp force might be more accurate on the newer machine. 2) If the mold is too small for the electric molding machine (uses less than 2/3 of the platen) then you can cause platen concavity resulting in additional force applied to the perimeter of the mold. 3) The newer machine may also provide a faster injection speed which would also affect the mold venting.

-Andy

Although adjusting the temperature of your hot runner drops can help balance your tool, there are a few alternative ways molders use to help balance their tool.

1) Adjustable drops – In many hot runner tools, the height of the hot runner drop can often be adjusted to increase or decrease the thickness of the specific drops

2) Use true balanced runner systems – Many hot runner manifolds are not built with balance and symmetry in mind. Additionally, most hot runner systems do not use features such as Beaumont’s Melt Flipper to balance the shear within the hot runner system.

3) Balance the clamping – All platens deflect, and many older platens will be somewhat concave. Check and measure these conditions to ensure spacing or additional bolster plates are not needed. Additionally, a review of parallelism during clamping can be very helpful.

4) Balance the cooling – In many injection molds, the cooling supply each individual cavity may not be properly balanced. This is very common when the part geometries are not symmetrical… resulting in variations the effectiveness of the cooling from part to part. You may want to measure and compare the coolant temperature and flow going to each region of the tool.

5) Balance of venting – This can be an often overlooked cause of cavity imbalance. The effectiveness of the melt entering the mold cavity is based much on the air’s ability to get out the plastic’s way. I have seem many molds with virtually no venting to interior cavities… or even caes where the inner cavities actually vent to the out cavities. causing all sorts of complications in gas removal.

When specifying new hot runner systems or new tooling, try to incorporate systems which can be easily adjusted. Many molders see great benefits to the additional control and flexibility brought forth through the use of valve and thermal gate systems.

-Andy