Bozilla Corp January 2009 Newsletter

Before I go and say 'absolutely not', I'd have to first state that there are many tools that are very small and will fit into an array of IMM's (Injection Molding Machines) with low clamp force. These are not going to be the problematic cases.

Selecting a machine based on clamp force (tonnage) is more common when you have a part with a large projected area; bumper fascias, housewares and many other items.

In today's economic climate, it's more important than ever to conserve energy. Many believe that using the smallest IMM is the best way to achieve this cost savings. However, there are reasons why a smaller machine isn't always the most efficient machine.

Reason 1: If an optimum process is the objective, then select a machine that does not allow the tool to blow open during that 'optimized' process.

I have had many customers come to me with concerns about the process asking why the part is warping or exhibiting cosmetic defects. Once I dig into the process, I typically find that the part is not packed sufficiently due to the tool blowing open. In order to keep the tool closed, they must pack with very little pressure for a very short time. This immediately spells problems and is caused because the tool is in an IMM that has too little clamp force.

Reason 2: You are able to make parts but the process window is so small that staying within the process window is impossible to maintain.

The inability to stay within a process window could be due to many problems especially since there are so many variables in the molding process. However, if the machine does not have sufficient clamp force to stay closed during an optimum molding process, concessions will be made and you may find yourself on the edge or on the outside of the molding window. Once this happens, it's typically a firefighting process for the duration of production for that tool. Firefighting typically proves inefficient with regards to waste, lost productivity, lost man hours, excessive energy consumption, etc.

Reason 3: Valve gates are closed prematurely during 2nd stage to prevent the clamp from blowing open.

Here is a common practice that often ends in poor part production. Valve gates are not as simple as many often think. When valve gates are sequenced, the 1st stage injection velocity must be profiled to accommodate the extra valve gates opening. Similarly, during 2nd stage, the valve gates must remain open until the part freezes up to the gate (very near the gate but not on the gate). If any valve gate is closed before the part freezes, the chances of having backflow increases dramatically. Furthermore, the part will not be packed in the region of the valve gate that has been closed. A part that is not packed sufficiently is susceptible to high volumetric shrinkage which is a precursor to warpage.

Reason 4: The reject rate is high.

A high reject rate may be due to the inability to stay within the optimum molding window resulting in a 'floating' process that changes because of any slight variation. The inability to stay within a molding window may be due to the clamp force controlling the process rather than the process settings controlling the process. Having high reject rates is bad news all together.

In conclusion, with many parts, mostly with large projected areas, it is more crucial than ever to spec out an IMM worthy of the process required for that part. This is not a simple hand calculation based on the type of resin used (X Tons of clamp force per square inch based on a specific type of resin). It becomes more complicated than that and is typically a function of the pressure distribution over the projected area during 2nd stage pack.

So how do you know what the required clamp force will be for your part? A flow analysis will provide the answer.

Bozilla Corp can perform a flow analysis to assist you in making the best choice for selecting an appropriate sized Injection Molding Machine for your project.

Contact Bozilla Corporation for your FEA and injection molding troubleshooting needs at and visit our website at www.BozillaCorporation.com.

We wish you a Safe and Prosperous New Year!