Andy Routsis: November 2009 Archives

Most platen deflection occurs within the stationary platen due to the fact that it receives the least amount of support. It also has a large hole cut out of the center for the sprue bushing.

There are typically two different instances in which deflection of the stationary platen occurs. The first deflection happens when the mold is clamped under full tonnage. The second deflection takes place as the polymer is injected and packed into the mold.

The Platen Deflection Test is used to determine the amount of stationary platen deflection that occurs during the molding cycle. Basically, a dial indicator mounted to the machine base, and is placed as close to the center of the platen as possible.

The results of this test should be retained with the machine’s records and used as a baseline for future deflection comparisons. This is valuable information - as it allows you to track the degradation of the stationary platen’s integrity over time.

You can often reduce platen deflection by increasing the nozzle forward force.

Our intelligent molder series of training covers this topic in much more detail. Also I discuss these tests in our free online webinars: http://www.traininteractive.com/free/webinar/player/

-Andy

Note: After a few follow-up questions it was determined that the part is full during first stage, and changes cannot be made to the shot size, transfer position, or fill time.

The short answer is... yes, weight can be a great way to control tolerances for a molded part.

Unfortunately, the root of your problem is the fact that the part is completely full during first stage, so the process cannot properly compensate for material variation. In this situation, you must fill the mold completely during first stage to meet the validation requirements. Even with sinks, you are actually packing the part during first stage fill. Since most materials will exhibit viscosity variations of approximately 5-10%... the degree of packing that occurs during first stage fill will vary greatly with material variation. These variations can result from machine changes, ambient temperature changes, humidity changes, as well as the obvious variations in the material.

In the long run, it is much more beneficial to establish a process by separating fill from pack. Filling with a short shot during first stage... and packing out the part during second stage will allow the part to fill a little more or less during first stage. completing fill and packing the out during the second stage will ensure velocity is not being used to perform the packing duties.

In your current process... you have a slight tolerance in your process, and I suggest you do your best to slightly increase the amount of material that enters the mold during fill when the weight is low, and slightly reduce that amount when the part is heavy.

In the long term, you should also try to focus on documenting and validating more process outputs rather than process inputs. When you validate all your process inputs, as well as all your process outputs... you eliminate all your opportunities to compensate for material variation.

-Andy

Here at A. Routsis Associates, Inc. we like to thank all of our readers and contributors for their support... but also mention that there are great blog entries in our archives many of the more recent readers may have missed...

A few of the popular archived entries are listed below:

Just to clarify for our readers... many injection molders will position insulation plates between the A and B clamping plates and the stationary and movable machine platens. The purpose of these plates are two fold... (1) to prevent the machine platen from influencing the heating and cooling of the two mold halves in an attempt to stabilize mold tempertaure. (2) to reduce the heating and expansion of the platens... which may affect mold safety on larger machines and clamp tonnage on toggle machines. Over time, this can also affect the squareness of the molding machine.

Most processes can benefit from these plates as they tend to eliminate variables in the process and reduce machine maintenance. There are two ways to go about adding insulation plates to the molds.

Wrong Way - Many injection molders solve these problems with generic plates they just wedge between the mold and the platen during setup. I am not in favor of these plates as they get damaged easily and can shift during production. If they get damaged, they can apply uneven clamp tonnage which will affect part quality and platen alignment. If the plate shifts, you can create a serious risk to both your employees and your equipment.

Right Way - Such plates should be individually fitted and bolted to both the A and B half of each mold. This approach will help prevent damage to the insulation, and ensure consistent performance in the application. This may seem like an expensive option initially, but the reduction in die setting time and uniformity of clamp tonnage should outweigh the initial expense.

These systems most benefit anyone operating a hot runner system, hot sprue, or high-temperature molding application with oil-coolant will benefit from an insulation plate between the clmap plates and the platens.

-Andy