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Improving Process Efficiency – Packing Pressure

Process efficiency is one of the largest concerns in the industry today. Improving the efficiency and reliability of a process is a great way to reduce overhead, this post specifically addresses optimization of packing pressure.

Packing Pressure Window: Assuming the transfer is developed with a 1st stage short shot, you should determine the minimum and maximum pressure which provides an acceptable part to determine the process window.
Packing Efficiency: Once you determine the range or acceptable packing pressures you can select the best packing pressure based on your application. For example if it is an engineering part, you might select a pressure in the middle. If it is a commodity part, you may use a pressure near the lower end of the window to reduce part weight and material usage by as much as 4% for thick walled parts.
-Andy

Improving Process Efficiency – Transfer

This is another installment in a series of posts on improving process efficiency. Using the appropriate transfer from 1st Stage Injection to 2nd Stage Packing will help reduce equipment wear and overall energy consumption.

Regarding Energy: When you transfer when the mold is completely full, the pressure in the barrel increases sharply. This increase is caused by the machine’s attempt to maintain the programmed screw velocity. This increase in pressure causes a significant rise in cavity pressure during injection. This rise in pressure causes the need for additional clamping force to counteract injection forces.
Regarding Process Consistency: The process should be designed to use a 5-10% short shot at the time of transfer with enough packing pressure to pack out the part without flash. When normal viscosity fluctuations occur, the process should remain short during injection while enough pressure is available during packing to fill and pack the mold. This will provide a more robust and reliable process that compensates for normal variation.
-Andy

Improving Process Efficiency – 1st Stage Injection

This is the first in a series of posts I will make on easy ways to improve efficiency and reduce costs. Reducing the 1st Stage Injection time is one of the easiest ways to reduce both the cycle time and energy required to inject the polymer.

Regarding Energy: Increasing the rate of injection will increase the shear applied to the polymer. As with all polymers, as the shear rate increases the polymer viscosity is reduced due to shear thinning. This reduction in viscosity causes the overall amount of energy required to fill the mold cavity.
Regarding Cycle Time: Decreasing the time required to fill the mold will cause the polymer within the mold to begin cooling faster. This will reduce the overall amount of cooling time necessary to prepare the part for ejection.
-Andy

A Note to Educational Institutions

Whenever you are soliciting donations from industry, always make the donor feel that you are extremely grateful. It is the job of every educational institution to ensure that their contributors feel appreciated and that they are not sending their money down a black hole.

Make it Visible: Place placards or signs on donated equipment and material stating who donated it and when. Always keep the area around the equipment clean and professional looking to demonstrate respect. If a piece of equipment is no longer needed, offer it back to the company before taking it off display (they might offer you something useful in exchange).
Show Your Appreciation: Give the donor a placard, certificate, etc. that they can display in their lobby or conference room. This keeps your institution in their minds and gives the added benefit of others becoming aware of your institution. If this is dated, it will prompt the donor to keep their placards up-to-date.
Spread the Word: When possible, get the contribution written up in either in a paper or on a website. You should also have a special section on your website where all donors are featured. This will give exposure as well as create some potential competition between donors.
-Andy

Slowing Down the Screw

If you have to slow the injection speed down to prevent overshooting or gas entrapment during injection; then you are probably filling the mold too much during first stage fill.

Comment: We can use one speed to fill most of the mold, but we always have to slow the screw down as it gets to the the end of fill to prevent flash and gas entrapment.
My Response: The best approach to complete the filling of the mold is to transfer to pressure-controlled packing and let the screw slow down as the mold fills and the pressure builds in the cavity. Actually, the point where you start your slowdown is likely to be a good transfer position.
Additional Thoughts: Keep in mind, you should transfer from fill to pack at the position where the mold cavity is 90-95% fill. This is based on the mold cavity being 100% full with 1st stage only (when the first cavity fills if multi-cavity).
-Andy

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