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Shorts, Sinks, & Flash

During a recent discussion, a manager related to me that their most common defects are shorts, sinks, and flash. She then stated that this is inherent to the molding process could not really be changed.

Shorts, Sinks, & Flash: Whenever these three are the top defects at a facility, it is because the molds are being filled too much during 1st Stage Fill. When the mold fills too much during first stage fill, the packing pressure is primarily used to ensure gate seal since the mold cavity is full. This only works when the material viscosity is identical to the time the process was established. When the viscosity drops, the mold will flash, when the viscosity rises, the mold will short or sink.
For more about this, please feel free to read:
-Andy

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

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