Welcome!

This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

VISIT OUR STORE
VISIT OUR WEBSITE

Converting Percentage to Pressure

It is relatively easy to convert a percentage setting on your controller to the actual pressure applied to the plastic. The most important factor is that you need to know the maximum pressure capacity.

Question: How do I convert a pressure measurement which is provided in percentage? When the panel says 50%, what is that in pressure?
My Response: Percentage is per-cent or per-hundred. This is a ratio with 100 as the maximum and the percentage as a fraction of that. 50% = 50/100 = half of the maximum. To calculate this, convert the percentage to a fraction 50% = 50/100 = 0.50 and then multiply this by the maximum machine capability.
Pressure = (percent / 100) x Maximum
-Andy

Documentation without 1st Stage Short

Essentially, you cannot generate reliable machine independent process documentation based on process outputs if the mold cavity fills completely during 1st stage injection.

Problem: I see many companies trying to utilize machine-independent process documentation while still filling the cavity completely during 1st stage injection. Since the mold cavity is both filled and packed during first stage injection  it can only be repeated when the material has the identical viscosity at the time the process was documented. Each time the material viscosity changes, the amount of filling and packing during first stage injection will change, requiring adjustments to the overall process. For this reason, machine-independent process documentation is only a moderately helpful tool when the cavity is 100% full at the time of transfer.
Resolution: Fill the mold 90 to 95% full during first stage injection and then pack out the part using 2nd stage packing pressure. This will allow you the benefit of documenting important repeatable process outputs such as 1st Stage Time, Melt Temperature, and 1st Stage Short Shot Weight.
-Andy

Glass-Filled Nylon with Regrind

Since the regrind has variable sized pieces,
additional drying time may be necessary to ensure the thicker chunks are fully
dried. The regrind process results in irregular-sized pieces which have different cross sections.


Question:  Most of our parts use an industrial grade resin ( recycled PA with GF) and often with 20% of  regrind resin. This means that the viscosity can vary significantly as well as the size of the “pellet”. Any recommendation when molding with recycled PAGF resin?

My Response: Since the regrind has variable sized pieces,
additional drying time may be necessary to ensure the thicker chunks are fully
dried. You should always take measures to reduce the shear applied to this
material. This will reduce the degree of polymer chain and additive degradation
which takes place in the barrel. If the parts have thin walls, you will have to use higher 2nd stage packing pressures than you currently use, this can be more than the transfer pressure for many molds (assuming a 1st stage short shot is used).

-Andy  

Hydraulic vs. Position Transfer

When filling the mold completely during 1st
stage fill, many processes respond well to pressure transfer when compared to
position transfer. In either case, you will see even better results with a 1st
stage short shot (90-95%) using position transfer.


Question: I recently moved to another company and all the setups are using Injection Pressure as trigger to change from fill phase (Velocity controlled) to Holding ( Pressure controlled). Note that there since there is no pressure sensor in the mold but the machine injection pressure is used. A Engineer here tells me that this was done because while using a screw position switching point they had too much variation of the cushion leading to short shot and flash. What are the pros and cons of a pressure switching point versus screw position switch point? 

Regarding Hydraulic Transfer: Hydraulic pressure transfer is used the fills
mold completely during first stage. Once the mold fills, the pressure begins to
increase rapidly and the pressure transfer method is used to identify this
pressure spike and then transfer. When filling the mold completely during 1st
stage fill, many processes respond well to pressure transfer when compared to
position transfer. In either case, you will see even better results with a 1st
stage short shot (90-95%) using position transfer.
My Recommendations: The process should run with position transfer to
best compensate for the viscosity changes. The part should be 90 to 95% short
shot based on part weight. If this is a multi-cavity mold, 100% full is the
weight of all parts when the first cavity becomes full (this ensures all mold
cavities are short shot). The packing pressure should then be used to compete
mold filling and pack out the parts without flash. You will likely use a
significantly higher packing pressure, but will gain more process consistency
as a result.

-Andy

Improving Process Efficiency – Packing Time

A 2nd Stage Packing study is a critical aspect to optimizing any molding process. You should perform a gate seal test for each of your molds to ensure you are not using wasting energy with excessive packing time.

Gate Seal Time: To determine the time required for the gate to seal requires a gate seal study. To perform this test, you mold a series of parts using different 2nd stage times and weigh each part (without gates and runners). When graphed, you will see the part weigh increase with packing time until the gate seals. Once the gate is sealed, the part weight should stabilize for a cold runner mold. If a hot runner gates directly into the part, the part weight will stop curving and become linear at the time of gate seal.
Packing Time: When gate seal is desired, the optimal time used for 2nd stage packing is 10% greater than the gate seal time. When a non-seal condition is desired, the 2nd stage packing time should 10% lower than the gate seal or less.
-Andy 

  • Recent Posts

  • Recent Comments

  • Archives

  • Categories

    • © 2016 All Rights Reserved | Powered by Wordpress | Made with ❤ by Themely