Welcome!

I typically ask for more specifics, but in this case it is more
about the philosophy behind making the change.

Whenever making wholesale changes to the tool steel, you must take a scientific approach.

An example of this approach would be as follows…

1. Verify the melt temperature

2. Verify the mold temperature

3. Check the vents (you can often reduce the tonnage or place a piece of tape on the parting line to test if the venting is adequate)

4. Check the nozzle and hot runner system for any obstructions… or improper sizing. Depending on the application, you can perform a pressure loss study, actually dismantle the components, or try replacing them with different sizes.

5. Perform an in-mold rheology study (the gate may be too small if the mold cannot fill after shear thinning takes place… but, if there is no shear thinning present on the rheology curve… the gate could even be too large!!!) I strongly recommend performing these in-mold rheology curves at both higher and lower melt and mold temperatures as these conditions can influence shear thinning and viscosity.

6. After obtaining all this data, and is the problem does not resolve itself in the process of discovery, you should review the facts with the designers and processors to determine the correct course of action.

As a engineer… I always prefer taking a scientific approach to the resolution to any defect. This prevents rash and costly decisions from being made.

-Andy

A material such as this is a flame retardant grade of PC/ABS. By itself, PC/ABS can be a difficult material to process as it tends to have a very narrow processing window. What many people do not realize is the thermal instability of flame retardants.

It may seem counter-intuitive… but flame retardants tend to have extremely high degrees of thermal instability! These flame retardants which were added to the material often degrade easily due to shear, temperature, and moisture.

Initially, the material should be dried, even if it comes in sealed bags… and you should be molding the part in a machine which is using 40-80% of the shot size to ensure a very low residence time. 

Next, measure the actual melt temperature and the dewpoint of the material at the feedthroat and then compare these to the recommendations. You should also reduce the screw speed & back pressure, and increase the screw delay to reduce shear and barrel residence time.

You can always increase the number of gates on the parting line… since more gates don’t cause flash. If you instill a schedule for the routine cleaning of the mold, this will also help prevent the gas buildup.

An in-mold rheology test will best determine when shear thinning occurs during fill, as well as visually demonstrate the fill time where the rate of filling degrades the polymer.

When your process is stabilized and functioning properly… you really need to document the process based on the process outputs such as melt temperature, fill time, part weight at end of fill, plastic pressures, etc.

The science behind today’s polymers can create some materials which really perform great in their applications, but require very delicate processing. When the correct process is reached it is critical that you document the process and not just the machine settings so it can be repeated.

-Andy

In a recent webinar, I received this question form one of our participants…

It depends… but typically larger machines should be levelled every 6 months. The smaller machines tend to be more stable and rigid, requiring levelling every 12 months.

What is even more important is the fact that you need to measure any machine that is new to your facility every 3 months for the first year. Many molders assume that newer machines require less attention, but it may take up to a year for the machine to settle in.

Keep in mind, you should always level the machine by placing the level on the tie bars. Although bubble levels can be used, a laser level is significantly more effective at measuring the levelness of the machine. Clean off the tie bar before measuring and use a grooved level so that it properly rests on the tie bar.

-Andy

A customer of ours asked this great question the other day…

• Area of part & cold runner is 19 in.sq.
• Fill Pressure Actual (@ 1”/s fill speed) = 900psi hydraulic
• Hold/Pack Pressure Actual = 650psi hydraulic
• Intensification ratio for the press is 15.88.

Assuming you are not 100% full during first stage… the 10,322 psi calculation would be more correct.

(10,322 lbs/in*in) x (19 in*in) / (2000 lbs/ton) =  ~98 tons.

This is because the pressure is not distributed across the mold cavity until the mold cavity is full… assuming mold filling is completed during 2nd stage packing.

Ultimately, the pressure losses that occur during fill actually reduce the actual pressure the mold cavity realizes so the 98 tons calculation would actually include a fudge factor for safety.

If you were to take a more exacting approach to this calculation, you could preform a pressure loss study to determine the actual pressure loss through the nozzle and sprue as well as during fill. From this pressure loss data you could estimate the average pressure distribution across the mold cavity and relate it to the 2nd stage packing pressure distribution.

In most cases, the simple approach used above would be satisfactory for the typical custom molder.. especially since it would accommodate a small fudge factor to compensate for variability and machine inconsistency.

Molders who perform fewer mold changes… or are purchasing a machine specifically for an application should perform a more detailed pressure study.

-Andy