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With I am asked how many vents should be added, I always say more. Each material has design guidelines for vent depth which are typically very helpful.

Ultimately, the more vents you can have on the part, the better. Obviously… strategic venting, such as a porous steel in a boss, can be very important; but getting as much gas out of the mold prior to that boss will minimize the amount of localized venting that is required. Also consider venting the runner, sprue, and cold slug wells.

Since the polymer only flashes as a result of thickness, the width of the vent has virtually no effect. As a result, you can make the vents relatively wide. I have seen large vents taking up high percentages of the parting line with great success.

One commonly overlooked aspect to venting is the vent drop. Behind any well designed vent is a substantive vent drop.  These drops are deep grooves, which channel the gas from behind the vent to the outside the mold. Think of the vent as a gate designed to transfer the gas from the mold to the vent drop.

Lastly, high clamp pressures may reduce the effectiveness of your vents… and may close them completely.

Many mold designers put extensive thought into the getting the polymer into the mold… but pay little attention to how the gas will get out of the way. Most of the molds I see with vent clogging problems often have thin vents, too few vents, no vent drop, or combinations of all three. Give the gas trapped within the mold many wide vents with a big vent drop to help it escape to the atmosphere.

-Andy

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