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The Depth of Eyebolts…

I received a quick question from a molder overseas via email…

NG
How deep should an eyebolt be screwed in when performing a mold change?

My Response
Although most bolt manufacturers recommend at least 1.5 times the bolt diameter… I believe threading the eyebolt at least 2 times the diameter will provide a much higher degree of confidence.

Most mold makers will thread the holes to a depth of 2.5 times the diameter or more… but it is always best to check the depth to make sure.
Additional Thoughts
When specifying your tooling, always make sure the eyebolt holes are at threaded at least 2.5 times the diameter of the eyebolt.
-Andy

Minimizing Cooling System Cost…

I was recently asked the question…
Tom
Is it worth investing in a highly conductive mold material like QC7 to minimize cooling time?
My Response
You can reduce your cooling time a couple ways. The first way to reduce cooling time is to increase your rate of heat transfer. That means you must increase the thermal conductivity of your mold material The second way to reduce your cooling time is to move your cooling lines closer to the mold cavity surface. By reducing the distance between your cooling lines and cavity surface you effectively reduce the thermal gradient between your cooling fluid and polymer melt.
My Thoughts
The amount you should invest into increasing the efficiency of your cooling is dependent on many factors. You must perform a cost analysis on your expected output with different cooling system designs. Use the expected cooling time and cost of each cooling system in your business plan to determine which design is most cost effective for you. 
Be aware that a thermal conductivity value five times greater than steel will not result in a five times greater cooling rate. The cooling rate will become limited by the heat conduction through the plastic and the heat convection to the mold coolant.
-Andy

Degradation of ETPU…

I received this question yesterday…

AS
We are using Isoplast in a hot runner system. The press was shut down. When the sprue, manifold, and barrel heaters were turned on, the material started oozing as a white material covered in black specs. What would cause this behavior?
My Response
The material you are processing is a ETPU (engineering thermoplastic polyurethane) which tends to be prone to degradation, especially when the material is not properly dried.
It is critical that you determined the dewpoint of the material at the hopper and compare the melt temperature, dewpoint, and drying recommendations of the material.
Additional Thoughts
Any time you are using a hot runner system, you are adding more heat and residence time to the material. If the hot runner design is not specifically tailored to reduce stagnation and hot spots, it is very likely that hangups and degradation will occur within the hot runner system.
-Andy

Using Processing Aides with PP…

As a response to an earlier blog, I received this follow-up question…
KD
Are there any additives on the market that help to lower melt flow
index MFI of polypropylene?

My Response
There are many additives that can be used with Polypropylene… but it requires research, and significant testing, into the application. I wish I could just recommend an additive, but I have encountered many additives which will either increase or decrease the number and size of semi-crystalline regions within the polymer.
Additional Thoughts
Even colorants will have a significant affect on the degree of crystallinity within the final molded part. For example, the color green might cause the part to be larger than specification while another color, from the same supplier, causes the part to be significantly smaller than specification.
I have seen small amounts of additives significantly affect the optimal tact temperature of a polymer and even cause a process to become auto-thermic where the front zones of the machine actually become higher than the zone temperature setpoint.
-Andy

PC+PET Blend Screw Slip and Feeding Problems

I was just asked the question…
Kevin

We are making some products using a PC+PET blend. We are having issues with material feeding / screw slip and screw stall. Do you have any idea what the exact issue may be?

My Response

Since you are processing a blend of two materials, the melting characteristics of each will be slightly different.
 
The goal during screw recovery is to maximize the material’s adhesion (tact) to the barrel in the feed zone to ensure the best conveyance to the transition zone.
 
The rear zone temperature is often lower than the final melt temperature in such blends. This is because only one component generally needs softening to ensure proper tact between the pellet and barrel while in the feed zone. Too much melting can often result in one component actually behaving like a lubricant which can inhibit conveyance.
 
The best way to ensure the optimal recovery is to perform a tact temperature study: 


My Thoughts
 
It may be beneficial to pull the screw and ensure there is no significant wear in the feed zone of the screw, this region of the screw may need polishing or surface treatment to ensure proper conveyance.
 
Also, make sure you are using either a recovery delay or suck-back before screw recovery to ensure there is no excessive pressure on the screw and material when recovery begins.
 
-Andy 

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