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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.

5 Rules of Processing

To put it bluntly, if your technicians and engineers are not following these 5 simple rules your company is losing money.

Rule 1: Change One Aspect at at Time

By allowing the change to take place, it allows you to be deliberate in your changes.

Rule 2: Make Significant Changes

If you believe something is going to help, make a big adjustment and see what happens – if it went to far, you can always dial it back a little.

Rule 3: If a Change does not Work, Change it Back

Unnecessary changes cause the process to drift further and further away from the process standard.

Rule 4: Document The Process Outputs

There should be a standard documented process for each mold or die as well as a process sheet generated each time 1st piece approval occurs.

note: this is separate from the setup sheet which contains all the machine settings.

Rule 5: Document Every Change

Process changes are important, but also include anything else that affects the system such as repairs, cleaning, mold adjustments, and equipment changes.

Conclusion

This list may seem simple, but there are very few companies in the plastic industry which consistently use all 5 of these steps. The processors who are actually following these simple steps every day are some of the most competitive companies in the plastics industry.

Quick Note on Safety with PVC & Acetal

For some reason we are encountering a large number of companies who’s employees are not familiar with extremely important safety concerns of running both PVC and Acetal (POM) in the same plant.

Concern:

Heated PVC gives off hydrochloric acid while heated, this acid will erode most metal surfaces that are not stainless or plated with protective coatings like chrome. Acetal gives off formaldehyde which is also corrosive. While both PVC and Acetal can be dangerous by themselves, if these two gasses combine they cause a violent chain reaction which typically results in an explosion as well as a cloud of deadly hydrochloric acid. PVC, or polyvinyl chlorides, can never be processed in the same machine that has processed Acetal, also known as POM or Polyoxymethylene. Likewise, a machine that was used for molding Acetal cannot be used for molding PVC. Any combination of these two materials produces a deadly gas which can also cause an explosion. In fact, just a little bit of one exposed to the other can cause a chain reaction which can blow the hopper or end-cap off of the molding machine. Be aware that no purging compound or procedure exists that will allow you to safely process these two materials in the same molding machine.

Moving Forward:

Every company which molds both PVC and Acetal (POM) must have fool-proof procedures in place to ensure these two materials are never processed in the same molding machine, delivered in the same supply tubes, mixed in the same mixers, blended in the same blenders, ground in the same grinders, or dried in the same driers. It is also critical to ensure these materials are not processed in proximity of each other as the mixing of the gasses these two materials produce creates a very dangerous, corrosive, and potentially deadly hydrochloric gas.

The Best Plastics Training is Online

Over the past 16 years, Routsis Training has generated hundreds of online training courses and advanced skills-development labs which run on all current operating systems, web browsers, as well as through our smart-device app.

Announcement:

As of June 30, 2021 Marcom will officially discontinue support for the commonly used CD/DVD-ROM based training software known as CourseWorks.

Invite:

If you currently have older CD/DVD-ROM based training at your facility, Routsis Training welcomes you to contact us for an online demo of our new and up to date online training course and skills-development labs focused on today’s industry best practices.

We have ongoing in-house training solutions available for individuals, global corporations, and every company in between.

Testing Volumetric Feed Systems

Don’t assume that when you are getting consistent feed with your volumetric feed system as this needs to be tested.

Situation:

A customer was having significant issues with a PVC twin screw profile extrusion line where the line would break often during production without any obvious cause as well as provide profiles which would vary in dimensions over time. During investigation it was found that the die pressure fluctuated up and down approximately 10% in 2-3 minute intervals.

Investigation:

After checking some basic items we tested the material feed system and found that a 25% increase in feed (16RPM to 20RPM) resulted in a doubling of material feed (140lbs/hr to 280lbs/hr). After further testing we located a speed which provided the correct amount of material in a consistent manner to get the machine back up and running. After this correction, the inconsistency of the profile decreased.

Additional Thoughts:

You often have to check your equipment to ensure it is functioning properly. Just because a material feeder is turning at a steady rate does not mean the material is being conveyed properly in a consistent manner.

Torque for Clamping Bolts

Torque calculators are a great resource for the Scientific Molder as long as they take in effect the low strength of the cast platen.

Question:

We are trying to figure out what torque to use on our bolts when clamping in molds. I read that the bolt size and hole diameter make all the difference. If I send you those sizes, could you recommend those values for me?

My Response:

The proper way to determine max bolt torque is to use a calculator which allows you to select or enter strength of the platen material.

The grade of cast steel varies, but generally it has the strength of low grade cast steel around 30,000psi or 200 MPa. Since the integrity of most of these platen holes are generally suspect, it is safest to use a lower strength value such as 15,000-20,000psi or 120-160MPa.

Using a calculator such as the one below using a 5/8” steel bolt mating into a custom material with a yield strength of 20000, the recommended torque is 65ft-lb with a max of 86ft-lb – this falls in line with normal industry recommendations of using 50-60ft-lbs for 5/8” bolts (assuming you have at least 150% of the bolt diameter engaged).

https://www.futek.com/bolttorque/american

Additional Thoughts:

Many people get confused because a 5/8″ bolt might be rated at hundreds of ft-lbs of torque, but this does not take in account the significantly weaker threads in the platen.