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Changing Colors, Rising Costs:How To Reduce Thermoplastic Molding Scrap

With The Right Purging Agent

Run Clean, Run Lean


2/6Run Clean, Run Lean

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Large amounts of plastic

scrap have a negative impact

on molders protability.

Wasting plastic resin dueto poorly designed tooling,

process capability, resin quality,

color changes and product

contamination is frustrating for

businesses seeking to be more

efcient with materials.

Increased frequency of material and color changes caused by the just-in-time production methodologyare often the root of the problem. With heightened customer demand for a wide variety of customized

products, many businesses must learn to manage rising plastic scrap rates. One of the best ways to

reduce scrap is to increase your focus on purging operations.

For example, reducing the time required to purge a 500-ton injection molding press with an

estimated burden rate of $98 per hour by 10 minutes per day results in $5,486 of savings per year.

Purging thermoplastic molding machines with less material in a minimal amount of time inuences

the competitiveness of many processors. When you factor in the total cost of your chosen purging

methods, this becomes a signicant advantage.

Using Preventative Purging To Reduce Scrap Rates

Preventative purging is a best practice for molders working to reduce scrap. Consider the following

example of a Tier 1 automotive injection molder that was experiencing high scrap rates:

A molding companys scrap rates were in the 10-15% range when they decided to conduct an

investigation. During the study, the molder charted defect rates and discovered that the number of

defects increased steadily after manually cleaning screws.

The rms only choice was to pull the screw a second time, but their defect rate inevitably rose

again until they adopted a preventative purging regimen. As the molder optimized the timing of theirpreventative program, they decreased scrap rates to under 1%.

It is a helpful rst step for molders experiencing high scrap rates to chart their number of defects and

investigate whether a preventative purging schedule makes a difference.

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Using An Adequate Amount Of Purging Agent

One common cause of increasing scrap rates is insufcient purging during downtime and

changeovers. When molders do not use enough purging compound, reject rates increase.

Molders should stop the practice of using a predetermined amount of purging compound for every

purge. Some companies tell machine operators to use a specic amount of purging agent, adopting

a one-size-ts-all approach.

However, the required amount of purging compound for each changeover varies by situation.

For example, the more time that has passed since a screw pull and manual cleaning, the more

contamination molders see in their system.

Especially in the case or injection molding machines,

carbon tends to accumulate in low-pressure areas

(dead spots) of injection molding machines such as

the following:

Nozzle area

Mixing sections in screws

Hot runner angles

Insulated tips

Vents

After each purge, the machine must be carefully

inspected to ensure it is purged sufciently.

Plastics manufacturers should train molding machine

operators to purge until cleaning is complete, within a

certain tolerance range, prior to contacting a supervisor.

Placing a machine back into production after using a

preset amount of purging compound is not advisable.

How To Use Purging Compounds With Mechanical Purging

When using a purging compound to achieve a sufcient purge, there are three basic tips operators

should follow:

1. Use the maximum safe screw speed while purging and displacing the purging compound

2. Use the maximum safe backpressure while purging and displacing the purging compound

3. Keep the screw completely forward while purging



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Using high screw speed and backpressure causes

greater agitation and turbulence, which increases

the purging compounds scrubbing and cleaning

capabilities. Additionally, keeping the screw forward

focuses the agitation of the screw and purging

compound at the front end of the machine where

color and carbon deposits are most often a problem.

Reducing Scrap With

Manual Cleaning

A critical component of molding machine purging

is the full manual cleaning of the screw. If screws, barrels, nozzles, dies or die heads have not been

cleaned in a long time, a purging compound may not be enough to reduce scrap rates. Purging

compounds work best when used as a preventative measure instead of a reactive measure. As a last

resort, machine operators should clean everything manually and then implement a purge program.

Difficulty Starting Up After Weekend Shutdowns

At the end of the work week, many molding machine operators simply turn off the barrel heaters and

believe that this is an effective shutdown. Upon starting the machine back up on Monday morning,

scrap rates are high and it takes a long time to begin production.

It is a common misconception that shutting the heat off causes the resin to cool down and solidify

harmlessly. In actuality, it may take hours for the plastic in the barrel to cool enough to stop

degrading. Over this period of time, carbon forms in the barrel and adheres to metal surfaces.

When operators shut the heat off with plastic in the barrel on Friday evening, carbon builds up over

the weekend and slows production on Monday morning. The most effective solution is to seal the

screw and barrelprior to shutdown.

Implementing Weekly Sealing

Sealing entails the use of a heat-stable purging compound during shutdowns and extended downtime,

including preventative maintenance periods or mold changes. Heat-sensitive resin is displaced by a

purging compound that prevents oxygen from entering the screw and barrel as the purge cools.
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At the time of startup, molders should run another barrel of purge through the system and change to

the next resin. Before long, production is on track.

Weekly sealing also decreases the need for maintenance purging. Carbonization occurs if resin

is kept under heat in the presence of oxygen. However, the degree of carbonization varies in each

situation. Some resins enter a danger zone more quickly than others.

While regular and preventative maintenance purging is advisable, it is highly benecial to use a heat-stable purging compound for temporary or extended shutdowns and sealing.

Nozzle And Check Ring Cleaning

After completing general purging procedures for cleaning the screw and barrel, it is recommended

that molders charge the machine with a small quantity of purging compound and perform short,

high-velocity injection shots.

This action blasts purging compounds into tight areas and difcult-to-reach spots that a purging

agent would not be able to access otherwise. Additional areas of your molding machine also benet

from this technique.

Mixing nozzles are well known for having color hang-up and degraded material hang-up due to

design. It is important that you clean machines thoroughly to prevent high scrap rates.

Hot Runners

Purging compound suppliers recommend different procedures to purge hot runners. Some mold

releases are not appropriate for hot runner cleaning, so plastics manufacturers should take this into

consideration when making a purchasing decision.

Other purging compounds, however, can be used in either a closed-mold or open-mold method

depending on the resins processed, mold design and cleaning difculty.



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About ASACLEAN

ASACLEAN purging compound is manufactured and distributed by Sun Plastech Inc., a subsidiary of Asahi Kasei Chemicals

Corp. Asahi Kasei is both ISO 9001 and ISO 14001 certied. To learn more about how ASACLEAN decreases your purge time

and increases your manufacturing efciency, visit www.asaclean.com.

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Ready to learn more about reducing scrap

and achieving an effective purge?

Call 1-800-787-4348 to speak with an

ASACLEAN expert at Sun Plastech Inc. today.

It is a best practice to raise hot runner temperatures,

particularly gate temperatures, in order to help

loosen deposits while purging. Short, high-velocity

shots are also helpful when purging using open-

mold procedures.

To purge valve-gated hot runners, machine operators

should shut off all the gates and place the pins in a

closed position in order to purge troublesome gates

individually. This technique causes greater pressure

to be delivered to each gate and allows for a more

thorough cleaning.

Correct purging procedures are instrumental for keeping thermoplastic molding machines running

efciently. For the most effective cleaning, follow these tips and techniques when purging and

conducting manual cleaning.

Molders who have questions about how to reduce scrap rates and machine downtime should

continue researching the best purging practices. An industry expert is also able to offer guidance

on preventative and maintenance purging procedures.
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