Tip durability failures are often attributable to the sealing surfaces. This includes the check ring outer diameter (OD) and the front seat on a check ring style tip, and the OD and ball / poppet on those style tips. Wear to the OD from the processing of abrasive resins often results in excessive clearance with the barrel wall inhibiting a strong cushion. Corrosion or abrasive wear on the ball or poppet decreases its ability to function properly. Likewise, a check ring, rear seat, ball, or poppet that has been exposed to unmelt will result in a compromised sealing surface and hence cushion failures.

Wear to the front seat on a sliding check ring tip is usually the result of excessive screw speeds. Visible as a notch on the seat and sometimes the front of the check ring, this type of wear results in excessive travel of the check ring. Causes of processing issues will often be evidenced by the inability of the check ring to find its seat within the injection phase, resulting in an inconsistent cushion. Note that these issues are different from the slippage resulting from excessive OD wear. Front seat wear may usually be resolved by lowering recovery RPM or going to a fixed tip design.

The above-described failures result from normal use. Even when optimal designs have been determined and the best base materials selected, the most careful processors may still wear out tips as a result of the resins they use. Corrosion failure is different from a design or specification failure, however. Processing problems with brand new tips, or that result when changing to a new resin may be easily corrected. MSI will always take a close look at your operation to determine whether a different tip design should be used or if an existing design might be modified for better results. In other cases, shot size, the resins processed, and possibly machine speeds will mandate a different tip style. Regardless of your processing environment, the sliding check ring style tip should always be your starting point since it is the most versatile and durable and has the fewest drawbacks when in use.

Note: Successful processing of PVC-R most often requires a smear tip due to its extreme sensitivity to changes in flow path. For more information on tips for these applications, please call us.

Recommendations

The great majority of tips sold by Molders Services are sliding check ring. Of these, the overwhelming majority are now 4-piece designs. Why? They simply last longer and are more reliable. With the front seat separate from the retainer, we can make the seat with a harder, more durable material. This not only gives us a more durable front seat, but also provides a lower Rc stud, which may be drilled out if broken – an option virtually impossible when a compromised three-piece tip stud cannot be freed from a screw.

MSI can and does make retainers from stainless steel for highly corrosive applications. It is this kind of retainer option versatility that allows our customers to enjoy the improved performance of 4-piece tips at less cost due to the potential of using less expensive steel for certain applications.

Ask us about the tip alternatives best suited for your organization.

Wear: Cause and Effects

As screws and barrels begin to wear, material will start to get lost over the flight OD during recovery due to increasingly excessive clearance with the barrel. This clearance will initially reduce recovery output and increase shot recovery times. As this wear continues, the “lost” material will build up a heat history, which leads to burning and degradation, negatively impacting part quality.

As degradation continues, corrosive gases also cause screw deterioration, little-by-little, resulting in more opportunity for material hang-ups and even more burning. Screw to barrel clearance becomes more critical as you move from the feed port to the discharge end, meaning you can have a great deal more clearance in your feed section than you can in your metering zone before experiencing any issues.

What to Look For

A good guideline to use is 0.015” total clearance in the metering section, 0.025” in the transition section, and 0.035” in the feed section. Once the wear on your components reaches these clearances, it’s probably a good idea to begin monitoring your process output and predicting when components will eventually need replacement. Much like the car you drive, many miles can sometimes be driven when the low fuel light comes on. Unfortunately, sometimes you can run out of gas! MSI is here to help ensure you’re never stranded.

Is Replacement Needed?

Bottom line? Two crucial factors should determine whether a screw or barrel are in need of replacement: shot recovery time and part quality. Once cycle times impact costs or rejects increase due to unacceptable quality, it’s time to consider component replacement. In addition, tips can wear on both the check ring OD and the front seat, causing cushion control issues and the need for replacement. If you are not running your components until these issues begin to surface, you’re not getting your money’s worth! After all, screws can start looking really ugly long before output and quality issues surface.

Monitoring Components for Wear

So – do you have the personnel and time to inspect, measure, and monitor your process data to assure you’re getting the most from all your components? Most don’t and many more recognize that they don’t have to by relying on the professionals at Molders Services.

Consider the kind of precision information revealed in one of MSI’s inspection reports. For example, several sections display zone-by-zone wear on a screw and barrel, and on the check ring OD, as well as the total clearance between the screw and barrel. Extensive comments sections include expert observations and recommendations. Compare this information with your existing process data to determine which components are causing issues and whether they are significant enough warrant replacement.

Don’t leave productivity and performance to chance. Call us. That’s why we’re here for you.