Leak Detection Fundamentals: Leak Testing PET Containers Produced by the Two-Step High Speed Stretch Blow Molding (SBM) Process

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Maintain production efficiency while ensuring the integrity of each container you produce.

The Stretch Blow Molding Process (SBM) is a method of producing PET (polytheylene terephthalate) containers that have enhanced physical properties such as strength, clarity and gas barrier characteristics. In the SBM process, a plastic such as PET is molded into a preform using the injection molding process. Preforms are typically produced with the neck and threads (the “finish”) at one end. After cooling, the preforms are fed into a reheat stretch blow molding machine and heated (typically via infrared) above their glass transition temperature, then blown using high pressure air into metal blow molds so that they realize their final shape. The preform is always stretched with a core rod; in some polymers, including PET, this stretching causes strain hardening of the resin, which enables the bottles to resist deforming under the pressures formed by carbonated beverages, which typically approach 60 psi.

Fundamental Facts

Common Defect Types in PET Containers:

  • Sealing Related Defects – The ALPS leak test seals and pressure checks every container, providing a functional test of every seal surface.
  • Holes – Pressure-based leak testing will detect leakage regardless of its location or appearance.
  • Height and Topload Capacity – Auxiliary inspections can be added to ALPS leak testers to detect containers outside a given height parameter, and to verify that the container does not deform at a specific sealing force.

The SBM process is ideal for high speed, efficient production of clear containers with precise neck finishes. Accordingly, the majority of consumer clear bottles are produced this way, including those used to contain carbonated soft drinks, water, sports drinks, juices, specialty beverages, ready-to-drink teas, dressings, sauces, and food oils.

As with any consumer product container, leak testing is imperative to ensuring consistent integrity. ALPS Inspection offers leak testing equipment appropriate for containers produced by the SBM process. ALPS testers can be integrated into your existing production line to maintain or increase efficiency and confirm quality in every container you produce.

ALPS NGR NexGen Rotary  Leak TestersALPS offers several NexGen Rotary leak testers for PET bottles produced at high speeds, developed based on experience running thousands of different container types. Many of the available features were designed specifically for PET with many of the applications for hot fill and custom bottle applications. Like all ALPS leak inspection equipment, the NexGen family of leak testers deliver fast and accurate results.

ALPS NexGen Rotary testers provide several benefits, including:

  • Scalability: Standard machine sizes available for speeds up to 1,200 BPM (72,000 BPH).
  • Quick-change no-tool change parts for rapid and efficient changeovers. A complete mechanical changeover can be completed in 10-15 minutes for even the largest machine size.
  • Leak Test Controller (LTC): Test heads on each station can be easily removed for maintenance and replaced with a test head from stock with a downtime of less than 30 seconds.

ALPS Speed-Glider  Leak Testers The Speed-Glider family of leak testers is appropriate for higher speed or higher sensitivity applications where single-station machines are not fast or sensitive enough. The Speed-Glider uses a timing screw to space the bottles for leak testing and to provide positive bottle handling. Moving test heads require fewer test stations to achieve higher speeds and greater test times, resulting in a more accurate leak test. The machine incorporates a single servo-driven slide to match the slide speed to the timing screw. Speed-Glider testers are available in configurations including 2, 3, 4, 5, 6, 7 and 8 stations for speeds from 30 up to 500 BPM. The conveyor underneath the leak tester is typically synchronized to the machine, by means of a 4-20 mA signal, to match the conveyor speed to the slide speed and ensure the most efficient performance.

ALPS Flex Pitch Multi-Station Moving Head  Leak TesterThe ALPS Flex Pitch leak tester is a multi-moving head, high speed leak tester that detects defects in wide range of bottle types. The tester mounts over an existing conveyor, so no line modifications or bottle transfers are required. The innovative design of the Flex Pitch tester maximizes flexibility, as test heads can be preset to the required pitch, enabling one-touch changeover between containers of varying diameters and making it particularly suitable for unusual container shapes. The machine is available in 2, 3 or 4 moving heads that offer 100 % leak testing and top seal surface defect detection at speeds up to 250 BPM.

Innovation from ALPS

In our drive for continuous improvement in package integrity, ALPS has developed capabilities that augment industry standard inspection practices, including a patented turntable vacuum system, topload deflection detection and laser height inspection options, as well as an integrated coding ad line integration options.

Turntable Vacuum System (U.S. Patent #7,559,232). The patented ALPS Turntable Vacuum System was designed to detect smaller pinholes in the gate of PET bottles. The standard ALPS leak test uses the pressure decay method, which detects a pressure drop from the inside of the container as air moves out through a hole. The bottom of the bottle must seal against the turntable to achieve the full effect. With the Turntable Vacuum System, a high vacuum is applied underneath each leak test station through a hole in the turntable. If there is a pinhole in the bottle, the vacuum draws air through the pinhole at a high rate, creating a greater pressure drop inside the bottle and allowing for the detection of significantly smaller holes during the duration of the leak test.

Topload Deflection Detection and Laser Height Inspection Options

The topload deflection detection option is a go/no-go inspection that uses an additional sensor on each probe cylinder. The sensor is adjusted to activate at a specific height when the test probe is sealed onto the bottle. If the container partially collapses as a result of the probe force, the switch turns off and the bottle is rejected. This method detects bottles that are grossly short or deformed due to a lack of topload capacity.

The laser height inspection option was designed as a result of customer demand for a method for monitoring container heights directly after bottles are blow molded. A through-beam laser array is mounted over an infeed or exit starwheel. As the container passed through the sensor, an “average height” is taken of readings from the top of the bottle. A small amount of vacuum is typically applied underneath the inspection zone, to assist in stabilizing the containers.

Coding Device Ready Option

ALPS has also developed a method for integrating laser coding of PET containers onto the inspection turntable. This takes advantage of the fact that containers’ positioning is highly controlled for leak testing. The coder is positioned on the infeed side of the testing machine approximately 90 degrees from the bottle entry point. Interface signals are provided to track containers, trigger the coder, and ensure that the coder is working.

Line Integration Options

In keeping with ALPS desire to help you maintain or increase your efficiency while ensuring the integrity of each container produced, ALPS leak testers are designed to integrate with a variety of production line types, including high-speed photo-eye, tabletop conveyor, and air conveyor designs. For high speed PET lines, ALPS leak testers are equipped with three upstream sensors to regulate the speed and keep the infeed timing screw full of containers for the most efficient handling. Testers are designed for easy installation onto an existing continuous tabletop conveyor. If the production line utilizes air conveyor, which is common for high speed PET, a tabletop conveyor section is required for the leak tester. In this case, bottles are fed directly from the air conveyor into the ALPS timing screw infeed. On the exit side of the tabletop conveyor, bottles are then transferred back onto the air conveyor.

Common Defect Types in PET Containers

There are a wide range of defect types that can be found in PET container production. Even the smallest defect can produce negative outcomes, including poor or inconsistent product quality and brand reputation. ALPS leak testing equipment is engineered to identify the following types of defects:

  • Sealing Related Defects (e.g., Short Shots, Nicks, Cocked Necks) – The ALPS leak test seals and pressure checks every container, providing a functional test of every seal surface. ALPS specifically uses the lowest test pressure in the industry, to enable low sealing forces and the ability to tune the Seal Surface Inspection. Alternate probe seals (softer or harder) and probe designs (fixed, tilting, etc.) can be provided to ensure the integrity of the most critical sealing related defects for a given container.
  • Holes (e.g., Gate Leaks, Gate Cracks, Contamination Defects, Misformed Bottles) – Pressure-based leak testing will detect leakage regardless of its location or appearance. The most common area for small leaks in a PET container is the Gate, which is the injection point on the bottom of the container. Leaks can occur anywhere in the bottle if there are material issues (e.g., contamination from recycled material) or process issues affecting the proper formation of the bottle.
  • Height and Topload Capacity – Auxiliary inspections can be added to ALPS leak testers to detect containers outside a given height parameter, and to verify that the container does not deform at a specific sealing force (topload). Height checks can also detect a “perfect short shot” which describes a defect caused in the pre-form injection stage. In a perfect short shot, the plastic doesn’t reach all the way to the far end of the injection mold, but it is even enough across the entire finish to pass leak inspection; while it is shorter than desired, it is not rejected. A typical short shot is low area in an otherwise normal finish, so the seal sits on the rest of the finish, but the low area doesn’t seal, so the container is rejected for a leak.

How Can ALPS Help?

No matter the type of container you produce, leak testing helps protect you and your customers from defects that can result in damaged products and damage to your brands; it also protects your financial investment by preventing waste and protecting inventory. ALPS equipment is designed to make leak testing easy and automatic, by integrating seamlessly into your Stretch Blow Molding production line and providing options to meet any number of specifications across container shape and size, testing methodology, and line speed.

ALPS understands the SBM process and will work with you to understand your individual inspection needs. With several machine types to choose from and a range of targeted testing and handling options to meet the leak testing needs of almost any line or container type, ALPS has the right leak tester for you.