Injection molding produces large quantities of parts by injecting molten plastic into a mold cavity. Two hardened steel plates closing together create a hollow, trapping the plastic between them. The molten plastic goes into the mold cavity under high pressure, where it cools and solidifies.
Extrusion blow molding machinery often uses blowpin assemblies. The blowpin assembly is responsible for transferring the molten plastic from the injection barrel to the mold cavity. The assembly must withstand the high-pressures and temperatures associated with injection molding and the repeated cycling during production.
We’ll explore several factors when choosing a blowpin assembly for your mold.
Purpose of the Blowpin Assembly
There are three main uses for a blowpin assembly. First, it helps compress air to inflate the parison inside the mold cavity using the blowpin stem’s inner channel. The container then takes on the profile of the mold cavity.
Second, you can use the blowpin’s tip to measure the distance between the inside and outside of the container’s neck. Lastly, you can also cut the flash neck, leaving a smooth, flat neck with the cutting ring touching the striker plate.
The cost is worth it if you buy blowpins made for a specific neck finish and container size. Cycle time savings alone can pay for them quickly.
Background of Blowpin Assembly Designs
Over time, various iterations of blowpin assemblies have resulted in significant advancements.
- Indirect Water-Cooled (IWC)
IWC blowpin assemblies were standard in processors some 20 to 30 years ago. The threaded blowpin tips of IWC blowpins have a cooling system where water circulates through the blowpin stem. You can insert a vented cutter spacer behind the cutting ring to release some pressurized air from the mold.
To aid in cooling the neck flash, you can cut out holes at a 30-degree taper in the spacer. These assemblies are great for necks 28mm and smaller with a loose tolerance or a spin-dome finish.
- Direct Water-Cooled (DWC)
The DWC blowpin assembly was the next iteration of this popular cooling system. DWC blowpins have a water-circulating tip threaded or pushed onto the stem; O-rings cool the tip directly.
By touching the tip, water pulls heat more effectively and quickly from the container neck, reducing cycle time. However, O-rings prevent exhaust/vent air from escaping through the tip.
- Full-Length Sleeve with NPT Port
Next-generation designs featured a full-length sleeve with an NPT port and tapered holes for direct air cooling. If your mold has a captured neck, consider using a spring-loaded deflashing sleeve.
A deflashing sleeve contains serrations on the bottom that grasp and extract moil from the molded container cleanly. As the moil goes past the stripper plate, it dislodges from the sleeve, leaving a top-flash-free container.
- Recirculating Air
Recirculating air was a feature added to the next-generation blowpin assemblies. Adding a tube with a thin wall to the assembly allows the blow air to recycle within the mold.
The blow air will go through the air tube’s inner channel. Spent air from the mold cavity escapes between the air tube’s outer wall and the water tube’s internal route. You can accelerate the cooling and form retention of the container by recirculating the air in the mold.
Modern extrusion blow molding machinery typically includes recirculating air option controls and timers. Close the exhaust valve and maintain full pressure to maximize blowing.
Suppose there’s an attached pressure relief valve to the top of a recirculating air blowpin system. In that case, you can use it independently of the extrusion blow moulding machine.
The Blowpin Tip
When designing a blowpin assembly, making the tip wall as thin as feasible is essential. You may swap parts between bottles with different neck diameters, ranging between 20 to 70 millimetres. Over 70 millimetres is indicative of a less common design.
When trying to achieve a snug fit at the neck, the blowpin tip is essential. DWC maintains a more uniform size, which is important for today’s precise neck finishes.
Best Materials for Blowpin Tips
- Beryllium-free copper is the most common material for blowpin tips.
- Copper is ideal for preventing the neck of a container from overheating.
- Aluminum is a good choice for cooling but is not very long-lasting.
- Stainless steel outlasts copper and aluminum but provides poor cooling.
- Highly polished steel is good if you work with Polypropylene or another “sticky” material.
But the best option, if in your budget, is copper with a nickel/Teflon coating for additional lubricity and longevity.
Adding steps to the blowpin tip allows air to blast closer to the neck, smoothing HDPE or comparable materials.
But what if your necks are oval instead of round? Conventional wisdom holds that an oval shape is best for cutting necks. You can reshape the blowpin tips by cutting an oval, a cheaper alternative for achieving the same result. This is now commonplace because of consistent results.
Finally, we recommend a stabilizer pin as a dummy when making an offset neck container. To accommodate this, there’s a small pocket on the mold. This is on the reverse side of the neck.
Mounting the stabilizer pin to the calibration plate doesn’t require as precise centering or alignment as the blowpin assembly.
Most pins are short shafts with a grooved or knurled ball at the end to grip the container. The container will not tilt or twist when you open the mold because of the stabilizer pin. It keeps the container linked to the blowpin system, allowing the mold to travel back under the extrusion head.
You should see increased profits if you incorporate these design considerations into your extrusion blow molding machine processing. It’s essential to keep in mind the adage, “It takes money to make money.” Acquiring blowpin assemblies for each mold set may initially seem excessive. If you create assemblies with the mold in mind, you’ll save time and money.
For more information on blow molding machines and resources, contact Pet All Manufacturing Inc. today.