Multiple preform injection molding technology with improved barrier properties

The PET industry has adopted a variety of barrier technologies. However, one of the technologies - multiple injections of PET preforms - has not been successful. With the joint efforts of partners, Netstal-Maschinen AG has now made this technology capable of producing high-quality preforms further developed.

In the PET industry, barrier technologies are increasingly receiving attention. In order to have a satisfactory barrier effect for PET packaging, there are currently three proven barrier solutions that people have adopted. Among them, the multi-layer method (also called "composite injection molding") is to inject the PET substrate first, and then to inject the barrier material through the thermal core to form an intermediate barrier layer. This method requires higher skills for operators and requires strict quality control. Another barrier solution is to mix the barrier material directly into the PET stock. The advantage is that the mixed material can be processed using a standard preform production system without taking any other technological measures, and the operator does not need to have any other knowledge. However, the disadvantage is that the incorporated barrier material tends to make the PET opaque. The third method is called “coating method”, ie, plasma coating is performed on the inside or outside of a blow-molded hollow body or preform, and the process is usually complicated.

In addition, there are other technical solutions for PET preform barriers, such as multiple injection molding, which are characterized by starting from the inner injection molding and then performing multi-layer injection molding during parallel processing. Unfortunately, in recent years, this technology has been ignored.

In the industry, multiple injection molding processes used to make preforms are often considered difficult and complex due to the fact that, first of all, each layer of the preform must be very thin. However, in actual production, the core may deviate from the center due to the pressure difference, so that an even distribution of the wall thickness cannot be achieved. In addition, when the first molding cycle is completed, the preform body must be transferred to another mold cavity for injection molding of the next layer. But so far, this process has still been accomplished through rotation, and this can easily lead to delamination between layers.

In order to solve these problems, Netstal-Maschinen AG has worked closely with the Spanish mold maker Molmasa, and Spain's raw material manufacturer Novapet. Through intensive work, it succeeded in making the multiple injection molding process economical, reliable and renewable.

The innovatively modified molds and machines now produce preforms with a wall thickness of only 1.2 mm in the first production cycle. Due to advances in raw material technology, the risk of stratified delamination is also avoided. When it is necessary to change the cavity, the rows of cores can move in parallel, replacing the original complex rotary motion. In short, these technological advances paved the way for the implementation of various barrier schemes and the application of other technologies. In the future, even a mixture of stretched polyethylene (OPP) and barrier materials can be used. In addition, other materials may be adhered to the first layer in order to obtain different color effects.

PRElactia preform

At the K2007 exhibition, a preform called “Prelactia” (a preform that absorbs light through a carbon-titanium additive) was first seen in the world. This is a light-blocking dairy preform produced using a multiple injection molding process.

In general, in order for the PET container to reflect ultraviolet light, white and TiO2 must be added, and the added concentration is generally about 12%. As a highly wear-resistant additive, TiO2 can make the container less susceptible to wear. However, since TiO2 is relatively expensive, it is generally possible to add gray or black in order to reduce the amount of TiO2. However, grey or black milk bottles do not sell well in the dairy market. In order to solve these problems, multiple injection molding is an ideal solution.

Multiple injection molding process

In the multi-injection production of PRElactia preforms, a SynErgy 2C2400-2150/900 injection molding machine with two injection units was used. The raw materials came from Novapet, the company's raw material for the manufacture of PRElactia preforms. It is called "Dairy2 complex". This material is a ready-to-use material with only 7.2% TiO2 added during the polymerization. For the thin layers of the preform, it provides very good fluidity and avoids delamination problems. In addition, the preform uses carbon-gray liquid color developed specifically for its inner layer, and the outer layer is still white. With this carbon-grey liquid color, a metal gray surface is formed inside the bottle after stretch blow molding, which is a key factor for fully absorbing light.

Molmasa has created an efficient mold for this multiple injection molding process design that does not move with the core rotation described above. Core transport is performed according to the principle of displacement, which means that the entire core plate will move up and down along a row of cavities. In this way, the mold core first enters the first mold cavity for injection molding, and then enters the adjacent mold cavity in the second production cycle to start the second layer injection molding. When the first part is completed, the mold opens and the core moves parallel to the adjacent row of larger mold cavities.

In general, the preform body produced in the first production cycle has no bottleneck, its wall thickness is 1.2 mm, and the base area has 3 small holes. In fact, these are the result of the core support, which prevents the movement of the core when filling the cavity, thus ensuring the uniformity of the thickness of the first layer. At present, this solution has been patented. In addition, when the rows of cores are transported for the second production cycle, the newly released cores will enter the mold cavity for the first injection. In the second stage of production, the still grey preform body will be coated with a 2 mm thick bright white colour, creating a bottle neck and threads, while the 3 holes in the base are also filled.

Cutting-edge valve technology is another innovation in this multi-injection process. With this technology, it is possible to alternately charge the outermost two rows of cores inserted only in the second production cycle. With the final opening of the mold, the preform is ejected, and its inner and outer layers are uniform in thickness and complete.

Perfect dairy product bottle

In order to test the practical effect of this innovative preform concept, Netstal-Maschinen AG first invited the French company PDG Plastiques to try PRElactia preforms. PDG Plastiques owns several blow molding machines and is very familiar with the production process of dairy packaging. The company quickly blown Prelactia preforms into bottles and tested them. After 6 months, 1000 bottles of UHT milk were evaluated according to strict testing standards. The results showed that the barrier performance was better than that of single-layer bottles, and none of the bottles had milk smelled. Based on this, in the fall of 2007, PDG Plastiques successfully put its first Netstal preform injection molding system into the manufacture of PRElactia preforms.

When holding a bottle blown from the PRElactia preform, the bottle is still pure white despite its gray interior. Compared with the multilayered bottle, the gray inner layer is generally not visible. If the bottle is cut, the matching of the material can be seen, and this matching also avoids the possibility of stratified delamination. The light measurement results show that the bottle can absorb up to 99.99% of the light, and can reflect the wavelength of 700nm light.

Obviously, multiple injection molding is a perfect solution developed by Netstal-Maschinen AG and its partners for PET packaging of dairy products.