Double Layer Moulding for Environment, Performance & Aesthetics

Nissei ASB Machine Co., Ltd. has maintained a healthy lead as the world’s leading supplier of 1-step injection stretch blow molding machines and molds for one important reason – continual innovation. After more than 40 years at the helm, the company’s founder and chairman, Mr. Daiichi Aoki, has maintained the company’s core policy of developing new molding techniques to expand its market in new directions.

Molded-In Decoration

If the container can be effectively decorated direct from the molding machine, downstream costs for printing and decoration may be either reduced or eliminated.

Simply molding clear or tinted material over a solid background color gives many interesting and unique visual effects that generate a feeling of depth. For enhanced appeal, by profiling the injection tooling, the boundary between the two layers can be made non-uniform to achieve stripes or various gradation effects either vertically or horizontally.

Molding Inside Out

Conventionally, the inner layer of the preform will be molded first with the second layer being injected over it. But, depending on the material combination, neck design or the desired visual effect it is equally possible to mold the outer layer first then inject the second layer to become the inner layer of the preform.

Food Safety with Recycled Resin

Regulations on food contact with recycled resins vary throughout the world, so the only way to be confident of global compliance is to eliminate the contact completely.

Multilayer co-injection molding, where more than one material is simultaneously injected, was pioneered by ASB in the early 1980’s and still finds a use today for a variety of applications mainly related to gas barrier, utilizing low percentages of secondary materials.

One of co-injection’s major drawbacks is the typically complex hot runner designs having high investment, and even higher maintenance costs. The molding process too, becomes more challenging requiring very careful attention to ensure even distribution of the secondary resins throughout the entire cavity set. With future legislation almost certainly requiring packaging to contain minimum prescribed percentages of recycled materials, process control is critical and co-injection cannot guarantee conformity. Although it can theoretically be used to inject recycled resin to avoid food contact, it was never designed for this purpose – and running at its maximum levels of around 15~20% of body weight there is a risk of encroachment into the preform’s inner surface or internal leakage in the hot runner.


Perhaps the most dramatic benefit of the process comes in producing functional, unique and eye-catching containers that cannot be molded by any similar process.

The usage life of many cosmetics such as mascara and lotions depend on maintaining a careful moisture balance within the product and for this reason, their packaging is often molded in polypropylene. In the Double Layer process, the option now exists to mold a different outer layer using PET or other resin to expand the opportunities for packaging designers to create completely new concepts. Furthermore, the thickness of the inner PP layer is virtually unlimited so very high moisture barrier can be assured.

Another common design used for premium cosmetics is the glass-like appearance for bottles and jars, but molded in safe plastics such as PET. Jars in the 30 to 100ml range are often preferred to have an extra thick base that enhances the brand image and shows off the product at its best.

These containers create two challenges – preform cooling and machine shotweight capacity. The Double Layer process solves these by; a) building up the preform in two, easier to cool, layers and b) the effective shot-weight capacity of the machine is doubled by the use of two standard sized injection units.

These very thick containers often require molding in high-priced resin grades having very low crystallization rates, but with the ability to mold two thinner layers with more effective cooling, it may be possible to shift to a lower cost resin while achieving the same clarity.

For added visual appeal, containers can be molded using two different colors to form each preform layer, and with tapered layer thickness, a gradation effect can be achieved in the blown container.



While Double Layer molding is most certainly a niche process, it further widens the 1-step molding possibilities for container design and manufacture where packaging has specific requirements for ecology, enhanced performance or eyecatching aesthetic design.

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