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issue: November 2003 APPLIANCE Magazine

Plastics Buyers' Guide
Multi-Shot Plastics Injection Molding Hits Target with Faster Processing and Wide Range of Design Possibilities


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A two-color cell phone case, a washing machine motor mount, a washable vacuum sweeper filter - all are produced with multi-shot injection molding, one of the hottest technologies in plastics processing today.

Multi-shot molding minimizes production and assembly operations, speeds part throughput, improves quality, and allows creation of innovative product features. Propelled by those advantages, multi-color/material molding is beginning to break into mainstream manufacturing in the U.S., leading to a four-fold increase in the sales of such machines in the last decade.

The technology for multi-shot molding is well developed, proven, and reliable. It evolved in Europe and is widely employed there. In the U.S. and Canada, however, it has been almost a trade secret - mainly confined to large proprietary manufacturers and protectively guarded. The number of sites has been limited, but the volume of parts produced by multi-shot molding is substantial, much more than generally realized.


Pictured is a vacuum filter produced in a Ferromatik Milacron K-TEC 200 two-component injection machine by multicomponent molder Innatech (Rochester, MI, U.S.). The part has nearly 1,100 holes, requiring precision control of fill. The body is molded of PPTF, with an integral TPV seal.

Changes in manufacturing strategies - particularly Asian competition and the growing emphasis on reducing in-process time and inventories - have made multi-shot molding more viable and cost-justifiable for a broad range of manufacturers, particularly in appliance parts. There are a variety of ways that multi-shot machines consolidate several processes into one operation, thus improving competitiveness.

1. Over-Molding Additional Colors and Product Features - Over-molding enables the use of multiple colors and materials in a plastic product - for greater appeal, distinctiveness, and perceived value - while retaining "one-stop" processing efficiencies. Similar to progressive stamping, the technique performs various molding operations in sequence. The first shot molds the core structure, the preform. This is then overmolded with different colors or materials - as many as four additional shots.

The over-molding can be cosmetic or functional. For example, simple color-in-color application was first developed several decades ago for typewriter keys. Molding in the color for the symbols on the keys eliminated a paint operation and improved quality by providing resistance to wear. That same process is used today to imprint logos or text permanently in a second color onto products.

Multi-shot molding also allows sound-dampening material, gaskets, and soft-touch handles to be added to a hard base.

2. In-Mold Assembly Reduces Manufacturing Cost - Beyond consolidating several molding operations on one machine, multi-shot molding can also be used to eliminate assembly operations. For example, a rigid part, such as a water pump housing, can be molded with a soft gasket, eliminating secondary handling and assembly. Quite often, the gasket also seals better from being molded to the base part before dimensional changes (post-mold shrinkage) can occur.

Complete articulating assemblies, such as adjustable air vent louvers, plastic hinges, and toys with movable and articulating parts, can be produced on one machine, with one setup. Intentional use of incompatible materials - materials that do not bond together - enables one element to be injected into another to achieve a mechanical fit, as with interlocking hinges or a ball-and-socket joint. This achieves true in-mold assembly.

3. Special Features for Greater Consumer Appeal - Multi-material molding is widely used to improve the tactile quality of consumer products and gain a higher market share. The fastest growing application for multi-shot molding, this technique utilizes thermoplastic "soft touch" elastomers over a rigid base part. Soft elastomer grips are being overmolded on power tools, tooth brushes, and cell phone cases, to name just a few products.

A leading toolmaker's products, for example, feature a dual-durometer handle - a hard polypropylene core with softer, thermoplastic rubber outer grip - that makes them easy to grasp. A novel sport camera uses a rubberized, knobby outer cover for positive grip underwater or on the move. The cover material also functions as an integral water seal effective to 14-ft underwater pressures.

Grab bars, handles, and cranks are being produced with filled engineering resins in the core for structural strength, while the skin can be a softer or more visually appealing material. Reversing the sequence, soft elastomers injected into the core of a sandwich part can reduce the "plastic sound" of a part to improve consumer appeal.

Dense material, such as barium sulfide, even powdered metal, can be injected into the core of a part to add weight or change the balance point. Increased weight often conveys a sense of higher value to the consumer.

Manufacturers also improve consumer appeal, function, and product differentiation with use of a second color or clear material.

4. Engineered Properties, Lower-Cost Materials and Methods - The chemical, heat-resisting, mechanical, and structural properties of different plastics for skins and cores (sandwich molding) can all be manipulated to achieve optimum properties for a part at low cost.

Electronic enclosures are being molded with EFI shielding sandwiched in the core. Lawn mower decks and lawn tractor parts are being produced with various core and skin plastics to reduce material cost, improve engineering properties, and add consumer appeal. Blowing agents or foamed materials are used to create a sound-deadening core for noise reduction.

Experiments and experience have both demonstrated that the structural properties of two inexpensive resins in a co-injected part can exceed the performance of a more expensive engineering resin in the same monolayer part.

5. Reduced Raw Material Costs - Co-injection molding can cut costs by allowing a manufacturer to use lower cost material, even scrap, in the core of a part, while reducing the amount of premium material in the skin. For example, the amount of UV-resistant resin (or colorant) needed for lawn tractor components can be reduced significantly by molding a virgin material skin over lower cost core resin.

Lower cost flammable resin can be used as a core with a non-flammable skin. Painted plastic scrap - normally worthless and unusable - can easily be recycled into the core of a part with virgin material skin. Simply put, reduced virgin material use cuts part-making cost for greater profit or better competitive position.

Machine Requirements

Multi-shot molding requires that many functions be controlled and driven simultaneously and independently, placing special demands on machine design. Cost-effective production speed requires simultaneous operation of the rotating mold, clamp, ejectors, extruder, etc. Rotating molds also consume a lot of oil. Demands on the control are high, as well, because multiple injection units need to operate simultaneously.

All this coordinated, concurrent, independent action requires a very fast control and a surge of hydraulic power. The advanced European machines used most widely in multicolor/material applications provide a hydraulic power reserve through a charged accumulator, eliminating the need for a much larger pump and its energy penalty.

Cost Justifying Multi-Shot Molding

A multi-color/material mold and machine, because of expanded capabilities and more complex design, requires greater capital outlay than conventional single-shot molding equipment. This requires aggressive utilization to maximize return on investment:

  • production requirements should be high enough to keep a machine in full production 5,000 to 6,000 hr per year

  • the part should have a long life (3 to 5 years)

    There is no easy rule of thumb for determining a cost premium to cover consolidating operations onto one machine. However, a variety of benefits or savings should be considered in justifying a multi-shot system:

  • In most cases, a single injection-molding machine with multiple injection units will cost less than an equivalent number of machines with single injection units. Likewise, a single two-color mold will usually cost less than two one-color molds.

  • Labor savings - a single machine operator for a multi-step process can have a major impact on payback calculations and lifecycle costs.

  • Faster inventory turns for savings on in-process inventory will vary from operation to operation, but can be the critical factor in the decision matrix.

  • Floor space, electricity, and other utility costs are also reduced by consolidating operations on one machine.

  • Part quality will be improved, because machine-to-machine transfers almost always degrade part quality due to post-mold cooling/shrinkage.

    Getting started

    Multicomponent molding is common in Europe, but still a relatively young technology in the U.S., although a pool of experienced operators is starting to develop. Multicomponent moldmakers in Europe have a lead, too, because they have simply been doing it longer, but a number of U.S. producers are rapidly gaining proficiency. Some capable European moldmakers are putting down roots in the U.S., as well.

    If a part is complex or new, without an established market, consider a mold supplier who can design/build a low-cavitation pilot mold and produce parts for evaluation and test marketing.

    Given the complexity of multi-shot systems and limited U.S. experience and exposure with them, companies usually prefer a turnkey system.

    Ferromatik Milacron's approach is to provide an initial feasibility evaluation and several proposed options for budgeting. Once the customer has decided the option that fits it best, Ferromatik Milacron develops the complete process (mold, tooling, machine, and even material handling). This is put through extensive performance trials to earn customer acceptance.

    This information is provided by Robert B. Hare, general manager of Ferromatik Milacron Europe, USA (Batavia, OH, U.S.).

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