Multi-Material Injection Molding | Seek n' Geek VI
The development of injection molding marks a revolution in manufacturing practices and an explosion in plastics research and development. Multi-material injection molding MMM provides even more advanced capabilities and benefits such as soft grip surfaces on rigid structures, selective compliance, multiple colors for aesthetic design, reduced assembly line time, and molding geometries not possible with single shot molding. MMM manufacturing techniques were pioneered by the automotive and consumer product industries for applications ranging from toothbrushes and hand tool grips to integrated seals and gaskets. Furthermore, integrated hard-soft combinations can significantly reduce tooling costs by eliminating undercut retaining features for assembling soft components. This paper will provide a practical, condensed introduction to multi-material molding processes, terminology, design challenges and applications.
Injection molding is a widely used manufacturing process in which a material is heated and then injected into a mold. Figure 1  shows a typical molding machine and injection process. The material, typically in pellet form is fed into the hopper, and transferred by the screw down the barrel through the heaters. For single-shot injections, once the material reaches temperature it is injected into the mold cavity, then cooling fluid runs through the mold to cool the part, the mold opens and the part is ejected.
Figure 1: A diagram of an injection molding machine showing the flow of material from hopper to molded part
Multi-Material Injection Molding
MMM manufacturing is an umbrella term for three different processes: multi-component injection molding MCM, multi-shot injection molding MSM, and over-molding.
Figure 2: Types of multi-material injection molding processes
Figure 3: On the left, a diagram of a multi-shot injection molding process, on the right a diagram showing co-injection molding, MCM
MCM, multi-component molding, is the simplest and most common MMM process. Materials are injected simultaneously or sequentially through the same or different gate locations into a single mold that is not changed or manipulated during the process. [2, p. 2] The interaction between the two materials creates a skin-core, or layered, configuration, shown in Figure 3 . Choosing simultaneous or sequential injection, and designing gate locations determines the final material composition of the part. For further depth, look to the experimental combinations correlated with simulated processes in Zoetelief et al. [5, p. 216]
MSM, multi-shot molding, a more complex and versatile process compared to MCM, sequentially injects the different materials into the mold, changing the mold cavity geometry between shots, with the component fixtured inside. Figure 2 notes that there are two methods for moving the mold cavity geometry, through a rotary platen or an index plate. The left image in Figure 3 shows a rotary platen MSM process, where the blue thermoplastic is injected first as the substrate, and then the mold is rotated 180° and the LSR is injected on top of the substrate.
Over-molding, the third type of MMM processing, forms a material layer around a premade part suspended within the mold cavity. [2, p. 2] Most commonly, the premade part is a type of rigid substrate, but a 2013 patent demonstrates the use of over-molding with batteries and electronics. 
As with all manufacturing processes there are trade-offs and advantages. Machine expenditures are an expensive overhead investment. One advantage of overmolded insert molding, is that it can use a conventional single shot injection molding machine, which lowers tooling costs. Alternatively, multi-shot needs a special injection molding machine with two or more barrels, but it significantly reduces cycle times and labor costs while providing high quality parts.
For more specifics on these manufacturing methods, two introductory texts on the subject are Plastic Injection Molding by Douglas Bryce  and Goodship and Love’s book titled Multi-Material Injection Moulding.  Manufacturers are also important sources of information on available processes, materials, and complex parts.
John Wesley Hyatt patented the first injection molding machine in 1872, which he had invented to mold celluloid billiard balls.  Throughout the early 20th century new plastics were discovered and invented. In 1907 Leo Baekeland discovered the first fully synthetic plastic Bakelite, in 1913 Fritz Klatte discovered vinyl chloride, the basis of PVC, and in 1922 Hermann Staudinger’s research described the polymerization process of macro molecules. Wallace Carothers created nylon in 1935, and two years later Otto Bayer discovered polyurethane.  The use of plastics to create low-cost mass manufactured parts accelerated with the onset of WWII. PTFE was discovered in 1938, polystyrene in 1948, and polycarbonate in 1953 by Herrman Schnell. Significantly, in 1946 James Watson Hendry invented an extrusion screw injection machine with better control and injection speed over prior art. 
In 1962, almost one hundred years after Hyatt’s patent G. Carozzo patented the first MMM process for injection molding composite articles, one application being automotive taillights with multiple colors.  In the 1950s and 60s several new thermoplastics were developed, including high-density polyethylenes resulting from the Phillips and Ziegler processes, as well as polypropylene, acetyl, ABS, and some polycarbonates.  In the 1970s co-injection molding, with high-pressure and sequential injection, was developed by ICI (England) to produce a foam-core “sandwich” structure to address challenges with surface swirl and post mold finishing operations with structural foam molding. 1979 marks a turning point at which plastic product overtook steel production. In 1980 Jules M. Hock and Donald S. de Vries developed a novel over-molding technique for a piston with a rigid core and integrated flexible seal, the patent is cited by 53 patents and lists 26 classifications. 
In the past three decades advances in plastic technology have grown exponentially, with the invention of new materials, faster more efficient processes, and more complex geometries. As an article in Plastics Technology articulates, the newest innovations are in the realm of the three ‘M’s – mega, micro, and multi.  Matthew Naitove and Jim Callari, executive editor and editorial director of Plastics Technology magazine published “50 Ideas that Changed Plastics” Brousseau et al articulate many of these advances for applications such as medical implants, drug delivery and diagnostic devices, micro engines etc. 
Materials and Geometry Considerations
All MMM processes are impacted by three key factors: materials, temperature and part geometry. Material selection, temperature optimization and revising part geometry are all part of the design for manufacture process. Modulating these key factors impacts final part quality and cost. Material compatibility impacts adhesion of the materials, while the temperature of the mold and the materials impacts material flow through the cavity as well as part cooling time. Optimizing temperatures leads to more efficient manufacturing cycles, while optimizing part geometry impacts the mold complexity and reliability of the interface between materials.
Good adhesion between the two or more materials in a multi-material part is essential for high quality parts. Adhesion can be attained through material compatibility bonding and/or part geometry, such as mechanical interlocks, shown in Figure 4, in which the second material is molded into the rigid substrate in such a way that the geometry resists separation . The best bonds between materials form when melt temperatures of the soft material closely match the melt temperature of the rigid substrate. Good bonding between materials distributes any applied loads over the full contact surface area, better than when the two materials are molded separately. 
Figure 4: Three types of Mechanical Interlocks
Common materials used in MMM processes include engineering plastics like ABS, polycarbonates, acetal and nylons, polyolefins like polypropylene and polyethylene, and thermoplastic elastomers TPEs which are elastic, flexible, and environmentally resistant.  Table 1 outlines useful properties of these materials, and Figure 5 characterizes the materials based on performance, cost, volume of manufacturing, and bonded structure. Protolabs, a leader in low volume molding, provides an informative list of stocked materials, features for each, and the tensile strength, flexural modulus, impact strength, and the max temperature before bending and softening.  The Protolabs chart provides an overview of the engineering plastics listed in Table 1 below. For materials selection and technical datasheets, the Society of Plastics engineers offers numerous resources including a materials database and plastic selector guide. 
For the materials in Table 1 below, and shown in Figure 5, crystalline versus amorphous structure impacts the chemical resistance, impact resistance, material flow and processing of materials. [8, p. 128] New compound materials include additives for strength, stiffness, temperature performance, appearance, and cost. An ASME blog article titled Metal to Plastic  gives examples like the addition of carbon and stainless steel improve conductivity and shielding, lubricant fillers improve wear and friction properties, mineral fillers improve electrical performance, weighted feel, sound dampening, and dimensional stability, and impact modifiers improve material toughness.
Table 1: Commonly used materials in MMM processes
Figure 5: Mapping materials by price, performance and structure
One additional consideration for handheld devices is the feel of a product, which is dependent on the material choice and geometry. Feel can be designed by designating part thickness and selecting materials with varying durometers, flexural modulus’ and coefficients of friction.  The actual stiffness of a part is dependent on the material properties and geometry, like thickness of the overmolded material. The flexural modulus measures a material’s resistance to bending, while Shore Hardness measures the surface hardness of a flexible material. The coefficient of friction, both static and kinetic, also impacts the grippy feel of a material.
Every manufacturer lists their own design guidelines and recommendations, but a set of guidelines specifically for multi-material manufacturing is coalescing. The work of Banerjee et al  strives to derive design rules for MMM and develop a computer model to analyze designs, identify challenges, and propose potential modifications using state transition digrams and failure mode matrices. The GLS Overmolding Design guide  lays out ubiquitous recommendations for MMM, many which are the same as conventional single material molding, but others which are unique to MMM processes. Uniform part wall thicknesses lead to best molding times and good bonding. [28, p. 6] Coring out thick sections minimizes shrinkage, reduces weight and also lowers cycle times. Gradual transitions between different thicknesses reduces flow problems like back fills and gas traps, and radii in sharp corners will reduce localized stresses. Avoiding deep, un-ventable blind pockets prevents air bubbles and residual stresses. Vertical faces on the exterior faces of the part need draft angles for smooth ejection.  Specific to hard-soft combinations with TPE, the overmolded thickness should be less than or equal to the thickness of the substrate. This prevents warpage of the substrate, an externality of material cooling and internal stresses. Furthermore, some undercuts are possible with TPE if it can flex during mold ejection. Table 2 outlines several desired outcomes for a high quality part and recommendations for each, connecting the geometry recommendations outlined above to features.
Table 2: Features for consideration in MMM design for manufacture
Manufacturing and Design Challenges
Choosing a manufacturing method necessitates weighting the number of parts needed, the local labor costs, available equipment, desired part quality and the specifications of the material selected. For example, insert molding can be done at high production volumes and is good when local labor costs are low. Whereas, MSM requires more labor, but facilitates high volume production. Another important consideration is that there is a significant difference in bond strength between insert and multi-shot molding, because of the difference in temperature of the substrate.
Substrate temperature is one of the factors impacting the bond strength and adhesion of materials. Also influential are the delay time between injections, the injection pressure, the melt temperature of each polymer and the mold temperature.  While thicker parts have more thermal mass and lose heat more slowly, thin walled parts are more desirable to ensure part quality, therefore it’s necessary to have short cycle times between shots to ensure adhesion between materials before cooling. The development of the rotary platen and index plate for multi-shot molding increased efficiency by decreasing cycle times and the reducing human labor in the process.
In addition to considering bonding, MMM processes are liable to the standard injection molding defects, including flash, bubbles (voids), brittleness, flow lines, discoloration, nonfill, excessive shrinkage, sink marks and warpage. These defects and common remedies are covered in Chapter 11 of Plastic Injection Molding copyrighted by the Society of Manufacturing Engineering.  Challenges particular to MMM are material incompatibility, interactions among cooling systems, gate placement, de-moldability, and ejection system problems. [2, p. 6]
Manufacturers are experts in understanding the interplay between all the factors described from material selection, to geometry considerations, and tuning the molding parameters to reliably and repeatedly produce precision quality parts. There are numerous competing companies that design and build injection mold machinery and equipment including Arburg, Inc., Boy Machines Inc., Engel Machinery, Inc. Foboha USA Inc, Husky Injection Molding Systems Ltd., JSW Plastics Machinery, Inc., Krauss Maffei Corp., MGS Mfg Group, Milacron Plastics Technologies, Negri Bossi North America, Nissei America, Inc., Sumitomo (SHI) Demag, Toshiba Machine Co., and Van Dorn Demag.
Applications of MMM and Conclusion
MMM manufacturing techniques can be used for applications ranging from plastic on plastic, rubber on plastic, plastic on metal, and rubber on metal. Plastic on plastic applications most often are aesthetic changes in color or resin for design, branding, or as a user guide. Rubber on plastic methods are used to create soft grip areas on toothbrushes, power tools, or medical devices and for integrating soft seals into structures. Rubber on plastic also creates compliance for living hinges. In a plastic on metal process, a cast, formed or machined metal piece is first inserted into the mold cavity, then plastic is molded around the piece. This method allows plastic handles to be molded directly onto metal tools and allows metal threaded inserts to be integrated into a plastic part. Finally, rubber on metal applications mold a soft rubber or TPE onto metal inserts for soft grips likes scissors, or an integrated wiring harness that includes the housing, grommets and seals manufactured in the same mold. These methods allow designers to create flexible areas on rigid parts (selective compliance), to design aesthetic and ergonomic soft grip surfaces, to reduce assembly line time, to integrate seals, and to use fewer fasteners and adhesives in assemblies.
As the automobile and aerospace industries relentlessly seek to reduce overall vehicle weight to improve fuel efficiency, more parts are designed for structural plastics instead of aluminum or steel. With structural design, plastics can balance high tensile strength with reduced part weight. Contrasted with metal parts, plastics also improve corrosion resistance and consolidate multiple metal parts into a single plastic part, reducing labor and fasteners needed for assembly. Furthermore, injection molding is a highly repeatable process with less material waste than machining, and overall lower manufacturing costs. The trend towards plastics manifests economically as well. “The global engineering plastics market [was] valued at $57.2 billion in 2014 and is expected to reach $91.78 billion by 2020 at a compound annual growth rate of 8.2%.” Overall, multi-material manufacturing, encompassing multi-component, multi-shot and over-molding techniques, gives engineers the greatest design flexibility and creativity.
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