Two-shot injection molding and the right materials put soft-touch grips and watertight seals on an expanding array of structural plastics.
By John Caamano
Edited by Jean M. Hoffman
There are times when hard plastics need softer qualities. Handles and knobs, for example, are often outfitted with pliant, nonslip grips that make them comfortable to use and limit shock and vibration for the operator. Likewise flexible, form-fitting components can be added between mated parts that would otherwise be too rigid to form watertight seals.
In the past, a design practice added soft thermoplastic-elastomer (TPE) overlays or rubber elements to molded parts. But doing so could limit design freedom and boost part cost. A recent alternative directly molds soft, yielding surfaces onto rigid thermoplastics. The technique — a two-step injection-molding process — first injects the hard material into the mold cavity, followed instantly by the second injection of the TPE.
Hard-soft, overmold processes create integrated seals, acoustic-damping elements, and nonslip, impact and vibration-absorbing surfaces. These overmolded parts have broad potential in automobiles, material handling, plumbing, heating and air conditioning, power tools, and audio systems. Automotive applications range from sealed housings to easy-to-grip passenger compartment controls. Other potential end uses include paper transport rollers in office equipment, plumbing seals and gaskets, and soft grips on power tools.
Parts containing hard-soft combinations also overcome limitations of conventional assembly techniques. For example, fitting seals that prevent moisture from penetrating inside electronic components is getting more difficult as electronic packages continue shrinking. Molded hard-soft combinations may become an alternative to conventional seals and gaskets.
And there is more good reason for combining soft and hard plastics. For instance, directly molding soft elements onto hard-plastic substrates consolidates two or more components thus eliminating additional assembly costs. Injection molding also expands design flexibility making difficult-to-assemble parts, such as complex gaskets for hard-to-reach places, in a single step. Hard-soft molding may also eliminate undercuts and other features that hold conventional, mechanically assembled soft components in place. This reduces tooling cost and may remove specialized cams and sliding actions from the mold.
Cost savings reportedly exceed 25% over conventional assembly methods, because assembly is done by the molding machine. Thus labor and inspection costs drop, as does a range of ancillary manufacturing expenditures such as allocated floor space and parts inventory.
Hard-soft molding is most economical for big parts, but can be effective for large runs of small parts. With small parts, eliminating secondary assembly can more than balance out the tooling and equipment costs required for two-component molding.
With two-shot injection molding, designers have a wide range of hard-soft combination options to meet performance requirements. Thermoplastics available for the hard component include engineering polymers such as acetal (POM), polybutylene terephthalate (PBT), and polyphenylene sulfide (PPS).
POM is dimensionally stable, tough, hard, stiff, and resists chemicals. It has a high tensile strength, low coefficient of friction, longterm stability, and good electrical properties. PBT offers flame-retardant grades that are also strong, hard, and stiff. And like POM, it has good long-term stability and electrical properties. PPS is also hard and stiff, can withstand service temperatures up to 240°C, and is inherently flame resistant.
TPEs are often the soft material of choice because they combine high elasticity, good flexibility at low temperatures, and exceptional chemical resistance. They are available in five major families:
TPE-A are polyamides such as polyetheramide.
TPE-E includes polyesters such as polyetherester and polyesterester.
TPE-O is a group of polyolefin-based elastomers blended with polypropylene (PP) and ethylene propylene diene monomer (EPDM) rubber. Cross-linked polyolefin TPEs are a subset that is often designated as TPE-V.
TPE-S includes block copolymer styrenes such as styrene ethylbutylene styrene (SEBS).
TPE-U is based on polyurethanes which include polyetherurethane and polyesterurethane.
Most TPEs have Shore-A hardness values >15 and Shore-D valves <75. They also serve well over a wide range of conditions. They withstand temperatures from –60 to 160°C and have good to excellent aging characteristics, depending on the TPE. Most resist hot water, acids, and alcohols.
A few of the TPEs, however, have limited resistance to fuels. Chemical and fuel-resistance characteristics can vary depending on temperature, time, type of exposure, and other environmental issues. Therefore, elastomers being considered should be tested in appropriate environments prior to specification and actual use.
Hard-soft combinations can also use nonpolar rubbers such as styrene butadiene, EPDM, and polar rubbers such as nitrile butadiene. These classic rubbers function over a wide thermal range and have good elongation and elastic memory. They have better resilience and resist chemicals and temperature better than many TPEs.
Adhesion promoters are often added to the TPEs and rubbers so they bond well to the hard substrate. The bond between the two materials forms after the soft material is injected over the hard material, and this bond often exceeds the tear strength of the elastomer.
Common hard-soft combinations include PP and a PP/EPDM-based rubber, as well as PP and SEBS. Many of these hard-soft combinations stand up well to highly demanding applications. Treated linear-PPS, for example, has been paired with vulcanized rubbers for use in engine compartments and brakes or with adhesion-modified silicone rubber via liquid-injection molding for high-temperature applications.
MAKING THE BOND
With two-shot injection molding, the best bond forms when the melt temperature of the soft material closely matches the melt temperature of the hard material. Molding can be done with two-component machines having vertical, L-position, piggyback, and other configurations. Systems typically use a rotating base and/or retracting-cores. Retracting core tools have sliding cores that, when closed, let the first shot enter from one direction. Opening the core then lets the second melt inject from another direction. Rotating tools receive the first material with the mold in one position. The tool rotates to its second position and injects the TPE material. Rotating tools have higher productivity, but retracting-core molds better maintain heat in the system because the mold doesn't open between shots.
Many parameters affect the bond between the two materials. The most critical include delay time between the two injections, injection pressure, melt temperature of each polymer, and mold temperature.
Other material, tooling, and processing factors also influence adhesion. Material factors include the moisture content, crystallization and heat-transfer rates, surface tension, molecular weight, and the reinforcements pigments, lubricants and stabilizers present.
The time between the first and second shots should be as short as reasonably possible to help maintain high substrate temperatures. Wall thicknesses of >3 mm help parts retain surface temperature better after the mold is opened. Thin-walled parts have less thermal mass and lose heat rapidly, so they may need to be processed at the shortest possible cycle time.
Molding conditions vary depending on materials, machines used, and part configuration. Optimizing the process for the best adhesion is often an empirical process. The quality of the bond depends on substrate cooling times, injection speed of the TPE, and hold pressures. For example, adhesion may be poor when the substrate is cooled too long before injecting the elastomer. Conversely, the bond may suffer if the substrate doesn't have enough time to form a fine, crystalline skin.
In other words, the time between shots should be long enough for the initial polymer to transition from a molten to a more stable, semicrystalline state, but short enough to prevent excessive cooling. The typical time between first and second injections is 2 to 5 sec. Molds may have to be heated if longer times are involved. Trials help define what material and mold temperatures work best.
One general guideline is that adhesion is strongest when the elastomer is injected at temperature and pressure high enough to ensure the mold packs out fully. Trapped gases in the mold can degrade adhesion, so soft materials may need to be dried prior to molding. In addition, the mold should be vented so gases can escape and screw retraction should be slow so entrapment of air is avoided.
Parts may warp when the temperature disparity between the first and second shots is too great, especially in long, thin parts. Warpage may occur if the hard plastic is thinner than the bonding elastomer. Because of this, such parts may need to be redesigned to compensate for warpage.
Adhesion between the two materials is weakest immediately after the second injection and improves as the combined part cools. Care should be taken not to stress the bond when the part is ejected from the mold. If mold release agents are used, they should not react with the elastomer or rubber.
Because they are made in a highly repeatable process, molded hard-soft parts can improve quality by reducing the potential for human error. Hard-soft parts may also reduce reject rates due to bond failure compared to conventional mechanically bonded parts. Because the bond between hard and soft elements extends over the contact surface, the load can be distributed over the entire surface area. By contrast, mechanically attached soft overlays are usually held by a few molded-in posts, which fit into holes in the substrate. The load here is only carried by the area of the posts that comprises the mechanical bond.
FINALLY, A SOFT TOUCH FOR ACETAL
Molding hard and soft materials together is an evolving field with new combinations still being developed. A recent entry is a SEBS grade that overcomes bonding issues with acetal.
The technique, reportedly the first of its kind, matches Ticona's Celcon and Hostaform acetal copolymers with an adhesion-modified SEBS called Thermolast-K from Kraiburg Corp., Duluth, Ga. The SEBS material is tailored to form a strong bond to the acetal by overcoming acetal's inherent lubricity. The SEBS combines high elasticity and flexibility at low temperature with resistance to water, acids, and alkalis. It also has a wide range of Shore hardness values (45A to 70D) and a relatively wide processing window.
The two-component molding process involves an acetal melt temperature of 200°C and a cooling time of at least 2 sec. The acetal substrate is held just below its crystallization temperature after molding. The SEBS is introduced via hot runners at 250 to 260°C. Residence time for the SEBS in the machine at these temperatures is kept as short as possible, in part by using small barrels, so the adhesion promoter does not degrade. Since SEBS flows well at 250°C, molders should adjust for jetting and entrapped air as the material enters the mold. The tool should be vented to release trapped air.
This hard-soft technology can be used to add long-lasting seals, gaskets, and nonslip and energy-absorbing elements to acetal parts. Its potential uses include seals for auto door-lock housings, sensors and sensor housings, as well as in medical devices, heating and air-conditioning systems, and audio components.