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Injection molding with two or more materials requires either a two-shot molding approach or a simultaneous coinjection technique. Regardless of the process used, molders face the same challenges in achieving high part quality. Three common problems with any multi-material process are insufficient chemical or mechanical bonding of the polymers, incomplete filling of one or more components, and flashing of one or more components.

These conditions can occur whether the materials combination is reinforced and unreinforced, solid and foamed, rigid and soft, virgin and regrind, pigmented and unpigmented, etc.

Multi-material molding and its problems and solutions is a complex subject that cannot be explored thoroughly in a short article. The accompanying table indicates the range of variables involved. A few of the more important factors bear a brief discussion.

Achieving a good bond between materials is often dependent on the temperature of the first material when the second is injected.

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Time and temperature
One cause of insufficient bonding between materials relates to the timing of the injection of the materials and the temperature of the first material when it is joined with the second. Too much cooling of the first material tends to weaken bonding. On the other hand, the first shot must be cooled enough not to be deformed or displaced when you shoot the second one. If the second shot comes too soon, while the first material is still soft, the second material can compress and flash over the first one, causing “splash marks.”

When running parts on two injection machines (molding the first shot on machine one and inserting it into the mold of the second machine), bonding is not apt to be as good as on a two-shot machine with rotating table. Even when using compatible materials, the delay time between the two shots is relatively long and the first shot is likely to be too cold. A higher part temperature is recommended for better chemical/mechanical bonding. Also, if the first shot picks up dust while being transferred to the second mold, bonding will also be negatively affected.

Apart from process conditions, material choice can greatly affect bonding. Some materials naturally tend to adhere better than others, and resin suppliers–particularly makers of TPEs—have been working hard to optimize certain grades for overmolding by increasing their range of adhesion to other polymers.

Additives and pigments can affect bonding. Glass fibers in one material can enhance bonding with the second. Fibers on the surface of the material promote a mechanical bond with the second shot.

Note that materials containing fillers like talc or calcium carbonate should be dried adequately. These fillers hold a lot of moisture, which can detract from bonding.

Elements of quality
To prevent underfilling or overfilling (and flashing) of either material, the shot-to-shot accuracy of the machine is obviously a critical factor. Shot variability of less than 0.3% to 0.5% is recommended. A machine with closed-loop injection-speed control is the best choice.

Sequential two-shot molding and simultaneous coinjection face many of the same challenges to part quality.

Next, pick a mold maker with experience in multi-material parts. You can save a lot of money if you have the mold designed well from the start. For example, it can be helpful to supplement the thermal/chemical bonding between two materials with a mechanical joint achieved by using undercuts or similar designs.

Make sure multi-cavity molds are well balanced. Hot-runner manifolds must be balanced too, and the number and size of drops must be sufficient for low-pressure filling.

Mold temperature is another important factor. Accurate control of the temperature is mandatory when running molds with core lifters for the second shot. Incorrect mold temperature can cause a lifter to wedge or jam, because of differential thermal expansion of the steel or steel/brass combination.

Operators must be well trained for successful multi-material molding. Wrong machine settings are often the culprits when parts don’t turn out right. Because of its complexities, only people who understand the process should be allowed to attempt corrections if something goes wrong.

Achieving a good bond between materials is often dependent on the temperature of the first material when the second is injected.