Fig 1: Vinyl inserts can be inserted and
bonded to automotive interior parts.

Fig 2: Film laminates used to replace paper or plastic labels won't fade, peel off, or cause a choking hazard.

Fig 3: A two-sided end-of-arm tool uses a pin-to-pin transfer of the inserted material into the mold

Fig 4: A two-sided end-of-arm tool secures
the molded parts with grippers

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Increasing customer demand and competition in the injection molding industry has led to the need for ingenuity and, thus, added complexity. Custom molders have to cut costs and offer better products to improve the bottom line and to keep the new business coming in. Insert molding is a way in which molders can accomplish both of these tasks and see a quick return on their automation investment.

Insert molding is a process by which an object, or insert, is placed into the mold between cycles and the plastic is formed around the insert. By doing so the object becomes a part of the final molded product or a detail of the insert is left on the plastic part. Insert molding is not new to the industry, but it is a process that can be overlooked as a viable and innovative solution to molding problems.

What are inserts and how are they used?
The materials used as inserts include a variety of metallic inserts, cloth, retaining rings and film. A metallic core can be inserted into the mold to become part of the molded part or to leave its profile in the molded part. These cores can be used to create threads in the molded part and an operator or automation fixture in the post-mold process can back them out. Metallic inserts can also be used to increase the strength of the molded part or to create a metallic threaded hole or through hole.

Cloth and vinyl can also be inserted into the mold so they can bond to the molded part (see Fig 1). The plastic penetrates the fibers or pores of the material and this creates a permanent bond. This process eliminates downstream trimming, gluing, and a pressing fixture and is especially useful for automotive interiors.

Film laminates are also getting more attention for insertion (see Fig 2). These in-mold decorations can replace paper or plastic labels and, because they become part of the final molding, the print and color do not fade or peel off. The film consists of several layers that include coatings, print, color, and a micro-porous backing sheet. The advantage of molding films onto parts is that the micro-porous backing sheet, just like fabrics, allows the plastic to flow into the pores and create a permanent bond.

Another advantage is that the film shrinks with the cooling part so you won't see any ripples on the surface. Other applications for film inserts include creating a wood-grained look or metallic finish to the molded part.

Automating the process
Until recently, one of the problems with insert molding was automating the process. Most insert molding is done by an operator, who manually loads the inserts onto the mold. The quality and consistency of the molded parts is, therefore, dependent on the operator and cycle time is usually compromised.

Automating the process varies in complexity depending on the application. If there is a robot on the press, the insertion can be done with an end-of-arm tool designed for the application. End-of-arm tools can use docking pins that locate in bushings in the mold to ensure the precision necessary for placing the insert into the mold. They can also carry an electrostatic charger to create an electric field on film inserts. Another option is to have the end-of-arm tool insert the material into the mold with one side and pull the molded part with the other (see Figs 3 and 4).

Another way of automating the insertion process, without a robot, is to use a Side-entry pick-and place unit to shuttle the inserts in from the side of the press. The pick-and-place can be controlled by the press or by an operator. And although the range of motion is more limited than that of a robot, the cost is substantially less.

The automation sequence begins with an operator loading the insert onto the end of-arm tool or a fixture that organizes the inserts and presents them to the end-of arm tool. The fixture can be a magazine of inserts or the operator can load the fixture between cycles. The fixtures can be fully guarded and include slides to shuttle the fixtures, light curtains, PLCs, etc.

Advantages of automating insert molding include eliminating operator involvement, making better products and molding parts that could not be possible or would be very difficult without an insertion. The automation equipment generally pays for itself quickly in reduced cycle times and consistently higher quality parts.

How to get started
The process begins with an evaluation of a job to see if insert molding can enhance the quality of the final molded part and reduce cycle time. Ask yourself if the automation equipment will pay for itself within the life of the job. Once this has been done, bids can be obtained from companies providing the inserts and from automation companies.

Depending on the degree of automation required, the equipment can usually be installed within a day or two so the down time is limited and the floor space required is generally no more than an operator's regular work station.

Reprinted with permission from Plastics Auxiliaries Magazine, November 1999