November/December 1999

Molding Your Future

In the Wiring Harness Industry

By Forest Rose, III
AmeriPlas Machinery Corp

Connectors—Grommets—Strain Reliefs — Plugs. Have you ever looked at the end of a wire and found one of these? Not the type held together with screws, nuts, and bolts; then assembled by hand, but the type that are all one piece with wire or terminals coming out on either end. Where do they come from? Various vendors, of course, but how do they get there? Is it part of the cable? Are the terminals pressed in when they are hot? Is it a long process, similar to potting? Does it take high level personnel to set up and run the process, like CNC machinery? The answers to these questions might amaze you.

The "magical" process that makes connectors, grommets, strain reliefs, and plugs is called insert molding, which is a special niche of injection molding. In the simplest sense, insert molding is the process in which you mold plastic around an object that has been placed in a mold. The shape of the mold and what is placed inside the mold determines what the end product will be. What separates insert molders from plain injection molding is that insert molders make high tech, high dollar parts and are always molding on or around something in the mold, while plain injection molders make high volumes of low dollar parts out of solid plastic.

Insert injection molding — becoming an important
part of the wiring harness industry.
 
 

How is the process of insert injection molding affecting the wire harness business? For many companies it has provided opportunities for increased sales and profit and a method for companies to control production and deliveries to their customers. Insert molding can give the wire harness manufacturer a different or additional way to manufacture and assemble a superior product with less parts and labor.

A good example of how insert molding can do all of this is the D-Sub Connector. This product has a terminal/contact housing, soldered onto a cable. The top and the bottom of the connector are placed around the terminal/contact housing and held together with screws. Finally, a cord restraint is added at the end to prevent the cable from pulling out. All of this has to be hand assembled and could have twelve separate parts.

If this part were insert molded, the terminal-contact housing would be placed into a mold and the part would be molded around it, forming the body of the connector and the cord restraint. The whole part would be sealed hermetically, and the cord restraint would be fused to the outer jacket of the wire. The result is a superior product with only two parts. (1) A Terminal-Contact Housing and (2) A Molded Connector Body. The labor is drastically reduced, and this factor alone reduces the cost of the product. In addition, you, the wire harness manufacturer, would not have to buy this part from a vendor, and you could produce only what is needed for the order; thus, enabling you to carry minimal stock.

D-Sub Connectors are not the only insert molded parts. Insert molded parts are limited only by your imagination. Common products are connectors with terminated ends, grommets molded around inserted wires, antennas with coils as inserts, solenoids with winding as inserts, switches with contacts as inserts, sensors with circuit boards as inserts, and sockets with terminals as inserts.

The list is endless and is constantly being expanded. Not mentioned above are the mechanical, medical, and decorative parts that are also insert molded. For example, you probably are standing on insert molded shoe soles, and you brush your teeth every morning with an insert molded toothbrush.

What is needed to make all these products can be grouped into six areas: (1) An Injection Molding Machine, (2) The Mold or Tooling, (3) Material or Plastic, (4) An Operator, (5) Inserts, and (6) Accessory Equipment. At first glance, this may seem overwhelming, but it is really no more complex than cutting, stripping and terminating wire.
 
 
 
 

#1 — The Injection Molding Machine

Injection molding machines come in various shapes and sizes to fit the customer's needs, but all have the same three basic parts:

(1) A Clamp,

(2) An Injection Unit, and

(3) A Mold Transfer Mechanism

The clamp on an injection molding machine is what holds the two halves of the mold together. The clamp on most insert molding machines is in the shape of the letter "C". That is why most of the insert molding machines in the wire industry are called C-Frame presses. They look similar to punch presses or terminal presses, but are much bigger. Clamps come in different sizes and are rated in Tons. As the size of the clamp increases, generally, the tonnage of the clamp increases.

When larger parts are molded, more tonnage or force is needed to keep the mold halves together as plastic is injected into the mold. The general rule for clamp size is three tons per square inch of part, measured as a cross section at the parting line. The most popular clamp size machine for the wire harness industry is a 30 Ton C-Frame, which will manufacture 2 pieces — 2.2" x 2.2" or 4 pieces — 1.5" x 1.5" or 8 pieces — 1.1" x 1.1".

The injection unit is the mechanism that converts the pelletized plastic into a hot semi-fluid liquid; then injects the plastic into the mold, where it cools quickly and takes on the shape of the part. Injection units come in a wide range of sizes and are rated by the maximum weight of material that can be injected or "shot" into a mold. Since different materials have different weights, the industry uses polystyrene as a standard. For a 30 Ton C-Frame injection molding machine, you usually don't need more that a 5 oz. injection unit. A 5 oz. injection unit would allow you to mold a solid block of PVC, 8 inches cubed (2"x2"x2") or 4 blocks, 2 inches cubed (1.25"x1.25"x1.25"). In the connector wire harness industry, a 3 oz. injection unit is very typical because electronic parts and connectors tend to be getting smaller. Injection units of 5 oz. or larger are usually needed for high power connectors such as range outlets or when a large number of parts are molded at one time.

The injection unit can be mounted two ways: (1) Horizontally, to inject material from the back, commonly called a parting line injection unit and (2) Vertically, to inject material through the top of the mold, commonly called "through the sprue" molding. Both of these layouts have advantages which come down to three factors: tool layout, part loading and mold building.

With a horizontally mounted injection unit or a parting line machine, the natural mold layout (and balancing of the mold) looks like a branching, family tree. This lines up the cavities in a row which makes it easier for an operator to load and allows the inserts with attached cables to be draped out of the front of the mold. Not all mold makers build parting line tooling. AmeriPlas designs and manufacturers molds in-house and works with mold makers across the country who specialize in parting line molds in order to remain competitive and to be on the cutting edge of mold design.

With a vertically mounted injection unit, the natural mold layout (and balancing of the mold) looks like a branching "H" pattern. This brings the cavities to the right and left and front and back. This does not lend itself to the molding of inserts with cables on them because the cables on the back side of the tool must be draped around the mold and this "problem" increases loading and handling time. The layout of the die lends itself to a more efficient use of space and works well in encapsulation, molding coils and contact bodies, where wire length is not such an issue. Unlike parting line tooling, every mold maker understands how to design and build molds for vertical injection machines (or through the sprue molds).

The final part of an injection molding machine is the mold transfer mechanism. The two standard mold transfer mechanisms are the shuttle table and the rotary table. These mechanisms are very important for safety and production reasons. For safety, you never want to be loading in the clamping area, and if the operator never enters the clamping area, that eliminates all the dangers associated with it. From the production side, anytime the machine is not molding parts, there is a loss in production. The idea of the mold transfer mechanism is to increase production by allowing the operator to load and unload parts while the machine is molding.

The shuttle table mounts two bottom halves of a mold and moves left to right. The operator normally unloads a finished part, puts in a new insert, and then the shuttle moves the mold under the clamp where the molding begins. The operator will then move to the other bottom half to unload the previously molded part and load new inserts. After loading, the shuttle moves the mold the other way, under the clamp where the molding is done again, and the process continues.

The shuttle table works particularly well for wiring harness manufacturers because the wire ends simply drape out of the front of the mold and follow the table back and forth. The back and forth motion of the shuttle table leaves nothing for long cables to get tangled on.

Rotary tables are either indexed in two or four positions or stations and have two or four mold halves attached. If four stations are used, you can use the #1 station for loading and unloading, the #2 station for molding, and the #3 and #4 stations for optional use. Common options are for auto unloading and part number stamping. If two stations are used, you can use the #1 station for loading and unloading and the #2 station for molding. The main advantage of the rotary table is that the operator does not have to move back and forth as with the shuttle table. But keep in mind, wire lengths, because with the rotary table, the operator has the additional task of routing long wires so the rotary table can index freely.

#2 — The Mold or Tooling

The mold or tooling, as it is sometimes referred to, is the real value-added ingredient in the process. The mold is what determines what the final product will be. For example, an injection molding machine could contain a mold the shape of a three pronged cord. The operator would insert a terminated, multi-conductor cable. The injection molding machine and mold would then produce an appliance power cord. If you change the mold, the same injection molding machine could mold antenna ends for cellular phones. For this example, you would use the same injection molding machine, but a different mold (the shape of an antenna), and the operator would insert coiled wire and the end bracket of the antenna. And, if by "magic," a second product, an antenna, can be produced.

Tooling should be universal, easily changeable, quick to set up, and flexible. AmeriPlas has developed a tooling system with those characteristics.

What does the tooling system look like and what are its parts?

1. Sprue Bushing — This is the place where the injection unit seals up against the tooling and allows the transfer of material (plastic) at high pressure to flow from the injection unit into the mold halves. It should be made out of hardened tool steel and should be replaceable because this is where the injection unit is brought up against the mold, and it can see millions of operations.

2. The "Runner" or Runner System is how the material is "tunneled" through the mold to get from the Sprue Bushing (or Entrance) to the different parts or cavities. The runners connect the parts together, and the operator must snap them off to separate the parts. The runners can be reground and can be added back with new material, depending on product and material specifications.

3. Cavities are where the parts are molded. They are the shape of your finished product. Everything in a cavity is important because it affects the way your part will look: from its shape, to its surface finish, to its size. Cavities should be made of tool steel, hardened and chrome plated.

4. Wire shut-offs are the part of the mold that pinches off on what is leaving the mold (wires) so that the plastic does not flow out. The fit of these shut-offs is very critical. It must be tight enough not to allow the high pressure plastic to flow out of the cavity, but not so tight that it damages what it is shutting off on. These shut-offs should be removable because they have a high risk of being damaged. Also, when molding around wire, it may be important to have different size shut-offs, as you change wire from vendor to vendor, or if a particular vendor has a wide spec on their insulation size. You want to have the flexibility to make that change in the mold.

5. Loading bars are a section of the mold usually in one end of the cavity that is lifted up by the machine to allow the operator to insert a wire or terminated leads into it. Then it is retracted back into place and a new part is ready to be molded. Loading bars and parting line tooling are signatures of insert molders and wire harness molders because that is what is needed to mold around a terminated wire or an insert.

6. Knockouts are pins or rods that come up through the cavities to eject or push out the parts after they have been molded. Before the machine molds, Knockout Pins and Loading Bars are retracted and sit flush with the cavities. Knockout Pins usually leave telltale signs in plastic parts, usually in the shape of the round rod that did not line up perfectly with the cavity.
 
 

#3 — The Material or Plastic

What a part is made of affects how it looks, feels and functions. Today's plastics cover a broad range of quantities from soft, rubbery, flesh-like materials to super high tech, engineering resins that are stronger than steel and sound like metal when they are knocked together. Most of these materials can be processed in the same injection molding machine.

Material selection can be a confusing process, especially if you do not have a method of attacking it. The easiest thing to do is to contact the customer. If a product is a part of your customer's product line, there might be material specifications that you have to match. In this case, our job is simple. You simply call the plastic supplier and place an order for the material. If you are developing the product, you need to know answers to these questions before you can talk to a process engineer. What are you molding around? Copper terminals? Extruded cable? How well does it need to bond? Does it need to be chemically resistant? What chemical must it resist? Should it be hard or soft? What durometer? Is flexibility important? How flexible? Is abrasion resistance important? Does it have to be flame resistant? If so, how much? Once you know the answers, then you can start to talk to an application engineer at any one of the plastic suppliers or at AmeriPlas.

If you are a little familiar with plastics, you know that plastics have long, tongue twisting names like Polyvinyl Chloride, Thermoplastic Elastomer, or Ultrahigh Molecular Weight Polyethylene. To help with the long-winded plastics names most are condensed with their initials like PVC for Polyvinyl Chloride or PUR/PU for Polyurethane. To make things even more confusing, every company that produces plastic has their own trade name for a plastic, like Valox (GE) or Santoprene (Advanced Elastomer System). For example, most people do not realize that Lexan is a trade name of GE. The real (generic) name for Lexan is Polycarbonated and, it can be purchased from many sources under various brand names, such as Merlon (Bayer) and Calibre (Dow). There are several reference books published yearly that can keep you updated on all the different brand names. They can be cross referenced to their generic names to help you keep things straight.

Who are some of the major general resin manufacturers? BASF, Bayer, DuPont, GE, and Teknor Apex. Also, there are specialty companies (General Polymer and Bruck) who will distribute the big brands in small quantities. Some (LNP, DSM Engineering Plastics) will custom mix and/or put in special additives such as colors, flame retardants and fillers.

When you purchase plastic in its raw form, it comes in dry pellet form about the size of BB's. It can be purchased in bags like you buy concrete or in drums, gaylord boxes or by rail cars to fill a silo. Usually you purchase the material in its natural color or in a few standard colors such as black or gray. The natural colors are usually off-white or clear.

Coloring can be easily added to the natural material to give you just about any color imaginable. The coloring agent comes in three forms. Each has its advantages. You can get coloring in concentrated pellets, powdered coloring or liquid color. Each must be handled differently. Concentrated pellets are measured with rates like 1 part color to 100 parts material; then mixed with the natural material. Powdered color is handled in a similar fashion, but because it is much finer, it has a more uniform coating, but it is much messier. Liquid coloring must be injected into the lower part of the hopper, which requires more equipment, but it allows you to instantly change colors without emptying the hopper.
 
 

#4 — The Operator

You need an operator to run an injection molding machine. The skill level of the operator can vary from a simple loader and unloader to a process setup person. From the setup through the operation an injection molding machine is no more complex than a crimping press. The same skill level and practices to mount tooling in a crimping press are used to mount molds in an injection molding machine. At the most basic level, an operator is needed to place the inserts into the mold, to press the cycle start button and then, to remove finished parts.

Typically, this cycle time will go from 10 to 30 seconds, yielding 2 to 8 pieces every cycle. Getting outputs of 15,000 pieces in a shift is a possibility. Usually, operator loading and unloading time is the determining factor of cycle time, unless the parts have sections over 1/4", causing the cure time to be over 15 seconds. Even so, longer cure times can be balanced with more cavities to load, making the operator loading time the limiting factor again.
 
 

#5 — Inserts

An insert is anything that is placed in the mold so that plastic can be molded on it or around it. Any insert that is used must have dimensional consistency. This is especially true if the insert is being shut off and on. Variations in the size of the insert will change the amount of material needed to fill the mold. This can cause the cavities to be partially filled (known as short shot), or can cause too much material in the cavities (known as flashing). Variations of an insert on a shutoff area can cause plastic to squirt down the inserts (also, known as flashing) or can even push the inserts out of place.
 
 

#6 — Accessory Equipment

An injection molding machine, a mold, plastic, and an operator loading inserts is what is needed to manufacture an insert molded part, but accessory equipment for the injection molding industry is as big as accessory equipment for the wiring harness manufacturer. Here is a listing of typical equipment that is acquired as a molding operation expands: Mold temperature controller to regulate mold temperature, grinders to regrind runners, automatic hopper loaders to keep hoppers full, material dryers to dry certain plastics prior to use, mixers to mix colorants into the plastic, color feeders to inject colorants into the hopper, chiller units to cool machinery, and silos to store plastic.
 
 

Conclusion

With connectors, grommets, strain reliefs and plugs, we have attempted to show that insert molding is not "magical", but is a manufacturing process that can benefit your company. With the increased use of the computer and modern communication and with electrical and electronic products becoming more and more common, the wiring harness manufacturers have added insert molding to their manufacturing mix.

To summarize, introduction of the injection molding process within your company can reduce labor efforts, increase your efficiency, produce a superior product, reduce your inventory and shorten lead times.

AmeriPlas specializes in insert molding, focusing on the electrical and electronic industries. Our expertise is in the design and manufacture of machinery, tooling, processing and accessories for molding. We can provide services to evaluate the feasibility of a molding project and have a "Concept to Turnkey" approach to injection molding. If you are a current insert molder or have a new project or product, AmeriPlas Machinery Corporation is the company that can "Build The Machines That Make You Money."

For further information about the AmeriPlas Vortex Machine or the injection molding process, contact AmeriPlas Machinery Corp, 13415 Union Avenue, Cleveland, OH 44120. Phone (216) 561-8950 or Fax (216) 561-8960.