Application Review: Epoxy Tooling

Process:
Epoxy Molds

Applicable Industries:
Aerospace, Toy Manufacturers, Medical, Sports Equipment Automotive, Computer & Consumer Products

Types of Masters Used:
Rapid Prototypes
Machined or Fabricated Patterns
Wax Sculptings
Types of Molds Made:
Investment Wax Injection Molds
Compression Molds
Liquid Injection Molds
Reaction Injection Molds
Plastic Injection Molds
Types and Quantities of Parts Made:
Polyurethane 100 to 600
Polyurea 100 to 600
Epoxy 50 to 300
Investment Wax Patterns 150 to 1000+
Low Melt Metal Alloy 50 to 700
Polyurethane Foam 1000 to 5000+
Silicone Rubber 200 to 5000+

Procedure:
Epoxy molds or aluminum epoxy molds are reasonably fast in comparison to machined molds and are a relatively inexpensive way to create prototype and production tooling. New epoxy products offer a much higher compression strength and heat resistance. If the molds are designed properly they can withstand injection and compression pressures with the use of aluminum standoffs or mold boxes.

A high strength mold can be achieved within a few weeks with a master pattern or rapid prototype. The steps taken to achieve this result are as follows: One must first create a pattern; as with castable silicones, these epoxy resins will reproduce surface detail and textures exactly. Therefore, the pattern is finished to the desired quality before making the mold. Typically an interim RTV mold will be made in order to create a urethane reproduction for the epoxy process. This is done because the master is usually destroyed in the epoxy molding process.

The parting lines are established in much the same way we would make a RTV tool. The epoxy is applied to one part of the mold at a time and then goes through a multilevel post curing process. The post curing is where the high strength is achieved. At this time, the mold is machined to hold square and any gates and vents that could not be molded in are applied. Often, intricate core detail will be added in with a machined aluminum or steel insert.

Curing time is dependent on the product and curing agent. Times range from 30 minutes to over 40 hours. Adding heat will speed up the curing process significantly. Aging the mold at room temperature for up to 72 hours, if possible, will increase the productive life of the mold.

Properties of epoxy such as compression strength and hardness vary considerably from product to product. Choosing the proper epoxy for a particular design is based on experience and manufacturers recommendations. The most important consideration is the intended production material.

Some considerations for the epoxy molds are: For injection molding, glass filled materials will rapidly erode the mold surface therefore reducing mold life greatly. Many part geometry's would need to be produced with a different mold process. Some thermoplastic materials have such a high viscosity that injection pressures can exceed the epoxy strength.

These types of molds can be used for very simple to very complex parts. Slides and loose pieces can be made in a similar manner to prototype injection molds. To date Harrington Product Development Center has made very intricate molds with as many as 5 pieces, including low melt metal coring, to pick up undercuts and mechanical details.