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August 13, 2000

7 Min Read
Prototype tooling for super-thin parts

Editor’s note: The Plastics Components Operations (PCO) of Pitney Bowes continues to push the edge of the envelope in product development and custom molding projects demanding super precision. It now has seven years of experience running its highly advanced Technoplas Super Injection Molding (SIM) system (see February 1995 IMM, pp. 78-82, for an initial report).

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Figure 1. Previously, this Ultem part, a power supply case for military night-vision goggles, cost $25/set to machine to spec after molding. The set also includes a cover. SIM Masterframe System tooling allowed Pitney Bowes to mold them for $2.70/set.

In a Molding 2000 conference presentation, PCO engineers brought the audience up to date on the SIM envelope’s present-day edge with four recent successes achieved since the program started designing molds to make unmoldable parts. The authors, all from Pitney Bowes, are Jeffrey J. Gray, senior engineer; Russell V. Pribanic, tooling specialist; and Timothy R. Erwin, technical advisor. The three engineers outlined PCO’s approach to SIM tooling and described a recent case history. 

SIM Basics
The principle of Technoplas SIM is fast filling, e.g., .01 second, through small gates monitored and controlled by a high-speed analog controller. The controller uses cavity pressure signal feedback to alter and control the molding process. The machine uses cavity pressure information to adjust machine mechanics to provide for uniform material conditions inside the cavity from shot to shot. 

SIM is ideal for producing parts with low internal stress in hard-to-mold resins, and parts with unusual or undesirable design characteristics. Examples are parts with extreme wall thickness transitions of thick to thin, thin walls, tight tolerances, and deep ribs or holes. 

Special attention is given to SIM mold design and construction. The bases must be vacuum tight and generally more rugged than normal. SIM molds require moldmaker capabilities with gaugemaker workmanship. 

Plates must be flat to within .0001 inch, and the base must have a vacuum-tight shutoff mechanism to permit closing the mold the last 7 to 10 mm under vacuum. SIM mold features include the following: 

  • Hardened mold frames with a vacuum-tight interface between plates.

  • A sealed and vacuum-tight ejector box.

  • A linear-bearing guided ejector assembly.

  • A vacuum-ring parting line.

  • A precision-fit transducer pin to relay cavity or runner pressure back to the transducer.

SIM Masterframe Systems
Obviously, these enhanced mold characteristics come at a price. SIM molds cost more than a conventional high-quality mold. One of the problems in expanding Pitney Bowes’ SIM business was the cost of product development—prototype molds cost almost as much as production units. Customers had to spend more than normal engineering dollars on developing tooling and products that might or might not work.

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Figure 2. Some 80 percent of Pitney Bowes' SIM work can be prototyped in its Masterframe mold frames, which incorporate all the expensive mold features required by SIM, but also use cavity/core inserts that can be quickly and inexpensively modified to make sure the product and tooling work.



To alleviate the large financial burden and help make SIM easier to evaluate as an injection molding process, Pitney Bowes developed a mold system incorporating all of the expensive production mold features into what is called a SIM Masterframe System (Figure 2). Eighty percent of PCO’s internal and external SIM work can be prototyped in these frames. 

The frames have two "A" sides (two-plate and three-plate), multiple transducer pin locations, vacuum-tight construction in H-13 steel, and parting line vacuum rings. Mold temperature control is via fittings into the "A" and "B" plates (and the runner plate in the three-plate version) that can be changed to run either water up to 200F or oil up to 450F. The temperature media is channeled directly around the cavity/core inserts. 

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Figure 3. Insert/ejector assemblies nest in tapered pockets in the Masterframe ejector plate. Hand-pick cores assisted by the ejector were used to mold the power supply cases, after Pitney Bowes found it difficult to remove the parts from two non-cammed lifters.

Super-precision Inserts
Insert sets can be two- or three-plate style, depending on the application, and are precision fit for vacuum and liquid integrity, and for molding accuracy across the parting line. A .0005-inch slide fit is typical. Both core and cavity inserts have baffled cooling/heating channels for the most direct temperature transfer possible. 

The insert ejector assembly (Figure 3) nests into a tapered pocket in the Masterframe ejector plate for precision fit and guidance. Hand-loaded inserts or lifters generally accomplish side pulls. 

Prototype SIM Masterframe mold insert sets reduce mold development costs by about 75 percent. And, when the prototyping stage is finished, they can be used for limited production while production molds are built. 

Case Study: Night-vision Components
This moldmaking and molding system was put to the test recently when Pitney Bowes used the Technoplas SIM system to mold two parts that were to fit together—a power supply case and cover that houses printed circuit boards, wiring, and an electronic load cell when assembled. The assemblies fit into night-vision goggles used by the military (See Figure 1 and Figure 4). 

Space was at a premium and they were hardly easy parts to mold. Key characteristics were thin walls, 0º to .5º draft, and tight tolerances. Also, they were specified in Ultem 1000 PEI resin. It cost $25/set to postmold machine them. Pitney Bowes SIM molded them for $2.70/set. 

Here is a closer look at the case tolerances (the cover’s another story): 

  • Length dimensions: 1.033 inches ±.002 inch overall length; 1.444-inch diameter +.000/-.002 inch. Length on the OD has 0º draft and includes the entire cavity. Length over the .469-inch ID has 0º draft. All other drafts on the core are .5º.

  • Wall thickness: The main wall thickness is .022 inch +.000/-.002 inch. The thinnest wall section is .019 inch.

  • Other features: A .010-inch-wide by .005-inch-deep air purge slot runs the length of the part. The case’s OD must be perpendicular to the top datum surface within .0015 inch.

As you might imagine, Pitney Bowes recommended prototyping it first. To save the customer money, Pitney Bowes decided to use a core/cavity set and ejection system sized to fit into its SIM Masterframe System. Two features in the part required side actions. PCO used two noncammed lifters. The lifters eased ejection. It originally intended to roll the case off these lifters manually after ejection. 

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Figure 4. Super injection molding requires super injection molds. Pitney Bowes has developed its own unit frame tooling system to reduce the prototyping costs involved in molding challenging parts like this extremely thin-wall, zero-draft Ultem part.

The case was subgated into the ID wall. Initial gate size was .010 inch in diameter. The runner had a .020-inch diameter. Mold shrinkage used was .007 inch/inch. Vent depth was .0015 inch. Cavity/core steels were in H-13, with cavity finishes of #3 and a 600 draw finish on the core. 

First shots yielded cases that had distortion on the top surface. Parts were releasing improperly on the "B" side core that forms the ID, cases could not be removed easily from the lifters, and the open end of the case furthest from the gate had a nonfill area. So, Pitney Bowes pursued the following modifications: 

  • Runner diameter was enlarged to .054 inch.

  • Gate diameters were enlarged to .020 inch—eventually replaced with a direct sprue into a .012-inch-thick diaphragm gate removed with a punch press.

  • Increased draw polish on core.

  • Changed lifters to hand-pick cores assisted by, but not tied to, the ejector.

The company also made additional hand-pick cores and redressed the cavity due to the wear of insert loading and unloading. Wear resulted in flash. The part was filled in .05 second.

After the modifications were made, cases were molded to print. The cases and covers were put into production with the prototype molds filling an initial order of 10,000 sets. Production molds are now running. Pitney Bowes says cost savings will allow its customer to pay for the molds in 10 1/2 months. 

Contact information
Pitney Bowes Inc.
Danbury, CT
Timothy R. Erwin
Phone: (203) 790-3647
Fax: (203) 790-3768
E-mail: [email protected]

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