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Handling the Pressure

Mon, 11/10/2008 - 6:52am
Michael Hansen, Ph.D.
The Project: Create a handle for a defibrillator durable enough to withstand exceptional performance requirements.
The Solution: Use computer-aided engineering and various flow simulation software to ensure that the final product met the specific characteristics necessary for success.




Michael Hansen, Ph.D. is senior technical development engineer for Mack Molding Co., a custom plastics molder and contract manufacturer. For nearly 20 years, he has worked extensively with the role of plastics processing in the design of gas-assisted injection molded parts. Dr. Hansen can be reached at 802-375-0390 or michael.hansen@mack.com.

A cross-section of the finished overmolded handle clearly illustrates the gas channel, polycarbonate substrate and TPU overmold.
The 911 caller complains of chest pain and shortness of breath. EMTs speed to the scene, ambulance defibrillator in tow. When they arrive, assessing and treating the patient is priority #1. Time is precious and the equipment must perform.

Designed to meet the specific demands and extreme conditions of the emergency medical services environment, this particular ambulance defibrillator features a suitcase-style design with an innovative, protective roll cage, allowing emergency personnel to carry and store the device easily. Lightweight, yet indestructible, it can be dropped off a building or run over by a truck without affecting operation.

The unit is protected on three sides with gas-assisted parts—two side rails and the front handle, which is the subject of this case study. The handle is used both to carry the unit and set it up at an angle. Therefore, it has to be durable and sturdy enough to carry the 15-lb device, yet comfortable and easy to grip for the emergency medical technician handling the product.

The Challenge
Figure 1: The shutoff that defines the boundary between the overmold (black) and the substrate results in a grooved element in the part.
A key element of the product’s exterior design is the light, hollow injection molded handle. The customer wanted a handle that was hollow, but stiff enough to pass rigorous tests. Specifications also included a soft-touch feel for easier transport and a snug grip. There could be no gas pinhole in the handle, and both the substrate and overmolding resins had to be capable of developing a chemical bond.

While hollow, the substrate also had to be structural enough to withstand a shutoff in the steel tool to prevent flash. To accomplish this, the tooled steel had to apply just the right amount of pressure to the substrate to stop the flow of the overmolding resin without crushing the substrate (Figure 1).

It’s important to remember that as the overmolding resin is injected into the tool, the substrate heats up as well. So the substrate must be rigid enough to survive the temperature rise without collapsing. This requires a consistent wall thickness distribution, both for the length of the part and its cross-section.

CAE Confirms Solution
Figure 2: The molding technique chosen for the gas-assist handle for the automated external defibrilator is full-shot molding with backspill.
To produce a handle that will pass all the requirements, the first step is to choose the right gas-assist process. In this case, the best technique is full-shot molding with backspill, which results in a complete filling of the cavity with melt, followed by gas injection and opening of the backspill cavities (Figure 2).

CAE (computer-aided engineering) was used, along with various flow simulation software, to determine the size of the backspill cavity, discreetly position the gas pin, and avoid a hole at the gating location. Flow simulations also clearly illustrated the remaining wall thickness distribution of the substrate part after gas was introduced into the process (Figure 3).

In addition, CAE helped to confirm the processing sequence and cycle time, minimizing material waste and trial and error at the press. CAE also helped to determine a tooling concept with backup options. For example, machining rather than welding a larger backspill area saves steel, which is costly. In the end, this allows for less expensive tool modifications.

Figure 3: Flow simulation software traced the path of the gas channel and illustrated wall thickness distribution.
The handle is molded in a 300-ton press because of tool size, which incorporates the backspill cavity, valve gate, gas pin, four tool actions, and a hydraulic shutoff between the runner and backspill cavity. The substrate is molded of modified polycarbonate resin for chemical resistance, flame retardancy, and ECO compliance. It is overmolded with thermoplastic polyurethane for abrasion, required shore hardness, impact resistance, flow length, and a comfortable grip.

Mack molds a total of 14 parts for this ambulance defibrillator unit, using thermoplastic polyurethane, polycarbonate, and modified ABS resins. All materials are high impact resins with high temperature resistance to ensure performance in the harshest of conditions.

Online
For additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com or Mack Molding Co. at www.mack.com.


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