Analyzing the Advantages
of Robotically Dispensed
FIP Gaskets and Seals
While robotic technology improves productivity and gasket performance,
up-front attention to component and mold design can lead to even higher gains.
By John Snyder
and Tom Chresand
Editors note: Traditionally, injection molders would have to manually adhere the die-cut peel and stick gaskets onto plastic parts molded for the automotive, appliance, drum/barrel and electronics industries. But recently, companies in the United States have begun to follow Europes lead in using robotic equipment for gasketing. Although this method offers savings in both time and labor, to receive the optimal benefits, plastic parts must be designed slightly different.
Robotic foam-in-place (FIP) gasketing is an established technology for automating the gasketing and sealing of component parts used in the manufacture of plastic automotive, appliance and electrical components.
Specific applications for this process include: automotive air exhausters and access covers, vacuum cleaners, air filters, dish washers, clothes dryers, dehumidifiers, air conditioners, freezers, refrigerators, ranges, air cleaners and computers.
But while many plastic parts are amenable to the use of FIP seals, up-front attention must be given to the mold and plastic part design. If done correctly, the benefits of the FIP seal and gasket approach will be reductions in labor costs, increased flexibility in both product and process design, and seamless, high performance gaskets.
The FIP Process
An FIP foam is a void-filled polymer matrix. The foam is applied robotically to a part as a liquid or semi-liquid and cures to a solid on the part. Foams can be one-component, such as hotmelt or urethane moisture cure; or two-component, such as polyurethane or silicone.
One-component foams are produced by mechanically mixing an inert gas, such as nitrogen, into the material, while two-component foams are produced by the production of gas in a water-based side reaction as the material is blended during the dispensing process. Selection of the foam material depends on the application and the performance specifications. However, two-component polyurethane foams are most widely used in the FIP process since they offer the widest range of physical properties.
The viscosity of different FIP materials can vary widely, from a paint-like consistency to a thick paste (thixotropic). The way in which these materials flow onto a flat surface or into a groove has implications for the part design.
For both one- and two-component foams, the liquid or semi-liquid material must be applied to the substrate accurately since curing will occur wherever the material is first laid down. The material cannot be adjusted or moved into position after the fact. For this reason, the material is almost always robotically applied. While round parts can sometimes be gasketed with a simple turntable, most applications require either a three- or six-axis robot.
It is the robotics requirement that actually leads to some of the major benefits of FIP gasketing: a reduction of labor and an increase in placement accuracy.
Fixturing is another important aspect of the FIP process. Parts must be moved into and out of the dispense station accurately and quickly in order to ensure high productivity, and they must be presented to the robot in optimal fashion. These factors also impact part design.
The Optimal Part Design
Since FIP materials can be applied either in a groove (liquid) or on a flat surface (semi-liquid paste), most component parts can be converted from manual gasketing to a FIP system or initially designed with the FIP approach in mind. The following are some part design guidelines:
While sloped regions as steep as 45 degrees can be accommodated, the seal should be dispensed onto a surface which is as consistently horizontal as possible. This will simplify the robotic requirements and reduce any potential slumping of the dispensed material. (A sloped application requires a thixotropic FIP material to avoid slumping.) The part should also be designed so that it sits flat. This will ensure that the material remains level during and immediately after foaming, thereby avoiding additional table fixture costs.
Whenever possible, a groove should be designed into the part to accept the gasket. A groove will allow use of lower viscosity liquid materials, which will self-level and avoid a visible knit line the point where the end of the gasket rejoins the beginning. A groove also protects the gasket from wear and abuse. The groove walls should be continuous and equal in height at all points so that the resulting gasket is uniform.
Narrow groove width is probably the most common design error. Material will not flow easily into a narrow groove, and air entrapment can occur. Also, the dispense nozzle must have a small inside diameter, which leads to a low flow rate and low robot speed. The overall result is a longer dispense cycle time.
Production rates can be significantly increased by use of an appropriate groove width. While groove widths as small as 1&Mac218;8 inch can be gasketed, the groove should preferably be from 3&Mac218;16 inch to 1 inch wide. Also, the groove width should be uniform around the part so that a constant robot speed and dispense rate can be maintained. This will lead to a gasket of uniform height.
Corners, both for flat and grooved parts, should be rounded so that the robot can maintain speed in the turns. The bottom of a groove should also be rounded so that air is not trapped while material flows into the groove. These design features lead to higher robot speed and increased production rates.
The gasket should be compressed between 35 to 45 percent to ensure proper sealing. Make sure that the tolerance between the mating surfaces is not a significant fraction of the gasket height. For example, if the tolerance between the parts is ± 0.030 inch (0.060 inch = 10 percent variation), the overall height of the gask
et should be 0.600 inch. Obviously, the less the part warpage the better.
Case Study: Straight But Not Narrow
An appliance manufacturer wanted to use FIP to gasket a freezer panel. The panel was 18 inches by 12 inches and had a 0.160 inch wide by 0.400 inch deep groove around the perimeter. The nozzle which was specified had a 0.150 inch outer diameter, 0.120 inch inner diameter and could accommodate a flow rate of about 1 g/second. This led to a cycle time of 15 seconds per part.
The material supplier recommended widening the groove to 0.210 inch and making it only 0.300 inch deep with a 0.030 inch radius between the groove walls and bottom. This allowed use of a 0.170 inch inner diameter nozzle (twice the cross-sectional area of the 0.120 inch inner diameter nozzle) which could produce a flow rate of 2 g/second. The result was a doubling of the robot speed and a halving of the cycle time to 7.5 seconds per part.
Thus, the production rate was doubled with no change in the amount of FIP material used.
Mating surfaces should have a radius edge or flange rather than a knife edge to minimize the chance of cutting the gasket surface when under compression.
FIP gasket materials do not adhere well to polyethylene, polypropylene, stainless steel, galvanized steel or aluminum. The plastic substrates require either a groove or surface pretreatment, while the metals typically require either a groove or priming.
Component parts should be free of obstacles, such as posts or flanges, which impede the movement of the robot or mix head.
The gasket surface should be free of rough surfaces created in the injection molding process such as knock-out marks or gussets. This reduces the chance of air entrapment that can lead to bubbles in the gasket surface and compromise the integrity of the seal. w
John Snyder is the president of Frazier Technologies. Tom Chresand is a senior applications engineer with H.B. Fuller Co.
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