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The Hot Tip Gate is the most common of all hot runner gates. Hot tip gates are typically located at the top of the part rather than on the parting line and are ideal for round or conical shapes where uniform flow is necessary. This gate leaves a small raised nub on the surface of the part. Hot tip gates are only used with hot runner molding systems. This means that, unlike cold runner systems, the plastic is ejected into the mold through a heated nozzle and then cooled to the proper thickness and shape in the mold.
The Direct or Sprue Gate is a manually trimmed gate that is used for single cavity molds of large cylindrical parts that require symmetrical filling. Direct gates are the easiest to design and have low cost and maintenance requirements. Direct gated parts are typically lower stressed and provide high strength. This gate leaves a large scar on the part at the point of contact.
Gate Location
To avoid problems from your gate location, below are some guidelines for choosing the proper gate location(s):
• Place gates at the heaviest cross section to allow for part packing and minimize voids & sink.
• Minimize obstructions in the flow path by placing gates away from cores & pins.
• Be sure that stress from the gate is in an area that will not affect part function or aesthetics.
o If you are using a plastic with a high shrink grade, the part may shrink near the gate causing “gate pucker” if there is high molded-in stress at the gate
• Be sure to allow for easy manual or automatic degating.
• Gate should minimize flow path length to avoid cosmetic flow marks.
• In some cases, it may be necessary to add a second gate to properly fill the parts.
• If filling problems occur with thin walled parts, add flow channels or make wall thickness adjustments to correct the flow.
Gates vary in size and shape depending upon the type of plastic being molded and the size of the part. Large parts will require larger gates to provide a bigger flow of resin to shorten the mold time. Small gates have a better appearance but take longer time to mold or may need to have higher pressure to fill correctly.
Wall Thickness
Prior to ejection from the mold, injection molded parts are cooled down from manufacturing temperatures so that they hold their shape when ejected. During the part cooling step of the molding process, changes in pressure, velocity and plastic viscosity should be minimized to avoid defects. Few aspects are more crucial during this period than wall thickness. This feature can have major effects on the cost, production speed and quality of the final parts.
Proper Wall Thickness:
Choosing the proper wall thickness for your part can have drastic effects on the cost and production speed of manufacturing. While there are no wall thickness restrictions, the goal is usually to choose the thinnest wall possible. Thinner walls use less material which reduces cost and take less time to cool, reducing cycle time.
The minimum wall thickness that can be used depends on the size and geometry of the part, structural requirements, and flow behavior of the resin. The wall thicknesses of an injection molded part generally range from 2mm – 4mm (0.080” – 0.160”). Thin wall injection molding can produce walls as thin as 0.5mm (0.020”). The chart below shows recommended wall thicknesses for common injection molding resins.
Uniform Wall Thickness:
Thick sections take longer to cool than thin ones. During the cooling process, if walls are an inconsistent thickness, the thinner walls will cool first while the thick walls are still solidifying. As the thick section cools, it shrinks around the already solid thinner section. This causes warping, twisting or cracking to occur where the two sections meet. To avoid this problem, try to design with completely uniform walls throughout the part. When uniform walls are not possible, then the change in thickness should be as gradual as possible. Wall thickness variations should not exceed 10% in high mold shrinkage plastics. Thickness transitions should be made gradually, on the order of 3 to 1. This gradual transition avoids stress concentrations and abrupt cooling differences.