Volume 7, Issue 1 - February 2011



Gate Design for Polycarbonate Medical Parts
by Mark Yeager, Principal Engineer / Engineering Consultant, Bayer MaterialScience LLC

Most part designers consider gate design to fall solely under the responsibility of the mold shop, and are therefore uninterested in gate design. This is short sighted. Poor gate design can reduce part performance and create cosmetics defects. Failure to identify a bad gate as the source of problems can lead to costly delays and part modifications. In addition, part design often dictates or limits the gate type, size and location, all factors that can contribute to gate related problems. Molders and part designers both need to be aware of proper gate design and the steps to follow to avoid problems.

Except for special cases, such as sprue-gated systems which have no runner, gates connect the runner to part. Gates perform two major functions, both of which require the gate thickness to be less than the thickness of the runner and part wall. First, gates freeze off and prevent pressurized material in the cavity from backing through the gate after the packing and holding phases of injection. Applied pressure from the press injection unit can stop earlier in the cycle, before the part and runner system solidifies, saving energy and press wear and tear. Secondly, gates provide a reduced thickness detachment point for easier separation of the part from the runner system.

Types
The Mold Design chapter in the Bayer MaterialScience Part and Mold Design Manual shows the most common gate types, and is a good reference for proper design of these gates. This article will limit its scope to cold runners only. Tunnel gates separate from the part automatically so avoid the costs of a secondary trimming step. They can feed directly into the part side wall where they will leave an oval gate vestige or they can feed into a post or tab that is off. The conical angle must be large enough to ease demolding and avoid excessive filling pressure. The angle to the parting line dictates the sharpness of the edge that trims the gate as well as the extent of the gate under cut. Below are design guidelines for both the conventional and modified tunnel gate. The modified gate design consumes less filling pressure and is gentler on the material.

Tunnel gates have several limitations. As the gate opening becomes too thick, the gate cuts off less cleanly, demolding forces become too high, and the gate steel can exhibit excessive wear or damage. Tunnel gate diameters for polycarbonate seldom exceed 0.100". This size limitation necessitates multiple gates, which generates more weld lines. The length of the weld lines can be minimized by placing gates in a tight cluster. Tunnel gates can normally only reach part features located close to a runner lying on the mold parting line i.e. along the part perimeter or along large openings in the part design.

Pinpoint gates are not limited to the edges of the part. Fed by 3-plate runners, they can reach most anywhere on surfaces oriented parallel to the parting mold parting plane. Pinpoint gates separate from the mold during mold opening and must be designed with defined break-off points. Like tunnel gates, the size is typically limited to under 0.100" diameter to avoid degating problems. In medium to large parts, multiple gates are usually needed for proper filling.

Pinpoint and tunnel gates have rounded opening. There is a wide array of gates with rectangular openings. These gates tend to have larger cross sections, consume less pressure, generate less gate shear, produce lower molding stresses, and create fewer flow-related defects. On the negative side, they require a mechanical trimming step, leave large gate vestige marks and are limited to locations along the part edge. Below are the guidelines for the common edge gate. The gate needs to be 1/2 to 2/3 the wall thickness for packing and the width is set to achieve an acceptable shear rate. The gate land, the length in the direction of flow, needs to be short to avoid excessive pressure loss and early freeze off. For common edge gates, the gate land should not exceed 0.060".

Two interesting variations on these gates are the filter bowl and diaphragm gates which feed the entire edge of an opening to avoid weld lines. The gates are then trimmed or punched out. These designs are appropriate for pressure vessels or cylindrical with high cosmetic requirements.

Size
Factors affecting optimum gate size include part thickness, part volume, filling speed, material properties and the number of gates. Gate thickness controls packing. To avoid sink and shrinkage-related defects, gates must remain open and free to inject additional material after the initial filling phase. Undersized gate thickness freeze off too quickly, which can lead to excessive molding shrinkage, dimensional problems and sink. Packing rings around the gate, as shown below, are clear signs of an undersized gate. The rings are formed as the gate freezes and reopens repeatedly during the packing phase. The gate thickness is defined as the smallest of the gate cross-section dimensions. For polycarbonates materials and their blends, we recommend a gate thickness of at least one half the part thickness. Two thirds is preferred for highly cosmetic parts.

Flow from the gate is supposed to proceed radially from the gate. When a thin gate feeds directly into the edge of a thicker wall, the flow can initially enter the mold cavity as an extruded strand without wetting or attaching to the cavity surface. The strand then typically folds back on itself until the strand bunches up in front of the gate. At that time, a radial flow front begins and the bunched strands are encapsulated by the flow front. This is called jetting. Jetting leaves a serpentine surface defect and weakens the part. Jetting can be avoided by orienting the gate so flow impinges on a wall surface or mold core. Alternatively, the gate thickness can be increased to 80% of the wall thickness. At this thickness, flow from the gate will quickly swell to the full wall thickness, and the flow front will attach to the cavity wall.

Volumetric flow rate and gate size control shear rate in the gate. Excessive gate shear can lead to haziness around the gate (gate blush) and streaky defects. Shear rate, measured in reciprocal seconds, is a measure of the shear applied to the material, particularly in areas of high flow velocity such as a gate. Bayer MaterialScience conducted studies to measure the shear rate at which gate blush first becomes objectionable. The table below shows the guideline limits we established for a variety of materials. We recommend 1/2 these values for critical transparent or cosmetic applications and most medical parts. Shear-related problems seldom occur below these limits.


Computer flow analysis can take into account the best filling speed and injection velocity profile when calculating the shear rate in the gate. A less accurate but simpler method is to calculate bulk shear rate using an estimated, uniform volumetric flow rate in the appropriate shear rate formula.

shear rate = 4Q/π r3 for round gates
shear rate = 6Q/wt2 for rectangular gates

where:
Q = flow rate (in3/sec)
r = gate radius (in.)
w = gate width (in.)
t = gate thickness (in.)

To calculate flow rate, divide the volume passing through the gate by the estimated time to fill just the part, not the part and runner. For parts with multiple gates, this will mean assigning a portion of the part volume to each gate.

To minimize packing and gate shear problems in rectangular gates, set the thickness at 1/2 to 2/3 the part wall thickness and adjust the gate width to achieve an acceptable shear rate. For round gates, adjust set the diameter at 1/2 to 2/3 the part thickness (for packing) or at the value needed to achieve an acceptable shear rate, which ever is greater. Increase the quantity of gates if the calculated gate size is too large to degate cleanly.


Location
Gate location can have a direct impact on part moldability, performance, appearance and cost. The location of the gate determines the filling pattern and maximum flow distance. Ideally the gate would be placed centrally to balance filling and minimize flow distance. The best gate location for filling may be unacceptable for a variety of reasons. It might place an unsightly gate mark on a cosmetic surface. Cavity layout restrictions or mechanisms in the mold such as slides or lifters may also prevent ideal gate placement.

Placing gates at multiple locations on the part can reduce the maximum flow length, improve packing and reduce the required filling pressure. Keep in mind that unless sequential hot runner gating is used, multiple gates produce weld lines where the flow fronts meet. Weld lines form where the flow fronts meet head on and usually extend until the angle between the merging flow fronts drops below 90 degrees. Flow simulation can show where the weld lines will occur. If there are locations which are cosmetically unacceptable, then repositioning the gates might move them to a more acceptable location.

In parts with thick and thin areas in the main wall, gates should feed into the thickest sections to avoid thin-to-thick filling. Flow should progress from thick-to-thin for proper packing, sink prevention, and dimensional control. The part designer may need to incorporate thickened flow channels in the part design to provide a continuous thick flow path from the gate to the thick wall sections. Alternatively the designer could modify the design to core out the thick sections to achieve a more uniform wall thickness, or build in gate accommodations in the thick areas. Perhaps the gate vestige in the thick section could be placed in a recess or be covered by a label.

Unfilled PC and PC blends exhibit nearly uniform shrinkage in the flow and cross-flow direction, so filling orientation is not a major concern. Fiber-filled grades exhibit considerably less shrinkage in the flow direction than in the cross-flow direction. This can lead to orientation-related warpage. With glass-filled grades, it is usually better to gate at the end so the fibers align lengthwise. The mechanical properties of fiber filled grades are better in the direction of fiber alignment. Lengthwise orientation can improve the mechanical performance of parts that see applied loads that place the part in tension or bending over the length dimension.

Delicate or unsupported mold cores can deflect during filling if the flow front progresses asymmetrically around the core. As flow advances sooner up one side of the core, filling pressure flexes the core and increases the wall thickness on that side and reduces it on the opposite side. Filling accelerates on the thick side and slows down on the opposite side causing a snow-balling effect that increases core deflection. Core flex changes the part wall thickness, creates possible part ejection problems and can lead to core cracking. Placing gates on both sides of the core can improve filling symmetry and correct core deflection.

The best gate position is often a compromise between mold design feasibility, appearance, and molding ease.

Summary
The gate type, size and location have a direct influence on molding ease, part quality and part cosmetics. The design of the part often dictates or limits the gating options, and the part designer needs to be aware of this. It is also important for all parties to correctly identify the gate as the culprit when gate-related problems arise.

Gates come in a variety of shapes sizes, each with advantages and disadvantages. There are often many options for gate placement. The best gating is usually a compromise between mold design feasibility, part appearance, material requirements and molding ease.

Gate thickness is most critical. The gate needs to be large enough to facilitate proper part packing. For polycarbonate and polycarbonate blends, the gate thickness should be 1/2 to 2/3 the part wall thickness. The gate also has to be large enough to stay within the shear rate limits of the material. The shear rate can be calculated if you know the part volume and can estimate the filling time.

Proper gate design can optimize part performance and appearance and prevent many common molding defects.

For more technical papers related to molding polycarbonate parts please visit our website.

If you are located in Europe or Asia, you may also be interested in visiting our Plastics TechCenter website.

 


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