Asahi Kasei Plastics compounds can be injection molded in virtually any configuration achievable. While it is true that the presence of the glass fillers may increase the stiffness of the melt, an increase in melt temperature and/or mold temperature will generally compensate for the reinforcement's melt stiffening effect. However, where melt and mold temperature may be restricted due to the material's thermal stability, dimensional considerations or cycle time objectives, the following guidelines for the design of the mold should be considered.

Nominal Wall Thickness

The melt viscosity (stiffness of the melt) is primarily determined by the melt viscosity of the base thermoplastic. Except in very highly loaded compounds, the influence of the reinforcement on the melt viscosity is secondary to that of the thermoplastic itself. Therefore, when determining minimum wall thickness requirements, the characteristics of the base thermoplastics should receive prime consideration.

Our Technical Service Personnel are available for consultation on questions of flow length and minimum wall stock requirements for specific applications and materials.

Recommended Part Thickness Bosses

Bosses are commonly incorporated in injection molded thermoplastic parts to accommodate the attachment of adjacent components.

The single most critical element of boss design is the selection of the correct bore diameter. This is particularly critical in reinforced thermoplastic parts. These parts are stiffer and less ductile than their unreinforced counterparts. Therefore, the bore diameter must be small enough to provide sufficient undercut for good fastener retention, but not so small as to require expansion of boss diameter.

Thread cutting fasteners are preferred to thread forming fasteners in the more rigid compounds as they minimize the requirement for boss wall deformation.

Ultrasonic insertion of threaded metal inserts is readily accomplished in thermoplastic parts. For parts that have increased elevated temperature stiffness, somewhat higher ultrasonic energy levels may be required.

Bosses should be designed to maintain wall thickness similar to the adjacent sections. Generous radii at the base of the boss are recommended to minimize stress concentration in this generally highly stressed region.

A minimum draft angle of 2 degrees is recommended for external boss surfaces. In general, no draft is required on the core pin. The bore depth should be kept to a minimum and the core pin chamfered and polished.

Ideally, the cross-sectional thickness of the boss should be no greater than that of the main body of the part. Gussets can be added to the boss to improve load bearing capacity.

Ribs

Ribs (linear projections normal to the part surface) are often employed to improve the load bearing capacity of structural parts while minimizing material usage. Several factors determine rib dimensions. From a structural standpoint, the height of the rib is the most significant factor influencing the ribs contribution to flexural load bearing. The flexural stiffness of a rib (the load a rib can support at a given deflection) is proportional to the cube of the rib height.

The rib height however is generally limited by several factors. They include:

•    Stress distribution: Several smaller ribs may be used in place of a higher one to achieve more uniform stress distribution and better structural stability.

•    Surface sink marks: To facilitate part removal it is necessary to incorporate relatively generous draft angles on the sides of ribs. For a given draft angle the width of the rib at its base is directly proportional to the ribs height. If the rib is too high, the thickness of its base may be great enough to create sink marks on the adjacent part surface.

•    Cycle time: The thinner rib cross-section and increased surface area of multiple shorter ribs often provide faster melt set-up than that required for fewer tall ribs with heavy base cross-sections.

A minimum draft angle of 5.0° is recommended to facilitate ejection of ribbed parts. The height of the rib will determine its base width and resultant cross sectional mass.

Draft

The mold shrinkage may be very different (usually lower) than that of a non-reinforced thermoplastic. This lowers the tendency of the molded part to shrink away from the wall of the mold cavity. In addition, reinforcements generally increase the stiffness of the molded part. As a result, parts have a limited ability to shrink away from undercut and/or zero draft regions of mold cavities.

Draft angles should be between 1 and 2 degrees greater than that required for the corresponding unreinforced thermoplastic. Textured mold surfaces may require an additional 1 to 2 degrees of draft to accommodate the resulting irregular part and mold surface.

Radii

Parts designed will benefit greatly from the use of generous radii at intersecting part surfaces. Extremely high stress loads may develop at sharp part corners during part ejection, handling and/or application. These loads can be significantly reduced by employing generous radii. A secondary function of part radii is to facilitate uniform material flow during cavity filling. Both properties and surface finish benefit from uniform cavity filling.

Inside radii should be as large as appearance and part function requirements permit. A minimum of 1/16 inch radius is mandatory if part strength is to be maintained at the intersection of the surfaces. Out-side radii should be sized to maintain uniform part wall thickness and minimize material stagnation during mold fill.

Adherence to basic part design practices is the first step toward insuring trouble free molding and part performance.

Part Geometry

The most important thing about part configuration is wall thickness. Heavy walls and changes in wall thickness should be avoided as much as possible. 

Non-uniform wall thicknesses will produce different amounts of shrinkage along with a greater potential for warpage.

If wall thickness must vary, the transition should be as gradual as possible. It is best to use large radii since sharp angles produce localized shear heating. This causes slower cooling, higher stress, and higher crystallinity around these points. Edges and corners should be generously radiused, and in order to keep the wall thickness constant, make the outside radius larger than the inside radius.

Draft angles are important because if the part does not come out of the mold smoothly, it can be deformed during ejection. Reinforced materials normally require more draft than unfilled materials. For more on this subject, please refer to the section on draft angles (see the DRAFT section above).

Several good software programs are available to simulate the effects of mold design and gate location. Most of Asahi Kasei Plastics grades have been characterized for usage with these programs. Please contact one of our Technical Service Personnel to obtain these characterizations and other information.

Surface Finish Considerations

The surface finish of an injection molded part varies considerably with:

•    Mold surface

•    Rheological characteristics of base thermoplastic

•    Type and level of reinforcement

•    Mold design and processing conditions

In general, best results are achieved if a low melt viscosity is maintained throughout the mold filling process. This is best accomplished by employing the following:

•    Moderate to fast injection speed

•    Moderately high melt temperature

•    Moderately high mold temperature

•    Part and gating design that facilitates uniform non turbulent flow of the material into the mold cavity

Mold Shrinkage

The factors influencing the mold shrinkage include:

•    Mold shrinkage characteristics of the base polymer

•    Type and level of reinforcement

•    Molding conditions

•    Polymer and orientation of the filler material

•    Part configuration

Polymers with inherently high mold shrinkage characteristics in the unreinforced state tend to display the greatest percentage of reduction in mold shrinkage when a reinforcement material is introduced. Mold shrinkage is reduced as the level of reinforcement is increased. Consequently, the influence of molding conditions on mold shrinkage typically experienced in unreinforced polymers is reduced.

Fibrous fillers tend to reduce mold shrinkage more than non-fibrous ones. In addition, the mold shrinkage of a glass filled plastic may be lower in the direction of material flow than in the cross flow direction. This phenomenon is called differential mold shrinkage. The tendency of a fibrous filler to align themselves in the direction of polymer flow during mold filling increases the composite's resistance to shrinkage in that direction during part cooling. Parts molded from composites whose base polymers have characteristically high mold shrinkages tend to display the greatest differential mold shrinkage in the direction of flow versus the cross direction of flow. Differential shrinkage may be diminished by the introduction of non-fibrous fillers into the composite.

The mold shrinkage values typically available are generated on test specimens of 1/8" thickness and measured in the fill direction. Due to the potential influence of the previously mentioned variables and part configuration on mold shrinkage the stated values should only be used as an estimate.

Our Technical Service Personnel are available for consultation on questions of mold shrinkage for specific applications and materials.

Warpage

Part warpage is caused by uneven stress distribution, a condition that can be produced by the combined effects of processing conditions, mold design and part geometry. Because of this complex web of variables, the amount and direction of warpage are much harder to predict than shrinkage. Non uniform shrinkage can cause these stresses and can be a function of the base resin (crystalline or amorphous), fiber orientation, changes in wall thickness, gate location, or any combination of many other things.

Our Technical Service Personnel are available to assist you with warpage issues.

Mold Design

The main things to consider, in order of importance are gate size and their location(s), cooling, and mold rigidity.

Gate location and size are very important. The location should be such that the part fills evenly, so as to provide the most uniform melt condition during filling and pressure profile during packing. This can be verified through the use of short shots. Normally, gates should not be located in thin sections or ahead of pinch points.

Large gates are preferred. Small gates promote shrinkage by causing too much shear in the material. Additionally, they can freeze off prematurely and terminate packing too soon. Low material density encourages shrinkage.

Adequate cooling is necessary, especially with crystalline resins since a longer cooling time leads to a higher level of crystallinity which, in turn, results in increased shrinkage and warpage. Additionally, the cooling lines need to be located in such a manner so as to provide even temperature distribution throughout the part. Hot spots can produce local shrinkage, which will cause warpage. Core pins, small tooling laminates, and areas around gates can all be troublesome hot spots. If it is not possible to locate cooling lines in the area, it might be feasible to use a highly conductive alloy such as beryllium-copper.

Mold rigidity can also be a factor. The mold needs to be rigid enough to withstand injection pressures. If not, backing off the injection pressure can reduce the packed-out density of the part and increase shrinkage. 

Because we cannot anticipate or control the different conditions under which this information and our products may be used, we do not guarantee the applicability or accuracy of this information or the suitability of our products in any given situation. The information and products referenced herein are intended for use by persons having technical skill and understanding, at their own discretion and risk. We cannot anticipate or control conditions of information and product usage. Users of our products should make their own tests to determine the suitability of each product for their particular purpose. WE MAKE NO REPRESENTATIONS OR WARRANTIES OF ANY KIND, INCLUDING ANY EXPRESS OR IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE. Also, statements concerning the possible use for our products are not intended to be nor are they recommendations to use our products in the infringement of any patent.