Plastic part design guideline

Design Guide Line

1. Ribs height and thickness

1-1. Heigh of Boss, H = 2.5D

Boss diameter, wall thickness, and height design parameters. While boss heights vary by design, the following guidelines will help avoid surface imperfections like sink marks and voids: the height of the boss should be no more than 2 1/2 times the diameter of the hole in the boss.

1-2. Rib-to-wall thickness ratios, 60%

Thin ribs on thicker walls may provide stiffness but also can result in sinking on the outside of the wall. To prevent sink, the thickness of the rib should be about half of the thickness of the wall. This rule-of-thumb guideline should help keep this from happening.

1-3. Typical wall thickness

Wall thik.		max[in]	min[mm]	max[mm]
ABS	0.045	0.140	1.143	3.556
PC	0.040	0.150	1.016	3.810
PP	0.025	0.150	0.635	3.810
PE	0.030	0.200	0.762	5.080
PU	0.080	0.750	2.032	19.050

* Fiber reinforced plastic allows for larger thicker parts

2. Minimizes Warpage

Warpage due to stresses in step transitions between wall thicknesses can be improved through the use of a ramp. The use of gussets can also provide support in corners to help avoid warping.

A. High stress concentrations
B. Reduced stress concentrations
C. Thinner walls result in shrinkage during cooling
D. Gussets provide additional support to reduce warpage

3. Avoiding sink

Thicker and non-uniform wall thicknesses can often result in sinks in the material due to the same solidification physics described above. The use of thinner, uniform wall thicknesses helps to avoid sink.

A. Boss in corner causes sink
B. Thinner walls on boss eliminates sink
C. Thick walls cause sink, warp & excess shrink
D. Thinner walls give accurate part

4. Process defect

BURN MARKS

 

Burn marks can be defined as small dark brown or black discolorations on the surface of a molded part, usually found at the end of the material flow path or in blind pockets.

Machine

Excessive Injection Speed or Pressure

Explanation: Injection speed and pressure determine how fast molten resin is injected into a mold. If either is too high, the resin is forced in so fast that trapped air and gases are not allowed time to be vented. Many of these gases are pushed to the edge of the flow front and become compressed to the point that they auto-ignite, burning the surrounding plastic. The burned areas appear as char marks on the molded part.

Solution: Reducing the injection speed or pressure will allow enough time for the gases or trapped air to escape through normal vent paths.

Excessive Back Pressure

Explanation: While most materials will benefit from some back pressure application, there is a limit to the amount needed for a specific material and product. The whole idea of applying back pressure is to mix the material better, making it denser and more oriented for flow. However, this very act of mixing may introduce air into the melt, which may be too much for the venting system to handle under normal conditions. The excess air may be compressed at the vent locations and auto-ignite, causing burn marks on the part. There is also the possibility that shearing action from too high a back pressure setting will degrade the material in the barrel and cause burning to occur.

Solution: Use minimum back pressure. All materials will benefit from approximately 50-psi back pressure, but some require up to 300 psi. The material supplier is the best source of information regarding proper back pressure settings for a specific material. When adjusting back pressure use increments of 10 psi.

High Screw Rotation Speed

Explanation: The turning of the screw brings fresh material into the heating cylinder and imparts a certain amount of heat to that material through rotary shear friction. The faster the screw turns, the greater the friction and shear heat created. If the speed is too high, the friction will cause the material to overheat and become thermally degraded. This causes small particles of charred material to form and these get pushed into the melt stream during injection. They will show up on the molded part as burned areas.

Solution: Adjust the screw rotation speed. An average speed should be approximately 100 rpm. But, specific materials require specific rotation speeds. Consult the material supplier for the proper speed for a specific resin. When adjusting up or down, do so in increments of 10 rpm.

Improper Compression Ratio of Screw

Explanation: Compression ratio is a value that indicates how much a screw will compress a material while it is being processed in the barrel of the machine. It is calculated by dividing the depth of the feed section (rear) of the screw by the depth of the metering section (front) and is usually less than 2 to 1. If the compression ratio is too great for the specific material being molded, the resin degrades thermally and the burned material flows into the molded part.

Solution: The material supplier can recommend the proper compression ratio for a specific resin. A general-purpose screw can usually be used to provide adequate compression but specific conditions may require a screw that is specially designed for a given material.

Faulty Temperature Controllers and Heater Bands

Explanation: Temperature controllers are sensitive units and should be checked and calibrated often (every 3 months maximum). Heater bands do wear out or burn out and normally there is no way of knowing. If controllers or heater bands are not functioning properly, the other controllers and heater bands must compensate. The result is localized overheating of material in the cylinder. This material can degrade and char and enter the melt stream to become molded into the plastic part.

Solution: Inspect and calibrate temperature controllers at least every 3 months. Inspect and replace damaged heater bands as necessary. In addition, look for broken or crimped wires, poor insulation, rusted areas on the heater bands, and poor electrical connections.

Excessive Barrel Temperatures

Explanation: When barrel temperatures are set too high, the resin will overheat and undergo thermal degradation. This degraded material will break loose, enter the melt stream, and become molded into the finished part.

Solution: Establish proper barrel temperatures and profile. The material supplier will provide accurate barrel temperature requirements. The profile should have the barrel temperatures increase progressively from rear to front.

MOLD

Improper Sprue Bushing-To-Nozzle Sizing

Explanation: If the sprue bushing diameter does not match the nozzle opening (or vice-versa) molecular shearing will occur at their junction and some of the material flowing through that area will degrade. The degraded material will enter the melt stream and be molded into the finished part.

Solution: Using bluing dye or thick paper, press the nozzle against the sprue bushing, and check the impression of the openings of each. They should be close to the same and not be off center. Replace the nozzle tip or the sprue bushing if they do not match. Re-center the heating cylinder to the mold if they are off center.

Improper Venting

Explanation: Air is trapped in a closed mold and incoming molten plastic will compress this air until it auto-ignites. This burns the surrounding plastic and results in charred material in the form of burn marks.

Solution: Vent the mold by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. Vents should take up approximately 30% of the perimeter of the molded part. Vent the runner, too. Any air that is trapped in the runner will be pushed into the part. Blind pockets can be vented using flush core pins or fake ejector pins and grinding a flat down the entire length of the pins.

Undersized Gates

Explanation: Gates are used to determine the flow of the material into the cavity. They are intended to cause a slight restriction and impart shearing heat to the plastic, as well as to control the speed at which the plastic enters. If the gate is too small, the restriction is too great and the material will overheat causing degradation. The degraded material shows up as burned resin in the finished part.

Solution: Size the gate according to the material supplier's recommendations. The gate should be as thin as possible to minimize cycle time, but as thick as necessary to reduce the tendency to degrade material. Gates should be installed in removable inserts so they can easily be altered or replaced.

MATERIAL

Excessive Use of Regrind

Explanation: Regrind melts at a lower temperature than virgin, and a regrind/virgin blend must be heated high enough to melt the virgin, which may degrade the regrind. For this reason, regrind use should be minimized if mixed with virgin material. However, regrind by itself can be used successfully by lowering the melt temperature.

Solution: Use 100% regrind, or, if mixing with virgin, limit the amount of regrind to 15% by weight. It may be necessary to use no regrind at all, especially in some medical and electronic products.

OPERATOR

Inconsistent Process Cycle

Explanation: It is possible that the machine operator is the cause of delayed or inconsistent cycles. This will result in excessive residence time and erratic heating of the material in the injection barrel. If such a condition exists, materials may degrade, resulting in locally burned resin.

Solution: If possible, run the machine on automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is really necessary. And, instruct all employees on the importance of maintaining consistent cycles.

5. Plastic Design Guideline

PLASTIC DESIGN GUIDE

API engineers and designers can work with you to develop and design your plastic injection molded component. To ensure a quality part, there are three major areas of focus throughout the design stage:

1. Proper plastic part design
2. Proper material selection plastic part design
3. Processing conditions for plastic injection molding

The designers and engineers at API have over 250 combined years of experience in designing parts for plastic injection molding, selecting materials, and processing resins (specializing in engineering and high performance resins). This guide was designed to demonstrate the basic elements of proper plastic part design.

DESIGNING PARTS FOR UP & DOWN MOLDING HELPS CONTROL COST

Designing a part that can be molded with a "straight pull" or "up & down" motion is a great way to keep the cost of the mold down. A straight pull mold is designed so that when the two halves (A side and B side) of the mold separate from each other, there is no plastic blocking the path of the metal in the direction of the pull. Undercuts on the part cause this blockage of path and require an action in the mold (cams, core pulls, etc.). Action in the mold can have a major impact on the cost (and overall size) of a mold.

UNIFORM WALL THICKNESS HELPS TO PREVENT DEFECTS

Proper wall thickness is one of the most fundamental requirements in designing a part for plastic injection molding. Plastic shrinks as it cools which can lead to defects such as sink marks, voids, stresses, and warping. Plastic resin solidifies in the mold nearer to the outside of the part (closest to the mold surface). Thick sections of a part tend to pull inward, creating stresses, sink marks, or voids. Since thinner sections cool quicker, stress can build in the part between thinner and thicker sections, resulting in part warpage.

DRAFT ALLOWS FOR PARTS TO RELEASE FROM THE MOLD

Draft is required on all parts in the direction of mold movement in order to allow parts to release or eject from the mold properly. Draft is the angle in which the part is tapered to allow it to release. As the part cools, it tends to want to shrink to the core side of the mold. Adding draft helps the part to release. Most parts or applications require a minimum of 1/2 to 1 degree, however 11/2 to 2 degrees is widely accepted as the norm.

HOLES ENHANCE PART FUNCTIONALITY AND REDUCE WEIGHT

Holes can be added to a part for functionality or to reduce overall part weight (coring). Core pins are typically used to form a hole, preventing the molten plastic from filling in that space. Through holes go all the way through a part. Blind holes do not completely go through a part. Core pins for a blind hole are only supported by one end, so there is a greater degree of difficulty in forming them without defect. Forming holes can lead to defects or have a negative impact on aesthetics. Since the molten plastic flows around the core pin, it can leave a weld line (which may be visible and/or be weaker than the remainder of the part).

1. The depth of a blind hole should be about two times the diameter of the core pin for small pins (less than 3/16") and four times the diameter of the core pin for pins greater than 3/16".
2. Distance from the edge of a hole to a vertical surface (edge or part or rib) or another hole should be at least two times the thickness of the part or at least the diameter of the core pin (hole).

3. Holes created in the direction of the opening/closing of the mold or parallel to the parting line are relatively easy to produce. Holes at different angles can be created, but may require special action in the mold utilizing core pulls or cams which can have significant cost impacts.

BOSSES AID IN ASSEMBLY AND MOUNTING

Bosses can be added to the part design for assembly, locating, or mounting of a part. Improper placement of a boss leads to uneven wall thickness and can have a negative impact on the aesthetics, shrinking, or strength of a part.

1. Wall thickness around a boss feature should be 55%–65% of the nominal wall thickness for thin walls (less than 1/8") or around 40% of the nominal wall thickness if greater than 1/8"
2. Boss height should be no more than 2 1/2 times the diameter of a hole in the boss

RIBS ENHANCE PART STRENGTH AND STABILITY

Ribs can be added to parts to add rigidity or stiffness. Adding ribs allows for a part to increase strength and bear a higher load. Ribs too have recommended guidelines to maximize functionality and minimize defects.

1. Rib thickness should be less than the wall thickness. Recommended thickness is 60% to 80% of the nominal wall.
2. Adding more ribs adds more strength or stiffness to a part. It is better to add more ribs than make larger or thicker ribs.
3. Ribs should be spaced at least two times the nominal wall thickness from one another.
4. Rib height should be less than three times the nominal wall thickness of the part.
5. If a thick rib is required, the center of the rib should be cored (cut out) to allow for uniform wall thickness.

ADDING A RADIUS REDUCES STRESS ON CORNERS

Radii should be added to angles to prevent sharp corners. Corners can lead to stresses, limit material flow, and often reduce part strength.

1. Inside radius of a corner should be at least half of the wall thickness
2. Outside radius of a corner should be equal to the part thickness plus the inside radius
3. A radius being added to a boss or rib should be 1/4 of the part thickness, no smaller than .015

Material selection and processing conditions are equally important factors in the proper design for a plastic injection molded component or part.

We understand that initially, material selection may seem overwhelming since there are so many materials to choose from. You can rest easy knowing that API has partnered with resin suppliers for over 60 years and has a multitude of experience in manufacturing plastic parts with even the most difficult-to-process resins. Please see our basic resin guidefor a few examples of commodity and engineering resins that API has become an expert in molding.

Give us a call today (516) 333-7577 to discuss your plastic design or request a quote today.

Undercut helps pull the part

A third approach is shown in Figure 6. Here the part is formed between a cavity in the A-side mold half and a core in the B-side. After cooling, the mold opens, leaving the part on the core. With nothing pressing against the outside of the part, the part wall is free to stretch as it is ejected off the B-side core, releasing the part with the inside ring intact.

Figure 6 – Mold can be made core-cavity, allowing room for the part to “bump-off” after the mold opens