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Guidelines for Part & Tool Design for Molding Rigid Clear Materials

Stiff-flowing polystyrenes, acrylics, and the newest copolyesters have molding limitations that must be accounted for

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By Mark Rosen
Corex Design Group Inc., Franklin Lakes, New Jersey, USA

Guidelines for Part & Tool Design for Molding Rigid Clear Materials

Stiff-flowing polystyrenes, acrylics, and the newest copolyesters have molding limitations that must be accounted for

Previous Article       Next Article

By Mark Rosen
Corex Design Group Inc., Franklin Lakes, New Jersey, USA

Guidelines for Part & Tool Design for Molding Rigid Clear Materials

Stiff-flowing polystyrenes, acrylics, and the newest copolyesters have molding limitations that must be accounted for

Previous Article       Next Article

By Mark Rosen
Corex Design Group Inc., Franklin Lakes, New Jersey, USA

Figure 1: A long clear part with issues of part cracking near the gate at mold-open (the issue was that ribs near the gate had too little draft).

Table 1: Flow Lengths (in Inches) for Various Wall Thicknesses (0.060-0.125"), for a Simple, Rectangular Part

Table 1: Flow Lengths (in Inches) for Various Wall Thicknesses (0.060-0.125"), for a Simple, Rectangular Part

Table 1: Flow Lengths (in Inches) for Various Wall Thicknesses (0.060-0.125"), for a Simple, Rectangular Part

Figure 2: Study of draft angle (left) vs. cooling time (right) vs. for 2.5” (63.5-mm) tall rib, 0.070” (1.8-mm) wide at the top (the material was Evonic Cyro R-30 impact-modified acrylic; analysis was run with Simpoe-Mold, Dassault Systems).

Figure 2: Study of draft angle (left) vs. cooling time (right) vs. for 2.5” (63.5-mm) tall rib, 0.070” (1.8-mm) wide at the top (the material was Evonic Cyro R-30 impact-modified acrylic; analysis was run with Simpoe-Mold, Dassault Systems).

Figure 2: Study of draft angle (left) vs. cooling time (right) vs. for 2.5” (63.5-mm) tall rib, 0.070” (1.8-mm) wide at the top (the material was Evonic Cyro R-30 impact-modified acrylic; analysis was run with Simpoe-Mold, Dassault Systems).

[Ed. note: The author can be reached at mrosen@corexdg.com or U.S. 201-970-9188; learn more about his services at the end of this article.]

 

The injection molding of rigid clear materials can often be a challenge due to their stiff-flowing nature and somewhat brittle mechanical properties. These materials can range from low-cost crystal polystyrene to higher-performance materials such as impact-modified acrylics or copolyesters. I’m often asked to troubleshoot molds for these clear, stiff materials, facing issues such as poor surface cosmetics, cracking at ejection (e.g., Figure 1), and underfilled regions of the part.

Although there are many rigid clear materials, the goal of this article is not to discuss the range of material options but to highlight some of the general requirements for part and tooling design and processing. Many of these materials have common traits in that they are relatively stiff-flowing, can degrade and off-gas when overheated or over-sheared, have a lower percent elongation at break and lower notched-Izod values, and can sometimes stick to hot polished surfaces. 

Whenever troubleshooting parts that are molded of these materials, I first focus on the following questions, which will be discussed in this article, along with some general rules for good part and mold design:

Fill Pressure

When running these materials, establishing the correct pack profile can be tricky. Higher injection pressures may be needed to overcome restrictive gating, longer flow lengths, and poor venting. Raising the melt temperatures to lower the fill pressures can cause issues with many clear materials. At too-high a temperature and/or extended residence times, the surface finish of the part can become worse and the material can become more gaseous. This can make part-sticking issues worse.

In an effort to reduce sticking of the part, reducing the pack/hold pressure and time can result in a part with “wavy” surfaces and/or increased warpage. The use of larger gates are recommended since this will reduce the fill pressures and allow for longer pack times, which is preferred over high pack pressures and shorter pack times required when thinner gates are used.

Many clear materials are stiffer-flowing materials with melt flow rate (MFR) values in the range of 3 to 8 g/10 min. (ASTM D1238). Also, being amorphous in nature, these materials require longer pack times. With this type of material, higher fill pressures should be avoided since this can introduce added stresses near the gate and increase the stresses on the part during part ejection. For these stiffer-flowing materials, as a general rule, it’s recommended to have a part design and gate location such that total pressure drop does not exceed around 15,000 psi (1030 bar). For design purposes, this results in a cavity pressure drop limit of around 10,000 psi (690 bar) and runner loss of no more than 5000 psi (340 bar). These pressure drops should confirmed by a 3-D mold-filling analysis by a skilled user. 

In Table 1, a simple rectangular part (cavity only) was analyzed by Simpoe-Mold (Dassault Systems) using a tetrahedral 3-D mesh to test the fill pressure at various thicknesses for three different clear materials. A pressure limit of 10,000 psi (690 bar) was used for the cavity fill pressure. Note that the higher performance impact-modified acrylic and copolyester are stiffer flowing than the crystal PS. 

Runners & Gating

For clear parts with optical surfaces, the use of cold runners with wide gates located on the side of the part is preferred over hot runners with gates in the middle of the part. This is because gates located in the middle of the part are more prone to stress marks, plus create concerns with a visible gate vestige. A combination of hot and cold runners can also be successfully used with side tab gates. For larger parts, sometimes center gates must be used to reduce the flow length. If so, larger valve gates or “hot sprue gate”-type tips are recommended.

First, try to use the minimum number of gates which will fill the part at a reasonable pressure. This results in a simpler flow pattern with fewer weld lines and flow hesitation. Run mold-fill analysis to optimize gate location, number of gates, and runner sizes. Gate in thicker wall section of the part, and try to locate gates so there’s a flat flow pattern at end of fill; this results in less acceleration of the melt front at the end of fill, which improves venting and part cosmetics. Other recommendations include:

Draft & Polish

For these materials, suppliers will typically recommend that parts be designed with a minimum draft angle of 1 to 3 degrees to reduce sticking during mold-open and part ejection. However, with taller ribs, using these larger draft angle recommendations can result in wall sections at the base of the ribs which are too thick, resulting in long cycle times, along with surface sink or voids.

In Figure 2, it can be seen that for a 2 ½” (76-mm) tall rib with a 0.070” (1.8 mm) top thickness, using a draft angle of 1° results in a reasonable estimated mold-closed time. This compares to uneconomical estimated mold-closed times of 68 and 80 seconds for 2° and 3°, respectively. 

The following are some general guidelines for draft and polish:

Venting

Good venting is key to molding clear parts. Molds with compromised venting will require slower fill time and higher fill pressures and be more prone to surface defects (see also the article “Good Venting” in the June 2015 Plastics Engineering for an overview of venting recommendations).

First, run a flow simulation to locate gates to minimize air traps, severe weld lines, and converging flow fronts at end of fill. And.... 

Cooling 

Many clear materials have a tendency to stick to hot mold surfaces, so the mold should have good cooling to minimize hot spots. Try to avoid part designs requiring long slender cores in the mold which will run hotter. Also, try to avoid tall ribs or slender cores on the front half of the mold, as these can result in the part sticking at mold open. With box-shaped parts, if possible, move the ejector pin locations to allow cooling of inside corners of the parts. With cold runner layouts, larger cold sprues should be used, and adding water in the mold to cool the sprue is recommended.

Summary

In summary, the molding of stiff clear materials can sometimes make it challenging to get acceptable part surface cosmetics and to not damage parts during mold-open and ejection. In this article, we discussed some rules to follow for part and tool design to help with these challenges.

To improve the odds of success, it’s recommended to include in the budget a detailed part and mold design review by a broadly skilled review team with expertise in materials, part design, tooling, and processing. This should also always include a filling analysis (by a skilled analyst using a higher-end analysis program). Parts molded with these clear rigid materials often have delays in sampling approvals, with expensive tooling changes often due to rushed schedules, lack of up-front engineering review, and attempts to save money with lower-cost tooling.

 

About the author… Mark Rosen is founder of Corex Design Group (www.corexdg.com), an award-winning plastics consulting firm consisting of plastics industry veterans available to assist companies with design, engineering, analysis, and technical marketing. He can be reached via e-mail at mrosen@corexdg.com or by phone at +1 201-970-9188.