An ABS junction box lid comes off the press with a shallow dent over each screw boss, and the cover no longer sits flat on its gasket. Turning up the hold pressure helps one shot and flashes the next. Junction box injection moulding on a 150 t press works well when wall thickness, boss design, resin grade and cooling are planned together, and this article covers each.
Here, boxes means wiring and junction boxes, meter and control housings, and small appliance shells.
Key Takeaways
- Cooling time rises with the square of wall thickness, so a wall 20% thinner needs about 36% less cooling, and a thick boss cools last.
- Flame ratings depend on thickness. Covestro rates its Makrolon 6555 flame-retardant PC UL 94 V-0 at 3.0 mm but only V-2 at 1.0 mm.
- Boxes and enclosures for household electrical accessories must meet IS 14772:2020 under the Electrical Accessories (Quality Control) Order, 2023.
- Dry the resin before moulding: INEOS gives 71 to 88 °C for 2 to 4 hours for Lustran ABS 248, and Covestro 120 °C for 2 to 3 hours for Makrolon 6555.
What Does Junction Box Injection Moulding Need From the Machine?
It needs enough clamp for the box's projected area in the chosen resin, enough opening stroke to eject a deep box, mould temperature control for engineering resins, and a stable process that holds wall thickness and flatness shot after shot. Depth and flatness can matter more than clamp.
A box's projected area is its open face, not its weight, so a deep box can need modest clamp for a heavy shot. A 150 × 150 mm box face is 225 cm²; in ABS at 2 to 4 US t/in² that needs about 63 to 126 t before any margin. The method is set out in the applications article in this series.
Stroke is the check people skip. As a rough check, not a rule, plan an opening stroke of at least twice the box depth plus room for the part to fall clear. A 100 mm deep box therefore wants more than 200 mm of clear opening.
Flame-retardant PC needs a mould temperature controller. Covestro recommends 80 to 120 °C for Makrolon 6555, far above chilled water, and an uneven mould temperature is one more way to warp a flat lid.
Why Does Wall Thickness Decide Cycle Time and Flatness?
Cooling can take the largest share of a housing's cycle, and RJG notes cooling time depends most on wall thickness, which is squared in the cooling equation. A wall 20% thinner needs about 36% less cooling, so uneven walls cool unevenly and pull a box out of flat.
Relative cooling time by wall thickness (squared relationship, 3.0 mm = 1.00)
| Wall thickness | Relative cooling time | What it means for a box |
|---|---|---|
| 2.0 mm | 0.44 | Fast cycle; check stiffness, knock-outs and flame rating |
| 2.5 mm | 0.69 | About 31% less cooling than 3.0 mm |
| 3.0 mm | 1.00 | Reference wall |
| 3.5 mm | 1.36 | About 36% more cooling; often a boss or rib root grown too thick |
RJG reports a trial on an eight-cavity ABS tensile bar mould, 3.3 mm thick, where about 85% of the cycle was spent cooling. It also notes only about 40% of the heat has to come out before the part can be ejected, and suggests adding about 20% to the calculated cooling time as a safety factor.
That is why the dent over the boss appears. A solid boss is a local thick section that keeps cooling after the walls around it have set, and it pulls the face in as it shrinks. Core the boss out, keep ribs thinner than the wall they support, and gate so the thick sections pack last.
Which Resin Suits Indoor Boxes, Outdoor Boxes and Housings?
ABS suits many indoor housings and small appliance shells, and PC suits parts that need impact strength, heat resistance or a flame rating at thin walls. Each grade has its own clamp factor, drying and mould temperature, so fix the grade before the mould steel is cut.
INEOS names small appliance housings among the typical uses of Lustran ABS 248, rates it UL 94 HB, and gives a mould temperature of 43 to 66 °C and a maximum regrind of 20%. HB is the lowest UL 94 class, so an ABS box that holds live parts may need a flame-retardant grade instead.
Covestro lists a glow wire flammability index of 960 °C for Makrolon 6555 at 1.5 and 3.0 mm, falling to 875 °C at 0.75 mm. IEC 60695-2-12 defines that index as the highest test temperature at which the material does not ignite, or goes out within 30 seconds without burning drips igniting the tissue below.
It is a material test, so the finished box is still tested on its own. Our position is firm: never thin a flame-rated wall to save cycle time without re-checking the rating at the new thickness. A cheaper cycle that fails a type test costs far more.
What Indian Rules Apply to Electrical Boxes?
Boxes and enclosures for household electrical accessories must meet IS 14772:2020 and carry the Standard Mark under the Electrical Accessories (Quality Control) Order, 2023. The standard covers marking, access to live parts, earthing provision, ingress of solids and water, and mechanical and thermal strength.
The Order was published as S.O. 43(E) dated 1 January 2024. It came into force six months after publication, with nine months allowed for small enterprises and twelve for micro enterprises, and goods made for export are excluded. Amendment orders can move dates, so check the latest notification before you plan production.
A BIS summary of IS 14772 says boxes are classified by material, installation type, inlets, clamping means, minimum installation temperature and degree of protection. Lids, covers, drain holes, knock-outs and screw fixings must be designed to keep out solids and water. For a moulder, those are tested features, not cosmetic ones.
Switches that fit into these boxes fall under IS 3854 and their own compulsory certification.
What Should You Send Before Moulding a Box or Housing?
Send the drawing or a sample, the resin grade and any flame or IP rating it must meet, the wall thickness at bosses and ribs, the box depth, the cavity count and your monthly volume. With those, clamp, stroke, cooling and output can be estimated together. See the 150 ton horizontal injection moulding machine page.
Frequently Asked Questions About Junction Box Injection Moulding
Why do screw bosses leave sink marks on a box lid?
A solid boss is a thick section joined to a thinner wall. The wall freezes first, and the boss keeps shrinking as it cools, pulling the outer face inward. Coring the boss, adding gussets instead of thickness, and packing through a gate near the thick area all reduce the dent on the visible side.
How much regrind can go back into an ABS housing?
Follow the resin supplier's limit for your grade. INEOS gives a maximum of 20% regrind for Lustran ABS 248. Keep regrind dry and clean, feed it at a steady ratio, and check that colour, impact strength and any flame rating still hold, because a certified box is approved on its declared material.
Does an outdoor box need a different resin from an indoor one?
Usually it needs a different grade. IS 14772 covers boxes for indoor and outdoor use, and outdoor boxes face sunlight, rain and wider temperature swings. A UV-stabilised grade, a sealing face that stays flat and a tested degree of protection matter more outdoors than gloss or colour.
Do knock-outs need special mould features?
Yes. A knock-out is a thin web that must hold during handling yet break cleanly on site. Its thickness, the groove around it and where the melt meets it decide how it behaves. Weld lines across a knock-out can make it break unevenly, so check gate position on the mould design.
Is the glow wire index the same as testing the finished box?
No. Under IEC 60695-2-12, the glow wire flammability index is measured on material plaques at set thicknesses. It helps you choose a grade, but the finished product is tested separately under its own standard. A box with thinner walls or sharp edges can behave differently from the plaque.
Related Articles
- What Can You Make on a 150 Ton Injection Moulding Machine?
- Three Screw Options on a 150 Ton Machine: Which One Fits Your Part?
- 150 Ton Machine for Small Containers and Mugs
Sources & References
- RJG Inc.: How to Determine Injection Molding Cooling Time
- Covestro: Makrolon 6555 product page
- Gazette of India (DPIIT), S.O. 43(E): Electrical Accessories (Quality Control) Order, 2023 (1 January 2024)
- Bureau of Indian Standards: IS 14772:2020 Boxes and Enclosures for Electrical Accessories: summary
Technical Disclaimer
Disclaimer: The information provided in this document is for general informational and educational purposes only. It is based on technical references and industry information available at the time of preparation and may be subject to change. It does not constitute engineering, technical, commercial or professional advice, nor does it constitute a quotation, specification, warranty or contractual commitment by Pratishna Engineers.
Actual injection moulding machine and process requirements vary according to the component design, material, mould, production volume, machine configuration and operating conditions. The information, examples, calculations and recommendations provided should be independently evaluated and verified for the specific application before making any equipment, process or production decision. Information obtained from third-party sources remains subject to the accuracy and limitations of those sources.
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