A thermoset preform behaves nothing like a thermoplastic sheet. You cannot re-melt it, you cannot re-flow a cold shot, and once the cure has advanced past a certain point the material in the tray is scrap — not rework, scrap. That is the reason preheating is treated as a controlled process step in compression and transfer molding rather than as a warm-up. A high frequency preheater heats the compound volumetrically, from the inside out, which is the one property that makes it more than a faster oven.
Why HF and Not an Oven
Ovens and infrared panels heat a thermoset preform from the surface inward. Heat has to conduct through a material that is a poor conductor by design, so the outside reaches temperature long before the core does. On a phenolic or melamine preform that is the worst possible profile: the skin is hot enough to start curing while the centre is still cold and stiff. You then press a preform that has a partly cured shell and a hard core, and no amount of press pressure will fix it.
Dielectric heating works the other way round. The radio frequency field acts on the polar groups throughout the volume, so the temperature profile is nearly flat from the surface to the centre. Two practical consequences follow: the material flows as one body rather than as a soft skin around a hard core, and the moisture trapped inside is driven out rather than sealed in under a cured surface.
| Method | Heat Profile | What Goes Wrong |
|---|---|---|
| Oven / hot air | Surface hot, core cold; slow and uneven | A cured skin forms before the core softens; drying happens only at the surface |
| Infrared | Very surface-weighted; depends on colour and emissivity | Dark compounds scorch on the face; light compounds under-heat |
| HF dielectric | Volumetric and near-uniform; fast | Runs away if the energy dose is not controlled — see below |
The Runaway You Have to Design Against
Thermosetting compounds are convenient to heat dielectrically because their loss factor is high. That same property is the hazard: for phenolic and melamine compounds the dielectric loss factor rises as the material warms, so a batch that is absorbing a given power at 40 °C absorbs more of it at 90 °C. Left on a fixed power setting, the last part of the cycle heats faster than the first, and the centre — the hottest region — reaches cure onset first.
The failure looks like a batch that felt right and then set hard in the tray, or a preform that flows well on the outside and short-shoots. Three habits prevent it, and all three are procedural rather than electronic:
What Preheating Actually Buys You
The benefits are usually listed as a series of claims. Read as process effects, they are all consequences of two things: lower and more uniform viscosity, and less water in the compound.
| Effect | Mechanism | Where You See It |
|---|---|---|
| Faster fill, lower pressure | Softened compound flows at lower viscosity, so it reaches cavity extremities before it starts to set | Fewer short shots; lower clamp and transfer pressure; less flash |
| No water marks or voids | Moisture leaves the compound during preheat instead of flashing to steam inside the cavity | Cleaner surfaces; fewer blisters near the gate — the defect that costs the most to inspect for |
| Lower internal stress | A uniform preform temperature means the part cures at a more even rate across its section | Less warpage and less cracking after ejection and in service |
| Shorter in-mold cure | Part of the heat the cure needs has already been delivered outside the mold | Shorter cycle, or the same cycle at a lower mold temperature — which is gentler on the tool |
| Better shot-weight control | Soft material compresses predictably, so the charge that goes into the cavity is repeatable | Less material waste per shot, and less weight variation between parts |
| Longer mold life | Softer compound and lower required pressure reduce mechanical and abrasive loading on gates and cavity edges | Slower gate wear on filled compounds, which are abrasive by nature |
Plate Type or Roller Type
The two configurations exist because the raw material arrives in two different forms, and mixing them up is the most common specification error.
| Configuration | Material Form | Practical Notes |
|---|---|---|
| Plate type | Pulverised or granular compound, spread in a tray or pressed into a preform | Batch process. Capacity is quoted as preheating weight, so the tray load and the electrode area have to match. Powder handling needs dust control around the machine. |
| Roller type | Solid or sheet material, fed through in a continuous strip | Throughput is set by feed speed and strip thickness rather than by batch weight. Consistent strip thickness matters more than consistent weight per piece. |
On the plate machines the numbers that decide fit are electrode size and preheating weight. A 310 × 360 mm electrode with a 1000 g batch and a 340 × 380 mm electrode with a 3000 g batch are two different production scales, not two options on the same machine: the second is for larger parts or higher shot rates, and it draws twice the input power to deliver them. The high frequency preheater range sets out both plate models and the roller alternative side by side.
Matching the Preheater to the Press
Preheating is a batch step feeding a cyclic machine, and that mismatch is where most lines lose output. The useful calculation is short and worth doing before the machine is ordered.
For a part close to the batch capacity the position is worse than the arithmetic suggests, because a 900 g part on a 1000 g machine leaves no room: you preheat one preform at a time and the preheat cycle is fully serial with the press. A 3000 g machine running three preforms side by side decouples the two, which is usually what makes a line flow rather than stutter.
Checking a Preheat Without a Thermometer
A surface temperature reading on a thermoset preform is misleading: the surface cools within seconds of leaving the machine and it was never the coldest point anyway. Three checks are more useful, and all of them can be done at the press.
| Check | Method | Reading |
|---|---|---|
| Flow pill | Press a weighed pill under a fixed load for a fixed time and measure the spread diameter | A stable diameter batch after batch means a stable preheat. A shrinking diameter means the compound is advancing. |
| Weight loss | Weigh the batch before and after preheating | Shows how much moisture left. A sudden change in loss usually means the material lot or the storage changed. |
| Section check | Cut a preform in half immediately after removal and compare core with surface | A visible colour or stiffness gradient means the preheat is too fast for the section thickness, or the batch is too thick for the electrode |
The flow pill is the one worth institutionalising. It converts a subjective judgement into a number the operator can compare against yesterday, and it catches the slow drift in electrode condition or tube output that otherwise shows up as an unexplained rise in short shots a month later. Tube behaviour is worth watching for the same reason — output falls gradually before it fails outright, which is covered under high frequency oscillator tubes.
Defects That Trace Back to the Preheater
| Molding Defect | Preheat Cause | Correction |
|---|---|---|
| Water marks and surface haze | Batch under-heated, so moisture never left the compound | Increase dose with the same batch weight; check that storage has not let the material pick up water |
| Short shots at the far end of the cavity | Preform too cold, or cooled between removal and loading | Shorten the preheat-to-load path before raising temperature; a long walk to the press costs more than a few seconds of heat |
| Compound setting in the tray | Over-heating, usually from a fixed power setting on a warm batch or a second pass | Control by dose and weight; never re-run an interrupted batch |
| Blisters near the gate | Moisture plus too fast a fill — heat was added but drying time was not | Longer lower-power preheat, or a hold; check compound storage and shelf life |
| Part weight varying between shots | Batch weight not controlled, or uneven density in the tray | Weigh every charge; level the powder in the tray rather than heaping it |
| Warpage and cracking in service | Uneven preform temperature, usually a batch too thick for the electrode to heat through | Reduce batch thickness, not batch time; spreading the charge wider heats more evenly than leaving it in a pile |
Melamine, Phenolic, Epoxy: The Differences That Matter
All three are thermosets with useful dielectric loss, but they do not share a recipe and they do not fail in the same way.
| Compound | Typical Products | Preheating Notes |
|---|---|---|
| Melamine | Tableware, trays, electrical accessories, decorative laminates | The most common material for this class of machine and the most forgiving. Moisture control dominates quality — this is where water marks are most visible. |
| Phenolic | Commutators, electrical components, handles, industrial parts | Darker compounds hide surface defects, so quality problems appear as internal voids rather than visible marks. Preheat uniformity matters more than absolute temperature. |
| Epoxy molding compound | Semiconductor packages, discrete devices, integrated circuits | The least forgiving. Filled and highly reactive, with a narrow window: under-heat gives incomplete fill of fine features, over-heat starts the cure before the compound reaches them. |
Treat published preheat temperatures as starting points only. The right figure depends on the compound, the filler loading, the preform size and even the season, because cold material entering the machine absorbs the same dose more slowly. Confirm the window with the compound supplier, then find it on your own machine with the flow pill. The wider context of how these materials behave in forming and molding is covered in the technical guide to mould machines, and the tooling that follows the preheat is described under moulds.
Five Questions Before You Specify
Two operational points sit outside the specification and are worth planning for. Powder handling around a plate machine needs extraction and housekeeping, because fine compound settles on hot surfaces and on the electrode. And the HF enclosure has to be treated as a safety item, not as a cover: the machine is running at 27.12 MHz with several kilowatts, and a shield that is removed for convenience is a hazard the next shift inherits. The components inside that enclosure are described in essential HF accessories.
Frequently Asked Questions
Tell Us the Charge Weight and the Shots per Hour
Chenghao Machinery builds high frequency preheaters for phenolic, melamine and epoxy molding compounds — plate type for pulverised material, roller type for solid stock, from 5 kW to 10 kW at 27.12 MHz. Send us the compound, the charge weight and your target shot rate, and we will tell you which model keeps up with the press.
Dongguan Shenghao Machinery Co., Ltd. (Chenghao Machinery) | sales11@chenghaointernational.com | +86 158 1765 5869
Dongguan, Guangdong, China